Role of the vertical pressure gradient in wave boundary layers
DEFF Research Database (Denmark)
Jensen, Karsten Lindegård; Sumer, B. Mutlu; Vittori, Giovanna
2014-01-01
By direct numerical simulation (DNS) of the flow in an oscillatory boundary layer, it is possible to obtain the pressure field. From the latter, the vertical pressure gradient is determined. Turbulent spots are detected by a criterion involving the vertical pressure gradient. The vertical pressure...... gradient is also treated as any other turbulence quantity like velocity fluctuations and statistical properties of the vertical pressure gradient are calculated from the DNS data. The presence of a vertical pressure gradient in the near bed region has significant implications for sediment transport....
Vertical ozone characteristics in urban boundary layer in Beijing.
Ma, Zhiqiang; Xu, Honghui; Meng, Wei; Zhang, Xiaoling; Xu, Jing; Liu, Quan; Wang, Yuesi
2013-07-01
Vertical ozone and meteorological parameters were measured by tethered balloon in the boundary layer in the summer of 2009 in Beijing, China. A total of 77 tethersonde soundings were taken during the 27-day campaign. The surface ozone concentrations measured by ozonesondes and TEI 49C showed good agreement, albeit with temporal difference between the two instruments. Two case studies of nocturnal secondary ozone maxima are discussed in detail. The development of the low-level jet played a critical role leading to the observed ozone peak concentrations in nocturnal boundary layer (NBL). The maximum of surface ozone was 161.7 ppbv during the campaign, which could be attributed to abundant precursors storage near surface layer at nighttime. Vertical distribution of ozone was also measured utilizing conventional continuous analyzers on 325-m meteorological observation tower. The results showed the NBL height was between 47 and 280 m, which were consistent with the balloon data. Southerly air flow could bring ozone-rich air to Beijing, and the ozone concentrations exceeded the China's hourly ozone standard (approximately 100 ppb) above 600 m for more than 12 h.
Surface influence upon vertical profiles in the nocturnal boundary layer
Garratt, J. R.
1983-05-01
Near-surface wind profiles in the nocturnal boundary layer, depth h, above relatively flat, tree-covered terrain are described in the context of the analysis of Garratt (1980) for the unstable atmospheric boundary layer. The observations at two sites imply a surface-based transition layer, of depth z *, within which the observed non-dimensional profiles Φ M 0 are a modified form of the inertial sub-layer relation Φ _M ( {{z L}} = ( {{{1 + 5_Z } L}} ) according to Φ _M^{{0}} ˜eq ( {{{1 + 5z} L}} )exp [ { - 0.7( {{{1 - z} z}_ * } )] , where z is height above the zero-plane displacement and L is the Monin-Obukhov length. At both sites the depth z * is significantly smaller than the appropriate neutral value ( z * N ) found from the previous analysis, as might be expected in the presence of a buoyant sink for turbulent kinetic energy.
Vertical transport of water in the Martian boundary layer
Zent, Aaron P.; Haberle, R. M.; Houben, Howard C.
1993-01-01
We are continuing our examination of the transport of H2O through the martian boundary layer, and we have written a one-dimensional numerical model of the exchange of H2O between the atmosphere and subsurface of Mars through the planetary boundary layer (PBL). Our goal is to explore the mechanisms of H2O exchange, and to elucidate the role played by the regolith in the local H2O budget. The atmospheric model includes effects of Coriolis, pressure gradient, and frictional forces for momentum, as well as radiation, sensible heat flux, and advection for heat. The model differs from Flasar and Goody by use of appropriate Viking-based physical constants and inclusion of the radiative effects of atmospheric dust. We specify the pressure gradient force or compute it from a simple slope model. The subsurface model accounts for conduction of heat and diffusion of H2O through a porous adsorbing medium in response to diurnal forcing. The model is initialized with depth-independent H2O concentrations (2 kg M(exp -3)) in the regolith, and a dry atmosphere. The model terminates when the atmospheric H2O column abundance stabilizes at 0.1 percent per sol.
International Nuclear Information System (INIS)
Ishak, Anuar; Nazar, Roslinda; Pop, Ioan
2008-01-01
The mixed convection boundary layer flow through a stable stratified porous medium bounded by a vertical surface is investigated. The external velocity and the surface temperature are assumed to vary as x m , where x is measured from the leading edge of the vertical surface and m is a constant. Numerical solutions for the governing Darcy and energy equations are obtained. The results indicate that the thermal stratification significantly affects the surface shear stress as well as the surface heat transfer, besides delays the boundary layer separation
International Nuclear Information System (INIS)
Druilhet, A.; Guedalia, D.; Fontan, J.
1980-01-01
Methods for determining the vertical exchange coefficients in the planetary boundary layer using the natural radioactive tracers radon ( 222 Rn), thoron ( 220 Rn), and ThB( 212 Pb) are presented here. Thoron (T/sub 1/2/ = 54 sec) was used for the surface layer. The main results are given for two applications: vertical exchange studies above a flat country and inside a vegetable canopy. Owing to its lifetime, radon can be used for the nocturnal stabilities that have an important concentration increase near the ground. For the planetary layer (0 to 2000m), ThB(T/sub 1/2/ = 10.6 hr) was mainly used
Two-phase gas bubble-liquid boundary layer flow along vertical and inclined surfaces
International Nuclear Information System (INIS)
Cheung, F.B.; Epstein, M.
1985-01-01
The behavior of a two-phase gas bubble-liquid boundary layer along vertical and inclined porous surfaces with uniform gas injection is investigated experimentally and analytically. Using argon gas and water as the working fluids, a photographical study of the two-phase boundary layer flow has been performed for various angles of inclination ranging from 45 0 to 135 0 and gas injection rates ranging from 0.01 to 0.1 m/s. An integral method has been employed to solve the system of equations governing the two-phase motion. The effects of the gas injection rate and the angle of inclination on the growth of the boundary layer have been determined. The predicted boundary layer thickness is found to be in good agreement with the experimental results. The calculated axial liquid velocity and the void fraction in the two-phase region are also presented along with the observed flow behavior
Wind tunnel study of a vertical axis wind turbine in a turbulent boundary layer flow
Rolin, Vincent; Porté-Agel, Fernando
2015-04-01
Vertical axis wind turbines (VAWTs) are in a relatively infant state of development when compared to their cousins the horizontal axis wind turbines. Very few studies have been carried out to characterize the wake flow behind VAWTs, and virtually none to observe the influence of the atmospheric boundary layer. Here we present results from an experiment carried out at the EPFL-WIRE boundary-layer wind tunnel and designed to study the interaction between a turbulent boundary layer flow and a VAWT. Specifically we use stereoscopic particle image velocimetry to observe and quantify the influence of the boundary layer flow on the wake generated by a VAWT, as well as the effect the VAWT has on the boundary layer flow profile downstream. We find that the wake behind the VAWT is strongly asymmetric, due to the varying aerodynamic forces on the blades as they change their position around the rotor. We also find that the wake adds strong turbulence levels to the flow, particularly on the periphery of the wake where vortices and strong velocity gradients are present. The boundary layer is also shown to cause greater momentum to be entrained downwards rather than upwards into the wake.
A theory for natural convection turbulent boundary layers next to heated vertical surfaces
International Nuclear Information System (INIS)
George, W.K. Jr.; Capp, S.P.
1979-01-01
The turbulent natural convection boundary layer next to a heated vertical surface is analyzed by classical scaling arguments. It is shown that the fully developed turbulent boundary layer must be treated in two parts: and outer region consisting of most of the boundary layer in which viscous and conduction terms are negligible and an inner region in which the mean convection terms are negligible. The inner layer is identified as a constant heat flux layer. A similarity analysis yields universal profiles for velocity and temperature in the outer and constant heat flux layers. An asymptotic matching of these profiles in an intermediate layer (the buoyant sublayer) yields analytical expressions for the buoyant sublayer profiles. Asymptotic heat transfer and friction laws are obtained for the fully developed boundary layers. Finally, conductive and thermo-viscous sublayers characterized by a linear variation of velocity and temperature are shown to exist at the wall. All predictions are seen to be in excellent agreement with the abundant experimental data. (author)
Two-phase gas bubble-liquid boundary layer flow along vertical and inclined surfaces
International Nuclear Information System (INIS)
Cheung, F.B.; Epstein, M.
1985-01-01
The behavior of a two-phase gas bubble liquid boundary layer along vertical and inclined porous surfaces with uniform gas injection is investigated experimentally and analytically. Using argon gas and water as the working fluids, a photographical study of the two-phase boundary layer flow has been performed for various angles of inclination ranging from 45 0 to 135 0 and gas injection rates ranging from 0.01 to 0.1 m/s. An integral method has been employed to solve the system of equations governing the two-phase motion. The effects of the gas injection rate and the angle of inclination on the growth of the boundary layer have been determined
International Nuclear Information System (INIS)
Tsuji, Toshihiro; Kajitani, Tsuyoshi; Nishino, Tatsuhiko
2007-01-01
An experimental study on heat transfer enhancement for a turbulent natural convection boundary layer in air along a vertical flat plate has been performed by inserting a long flat plate in the spanwise direction (simple heat transfer promoter) and short flat plates aligned in the spanwise direction (split heat transfer promoter) with clearances into the near-wall region of the boundary layer. For a simple heat transfer promoter, the heat transfer coefficients increase by a peak value of approximately 37% in the downstream region of the promoter compared with those in the usual turbulent natural convection boundary layer. It is found from flow visualization and simultaneous measurements of the flow and thermal fields with hot- and cold-wires that such increase of heat transfer coefficients is mainly caused by the deflection of flows toward the outer region of the boundary layer and the invasion of low-temperature fluids from the outer region to the near-wall region with large-scale vortex motions riding out the promoter. However, heat transfer coefficients for a split heat transfer promoter exhibit an increase in peak value of approximately 60% in the downstream region of the promoter. Flow visualization and PIV measurements show that such remarkable heat transfer enhancement is attributed to longitudinal vortices generated by flows passing through the clearances of the promoter in addition to large-scale vortex motions riding out the promoter. Consequently, it is concluded that heat transfer enhancement of the turbulent natural convection boundary layer can be substantially achieved in a wide area of the turbulent natural convection boundary layer by employing multiple column split heat transfer promoters. It may be expected that the heat transfer enhancement in excess of approximately 40% can be accomplished by inserting such promoters
The vertical structure of the Saharan boundary layer: Observations and modelling
Garcia-Carreras, L.; Parker, D. J.; Marsham, J. H.; Rosenberg, P.; Marenco, F.; Mcquaid, J.
2012-04-01
The vertical structure of the Saharan atmospheric boundary layer (SABL) is investigated with the use of aircraft data from the Fennec observational campaign, and high-resolution large-eddy model (LEM) simulations. The SABL is one of the deepest on Earth, and crucial in controlling the vertical redistribution and long-range transport of dust in the Sahara. The SABL is typically made up of an actively growing convective region driven by high sensible heating at the surface, with a deep, near-neutrally stratified Saharan residual layer (SRL) above it, which is mostly well mixed in humidity and temperature and reaches a height of ~500hPa. These two layers are usually separated by a weak (≤1K) temperature inversion, making the vertical structure very sensitive to the surface fluxes. Large-eddy model (LEM) simulations initialized with radiosonde data from Bordj Bardji Mokhtar (BBM), southern Algeria, are used to improve our understanding of the turbulence structure of the stratification of the SABL, and any mixing or exchanges between the different layers. The model can reproduce the typical SABL structure from observations, and a tracer is used to illustrate the growth of the convective boundary layer into the residual layer above. The heat fluxes show a deep entrainment zone between the convective region and the SRL, potentially enhanced by the combination of a weak lid and a neutral layer above. The horizontal variability in the depth of the convective layer was also significant even with homogeneous surface fluxes. Aircraft observations from a number of flights are used to validate the model results, and to highlight the variability present in a more realistic setting, where conditions are rarely homogeneous in space. Stacked legs were performed to get an estimate of the mean flux profile of the boundary layer, as well as the variations in the vertical structure of the SABL with heterogeneous atmospheric and surface conditions. Regular radiosondes from BBM put
Amazon boundary layer aerosol concentration sustained by vertical transport during rainfall.
Wang, Jian; Krejci, Radovan; Giangrande, Scott; Kuang, Chongai; Barbosa, Henrique M J; Brito, Joel; Carbone, Samara; Chi, Xuguang; Comstock, Jennifer; Ditas, Florian; Lavric, Jost; Manninen, Hanna E; Mei, Fan; Moran-Zuloaga, Daniel; Pöhlker, Christopher; Pöhlker, Mira L; Saturno, Jorge; Schmid, Beat; Souza, Rodrigo A F; Springston, Stephen R; Tomlinson, Jason M; Toto, Tami; Walter, David; Wimmer, Daniela; Smith, James N; Kulmala, Markku; Machado, Luiz A T; Artaxo, Paulo; Andreae, Meinrat O; Petäjä, Tuukka; Martin, Scot T
2016-11-17
The nucleation of atmospheric vapours is an important source of new aerosol particles that can subsequently grow to form cloud condensation nuclei in the atmosphere. Most field studies of atmospheric aerosols over continents are influenced by atmospheric vapours of anthropogenic origin (for example, ref. 2) and, in consequence, aerosol processes in pristine, terrestrial environments remain poorly understood. The Amazon rainforest is one of the few continental regions where aerosol particles and their precursors can be studied under near-natural conditions, but the origin of small aerosol particles that grow into cloud condensation nuclei in the Amazon boundary layer remains unclear. Here we present aircraft- and ground-based measurements under clean conditions during the wet season in the central Amazon basin. We find that high concentrations of small aerosol particles (with diameters of less than 50 nanometres) in the lower free troposphere are transported from the free troposphere into the boundary layer during precipitation events by strong convective downdrafts and weaker downward motions in the trailing stratiform region. This rapid vertical transport can help to maintain the population of particles in the pristine Amazon boundary layer, and may therefore influence cloud properties and climate under natural conditions.
Vertical distribution of ozone and VOCs in the low boundary layer of Mexico City
Directory of Open Access Journals (Sweden)
E. Velasco
2008-06-01
Full Text Available The evolution of ozone (O_{3} and 13 volatile organic compounds (VOCs in the boundary layer of Mexico City was investigated during 2000–2004 to improve our understanding of the complex interactions between those trace gases and meteorological variables, and their influence on the air quality of a polluted megacity. A tethered balloon, fitted with electrochemical and meteorological sondes, was used to obtain detailed vertical profiles of O_{3} and meteorological parameters up to 1000 m above ground during part of the diurnal cycle (02:00–18:00 h. VOCs samples were collected up to 200 m by pumping air to canisters with a Teflon tube attached to the tether line. Overall, features of these profiles were found to be consistent with the formation of an upper residual layer during nighttime carrying over a fraction of the O_{3} from the previous day that contributes to the background concentration in surrounding regions. At the same time the release of heat stored in the urban surface forms a shallow unstable layer close to the ground, where the nocturnal emissions are trapped. After sunrise an O_{3} balance is determined by photochemical production, entrainment from the upper residual layer and destruction by titration with nitric oxide, delaying the ground-level O_{3} rise by 2 h. The subsequent evolution of the conductive boundary layer and vertical distribution of pollutants are discussed in terms of the energy balance, the presence of turbulence and the atmospheric stability.
Amazon boundary layer aerosol concentration sustained by vertical transport during rainfall
Energy Technology Data Exchange (ETDEWEB)
Wang, Jian; Krejci, Radovan; Giangrande, Scott; Kuang, Chongai; Barbosa, Henrique M. J.; Brito, Joel; Carbone, Samara; Chi, Xuguang; Comstock, Jennifer; Ditas, Florian; Lavric, Jost; Manninen, Hanna E.; Mei, Fan; Moran-Zuloaga, Daniel; Pöhlker, Christopher; Pöhlker, Mira L.; Saturno, Jorge; Schmid, Beat; Souza, Rodrigo A. F.; Springston, Stephen R.; Tomlinson, Jason M.; Toto, Tami; Walter, David; Wimmer, Daniela; Smith, James N.; Kulmala, Markku; Machado, Luiz A. T.; Artaxo, Paulo; Andreae, Meinrat O.; Petäjä, Tuukka; Martin, Scot T.
2016-10-24
A necessary prerequisite of cloud formation, aerosol particles represent one of the largest uncertainties in computer simulations of climate change1,2, in part because of a poor understanding of processes under natural conditions3,4. The Amazon rainforest is one of the few continental regions where aerosol particles and their precursors can be studied under near-natural conditions5-7. Cloud condensation nuclei (CCN) in clean Amazonia are mostly produced by the growth of smaller particles in the boundary layer8-10, whereas these smaller particles themselves 31 appear to be produced elsewhere5,11. Key questions are in what part of the atmosphere they might 32 be produced and what could be the transport processes that deliver them to the boundary layer, where they grow into CCN. Here, using recent aircraft measurements above central Amazonia, we show high concentrations of small particles in the lower free troposphere. The particle size spectrum shifts towards larger sizes with decreasing altitude, implying particle growth as air descends from the free troposphere towards Earth's surface. Complementary measurements at ground sites show that free tropospheric air having high concentrations of small particles (diameters of less than 50 nm) is transported into the boundary layer during precipitation events, both by strong convective downdrafts and by weaker downward motions in the trailing stratiform region. This vertical transport helps maintain the population of small particles and ultimately CCN in the boundary layer, thereby playing an important role in controlling the climate state under natural conditions. In contrast, this mechanism becomes masked under polluted conditions, which sometimes prevail at times in Amazonia as well as over other tropical continental regions5,12.
Interaction of an Artificially Thickened Boundary Layer with a Vertically Mounted Pitching Airfoil
Hohman, Tristen; Smits, Alexander; Martinelli, Luigi
2011-11-01
Wind energy represents a large portion of the growing market in alternative energy technologies and the current landscape has been dominated by the more prevalent horizontal axis wind turbine. However, there are several advantages to the vertical axis wind turbine (VAWT) or Darrieus type design and yet there is much to be understood about how the atmospheric boundary layer (ABL) affects their performance. In this study the ABL was simulated in a wind tunnel through the use of elliptical shaped vortex generators, a castellated wall, and floor roughness elements as described in the method of Counihan (1967) and then verified its validity by hot wire measurement of the mean velocity profile as well as the turbulence intensity. The motion of an blade element around a vertical axis is approximated through the use of a pitching airfoil. The wake of the airfoil is investigated through hot wire anemometry in both uniform flow and in the simulated boundary layer both at Re = 1 . 37 ×105 based on the chord of the airfoil. Sponsored by Hopewell Wind Power (Hong Kong) Limited.
International Nuclear Information System (INIS)
Nakao, Keisuke; Hattori, Yasuo; Suto, Hitoshi
2017-01-01
Highlights: • A large-eddy simulation of a spatially developing natural convection boundary layer is conducted. • First- and second-order moments of the heat and momentum showed a reasonable agreement with past experiments. • Coherent structure of turbulent vortex inherent in this boundary layer is discussed. - Abstract: Large-eddy simulation (LES) on a spatially developing natural convection boundary layer along a vertical heated plate was conducted. The heat transfer rate, friction velocity, mean velocity and temperature, and second-order turbulent properties both in the wall-normal and the stream-wise direction showed reasonable agreement with the findings of past experiments. The spectrum of velocity and temperature fluctuation showed a -2/3-power decay slope and -2-power decay slope respectively. Quadrant analysis revealed the inclination on Q1 and Q3 in the Reynolds stress and turbulent heat flux, changing their contribution along the distance from the plate surface. Following the convention, we defined the threshold region where the stream-wise mean velocity takes local maximum, the inner layer which is closer to the plate than the threshold region, the outer layer which is farther to the plate than the threshold region. The space correlation of stream-wise velocity tilted the head toward the wall in the propagating direction in the outer layer; on the other hand, the correlated motion had little inclination in the threshold region. The time history of the second invariant of gradient tensor Q revealed that the vortex strength oscillates both in the inner and the outer layers in between the laminar and the transition region. In the turbulent region, the vortex was often dominant in the outer layer. Instantaneous three-dimensional visualization of Q revealed the existence of high-speed fluid parcels associated with arch-shape vortices. These results were considered as an intrinsic structure in the outer layer, which is symmetrical to the structure of
Energy Technology Data Exchange (ETDEWEB)
Berg, Larry K. [Pacific Northwest National Laboratory, Richland, Washington; Newsom, Rob K. [Pacific Northwest National Laboratory, Richland, Washington; Turner, David D. [Global Systems Division, NOAA/Earth System Research Laboratory, Boulder, Colorado
2017-09-01
One year of Coherent Doppler Lidar (CDL) data collected at the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) site in Oklahoma is analyzed to provide profiles of vertical velocity variance, skewness, and kurtosis for cases of cloud-free convective boundary layers. The variance was scaled by the Deardorff convective velocity scale, which was successful when the boundary layer depth was stationary but failed in situations when the layer was changing rapidly. In this study the data are sorted according to time of day, season, wind direction, surface shear stress, degree of instability, and wind shear across the boundary-layer top. The normalized variance was found to have its peak value near a normalized height of 0.25. The magnitude of the variance changes with season, shear stress, and degree of instability, but was not impacted by wind shear across the boundary-layer top. The skewness was largest in the top half of the boundary layer (with the exception of wintertime conditions). The skewness was found to be a function of the season, shear stress, wind shear across the boundary-layer top, with larger amounts of shear leading to smaller values. Like skewness, the vertical profile of kurtosis followed a consistent pattern, with peak values near the boundary-layer top (also with the exception of wintertime data). The altitude of the peak values of kurtosis was found to be lower when there was a large amount of wind shear at the boundary-layer top.
Bendl, Jan; Hovorka, Jan
2017-12-01
The planetary boundary layer is a dynamic system with turbulent flow where horizontal and vertical air mixing depends mainly on the weather conditions and geomorphology. Normally, air temperature from the Earth surface decreases with height but inversion situation may occur, mainly during winter. Pollutant dispersion is poor during inversions so air pollutant concentration can quickly rise, especially in urban closed valleys. Air pollution was evaluated by WHO as a human carcinogen (mostly by polycyclic aromatic hydrocarbons) and health effects are obvious. Knowledge about inversion layer height is important for estimation of the pollution impact and it can give us also information about the air pollution sources. Temperature and relative humidity vertical profiles complement ground measurements. Ground measurements were conducted to characterize comprehensively urban airshed in Svermov, residential district of the city of Kladno, about 30 km NW of Prague, from the 2nd Feb. to the 3rd of March 2016. The Svermov is an air pollution hot-spot for long time benzo[a]pyrene (B[a]P) limit exceedances, reaching the highest B[a]P annual concentration in Bohemia - west part of the Czech Republic. Since the Svermov sits in a shallow valley, frequent vertical temperature inversion in winter and low emission heights of pollution sources prevent pollutant dispersal off the valley. Such orography is common to numerous small settlements in the Czech Republic. Ground measurements at the sports field in the Svermov were complemented by temperature and humidity vertical profiles acquired by a Vaisala radiosonde positioned at tethered He-filled balloon. Total number of 53 series of vertical profiles up to the height of 300 m was conducted. Meteorology parameters were acquired with 4 Hz frequency. The measurements confirmed frequent early-morning and night formation of temperature inversion within boundary layer up to the height of 50 m. This rather shallow inversion had significant
Wang, Chenggang; Cao, Le
2016-04-01
Air pollution occurring in the atmospheric boundary layer is a kind of weather phenomenon which decreases the visibility of the atmosphere and results in poor air quality. Recently, the occurrence of the heavy air pollution events has become more frequent all over Asia, especially in Mid-Eastern China. In December 2015, the most severe air pollution in recorded history of China occurred in the regions of Yangtze River Delta and Beijing-Tianjin-Hebei. More than 10 days of severe air pollution (Air Quality Index, AQI>200) appeared in many large cities of China such as Beijing, Tianjin, Shijiazhuang and Baoding. Thus, the research and the management of the air pollution has attracted most attentions in China. In order to investigate the formation, development and dissipation of the air pollutions in China, a field campaign has been conducted between January 1, 2015 and January 28, 2015 in Yangtze River Delta of China, aiming at a intensive observation of the vertical structure of the air pollutants in the atmospheric boundary layer during the time period with heavy pollution. In this study, the observation data obtained in the field campaign mentioned above is analyzed. The characteristics of the atmospheric boundary layer and the vertical distribution of air pollutants in the city Dongshan located in the center of Lake Taihu are shown and discussed in great detail. It is indicated that the stability of the boundary layer is the strongest during the nighttime and the early morning of Dongshan. Meanwhile, the major air pollutants, PM2.5 and PM10 in the boundary layer, reach their maximum values, 177.1μg m-3 and 285μg m-3 respectively. The convective boundary layer height in the observations ranges from approximately 700m to 1100m. It is found that the major air pollutants tend to be confined in a relatively shallow boundary layer, which represents that the boundary layer height is the dominant factor for controlling the vertical distribution of the air pollutants. In
Vertical axis wind turbine wake in boundary layer flow in a wind tunnel
Rolin, Vincent; Porté-Agel, Fernando
2016-04-01
A vertical axis wind turbine is placed in a boundary layer flow in a wind tunnel, and its wake is investigated. Measurements are performed using an x-wire to measure two components of velocity and turbulence statistics in the wake of the wind turbine. The study is performed at various heights and crosswind positions in order to investigate the full volume of the wake for a range of tip speed ratios. The velocity deficit and levels of turbulence in the wake are related to the performance of the turbine. The asymmetric incoming boundary layer flow causes the rate of recovery in the wake to change as a function of height. Higher shear between the wake and unperturbed flow occurs at the top edge of the wake, inducing stronger turbulence and mixing in this region. The difference in flow relative to the blades causes the velocity deficit and turbulence level to change as a function of crosswind position behind the rotor. The relative difference diminishes with increasing tip speed ratio. Therefore, the wake becomes more homogeneous as tip speed ratio increases.
International Nuclear Information System (INIS)
Abedin, Mohammad Zoynal; Tsuji, Toshihiro; Lee, Jinho
2012-01-01
Highlights: ► A time-developing direct numerical simulations are done for water along a heated vertical plate. ► The objective is to see the effects of free streams on the combined-convection boundary layers. ► There are no reports for water with direct numerical simulation in this regards. ► An experiment is also conducted on the transitional and turbulent boundary layer in water. ► This is to collect informations on the integral thickness of the velocity boundary layer. - Abstract: Time-developing thermally-driven boundary layers created by imposing aiding and opposing freestreams on the natural-convection boundary layer in water along a heated vertical flat plate have been examined with a direct numerical simulation to clarify their transition and turbulence behaviors. The numerical results for aiding flow reveal that the transition begins at a thick laminar boundary layer due to the delay of the transition and large-scale vortexes centering on the spanwise direction are followed, while, for opposing flow, the transition begins at a thin laminar boundary layer due to the quickening of the transition and relatively small-scale vortexes are generated with the progress of transition. To improve the significance of the present numerical results, the association of turbulence statistics between time- and space-developing flows has been investigated. Consequently, the numerical results for time-developing flow are converted to those for space-developing flow through the integral thickness of the velocity boundary layer for pure natural convection, and thus the regimes of boundary layer flows can be quantitatively assessed. Moreover, the turbulence statistics and the flow structures in the thermally-driven boundary layers are also presented.
Vertical Structure of the Urban Boundary Layer over Marseille Under Sea-Breeze Conditions
Lemonsu, Aude; Bastin, Sophie; Masson, Valéry; Drobinski, Philippe
2006-03-01
During the UBL-ESCOMPTE program (June July 2001), intensive observations were performed in Marseille (France). In particular, a Doppler lidar, located in the north of the city, provided radial velocity measurements on a 6-km radius area in the lowest 3 km of the troposphere. Thus, it is well adapted to document the vertical structure of the atmosphere above complex terrain, notably in Marseille, which is bordered by the Mediterranean sea and framed by numerous massifs. The present study focuses on the last day of the intensive observation period 2 (26 June 2001), which is characterized by a weak synoptic pressure gradient favouring the development of thermal circulations. Under such conditions, a complex stratification of the atmosphere is observed. Three-dimensional numerical simulations, with the Méso-NH atmospheric model including the town energy balance (TEB) urban parameterization, are conducted over south-eastern France. A complete evaluation of the model outputs was already performed at both regional and city scales. Here, the 250-m resolution outputs describing the vertical structure of the atmosphere above the Marseille area are compared to the Doppler lidar data, for which the spatial resolution is comparable. This joint analysis underscores the consistency between the atmospheric boundary layer (ABL) observed by the Doppler lidar and that modelled by Méso-NH. The observations and simulations reveal the presence of a shallow sea breeze (SSB) superimposed on a deep sea breeze (DSB) above Marseille during daytime. Because of the step-like shape of the Marseille coastline, the SSB is organized in two branches of different directions, which converge above the city centre. The analysis of the 250-m wind fields shows evidence of the role of the local topography on the local dynamics. Indeed, the topography tends to reinforce the SSB while it weakens the DSB. The ABL is directly affected by the different sea-breeze circulations, while the urban effects appear
Directory of Open Access Journals (Sweden)
Benjamin L. Hemingway
2017-09-01
Full Text Available The lowest portion of the Earth’s atmosphere, known as the atmospheric boundary layer (ABL, plays an important role in the formation of weather events. Simple meteorological measurements collected from within the ABL, such as temperature, pressure, humidity, and wind velocity, are key to understanding the exchange of energy within this region, but conventional surveillance techniques such as towers, radar, weather balloons, and satellites do not provide adequate spatial and/or temporal coverage for monitoring weather events. Small unmanned aircraft, or aerial, systems (sUAS provide a versatile, dynamic platform for atmospheric sensing that can provide higher spatio-temporal sampling frequencies than available through most satellite sensing methods. They are also able to sense portions of the atmosphere that cannot be measured from ground-based radar, weather stations, or weather balloons and have the potential to fill gaps in atmospheric sampling. However, research on the vertical sampling scales for collecting atmospheric measurements from sUAS and the variabilities of these scales across atmospheric phenomena (e.g., temperature and humidity is needed. The objective of this study is to use variogram analysis, a common geostatistical technique, to determine optimal spatial sampling scales for two atmospheric variables (temperature and relative humidity captured from sUAS. Results show that vertical sampling scales of approximately 3 m for temperature and 1.5–2 m for relative humidity were sufficient to capture the spatial structure of these phenomena under the conditions tested. Future work is needed to model these scales across the entire ABL as well as under variable conditions.
Vertical structure of the boundary layer. a comparison between land and sea
Digital Repository Service at National Institute of Oceanography (India)
RameshKumar, M.R.; Sadhuram, Y.
The structure and the energetics of the boundary layer have been studied simultaneously over land and sea off Goa Coast just before the onset of south west monsoon. The Radiosonde data obtained at 00 GMT (0530 IST) are used in the present...
Energy Technology Data Exchange (ETDEWEB)
Ohk, Seung Min; Chung, Bum Jin [Kyunghee University, Yongin (Korea, Republic of)
2016-05-15
The Passive Cooling System (PCS) driven by natural forces drew research attention since Fukushima nuclear power plant accident. This study investigated the natural convection heat transfer inside of vertical pipe with emphasis on the phenomena regarding the boundary layer interaction. Numerical calculations were carried out using FLUENT 6.3. Experiments were performed for the parts of the cases to explore the accuracy of calculation. Based on the analogy, heat transfer experiment is replaced by mass transfer experiment using sulfuric acid copper sulfate (CuSO{sub 4}. H{sub 2}SO{sub 4}) electroplating system. The natural convection heat transfer inside a vertical pipe is studied experimentally and numerically. Experiments were carried out using sulfuric acid-copper sulfate (H{sub 2}SO{sub 4}-CuSO{sub 4}) based on the analogy concept between heat and mass transfer system. Numerical analysis was carried out using FLUENT 6.3. It is concluded that the boundary layer interaction along the flow passage influences the heat transfer, which is affected by the length, diameter, and Prandtl number. For the large diameter and high Prandtl number cases, where the thermal boundary layers do not interfered along the pipe, the heat transfer agreed with vertical flat plate for laminar and turbulent natural convection correlation within 8%. When the flow becomes steady state, the forced convective flow appears in the bottom of the vertical pipe and natural convection flow appears near the exit. It is different behavior from the flow on the parallel vertical flat plates. Nevertheless, the heat transfer was not different greatly compared with those of vertical plate.
Directory of Open Access Journals (Sweden)
Abid Hussanan
Full Text Available In this paper, the heat transfer effect on the unsteady boundary layer flow of a Casson fluid past an infinite oscillating vertical plate with Newtonian heating is investigated. The governing equations are transformed to a systems of linear partial differential equations using appropriate non-dimensional variables. The resulting equations are solved analytically by using the Laplace transform method and the expressions for velocity and temperature are obtained. They satisfy all imposed initial and boundary conditions and reduce to some well-known solutions for Newtonian fluids. Numerical results for velocity, temperature, skin friction and Nusselt number are shown in various graphs and discussed for embedded flow parameters. It is found that velocity decreases as Casson parameters increases and thermal boundary layer thickness increases with increasing Newtonian heating parameter.
Hussanan, Abid; Zuki Salleh, Mohd; Tahar, Razman Mat; Khan, Ilyas
2014-01-01
In this paper, the heat transfer effect on the unsteady boundary layer flow of a Casson fluid past an infinite oscillating vertical plate with Newtonian heating is investigated. The governing equations are transformed to a systems of linear partial differential equations using appropriate non-dimensional variables. The resulting equations are solved analytically by using the Laplace transform method and the expressions for velocity and temperature are obtained. They satisfy all imposed initial and boundary conditions and reduce to some well-known solutions for Newtonian fluids. Numerical results for velocity, temperature, skin friction and Nusselt number are shown in various graphs and discussed for embedded flow parameters. It is found that velocity decreases as Casson parameters increases and thermal boundary layer thickness increases with increasing Newtonian heating parameter.
Vertical structure of atmospheric boundary layer over Ranchi during the summer monsoon season
Chandra, Sagarika; Srivastava, Nishi; Kumar, Manoj
2018-04-01
Thermodynamic structure and variability in the atmospheric boundary layer have been investigated with the help of balloon-borne GPS radiosonde over a monsoon trough station Ranchi (Lat. 23°45'N, Long. 85°43'E, India) during the summer monsoon season (June-September) for a period of 2011-2013. Virtual potential temperature gradient method is used for the determination of mixed layer height (MLH). The MLH has been found to vary in the range of 1000-1300 m during the onset, 600-900 m during the active and 1400-1750 m during the break phase of monsoon over this region. Inter-annual variations noticed in MLH could be associated with inter-annual variability in convection and rainfall prevailing over the region. Along with the MLH, the cloud layer heights are also derived from the thermodynamic profiles for the onset, active and break phases of monsoon. Cloud layer height varied a lot during different phases of the monsoon. For the determination of boundary-layer convection, thermodynamic parameter difference (δθ = θ es- θ e) between saturated equivalent potential temperature (θ es ) and equivalent potential temperature (θ e) is used. It is a good indicator of convection and indicates the intense and suppressed convection during different phases of monsoon.
Energy Technology Data Exchange (ETDEWEB)
Iga, Keita, E-mail: iga@aori.u-tokyo.ac.jp [Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8564 (Japan)
2017-12-15
Axisymmetric flow in a cylindrical tank over a rotating bottom is investigated and its approximate solution with an analytic expression is obtained. The interior region, comprising the majority of the fluid, consists of two sub-regions. It is easily shown that a rigid-body rotational flow with the same rotation rate as that of the bottom is formed in the inner interior and that a potential flow with constant angular momentum occurs in the outer interior sub-region. However, the radius that divides these two sub-regions has not been determined. To determine this radius, the structures of the boundary layers are investigated in detail. These boundary layers surround the interior regions, and include the boundaries between the interior region and the side wall of the tank, between the interior and the bottom, and between the inner and outer interior sub-regions. By connecting the flows in the boundary layers, the vertical circulation as a whole is established, and consequently the radius dividing the two interior sub-regions is successfully determined as a function of the aspect ratio of the water layer region. This axisymmetric flow will be utilized as the basic state for investigating theoretically various non-axisymmetric phenomena observed in laboratory experiments. (paper)
International Nuclear Information System (INIS)
Jha, P.K.
1986-01-01
An attempt has been made to study the problem of free convection hydromagnetic flow of an elastico-viscous fluid past a porous vertical plate in a rotating frame of reference taking ohmic and viscous dissipations into account in the presence of Hall current. The nature of velocity profile shows the existence of multiple boundary layers. Their 'thickness' is seen to decrease with increasing values of Ekman, Hartman and Prandtl numbers and Hall parameter. The graphical study reveals that the increasing values of Hall parameter and Ekman number (for a fixed large value of Hall parameter) exert opposite influence on the flow. (author). 11 refs., 2 tables
DEFF Research Database (Denmark)
Draxl, Caroline; Hahmann, Andrea N.; Pena Diaz, Alfredo
2014-01-01
with different PBL parameterizations at one coastal site over western Denmark. The evaluation focuses on determining which PBL parameterization performs best for wind energy forecasting, and presenting a validation methodology that takes into account wind speed at different heights. Winds speeds at heights...... regarding wind energy at these levels partly depends on the formulation and implementation of planetary boundary layer (PBL) parameterizations in these models. This study evaluates wind speeds and vertical wind shears simulated by theWeather Research and Forecasting model using seven sets of simulations...
Fichtl, G. H.
1971-01-01
Statistical estimates of wind shear in the planetary boundary layer are important in the design of V/STOL aircraft, and for the design of the Space Shuttle. The data analyzed in this study consist of eleven sets of longitudinal turbulent velocity fluctuation time histories digitized at 0.2 sec intervals with approximately 18,000 data points per time history. The longitudinal velocity fluctuations were calculated with horizontal wind and direction data collected at the 18-, 30-, 60-, 90-, 120-, and 150-m levels. The data obtained confirm the result that Eulerian time spectra transformed to wave-number spectra with Taylor's frozen eddy hypothesis possess inertial-like behavior at wave-numbers well out of the inertial subrange.
Directory of Open Access Journals (Sweden)
Norfifah Bachok
Full Text Available The steady boundary layer flow of a viscous and incompressible fluid over a moving vertical flat plate in an external moving fluid with viscous dissipation is theoretically investigated. Using appropriate similarity variables, the governing system of partial differential equations is transformed into a system of ordinary (similarity differential equations, which is then solved numerically using a Maple software. Results for the skin friction or shear stress coefficient, local Nusselt number, velocity and temperature profiles are presented for different values of the governing parameters. It is found that the set of the similarity equations has unique solutions, dual solutions or no solutions, depending on the values of the mixed convection parameter, the velocity ratio parameter and the Eckert number. The Eckert number significantly affects the surface shear stress as well as the heat transfer rate at the surface.
Energy Technology Data Exchange (ETDEWEB)
Ishioka, Futoshi; Lee, D.I; Taniguchi, Takashi; Kikuchi,Katsuhiro
1988-09-30
In general, it is difficult to accurately understand the behavior of aerosol particles in the boundary layer above urban areas because aerosol sources are influenced by time-dependent factors and local climate. To overcome this difficulty, a particle counter which can count Mie particles with diameters of 0.3 /mu/m or more in five diameter ranges was installed on a large tehered balloon. With this method, the vertical distribution of aerosol concentration was measured in several areas different in meteorological condition, and the dependence of the particle behavior on particle diameter was studied. As a result, it has been revealed that the results of the observations explained above agree with the results of studies conducted in the past, but that dependence on particle diameter is not significant. 37 references, 21 figures, 1 table.
International Nuclear Information System (INIS)
Colls, J.J.; Micallef, A.
1999-01-01
Concentrations and vertical profiles of various fractions of airborne particulate matter (suspended particulate matter (SPM), PM 10 and PM 2.5 ) have been measured over the first three metres from ground in a street canyon. Measurements were carried out using automated near real-time apparatus called the Kinetic Sequential Sampling (KSS) system. KSS system is essentially an electronically-controlled lift carrying a real-time particle monitor for sampling air sequentially, at different heights within the breathing zone, which includes all heights within the surface layer of a street canyon at which people may breathe. Data is automatically logged at the different receptor levels, for the determination of the average vertical concentration profile of airborne particulate matter. For measuring the airborne particle concentration, a Grimm Dust Monitor 1.104/5 was used. The recorded data also allows for time series analysis of airborne particulate matter concentration at different heights. Time series data and hourly-average vertical concentration profiles in the boundary layer of the confines of a street are thought to be mainly determined by traffic emissions and traffic associated processes. Hence the measured data were compared with results of a street canyon emission-dispersion model in time and space. This Street Level Air Quality (SLAQ) model employs the plume-box technique and includes modules for simulating vehicle-generated effects such as thermally- and mechanically-generated turbulence and resuspension of road dust. Environmental processes, such as turbulence resulting from surface sensible heat and the formation of sulphate aerosol from sulphur dioxide exhaust emissions, are taken into account. The paper presents an outline description of the measuring technique and model used, and a comparison of the measured and modelled data
Lee, Z. S.; Baas, A. C.; Jackson, D.; Cooper, J. A.; Lynch, K.; Delgado-Fernandez, I.; Beyers, M.
2010-12-01
Recent studies have suggested the significant role of boundary layer turbulence and coherent flow structures on sand transport by wind over beaches and desert dunes. Widespread use of sonic anemometry and high-frequency sand transport sensors and traps have facilitated a move beyond the basic monitoring of shear velocities and bulk sediment transport to more detailed measurements at much higher spatio-temporal resolutions. In this paper we present results of a small-scale point-location field study of boundary layer turbulence and shear stresses conducted under obliquely onshore winds over a beach at Magilligan Strand, Northern Ireland. High-frequency (25 Hz) 3D wind vector measurements were collected at five different heights between 0.13 and 1.67 metres above the bed using sonic anemometry for durations of several hours, and the associated sand transport response was measured using an array of Safires. The wind data are used to investigate the vertical structure of Reynolds shear stresses and burst-sweep event characteristics, as well as a comparison with the standard logarithmic (law-of-the-wall) wind profile. The study explores the identification and selection of a characteristic event duration based on integral time-scales as well as spectral analysis, and includes an assessment of the issues involved with data rotations for yaw, pitch, and roll corrections relative to flow streamlines, and the subsequently derived turbulence parameters based on fluctuating vector components (u’, v’, w’). Results show how the contributions to shear stress and the average pitch of bursts and sweeps changes as a function of height above the bed, indicating the transformation of top-down turbulent eddies as they travel toward the surface. A comparison between the turbulence data and the synchronous sand transport events, meanwhile, reveals the potential effects of enhanced saltation layer roughness feedback on eddies close to the bed.
Steeneveld, G.J.
2012-01-01
Understanding and prediction of the stable atmospheric boundary layer is a challenging task. Many physical processes are relevant in the stable boundary layer, i.e. turbulence, radiation, land surface coupling, orographic turbulent and gravity wave drag, and land surface heterogeneity. The development of robust stable boundary layer parameterizations for use in NWP and climate models is hampered by the multiplicity of processes and their unknown interactions. As a result, these models suffer ...
A Large-Eddy Simulation Study of Vertical Axis Wind Turbine Wakes in the Atmospheric Boundary Layer
Shamsoddin, Sina; Porté-Agel, Fernando
2017-04-01
In a future sustainable energy vision, in which diversified conversion of renewable energies is essential, vertical axis wind turbines (VAWTs) exhibit some potential as a reliable means of wind energy extraction alongside conventional horizontal axis wind turbines (HAWTs). Nevertheless, there is currently a relative shortage of scientific, academic and technical investigations of VAWTs as compared to HAWTs. Having this in mind, in this work, we aim to, for the first time, study the wake of a single VAWT placed in the atmospheric boundary layer using large-eddy simulation (LES). To do this, we use a previously-validated LES framework in which an actuator line model (ALM) is incorporated. First, for a typical three- and straight-bladed 1-MW VAWT design, the variation of the power coefficient with both the chord length of the blades and the tip-speed ratio is analyzed by performing 117 simulations using LES-ALM. The optimum combination of solidity (defined as Nc/R, where N is the number of blades, c is the chord length and R is the rotor radius) and tip-speed ratio is found to be 0.18 and 4.5, respectively. Subsequently, the wake of a VAWT with these optimum specifications is thoroughly examined by showing different relevant mean and turbulence wake flow statistics. It is found that for this case, the maximum velocity deficit at the equator height of the turbine occurs 2.7 rotor diameters downstream of the center of the turbine, and only after that point, the wake starts to recover. Moreover, it is observed that the maximum turbulence intensity (TI) at the equator height of the turbine occurs at a distance of about 3.8 rotor diameters downstream of the turbine. As we move towards the upper and lower edges of the turbine, the maximum TI (at a certain height) increases, and its location moves relatively closer to the turbine. Furthermore, whereas both TI and turbulent momentum flux fields show clear vertical asymmetries (with larger magnitudes at the upper wake edge
A Large-Eddy Simulation Study of Vertical Axis Wind Turbine Wakes in the Atmospheric Boundary Layer
Directory of Open Access Journals (Sweden)
Sina Shamsoddin
2016-05-01
Full Text Available In a future sustainable energy vision, in which diversified conversion of renewable energies is essential, vertical axis wind turbines (VAWTs exhibit some potential as a reliable means of wind energy extraction alongside conventional horizontal axis wind turbines (HAWTs. Nevertheless, there is currently a relative shortage of scientific, academic and technical investigations of VAWTs as compared to HAWTs. Having this in mind, in this work, we aim to, for the first time, study the wake of a single VAWT placed in the atmospheric boundary layer using large-eddy simulation (LES. To do this, we use a previously-validated LES framework in which an actuator line model (ALM is incorporated. First, for a typical three- and straight-bladed 1-MW VAWT design, the variation of the power coefficient with both the chord length of the blades and the tip-speed ratio is analyzed by performing 117 simulations using LES-ALM. The optimum combination of solidity (defined as N c / R , where N is the number of blades, c is the chord length and R is the rotor radius and tip-speed ratio is found to be 0.18 and 4.5, respectively. Subsequently, the wake of a VAWT with these optimum specifications is thoroughly examined by showing different relevant mean and turbulence wake flow statistics. It is found that for this case, the maximum velocity deficit at the equator height of the turbine occurs 2.7 rotor diameters downstream of the center of the turbine, and only after that point, the wake starts to recover. Moreover, it is observed that the maximum turbulence intensity (TI at the equator height of the turbine occurs at a distance of about 3.8 rotor diameters downstream of the turbine. As we move towards the upper and lower edges of the turbine, the maximum TI (at a certain height increases, and its location moves relatively closer to the turbine. Furthermore, whereas both TI and turbulent momentum flux fields show clear vertical asymmetries (with larger magnitudes at the
Steeneveld, G.J.
2012-01-01
Understanding and prediction of the stable atmospheric boundary layer is a challenging task. Many physical processes are relevant in the stable boundary layer, i.e. turbulence, radiation, land surface coupling, orographic turbulent and gravity wave drag, and land surface heterogeneity. The
Development of boundary layers
International Nuclear Information System (INIS)
Herbst, R.
1980-01-01
Boundary layers develop along the blade surfaces on both the pressure and the suction side in a non-stationary flow field. This is due to the fact that there is a strongly fluctuating flow on the downstream blade row, especially as a result of the wakes of the upstream blade row. The author investigates the formation of boundary layers under non-stationary flow conditions and tries to establish a model describing the non-stationary boundary layer. For this purpose, plate boundary layers are measured, at constant flow rates but different interferent frequency and variable pressure gradients. By introducing the sample technique, measurements of the non-stationary boundary layer become possible, and the flow rate fluctuation can be divided in its components, i.e. stochastic turbulence and periodical fluctuation. (GL) [de
Kudo, Rei; Aoyagi, Toshinori; Nishizawa, Tomoaki
2018-05-01
Vertical profiles of the aerosol physical and optical properties, with a focus on seasonal means and on transport events, were investigated in Tsukuba, Japan, by a synergistic remote sensing method that uses lidar and sky radiometer data. The retrieved aerosol vertical profiles of the springtime mean and five transport events were input to our developed one-dimensional atmospheric model, and the impacts of the aerosol vertical profiles on the evolution of the atmospheric boundary layer (ABL) were studied by numerical sensitivity experiments. The characteristics of the aerosol vertical profiles in Tsukuba are as follows: (1) the retrieval results in the spring showed that aerosol optical thickness at 532 nm in the free atmosphere (FA) was 0.13, greater than 0.08 in the ABL owing to the frequent occurrence of transported aerosols in the FA. In other seasons, optical thickness in the FA was almost the same as that in the ABL. (2) The aerosol optical and physical properties in the ABL showed a dependency on the extinction coefficient. With an increase in the extinction coefficient from 0.00 to 0.24 km-1, the Ångström exponent increased from 0.0 to 2.0, the single-scattering albedo increased from 0.87 to 0.99, and the asymmetry factor decreased from 0.75 to 0.50. (3) The large variability in the physical and optical properties of aerosols in the FA were attributed to transport events, during which the transported aerosols consisted of varying amounts of dust and smoke particles depending on where they originated (China, Mongolia, or Russia). The results of the numerical sensitivity experiments using the aerosol vertical profiles of the springtime mean and five transport events in the FA are as follows: (1) numerical sensitivity experiments based on simulations conducted with and without aerosols showed that aerosols caused the net downward radiation and the sensible and latent heat fluxes at the surface to decrease. The decrease in temperature in the ABL (-0.2 to -0
Cheng, Wan; Pullin, D. I.; Samtaney, Ravi
2015-01-01
We describe large-eddy simulations of turbulent boundary-layer flow over a flat plate at high Reynolds number in the presence of an unsteady, three-dimensional flow separation/reattachment bubble. The stretched-vortex subgrid-scale model is used
Cheng, Wan
2015-06-30
We describe large-eddy simulations of turbulent boundary-layer flow over a flat plate at high Reynolds number in the presence of an unsteady, three-dimensional flow separation/reattachment bubble. The stretched-vortex subgrid-scale model is used in the main flow domain combined with a wall-model that is a two-dimensional extension of that developed by Chung & Pullin (2009). Flow separation and re-attachment of the incoming boundary layer is induced by prescribing wall-normal velocity distribution on the upper boundary of the flow domain that produces an adverse-favorable stream-wise pressure distribution at the wall. The LES predicts the distribution of mean shear stress along the wall including the interior of the separation bubble. Several properties of the separation/reattachment flow are discussed.
Trowbridge, John H; Lentz, Steven J
2018-01-03
The oceanic bottom boundary layer extracts energy and momentum from the overlying flow, mediates the fate of near-bottom substances, and generates bedforms that retard the flow and affect benthic processes. The bottom boundary layer is forced by winds, waves, tides, and buoyancy and is influenced by surface waves, internal waves, and stratification by heat, salt, and suspended sediments. This review focuses on the coastal ocean. The main points are that (a) classical turbulence concepts and modern turbulence parameterizations provide accurate representations of the structure and turbulent fluxes under conditions in which the underlying assumptions hold, (b) modern sensors and analyses enable high-quality direct or near-direct measurements of the turbulent fluxes and dissipation rates, and (c) the remaining challenges include the interaction of waves and currents with the erodible seabed, the impact of layer-scale two- and three-dimensional instabilities, and the role of the bottom boundary layer in shelf-slope exchange.
Trowbridge, John H.; Lentz, Steven J.
2018-01-01
The oceanic bottom boundary layer extracts energy and momentum from the overlying flow, mediates the fate of near-bottom substances, and generates bedforms that retard the flow and affect benthic processes. The bottom boundary layer is forced by winds, waves, tides, and buoyancy and is influenced by surface waves, internal waves, and stratification by heat, salt, and suspended sediments. This review focuses on the coastal ocean. The main points are that (a) classical turbulence concepts and modern turbulence parameterizations provide accurate representations of the structure and turbulent fluxes under conditions in which the underlying assumptions hold, (b) modern sensors and analyses enable high-quality direct or near-direct measurements of the turbulent fluxes and dissipation rates, and (c) the remaining challenges include the interaction of waves and currents with the erodible seabed, the impact of layer-scale two- and three-dimensional instabilities, and the role of the bottom boundary layer in shelf-slope exchange.
Schlichting (Deceased), Hermann
2017-01-01
This new edition of the near-legendary textbook by Schlichting and revised by Gersten presents a comprehensive overview of boundary-layer theory and its application to all areas of fluid mechanics, with particular emphasis on the flow past bodies (e.g. aircraft aerodynamics). The new edition features an updated reference list and over 100 additional changes throughout the book, reflecting the latest advances on the subject.
Stagg, G W; Parker, N G; Barenghi, C F
2017-03-31
We model the superfluid flow of liquid helium over the rough surface of a wire (used to experimentally generate turbulence) profiled by atomic force microscopy. Numerical simulations of the Gross-Pitaevskii equation reveal that the sharpest features in the surface induce vortex nucleation both intrinsically (due to the raised local fluid velocity) and extrinsically (providing pinning sites to vortex lines aligned with the flow). Vortex interactions and reconnections contribute to form a dense turbulent layer of vortices with a nonclassical average velocity profile which continually sheds small vortex rings into the bulk. We characterize this layer for various imposed flows. As boundary layers conventionally arise from viscous forces, this result opens up new insight into the nature of superflows.
The Atmospheric Boundary Layer
Garratt, J. R.
1994-05-01
A comprehensive and lucid account of the physics and dynamics of the lowest one to two kilometers of the Earth's atmosphere in direct contact with the Earth's surface, known as the atmospheric boundary layer (ABL). Dr. Garratt emphasizes the application of the ABL problems to numerical modeling of the climate, which makes this book unique among recent texts on the subject. He begins with a brief introduction to the ABL before leading to the development of mean and turbulence equations and the many scaling laws and theories that are the cornerstone of any serious ABL treatment. Modeling of the ABL is crucially dependent for its realism on the surface boundary conditions, so chapters four and five deal with aerodynamic and energy considerations, with attention given to both dry and wet land surfaces and the sea. The author next treats the structure of the clear-sky, thermally stratified ABL, including the convective and stable cases over homogeneous land, the marine ABL, and the internal boundary layer at the coastline. Chapter seven then extends this discussion to the cloudy ABL. This is particularly relevant to current research because the extensive stratocumulus regions over the subtropical oceans and stratus regions over the Arctic have been identified as key players in the climate system. In the final chapters, Dr. Garratt summarizes the book's material by discussing appropriate ABL and surface parameterization schemes in general circulation models of the atmosphere that are being used for climate stimulation.
International Nuclear Information System (INIS)
Hady, F. M.; Ibrahim, F. S.; Abdel-Gaied, S. M.; Eid, M. R.
2011-01-01
The effect of yield stress on the free convective heat transfer of dilute liquid suspensions of nanofluids flowing on a vertical plate saturated in porous medium under laminar conditions is investigated considering the nanofluid obeys the mathematical model of power-law. The model used for non-Newtonian nanofluid incorporates the effects of Brownian motion and thermophoresis. The governing boundary- layer equations are cast into dimensionless system which is solved numerically using a deferred correction technique and Newton iteration. This solution depends on yield stress parameter Ω, a power-law index n, Lewis number Le, a buoyancy-ratio number Nr, a Brownian motion number Nb, and a thermophoresis number Nt. Analyses of the results found that the reduced Nusselt and Sherwood numbers are decreasing functions of the higher yield stress parameter for each dimensionless numbers, n and Le, except the reduced Sherwood number is an increasing function of higher Nb for different values of yield stress parameter
Directory of Open Access Journals (Sweden)
Asterios Pantokratoras
2008-01-01
Full Text Available Exact analytical solutions of boundary layer flows along a vertical porous plate with uniform suction are derived and presented in this paper. The solutions concern the Blasius, Sakiadis, and Blasius-Sakiadis flows with buoyancy forces combined with either MHD Lorentz or EMHD Lorentz forces. In addition, some exact solutions are presented specifically for water in the temperature range of 0∘C≤≤8∘C, where water density is nearly parabolic. Except for their use as benchmarking means for testing the numerical solution of the Navier-Stokes equations, the presented exact solutions with EMHD forces have use in flow separation control in aeronautics and hydronautics, whereas the MHD results have applications in process metallurgy and fusion technology. These analytical solutions are valid for flows with strong suction.
DEFF Research Database (Denmark)
Sempreviva, Anna Maria; Schiano, M.E.; Pensieri, S.
2010-01-01
sensors mounted on the buoy ODAS Italia1 located in the centre of the Gulf of Genoa. The evolution of the height (zi) of the MABL was monitored using radiosondes and a ceilometer on board of the N/O Urania. Here, we present the database and an uncommon case study of the evolution of the vertical structure...
HEAT AND MASS TRANSFER FOR VISCO-ELASTIC MHD BOUNDARY LAYER FLOW PAST A VERTICAL FLAT PLATE
Rita Choudhury; Hridi Ranjan Deb
2012-01-01
The two-dimensional free convection flow of visco-elastic and electrically conducting fluid past a vertical impermeable flat plate is considered in presence of a uniform transverse magnetic field. The governing equations are reduced to ordinary differential equation by introducing appropriate co-ordinate transformation. The analytical expressions for the velocity, temperature and species concentration fields have been obtained. The corresponding expressions for the non-dimensional rates of he...
Hay, Alex E.; Zedel, Len; Cheel, Richard; Dillon, Jeremy
2012-03-01
Results are presented from an experimental investigation of rough turbulent oscillatory boundary layers using a prototype wideband bistatic coherent Doppler profiler. The profiler operates in the 1.2 MHz to 2.3 MHz frequency band and uses software-defined radio technologies for digital control of the frequency content and shape of the transmit pulse and for digital complex demodulation of the received signals. Velocity profiles are obtained at sub-millimeter range resolution and 100 Hz profiling rates (each profile being an ensemble average of 10 pulse pairs). The measurements were carried out above beds of fixed sand or gravel particles, with median grain diameters of 0.37 mm and 3.9 mm, respectively, oscillating sinusoidally at a 10 s period through excursions of 0.75 m to 1.5 m. The resulting vertical profiles of horizontal velocity magnitude and phase, with the vertical axis scaled by ℓ = κu∗m/ω, are comparable to similarly scaled profiles obtained using laser Doppler anemometry by Sleath (1987) and Jensen (1988). A key objective of the comparisons between the previous experiments and those reported here was to establish how close to the bed reliable velocity measurements can be made with the sonar. This minimum distance above the bed is estimated to be 5 ± 1 mm, a value approaching the 3 to 4 mm limit set by the path of least time.
The Plasmasphere Boundary Layer
Directory of Open Access Journals (Sweden)
D. L. Carpenter
2004-12-01
Full Text Available As an inner magnetospheric phenomenon the plasmapause region is of interest for a number of reasons, one being the occurrence there of geophysically important interactions between the plasmas of the hot plasma sheet and of the cool plasmasphere. There is a need for a conceptual framework within which to examine and discuss these interactions and their consequences, and we therefore suggest that the plasmapause region be called the Plasmasphere Boundary Layer, or PBL. Such a term has been slow to emerge because of the complexity and variability of the plasma populations that can exist near the plasmapause and because of the variety of criteria used to identify the plasmapause in experimental data. Furthermore, and quite importantly in our view, a substantial obstacle to the consideration of the plasmapause region as a boundary layer has been the longstanding tendency of textbooks on space physics to limit introductory material on the plasmapause phenomenon to zeroth order descriptions in terms of ideal MHD theory, thus implying that the plasmasphere is relatively well understood. A textbook may introduce the concept of shielding of the inner magnetosphere from perturbing convection electric fields, but attention is not usually paid to the variety of physical processes reported to occur in the PBL, such as heating, instabilities, and fast longitudinal flows, processes which must play roles in plasmasphere dynamics in concert with the flow regimes associated with the major dynamo sources of electric fields. We believe that through the use of the PBL concept in future textbook discussions of the plasmasphere and in scientific communications, much progress can be made on longstanding questions about the physics involved in the formation of the plasmapause and in the cycles of erosion and recovery of the plasmasphere.
Key words. Magnetospheric physics (plasmasphere; plasma convection; MHD waves and instabilities
The Plasmasphere Boundary Layer
Directory of Open Access Journals (Sweden)
D. L. Carpenter
2004-12-01
Full Text Available As an inner magnetospheric phenomenon the plasmapause region is of interest for a number of reasons, one being the occurrence there of geophysically important interactions between the plasmas of the hot plasma sheet and of the cool plasmasphere. There is a need for a conceptual framework within which to examine and discuss these interactions and their consequences, and we therefore suggest that the plasmapause region be called the Plasmasphere Boundary Layer, or PBL. Such a term has been slow to emerge because of the complexity and variability of the plasma populations that can exist near the plasmapause and because of the variety of criteria used to identify the plasmapause in experimental data. Furthermore, and quite importantly in our view, a substantial obstacle to the consideration of the plasmapause region as a boundary layer has been the longstanding tendency of textbooks on space physics to limit introductory material on the plasmapause phenomenon to zeroth order descriptions in terms of ideal MHD theory, thus implying that the plasmasphere is relatively well understood. A textbook may introduce the concept of shielding of the inner magnetosphere from perturbing convection electric fields, but attention is not usually paid to the variety of physical processes reported to occur in the PBL, such as heating, instabilities, and fast longitudinal flows, processes which must play roles in plasmasphere dynamics in concert with the flow regimes associated with the major dynamo sources of electric fields. We believe that through the use of the PBL concept in future textbook discussions of the plasmasphere and in scientific communications, much progress can be made on longstanding questions about the physics involved in the formation of the plasmapause and in the cycles of erosion and recovery of the plasmasphere. Key words. Magnetospheric physics (plasmasphere; plasma convection; MHD waves and instabilities
Analysis of turbulent boundary layers
Cebeci, Tuncer
1974-01-01
Analysis of Turbulent Boundary Layers focuses on turbulent flows meeting the requirements for the boundary-layer or thin-shear-layer approximations. Its approach is devising relatively fundamental, and often subtle, empirical engineering correlations, which are then introduced into various forms of describing equations for final solution. After introducing the topic on turbulence, the book examines the conservation equations for compressible turbulent flows, boundary-layer equations, and general behavior of turbulent boundary layers. The latter chapters describe the CS method for calculati
Directory of Open Access Journals (Sweden)
A. Mahdy
2015-03-01
Full Text Available A numerical study of laminar magnetohydrodynamic thermosolutal Marangoni convection along a vertical surface in the presence of the Soret and Dufour effects has been performed. The diffusion-thermo implies that the heat transfer is induced by concentration gradient, and thermo-diffusion implies that the mass diffusion is induced by thermal gradient. In conformity to actuality, it is assumed that the surface tension varies linearly with both the temperature and concentration and that both interface temperature and concentration are quadratic functions of the interface arc length x. The general governing partial differential equations are converted into nonlinear ordinary differential equations using unique similarity transformations. The aim of this study is to investigate the effects of Hartmann number (0 ≤ M ≤ 5, thermosolutal surface tension ratio (0 ≤ R ≤ 5, Soret parameter (0.1 ≤ Sr ≤ 2, Dufour parameter (0.03 ≤ Du ≤ 0.6, Prandtl number (0.72 ≤ Pr ≤ 10 and Schmidt number (0.3 ≤ Sc ≤ 3 on the fluid velocity heat and mass transfer. It is found that, both of temperature and concentration gradient at the wall increases as the thermosolutal surface tension ratio increases. Also, the increase in Prandtl number results in an enhancement in the heat transfer at the wall.
Modeling the summertime Arctic cloudy boundary layer
Energy Technology Data Exchange (ETDEWEB)
Curry, J.A.; Pinto, J.O. [Univ. of Colorado, Boulder, CO (United States); McInnes, K.L. [CSIRO Division of Atmospheric Research, Mordialloc (Australia)
1996-04-01
Global climate models have particular difficulty in simulating the low-level clouds during the Arctic summer. Model problems are exacerbated in the polar regions by the complicated vertical structure of the Arctic boundary layer. The presence of multiple cloud layers, a humidity inversion above cloud top, and vertical fluxes in the cloud that are decoupled from the surface fluxes, identified in Curry et al. (1988), suggest that models containing sophisticated physical parameterizations would be required to accurately model this region. Accurate modeling of the vertical structure of multiple cloud layers in climate models is important for determination of the surface radiative fluxes. This study focuses on the problem of modeling the layered structure of the Arctic summertime boundary-layer clouds and in particular, the representation of the more complex boundary layer type consisting of a stable foggy surface layer surmounted by a cloud-topped mixed layer. A hierarchical modeling/diagnosis approach is used. A case study from the summertime Arctic Stratus Experiment is examined. A high-resolution, one-dimensional model of turbulence and radiation is tested against the observations and is then used in sensitivity studies to infer the optimal conditions for maintaining two separate layers in the Arctic summertime boundary layer. A three-dimensional mesoscale atmospheric model is then used to simulate the interaction of this cloud deck with the large-scale atmospheric dynamics. An assessment of the improvements needed to the parameterizations of the boundary layer, cloud microphysics, and radiation in the 3-D model is made.
Costigliola, V.
2010-09-01
It has long been known that specific atmospheric processes, such as weather and longer-term climatic fluctuations, affect human health. The biometeorological literature refers to this relationship as meteorotropism, defined as a change in an organism that is correlated with a change in atmospheric conditions. Plenty of (patho)physiological functions are affected by those conditions - like the respiratory diseases - and currently it is difficult to put any limits for pathologies developed in reply. Nowadays the importance of atmospheric boundary layer and health is increasingly recognised. A number of epidemiologic studies have reported associations between ambient concentrations of air pollution, specifically particulate pollution, and adverse health effects, even at the relatively low concentrations of pollution found. Since 1995 there have been over twenty-one studies from four continents that have explicitly examined the association between ambient air pollutant mixes and daily mortality. Statistically significant and positive associations have been reported in data from various locations around the world, all with varying air pollutant concentrations, weather conditions, population characteristics and public health policies. Particular role has been given to atmospheric boundary layer processes, the impact of which for specific patient-cohort is, however, not well understood till now. Assessing and monitoring air quality are thus fundamental to improve Europe's welfare. One of current projects run by the "European Medical Association" - PASODOBLE will develop and demonstrate user-driven downstream information services for the regional and local air quality sectors by combining space-based and in-situ data with models in 4 thematic service lines: - Health community support for hospitals, pharmacies, doctors and people at risk - Public information for regions, cities, tourist industry and sporting event organizers - Compliance monitoring support on particulate
Directory of Open Access Journals (Sweden)
M.A. Imran
2018-03-01
Full Text Available The aim of this article is to investigate the unsteady natural convection flow of Maxwell fluid with fractional derivative over an exponentially accelerated infinite vertical plate. Moreover, slip condition, radiation, MHD and Newtonian heating effects are also considered. A modern definition of fractional derivative operator recently introduced by Caputo and Fabrizio has been used to formulate the fractional model. Semi analytical solutions of the dimensionless problem are obtained by employing Stehfest’s and Tzou’s algorithms in order to find the inverse Laplace transforms for temperature and velocity fields. Temperature and rate of heat transfer for non-integer and integer order derivatives are computed and reduced to some known solutions from the literature. Finally, in order to get insight of the physical significance of the considered problem regarding velocity and Nusselt number, some graphical illustrations are made using Mathcad software. As a result, in comparison between Maxwell and viscous fluid (fractional and ordinary we found that viscous (fractional and ordinary fluids are swiftest than Maxwell (fractional and ordinary fluids. Keywords: Free convection, Slip, Maxwell fluid, Newtonian heating, Exponentially accelerated plate, Caputo-Fabrizio fractional derivatives, Stehfest’s and Tzou’s algorithms
Imran, M. A.; Riaz, M. B.; Shah, N. A.; Zafar, A. A.
2018-03-01
The aim of this article is to investigate the unsteady natural convection flow of Maxwell fluid with fractional derivative over an exponentially accelerated infinite vertical plate. Moreover, slip condition, radiation, MHD and Newtonian heating effects are also considered. A modern definition of fractional derivative operator recently introduced by Caputo and Fabrizio has been used to formulate the fractional model. Semi analytical solutions of the dimensionless problem are obtained by employing Stehfest's and Tzou's algorithms in order to find the inverse Laplace transforms for temperature and velocity fields. Temperature and rate of heat transfer for non-integer and integer order derivatives are computed and reduced to some known solutions from the literature. Finally, in order to get insight of the physical significance of the considered problem regarding velocity and Nusselt number, some graphical illustrations are made using Mathcad software. As a result, in comparison between Maxwell and viscous fluid (fractional and ordinary) we found that viscous (fractional and ordinary) fluids are swiftest than Maxwell (fractional and ordinary) fluids.
Sublayer of Prandtl Boundary Layers
Grenier, Emmanuel; Nguyen, Toan T.
2018-03-01
The aim of this paper is to investigate the stability of Prandtl boundary layers in the vanishing viscosity limit {ν \\to 0} . In Grenier (Commun Pure Appl Math 53(9):1067-1091, 2000), one of the authors proved that there exists no asymptotic expansion involving one of Prandtl's boundary layer, with thickness of order {√{ν}} , which describes the inviscid limit of Navier-Stokes equations. The instability gives rise to a viscous boundary sublayer whose thickness is of order {ν^{3/4}} . In this paper, we point out how the stability of the classical Prandtl's layer is linked to the stability of this sublayer. In particular, we prove that the two layers cannot both be nonlinearly stable in L^∞. That is, either the Prandtl's layer or the boundary sublayer is nonlinearly unstable in the sup norm.
Boundary Layer Control on Airfoils.
Gerhab, George; Eastlake, Charles
1991-01-01
A phenomena, boundary layer control (BLC), produced when visualizing the fluidlike flow of air is described. The use of BLC in modifying aerodynamic characteristics of airfoils, race cars, and boats is discussed. (KR)
The laminar boundary layer equations
Curle, N
2017-01-01
Thorough introduction to boundary layer problems offers an ordered, logical presentation accessible to undergraduates. The text's careful expositions of the limitations and accuracy of various methods will also benefit professionals. 1962 edition.
Removing Boundary Layer by Suction
Ackeret, J
1927-01-01
Through the utilization of the "Magnus effect" on the Flettner rotor ship, the attention of the public has been directed to the underlying physical principle. It has been found that the Prandtl boundary-layer theory furnishes a satisfactory explanation of the observed phenomena. The present article deals with the prevention of this separation or detachment of the flow by drawing the boundary layer into the inside of a body through a slot or slots in its surface.
Tokamak plasma boundary layer model
International Nuclear Information System (INIS)
Volkov, T.F.; Kirillov, V.D.
1983-01-01
A model has been developed for the limiter layer and for the boundary region of the plasma column in a tokamak to facilitate analytic calculations of the thickness of the limiter layers, the profiles and boundary values of the temperature and the density under various conditions, and the difference between the electron and ion temperatures. This model can also be used to analyze the recycling of neutrals, the energy and particle losses to the wall and the limiter, and other characteristics
Numerical simulation of tsunami-scale wave boundary layers
DEFF Research Database (Denmark)
Williams, Isaac A.; Fuhrman, David R.
2016-01-01
This paper presents a numerical study of the boundary layer flow and properties induced by tsunami-scalewaves. For this purpose, an existing one-dimensional vertical (1DV) boundary layer model, based on the horizontal component of the incompressible Reynolds-averaged Navier–Stokes (RANS) equation...
Asymptotic analysis and boundary layers
Cousteix, Jean
2007-01-01
This book presents a new method of asymptotic analysis of boundary-layer problems, the Successive Complementary Expansion Method (SCEM). The first part is devoted to a general comprehensive presentation of the tools of asymptotic analysis. It gives the keys to understand a boundary-layer problem and explains the methods to construct an approximation. The second part is devoted to SCEM and its applications in fluid mechanics, including external and internal flows. The advantages of SCEM are discussed in comparison with the standard Method of Matched Asymptotic Expansions. In particular, for the first time, the theory of Interactive Boundary Layer is fully justified. With its chapter summaries, detailed derivations of results, discussed examples and fully worked out problems and solutions, the book is self-contained. It is written on a mathematical level accessible to graduate and post-graduate students of engineering and physics with a good knowledge in fluid mechanics. Researchers and practitioners will estee...
Benthic boundary layer modelling studies
International Nuclear Information System (INIS)
Richards, K.J.
1984-01-01
A numerical model has been developed to study the factors which control the height of the benthic boundary layer in the deep ocean and the dispersion of a tracer within and directly above the layer. This report covers tracer clouds of horizontal scales of 10 to 100 km. The dispersion of a tracer has been studied in two ways. Firstly, a number of particles have been introduced into the flow. The trajectories of these particles provide information on dispersion rates. For flow conditions similar to those observed in the abyssal N.E. Atlantic the diffusivity of a tracer was found to be 5 x 10 6 cm 2 s -1 for a tracer within the boundary layer and 8 x 10 6 cm 2 s -1 for a tracer above the boundary layer. The results are in accord with estimates made from current meter measurements. The second method of studying dispersion was to calculate the evolution of individual tracer clouds. Clouds within and above the benthic boundary layer often show quite different behaviour from each other although the general structure of the clouds in the two regions were found to have no significant differences. (author)
International Nuclear Information System (INIS)
Colette, A.
2005-12-01
Closing the tropospheric ozone budget requires a better understanding of the role of transport processes from the major reservoirs: the planetary boundary layer and the stratosphere. Case studies lead to the identification of mechanisms involved as well as their efficiency. However, their global impact on the budget must be addressed on a climatological basis. This manuscript is thus divided in two parts. First, we present case studies based on ozone LIDAR measurements performed during the ESCOMPTE campaign. This work consists in a data analysis investigation by means of a hybrid - Lagrangian study involving: global meteorological analyses, Lagrangian particle dispersion computation, and mesoscale, chemistry - transport, and Lagrangian photochemistry modeling. Our aim is to document the amount of observed ozone variability related to transport processes and, when appropriate, to infer the role of tropospheric photochemical production. Second, we propose a climatological analysis of the respective impact of transport from the boundary-layer and from the tropopause region on the tropospheric ozone budget. A multivariate analysis is presented and compared to a trajectography approach. Once validated, this algorithm is applied to the whole database of ozone profiles collected above Europe during the past 30 years in order to discuss the seasonal, geographical and temporal variability of transport processes as well as their impact on the tropospheric ozone budget. The variability of turbulent mixing and its impact on the persistence of tropospheric layers will also be discussed. (author)
Large Eddy Simulation of the ventilated wave boundary layer
DEFF Research Database (Denmark)
Lohmann, Iris P.; Fredsøe, Jørgen; Sumer, B. Mutlu
2006-01-01
A Large Eddy Simulation (LES) of (1) a fully developed turbulent wave boundary layer and (2) case 1 subject to ventilation (i.e., suction and injection varying alternately in phase) has been performed, using the Smagorinsky subgrid-scale model to express the subgrid viscosity. The model was found...... slows down the flow in the full vertical extent of the boundary layer, destabilizes the flow and decreases the mean bed shear stress significantly; whereas suction generally speeds up the flow in the full vertical extent of the boundary layer, stabilizes the flow and increases the mean bed shear stress...
Diagnosis of boundary-layer circulations.
Beare, Robert J; Cullen, Michael J P
2013-05-28
Diagnoses of circulations in the vertical plane provide valuable insights into aspects of the dynamics of the climate system. Dynamical theories based on geostrophic balance have proved useful in deriving diagnostic equations for these circulations. For example, semi-geostrophic theory gives rise to the Sawyer-Eliassen equation (SEE) that predicts, among other things, circulations around mid-latitude fronts. A limitation of the SEE is the absence of a realistic boundary layer. However, the coupling provided by the boundary layer between the atmosphere and the surface is fundamental to the climate system. Here, we use a theory based on Ekman momentum balance to derive an SEE that includes a boundary layer (SEEBL). We consider a case study of a baroclinic low-level jet. The SEEBL solution shows significant benefits over Ekman pumping, including accommodating a boundary-layer depth that varies in space and structure, which accounts for buoyancy and momentum advection. The diagnosed low-level jet is stronger than that determined by Ekman balance. This is due to the inclusion of momentum advection. Momentum advection provides an additional mechanism for enhancement of the low-level jet that is distinct from inertial oscillations.
Exploring the magnetospheric boundary layer
International Nuclear Information System (INIS)
Hapgood, M.A.; Bryant, D.A.
1992-01-01
We show how, for most crossings of the boundary layer, one can construct a 'transition parameter', based on electron density and temperature, which orders independent plasma measurements into well-defined patterns which are consistent from case to case. We conclude that there is a gradual change in the balance of processes which determine the structure of the layer and suggest that there is no advantage in dividing the layer into different regions. We further conclude that the mixing processes in layer act in an organised way to give the consistent patterns revealed by the transition parameter. More active processes must sometimes take to give the extreme values (e.g. in velocity) which are seen in some crossings
A global boundary-layer height climatology
Energy Technology Data Exchange (ETDEWEB)
Dop, H. van; Krol, M.; Holtslag, B. [Inst. for Marine and Atmospheric Research Utrecht, IMAU, Utrecht (Netherlands)
1997-10-01
In principle the ABL (atmospheric boundary layer) height can be retrieved from atmospheric global circulation models since they contain algorithms which determine the intensity of the turbulence as a function of height. However, these data are not routinely available, or on a (vertical) resolution which is too crude in view of the application. This justifies the development of a separate algorithm in order to define the ABL. The algorithm should include the generation of turbulence by both shear and buoyancy and should be based on readily available atmospheric parameters. There is obviously a wide application for boundary heights in off-line global and regional chemistry and transport modelling. It is also a much used parameter in air pollution meteorology. In this article we shall present a theory which is based on current insights in ABL dynamics. The theory is applicable over land and sea surfaces in all seasons. The theory is (for various reasons) not valid in mountainous areas. In areas where boundary-layer clouds or deep cumulus convection are present the theory does not apply. However, the same global atmospheric circulation models contain parameterizations for shallow and deep convection from which separate estimates can be obtained for the extent of vertical mixing. (au)
Turbulent Helicity in the Atmospheric Boundary Layer
Chkhetiani, Otto G.; Kurgansky, Michael V.; Vazaeva, Natalia V.
2018-05-01
We consider the assumption postulated by Deusebio and Lindborg (J Fluid Mech 755:654-671, 2014) that the helicity injected into the Ekman boundary layer undergoes a cascade, with preservation of its sign (right- or alternatively left-handedness), which is a signature of the system rotation, from large to small scales, down to the Kolmogorov microscale of turbulence. At the same time, recent direct field measurements of turbulent helicity in the steppe region of southern Russia near Tsimlyansk Reservoir show the opposite sign of helicity from that expected. A possible explanation for this phenomenon may be the joint action of different scales of atmospheric flows within the boundary layer, including the sea-breeze circulation over the test site. In this regard, we consider a superposition of the classic Ekman spiral solution and Prandtl's jet-like slope-wind profile to describe the planetary boundary-layer wind structure. The latter solution mimics a hydrostatic shallow breeze circulation over a non-uniformly heated surface. A 180°-wide sector on the hodograph plane exists, within which the relative orientation of the Ekman and Prandtl velocity profiles favours the left rotation with height of the resulting wind velocity vector in the lowermost part of the boundary layer. This explains the negative (left-handed) helicity cascade toward small-scale turbulent motions, which agrees with the direct field measurements of turbulent helicity in Tsimlyansk. A simple turbulent relaxation model is proposed that explains the measured positive values of the relatively minor contribution to turbulent helicity from the vertical components of velocity and vorticity.
Review: the atmospheric boundary layer
Garratt, J. R.
1994-10-01
An overview is given of the atmospheric boundary layer (ABL) over both continental and ocean surfaces, mainly from observational and modelling perspectives. Much is known about ABL structure over homogeneous land surfaces, but relatively little so far as the following are concerned, (i) the cloud-topped ABL (over the sea predominantly); (ii) the strongly nonhomogeneous and nonstationary ABL; (iii) the ABL over complex terrain. These three categories present exciting challenges so far as improved understanding of ABL behaviour and improved representation of the ABL in numerical models of the atmosphere are concerned.
The surface roughness and planetary boundary layer
Telford, James W.
1980-03-01
Applications of the entrainment process to layers at the boundary, which meet the self similarity requirements of the logarithmic profile, have been studied. By accepting that turbulence has dominating scales related in scale length to the height above the surface, a layer structure is postulated wherein exchange is rapid enough to keep the layers internally uniform. The diffusion rate is then controlled by entrainment between layers. It has been shown that theoretical relationships derived on the basis of using a single layer of this type give quantitatively correct factors relating the turbulence, wind and shear stress for very rough surface conditions. For less rough surfaces, the surface boundary layer can be divided into several layers interacting by entrainment across each interface. This analysis leads to the following quantitatively correct formula compared to published measurements. 1 24_2004_Article_BF00877766_TeX2GIFE1.gif {σ _w }/{u^* } = ( {2/{9Aa}} )^{{1/4}} ( {1 - 3^{{1/2}{ a/k{d_n }/z{σ _w }/{u^* }z/L} )^{{1/4}} = 1.28(1 - 0.945({{σ _w }/{u^* }}}) {{z/L}})^{{1/4 where u^* = ( {{tau/ρ}}^{{1/2}}, σ w is the standard deviation of the vertical velocity, z is the height and L is the Obukhov scale lenght. The constants a, A, k and d n are the entrainment constant, the turbulence decay constant, Von Karman's constant, and the layer depth derived from the theory. Of these, a and A, are universal constants and not empirically determined for the boundary layer. Thus the turbulence needed for the plume model of convection, which resides above these layers and reaches to the inversion, is determined by the shear stress and the heat flux in the surface layers. This model applies to convection in cool air over a warm sea. The whole field is now determined except for the temperature of the air relative to the water, and the wind, which need a further parameter describing sea surface roughness. As a first stop to describing a surface where roughness elements
Miller, Daniel J.; Zhang, Zhibo; Ackerman, Andrew S.; Platnick, Steven; Baum, Bryan A.
2018-01-01
Passive optical retrievals of cloud liquid water path (LWP), like those implemented for Moderate Resolution Imaging Spectroradiometer (MODIS), rely on cloud vertical profile assumptions to relate optical thickness (τ) and effective radius (re) retrievals to LWP. These techniques typically assume that shallow clouds are vertically homogeneous; however, an adiabatic cloud model is plausibly more realistic for shallow marine boundary layer cloud regimes. In this study a satellite retrieval simulator is used to perform MODIS-like satellite retrievals, which in turn are compared directly to the large-eddy simulation (LES) output. This satellite simulator creates a framework for rigorous quantification of the impact that vertical profile features have on LWP retrievals, and it accomplishes this while also avoiding sources of bias present in previous observational studies. The cloud vertical profiles from the LES are often more complex than either of the two standard assumptions, and the favored assumption was found to be sensitive to cloud regime (cumuliform/stratiform). Confirming previous studies, drizzle and cloud top entrainment of dry air are identified as physical features that bias LWP retrievals away from adiabatic and toward homogeneous assumptions. The mean bias induced by drizzle-influenced profiles was shown to be on the order of 5–10 g/m2. In contrast, the influence of cloud top entrainment was found to be smaller by about a factor of 2. A theoretical framework is developed to explain variability in LWP retrievals by introducing modifications to the adiabatic re profile. In addition to analyzing bispectral retrievals, we also compare results with the vertical profile sensitivity of passive polarimetric retrieval techniques. PMID:29637042
Miller, Daniel J; Zhang, Zhibo; Ackerman, Andrew S; Platnick, Steven; Baum, Bryan A
2016-04-27
Passive optical retrievals of cloud liquid water path (LWP), like those implemented for Moderate Resolution Imaging Spectroradiometer (MODIS), rely on cloud vertical profile assumptions to relate optical thickness ( τ ) and effective radius ( r e ) retrievals to LWP. These techniques typically assume that shallow clouds are vertically homogeneous; however, an adiabatic cloud model is plausibly more realistic for shallow marine boundary layer cloud regimes. In this study a satellite retrieval simulator is used to perform MODIS-like satellite retrievals, which in turn are compared directly to the large-eddy simulation (LES) output. This satellite simulator creates a framework for rigorous quantification of the impact that vertical profile features have on LWP retrievals, and it accomplishes this while also avoiding sources of bias present in previous observational studies. The cloud vertical profiles from the LES are often more complex than either of the two standard assumptions, and the favored assumption was found to be sensitive to cloud regime (cumuliform/stratiform). Confirming previous studies, drizzle and cloud top entrainment of dry air are identified as physical features that bias LWP retrievals away from adiabatic and toward homogeneous assumptions. The mean bias induced by drizzle-influenced profiles was shown to be on the order of 5-10 g/m 2 . In contrast, the influence of cloud top entrainment was found to be smaller by about a factor of 2. A theoretical framework is developed to explain variability in LWP retrievals by introducing modifications to the adiabatic r e profile. In addition to analyzing bispectral retrievals, we also compare results with the vertical profile sensitivity of passive polarimetric retrieval techniques.
Convection Cells in the Atmospheric Boundary Layer
Fodor, Katherine; Mellado, Juan-Pedro
2017-04-01
In dry, shear-free convective boundary layers (CBLs), the turbulent flow of air is known to organise itself on large scales into coherent, cellular patterns, or superstructures, consisting of fast, narrow updraughts and slow, wide downdraughts which together form circulations. Superstructures act as transport mechanisms from the surface to the top of the boundary layer and vice-versa, as opposed to small-scale turbulence, which only modifies conditions locally. This suggests that a thorough investigation into superstructure properties may help us better understand transport across the atmospheric boundary layer as a whole. Whilst their existence has been noted, detailed studies into superstructures in the CBL have been scarce. By applying methods which are known to successfully isolate similar large-scale patterns in turbulent Rayleigh-Bénard convection, we can assess the efficacy of those detection techniques in the CBL. In addition, through non-dimensional analysis, we can systematically compare superstructures in various convective regimes. We use direct numerical simulation of four different cases for intercomparison: Rayleigh-Bénard convection (steady), Rayleigh-Bénard convection with an adiabatic top lid (quasi-steady), a stably-stratified CBL (quasi-steady) and a neutrally-stratified CBL (unsteady). The first two are non-penetrative and the latter two penetrative. We find that although superstructures clearly emerge from the time-mean flow in the non-penetrative cases, they become obscured by temporal averaging in the CBL. This is because a rigid lid acts to direct the flow into counter-rotating circulation cells whose axis of rotation remains stationary, whereas a boundary layer that grows in time and is able to entrain fluid from above causes the circulations to not only grow in vertical extent, but also to move horizontally and merge with neighbouring circulations. Spatial filtering is a useful comparative technique as it can be performed on boundary
Boundary layer attenuation in turbulent sodium flows
International Nuclear Information System (INIS)
Tenchine, D.
1994-01-01
Temperature fluctuations are produced in the sodium coolant of Liquid Metal Reactors when flows at different temperatures are mixing. That occurs in various areas of the reactor plant, in the primary and the secondary circuits. This paper deals with secondary circuit pipings, specifically the Superphenix steam generator outlet. The possibility of thermal striping in this area is studied because of the mixing of a main 'hot' flow surrounded by a smaller 'cold' flow in the vertical pipe located below the steam generator. This work was developed in the frame of a collaboration between CEA, EDF and FRAMATOME. The purpose of our study is to measure temperature fluctuations in the fluid and on the structures, on a sodium reduced scale model of the outlet region of the steam generator. We want to evidence the boundary layer attenuation by comparing wall and fluid measurements. From these experimental data, we shall propose a methodology to predict the boundary layer attenuation and the temperature fluctuations at the surface of the structure, for pipe flow configurations
Boundary-Layer Characteristics Over a Coastal Megacity
Melecio-Vazquez, D.; Ramamurthy, P.; Arend, M.; Moshary, F.; Gonzalez, J.
2017-12-01
Boundary-layer characteristics over New York City are analyzed for various local and synoptic conditions over several seasons. An array of vertical profilers, including a Doppler LiDAR, a micro-pulse LiDAR and a microwave radiometer are used to observe the structure and evolution of the boundary-layer. Additionally, an urbanized Weather Research and Forecasting (uWRF) model coupled to a high resolution landcover/land-use database is used to study the spatial variability in boundary layer characteristics. The summer daytime averaged potential temperature profile from the microwave radiometer shows the presence of a thermal internal boundary layer wherein a superadiabatic layer lies underneath a stable layer instead of a mixed-layer. Both the winter daytime and nighttime seasonal averages show that the atmosphere remains unstable near the surface and does not reach stable conditions during the nighttime. The mixing ratio seasonal averages show peaks in humidity near 200-m and 1100-m, above instrument level, which could result from sea breeze and anthropogenic sources. Ceilometer measurements show a high degree of variability in boundary layer height depending on wind direction. Comparison with uWRF results show that the model tends to overestimate convective efficiency for selected summer and winter cases and therefore shows a much deeper thermal boundary layer than the observed profiles. The model estimates a less humid atmosphere than seen in observations.
Experimental investigation of wave boundary layer
DEFF Research Database (Denmark)
Sumer, B. Mutlu
2003-01-01
A review is presented of experimental investigation of wave boundary layer. The review is organized in six main sections. The first section describes the wave boundary layer in a real-life environment and its simulation in the laboratory in an oscillating water tunnel and in a water tank...... with an oscillating seabed. A brief account is given of measured quantities, measurement techniques (LDA, PIV, flow visualization) and limitations/constraints in the experimental investigation of the wave boundary layer in the laboratory. The second section concentrates on uniform oscillating boundary layers...
Boundary layers of the earth's outer magnetosphere
Eastman, T. E.; Frank, L. A.
1984-01-01
The magnetospheric boundary layer and the plasma-sheet boundary layer are the primary boundary layers of the earth's outer magnetosphere. Recent satellite observations indicate that they provide for more than 50 percent of the plasma and energy transport in the outer magnetosphere although they constitute less than 5 percent by volume. Relative to the energy density in the source regions, plasma in the magnetospheric boundary layer is predominantly deenergized whereas plasma in the plasma-sheet boundary layer has been accelerated. The reconnection hypothesis continues to provide a useful framework for comparing data sampled in the highly dynamic magnetospheric environment. Observations of 'flux transfer events' and other detailed features near the boundaries have been recently interpreted in terms of nonsteady-state reconnection. Alternative hypotheses are also being investigated. More work needs to be done, both in theory and observation, to determine whether reconnection actually occurs in the magnetosphere and, if so, whether it is important for overall magnetospheric dynamics.
Boundary layers of the earth's outer magnetosphere
International Nuclear Information System (INIS)
Eastman, T.E.; Frank, L.A.
1984-01-01
The magnetospheric boundary layer and the plasma-sheet boundary layer are the primary boundary layers of the earth's outer magnetosphere. Recent satellite observations indicate that they provide for more than 50 percent of the plasma and energy transport in the outer magnetosphere although they constitute less than 5 percent by volume. Relative to the energy density in the source regions, plasma in the magnetospheric boundary layer is predominantly deenergized whereas plasma in the plasma-sheet boundary layer has been accelerated. The reconnection hypothesis continues to provide a useful framework for comparing data sampled in the highly dynamic magnetospheric environment. Observations of flux transfer events and other detailed features near the boundaries have been recently interpreted in terms of nonsteady-state reconnection. Alternative hypotheses are also being investigated. More work needs to be done, both in theory and observation, to determine whether reconnection actually occurs in the magnetosphere and, if so, whether it is important for overall magnetospheric dynamics. 30 references
Axisymmetric free convection boundary-layer flow past slender bodies
Kuiken, H.K.
1968-01-01
Radial curvature effects on axisymmetric free convection boundary-layer flow are investigated for vertical cylinders and cones for some special non-uniform temperature differences between the surface and the ambient fluid. The solution is given as a power series expansion, the first term being equal
Boundary Layer Flows in Porous Media with Lateral Mass Flux
DEFF Research Database (Denmark)
Nemati, H; H, Bararnia; Noori, F
2015-01-01
Solutions for free convection boundary layers on a heated vertical plate with lateral mass flux embedded in a saturated porous medium are presented using the Homotopy Analysis Method and Shooting Numerical Method. Homotopy Analysis Method yields an analytic solution in the form of a rapidly...
Ferdows, M.
2017-03-10
A steady two-dimensional free convective flow of a viscous incompressible fluid along a vertical stretching sheet with the effect of magnetic field, radiation and variable thermal conductivity in porous media is analyzed. The nonlinear partial differential equations, governing the flow field under consideration, have been transformed by a similarity transformation into a systemof nonlinear ordinary differential equations and then solved numerically. Resulting non-dimensional velocity and temperature profiles are then presented graphically for different values of the parameters. Finally, the effects of the pertinent parameters, which are of physical and engineering interest, are examined both in graphical and tabular form.
Modelling stable atmospheric boundary layers over snow
Sterk, H.A.M.
2015-01-01
Thesis entitled:
Modelling Stable Atmospheric Boundary Layers over Snow
H.A.M. Sterk
Wageningen, 29th of April, 2015
Summary
The emphasis of this thesis is on the understanding and forecasting of the Stable Boundary Layer (SBL) over snow-covered surfaces. SBLs
Suction of MHD boundary layer flows
International Nuclear Information System (INIS)
Rao, B.N.
1985-01-01
The boundary layer growth with tensor electrical conductivity and the transpiration number has been examined using local nonsimilarity solutions method. It is found that suction will cause the increase in wall shearing stress and decrease in thicknesses of the boundary layer. (Auth.)
Measurements in a synthetic turbulent boundary layer
Arakeri, J. H.; Coles, D. E.
Some measurements in a synthetic turbulent boundary layer (SBL) are reported. The main diagnostic tool is an X-wire probe. The velocity of the large eddies is determined to be 0.842 times the freestream velocity. The mean properties of the SBL are reasonably close to those of a natural turbulent boundary layer. The large eddy in the SBL appears to be a pair of counterrotating eddies in the stream direction, inclined at a shallow angle and occupying much of the boundary-layer thickness.
Longitudinal vortices in a transitioning boundary layer
International Nuclear Information System (INIS)
Anders, J.B.; Backwelder, R.F.
1980-01-01
Naturally occurring spanwise variations of the streamwise velocity component, characteristic of longitudinal vortices embedded in a transitioning boundary layer were explored using hot-wire anemometers. A vibrating ribbon introduced stable or unstable Tollmien-Schlichting waves into the laminar boundary layer. These damped or growing disturbances always developed a strong three-dimensional pattern even though no spanwise perturbations were artificially induced. Changing the radius of the leading edge and other modifications to the flat plate, wind tunnel and boundary layer did not alter the spanwise wavelength of the vortices. (orig.)
International Nuclear Information System (INIS)
Shashkov, Alexander; Higuchi, Kaz; Chan, Douglas
2007-01-01
During the period of July 8-13, 2002, we collected vertical profiles by aircraft of meteorological variables and atmospheric CO 2 over the OBS (old black spruce) site located in Boreal Ecosystem Research and Monitoring Sites in Northern Saskatchewan, Canada. We have used the data from the morning and afternoon flights to calculate the regional daily afternoon CO 2 flux for the days July 8-11. These daily fluxes were then compared to those obtained by the boundary layer budget method and by the eddy covariance measurements on the tower at the OBS site. We identified the importance of changes in the CO 2 concentration by advection to the flux estimates. In addition, we provide arguments to suggest that subseasonal temporal averaging might not, at least in some cases, eliminate advective bias contribution to the flux estimates. Because the advective influence is large and highly directional, even on seasonal and interannual timescales, it is advisable that flux estimates based on CO 2 concentration change at a site contain dynamic description of an air parcel transport history
A variable K - planetary boundary layer model
International Nuclear Information System (INIS)
Misra, P.K.
1976-07-01
The steady-state, homogeneous and barotropic equations of motion within the planetary boundary layer are solved with the assumption that the coefficient of eddy viscosity varies as K(Z) = K 0 (1-Z/h)sup(p), where h is the height of the boundary layer and p a parameter which depends on the atmospheric stability. The solutions are compared with the observed velocity profiles based on the Wangara data. They compare favourably. (author)
Comments on Hypersonic Boundary-Layer Transition
1990-09-01
mechanism by which boundary-layer disturbance growth is generally initiated and establishes the initial distur- banca amplitude at the onset of disturbance...Patankar, S. V., and Spalding, P. B., Heat and Mass Transfer in Boundary Lavers, CRC Press , Cleveland, Ohio, 1968. 87. Neumann, R. D., and Patterson, .J. 1
DEFF Research Database (Denmark)
Chivaee, Hamid Sarlak; Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming
2012-01-01
Large eddy simulation (LES) of flow in a wind farm is studied in neutral as well as thermally stratified atmospheric boundary layer (ABL). An approach has been practiced to simulate the flow in a fully developed wind farm boundary layer. The approach is based on the Immersed Boundary Method (IBM......) and involves implementation of an arbitrary prescribed initial boundary layer (See [1]). A prescribed initial boundary layer profile is enforced through the computational domain using body forces to maintain a desired flow field. The body forces are then stored and applied on the domain through the simulation...... and the boundary layer shape will be modified due to the interaction of the turbine wakes and buoyancy contributions. The implemented method is capable of capturing the most important features of wakes of wind farms [1] while having the advantage of resolving the wall layer with a coarser grid than typically...
Shock-like structures in the tropical cyclone boundary layer
Williams, Gabriel J.; Taft, Richard K.; McNoldy, Brian D.; Schubert, Wayne H.
2013-06-01
This paper presents high horizontal resolution solutions of an axisymmetric, constant depth, slab boundary layer model designed to simulate the radial inflow and boundary layer pumping of a hurricane. Shock-like structures of increasing intensity appear for category 1-5 hurricanes. For example, in the category 3 case, the u>(∂u/∂r>) term in the radial equation of motion produces a shock-like structure in the radial wind, i.e., near the radius of maximum tangential wind the boundary layer radial inflow decreases from approximately 22 m s-1 to zero over a radial distance of a few kilometers. Associated with this large convergence is a spike in the radial distribution of boundary layer pumping, with updrafts larger than 22 m s-1 at a height of 1000 m. Based on these model results, it is argued that observed hurricane updrafts of this magnitude so close to the ocean surface are attributable to the dry dynamics of the frictional boundary layer rather than moist convective dynamics. The shock-like structure in the boundary layer radial wind also has important consequences for the evolution of the tangential wind and the vertical component of vorticity. On the inner side of the shock the tangential wind tendency is essentially zero, while on the outer side of the shock the tangential wind tendency is large due to the large radial inflow there. The result is the development of a U-shaped tangential wind profile and the development of a thin region of large vorticity. In many respects, the model solutions resemble the remarkable structures observed in the boundary layer of Hurricane Hugo (1989).
Boundary-layer effects in droplet splashing
Riboux, Guillaume; Gordillo, Jose Manuel
2017-11-01
A drop falling onto a solid substrate will disintegrate into smaller parts when its impact velocity exceeds the so called critical velocity for splashing. Under these circumstances, the very thin liquid sheet ejected tangentially to the solid after the drop touches the substrate, lifts off as a consequence of the aerodynamic forces exerted on it and finally breaks into smaller droplets, violently ejected radially outwards, provoking the splash. Here, the tangential deceleration experienced by the fluid entering the thin liquid sheet is investigated making use of boundary layer theory. The velocity component tangent to the solid, computed using potential flow theory provides the far field boundary condition as well as the pressure gradient for the boundary layer equations. The structure of the flow permits to find a self similar solution of the boundary layer equations. This solution is then used to calculate the boundary layer thickness at the root of the lamella as well as the shear stress at the wall. The splash model presented in, which is slightly modified to account for the results obtained from the boundary layer analysis, provides a very good agreement between the measurements and the predicted values of the critical velocity for the splash.
Ground observations of magnetospheric boundary layer phenomena
International Nuclear Information System (INIS)
McHenry, M.A.; Clauer, C.R.; Friis-Christensen, E.; Newell, P.T.; Kelly, J.D.
1990-01-01
Several classes of traveling vortices in the dayside ionospheric convection have been detected and tracked using the Greenland magnetometer chain (Friis-Christensen et al., 1988, McHenry et al., 1989). One class observed during quiet times consists of a continuous series of vortices moving generally anti-sunward for several hours at a time. The vortices strength is seen to be approximately steady and neighboring vortices rotate in opposite directions. Sondrestrom radar observations show that the vortices are located at the ionospheric convection reversal boundary. Low altitude DMSP observations indicate the vortices are on field lines which map to the inner edge of the low latitude boundary layer. Because the vortices are conjugate to the boundary layer, repeat in a regular fashion and travel antisunward, the authors argue that this class of vortices is caused by the Kelvin-Helmholtz instability of the inner edge of the magnetospheric boundary layer
Stability of spatially developing boundary layers
Govindarajan, Rama
1993-07-01
A new formulation of the stability of boundary-layer flows in pressure gradients is presented, taking into account the spatial development of the flow. The formulation assumes that disturbance wavelength and eigenfunction vary downstream no more rapidly than the boundary-layer thickness, and includes all terms of O(1) and O(R(exp -1)) in the boundary-layer Reynolds number R. Although containing the Orr-Sommerfeld operator, the present approach does not yield the Orr-Sommerfeld equation in any rational limit. In Blasius flow, the present stability equation is consistent with that of Bertolotti et al. (1992) to terms of O(R(exp -1)). For the Falkner-Skan similarity solutions neutral boundaries are computed without the necessity of having to march in space. Results show that the effects of spatial growth are striking in flows subjected to adverse pressure gradients.
Diamagnetic boundary layers: a kinetic theory
International Nuclear Information System (INIS)
Lemaire, J.; Burlaga, L.F.
1976-01-01
A kinetic theory for boundary layers associated with MHD tangential 'discontinuities' in a collisionless magnetized plasma such as those observed in the solar wind is presented. The theory consists of finding self-consistent solutions of Vlasov's equation and Maxwell's equation for stationary, one-dimensional boundary layers separating two Maxwellian plasma states. Layers in which the current is carried by electrons are found to have a thickness of the order of a few electron gyroradii, but the drift speed of the current-carrying electrons is found to exceed the Alfven speed, and accordingly such layers are not stable. Several types of layers, in which the current is carried by protons are discussed; in particular, cases in which the magnetic field intensity and/or direction changed across the layer were considered. In every case, the thickness was of the order of a few proton gyroradii and the field changed smoothly , although the characteristics depended somewhat on the boundary conditions. The drift speed was always less than the Alfven speed, consistent with stability of such structures. The results are consistent with the observations of boundary layers in the solar wind near 1 AU. (Auth.)
On the modeling of electrical boundary layer (electrode layer) and ...
Indian Academy of Sciences (India)
In the first part of the paper, equations and methodology are discussed and in the second, we discuss results. 2. Methodology. In the atmospheric electricity, the earth's surface is one electrode and electrode layer or electrical boundary layer is a region near the surface of the earth in which profiles of atmospheric electrical.
Atmospheric Boundary Layer Modeling for Combined Meteorology and Air Quality Systems
Atmospheric Eulerian grid models for mesoscale and larger applications require sub-grid models for turbulent vertical exchange processes, particularly within the Planetary Boundary Layer (PSL). In combined meteorology and air quality modeling systems consistent PSL modeling of wi...
Problems of matter-antimatter boundary layers
International Nuclear Information System (INIS)
Lehnert, B.
1975-01-01
This paper outlines the problems of the quasi-steady matter-antimatter boundary layers discussed in Klein-Alfven's cosmological theory, and a crude model of the corresponding ambiplasma balance is presented: (i) at interstellar particle densities, no well-defined boundary layer can exist in presence of neutral gas, nor can such a layer be sustained in an unmagnetized fully ionized ambiplasma. (ii) Within the limits of applicability of the present model, sharply defined boundary layers are under certain conditions found to exist in a magnetized ambiplasma. Thus, at beta values less than unity, a steep pressure drop of the low-energy components of matter and antimatter can be balanced by a magnetic field and the electric currents in the ambiplasma. (iii) The boundary layer thickness is of the order of 2x 0 approximately 10/BT 0 sup(1/4) meters, where B is the magnetic field strength in MKS units and T 0 the characteristic temperature of the low-energy components in the layer. (Auth.)
Boundary layer heights derived from velocity spectra
Energy Technology Data Exchange (ETDEWEB)
Hoejstrup, J.; Barthelmie, R.J. [Risoe National Lab., Roskilde (Denmark); Kaellstrand, B. [Univ. of Uppsala, Uppsala (Sweden)
1997-10-01
It is a well-known fact that the height of the mixed layer determines the size of the largest and most energetic eddies that can be observed in the unstable boundary layer, and consequently a peak can be observed in the power spectra of the along-wind velocity component at scales comparable to the mixed layer depth. We will now show how the mixed layer depth can be derived from the u-specta and the results will be compared with direct measurements using pibal and tethersonde measurements. (au)
Magnetohydrodynamic boundary layer on a wedge
International Nuclear Information System (INIS)
Rao, B.N.; Mittal, M.L.
1981-01-01
The effects of the Hall and ionslip currents on the gas-dynamic boundary layer are investigated in view of the increasing prospects for using the MHD principle in electric power generation. The currents are included in the analysis using the generalized Ohm's law (Sherman and Sutton, 1964), and the resulting two nonlinear coupled equations are solved using a modification in the method suggested by Nachtsheim and Swigert (1965), Dewey and Gross (1967), and Steinheuer (1968). Solutions are presented for the incompressible laminar boundary-layer equations in the absence and the presence of the load parameter, and for the pressure gradient parameter for flow separation
Self-similar magnetohydrodynamic boundary layers
Energy Technology Data Exchange (ETDEWEB)
Nunez, Manuel; Lastra, Alberto, E-mail: mnjmhd@am.uva.e [Departamento de Analisis Matematico, Universidad de Valladolid, 47005 Valladolid (Spain)
2010-10-15
The boundary layer created by parallel flow in a magnetized fluid of high conductivity is considered in this paper. Under appropriate boundary conditions, self-similar solutions analogous to the ones studied by Blasius for the hydrodynamic problem may be found. It is proved that for these to be stable, the size of the Alfven velocity at the outer flow must be smaller than the flow velocity, a fact that has a ready physical explanation. The process by which the transverse velocity and the thickness of the layer grow with the size of the Alfven velocity is detailed.
Self-similar magnetohydrodynamic boundary layers
International Nuclear Information System (INIS)
Nunez, Manuel; Lastra, Alberto
2010-01-01
The boundary layer created by parallel flow in a magnetized fluid of high conductivity is considered in this paper. Under appropriate boundary conditions, self-similar solutions analogous to the ones studied by Blasius for the hydrodynamic problem may be found. It is proved that for these to be stable, the size of the Alfven velocity at the outer flow must be smaller than the flow velocity, a fact that has a ready physical explanation. The process by which the transverse velocity and the thickness of the layer grow with the size of the Alfven velocity is detailed.
Diffusion processes in the magnetopause boundary layer
International Nuclear Information System (INIS)
Tsurutani, B.T.; Thorne, R.M.
1982-01-01
Anomalous cross-field diffusion of magnetosheath ions and electrons is a direct consequence of cyclotron-resonant scattering by electrostatic and electromagnetic emissions which are continuously present within the magnetopause boundary layer. Expressions for the rate of cross-field diffusion involving either type of wave are developed and expressed in terms of the absolute upper limit referred to as Bohm diffusion. For the typical average intensity of waves observed in the boundary layer, resonant electron cross-field diffusion is always insignificant. However, magnetosheath ions, resonant with low frequency electrostatic waves, may be transported inward at a rate approaching one tenth the Bohm rate (D/sub perpendiculartsperpendicular/roughly-equal10 3 km 2 /s). While this is not the only mechanism capable of explaining the presence of the low latitude boundary layer it is adequate to account for the typical boundary layer thickness and it should occur at all local times and under all interplanetary conditions. It consequently provides a continuous mechanism for significant mass and momentum transfer across the magnetopause under conditions when field merging is inoperative
Nonlinear Transient Growth and Boundary Layer Transition
Paredes, Pedro; Choudhari, Meelan M.; Li, Fei
2016-01-01
Parabolized stability equations (PSE) are used in a variational approach to study the optimal, non-modal disturbance growth in a Mach 3 at plate boundary layer and a Mach 6 circular cone boundary layer. As noted in previous works, the optimal initial disturbances correspond to steady counter-rotating streamwise vortices, which subsequently lead to the formation of streamwise-elongated structures, i.e., streaks, via a lift-up effect. The nonlinear evolution of the linearly optimal stationary perturbations is computed using the nonlinear plane-marching PSE for stationary perturbations. A fully implicit marching technique is used to facilitate the computation of nonlinear streaks with large amplitudes. To assess the effect of the finite-amplitude streaks on transition, the linear form of plane- marching PSE is used to investigate the instability of the boundary layer flow modified by spanwise periodic streaks. The onset of bypass transition is estimated by using an N- factor criterion based on the amplification of the streak instabilities. Results show that, for both flow configurations of interest, streaks of sufficiently large amplitude can lead to significantly earlier onset of transition than that in an unperturbed boundary layer without any streaks.
Vortex sheet approximation of boundary layers
International Nuclear Information System (INIS)
Chorin, A.J.
1978-01-01
a grid free method for approximating incomprssible boundary layers is introduced. The computational elements are segments of vortex sheets. The method is related to the earlier vortex method; simplicity is achieved at the cost of replacing the Navier-Stokes equations by the Prandtl boundary layer equations. A new method for generating vorticity at boundaries is also presented; it can be used with the earlier voartex method. The applications presented include (i) flat plate problems, and (ii) a flow problem in a model cylinder- piston assembly, where the new method is used near walls and an improved version of the random choice method is used in the interior. One of the attractive features of the new method is the ease with which it can be incorporated into hybrid algorithms
Characterization of the atmospheric boundary layer from radiosonde ...
Indian Academy of Sciences (India)
In this paper, a comparison of two methods for the calculation of the height of atmospheric boundary layer (ABL) ... Boundary layer; GPS sonde; mixed layer height; turbulent flow depth. J. Earth Syst. ..... for her PhD research work. References.
Evaluation of the Atmospheric Boundary-Layer Electrical Variability
Anisimov, Sergey V.; Galichenko, Sergey V.; Aphinogenov, Konstantin V.; Prokhorchuk, Aleksandr A.
2017-12-01
Due to the chaotic motion of charged particles carried by turbulent eddies, electrical quantities in the atmospheric boundary layer (ABL) have short-term variability superimposed on long-term variability caused by sources from regional to global scales. In this study the influence of radon exhalation rate, aerosol distribution and turbulent transport efficiency on the variability of fair-weather atmospheric electricity is investigated via Lagrangian stochastic modelling. For the mid-latitude lower atmosphere undisturbed by precipitation, electrified clouds, or thunderstorms, the model is capable of reproducing the diurnal variation in atmospheric electrical parameters detected by ground-based measurements. Based on the analysis of field observations and numerical simulation it is found that the development of the convective boundary layer, accompanied by an increase in turbulent kinetic energy, forms the vertical distribution of radon and its decaying short-lived daughters to be approximately coincident with the barometric law for several eddy turnover times. In the daytime ABL the vertical distribution of atmospheric electrical conductivity tends to be uniform except within the surface layer, due to convective mixing of radon and its radioactive decay products. At the same time, a decrease in the conductivity near the ground is usually observed. This effect leads to an enhanced ground-level atmospheric electric field compared to that normally observed in the nocturnal stably-stratified boundary layer. The simulation showed that the variability of atmospheric electric field in the ABL associated with internal origins is significant in comparison to the variability related to changes in global parameters. It is suggested that vertical profiles of electrical quantities can serve as informative parameters on ABL turbulent dynamics and can even more broadly characterize the state of the environment.
Hairpin vortices in turbulent boundary layers
International Nuclear Information System (INIS)
Eitel-Amor, G; Schlatter, P; Flores, O
2014-01-01
The present work addresses the question whether hairpin vortices are a dominant feature of near-wall turbulence and which role they play during transition. First, the parent-offspring mechanism is investigated in temporal simulations of a single hairpin vortex introduced in a mean shear flow corresponding to turbulent channels and boundary layers up to Re τ = 590. Using an eddy viscosity computed from resolved simulations, the effect of a turbulent background is also considered. Tracking the vortical structure downstream, it is found that secondary hairpins are created shortly after initialization. Thereafter, all rotational structures decay, whereas this effect is enforced in the presence of an eddy viscosity. In a second approach, a laminar boundary layer is tripped to transition by insertion of a regular pattern of hairpins by means of defined volumetric forces representing an ejection event. The idea is to create a synthetic turbulent boundary layer dominated by hairpin-like vortices. The flow for Re τ < 250 is analysed with respect to the lifetime of individual hairpin-like vortices. Both the temporal and spatial simulations demonstrate that the regeneration process is rather short-lived and may not sustain once a turbulent background has formed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former DNS studies is an outer layer phenomenon not being connected to the onset of near-wall turbulence.
Comparison of Large Eddy Simulations of a convective boundary layer with wind LIDAR measurements
DEFF Research Database (Denmark)
Pedersen, Jesper Grønnegaard; Kelly, Mark C.; Gryning, Sven-Erik
2012-01-01
Vertical profiles of the horizontal wind speed and of the standard deviation of vertical wind speed from Large Eddy Simulations of a convective atmospheric boundary layer are compared to wind LIDAR measurements up to 1400 m. Fair agreement regarding both types of profiles is observed only when...
BOREAS AFM-6 Boundary Layer Height Data
Wilczak, James; Hall, Forrest G. (Editor); Newcomer, Jeffrey A. (Editor); Smith, David E. (Technical Monitor)
2000-01-01
The Boreal Ecosystem-Atmosphere Study (BOREAS) Airborne Fluxes and Meteorology (AFM)-6 team from National Oceanic and Atmospheric Adminsitration/Environment Technology Laboratory (NOAA/ETL) operated a 915-MHz wind/Radio Acoustic Sounding System (RASS) profiler system in the Southern Study Area (SSA) near the Old Jack Pine (OJP) site. This data set provides boundary layer height information over the site. The data were collected from 21 May 1994 to 20 Sep 1994 and are stored in tabular ASCII files. The boundary layer height data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The data files are available on a CD-ROM (see document number 20010000884).
Energy Technology Data Exchange (ETDEWEB)
Schlichting, Hermann [Technische Univ. Braunschweig (Germany). Inst. fuer Stroemungsmechanik; Gersten, Klaus [Bochum Univ. (Germany). Lehrstuhl fuer Thermodynamik und Stroemungsmechanik
2017-03-01
This new edition of the near-legendary textbook by Schlichting and revised by Gersten presents a comprehensive overview of boundary-layer theory and its application to all areas of fluid mechanics, with particular emphasis on the flow past bodies (e.g. aircraft aerodynamics). The new edition features an updated reference list and over 100 additional changes throughout the book, reflecting the latest advances on the subject.
Progress in modeling hypersonic turbulent boundary layers
Zeman, Otto
1993-01-01
A good knowledge of the turbulence structure, wall heat transfer, and friction in turbulent boundary layers (TBL) at high speeds is required for the design of hypersonic air breathing airplanes and reentry space vehicles. This work reports on recent progress in the modeling of high speed TBL flows. The specific research goal described here is the development of a second order closure model for zero pressure gradient TBL's for the range of Mach numbers up to hypersonic speeds with arbitrary wall cooling requirements.
Coupled wake boundary layer model of windfarms
Stevens, Richard; Gayme, Dennice; Meneveau, Charles
2014-11-01
We present a coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a windfarm. The model couples the traditional, industry-standard wake expansion/superposition approach with a top-down model for the overall windfarm boundary layer structure. Wake models capture the effect of turbine positioning, while the top-down approach represents the interaction between the windturbine wakes and the atmospheric boundary layer. Each portion of the CWBL model requires specification of a parameter that is unknown a-priori. The wake model requires the wake expansion rate, whereas the top-down model requires the effective spanwise turbine spacing within which the model's momentum balance is relevant. The wake expansion rate is obtained by matching the mean velocity at the turbine from both approaches, while the effective spanwise turbine spacing is determined from the wake model. Coupling of the constitutive components of the CWBL model is achieved by iterating these parameters until convergence is reached. We show that the CWBL model predictions compare more favorably with large eddy simulation results than those made with either the wake or top-down model in isolation and that the model can be applied successfully to the Horns Rev and Nysted windfarms. The `Fellowships for Young Energy Scientists' (YES!) of the Foundation for Fundamental Research on Matter supported by NWO, and NSF Grant #1243482.
Turbulent fluxes in stably stratified boundary layers
International Nuclear Information System (INIS)
L'vov, Victor S; Procaccia, Itamar; Rudenko, Oleksii
2008-01-01
We present here an extended version of an invited talk we gave at the international conference 'Turbulent Mixing and Beyond'. The dynamical and statistical description of stably stratified turbulent boundary layers with the important example of the stable atmospheric boundary layer in mind is addressed. Traditional approaches to this problem, based on the profiles of mean quantities, velocity second-order correlations and dimensional estimates of the turbulent thermal flux, run into a well-known difficulty, predicting the suppression of turbulence at a small critical value of the Richardson number, in contradiction to observations. Phenomenological attempts to overcome this problem suffer from various theoretical inconsistencies. Here, we present an approach taking into full account all the second-order statistics, which allows us to respect the conservation of total mechanical energy. The analysis culminates in an analytic solution of the profiles of all mean quantities and all second-order correlations, removing the unphysical predictions of previous theories. We propose that the approach taken here is sufficient to describe the lower parts of the atmospheric boundary layer, as long as the Richardson number does not exceed an order of unity. For much higher Richardson numbers, the physics may change qualitatively, requiring careful consideration of the potential Kelvin-Helmoholtz waves and their interaction with the vortical turbulence.
Scaling the heterogeneously heated convective boundary layer
Van Heerwaarden, C.; Mellado, J.; De Lozar, A.
2013-12-01
We have studied the heterogeneously heated convective boundary layer (CBL) by means of large-eddy simulations (LES) and direct numerical simulations (DNS). What makes our study different from previous studies on this subject are our very long simulations in which the system travels through multiple states and that from there we have derived scaling laws. In our setup, a stratified atmosphere is heated from below by square patches with a high surface buoyancy flux, surrounded by regions with no or little flux. By letting a boundary layer grow in time we let the system evolve from the so-called meso-scale to the micro-scale regime. In the former the heterogeneity is large and strong circulations can develop, while in the latter the heterogeneity is small and does no longer influence the boundary layer structure. Within each simulation we can now observe the formation of a peak in kinetic energy, which represents the 'optimal' heterogeneity size in the meso-scale, and the subsequent decay of the peak and the development towards the transition to the micro-scale. We have created a non-dimensional parameter space that describes all properties of this system. By studying the previously described evolution for different combinations of parameters, we have derived three important conclusions. First, there exists a horizontal length scale of the heterogeneity (L) that is a function of the boundary layer height (h) and the Richardson (Ri) number of the inversion at the top of the boundary layer. This relationship has the form L = h Ri^(3/8). Second, this horizontal length scale L allows for expressing the time evolution, and thus the state of the system, as a ratio of this length scale and the distance between two patches Xp. This ratio thus describes to which extent the circulation fills up the space that exists between two patch centers. The timings of the transition from the meso- to the micro-scale collapse under this scaling for all simulations sharing the same flux
Numerical Simulations of Hypersonic Boundary Layer Transition
Bartkowicz, Matthew David
Numerical schemes for supersonic flows tend to use large amounts of artificial viscosity for stability. This tends to damp out the small scale structures in the flow. Recently some low-dissipation methods have been proposed which selectively eliminate the artificial viscosity in regions which do not require it. This work builds upon the low-dissipation method of Subbareddy and Candler which uses the flux vector splitting method of Steger and Warming but identifies the dissipation portion to eliminate it. Computing accurate fluxes typically relies on large grid stencils or coupled linear systems that become computationally expensive to solve. Unstructured grids allow for CFD solutions to be obtained on complex geometries, unfortunately, it then becomes difficult to create a large stencil or the coupled linear system. Accurate solutions require grids that quickly become too large to be feasible. In this thesis a method is proposed to obtain more accurate solutions using relatively local data, making it suitable for unstructured grids composed of hexahedral elements. Fluxes are reconstructed using local gradients to extend the range of data used. The method is then validated on several test problems. Simulations of boundary layer transition are then performed. An elliptic cone at Mach 8 is simulated based on an experiment at the Princeton Gasdynamics Laboratory. A simulated acoustic noise boundary condition is imposed to model the noisy conditions of the wind tunnel and the transitioning boundary layer observed. A computation of an isolated roughness element is done based on an experiment in Purdue's Mach 6 quiet wind tunnel. The mechanism for transition is identified as an instability in the upstream separation region and a comparison is made to experimental data. In the CFD a fully turbulent boundary layer is observed downstream.
Simulation and optimal control of wind-farm boundary layers
Meyers, Johan; Goit, Jay
2014-05-01
In large wind farms, the effect of turbine wakes, and their interaction leads to a reduction in farm efficiency, with power generated by turbines in a farm being lower than that of a lone-standing turbine by up to 50%. In very large wind farms or `deep arrays', this efficiency loss is related to interaction of the wind farms with the planetary boundary layer, leading to lower wind speeds at turbine level. Moreover, for these cases it has been demonstrated both in simulations and wind-tunnel experiments that the wind-farm energy extraction is dominated by the vertical turbulent transport of kinetic energy from higher regions in the boundary layer towards the turbine level. In the current study, we investigate the use of optimal control techniques combined with Large-Eddy Simulations (LES) of wind-farm boundary layer interaction for the increase of total energy extraction in very large `infinite' wind farms. We consider the individual wind turbines as flow actuators, whose energy extraction can be dynamically regulated in time so as to optimally influence the turbulent flow field, maximizing the wind farm power. For the simulation of wind-farm boundary layers we use large-eddy simulations in combination with actuator-disk and actuator-line representations of wind turbines. Simulations are performed in our in-house pseudo-spectral code SP-Wind that combines Fourier-spectral discretization in horizontal directions with a fourth-order finite-volume approach in the vertical direction. For the optimal control study, we consider the dynamic control of turbine-thrust coefficients in an actuator-disk model. They represent the effect of turbine blades that can actively pitch in time, changing the lift- and drag coefficients of the turbine blades. Optimal model-predictive control (or optimal receding horizon control) is used, where the model simply consists of the full LES equations, and the time horizon is approximately 280 seconds. The optimization is performed using a
Diffusive boundary layers over varying topography
Dell, R. W.; Pratt, L. J.
2015-01-01
Diffusive bottom boundary layers can produce upslope flows in a stratified fluid. Accumulating observations suggest that these boundary layers may drive upwelling and mixing in mid-ocean ridge flank canyons. However, most studies of diffusive bottom
Acoustic explorations of the upper ocean boundary layer
Vagle, Svein
2005-04-01
The upper ocean boundary layer is an important but difficult to probe part of the ocean. A better understanding of small scale processes at the air-sea interface, including the vertical transfer of gases, heat, mass and momentum, are crucial to improving our understanding of the coupling between atmosphere and ocean. Also, this part of the ocean contains a significant part of the total biomass at all trophic levels and is therefore of great interest to researchers in a range of different fields. Innovative measurement plays a critical role in developing our understanding of the processes involved in the boundary layer, and the availability of low-cost, compact, digital signal processors and sonar technology in self-contained and cabled configurations has led to a number of exciting developments. This talk summarizes some recent explorations of this dynamic boundary layer using both active and passive acoustics. The resonant behavior of upper ocean bubbles combined with single and multi-frequency broad band active and passive devices are now giving us invaluable information on air-sea gas transfer, estimation of biological production, marine mammal behavior, wind speed and precipitation, surface and internal waves, turbulence, and acoustic communication in the surf zone.
Microstructure of Turbulence in the Stably Stratified Boundary Layer
Sorbjan, Zbigniew; Balsley, Ben B.
2008-11-01
The microstructure of a stably stratified boundary layer, with a significant low-level nocturnal jet, is investigated based on observations from the CASES-99 campaign in Kansas, U.S.A. The reported, high-resolution vertical profiles of the temperature, wind speed, wind direction, pressure, and the turbulent dissipation rate, were collected under nocturnal conditions on October 14, 1999, using the CIRES Tethered Lifting System. Two methods for evaluating instantaneous (1-sec) background profiles are applied to the raw data. The background potential temperature is calculated using the “bubble sort” algorithm to produce a monotonically increasing potential temperature with increasing height. Other scalar quantities are smoothed using a running vertical average. The behaviour of background flow, buoyant overturns, turbulent fluctuations, and their respective histograms are presented. Ratios of the considered length scales and the Ozmidov scale are nearly constant with height, a fact that can be applied in practice for estimating instantaneous profiles of the dissipation rate.
Boundary layers and scaling relations in natural thermal convection
Shishkina, Olga; Lohse, Detlef; Grossmann, Siegfried
2017-11-01
We analyse the boundary layer (BL) equations in natural thermal convection, which includes vertical convection (VC), where the fluid is confined between two differently heated vertical walls, horizontal convection (HC), where the fluid is heated at one part of the bottom plate and cooled at some other part, and Rayleigh-Benard convection (RBC). For BL dominated regimes we derive the scaling relations of the Nusselt and Reynolds numbers (Nu, Re) with the Rayleigh and Prandtl numbers (Ra, Pr). For VC the scaling relations are obtained directly from the BL equations, while for HC they are derived by applying the Grossmann-Lohse theory to the case of VC. In particular, for RBC with large Pr we derive Nu Pr0Ra1/3 and Re Pr-1Ra2/3. The work is supported by the Deutsche Forschungsgemeinschaft (DFG) under the Grant Sh 405/4 - Heisenberg fellowship.
Boundary layer height estimation by sodar and sonic anemometer measurements
International Nuclear Information System (INIS)
Contini, D; Cava, D; Martano, P; Donateo, A; Grasso, F M
2008-01-01
In this paper an analysis of different methods for the calculation of the boundary layer height (BLH) using sodar and ultrasonic anemometer measurements is presented. All the methods used are based on single point surface measurements. In particular the automatic spectral routine developed for Remtech sodar is compared with the results obtained with the parameterization of the vertical velocity variance, with the calculation of a prognostic model and with a parameterization based on horizontal velocity spectra. Results indicate that in unstable conditions the different methods provide similar pattern, with BLH relatively low, even if the parameterization of the vertical velocity variance is affected by a large scatter that limits its efficiency in evaluating the BLH. In stable nocturnal conditions the performances of the Remtech routine are lower with respect to the ones in unstable conditions. The spectral method, applied to sodar or sonic anemometer data, seems to be the most promising in order to develop an efficient routine for BLH determination
Thorpe method applied to planetary boundary layer data
International Nuclear Information System (INIS)
Gonzalez-Nieto, P.L.; Cano, J.L.; Tijera, M.; Cano, D.
2008-01-01
Turbulence affects the dynamics of atmospheric processes by enhancing the transport of mass, heat, humidity and pollutants. The global objective for our work is to analyze some direct turbulent descriptors which reflect the mixing processes in the atmospheric boundary layer (ABL). In this paper we present results related to the Thorpe displacements d Τ , the maximum Thorpe displacement (d Τ ) max and the Thorpe scale L Τ , the Ozmidov scale and their time evolution in the ABL during a day cycle. A tethered balloon was used to obtain vertical profiles of the atmospheric physical magnitudes up to 1000 m. We discuss the vertical and horizontal variability and how different descriptors are related to atmospheric mixing.
The inland boundary layer at low latitudes
Garratt, J. R.
1985-08-01
Observations from the Koorin boundary-layer experiment in Australia (latitude 16 °S) were analysed in a study of the nocturnal jet development. For geostrophic winds in the range 10 20 m s-1, ageostrophic wind magnitudes of 5 10m s-1 were common above the surface layer near sunset, with cross-isobar flow angles of about 40 °. The jet that then developed by midnight was probably the result of these large ageostrophic winds, strong surface cooling and favourable baroclinity and sloping terrain. The analysis is supported by numerical model calculations with special emphasis on the role of long-wave radiative cooling on turbulent decay. Decay is rapid in the presence of radiation, although there is little influence on stress divergence levels. Evidence of sea-breeze influences on the jet evolution, and on features of deeply penetrating sea breezes in general, will be presented and discussed in part 2 of this study (submitted to Boundary-Layer Meteorol.).
A Formula for the Depth of the Stable Boundary layer: Evaluation and Dimensional Analysis
Steeneveld, G.J.; Wiel, van de B.J.H.; Holtslag, A.A.M.
2006-01-01
The height (h) of the stable boundary layer (SBL) is of major importance to understand the relevant processes that govern the SBL development. The SBL depth is the layer in which turbulence transport takes place, and thus governs the vertical structure of the lower atmosphere. Furthermore, release
A Coordinate Transformation for Unsteady Boundary Layer Equations
Directory of Open Access Journals (Sweden)
Paul G. A. CIZMAS
2011-12-01
Full Text Available This paper presents a new coordinate transformation for unsteady, incompressible boundary layer equations that applies to both laminar and turbulent flows. A generalization of this coordinate transformation is also proposed. The unsteady boundary layer equations are subsequently derived. In addition, the boundary layer equations are derived using a time linearization approach and assuming harmonically varying small disturbances.
Boundary layer flow past a circular cylinder in axial flow
International Nuclear Information System (INIS)
Sawchuk, S.P.; Zamir, M.; Camiletti, S.E.
1985-01-01
This paper discusses a study of the laminar boundary layer on a semi-infinite circular cylinder in axial incompressible flow. Unlike previous studies, the present study investigates a full range of this boundary layer problem to determine skin friction, heat transfer and other integral properties of the boundary layer
The internal boundary layer — A review
Garratt, J. R.
1990-03-01
A review is given of relevant work on the internal boundary layer (IBL) associated with: (i) Small-scale flow in neutral conditions across an abrupt change in surface roughness, (ii) Small-scale flow in non-neutral conditions across an abrupt change in surface roughness, temperature or heat/moisture flux, (iii) Mesoscale flow, with emphasis on flow across the coastline for both convective and stably stratified conditions. The major theme in all cases is on the downstream, modified profile form (wind and temperature), and on the growth relations for IBL depth.
Boundary Layer Depth In Coastal Regions
Porson, A.; Schayes, G.
The results of earlier studies performed about sea breezes simulations have shown that this is a relevant feature of the Planetary Boundary Layer that still requires effort to be diagnosed properly by atmospheric models. Based on the observations made during the ESCOMPTE campaign, over the Mediterranean Sea, different CBL and SBL height estimation processes have been tested with a meso-scale model, TVM. The aim was to compare the critical points of the BL height determination computed using turbulent kinetic energy profile with some other standard evaluations. Moreover, these results have been analysed with different mixing length formulation. The sensitivity of formulation is also analysed with a simple coastal configuration.
Vortex Generators to Control Boundary Layer Interactions
Babinsky, Holger (Inventor); Loth, Eric (Inventor); Lee, Sang (Inventor)
2014-01-01
Devices for generating streamwise vorticity in a boundary includes various forms of vortex generators. One form of a split-ramp vortex generator includes a first ramp element and a second ramp element with front ends and back ends, ramp surfaces extending between the front ends and the back ends, and vertical surfaces extending between the front ends and the back ends adjacent the ramp surfaces. A flow channel is between the first ramp element and the second ramp element. The back ends of the ramp elements have a height greater than a height of the front ends, and the front ends of the ramp elements have a width greater than a width of the back ends.
Panahifar, Hossein; Khalesifard, Hamid
2018-04-01
The vertical structure of the atmospheric boundary layer (ABL) has been studied by use of a depolarized LiDAR over Tehran, Iran. The boundary layer height (BLH) remains under 1km, and its retrieval from LiDAR have been compared with sonding measurements and meteorological model outputs. It is also shown that the wind speed and direction as well as topography lead to the persistence of air pollution in Tehran. The situation aggravate in fall and winter due to temperature inversion.
Optimal Growth in Hypersonic Boundary Layers
Paredes, Pedro; Choudhari, Meelan M.; Li, Fei; Chang, Chau-Lyan
2016-01-01
The linear form of the parabolized linear stability equations is used in a variational approach to extend the previous body of results for the optimal, nonmodal disturbance growth in boundary-layer flows. This paper investigates the optimal growth characteristics in the hypersonic Mach number regime without any high-enthalpy effects. The influence of wall cooling is studied, with particular emphasis on the role of the initial disturbance location and the value of the spanwise wave number that leads to the maximum energy growth up to a specified location. Unlike previous predictions that used a basic state obtained from a self-similar solution to the boundary-layer equations, mean flow solutions based on the full Navier-Stokes equations are used in select cases to help account for the viscous- inviscid interaction near the leading edge of the plate and for the weak shock wave emanating from that region. Using the full Navier-Stokes mean flow is shown to result in further reduction with Mach number in the magnitude of optimal growth relative to the predictions based on the self-similar approximation to the base flow.
Flow Visualization in Supersonic Turbulent Boundary Layers.
Smith, Michael Wayne
This thesis is a collection of novel flow visualizations of two different flat-plate, zero pressure gradient, supersonic, turbulent boundary layers (M = 2.8, Re _theta ~ 82,000, and M = 2.5, Re_ theta ~ 25,000, respectively). The physics of supersonic shear flows has recently drawn increasing attention with the renewed interest in flight at super and hypersonic speeds. This work was driven by the belief that the study of organized, Reynolds -stress producing turbulence structures will lead to improved techniques for the modelling and control of high-speed boundary layers. Although flow-visualization is often thought of as a tool for providing qualitative information about complex flow fields, in this thesis an emphasis is placed on deriving quantitative results from image data whenever possible. Three visualization techniques were applied--'selective cut-off' schlieren, droplet seeding, and Rayleigh scattering. Two experiments employed 'selective cut-off' schlieren. In the first, high-speed movies (40,000 fps) were made of strong density gradient fronts leaning downstream at between 30^circ and 60^ circ and travelling at about 0.9U _infty. In the second experiment, the same fronts were detected with hot-wires and imaged in real time, thus allowing the examination of the density gradient fronts and their associated single-point mass -flux signals. Two experiments employed droplet seeding. In both experiments, the boundary layer was seeded by injecting a stream of acetone through a single point in the wall. The acetone is atomized by the high shear at the wall into a 'fog' of tiny (~3.5mu m) droplets. In the first droplet experiment, the fog was illuminated with copper-vapor laser sheets of various orientations. The copper vapor laser pulses 'froze' the fog motion, revealing a variety of organized turbulence structures, some with characteristic downstream inclinations, others with large-scale roll-up on the scale of delta. In the second droplet experiment, high
Bypass transition in compressible boundary layers
Vandervegt, J. J.
1992-01-01
Transition to turbulence in aerospace applications usually occurs in a strongly disturbed environment. For instance, the effects of free-stream turbulence, roughness and obstacles in the boundary layer strongly influence transition. Proper understanding of the mechanisms leading to transition is crucial in the design of aircraft wings and gas turbine blades, because lift, drag and heat transfer strongly depend on the state of the boundary layer, laminar or turbulent. Unfortunately, most of the transition research, both theoretical and experimental, has focused on natural transition. Many practical flows, however, defy any theoretical analysis and are extremely difficult to measure. Morkovin introduced in his review paper the concept of bypass transition as those forms of transition which bypass the known mechanisms of linear and non-linear transition theories and are currently not understood by experiments. In an effort to better understand the mechanisms leading to transition in a disturbed environment, experiments are conducted studying simpler cases, viz. the effects of free stream turbulence on transition on a flat plate. It turns out that these experiments are very difficult to conduct, because generation of free stream turbulence with sufficiently high fluctuation levels and reasonable homogeneity is non trivial. For a discussion see Morkovin. Serious problems also appear due to the fact that at high Reynolds numbers the boundary layers are very thin, especially in the nose region of the plate where the transition occurs, which makes the use of very small probes necessary. The effects of free-stream turbulence on transition are the subject of this research and are especially important in a gas turbine environment, where turbulence intensities are measured between 5 and 20 percent, Wang et al. Due to the fact that the Reynolds number for turbine blades is considerably lower than for aircraft wings, generally a larger portion of the blade will be in a laminar
On the nature of the plasma sheet boundary layer
Energy Technology Data Exchange (ETDEWEB)
Hones, E.W. Jr. (Mission Research Corp., Los Alamos, NM (USA) Los Alamos National Lab., NM (USA))
1990-01-01
The regions of the plasma sheet adjacent to the north and south lobes of the magnetotail have been described by many experimenters as locations of beams of energetic ions and fast-moving plasma directed primarily earthward and tailward along magnetic field lines. Measurements taken as satellites passed through one or the other of these boundary layers have frequently revealed near-earth mirroring of ions and a vertical segregation of velocities of both earthward-moving and mirroring ions with the fastest ions being found nearest the lobe-plasma sheet interface. These are features expected for particles from a distant tail source {bar E} {times} {bar B} drifting in a dawn-to-dusk electric field and are consistent with the source being a magnetic reconnection region. The plasma sheet boundary layers are thus understood as separatrix layers, bounded at their lobeward surfaces by the separatrices from the distant neutral line. This paper will review the observations that support this interpretation. 10 refs., 7 figs.
3D LDV Measurements in Oscillatory Boundary Layers
Mier, J. M.; Garcia, M. H.
2012-12-01
focal length remained constant and coincidence in the measurement volume for all 3 components was maintained when traversing the probes along the measurement profiles. Results show the existence of high turbulence levels inside the boundary layer up to about 30 mm away from the bottom. The streamwise component u shows greater intensities closer to the bottom and ahead of the freestream velocity maximum. On the contrary, the vertical component w shows smaller values of turbulent intensity, located higher up in the profile and lagging with respect to the freestream velocity maximum. Meanwhile, the spanwise component v shows similar intensities than w, happening in phase with it, but distributed all along the boundary layer, overlapping the areas of greater intensity of u and w. In addition, wall shear stress and other turbulent magnitudes related to the boundary layer were analyzed from the experimental results obtained through this research.
Ouwersloot, H.G.; Vilà-Guerau de Arellano, J.; Nölscher, A.C.; Krol, M.C.; Ganzeveld, L.N.; Breitenberger, C.; Mammarella, I.; Williams, J.; Lelieveld, J.
2012-01-01
We studied the atmospheric boundary layer (ABL) dynamics and the impact on atmospheric chemistry during the HUMPPA-COPEC-2010 campaign. We used vertical profiles of potential temperature and specific moisture, obtained from 132 radio soundings, to determine the main boundary layer characteristics
Role of residual layer and large-scale phenomena on the evolution of the boundary layer
Blay, E.; Pino, D.; Vilà-Guerau de Arellano, J.; Boer, van de A.; Coster, de O.; Faloona, I.; Garrouste, O.; Hartogensis, O.K.
2012-01-01
Mixed-layer theory and large-eddy simulations are used to analyze the dynamics of the boundary layer on two intensive operational periods during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign: 1st and 2nd of July 2011, when convective boundary layers (CBLs) were observed.
Soot and radiation in combusting boundary layers
Energy Technology Data Exchange (ETDEWEB)
Beier, R.A.
1981-12-01
In most fires thermal radiation is the dominant mode of heat transfer. Carbon particles within the fire are responsible for most of this emitted radiation and hence warrant quantification. As a first step toward understanding thermal radiation in full scale fires, an experimental and theoretical study is presented for a laminar combusting boundary layer. Carbon particulate volume fraction profiles and approximate particle size distributions are experimentally determined in both free and forced flow for several hydrocarbon fuels and PMMA (polymethylmethacrylate). A multiwavelength laser transmission technique determines a most probable radius and a total particle concentration which are two unknown parameters in an assumed Gauss size distribution. A sooting region is observed on the fuel rich side of the main reaction zone. For free flow, all the flames are in air, but the free stream ambient oxygen mass fraction is a variable in forced flow. To study the effects of radiation heat transfer, a model is developed for a laminar combusting boundary layer over a pyrolyzing fuel surface. An optically thin approximation simplifies the calculation of the radiant energy flux at the fuel surface. For the free flames in air, the liquid fuel soot volume fractions, f/sub v/, range from f/sub v/ approx. 10/sup -7/ for n-heptane, a paraffin, to f/sub v/ approx. 10/sup -7/ for toluene, an aromatic. The PMMA soot volume fractions, f/sub v/ approx. 5 x 10/sup -7/, are approximately the same as the values previously reported for pool fires. Soot volume fraction increases monotonically with ambient oxygen mass fraction in the forced flow flames. For all fuels tested, a most probable radius between 20 nm and 80 nm is obtained which varies only slightly with oxygen mass fraction, streamwise position, or distance normal to the fuel surface. The theoretical analysis yields nine dimensionless parameters, which control the mass flux rate at the pyrolyzing fuel surface.
Properties of the TEXTOR boundary layer
International Nuclear Information System (INIS)
Bogen, P.; Hartwig, H.; Hintz, E.; Hoethker, K.; Lie, Y.T.; Pospieszczyk, A.; Samm, U.
1984-01-01
First measurements on the TEXTOR boundary layer are reported. The hydrogen recycling in front of the four limiter segments has been studied by means of a CCD-camera, which proved to be a good instrument to center the discharge for symmetric plasma-limiter contact. The composition of the neutral fluxes from the limiter have been measured: oxygen fluxes are about a factor of ten higher than the metal fluxes; within the error limits the composition does not change with varying limiter radius. Electron densities in the scrape-off layer away from the limiter have been determined by injecting an Li-atom beam from a thermal source and by observing its emission as a function of radius. Similar measurements have been made in front of the limiter with sputtered Cr and O atoms. Both methods gave for the magnetic surface of the limiter radius nsub(e) approx.= 1 x 10 12 /cm 23 . Infrared observations of a test limiter with a CCD-camera and a PbSe-detector have been performed to record the thermal loads. About 10% of the input power flows to the limiter. (orig.)
Transition to turbulence in the Hartmann boundary layer
Energy Technology Data Exchange (ETDEWEB)
Thess, A.; Krasnov, D.; Boeck, T.; Zienicke, E. [Dept. of Mechanical Engineering, Ilmenau Univ. of Tech. (Germany); Zikanov, O. [Dept. of Mechanical Engineering, Univ. of Michigan, Dearborn, MI (United States); Moresco, P. [School of Physics and Astronomy, The Univ. of Manchester (United Kingdom); Alboussiere, T. [Lab. de Geophysique Interne et Tectonophysique, Observatoire des Science de l' Univers de Grenoble, Univ. Joseph Fourier, Grenoble (France)
2007-07-01
The Hartmann boundary layer is a paradigm of magnetohydrodynamic (MHD) flows. Hartmann boundary layers develop when a liquid metal flows under the influence of a steady magnetic field. The present paper is an overview of recent successful attempts to understand the mechanisms by which the Hartmann layer undergoes a transition from laminar to turbulent flow. (orig.)
Analytical solution for the convectively-mixed atmospheric boundary layer
Ouwersloot, H.G.; Vilà-Guerau de Arellano, J.
2013-01-01
Based on the prognostic equations of mixed-layer theory assuming a zeroth order jump at the entrainment zone, analytical solutions for the boundary-layer height evolution are derived with different degrees of accuracy. First, an exact implicit expression for the boundary-layer height for a situation
Airfoil boundary layer separation and control at low Reynolds numbers
Energy Technology Data Exchange (ETDEWEB)
Yarusevych, S.; Sullivan, P.E. [University of Toronto, Department of Mechanical and Industrial Engineering, Toronto, ON (Canada); Kawall, J.G. [Ryerson University, Department of Mechanical and Industrial Engineering, Toronto, ON (Canada)
2005-04-01
The boundary layer separation on a NACA 0025 airfoil was studied experimentally via hot-wire anemometry and surface pressure measurements. The results provide added insight into periodic boundary layer control, suggesting that matching the excitation frequency with the most amplified disturbance in the separated shear layer is optimal for improving airfoil performance. (orig.)
Planetary Boundary Layer Dynamics over Reno, Nevada in Summer
Liming, A.; Sumlin, B.; Loria Salazar, S. M.; Holmes, H.; Arnott, W. P.
2014-12-01
Quantifying the height of the planetary boundary layer (PBL) is important to understand the transport behavior, mixing, and surface concentrations of air pollutants. In Reno, NV, located in complex, mountainous terrain with high desert climate, the daytime boundary layer can rise to an estimated 3km or more on a summer day due to surface heating and convection. The nocturnal boundary layer, conversely, tends to be much lower and highly stable due to radiative cooling from the surface at night and downslope flow of cool air from nearby mountains. With limited availability of radiosonde data, current estimates of the PBL height at any given time or location are potentially over or underestimated. To better quantify the height and characterize the PBL physics, we developed portable, lightweight sensors that measure CO2 concentrations, temperature, pressure, and humidity every 5 seconds. Four of these sensors are used on a tethered balloon system to monitor CO2 concentrations from the surface up to 300m. We will combine this data with Radio Acoustic Sounding System (RASS) data that measures vertical profiles of wind speed, temperature, and humidity from 40m to 400m. This experiment will characterize the diurnal evolution of CO2 concentrations at multiple heights in the PBL, provide insight into PBL physics during stability transition periods at sunrise and sunset, and estimate the nighttime PBL depth during August in Reno. Further, we expect to gain a better understanding of the impact of mixing volume changes (i.e., PBL height) on air quality and pollution concentrations in Reno. The custom portable sensor design will also be presented. It is expected that these instruments can be used for indoor or outdoor air quality studies, where lightness, small size, and battery operation can be of benefit.
Stability of boundary layer flow based on energy gradient theory
Dou, Hua-Shu; Xu, Wenqian; Khoo, Boo Cheong
2018-05-01
The flow of the laminar boundary layer on a flat plate is studied with the simulation of Navier-Stokes equations. The mechanisms of flow instability at external edge of the boundary layer and near the wall are analyzed using the energy gradient theory. The simulation results show that there is an overshoot on the velocity profile at the external edge of the boundary layer. At this overshoot, the energy gradient function is very large which results in instability according to the energy gradient theory. It is found that the transverse gradient of the total mechanical energy is responsible for the instability at the external edge of the boundary layer, which induces the entrainment of external flow into the boundary layer. Within the boundary layer, there is a maximum of the energy gradient function near the wall, which leads to intensive flow instability near the wall and contributes to the generation of turbulence.
Boundary layer structure over areas of heterogeneous heat fluxes
International Nuclear Information System (INIS)
Doran, J.C.; Barnes, F.J.; Coulter, R.L.; Crawford, T.L.
1993-01-01
In general circulation models (GCMs), some properties of a grid element are necessarily considered homogeneous. That is, for each grid volume there is associated a particular combination of boundary layer depth, vertical profiles of wind and temperature, surface fluxes of sensible and latent heat, etc. In reality, all of these quantities may exhibit significant spatial variations the grid area, and the larger the area the greater the likely variations. In balancing the benefits of higher resolution against increased computational time and expense, it is useful to consider what the consequences of such subgrid-scale variability may be. Moreover, in interpreting the results of a simulation, one must be able to define an appropriate average value over a grid. There are two aspects of this latter problem: (1) in observations, how does one take a set of discrete or volume-averaged measurements and relate these to properties of the entire domain, and (2) in computations, how can subgrid-scale features be accounted for in the model parameterizations? To address these and related issues, two field campaigns were carried out near Boardman, Oregon, in June 1991 and 1992. These campaigns were designed to measure the surface fluxes of latent and sensible heat over adjacent areas with strongly contrasting surface types and to measure the response of the boundary layer to those fluxes. This paper discusses some initial findings from those campaigns
Prediction of boundary-layer transition caused by crossflow disturbances
Nomura, Toshiyuki; 野村 聡幸
1999-01-01
A prediction system for boundary layer transition is developed which consists of the Navier-Stokes code computing a compressible boundary layer, the linear PSE (Parabolized Stability Equations) code computing the spatial growth of a disturbance, and the N-factor code integrating the growth rate. The system is applied to the case that the transition of the compressible boundary layer on a swept cylinder is caused by cross flow disturbances which have the same spanwise wavelength as observed in...
INCOMPRESSIBLE LAMINAR BOUNDARY LAYER CONTROL BY BLOWING AND SUCTION
AZZEDINE NAHOUI; LAKHDAR BAHI
2013-01-01
A two-dimensional incompressible laminar boundary layer and its control using blowing and suction over a flat plate and around the NACA 0012 and 661012 profiles, is studied numerically. The study is based on the Prandtl boundary layer model using the finite differences method and the Crank-Nicolson scheme. The velocity distribution, the boundary layer thickness and the friction coefficient, are determined and presented with and without control. The application of the control technique, has de...
Park, G. I.; Wallace, J.; Wu, X.; Moin, P.
2010-11-01
Using a recent DNS of a flat-plate boundary layer, statistics of turbulence in transition at Reθ= 500 where spots merge (distributions of the mean velocity, rms velocity and vorticity fluctuations, Reynolds shear stress, kinetic energy production and dissipation rates and enstrophy) have been compared to these statistics for the developed boundary layer turbulence at Reθ= 1850. When the distributions in the transitional region, determined in narrow planes 0.03 Reθ wide, exclude regions and times when the flow is not turbulent, they closely resemble those in the developed turbulent state at the higher Reynolds number, especially in the buffer and sublayers. The skin friction coefficient, determined in this conditional manner in the transitional flow is, of course, much larger than that obtained by including both turbulent and non-turbulent information there, and is consistent with a value obtained by extrapolating from the developed turbulent region. We are attempting to perform this data analysis even further upstream in the transitioning flow at Reθ= 300 where the turbulent spots are individuated. These results add further evidence to support the view that the structure of a developed turbulent boundary layer is little different from its structure in its embryonic form in turbulent spots. *CTR 2010 Summer Program research.
Numerical Investigation of a Heated, Sheared Planetary Boundary Layer
Liou, Yu-Chieng
1996-01-01
A planetary boundary layer (PBL) developed on 11 July, 1987 during the First International Satellites Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) is investigated numerically by a two dimensional and a three dimensional large eddy simulation (LES) model. Most of the simulated mean and statistical properties are utilized to compare or verify against the observational results extracted from single Doppler lidar scans conducted by Gal-Chen et al. (1992) on the same day. Through the methods of field measurements and numerical simulations, it is found that this PBL, in contrast to the well-known convective boundary layer (CBL), is driven by not only buoyancy but also wind shear. Large eddies produced by the surface heating, as well as internal gravity waves excited by the convection, are both present in the boundary layer. The most unique feature is that in the stable layer, the momentum flux ({overlinerm u^' w^'}), transported by the gravity waves, is counter-gradient. The occurrence of this phenomenon is interpreted by Gal-Chen et al. (1992) using the theory of critical layer singularity, and is confirmed by the numerical simulations in this study. Qualitative agreements are achieved between the model-generated and lidar-derived results. However, quantitative comparisons are less satisfactory. The most serious discrepancy is that in the stable layer the magnitudes of the observed momentum flux ({overlinerm u^ ' w^'}) and vertical velocity variance ({overlinerm w^'^2}) are much larger than their simulated counterparts. Nevertheless, through the technique of numerical simulation, evidence is collected to show inconsistencies among the observations. Thus, the lidar measurements of {overline rm u^' w^'} and {overlinerm w^ '^2} seem to be doubtful. A Four Dimensional Data Assimilation (FDDA) experiment is performed in order to connect the evolution of the model integration with the observations. The results indicate that the dynamical relaxation
Pre-LBA Rondonia Boundary Layer Experiment (RBLE) Data
National Aeronautics and Space Administration — The atmospheric boundary layer (ABL) is the layer of air closest to the ground which is directly influenced on a daily basis by the heating and cooling of the...
Pre-LBA Rondonia Boundary Layer Experiment (RBLE) Data
National Aeronautics and Space Administration — ABSTRACT: The atmospheric boundary layer (ABL) is the layer of air closest to the ground which is directly influenced on a daily basis by the heating and cooling of...
Stable boundary-layer regimes at dome C, Antarctica : observation and analysis
Vignon, E.; van de Wiel, B.J.H.; van Hooijdonk, I.G.S.; Genthon, C.; van der Linden, S.J.A.; van Hooft, J.A.; Baas, P.; Maurel, W.; Traullé, O.; Casasanta, G.
2017-01-01
Investigation of meteorological measurements along a 45 m tower at Dome C on the high East Antarctic Plateau revealed two distinct stable boundary layer (SBL) regimes at this location. The first regime is characterized by strong winds and continuous turbulence. It results in full vertical coupling
Local Similarity in the Stable Boundary Layer and Mixing-Length Approaches : Consistency of Concepts
Van de Wiel, B.J.H.; Moene, A.F.; De Ronde, W.H.; Jonker, H.J.J.
2008-01-01
In stably stratified flows vertical movement of eddies is limited by the fact that kinetic energy is converted into potential energy, leading to a buoyancy displacement scale z B . Our new mixing-length concept for turbulent transport in the stable boundary layer follows a rigid-wall analogy, in the
Local similarity in the stable boundary layer and mixing-length approaches: consistency of concepts
Wiel, van de B.J.H.; Moene, A.F.; Ronde, W.H.; Jonker, H.J.J.
2008-01-01
In stably stratified flows vertical movement of eddies is limited by the fact that kinetic energy is converted into potential energy, leading to a buoyancy displacement scale z B . Our new mixing-length concept for turbulent transport in the stable boundary layer follows a rigid-wall analogy, in the
Local similarity in the stable boundary layer and mixing-length approaches : consistency of concepts
Wiel, van de B.J.H.; Moene, A.F.; Ronde, de W.H.; Jonker, H.J.J.
2008-01-01
In stably stratified flows vertical movement of eddies is limited by the fact that kinetic energy is converted into potential energy, leading to a buoyancy displacement scale zB. Our new mixing-length concept for turbulent transport in the stable boundary layer follows a rigid-wall analogy, in the
Comparison of WRF local and nonlocal boundary layer Physics in Greater Kuala Lumpur, Malaysia
Ooi, M. C. G.; Chan, A.; Kumarenthiran, S.; Morris, K. I.; Oozeer, M. Y.; Islam, M. A.; Salleh, S. A.
2018-02-01
The urban boundary layer (UBL) is the internal advection layer of atmosphere above urban region which determines the exchanges of momentum, water and other atmospheric constituents between the urban land surface and the free troposphere. This paper tested the performance of three planetary boundary layer (PBL) physics schemes of Weather Research and Forecast (WRF) software to ensure the appropriate representation of vertical structure of UBL in Greater Kuala Lumpur (GKL). Comparison was conducted on the performance of respective PBL schemes to generate vertical and near-surface weather profile and rainfall. Mellor-Yamada- Janjíc (MYJ) local PBL scheme coupled with Eta MM5 surface layer scheme was found to predict the near-surface temperature and wind profile and mixing height better than the nonlocal schemes during the intermonsoonal period with least influences of the synoptic background weather.
Internal and external 2-d boundary layer flows
Crawford, M. E.; Kays, W. M.
1978-01-01
Computer program computes general two dimensional turbulent boundary-layer flow using finite-difference techniques. Structure allows for user modification to accommodate unique problems. Program should prove useful in many applications where accurate boundary-layer flow calculations are required.
High frequency ground temperature fluctuation in a Convective Boundary Layer
Garai, A.; Kleissl, J.; Lothon, M.; Lohou, F.; Pardyjak, E.; Saïd, F.; Cuxart, J.; Steeneveld, G.J.; Yaguë, C.; Derrien, S.; Alexander, D.; Villagrasa, D.M.
2012-01-01
To study influence of the turbulent structures in the convective boundary layer (CBL) on the ground temperature, during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) observational campaign, high frequency ground temperature was recorded through infra-red imagery from 13 June - 8
The turning of the wind in the atmospheric boundary layer
DEFF Research Database (Denmark)
Pena Diaz, Alfredo; Gryning, Sven-Erik; Floors, Rogier Ralph
2014-01-01
Here we use accurate observations of the wind speed vector to analyze the behavior with height of the wind direction. The observations are a combination of tall meteorological mast and long-range wind lidar measurements covering the entire atmospheric boundary layer. The observations were performed...... winds underpredict the turning of the wind and the boundary-layer winds in general....
Transonic shock wave. Boundary layer interaction at a convex wall
Koren, B.; Bannink, W.J.
1984-01-01
A standard finite element procedure has been applied to the problem of transonic shock wave – boundary layer interaction at a convex wall. The method is based on the analytical Bohning-Zierep model, where the boundary layer is perturbed by a weak normal shock wave which shows a singular pressure
On hairpin vortices in a transitional boundary layer
Directory of Open Access Journals (Sweden)
Uruba Václav
2012-04-01
Full Text Available In the presented paper the results of experiments on transitional boundary layer are presented. The boundary layer was generated on smooth flat wall with zero pressure gradient forming one side of the channel of rectangular cross section. The hairpin vortices, packets of hairpin vortices, turbulent spots and calmed regions were experimentally investigated using time-resolved PIV technique.
International Nuclear Information System (INIS)
Jun, Y. M.; Chung, J. T.
2000-01-01
The working fluid from the combustor to the turbine stage of a gas turbine makes various boundary layer thickness. Since the inlet boundary layer thickness is one of the important factors that affect the turbine efficiency, It is necessary to investigate secondary flow and loss with various boundary layer thickness conditions. In the present study, the effect of various inlet boundary layer thickness on secondary flow and loss and the proper height of the boundary layer fences for various boundary layer thickness were investigated. Measurements of secondary flow velocity and total pressure loss within and downstream of the passage were taken under 5 boundary layer thickness conditions, 16, 36, 52, 69, 110mm. It was found that total pressure loss and secondary flow areas were increased with increase of thickness but they were maintained almost at the same position. At the following research about the boundary layer fences, 1/6, 1/3, 1/2 of each inlet boundary layer thickness and 12mm were used as the fence heights. As a result, it was observed that the proper height of the fences was generally constant since the passage vortex remained almost at the same position. Therefore once the geometry of a cascade is decided, the location of the passage vortex and the proper fence height are appeared to be determined at the same time. When the inlet boundary layer thickness is relatively small, the loss caused by the proper fence becomes bigger than end wall loss so that it dominates secondary loss. In these cases the proper fence height is decided not by the cascade geometry but by the inlet boundary layer thickness as previous investigations
Boundary Layer Control of Rotating Convection Systems
King, E. M.; Stellmach, S.; Noir, J.; Hansen, U.; Aurnou, J. M.
2008-12-01
Rotating convection is ubiquitous in the natural universe, and is likely responsible for planetary processes such magnetic field generation. Rapidly rotating convection is typically organized by the Coriolis force into tall, thin, coherent convection columns which are aligned with the axis of rotation. This organizational effect of rotation is thought to be responsible for the strength and structure of magnetic fields generated by convecting planetary interiors. As thermal forcing is increased, the relative influence of rotation weakens, and fully three-dimensional convection can exist. It has long been assumed that rotational effects will dominate convection dynamics when the ratio of buoyancy to the Coriolis force, the convective Rossby number, Roc, is less than unity. We investigate the influence of rotation on turbulent Rayleigh-Benard convection via a suite of coupled laboratory and numerical experiments over a broad parameter range: Rayleigh number, 10310; Ekman number, 10-6≤ E ≤ ∞; and Prandtl number, 1≤ Pr ≤ 100. In particular, we measure heat transfer (as characterized by the Nusselt number, Nu) as a function of the Rayleigh number for several different Ekman and Prandtl numbers. Two distinct heat transfer scaling regimes are identified: non-rotating style heat transfer, Nu ~ Ra2/7, and quasigeostrophic style heat transfer, Nu~ Ra6/5. The transition between the non-rotating regime and the rotationally dominant regime is described as a function of the Ekman number, E. We show that the regime transition depends not on the global force balance Roc, but on the relative thicknesses of the thermal and Ekman boundary layers. The transition scaling provides a predictive criterion for the applicability of convection models to natural systems such as Earth's core.
Destiny of earthward streaming plasma in the plasmasheet boundary layer
Green, J. L.; Horwitz, J. L.
1986-01-01
The dynamics of the earth's magnetotail have been investigated, and it has become clear that the plasmasheet boundary layer field lines map into the Region I Field-Aligned Currents (FAC) of the auroral zone. It is pointed out that the role of earthward streaming ions in the plasmasheet boundary layer may be of fundamental importance in the understanding of magnetotail dynamics, auroral zone physics, and especially for ionospheric-magnetospheric interactions. The present paper has the objective to evaluate propagation characteristics for the earthward streaming ions observed in the plasmasheet boundary layer. An investigation is conducted of the propagation characteristics of protons in the plasmasheet boundary layer using independent single particle dynamics, and conclusions are discussed. The density of earthward streaming ions found in the plasmasheet boundary layer should include the ring current as well as the auroral zone precipitaiton and inner plasmasheet regions of the magnetosphere.
Pitot-probe displacement in a supersonic turbulent boundary layer
Allen, J. M.
1972-01-01
Eight circular pitot probes ranging in size from 2 to 70 percent of the boundary-layer thickness were tested to provide experimental probe displacement results in a two-dimensional turbulent boundary layer at a nominal free-stream Mach number of 2 and unit Reynolds number of 8 million per meter. The displacement obtained in the study was larger than that reported by previous investigators in either an incompressible turbulent boundary layer or a supersonic laminar boundary layer. The large probes indicated distorted Mach number profiles, probably due to separation. When the probes were small enough to cause no appreciable distortion, the displacement was constant over most of the boundary layer. The displacement in the near-wall region decreased to negative displacement in some cases. This near-wall region was found to extend to about one probe diameter from the test surface.
Convective instability in a time-dependent buoyancy driven boundary layer
Energy Technology Data Exchange (ETDEWEB)
Brooker, A.M.H.; Patterson, J.C.; Graham, T.; Schoepf, W. [University of Western Australia, Nedlands (Australia). Centre for Water Research
2000-01-01
The stability of the parallel time-dependent boundary layer adjacent to a suddenly heated vertical wall is described. The flow is investigated through experiments in water, through direct numerical simulation and also through linear stability analysis. The full numerical simulation of the flow shows that small perturbations to the wall boundary conditions, that are also present in the experimental study, are responsible for triggering the instability. As a result, oscillatory behaviour in the boundary layer is observed well before the transition to a steady two-dimensional flow begins. The properties of the observed oscillations are compared with those predicted by a linear stability analysis of the unsteady boundary layer using a quasi-stationary assumption and also using non-stationary assumptions by the formulation of parabolized equations (PSE). (Author)
Ozone and meteorological boundary-layer conditions at Summit, Greenland, during 3-21 June 2000
Energy Technology Data Exchange (ETDEWEB)
Helmig, D.; Boulter, J.; David, D.; Birks, J.W.; Cullen, N.J.; Steffen, K. [University of Colorado, Boulder, CO (United States). Cooperative Institute for Research in Environmental Sciences; Johnson, B.J.; Oltmans, S.J. [National Oceanic and Atmospheric Administration, Boulder, CO (United States). Climate Monitoring and Diagnostics Laboratory
2002-06-01
The temporal and spatial distributions of boundary-layer ozone were studied during June 2000 at Summit, Greenland, using surface-level measurements and vertical profiling from a tethered balloon platform. Three weeks of continuous ozone surface data, 133 meteorological vertical profile data and 82 ozone vertical profile data sets were collected from the surface to a maximum altitude of 1400 m above ground. The lower atmosphere at Summit was characterized by the prevalence of strong stable conditions with strong surface temperature inversions. These inversions reversed to neutral to slightly unstable conditions between {approx} 9.00 and 18.00 h local time with the formation of shallow mixing heights of {approx} 70-250 m above the surface. The surface ozone mixing ratio ranged from 39 to 68 ppbv and occasionally had rapid changes of up to 20 ppb in 12 h. The diurnal mean ozone mixing ratio showed diurnal trends indicating meteorological and photochemical controls of surface ozone. Vertical profiles were within the range of 37-76 ppb and showed strong stratification in the lower troposphere. A high correlation of high ozone/low water vapor air masses indicated the transport of high tropospheric/low stratospheric air into the lower boundary layer. An {approx} 0.1-3 ppb decline of the ozone mixing ratio towards the surface was frequently observed within the neutrally stable mixed layer during midday hours. These data suggest that the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by photochemical processes and/or transport phenomena that follow diurnal dependencies. With 37 ppb of ozone being the lowest mixing ratio measured in all data no evidence was seen for the occurrence of ozone depletion episodes similar to those that have been reported within the boundary layer at coastal Arctic sites during springtime
Diffusive boundary layers over varying topography
Dell, R. W.
2015-03-25
Diffusive bottom boundary layers can produce upslope flows in a stratified fluid. Accumulating observations suggest that these boundary layers may drive upwelling and mixing in mid-ocean ridge flank canyons. However, most studies of diffusive bottom boundary layers to date have concentrated on constant bottom slopes. We present a study of how diffusive boundary layers interact with various idealized topography, such as changes in bottom slope, slopes with corrugations and isolated sills. We use linear theory and numerical simulations in the regional ocean modeling system (ROMS) model to show changes in bottom slope can cause convergences and divergences within the boundary layer, in turn causing fluid exchanges that reach far into the overlying fluid and alter stratification far from the bottom. We also identify several different regimes of boundary-layer behaviour for topography with oceanographically relevant size and shape, including reversing flows and overflows, and we develop a simple theory that predicts the regime boundaries, including what topographies will generate overflows. As observations also suggest there may be overflows in deep canyons where the flow passes over isolated bumps and sills, this parameter range may be particularly significant for understanding the role of boundary layers in the deep ocean.
Structure of the low-latitude boundary layer
International Nuclear Information System (INIS)
Sckopke, N.; Paschmann, G.; Haerendel, G.; Sonnerup, B.U.O.; Bame, S.J.; Forbes, T.G.; Hones, E.W. Jr.; Russell, C.T.
1981-01-01
Observations at high temporal resolution of the frontside magnetopause and plasma boundary layer, made with the Los Alamos Scientific Laboratory/Max-Planck-Institut, Institut fuer Extraterrestrische Physik, fast plasma analyzer on board the Isee 1 and 2 spacecraft, have revealed a complex quasi-periodic structure of some of the observed boundary layers: cool tailward streaming boundary layer plasma is seen intermittently, with intervening periods of hot tenuous plasma which has properties similar to the magnetospheric population. While individual encounters with the boundary layer plasma last only a few minutes, the total observation time may extend over 1 hour or more. One such crossing, at 0800 hours local time and 40 0 northern GSM latitude, is examined in detail, including a quantitative comparison of the boundary layer entry and exit times of the two spacecraft. The data are found to be compatible with a boundary layer that is always attached to the magnetopause but where the layer thickness has a large-scale spatial modulation pattern which travels tailward past the spacecraft. Included are periods when the thickness is essentially zero and others when it is of the order of 1 R/sub E/. The duration of these periods is highly variable but is typically in the range of 2--5 min, corresponding to a distance along the magnetopause of the order of 3--8 R/sub E/. The observed boundary layer features include a steep density gradient at the magnetopause, with an approximately constant boundary layer plasma density amounting to about 25% of the magnetosheath density, and a second abrupt density decrease at the inner edge of the layer. It also appears that the purely magnetospheric plasma is ocassionally separated from the boundary layer by a halo region in which the plasma density is somewhat higher, and the temperature somewhat lower, than in the magnetosphere. A tentative model is proposed
Characteristics of the magnetospheric boundary layer and magnetopause layer as observed by Imp 6
International Nuclear Information System (INIS)
Eastman, T.E.; Hones, E.W. Jr.
1979-01-01
Imp 6 observations of the low-latitude magnetospheric boundary layer indicate that the plasma within it is supplied primarily by direct entry of magnetosheath plasma across the magnetopause layer. We define the magnetopause layer as the current layer (separating the magnetosheath from the boundary layer) through which the magnetic field shifts in direction. High temporal resolution (3-s average) data reveal that in a majority of Imp 6 magnetopause crossing, no distinct changes in electron density or energry spectra are observed at the magne opause layer. In all Imp 6 crossings, some magnetosheathlike plasma is observed earthward of the magnetopause layer, implying the existence of a boundary layer. Boundary layer electron energy spectra are often virtually indistinguishable from the adjacent magnetosheath spectra. Low-latitude boundary layer bulk plasma flow as observed by Imp 6 almost always has an antisunward component and often has a significant cross-field component. The boundary layer thickness is highly variable and is generally much larger than the magnetopause layer thickness. Energetic electron pitch angle distributions indicate that the low-latitude boundary layers is normally on closed field lines. We conclude that diffusive as well as nondiffusive processes probably contribute to the entry of magnetosheath plasma into the boundary layer
Microbubble drag reduction in liquid turbulent boundary layers
International Nuclear Information System (INIS)
Merkle, C.L.; Deutsch, S.
1992-01-01
The interactions between a dense cloud of small bubbles and a liquid turbulent boundary layer are reviewed on the basis of available experimental observations to understand and quantify their capability for reducing skin friction. Gas bubbles are generally introduced into the boundary layer by injection through a porous surface or by electrolysis. After injection, the bubbles stay near the wall in boundary-layer-like fashion giving rise to strong gradients in both velocity and gas concentration. In general, the magnitude of the skin friction reduction increases as the volume of bubbles in the boundary layer is increased until a maximum skin friction reduction of typically 80-90% of the undisturbed skin friction level is reached. The volumetric gas flow required for this maximum is nominally equal to the volume flow of the liquid in the boundary layer. Bubble size estimates indicate that in most microbubble experiments the bubbles have been intermediate in size between the inner and outer scales of the undisturbed boundary layer. Additional studies with other nondimensional bubble sizes would be useful. However, the bubble size is most likely controlled by the injection process, and considerably different conditions would be required to change this ratio appreciably. The trajectories of the bubble clouds are primarily determined by the random effects of turbulence and bubble-bubble interactions. The effects of buoyancy represent a weaker effect. The trajectories are unlike the deterministic trajectory of an individual bubble in a time-averaged boundary layer. Bubbles are most effective in high speed boundary layers and, for the bubble sizes tested to date, produce an effect that persists for some on hundred boundary layer thicknesses. Modeling suggests that microbubbles reduce skin friction by increasing the turbulence Reynolds number in the buffer layer in a manner similar to polymers
Belyaev, Mikhail A.; Quataert, Eliot
2018-04-01
We present unstratified 3D MHD simulations of an accretion disk with a boundary layer (BL) that have a duration ˜1000 orbital periods at the inner radius of the accretion disk. We find the surprising result that angular momentum piles up in the boundary layer, which results in a rapidly rotating belt of accreted material at the surface of the star. The angular momentum stored in this belt increases monotonically in time, which implies that angular momentum transport mechanisms in the BL are inefficient and do not couple the accretion disk to the star. This is in spite of the fact that magnetic fields are advected into the BL from the disk and supersonic shear instabilities in the BL excite acoustic waves. In our simulations, these waves only carry a small fraction (˜10%) of the angular momentum required for steady state accretion. Using analytical theory and 2D viscous simulations in the R - ϕ plane, we derive an analytical criterion for belt formation to occur in the BL in terms of the ratio of the viscosity in the accretion disk to the viscosity in the BL. Our MHD simulations have a dimensionless viscosity (α) in the BL that is at least a factor of ˜100 smaller than that in the disk. We discuss the implications of these results for BL dynamics and emission.
The thermodynamic evolution of the hurricane boundary layer during eyewall replacement cycles
Williams, Gabriel J.
2017-12-01
Eyewall replacement cycles (ERCs) are frequently observed during the lifecycle of mature tropical cyclones. Although the kinematic structure and intensity changes during an ERC have been well-documented, comparatively little research has been done to examine the evolution of the tropical cyclone boundary layer (TCBL) during an ERC. This study will examine how the inner core thermal structure of the TCBL is affected by the presence of multiple concentric eyewalls using a high-resolution moist, hydrostatic, multilayer diagnostic boundary layer model. Within the concentric eyewalls above the cloud base, latent heat release and vertical advection (due to the eyewall updrafts) dominate the heat and moisture budgets, whereas vertical advection (due to subsidence) and vertical diffusion dominate the heat and moisture budgets for the moat region. Furthermore, it is shown that the development of a moat region within the TCBL depends sensitively on the moat width in the overlying atmosphere and the relative strength of the gradient wind field in the overlying atmosphere. These results further indicate that the TCBL contributes to outer eyewall formation through a positive feedback process between the vorticity in the nascent outer eyewall, boundary layer convergence, and boundary layer moist convection.
Large Eddy Simulations of a Bottom Boundary Layer Under a Shallow Geostrophic Front
Bateman, S. P.; Simeonov, J.; Calantoni, J.
2017-12-01
The unstratified surf zone and the stratified shelf waters are often separated by dynamic fronts that can strongly impact the character of the Ekman bottom boundary layer. Here, we use large eddy simulations to study the turbulent bottom boundary layer associated with a geostrophic current on a stratified shelf of uniform depth. The simulations are initialized with a spatially uniform vertical shear that is in geostrophic balance with a pressure gradient due to a linear horizontal temperature variation. Superposed on the temperature front is a stable vertical temperature gradient. As turbulence develops near the bottom, the turbulence-induced mixing gradually erodes the initial uniform temperature stratification and a well-mixed layer grows in height until the turbulence becomes fully developed. The simulations provide the spatial distribution of the turbulent dissipation and the Reynolds stresses in the fully developed boundary layer. We vary the initial linear stratification and investigate its effect on the height of the bottom boundary layer and the turbulence statistics. The results are compared to previous models and simulations of stratified bottom Ekman layers.
Diagnostic analysis of turbulent boundary layer data by a trivariate Lagrangian partitioning method
Energy Technology Data Exchange (ETDEWEB)
Welsh, P.T. [Florida State Univ., Tallahassee, FL (United States)
1994-12-31
The rapid scientific and technological advances in meteorological theory and modeling predominantly have occurred on the large (or synoptic) scale flow characterized by the extratropical cyclone. Turbulent boundary layer flows, in contrast, have been slower in developing both theoretically and in accuracy for several reasons. There are many existing problems in boundary layer models, among them are limits to computational power available, the inability to handle countergradient fluxes, poor growth matching to real boundary layers, and inaccuracy in calculating the diffusion of scalar concentrations. Such transport errors exist within the boundary layer as well as into the free atmosphere above. This research uses a new method, which can provide insight into these problems, and ultimately improve boundary layer models. There are several potential applications of the insights provided by this approach, among them are estimation of cloud contamination of satellite remotely sensed surface parameters, improved flux and vertical transport calculations, and better understanding of the diurnal boundary layer growth process and its hysteresis cycle.
DEFF Research Database (Denmark)
Bertagnolio, Franck; Fischer, Andreas; Zhu, Wei Jun
2014-01-01
The modeling of the surface pressure spectrum beneath a turbulent boundary layer is investigated, focusing on the case of airfoil flows and associated trailing edge noise prediction using the so-called TNO model. This type of flow is characterized by the presence of an adverse pressure gradient...... along the airfoil chord. It is shown that discrepancies between measurements and results from the TNO model increase as the pressure gradient increases. The original model is modified by introducing anisotropy in the definition of the turbulent vertical velocity spectrum across the boundary layer...
Bristled shark skin: a microgeometry for boundary layer control?
International Nuclear Information System (INIS)
Lang, A W; Hidalgo, P; Westcott, M; Motta, P
2008-01-01
There exists evidence that some fast-swimming shark species may have the ability to bristle their scales during fast swimming. Experimental work using a water tunnel facility has been performed to investigate the flow field over and within a bristled shark skin model submerged within a boundary layer to deduce the possible boundary layer control mechanisms being used by these fast-swimming sharks. Fluorescent dye flow visualization provides evidence of the formation of embedded cavity vortices within the scales. Digital particle image velocimetry (DPIV) data, used to evaluate the cavity vortex formation and boundary layer characteristics close to the surface, indicate increased momentum in the slip layer forming above the scales. This increase in flow velocity close to the shark's skin is indicative of boundary layer control mechanisms leading to separation control and possibly transition delay for the bristled shark skin microgeometry
Bristled shark skin: a microgeometry for boundary layer control?
Energy Technology Data Exchange (ETDEWEB)
Lang, A W; Hidalgo, P; Westcott, M [Aerospace Engineering and Mechanics Department, University of Alabama, Box 870280, Tuscaloosa, AL 35487 (United States); Motta, P [Biology Department, University of South Florida, 4202 East Fowler Avenue, Tampa, FL 33620 (United States)], E-mail: alang@eng.ua.edu
2008-12-01
There exists evidence that some fast-swimming shark species may have the ability to bristle their scales during fast swimming. Experimental work using a water tunnel facility has been performed to investigate the flow field over and within a bristled shark skin model submerged within a boundary layer to deduce the possible boundary layer control mechanisms being used by these fast-swimming sharks. Fluorescent dye flow visualization provides evidence of the formation of embedded cavity vortices within the scales. Digital particle image velocimetry (DPIV) data, used to evaluate the cavity vortex formation and boundary layer characteristics close to the surface, indicate increased momentum in the slip layer forming above the scales. This increase in flow velocity close to the shark's skin is indicative of boundary layer control mechanisms leading to separation control and possibly transition delay for the bristled shark skin microgeometry.
Allabakash, S.; Yasodha, P.; Bianco, L.; Venkatramana Reddy, S.; Srinivasulu, P.; Lim, S.
2017-09-01
This paper presents the efficacy of a "tuned" fuzzy logic method at determining the height of the boundary layer using the measurements from a 1280 MHz lower atmospheric radar wind profiler located in Gadanki (13.5°N, 79°E, 375 mean sea level), India, and discusses the diurnal and seasonal variations of the measured convective boundary layer over this tropical station. The original fuzzy logic (FL) method estimates the height of the atmospheric boundary layer combining the information from the range-corrected signal-to-noise ratio, the Doppler spectral width of the vertical velocity, and the vertical velocity itself, measured by the radar, through a series of thresholds and rules, which did not prove to be optimal for our radar system and geographical location. For this reason the algorithm was tuned to perform better on our data set. Atmospheric boundary layer heights obtained by this tuned FL method, the original FL method, and by a "standard method" (that only uses the information from the range-corrected signal-to-noise ratio) are compared with those obtained from potential temperature profiles measured by collocated Global Positioning System Radio Sonde during years 2011 and 2013. The comparison shows that the tuned FL method is more accurate than the other methods. Maximum convective boundary layer heights are observed between 14:00 and 15:00 local time (LT = UTC + 5:30) for clear-sky days. These daily maxima are found to be lower during winter and postmonsoon seasons and higher during premonsoon and monsoon seasons, due to net surface radiation and convective processes over this region being more intense during premonsoon and monsoon seasons and less intense in winter and postmonsoon seasons.
INCOMPRESSIBLE LAMINAR BOUNDARY LAYER CONTROL BY BLOWING AND SUCTION
Directory of Open Access Journals (Sweden)
AZZEDINE NAHOUI
2013-12-01
Full Text Available A two-dimensional incompressible laminar boundary layer and its control using blowing and suction over a flat plate and around the NACA 0012 and 661012 profiles, is studied numerically. The study is based on the Prandtl boundary layer model using the finite differences method and the Crank-Nicolson scheme. The velocity distribution, the boundary layer thickness and the friction coefficient, are determined and presented with and without control. The application of the control technique, has demonstrated its positive effect on the transition point and the friction coefficient. Both control procedures are compared for different lengths, speeds and angles of blowing and suction.
The atmospheric boundary layer — advances in knowledge and application
Garratt, J. R.; Hess, G. D.; Physick, W. L.; Bougeault, P.
1996-02-01
We summarise major activities and advances in boundary-layer knowledge in the 25 years since 1970, with emphasis on the application of this knowledge to surface and boundary-layer parametrisation schemes in numerical models of the atmosphere. Progress in three areas is discussed: (i) the mesoscale modelling of selected phenomena; (ii) numerical weather prediction; and (iii) climate simulations. Future trends are identified, including the incorporation into models of advanced cloud schemes and interactive canopy schemes, and the nesting of high resolution boundary-layer schemes in global climate models.
Change of Surface Roughness and Planetary Boundary Layer
DEFF Research Database (Denmark)
Jensen, Niels Otto
1978-01-01
The ratio between upstream and far downstream surface friction velocities relative to a change in surface roughness is given on the basis of results from surface Rossby number similarity theory. By simple theories for the internal boundary layer, which are found to compare quite well with recent...... numerical results from higher-order closure models, it is found that, even at a downwind distance such that the internal boundary layer has grown to the full height of the planetary boundary layers, the surface stress still considerably exceeds the equilibrium value...
Effect of externally generated turbulence on wave boundary layer
DEFF Research Database (Denmark)
Fredsøe, Jørgen; Sumer, B. Mutlu; Kozakiewicz, A.
2003-01-01
This experimental study deals with the effect of externally generated turbulence on the oscillatory boundary layer to simulate the turbulence in the wave boundary layer under broken waves in the swash zone. The subject has been investigated experimentally in a U-shaped, oscillating water tunnel...... results. The mean and turbulence quantities in the outer flow region are increased substantially with the introduction of the grids. It is shown that the externally generated turbulence is able to penetrate the bed boundary layer, resulting in an increase in the bed shear stress, and therefore...
Coherent structures in wave boundary layers. Part 2. Solitary motion
DEFF Research Database (Denmark)
Sumer, B. Mutlu; Jensen, Palle Martin; Sørensen, Lone B.
2010-01-01
This study continues the investigation of wave boundary layers reported by Carstensen, Sumer & Fredsøe (J. Fluid Mech., 2010, part 1 of this paper). The present paper summarizes the results of an experimental investigation of turbulent solitary wave boundary layers, simulated by solitary motion...... the boundary-layer flow experiences a regular array of vortex tubes near the bed over a short period of time during the deceleration stage; and (iii) transitional regime characterized with turbulent spots, revealed by single/multiple, or, sometimes, quite dense spikes in the bed shear stress traces...
Size distributions of boundary-layer clouds
Energy Technology Data Exchange (ETDEWEB)
Stull, R.; Berg, L.; Modzelewski, H. [Univ. of Wisconsin, Madison, WI (United States)
1996-04-01
Scattered fair-weather clouds are triggered by thermals rising from the surface layer. Not all surface layer air is buoyant enough to rise. Also, each thermal has different humidities and temperatures, resulting in interthermal variability of their lifting condensation levels (LCL). For each air parcel in the surface layer, it`s virtual potential temperature and it`s LCL height can be computed.
The boundary condition for vertical velocity and its interdependence with surface gas exchange
Kowalski, Andrew S.
2017-07-01
The law of conservation of linear momentum is applied to surface gas exchanges, employing scale analysis to diagnose the vertical velocity (w) in the boundary layer. Net upward momentum in the surface layer is forced by evaporation (E) and defines non-zero vertical motion, with a magnitude defined by the ratio of E to the air density, as w = E/ρ. This is true even right down at the surface where the boundary condition is w|0 = E/ρ|0 (where w|0 and ρ|0 represent the vertical velocity and density of air at the surface). This Stefan flow velocity implies upward transport of a non-diffusive nature that is a general feature of the troposphere but is of particular importance at the surface, where it assists molecular diffusion with upward gas migration (of H2O, for example) but opposes that of downward-diffusing species like CO2 during daytime. The definition of flux-gradient relationships (eddy diffusivities) requires rectification to exclude non-diffusive transport, which does not depend on scalar gradients. At the microscopic scale, the role of non-diffusive transport in the process of evaporation from inside a narrow tube - with vapour transport into an overlying, horizontal airstream - was described long ago in classical mechanics and is routinely accounted for by chemical engineers, but has been neglected by scientists studying stomatal conductance. Correctly accounting for non-diffusive transport through stomata, which can appreciably reduce net CO2 transport and marginally boost that of water vapour, should improve characterisations of ecosystem and plant functioning.
The boundary condition for vertical velocity and its interdependence with surface gas exchange
Directory of Open Access Journals (Sweden)
A. S. Kowalski
2017-07-01
Full Text Available The law of conservation of linear momentum is applied to surface gas exchanges, employing scale analysis to diagnose the vertical velocity (w in the boundary layer. Net upward momentum in the surface layer is forced by evaporation (E and defines non-zero vertical motion, with a magnitude defined by the ratio of E to the air density, as w = E/ρ. This is true even right down at the surface where the boundary condition is w|0 = E/ρ|0 (where w|0 and ρ|0 represent the vertical velocity and density of air at the surface. This Stefan flow velocity implies upward transport of a non-diffusive nature that is a general feature of the troposphere but is of particular importance at the surface, where it assists molecular diffusion with upward gas migration (of H2O, for example but opposes that of downward-diffusing species like CO2 during daytime. The definition of flux–gradient relationships (eddy diffusivities requires rectification to exclude non-diffusive transport, which does not depend on scalar gradients. At the microscopic scale, the role of non-diffusive transport in the process of evaporation from inside a narrow tube – with vapour transport into an overlying, horizontal airstream – was described long ago in classical mechanics and is routinely accounted for by chemical engineers, but has been neglected by scientists studying stomatal conductance. Correctly accounting for non-diffusive transport through stomata, which can appreciably reduce net CO2 transport and marginally boost that of water vapour, should improve characterisations of ecosystem and plant functioning.
Reactive boundary layers in metallic rolling contacts
Energy Technology Data Exchange (ETDEWEB)
Burbank, John
2016-05-01
more thorough investigation into the effects of residual austenite on the properties of this material. The high-performance alternative steels, 36NiCrMoV1-5-7 (hot working steel) and 45SiCrMo6 (spring steel), were heat treated as recommended by their respective manufacturers, and were not case-hardened. The selection of materials with and materials without case-hardening allows for an investigation into whether or not case-hardening is even necessary to deliver acceptable friction behaviour and wear performance. Elemental analyses were conducted by multiple methods to ensure accurate results. Residual austenite contents of the steels and the depth profiles of residual stresses were determined by X-Ray diffraction (XRD), for 20MnCr5 ranging from approximately 6 - 14 vol.%, and under 2 vol.% for the alternative alloys. Hardness profiles were taken from the testing surfaces into the material core. The carburization of 20MnCr5 led to higher hardness and the greater concentration of carbon in the carburization zone more representative of a hardened SAE E52100, or 100Cr6/102Cr6, than of a non-case-hardened 20MnCr5. Residual stresses from machining and case-hardening were measured directly at the sample surface. The high-performance steels fulfilled manufacturer expectations in terms of elemental content, with hardness values between 50 - 55 HRC and strongly martensitic microstructure character. With characterization of the chosen materials complete, the materials could then be subjected to pre-conditioning. The first pre-conditioning method involved targeted generation of cold work hardening as induced boundary layers to protect the contact zone against wear. Work hardening was identified both by variations in residual stress profiles, i.e. the introduction of beneficial compressive residual stresses, and hardness increases in the contact zone, providing enhanced wear resistance. Parameters for work hardening were further optimized to reduce damage to the surface substrates
Reactive boundary layers in metallic rolling contacts
International Nuclear Information System (INIS)
Burbank, John
2016-01-01
thorough investigation into the effects of residual austenite on the properties of this material. The high-performance alternative steels, 36NiCrMoV1-5-7 (hot working steel) and 45SiCrMo6 (spring steel), were heat treated as recommended by their respective manufacturers, and were not case-hardened. The selection of materials with and materials without case-hardening allows for an investigation into whether or not case-hardening is even necessary to deliver acceptable friction behaviour and wear performance. Elemental analyses were conducted by multiple methods to ensure accurate results. Residual austenite contents of the steels and the depth profiles of residual stresses were determined by X-Ray diffraction (XRD), for 20MnCr5 ranging from approximately 6 - 14 vol.%, and under 2 vol.% for the alternative alloys. Hardness profiles were taken from the testing surfaces into the material core. The carburization of 20MnCr5 led to higher hardness and the greater concentration of carbon in the carburization zone more representative of a hardened SAE E52100, or 100Cr6/102Cr6, than of a non-case-hardened 20MnCr5. Residual stresses from machining and case-hardening were measured directly at the sample surface. The high-performance steels fulfilled manufacturer expectations in terms of elemental content, with hardness values between 50 - 55 HRC and strongly martensitic microstructure character. With characterization of the chosen materials complete, the materials could then be subjected to pre-conditioning. The first pre-conditioning method involved targeted generation of cold work hardening as induced boundary layers to protect the contact zone against wear. Work hardening was identified both by variations in residual stress profiles, i.e. the introduction of beneficial compressive residual stresses, and hardness increases in the contact zone, providing enhanced wear resistance. Parameters for work hardening were further optimized to reduce damage to the surface substrates of the
Numerical simulations of the stratified oceanic bottom boundary layer
Taylor, John R.
Numerical simulations are used to consider several problems relevant to the turbulent oceanic bottom boundary layer. In the first study, stratified open channel flow is considered with thermal boundary conditions chosen to approximate a shallow sea. Specifically, a constant heat flux is applied at the free surface and the lower wall is assumed to be adiabatic. When the surface heat flux is strong, turbulent upwellings of low speed fluid from near the lower wall are inhibited by the stable stratification. Subsequent studies consider a stratified bottom Ekman layer over a non-sloping lower wall. The influence of the free surface is removed by using an open boundary condition at the top of the computational domain. Particular attention is paid to the influence of the outer layer stratification on the boundary layer structure. When the density field is initialized with a linear profile, a turbulent mixed layer forms near the wall, which is separated from the outer layer by a strongly stable pycnocline. It is found that the bottom stress is not strongly affected by the outer layer stratification. However, stratification reduces turbulent transport to the outer layer and strongly limits the boundary layer height. The mean shear at the top of the boundary layer is enhanced when the outer layer is stratified, and this shear is strong enough to cause intermittent instabilities above the pycnocline. Turbulence-generated internal gravity waves are observed in the outer layer with a relatively narrow frequency range. An explanation for frequency content of these waves is proposed, starting with an observed broad-banded turbulent spectrum and invoking linear viscous decay to explain the preferential damping of low and high frequency waves. During the course of this work, an open-source computational fluid dynamics code has been developed with a number of advanced features including scalar advection, subgrid-scale models for large-eddy simulation, and distributed memory
Rough-wall turbulent boundary layers with constant skin friction
Sridhar, A.; Pullin, D. I.; Cheng, W.
2017-01-01
A semi-empirical model is presented that describes the development of a fully developed turbulent boundary layer in the presence of surface roughness with length scale ks that varies with streamwise distance x . Interest is centred on flows
CFD simulation of the atmospheric boundary layer: wall function problems
Blocken, B.J.E.; Stathopoulos, T.; Carmeliet, J.
2007-01-01
Accurate Computational Fluid Dynamics (CFD) simulations of atmospheric boundary layer (ABL) flow are essential for a wide variety of atmospheric studies including pollutant dispersion and deposition. The accuracy of such simulations can be seriously compromised when wall-function roughness
Global instabilities and transient growth in Blasius boundary-layer ...
Indian Academy of Sciences (India)
boundary-layer flow warrants attention. .... double prime indicates a dummy variable, while R and S respectively denote integration in the ..... (labelled) but it also features an unstable structural mode labelled S that ..... theory and experiment.
Accretion disc boundary layers - geometrically and optically thin case
International Nuclear Information System (INIS)
Regev, Oded; Hougerat, A.A.
1988-01-01
The method of matched asymptotic expansions is applied to an optically and geometrically thin boundary layer between an accretion disc and the accreting star. Analytical solutions are presented for a particular viscosity prescription in the boundary layer. For a typical example we find that the disc closely resembles standard steady-disc theory. It is identical to it everywhere save a narrow boundary layer, where the temperature increases rapidly inward (by an order of magnitude), the angular velocity achieves maximum and decreases to its surface value and other variables also undergo rapid changes. This and previous work can now be used to calculate the emission from accretion discs including the boundary layers for a wide range of parameters. (author)
Boundary layer energies for nonconvex discrete systems
Scardia, L.; Schlömerkemper, A.; Zanini, C.
2011-01-01
In this work we consider a one-dimensional chain of atoms which interact through nearest and next-to-nearest neighbour interactions of Lennard-Jones type. We impose Dirichlet boundary conditions and in addition prescribe the deformation of the second and last but one atoms of the chain. This
Boundary-Layer Bypass Transition Over Large-Scale Bodies
2016-12-16
behaviour of the velocity and pressure changes with the curvature. This work aims to extend the results of the flat-plate boundary layer to a Rankine...example, consume an enormous amount of energy due to friction, many works have been directed to the suppression of transitional boundary layer disturbances...decrease of the enormous amount of energy consumed by airplanes during flight, moreover flight costs and aerodynamic noise could be reduced and number
On Hydromagnetic Stresses in Accretion Disk Boundary Layers
DEFF Research Database (Denmark)
Pessah, Martin Elias; Chan, Chi-kwan
2012-01-01
Detailed calculations of the physical structure of accretion disk boundary layers, and thus their inferred observational properties, rely on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear...... of efficient angular momentum transport in the inner disk regions. This suggests that the detailed structure of turbulent MHD accretion disk boundary layers could differ appreciably from those derived within the standard framework of turbulent shear viscosity...
Control of Boundary Layers for Aero-optical Applications
2015-06-23
with some difficulty) from hot-wire velocity measurements, or computed directly from CFD results (e.g. Wang & Wang, 2012). Several different density...of experimental and computational research, especially applied to supersonic and hypersonic boundary layers; see Smits & Dussauge (1996), Spina et...Duan, L., Beekman, I. and Martin, M.P. (2010) Direct Numerical Simulation of Hypersonic Turbulent Boundary Layers. Part 2. Effect of Wall
MPLNET V3 Cloud and Planetary Boundary Layer Detection
Lewis, Jasper R.; Welton, Ellsworth J.; Campbell, James R.; Haftings, Phillip C.
2016-01-01
The NASA Micropulse Lidar Network Version 3 algorithms for planetary boundary layer and cloud detection are described and differences relative to the previous Version 2 algorithms are highlighted. A year of data from the Goddard Space Flight Center site in Greenbelt, MD consisting of diurnal and seasonal trends is used to demonstrate the results. Both the planetary boundary layer and cloud algorithms show significant improvement of the previous version.
Unequilibrium kinetic of collisionless boundary layers in binary plasmas
International Nuclear Information System (INIS)
Kotelnikov, V.A.; Nikolaev, F.A.; Cherepanov, V.V.
1985-01-01
Relaxation processes of kinetic nonequilibrium collisionless boundary layers near spherical charged full absorbing surfaces in binary low-temperature plasmas are investigated. The effect of magnetic field on relaxation processes was neglected. The dynamics of components of the ionized gas was treated near the boundary layer. The potential distribution and the space dependence of concentration were calculated numerically. These results agree well with the experimental data. (D.Gy.)
Development of Robust Boundary Layer Controllers
National Research Council Canada - National Science Library
Speyer, Jason
2002-01-01
.... The three-dimensional Navier-Stokes equations of channel flow, linearized about a Poisueille profile, and Oberbeck-Boussinesq equations of a layer of fluid, linearized about the no motion state...
Short climatology of the atmospheric boundary layer using acoustic methods
International Nuclear Information System (INIS)
Schubert, J.F.
1975-06-01
A climatology of the boundary layer of the atmosphere at the Savannah River Laboratory is being compiled using acoustic methods. The atmospheric phenomenon as depicted on the facsimile recorder is classified and then placed into one of sixteen categories. After classification, the height of the boundary layer is measured. From this information, frequency tables of boundary layer height and category are created and then analyzed for the percentage of time that each category was detected by the acoustic sounder. The sounder also accurately depicts the diurnal cycle of the boundary layer and, depending on the sensitivity of the system, shows microstructure that is normally unavailable using other methods of profiling. The acoustic sounder provides a means for continuous, real time measurements of the time rate of change of the depth of the boundary layer. This continuous record of the boundary layer with its convective cells, gravity waves, inversions, and frontal system passages permits the synoptic and complex climatology of the local area to be compiled. (U.S.)
Turbulent boundary layer in high Rayleigh number convection in air.
du Puits, Ronald; Li, Ling; Resagk, Christian; Thess, André; Willert, Christian
2014-03-28
Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bénard experiment are presented for the Rayleigh number Ra=1.4×1010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I=0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re≈200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bénard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal.
Turbulent transport of large particles in the atmospheric boundary layer
Richter, D. H.; Chamecki, M.
2017-12-01
To describe the transport of heavy dust particles in the atmosphere, assumptions must typically be made in order to connect the micro-scale emission processes with the larger-scale atmospheric motions. In the context of numerical models, this can be thought of as the transport process which occurs between the domain bottom and the first vertical grid point. For example, in the limit of small particles (both low inertia and low settling velocity), theory built upon Monin-Obukhov similarity has proven effective in relating mean dust concentration profiles to surface emission fluxes. For increasing particle mass, however, it becomes more difficult to represent dust transport as a simple extension of the transport of a passive scalar due to issues such as the crossing trajectories effect. This study focuses specifically on the problem of large particle transport and dispersion in the turbulent boundary layer by utilizing direct numerical simulations with Lagrangian point-particle tracking to determine under what, if any, conditions the large dust particles (larger than 10 micron in diameter) can be accurately described in a simplified Eulerian framework. In particular, results will be presented detailing the independent contributions of both particle inertia and particle settling velocity relative to the strength of the surrounding turbulent flow, and consequences of overestimating surface fluxes via traditional parameterizations will be demonstrated.
Benthic boundary layer. IOS observational and modelling programme
International Nuclear Information System (INIS)
Saunders, P.M.; Richards, K.J.
1985-01-01
Near bottom currents, measured at three sites in the N.E. Atlantic, reveal the eddying characteristics of the flow. Eddies develop, migrate and decay in ways best revealed by numerical modelling simulations. Eddies control the thickness of the bottom mixed layer by accumulating and thickening or spreading and thinning the bottom waters. At the boundaries of eddies benthic fronts form providing a path for upward displacement of the bottom water. An experiment designed to estimate vertical diffusivity is performed. The flux of heat into the bottom of the Iberian basin through Discovery Gap is deduced from year long current measurements. The flux is supposed balanced by geothermal heating through the sea floor and diapycnal diffusion in the water. A diffusivity of 1.5 to 4 cm 2 s -1 is derived for the bottom few hundred meters of the deep ocean. Experiments to estimate horizontal diffusivity are described. If a tracer is discharged from the sea bed the volume of sea water in which it is found increases with time and after 20 years will fill an ocean basin of side 1000 km to a depth of only 1 to 2 km. (author)
Boundary layer parameterizations and long-range transport
International Nuclear Information System (INIS)
Irwin, J.S.
1992-01-01
A joint work group between the American Meteorological Society (AMS) and the EPA is perusing the construction of an air quality model that incorporates boundary layer parameterizations of dispersion and transport. This model could replace the currently accepted model, the Industrial Source Complex (ISC) model. The ISC model is a Gaussian-plume multiple point-source model that provides for consideration of fugitive emissions, aerodynamic wake effects, gravitational settling and dry deposition. A work group of several Federal and State agencies is perusing the construction of an air quality modeling system for use in assessing and tracking visibility impairment resulting from long-range transport of pollutants. The modeling system is designed to use the hourly vertical profiles of wind, temperature and moisture resulting from a mesoscale meteorological processor that employs four dimensional data assimilation (FDDA). FDDA involves adding forcing functions to the governing model equations to gradually ''nudge'' the model state toward the observations (12-hourly upper air observations of wind, temperature and moisture, and 3-hourly surface observations of wind and moisture). In this way it is possible to generate data sets whose accuracy, in terms of transport, precipitation, and dynamic consistency is superior to both direct interpolation of synoptic-scale analyses of observations and purely predictive mode model result. (AB) ( 19 refs.)
Turbulent Boundary Layers - Experiments, Theory and Modelling
1980-01-01
anemometry, London, Academic Press, 1976. 7. H.R.E. van Maanen, K. van der Molen and J. Blom, "Reduction of ambiguity noise in laser-Doppler...Raumfahrttechnik Hochschule der Bundeswehr München 8014 Neubiberg — Germany Professor Dr J.L. Van Ingen Dept. of Aerospace Engineering Delft...proximity to a solid boundary. J.Fluid Mech.12, 388 - 397, 1962. (21) Van Thin N., Messungen mit einem Hitzdraht in einer turbulenten Strömung in der
Analysis and Modeling of Boundary Layer Separation Method (BLSM).
Pethő, Dóra; Horváth, Géza; Liszi, János; Tóth, Imre; Paor, Dávid
2010-09-01
Nowadays rules of environmental protection strictly regulate pollution material emission into environment. To keep the environmental protection laws recycling is one of the useful methods of waste material treatment. We have developed a new method for the treatment of industrial waste water and named it boundary layer separation method (BLSM). We apply the phenomena that ions can be enriched in the boundary layer of the electrically charged electrode surface compared to the bulk liquid phase. The main point of the method is that the boundary layer at correctly chosen movement velocity can be taken out of the waste water without being damaged, and the ion-enriched boundary layer can be recycled. Electrosorption is a surface phenomenon. It can be used with high efficiency in case of large electrochemically active surface of electrodes. During our research work two high surface area nickel electrodes have been prepared. The value of electrochemically active surface area of electrodes has been estimated. The existence of diffusion part of the double layer has been experimentally approved. The electrical double layer capacity has been determined. Ion transport by boundary layer separation has been introduced. Finally we have tried to estimate the relative significance of physical adsorption and electrosorption.
Field emission from vertically aligned few-layer graphene
International Nuclear Information System (INIS)
Malesevic, Alexander; Kemps, Raymond; Vanhulsel, Annick; Chowdhury, Manish Pal; Volodin, Alexander; Van Haesendonck, Chris
2008-01-01
The electric field emission behavior of vertically aligned few-layer graphene was studied in a parallel plate-type setup. Few-layer graphene was synthesized in the absence of any metallic catalyst by microwave plasma enhanced chemical vapor deposition with gas mixtures of methane and hydrogen. The deposit consists of nanostructures that are several micrometers wide, highly crystalline stacks of four to six atomic layers of graphene, aligned vertically to the substrate surface in a high density network. The few-layer graphene is found to be a good field emitter, characterized by turn-on fields as low as 1 V/μm and field amplification factors up to several thousands. We observe a clear dependence of the few-layer graphene field emission behavior on the synthesis parameters: Hydrogen is identified as an efficient etchant to improve field emission, and samples grown on titanium show lower turn-on field values and higher amplification factors when compared to samples grown on silicon
FOREWORD: International Conference on Planetary Boundary Layer and Climate Change
Djolov, G.; Esau, I.
2010-05-01
structural uncertainties is hard to reduce and this could be one of the reasons determining slow progress in narrowing the climate model uncertainty range over the last 30 years (Knutti and Hagerl, Nature Geoscience, 2008). One of the most prominent structural uncertainties in the ongoing transient climate change is related to poor understanding and hence incorrect modelling of the turbulent physics and dynamics processes in the planetary boundary layer. Nevertheless, the climate models continue to rely on physically incorrect boundary layer parameterizations (Cuxart et al., BLM, 2006), whose erroneous dynamical response in the climate models may lead to significant abnormalities in simulated climate. At present, international efforts in theoretical understanding of the turbulent mixing have resulted in significant progress in turbulence simulation, measurements and parameterizations. However, this understanding has not yet found its way to the climate research community. Vice versa, climate research is not usually addressed by the boundary layer research community. The gap needs to be closed in order to crucially complete the scientific basis of climate change studies. The focus of the proposed forum could be formulated as follows: The planetary boundary layer determines several key parameters controlling the Earth's climate system but being a dynamic sub-system, just a layer of turbulent mixing in the atmosphere/ocean, it is also controlled by the climate system and its changes. Such a dynamic relationship causes a planetary boundary layer feedback (PBL-feedback) which could be defined as the response of the surface air temperature on changes in the vertical turbulent mixing. The forum participants have discussed both climatological and fluid dynamic aspects of this response, in order to quantify their role in the Earth's transient heat uptake and its representation in climate models. The choice of the forum location and dates are motivated by the role of tropical oceans
Beta limitation of matter-antimatter boundary layers
International Nuclear Information System (INIS)
Lehnert, B.
1987-08-01
A model has earlier been proposed for a boundary layer which separates a cloud of matter from one of antimatter in a magnetized ambiplasma. In this model steady pressure equilibrium ceases to exist when a certain beta limit is exceeded. The latter is defined as the ratio between the ambiplasma and magnetic field pressures which balance each other in the boundary layer. Thus, at an increasing density, the high-energy particles created by annihilation within the layer are 'pumped up' to a pressure which cannot be balanced by a given magnetic field. The boundary layer then 'disrupts'. The critical beta limit thus obtained falls within the observed parameter ranges of galaxies and other large cosmical objects. Provided that the considered matter-antimatter balance holds true, this limit is thus expected to impose certain existence conditions on matter-antimatter boundary layers. Such a limitation may apply to certain cosmical objects and cosmological models. The maximum time scale for the corresponding disruption development has been estimated to be in the range from about 10 -4 to 10 2 seconds for boundary layers at ambiplasma particle densities in the range from 10 4 to 10 -2 m -3 , respectively. (author)
Observations of the Early Evening Boundary-Layer Transition Using a Small Unmanned Aerial System
Bonin, Timothy; Chilson, Phillip; Zielke, Brett; Fedorovich, Evgeni
2013-01-01
The evolution of the lower portion of the planetary boundary layer is investigated using the Small Multifunction Research and Teaching Sonde (SMARTSonde), an unmanned aerial vehicle developed at the University of Oklahoma. The study focuses on the lowest 200 m of the atmosphere, where the most noticeable thermodynamic changes occur during the day. Between October 2010 and February 2011, a series of flights was conducted during the evening hours on several days to examine the vertical structure of the lower boundary layer. Data from a nearby Oklahoma Mesonet tower was used to supplement the vertical profiles of temperature, humidity, and pressure, which were collected approximately every 30 min, starting 2 h before sunset and continuing until dusk. From the profiles, sensible and latent heat fluxes were estimated. These fluxes were used to diagnose the portion of the boundary layer that was most affected by the early evening transition. During the transition period, a shallow cool and moist layer near the ground was formed, and as the evening progressed the cooling affected an increasingly shallower layer just above the surface.
Coherent structures in wave boundary layers. Part 1. Oscillatory motion
DEFF Research Database (Denmark)
Carstensen, Stefan; Sumer, B. Mutlu; Fredsøe, Jørgen
2010-01-01
This work concerns oscillatory boundary layers over smooth beds. It comprises combined visual and quantitative techniques including bed shear stress measurements. The experiments were carried out in an oscillating water tunnel. The experiments reveal two significant coherent flow structures: (i......) Vortex tubes, essentially two-dimensional vortices close to the bed extending across the width of the boundary-layer flow, caused by an inflectional-point shear layer instability. The imprint of these vortices in the bed shear stress is a series of small, insignificant kinks and dips. (ii) Turbulent...... spots, isolated arrowhead-shaped areas close to the bed in an otherwise laminar boundary layer where the flow ‘bursts’ with violent oscillations. The emergence of the turbulent spots marks the onset of turbulence. Turbulent spots cause single or multiple violent spikes in the bed shear stress signal...
Definition of Turbulent Boundary-Layer with Entropy Concept
Directory of Open Access Journals (Sweden)
Zhao Rui
2016-01-01
Full Text Available The relationship between the entropy increment and the viscosity dissipation in turbulent boundary-layer is systematically investigated. Through theoretical analysis and direct numerical simulation (DNS, an entropy function fs is proposed to distinguish the turbulent boundary-layer from the external flow. This approach is proved to be reliable after comparing its performance in the following complex flows, namely, low-speed airfoil flows with different wall temperature, supersonic cavity-ramp flow dominated by the combination of free-shear layer, larger recirculation and shocks, and the hypersonic flow past an aeroplane configuration. Moreover, fs is deduced from the point of energy, independent of any particular turbulent quantities. That is, this entropy concept could be utilized by other engineering applications related with turbulent boundary-layer, such as turbulence modelling transition prediction and engineering thermal protection.
Response of neutral boundary-layers to changes of roughness
DEFF Research Database (Denmark)
Sempreviva, Anna Maria; Larsen, Søren Ejling; Mortensen, Niels Gylling
1990-01-01
boundary layer where again the drag laws can be used to estimate the surface wind. To study this problem, data have been sampled for two years from four 30-m meteorological masts placed from 0 to 30 km inland from the North Sea coast of Jutland in Denmark. The present analysis is limited to neutral......When air blows across a change in surface roughness, an internal boundary layer (IBL) develops within which the wind adapts to the new surface. This process is well described for short fetches, > 1 km. However, few data exist for large fetches on how the IBL grows to become a new equilibrium...... stratification, and the surface roughness is the main parameter. The analysis of wind data and two simple models, a surface layer and a planetary boundary layer (PBL) model, are described. Results from both models are discussed and compared with data analysis. Model parameters have been evaluated and the model...
Directory of Open Access Journals (Sweden)
J. H. Marsham
2008-12-01
Full Text Available Observations of the Saharan boundary layer, made during the GERBILS field campaign, show that mesoscale land surface temperature variations (which were related to albedo variations induced mesoscale circulations. With weak winds along the aircraft track, land surface temperature anomalies with scales of greater than 10 km are shown to significantly affect boundary-layer temperatures and winds. Such anomalies are expected to affect the vertical mixing of the dusty and weakly stratified Saharan Residual Layer (SRL. Mesoscale variations in winds are also shown to affect dust loadings in the boundary layer.
Using the aircraft observations and data from the COSMO model, a region of local dust uplift, with strong along-track winds, was identified in one low-level flight. Large eddy model (LEM simulations based on this location showed linearly organised boundary-layer convection. Calculating dust uplift rates from the LEM wind field showed that the boundary-layer convection increased uplift by approximately 30%, compared with the uplift rate calculated neglecting the convection. The modelled effects of boundary-layer convection on uplift are shown to be larger when the boundary-layer wind is decreased, and most significant when the mean wind is below the threshold for dust uplift and the boundary-layer convection leads to uplift which would not otherwise occur.
Both the coupling of albedo features to the atmosphere on the mesoscale, and the enhancement of dust uplift by boundary-layer convection are unrepresented in many climate models, but may have significant impacts on the vertical transport and uplift of desert dust. Mesoscale effects in particular tend to be difficult to parametrise.
Particle motion in atmospheric boundary layers of Mars and Earth
White, B. R.; Iversen, J. D.; Greeley, R.; Pollack, J. B.
1975-01-01
To study the eolian mechanics of saltating particles, both an experimental investigation of the flow field around a model crater in an atmospheric boundary layer wind tunnel and numerical solutions of the two- and three-dimensional equations of motion of a single particle under the influence of a turbulent boundary layer were conducted. Two-dimensional particle motion was calculated for flow near the surfaces of both Earth and Mars. For the case of Earth both a turbulent boundary layer with a viscous sublayer and one without were calculated. For the case of Mars it was only necessary to calculate turbulent boundary layer flow with a laminar sublayer because of the low values of friction Reynolds number; however, it was necessary to include the effects of slip flow on a particle caused by the rarefied Martian atmosphere. In the equations of motion the lift force functions were developed to act on a single particle only in the laminar sublayer or a corresponding small region of high shear near the surface for a fully turbulent boundary layer. The lift force functions were developed from the analytical work by Saffman concerning the lift force acting on a particle in simple shear flow.
Coupled vs. decoupled boundary layers in VOCALS-REx
Directory of Open Access Journals (Sweden)
C. R. Jones
2011-07-01
Full Text Available We analyze the extent of subtropical stratocumulus-capped boundary layer decoupling and its relation to other boundary-layer characteristics and forcings using aircraft observations from VOCALS-REx along a swath of the subtropical southeast Pacific Ocean running west 1600 km from the coast of Northern Chile. We develop two complementary and consistent measures of decoupling. The first is based on boundary layer moisture and temperature stratification in flight profiles from near the surface to above the capping inversion, and the second is based the difference between the lifted condensation level (LCL and a mean lidar-derived cloud base measured on flight legs at 150 m altitude. Most flights took place during early-mid morning, well before the peak in insolation-induced decoupling.
We find that the boundary layer is typically shallower, drier, and well mixed near the shore, and tends to deepen, decouple, and produce more drizzle further offshore to the west. Decoupling is strongly correlated to the "mixed layer cloud thickness", defined as the difference between the capping inversion height and the LCL; other factors such as wind speed, cloud droplet concentration, and inversion thermodynamic jumps have little additional explanatory power. The results are broadly consistent with the deepening-warming theory of decoupling.
In the deeper boundary layers observed well offshore, there was frequently nearly 100 % boundary-layer cloud cover despite pronounced decoupling. The cloud cover was more strongly correlated to a κ parameter related to the inversion jumps of humidity and temperature, though the exact functional relation is slightly different than found in prior large-eddy simulation studies.
Pantazis, Alexandros; Papayannis, Alexandros; Georgoussis, Georgios
2018-04-01
In this paper we present a development of novel algorithms and techniques implemented within the Laser Remote Sensing Laboratory (LRSL) of the National Technical University of Athens (NTUA), in collaboration with Raymetrics S.A., in order to incorporate them into a 3-Dimensional (3D) lidar. The lidar is transmitting at 355 nm in the eye safe region and the measurements then are transposed to the visual range at 550 nm, according to the World Meteorological Organization (WMO) and the International Civil Aviation Organization (ICAO) rules of daytime visibility. These algorithms are able to provide horizontal, slant and vertical visibility for tower aircraft controllers, meteorologists, but also from pilot's point of view. Other algorithms are also provided for detection of atmospheric layering in any given direction and vertical angle, along with the detection of the Planetary Boundary Layer Height (PBLH).
International Nuclear Information System (INIS)
Moppert, C.
2006-02-01
The aim of this study is to analyse the vertical structure of the low troposphere during the ESCOMPTE campaign in relation with transport and diffusion of pollutants. This analysis shows the difficulty to define a boundary layer. It allows us to highlight a complex superposition of several internal boundary layers, particularly near the coast. The study of the layer where pollution may be accumulated or diluted pointed out the fact that pollution is trapped near the surface, close to the coastline under sea-breeze conditions whereas it is advected over the mountains where the boundary layers are deeper. During sea-breeze conditions, the ozone concentration is paradoxically weak near the sources at the coastline (titration). Over the mountains, the strong developments of the boundary layers result in a mixing between the highly polluted low troposphere and the surface which enhances the ozone concentration. (author)
Structure of reconnection boundary layers in incompressible MHD
International Nuclear Information System (INIS)
Sonnerup, B.U.Oe.; Wang, D.J.
1987-01-01
The incompressible MHD equations with nonvanishing viscosity and resistivity are simplified by use of the boundary layer approximation to describe the flow and magnetic field in the exit flow regions of magnetic field reconnection configurations when the reconnection rate is small. The conditions are derived under which self-similar solutions exist of the resulting boundary layer equations. For the case of zero viscosity and resistivity, the equations describing such self-similar layers are then solved in terms of quadratures, and the resulting flow and field configurations are described. Symmetric solutions, relevant, for example, to reconnection in the geomagnetic tail, as well as asymmetric solutions, relevant to reconnection at the earth's magnetopause, are found to exist. The nature of the external solutions to which the boundary layer solutions should be matched is discussed briefly, but the actual matching, which is to occur at Alfven-wave characteristic curves in the boundary layer solutions, is not carried out. Finally, it is argued that the solutions obtained may also be used to describe the structure of the intense vortex layers observed to occur at magnetic separatrices in computer simulations and in certain analytical models of the reconnection process
Lidar-Observed Stress Vectors and Veer in the Atmospheric Boundary Layer
DEFF Research Database (Denmark)
Berg, Jacob; Mann, Jakob; Patton, Edward G.
2013-01-01
This study demonstrates that a pulsed wind lidar is a reliable instrument for measuring angles between horizontal vectors of significance in the atmospheric boundary layer. Three different angles are considered: the wind turning, the angle between the stress vector and the mean wind direction......, and the angle between the stress vector and the vertical gradient of the mean velocity vector. The latter is assumed to be zero by the often applied turbulent-viscosity hypothesis, so that the stress vector can be described through the vertical gradient of velocity. In the atmospheric surface layer, where...... the Coriolis force is negligible, this is supposedly a good approximation. High-resolution large-eddy simulation data show that this is indeed the case even beyond the surface layer. In contrast, through analysis of WindCube lidar measurements supported by sonic measurements, the study shows that it is only...
Entropy Generation in Steady Laminar Boundary Layers with Pressure Gradients
Directory of Open Access Journals (Sweden)
Donald M. McEligot
2014-07-01
Full Text Available In an earlier paper in Entropy [1] we hypothesized that the entropy generation rate is the driving force for boundary layer transition from laminar to turbulent flow. Subsequently, with our colleagues we have examined the prediction of entropy generation during such transitions [2,3]. We found that reasonable predictions for engineering purposes could be obtained for flows with negligible streamwise pressure gradients by adapting the linear combination model of Emmons [4]. A question then arises—will the Emmons approach be useful for boundary layer transition with significant streamwise pressure gradients as by Nolan and Zaki [5]. In our implementation the intermittency is calculated by comparison to skin friction correlations for laminar and turbulent boundary layers and is then applied with comparable correlations for the energy dissipation coefficient (i.e., non-dimensional integral entropy generation rate. In the case of negligible pressure gradients the Blasius theory provides the necessary laminar correlations.
Boundary layer for non-newtonian fluids on curved surfaces
International Nuclear Information System (INIS)
Stenger, N.
1981-04-01
By using the basic equation of fluid motion (conservation of mass and momentum) the boundary layer parameters for a Non-Newtonian, incompressible and laminar fluid flow, has been evaluated. As a test, the flat plate boundary layer is first analized and afterwards, a case with pressure gradient, allowing separation, is studied. In the case of curved surfaces, the problem is first developed in general and afterwards particularized to a circular cylinder. Finally suction and slip in the flow interface are examined. The power law model is used to represent the stress strain relationship in Non-Newtonian flow. By varying the fluid exponent one can then, have an idea of how the Non-Newtonian behavior of the flow influences the parameters of the boundary layer. Two equations, in an appropriate coordinate system have been obtained after an order of magnitude analysis of the terms in the equations of motion is performed. (Author) [pt
Rotor blade boundary layer measurement hardware feasibility demonstration
Clark, D. R.; Lawton, T. D.
1972-01-01
A traverse mechanism which allows the measurement of the three dimensional boundary layers on a helicopter rotor blade has been built and tested on a full scale rotor to full scale conditions producing centrifugal accelerations in excess of 400 g and Mach numbers of 0.6 and above. Boundary layer velocity profiles have been measured over a range of rotor speeds and blade collective pitch angles. A pressure scanning switch and transducer were also tested on the full scale rotor and found to be insensitive to centrifugal effects within the normal main rotor operating range. The demonstration of the capability to measure boundary layer behavior on helicopter rotor blades represents the first step toward obtaining, in the rotating system, data of a quality comparable to that already existing for flows in the fixed system.
Theoretical skin-friction law in a turbulent boundary layer
International Nuclear Information System (INIS)
Cheskidov, A.
2005-01-01
We study transitional and turbulent boundary layers using a turbulent velocity profile equation recently derived from the Navier-Stokes-alpha and Leray-alpha models. From this equation we obtain a theoretical prediction of the skin-friction coefficient in a wide range of Reynolds numbers based on momentum thickness, and deduce the maximal value of c f max =0.0063 for turbulent velocity profiles. A two-parameter family of solutions to the equation matches experimental data in the transitional boundary layers with different free-stream turbulence intensity, while one-parameter family of solutions, obtained using our skin-friction coefficient law, matches experimental data in the turbulent boundary layer for moderately large Reynolds numbers
Vortex Generator Induced Flow in a High Re Boundary Layer
DEFF Research Database (Denmark)
Velte, Clara Marika; Braud, C.; Coudert, S.
2014-01-01
Stereoscopic Particle Image Velocimetry measurements have been conducted in cross-planes behind three different geometries of Vortex Generators (VGs) in a high Reynolds number boundary layer. The VGs have been mounted in a cascade producing counter-rotating vortices and the downstream flow...... development was examined. Three VG geometries were investigated: rectangular, triangular and cambered. The various VG geometries tested are seen to produce different impacts on the boundary layer flow. Helical symmetry of the generated vortices is confirmed for all investigated VG geometries in this high...... Reynolds number boundary layer. From the parameters resulting from this analysis, it is observed at the most upstream measurement position that the rectangular and triangular VGs produce vortices of similar size, strength and velocity induction whilst the cambered VGs produce smaller and weaker vortices...
Vortex Generator Induced Flow in a High Re Boundary Layer
DEFF Research Database (Denmark)
Velte, Clara Marika; Braud, C.; Coudert, S.
2012-01-01
Stereoscopic Particle Image Velocimetry measurements have been conducted in cross-planes behind three different geometries of Vortex Generators (VGs) in a high Reynolds number boundary layer. The VGs have been mounted in a cascade producing counter-rotating vortices and the downstream flow...... development was examined. Three VG geometries were investigated: rectangular, triangular and cambered. The various VG geometries tested are seen to produce different impacts on the boundary layer flow. Helical symmetry of the generated vortices is confirmed for all investigated VG geometries in this high...... Reynolds number boundary layer. From the parameters resulting from this analysis, it is observed at the most upstream measurement position that the rectangular and triangular VGs produce vortices of similar size, strength and velocity induction whilst the cambered VGs produce smaller and weaker vortices...
Defects and boundary layers in non-Euclidean plates
International Nuclear Information System (INIS)
Gemmer, J A; Venkataramani, S C
2012-01-01
We investigate the behaviour of non-Euclidean plates with constant negative Gaussian curvature using the Föppl–von Kármán reduced theory of elasticity. Motivated by recent experimental results, we focus on annuli with a periodic profile. We prove rigorous upper and lower bounds for the elastic energy that scales like the thickness squared. In particular we show that are only two types of global minimizers—deformations that remain flat and saddle shaped deformations with isolated regions of stretching near the edge of the annulus. We also show that there exist local minimizers with a periodic profile that have additional boundary layers near their lines of inflection. These additional boundary layers are a new phenomenon in thin elastic sheets and are necessary to regularize jump discontinuities in the azimuthal curvature across lines of inflection. We rigorously derive scaling laws for the width of these boundary layers as a function of the thickness of the sheet. (paper)
Boundary layer on a flat plate with suction
International Nuclear Information System (INIS)
Favre, A.; Dumas, R.; Verollet, E.
1961-01-01
This research done in wind tunnel concerns the turbulent boundary layer of a porous flat plate with suction. The porous wall is 1 m long and begins 1 m downstream of the leading edge. The Reynolds number based on the boundary layer thickness is of the order of 16.300. The suction rate defined as the ratio of the velocity perpendicular to the wall to the external flow velocity ranges from 0 to 2 per cent. The pressure gradient can be controlled. The mean velocity profiles have been determined for various positions and suction rates by means of total pressure probes together with the intensities of the turbulent velocity fluctuations components, energy spectra and correlations by means of hot wire anemometers, spectral analyser and correlator. The stream lines, the values of the viscous and turbulent shear stresses, of the local wall friction, of the turbulent energy production term, with some information on the dissipation of the energy have been derived from these measurements. For these data the integral of equation of continuity in boundary layer have been drawn. The suction effects on the boundary layer are important. The suction thoroughly alters the mean velocity profiles by increasing the viscous shear stresses near the wall and decreasing them far from the wall, it diminishes the longitudinal and transversal turbulence intensities, the turbulent shear stresses, and the production of energy of turbulence. These effects are much stressed in the inner part of the boundary layer. On the other hand the energy spectra show that the turbulence scale is little modified, the boundary layer thickness being not much diminished by the suction. The suction effects can be appreciated by comparing twice the suction rate to the wall friction coefficient (assumed airtight), quite noticeable as soon as the rate is about unity, they become very important when it reaches ten. (author) [fr
Unsteady turbulent boundary layers in swimming rainbow trout.
Yanase, Kazutaka; Saarenrinne, Pentti
2015-05-01
The boundary layers of rainbow trout, Oncorhynchus mykiss, swimming at 1.02±0.09 L s(-1) (mean±s.d., N=4), were measured by the particle image velocimetry (PIV) technique at a Reynolds number of 4×10(5). The boundary layer profile showed unsteadiness, oscillating above and beneath the classical logarithmic law of the wall with body motion. Across the entire surface regions that were measured, local Reynolds numbers based on momentum thickness, which is the distance that is perpendicular to the fish surface through which the boundary layer momentum flows at free-stream velocity, were greater than the critical value of 320 for the laminar-to-turbulent transition. The skin friction was dampened on the convex surface while the surface was moving towards a free-stream flow and increased on the concave surface while retreating. These observations contradict the result of a previous study using different species swimming by different methods. Boundary layer compression accompanied by an increase in local skin friction was not observed. Thus, the overall results may not support absolutely the Bone-Lighthill boundary layer thinning hypothesis that the undulatory motions of swimming fish cause a large increase in their friction drag because of the compression of the boundary layer. In some cases, marginal flow separation occurred on the convex surface in the relatively anterior surface region, but the separated flow reattached to the fish surface immediately downstream. Therefore, we believe that a severe impact due to induced drag components (i.e. pressure drag) on the swimming performance, an inevitable consequence of flow separation, was avoided. © 2015. Published by The Company of Biologists Ltd.
Receptivity of Hypersonic Boundary Layers over Straight and Flared Cones
Balakumar, Ponnampalam; Kegerise, Michael A.
2010-01-01
The effects of adverse pressure gradients on the receptivity and stability of hypersonic boundary layers were numerically investigated. Simulations were performed for boundary layer flows over a straight cone and two flared cones. The steady and the unsteady flow fields were obtained by solving the two-dimensional Navier-Stokes equations in axi-symmetric coordinates using the 5th order accurate weighted essentially non-oscillatory (WENO) scheme for space discretization and using third-order total-variation-diminishing (TVD) Runge-Kutta scheme for time integration. The mean boundary layer profiles were analyzed using local stability and non-local parabolized stability equations (PSE) methods. After the most amplified disturbances were identified, two-dimensional plane acoustic waves were introduced at the outer boundary of the computational domain and time accurate simulations were performed. The adverse pressure gradient was found to affect the boundary layer stability in two important ways. Firstly, the frequency of the most amplified second-mode disturbance was increased relative to the zero pressure gradient case. Secondly, the amplification of first- and second-mode disturbances was increased. Although an adverse pressure gradient enhances instability wave growth rates, small nose-tip bluntness was found to delay transition due to the low receptivity coefficient and the resulting weak initial amplitude of the instability waves. The computed and measured amplitude-frequency spectrums in all three cases agree very well in terms of frequency and the shape except for the amplitude.
The structure of the stably stratified internal boundary layer in offshore flow over the sea
Garratt, J. R.; Ryan, B. F.
1989-04-01
Observations obtained mainly from a research aircraft are presented of the mean and turbulent structure of the stably stratified internal boundary layer (IBL) over the sea formed by warm air advection from land to sea. The potential temperature and humidity fields reveal the vertical extent of the IBL, for fetches out to several hundred of kilometres, geostrophic winds of 20 25 m s-1, and potential temperature differences between undisturbed continental air and the sea surface of 7 to 17 K. The dependence of IBL depth on these external parameters is discussed in the context of the numerical results of Garratt (1987), and some discrepancies are noted. Wind observations show the development of a low-level wind maximum (wind component normal to the coast) and rotation of the wind to smaller cross-isobar flow angles. Potential temperature (θ) profiles within the IBL reveal quite a different structure to that found in the nocturnal boundary layer (NBL) over land. Over the sea, θ profiles have large positive curvature with vertical gradients increasing monotonically with height; this reflects the dominance of turbulent cooling within the layer. The behaviour is consistent with known behaviour in the NBL over land where curvature becomes negative (vertical gradients of θ decreasing with height) as radiative cooling becomes dominant. Turbulent properties are discussed in terms of non-dimensional quantities, normalised by the surface friction velocity, as functions of normalised height using the IBL depth. Vertical profiles of these and the normalised wavelength of the spectral maximum agree well with known results for the stable boundary layer over land (Caughey et al., 1979).
Electrical sensing of the dynamical structure of the planetary boundary layer
Nicoll, K. A.; Harrison, R. G.; Silva, H. G.; Salgado, R.; Melgâo, M.; Bortoli, D.
2018-04-01
Turbulent and convective processes within the planetary boundary layer are responsible for the transport of moisture, momentum and particulate matter, but are also important in determining the electrical charge transport of the lower atmosphere. This paper presents the first high resolution vertical charge profiles during fair weather conditions, obtained with instrumented radiosonde balloons over Alqueva, Portugal during the summer of 2014. The short intervals (4 h) between balloon flights enabled the diurnal variation in the vertical profile of charge within the boundary layer to be examined in detail, with much smaller charges (up to 20 pC m- 3) observed during stable night time periods than during the day. Following sunrise, the evolution of the charge profile was complex, demonstrating charged ultrafine aerosol, lofted upwards by daytime convection. This produced charge up to 92 pC m- 3 up to 500 m above the surface. The diurnal variation in the integrated column of charge above the site tracked closely with the diurnal variation in near surface charge as derived from a nearby electric field sensor, confirming the importance of the link between surface charge generation processes and aloft. The local aerosol vertical profiles were estimated using backscatter measurements from a collocated ceilometer. These were utilised in a simple model to calculate the charge expected due to vertical conduction current flow in the global electric circuit through aerosol layers. The analysis presented here demonstrates that charge can provide detailed information about boundary layer transport, particularly in regard to the ultrafine aerosol structure, that conventional thermodynamic and ceilometer measurements do not.
An Innovative Flow-Measuring Device: Thermocouple Boundary Layer Rake
Hwang, Danny P.; Fralick, Gustave C.; Martin, Lisa C.; Wrbanek, John D.; Blaha, Charles A.
2001-01-01
An innovative flow-measuring device, a thermocouple boundary layer rake, was developed. The sensor detects the flow by using a thin-film thermocouple (TC) array to measure the temperature difference across a heater strip. The heater and TC arrays are microfabricated on a constant-thickness quartz strut with low heat conductivity. The device can measure the velocity profile well into the boundary layer, about 65 gm from the surface, which is almost four times closer to the surface than has been possible with the previously used total pressure tube.
Leading edge effect in laminar boundary layer excitation by sound
International Nuclear Information System (INIS)
Leehey, P.; Shapiro, P.
1980-01-01
Essentially plane pure tone sound waves were directed downstream over a heavily damped smooth flat plate installed in a low turbulence (0.04%) subsonic wind tunnel. Laminar boundary layer disturbance growth rates were measured with and without sound excitation and compared with numerical results from spatial stability theory. The data indicate that the sound field and Tollmien-Schlichting (T-S) waves coexist with comparable amplitudes when the latter are damped; moreover, the response is linear. Higher early growth rates occur for excitation by sound than by stream turbulence. Theoretical considerations indicate that the boundary layer is receptive to sound excitation primarily at the test plate leading edge. (orig.)
Lower Atmospheric Boundary Layer Experiment (LABLE) Final Campaign Report
Energy Technology Data Exchange (ETDEWEB)
Klein, P [University of Oklahoma - School of Meteorology; Bonin, TA; Newman, JF [National Renewable Energy Laboratory; Turner, DD [National Oceanic and Atmospheric Administration; Chilson, P [University of Oklahoma; Blumberg, WG [University of Oklahoma; Mishra, S; Wainwright, CE; Carney, M [University of Oklahoma - School of Meteorology; Jacobsen, EP [University of Oklahoma; Wharton, S [Lawrence Livermore National Laboratory
2015-11-01
The Lower Atmospheric Boundary Layer Experiment (LABLE) included two measurement campaigns conducted at the Atmospheric Radiation Measurement (ARM) Southern Great Plains site in Oklahoma during 2012 and 2013. LABLE was designed as a multi-phase, low-cost collaboration among the University of Oklahoma, the National Severe Storms Laboratory, Lawrence Livermore National Laboratory, and the ARM program. A unique aspect was the role of graduate students in LABLE. They served as principal investigators and took the lead in designing and conducting experiments using different sampling strategies to best resolve boundary-layer phenomena.
Interactive boundary-layer calculations of a transonic wing flow
Kaups, Kalle; Cebeci, Tuncer; Mehta, Unmeel
1989-01-01
Results obtained from iterative solutions of inviscid and boundary-layer equations are presented and compared with experimental values. The calculated results were obtained with an Euler code and a transonic potential code in order to furnish solutions for the inviscid flow; they were interacted with solutions of two-dimensional boundary-layer equations having a strip-theory approximation. Euler code results are found to be in better agreement with the experimental data than with the full potential code, especially in the presence of shock waves, (with the sole exception of the near-tip region).
Analysis of diabatic flow modification in the internal boundary layer
DEFF Research Database (Denmark)
Floors, Rogier; Gryning, Sven-Erik; Pena Diaz, Alfredo
2011-01-01
Measurements at two meteorological masts in Denmark, Horns Rev in the sea and Høvsøre near the coastline on land, are used to analyze the behaviour of the flow after a smooth-to-rough change in surface conditions. The study shows that the wind profile within the internal boundary layer is control......Measurements at two meteorological masts in Denmark, Horns Rev in the sea and Høvsøre near the coastline on land, are used to analyze the behaviour of the flow after a smooth-to-rough change in surface conditions. The study shows that the wind profile within the internal boundary layer...
Experimental demonstration of the Rayleigh acoustic viscous boundary layer theory.
Castrejón-Pita, J R; Castrejón-Pita, A A; Huelsz, G; Tovar, R
2006-03-01
Amplitude and phase velocity measurements on the laminar oscillatory viscous boundary layer produced by acoustic waves are presented. The measurements were carried out in acoustic standing waves in air with frequencies of 68.5 and 114.5 Hz using laser Doppler anemometry and particle image velocimetry. The results obtained by these two techniques are in good agreement with the predictions made by the Rayleigh viscous boundary layer theory and confirm the existence of a local maximum of the velocity amplitude and its expected location.
Effects of shock on hypersonic boundary layer stability
Pinna, F.; Rambaud, P.
2013-06-01
The design of hypersonic vehicles requires the estimate of the laminar to turbulent transition location for an accurate sizing of the thermal protection system. Linear stability theory is a fast scientific way to study the problem. Recent improvements in computational capabilities allow computing the flow around a full vehicle instead of using only simplified boundary layer equations. In this paper, the effect of the shock is studied on a mean flow provided by steady Computational Fluid Dynamics (CFD) computations and simplified boundary layer calculations.
Conference on Boundary and Interior Layers : Computational and Asymptotic Methods
2015-01-01
This volume offers contributions reflecting a selection of the lectures presented at the international conference BAIL 2014, which was held from 15th to 19th September 2014 at the Charles University in Prague, Czech Republic. These are devoted to the theoretical and/or numerical analysis of problems involving boundary and interior layers and methods for solving these problems numerically. The authors are both mathematicians (pure and applied) and engineers, and bring together a large number of interesting ideas. The wide variety of topics treated in the contributions provides an excellent overview of current research into the theory and numerical solution of problems involving boundary and interior layers. .
Rough-wall turbulent boundary layers with constant skin friction
Sridhar, A.
2017-03-28
A semi-empirical model is presented that describes the development of a fully developed turbulent boundary layer in the presence of surface roughness with length scale ks that varies with streamwise distance x . Interest is centred on flows for which all terms of the von Kármán integral relation, including the ratio of outer velocity to friction velocity U+∞≡U∞/uτ , are streamwise constant. For Rex assumed large, use is made of a simple log-wake model of the local turbulent mean-velocity profile that contains a standard mean-velocity correction for the asymptotic fully rough regime and with assumed constant parameter values. It is then shown that, for a general power-law external velocity variation U∞∼xm , all measures of the boundary-layer thickness must be proportional to x and that the surface sand-grain roughness scale variation must be the linear form ks(x)=αx , where x is the distance from the boundary layer of zero thickness and α is a dimensionless constant. This is shown to give a two-parameter (m,α) family of solutions, for which U+∞ (or equivalently Cf ) and boundary-layer thicknesses can be simply calculated. These correspond to perfectly self-similar boundary-layer growth in the streamwise direction with similarity variable z/(αx) , where z is the wall-normal coordinate. Results from this model over a range of α are discussed for several cases, including the zero-pressure-gradient ( m=0 ) and sink-flow ( m=−1 ) boundary layers. Trends observed in the model are supported by wall-modelled large-eddy simulation of the zero-pressure-gradient case for Rex in the range 108−1010 and for four values of α . Linear streamwise growth of the displacement, momentum and nominal boundary-layer thicknesses is confirmed, while, for each α , the mean-velocity profiles and streamwise turbulent variances are found to collapse reasonably well onto z/(αx) . For given α , calculations of U+∞ obtained from large-eddy simulations are streamwise
Experimental measurements and modelling of the WEGA boundary layer plasma
International Nuclear Information System (INIS)
El Shaer, M.; Ichtchenko, G.
1983-02-01
The boundary layer of the WEGA Tokamak has been investigated by using specific diagnostics: movable 4 mm microwave interferometer, several types of movable and fixed probes, Katsumata probe, and multigrid electrostatic analyzer. During the RF heating at the lower hybrid frequency, some modifications in the parameters of the boundary layer are observed which are interpreted by the ponderomotive force effects. A comparison between the measured reflection coefficients of the grill waveguides and their predicted values by a coupling theory (not taking into account the real conditions facing the Grill) is presented. A diffusion model was also made to describe this particular region and to fit the experimental results
Fast Fermi acceleration in the plasma sheet boundary layer
International Nuclear Information System (INIS)
Wu, C.S.; Lui, A.T.Y.
1989-01-01
A longstanding question in the field of magnetospheric physics is the source of the energetic particles which are commonly observed along the plasma sheet boundary layer (PSBL). Several models have been suggested for the acceleration of these particles. We suggest a means by which the fast Fermi acceleration mechanism [Wu, 1984] can accelerate electrons at the plasma sheet and perhaps account for some of the observations. We propose the following: A localized hydromagnetic disturbance propagating through the tail lobe region impinges upon the PSBL deforming it and displacing it in towards the central plasma sheet. The boundary layer can then act like a moving magnetic mirror. If the disturbance is propagating nearly perpendicular to the layer then its velocity projected parallel to the layer (and the magnetic field) can be very large resulting in significant acceleration of reflected particles. copyright American Geophysical Union 1989
Energy Technology Data Exchange (ETDEWEB)
Vijayakumar, Ganesh [National Renewable Energy Lab. (NREL), Golden, CO (United States); Pennsylvania State Univ., University Park, PA (United States); Brasseur, James [Pennsylvania State Univ., University Park, PA (United States); Univ. of Colorado, Boulder, CO (United States); Lavely, Adam; Jayaraman, Balaji; Craven, Brent
2016-01-04
We describe the response of the NREL 5 MW wind turbine blade boundary layer to the passage of atmospheric turbulence using blade-boundary-layer-resolved computational fluid dynamics with hybrid URANS-LES modeling.
Modeling Turbulence Generation in the Atmospheric Surface and Boundary Layers
2015-10-01
hydrostatic equation: dP dz = −ρa g −→ ∫ ZI 0 ρa dz = − 1 g ∫ dP = + 1 g [P (0)− P (ZI)]. (6.14) The pressure at the surface is... surface pressure is estimated, we can compute a vertical pressure profile using the hydrostatic equation and a selected temperature profile based on dP... surface -layer atmosphere. By surface layer what is intended is a layer of foliage plus the surface itself. That is, a flat ground surface that
Plasma-based actuators for turbulent boundary layer control in transonic flow
Budovsky, A. D.; Polivanov, P. A.; Vishnyakov, O. I.; Sidorenko, A. A.
2017-10-01
The study is devoted to development of methods for active control of flow structure typical for the aircraft wings in transonic flow with turbulent boundary layer. The control strategy accepted in the study was based on using of the effects of plasma discharges interaction with miniature geometrical obstacles of various shapes. The conceptions were studied computationally using 3D RANS, URANS approaches. The results of the computations have shown that energy deposition can significantly change the flow pattern over the obstacles increasing their influence on the flow in boundary layer region. Namely, one of the most interesting and promising data were obtained for actuators basing on combination of vertical wedge with asymmetrical plasma discharge. The wedge considered is aligned with the local streamlines and protruding in the flow by 0.4-0.8 of local boundary layer thickness. The actuator produces negligible distortion of the flow at the absence of energy deposition. Energy deposition along the one side of the wedge results in longitudinal vortex formation in the wake of the actuator providing momentum exchange in the boundary layer. The actuator was manufactured and tested in wind tunnel experiments at Mach number 1.5 using the model of flat plate. The experimental data obtained by PIV proved the availability of the actuator.
Extreme Vertical Gusts in the Atmospheric Boundary Layer
2015-07-01
Density Currents Sea breezes, thunderstorm outflows and cold fronts are all physically similar, but differ in size22. They cause lift as the cold air...the consideration of sea breezes and other density currents. Moreover, some thunderstorm outflows run far ahead of their parent storm and may even...substantially opaque to these frequencies means most of the infra- red radiation from water molecules to space is coming from the top of the cloud. On
Vertical pressure gradient and particle motions in wave boundary layers
DEFF Research Database (Denmark)
Jensen, Karsten Lindegård
. The experiment is conducted in a oscillating water tunnel, for both smooth bed and rough bed. The particle motion is determined by utilizing particle tracking base on a video recording of the particle motion in the flow. In the oscillatory flow, in contrast to steady current, the particle motion is a function...
Temperature boundary layer profiles in turbulent Rayleigh-Benard convection
Ching, Emily S. C.; Emran, Mohammad S.; Horn, Susanne; Shishkina, Olga
2017-11-01
Classical boundary-layer theory for steady flows cannot adequately describe the boundary layer profiles in turbulent Rayleigh-Benard convection. We have developed a thermal boundary layer equation which takes into account fluctuations in terms of an eddy thermal diffusivity. Based on Prandtl's mixing length ideas, we relate the eddy thermal diffusivity to the stream function. With this proposed relation, we can solve the thermal boundary layer equation and obtain a closed-form expression for the dimensionless mean temperature profile in terms of two independent parameters: θ(ξ) =1/b∫0b ξ [ 1 +3a3/b3(η - arctan(η)) ] - c dη , where ξ is the similarity variable and the parameters a, b, and c are related by the condition θ(∞) = 1 . With a proper choice of the parameters, our predictions of the temperature profile are in excellent agreement with the results of our direct numerical simulations for a wide range of Prandtl numbers (Pr), from Pr=0.01 to Pr=2547.9. OS, ME and SH acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) under Grants Sh405/4-2 (Heisenberg fellowship), Sh405/3-2 and Ho 5890/1-1, respectively.
Hundred years of the boundary layer – Some aspects
Indian Academy of Sciences (India)
2005-08-02
Aug 2, 2005 ... from the wall and separation of the boundary layer, which in turn enables proper ... design, which performed better and consumed only one-third the power .... turbulent flow and also to free shear flows like wakes and jets.
Conserved variable analysis of the marine boundary layer and air
Indian Academy of Sciences (India)
The present study is based on the observed features of the MBL (Marine Boundary Layer) during the Bay of Bengal and Monsoon Experiment (BOBMEX) - Pilot phase. Conserved Variable Analysis (CVA) of the conserved variables such as potential temperature, virtual potential temperature, equivalent potential temperature ...
Exact solution of nonsteady thermal boundary layer equation
International Nuclear Information System (INIS)
Dorfman, A.S.
1995-01-01
There are only a few exact solutions of the thermal boundary layer equation. Most of them are derived for a specific surface temperature distribution. The first exact solution of the steady-state boundary layer equation was given for a plate with constant surface temperature and free-stream velocity. The same problem for a plate with polynomial surface temperature distribution was solved by Chapmen and Rubesin. Levy gave the exact solution for the case of a power law distribution of both surface temperature and free-stream velocity. The exact solution of the steady-state boundary layer equation for an arbitrary surface temperature and a power law free-stream velocity distribution was given by the author in two forms: of series and of the integral with an influence function of unheated zone. A similar solution of the nonsteady thermal boundary layer equation for an arbitrary surface temperature and a power law free-stream velocity distribution is presented here. In this case, the coefficients of series depend on time, and in the limit t → ∞ they become the constant coefficients of a similar solution published before. This solution, unlike the one presented here, does not satisfy the initial conditions at t = 0, and, hence, can be used only in time after the beginning of the process. The solution in the form of a series becomes a closed-form exact solution for polynomial surface temperature and a power law free-stream velocity distribution. 7 refs., 2 figs
Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain
Directory of Open Access Journals (Sweden)
Stefano Serafin
2018-03-01
Full Text Available The exchange of heat, momentum, and mass in the atmosphere over mountainous terrain is controlled by synoptic-scale dynamics, thermally driven mesoscale circulations, and turbulence. This article reviews the key challenges relevant to the understanding of exchange processes in the mountain boundary layer and outlines possible research priorities for the future. The review describes the limitations of the experimental study of turbulent exchange over complex terrain, the impact of slope and valley breezes on the structure of the convective boundary layer, and the role of intermittent mixing and wave–turbulence interaction in the stable boundary layer. The interplay between exchange processes at different spatial scales is discussed in depth, emphasizing the role of elevated and ground-based stable layers in controlling multi-scale interactions in the atmosphere over and near mountains. Implications of the current understanding of exchange processes over mountains towards the improvement of numerical weather prediction and climate models are discussed, considering in particular the representation of surface boundary conditions, the parameterization of sub-grid-scale exchange, and the development of stochastic perturbation schemes.
A mathematical model of turbulence for turbulent boundary layers
International Nuclear Information System (INIS)
Pereira Filho, H.D.V.
1977-01-01
Equations to the so called Reynolds stress-tensor (kinetic turbulent energy) and dissipation rate are developed and a turbulence flux approximation used. Our ideia here is to use those equations in order to develop an economical and fast numeircal procedure for computation of turbulent boundary layer. (author) [pt
Body surface adaptations to boundary-layer dynamics
Videler, J.J.
1995-01-01
Evolutionary processes have adapted nektonic animals to interact efficiently with the water that surrounds them. Not all these adaptations serve the same purpose. This paper concentrates on reduction of drag due to friction in the boundary layer close to the body surface. Mucus, compliant skins,
Wave boundary layer over a stone-covered bed
DEFF Research Database (Denmark)
Dixen, Martin; Hatipoglu, Figen; Sumer, B. Mutlu
2008-01-01
This paper summarizes the results of an experimental investigation on wave boundary layers over a bed with large roughness, simulating stone/rock/armour block cover on the sea bottom. The roughness elements used in the experiments were stones the size of 1.4cm and 3.85cm in one group of experiments...
Stability of hypersonic boundary-layer flows with chemistry
Reed, Helen L.; Stuckert, Gregory K.; Haynes, Timothy S.
1993-01-01
The effects of nonequilibrium chemistry and three dimensionality on the stability characteristics of hypersonic flows are discussed. In two-dimensional (2-D) and axisymmetric flows, the inclusion of chemistry causes a shift of the second mode of Mack to lower frequencies. This is found to be due to the increase in size of the region of relative supersonic flow because of the lower speeds of sound in the relatively cooler boundary layers. Although this shift in frequency is present in both the equilibrium and nonequilibrium air results, the equilibrium approximation predicts modes which are not observed in the nonequilibrium calculations (for the flight conditions considered). These modes are superpositions of incoming and outgoing unstable disturbances which travel supersonically relative to the boundary-layer edge velocity. Such solutions are possible because of the finite shock stand-off distance. Their corresponding wall-normal profiles exhibit an oscillatory behavior in the inviscid region between the boundary-layer edge and the bow shock. For the examination of three-dimensional (3-D) effects, a rotating cone is used as a model of a swept wing. An increase of stagnation temperature is found to be only slightly stabilizing. The correlation of transition location (N = 9) with parameters describing the crossflow profile is discussed. Transition location does not correlate with the traditional crossflow Reynolds number. A new parameter that appears to correlate for boundary-layer flow was found. A verification with experiments on a yawed cone is provided.
Towards Adaptive Grids for Atmospheric Boundary-Layer Simulations
van Hooft, J. Antoon; Popinet, Stéphane; van Heerwaarden, Chiel C.; van der Linden, Steven J. A.; de Roode, Stephan R.; van de Wiel, Bas J. H.
2018-02-01
We present a proof-of-concept for the adaptive mesh refinement method applied to atmospheric boundary-layer simulations. Such a method may form an attractive alternative to static grids for studies on atmospheric flows that have a high degree of scale separation in space and/or time. Examples include the diurnal cycle and a convective boundary layer capped by a strong inversion. For such cases, large-eddy simulations using regular grids often have to rely on a subgrid-scale closure for the most challenging regions in the spatial and/or temporal domain. Here we analyze a flow configuration that describes the growth and subsequent decay of a convective boundary layer using direct numerical simulation (DNS). We validate the obtained results and benchmark the performance of the adaptive solver against two runs using fixed regular grids. It appears that the adaptive-mesh algorithm is able to coarsen and refine the grid dynamically whilst maintaining an accurate solution. In particular, during the initial growth of the convective boundary layer a high resolution is required compared to the subsequent stage of decaying turbulence. More specifically, the number of grid cells varies by two orders of magnitude over the course of the simulation. For this specific DNS case, the adaptive solver was not yet more efficient than the more traditional solver that is dedicated to these types of flows. However, the overall analysis shows that the method has a clear potential for numerical investigations of the most challenging atmospheric cases.
A parametric study of adverse pressure gradient turbulent boundary layers
International Nuclear Information System (INIS)
Monty, J.P.; Harun, Z.; Marusic, I.
2011-01-01
There are many open questions regarding the behaviour of turbulent boundary layers subjected to pressure gradients and this is confounded by the large parameter space that may affect these flows. While there have been many valuable investigations conducted within this parameter space, there are still insufficient data to attempt to reduce this parameter space. Here, we consider a parametric study of adverse pressure gradient turbulent boundary layers where we restrict our attention to the pressure gradient parameter, β, the Reynolds number and the acceleration parameter, K. The statistics analyzed are limited to the streamwise fluctuating velocity. The data show that the mean velocity profile in strong pressure gradient boundary layers does not conform to the classical logarithmic law. Moreover, there appears to be no measurable logarithmic region in these cases. It is also found that the large-scale motions scaling with outer variables are energised by the pressure gradient. These increasingly strong large-scale motions are found to be the dominant contributor to the increase in turbulence intensity (scaled with friction velocity) with increasing pressure gradient across the boundary layer.
Chemical boundary layers in CVD II. Reversible reactions
Croon, de M.H.J.M.; Giling, L.J.
1990-01-01
In addition to irreversible reactions, which were treated in part I, reversible reactions in the gas phase have beenstudied using the concept of the chemical boundary layer. The analysis is given for the situations in which either the forwardor the back reaction is dominant. Two conceptual models
The use of a wave boundary layer model in SWAN
DEFF Research Database (Denmark)
Du, Jianting; Bolaños, Rodolfo; Larsén, Xiaoli Guo
2017-01-01
A Wave Boundary Layer Model (WBLM) is implemented in the third-generation ocean wave model SWAN to improve the wind-input source function under idealized, fetch-limited condition. Accordingly, the white capping dissipation parameters are re-calibrated to fit the new wind-input source function...
Atmospheric boundary layer evening transitions over West Texas
A systemic analysis of the atmospheric boundary layer behavior during some evening transitions over West Texas was done using the data from an extensive array of instruments which included small and large aperture scintillometers, net radiometers, and meteorological stations. The analysis also comp...
Hair receptor sensitivity to changes in laminar boundary layer shape
International Nuclear Information System (INIS)
Dickinson, B T
2010-01-01
Biologists have shown that bat wings contain distributed arrays of flow-sensitive hair receptors. The hair receptors are hypothesized to feedback information on airflows over the bat wing for enhanced stability or maneuverability during flight. Here, we study the geometric specialization of hair-like structures for the detection of changes in boundary layer velocity profiles (shapes). A quasi-steady model that relates the flow velocity profile incident on the longitudinal axis of a hair to the resultant moment and shear force at the hair base is developed. The hair length relative to the boundary layer momentum thickness that maximizes the resultant moment and shear-force sensitivity to changes in boundary layer shape is determined. The sensitivity of the resultant moment and shear force is shown to be highly dependent on hair length. Hairs that linearly taper to a point are shown to provide greater output sensitivity than hairs of uniform cross-section. On an order of magnitude basis, the computed optimal hair lengths are in agreement with the range of hair receptor lengths measured on individual bat species. These results support the hypothesis that bats use hair receptors for detecting changes in boundary layer shape and provide geometric guidelines for artificial hair sensor design and application.
Fifty Years of Boundary-Layer Theory and Experiment
Dryden, Hugh L.
1955-01-01
The year 1954 marked the 50th anniversary of the Prandtl boundary-layer theory from which we may date the beginning of man's understanding of the dynamics of real fluids. A backward look at this aspect of the history of the last 50 years may be instructive. This paper (1) attempts to compress the events of those 50 years into a few thousand words, to tell in this brief space the interesting story of the development of a new concept, its slow acceptance and growth, its spread from group to group within its country of origin, and its diffusion to other countries of the world. The original brief paper of Prandtl (2) was presented at the Third International Mathematical Congress at Heidelberg in 1904 and published in the following year. It was an attempt to explain the d'Alembert paradox, namely, that the neglect of the small friction of air in the theory resulted in the prediction of zero resistance to motion. Prandtl set himself the task of computing the motion of a fluid of small friction, so small that its effect could be neglected everywhere except where large velocity differences were present or a cumulative effect of friction occurred This led to the concept of boundary layer, or transition layer, near the wall of a body immersed in a fluid stream in which the velocity rises from zero to the free-stream value. It is interesting that Prandtl used the term Grenzsehicht (boundary layer) only once and the term Ubergangsschicht (transition layer) seven times in the brief article. Later writers also used Reibungsschicht (friction layer), but most writers today use Grenzschicht (boundary layer).
Pressure Fluctuations Induced by a Hypersonic Turbulent Boundary Layer
Duan, Lian; Choudhari, Meelan M.; Zhang, Chao
2016-01-01
Direct numerical simulations (DNS) are used to examine the pressure fluctuations generated by a spatially-developed Mach 5.86 turbulent boundary layer. The unsteady pressure field is analyzed at multiple wall-normal locations, including those at the wall, within the boundary layer (including inner layer, the log layer, and the outer layer), and in the free stream. The statistical and structural variations of pressure fluctuations as a function of wall-normal distance are highlighted. Computational predictions for mean velocity pro les and surface pressure spectrum are in good agreement with experimental measurements, providing a first ever comparison of this type at hypersonic Mach numbers. The simulation shows that the dominant frequency of boundary-layer-induced pressure fluctuations shifts to lower frequencies as the location of interest moves away from the wall. The pressure wave propagates with a speed nearly equal to the local mean velocity within the boundary layer (except in the immediate vicinity of the wall) while the propagation speed deviates from the Taylor's hypothesis in the free stream. Compared with the surface pressure fluctuations, which are primarily vortical, the acoustic pressure fluctuations in the free stream exhibit a significantly lower dominant frequency, a greater spatial extent, and a smaller bulk propagation speed. The freestream pressure structures are found to have similar Lagrangian time and spatial scales as the acoustic sources near the wall. As the Mach number increases, the freestream acoustic fluctuations exhibit increased radiation intensity, enhanced energy content at high frequencies, shallower orientation of wave fronts with respect to the flow direction, and larger propagation velocity.
Air Entrainment and Surface Ripples in a Turbulent Ship Hull Boundary Layer
Masnadi, Naeem; Erinin, Martin; Duncan, James H.
2017-11-01
The air entrainment and free-surface fluctuations caused by the interaction of a free surface and the turbulent boundary layer of a vertical surface-piercing plate is studied experimentally. In this experiment, a meter-wide stainless steel belt travels horizontally in a loop around two rollers with vertically oriented axes. This belt device is mounted inside a large water tank with the water level set just below the top edge of the belt. The belt, rollers, and supporting frame are contained within a sheet metal box to keep the device dry except for one 6-meter-long straight test section. The belt is accelerated suddenly from rest until reaching constant speed in order to create a temporally evolving boundary layer analogous to the spatially evolving boundary layer that would exist along a surface-piercing towed flat plate. Surface ripples are measured using a cinematic laser-induced fluorescence technique with the laser sheet oriented parallel or normal to the belt surface. Air entrainment events and bubble motions are recorded from underneath the water surface using a stereo imaging system. Measurements of small bubbles, that tend to stay submerged for a longer time, are planned via a high-speed digital in-line holographic system. The support of the Office of Naval Research is gratefully acknowledged.
Self-consistent model of the low-latitude boundary layer
International Nuclear Information System (INIS)
Phan, T.D.; Sonnerup, B.U.Oe.; Lotko, W.
1989-01-01
A simple two-dimensional, steady state, viscous model of the dawnside and duskside low-latitude boundary layer (LLBL) has been developed. It incorporates coupling to the ionosphere via field-aligned currents and associated field-aligned potential drops, governed by a simple conductance law, and it describes boundary layer currents, magnetic fields, and plasma flow in a self-consistent manner. The magnetic field induced by these currents leads to two effects: (1) a diamagnetic depression of the magnetic field in the equatorial region and (2) bending of the field lines into parabolas in the xz plane with their vertices in the equatorial plane, at z = 0, and pointing in the flow direction, i.e., tailward. Both effects are strongest at the magnetopause edge of the boundary layer and vanish at the magnetospheric edge. The diamagnetic depression corresponds to an excess of plasma pressure in the equatorial boundary layer near the magnetopause. The boundary layer structure is governed by a fourth-order, nonlinear, ordinary differential equation in which one nondimensional parameter, the Hartmann number M, appears. A second parameter, introduced via the boundary conditions, is a nondimensional flow velocity v 0 * at the magnetopause. Numerical results from the model are presented and the possible use of observations to determine the model parameters is discussed. The main new contribution of the study is to provide a better description of the field and plasma configuration in the LLBL itself and to clarify in quantitative terms the circumstances in which induced magnetic fields become important
International Nuclear Information System (INIS)
Fieg, G.
1975-02-01
This work deals with the hydrodynamics of laminar and turbulent free convection boundary layers on a vertical flat isothermal plate. Both for the laminar and turbulent region there is a good agreement with previous experimental and theoretical investigations. From these experiments one can draw important conclusions to the growth of instabilities in the transition region which lead to turbulence. (orig.) [de
Convective Cold Pool Structure and Boundary Layer Recovery in DYNAMO
Savarin, A.; Chen, S. S.; Kerns, B. W.; Lee, C.; Jorgensen, D. P.
2012-12-01
One of the key factors controlling convective cloud systems in the Madden-Julian Oscillation (MJO) over the tropical Indian Ocean is the property of the atmospheric boundary layer. Convective downdrafts and precipitation from the cloud systems produce cold pools in the boundary layer, which can inhibit subsequent development of convection. The recovery time is the time it takes for the boundary layer to return to pre convective conditions. It may affect the variability of the convection on various time scales during the initiation of MJO. This study examines the convective cold pool structure and boundary layer recovery using the NOAA WP-3D aircraft observations, include the flight-level, Doppler radar, and GPS dropsonde data, collected during the Dynamics of MJO (DYNAMO) field campaign from November-December 2011. The depth and strength of convective cold pools are defined by the negative buoyancy, which can be computed from the dropsonde data. Convective downdraft can be affected by environmental water vapor due to entrainment. Mid-level dry air observed during the convectively suppressed phase of MJO seems to enhance convective downdraft, making the cold pools stronger and deeper. Recovery of the cold pools in the boundary layer is determined by the strength and depth of the cold pools and also the air-sea heat and moisture fluxes. Given that the water vapor and surface winds are distinct for the convectively active and suppressed phases of MJO over the Indian Ocean, the aircraft data are stratified by the two different large-scale regimes of MJO. Preliminary results show that the strength and depth of the cold pools are inversely correlated with the surrounding mid-level moisture. During the convectively suppressed phase, the recovery time is ~5-20 hours in relative weak wind condition with small air-sea fluxes. The recovery time is generally less than 6 hours during the active phase of MJO with moist mid-levels and stronger surface wind and air-sea fluxes.
Mean flow structure of non-equilibrium boundary layers with adverse ...
Indian Academy of Sciences (India)
According to them, an equilibrium boundary layer might exist if the pressure ... of adverse pressure gradient on the turbulent boundary layer at the flat plate for ..... of a constant-pressure turbulent layer to the sudden application of an sudden.
New Theories on Boundary Layer Transition and Turbulence Formation
Directory of Open Access Journals (Sweden)
Chaoqun Liu
2012-01-01
Full Text Available This paper is a short review of our recent DNS work on physics of late boundary layer transition and turbulence. Based on our DNS observation, we propose a new theory on boundary layer transition, which has five steps, that is, receptivity, linear instability, large vortex structure formation, small length scale generation, loss of symmetry and randomization to turbulence. For turbulence generation and sustenance, the classical theory, described with Richardson's energy cascade and Kolmogorov length scale, is not observed by our DNS. We proposed a new theory on turbulence generation that all small length scales are generated by “shear layer instability” through multiple level ejections and sweeps and consequent multiple level positive and negative spikes, but not by “vortex breakdown.” We believe “shear layer instability” is the “mother of turbulence.” The energy transferring from large vortices to small vortices is carried out by multiple level sweeps, but does not follow Kolmogorov's theory that large vortices pass energy to small ones through vortex stretch and breakdown. The loss of symmetry starts from the second level ring cycle in the middle of the flow field and spreads to the bottom of the boundary layer and then the whole flow field.
Linear segmentation algorithm for detecting layer boundary with lidar.
Mao, Feiyue; Gong, Wei; Logan, Timothy
2013-11-04
The automatic detection of aerosol- and cloud-layer boundary (base and top) is important in atmospheric lidar data processing, because the boundary information is not only useful for environment and climate studies, but can also be used as input for further data processing. Previous methods have demonstrated limitations in defining the base and top, window-size setting, and have neglected the in-layer attenuation. To overcome these limitations, we present a new layer detection scheme for up-looking lidars based on linear segmentation with a reasonable threshold setting, boundary selecting, and false positive removing strategies. Preliminary results from both real and simulated data show that this algorithm cannot only detect the layer-base as accurate as the simple multi-scale method, but can also detect the layer-top more accurately than that of the simple multi-scale method. Our algorithm can be directly applied to uncalibrated data without requiring any additional measurements or window size selections.
Hasel, M.; Kottmeier, Ch.; Corsmeier, U.; Wieser, A.
2005-03-01
Using the new high-frequency measurement equipment of the research aircraft DO 128, which is described in detail, turbulent vertical fluxes of ozone and nitric oxide have been calculated from data sampled during the ESCOMPTE program in the south of France. Based on airborne turbulence measurements, radiosonde data and surface energy balance measurements, the convective boundary layer (CBL) is examined under two different aspects. The analysis covers boundary-layer convection with respect to (i) the control of CBL depth by surface heating and synoptic scale influences, and (ii) the structure of convective plumes and their vertical transport of ozone and nitric oxides. The orographic structure of the terrain causes significant differences between planetary boundary layer (PBL) heights, which are found to exceed those of terrain height variations on average. A comparison of boundary-layer flux profiles as well as mean quantities over flat and complex terrain and also under different pollution situations and weather conditions shows relationships between vertical gradients and corresponding turbulent fluxes. Generally, NO x transports are directed upward independent of the terrain, since primary emission sources are located near the ground. For ozone, negative fluxes are common in the lower CBL in accordance with the deposition of O 3 at the surface. The detailed structure of thermals, which largely carry out vertical transports in the boundary layer, are examined with a conditional sampling technique. Updrafts mostly contain warm, moist and NO x loaded air, while the ozone transport by thermals alternates with the background ozone gradient. Evidence for handover processes of trace gases to the free atmosphere can be found in the case of existing gradients across the boundary-layer top. An analysis of the size of eddies suggests the possibility of some influence of the heterogeneous terrain in mountainous area on the length scales of eddies.
Adeniyan, A.,
2013-01-01
The numerical investigation of a stagnation point boundary layer flow , mass and heat transfer of a steady two dimensional , incompressible , viscous electrically conducting, chemically reacting laminar fluid over a vertical convectively heated , electrically neutral flat plate exposed to a transverse uniform magnetic field has been carried out to examine the influence of the simultaneous presence of the effects of a convective boundary condition, chemical reaction, heat transfer and suctio...
Examination of uniform momentum zones in hypersonic turbulent boundary layers
Williams, Owen; Helm, Clara; Martin, Pino
2017-11-01
The presence of uniform momentum zones (UMZs) separated by regions of high shear is now well-established in incompressible flows, with the mean number of such zones increasing in a log-linear fashion with Reynolds number. While known to be present in supersonic and hypersonic boundary layers, the properties of these UMZs and the appropriate Reynolds number for comparison with incompressible results have not previously been investigated. A large, previously published DNS database of hypersonic boundary layers is used in this investigation, with Mach numbers up to 12 and wall temperatures from cold to adiabatic, resulting in a wide range of outer layer Reynolds numbers. UMZs are examined using a range of parameters in both conventional inner and semi-local scalings, and Reynolds number trends examined.
Provenance of the K/T boundary layers
International Nuclear Information System (INIS)
Hildebrand, A.R.; Boynton, W.V.
1988-01-01
An array of chemical, physical and isotopic evidence indicates that an impact into oceanic crust terminated the Cretaceous Period. Approximately 1500 cu km of debris, dispersed by the impact fireball, fell out globally in marine and nonmarine environments producing a 2 to 4 mm thick layer (fireball layer). In North American locales, the fireball layer overlies a 15 to 25 mm thick layer of similar but distinct composition. This 15 to 25 mm layer (ejecta layer) may represent approximately 1000 cu km of lower energy ejecta from a nearby impact site. Isotopic and chemical evidence supports a mantle provenance for the bulk of the layers. The extraordinary REE pattern of the boundary clays was modelled as a mixture of oceanic crust, mantle, and approximately 10 percent continental material. The results are presented. If the siderophiles of the ejecta layer were derived solely from the mantle, a test may be available to see if the siderophile element anomaly of the fireball layer had an extraterrestrial origin. Radiogenic Os-187 is depleted in the mantle relative to an undifferentiated chondritic source. Os-187/Os-186 ratios of 1.049 and 1.108 were calculated for the ejecta and fireball layers, respectively
Determination of boundary layer top on the basis of the characteristics of atmospheric particles
Liu, Boming; Ma, Yingying; Gong, Wei; Zhang, Ming; Yang, Jian
2018-04-01
The planetary boundary layer (PBL) is the lowest layer of the atmosphere that can be directly influenced with the Earth's surface. This layer can also respond to surface forcing. The determination of the PBL is significant to environmental and climate research. PBL can also serve as an input parameter for further data processing with atmospheric models. Traditional detection algorithms are susceptible to errors associated with the vertical distribution of aerosol concentrations. To overcome this limitation, a maximum difference search (MDS) algorithm was proposed to calculate the top of the boundary layer based on differences in particle characteristics. The top positions of the PBL from MDS algorithm under different convection states were compared with those from conventional methods. Experimental results demonstrated that the MDS method can determine the top of the boundary layer precisely. The proposed algorithm can also be used to calculate the top of the PBL accurately under weak convection conditions where the traditional methods cannot be applied. Finally, experimental data from June 2015 to December 2015 were analysed to verify the reliability of the MDS algorithm. The correlation coefficients R2 (RMSE) between the results of MDS algorithm and radiosonde measurements were 0.53 (115 m), 0.79 (141 m) and 0.96 (43 m) under weak, moderate and strong convections, respectively. These findings indicated that the proposed method possessed a good feasibility and stability.
The height of the atmospheric boundary layer during unstable conditions
Energy Technology Data Exchange (ETDEWEB)
Gryning, S.E.
2005-11-01
The height of the convective atmospheric boundary layer, also called the mixed-layer, is one of the fundamental parameters that characterise the structure of the atmosphere near the ground. It has many theoretical and practical applications such as the prediction of air pollution concentrations, surface temperature and the scaling of turbulence. However, as pointed out by Builtjes (2001) in a review paper on Major Twentieth Century Milestones in Air Pollution Modelling and Its Application, the weakest point in meteorology data is still the determination of the height of the mixed-layer, the so-called mixing height. A simple applied model for the height of the mixed-layer over homogeneous terrain is suggested in chapter 2. It is based on a parameterised budget for the turbulent kinetic energy. In the model basically three terms - the spin-up term and the production of mechanical and convective turbulent kinetic energy - control the growth of the mixed layer. The interplay between the three terms is related to the meteorological conditions and the height of the mixed layer. A stable layer, the so-called entrainment zone, which is confined between the mixed layer and the free air above, caps the mixed layer. A parameterisation of the depth of the entrainment zone is also suggested, and used to devise a combined model for the height of the mixed layer and the entrainment zone. Another important aspect of the mixed layer development exists in coastal areas where an internal boundary layer forms downwind from the coastline. A model for the growth of the internal boundary layer is developed in analogy with the model for mixed layer development over homogeneous terrain. The strength of this model is that it can operate on a very fine spatial resolution with minor computer resources. Chapter 3 deals with the validation of the models. It is based in parts on data from the literature, and on own measurements. For the validation of the formation of the internal boundary layer
Planetary boundary layer and circulation dynamics at Gale Crater, Mars
Fonseca, Ricardo M.; Zorzano-Mier, María-Paz; Martín-Torres, Javier
2018-03-01
The Mars implementation of the Planet Weather Research and Forecasting (PlanetWRF) model, MarsWRF, is used here to simulate the atmospheric conditions at Gale Crater for different seasons during a period coincident with the Curiosity rover operations. The model is first evaluated with the existing single-point observations from the Rover Environmental Monitoring Station (REMS), and is then used to provide a larger scale interpretation of these unique measurements as well as to give complementary information where there are gaps in the measurements. The variability of the planetary boundary layer depth may be a driver of the changes in the local dust and trace gas content within the crater. Our results show that the average time when the PBL height is deeper than the crater rim increases and decreases with the same rate and pattern as Curiosity's observations of the line-of-sight of dust within the crater and that the season when maximal (minimal) mixing is produced is Ls 225°-315° (Ls 90°-110°). Thus the diurnal and seasonal variability of the PBL depth seems to be the driver of the changes in the local dust content within the crater. A comparison with the available methane measurements suggests that changes in the PBL depth may also be one of the factors that accounts for the observed variability, with the model results pointing towards a local source to the north of the MSL site. The interaction between regional and local flows at Gale Crater is also investigated assuming that the meridional wind, the dynamically important component of the horizontal wind at Gale, anomalies with respect to the daily mean can be approximated by a sinusoidal function as they typically oscillate between positive (south to north) and negative (north to south) values that correspond to upslope/downslope or downslope/upslope regimes along the crater rim and Mount Sharp slopes and the dichotomy boundary. The smallest magnitudes are found in the northern crater floor in a region that
International Nuclear Information System (INIS)
Molnary, Leslie de
2002-01-01
A two layer bulk model is used to simulate numerically the time and spatial evolution of concentration of radionuclides in the Planetary Boundary Layer (PBL) for convective and stable conditions. In this model, the closure hypothesis are based on the integrated version of the Turbulent Kinetics Energy equation. This type of model was adopted here because it is numerically simple to be applied operationally in routine and emergency support systems of atmospheric releases at nuclear power plants, and the hypothesis of the efficiency of the vertical mixing seems to be physically reasonable to simulate PBL evolution for high wind conditions and stable conditions in subtropical latitudes regions. In order to validate the model, numerical simulations were carried out with initial and boundary conditions based on vertical profiles of temperatures and horizontal wind speed and direction obtained from tethered balloon soundings, synoptic charts at 850 hPa and surface observations. Comparisons between a 24 hour long numerical simulation and observations indicate that the model is capable of reproduce the diurnal evolution of temperature and horizontal wind during the convective regime. During stable conditions, the slab model was able to simulate the intensity of the surface inversion as a difference between the mixed layer and the surface temperature. The simulated mixed layer height matches with observations during the convective and stable regime. (author)
Lidar observations of marine boundary-layer winds and heights: a preliminary study
DEFF Research Database (Denmark)
Peña, Alfredo; Gryning, Sven-Erik; Floors, Rogier Ralph
2015-01-01
the highest data availability (among the three sites) and a very good agreement with the observations of wind speed and direction from cup anemometers and vanes from the platform's tower. The wind lidar was also able to perform measurements under a winter storm where 10-s gusts were observed above 60 m s 1......Here we describe a nearly 1-yr meteorological campaign, which was carried out at the FINO3 marine research platform on the German North Sea, where a pulsed wind lidar and a ceilometer were installed besides the platform's 105-m tower and measured winds and the aerosol backscatter in the entire...... marine atmospheric boundary layer. The campaign was the last phase of a research project, in which the vertical wind profile in the atmospheric boundary layer was firstly investigated on a coastal and a semi-urban site. At FINO3 the wind lidar, which measures the wind speed up to 2000 m, shows...
Zak, J. Allen; Rodgers, William G., Jr.
1997-01-01
As part of the NASA Terminal Area Productivity Program, Langley Research Center embarked on a series of field measurements of wake vortex characteristics and associated atmospheric boundary layer properties. One measurement period was at the Memphis International Airport in August 1995. Atmospheric temperature, humidity, winds, turbulence, radiation, and soil properties were measured from a variety of sensor systems and platforms including sodars, profilers, aircraft and towers. This research focused on: (1) changes that occurred in tower data during sunrise and sunset transitions, (2) vertical variation of temperature and cross-head winds at selected times utilizing combinations of sensors, and (3) changes measured by an OV-10 aircraft during approaches and level flights. Significant but not unusual changes are documented and discussed in terms of expected boundary layer behavior. Questions on measurement and prediction of these changes from existing and near-term capabilities are discussed in the context of a future Aircraft Vortex Spacing System.
Radiation forcing by the atmospheric aerosols in the nocturnal boundary layer
Singh, D. K.; Ponnulakshami, V. K.; Mukund, V.; Subramanian, G.; Sreenivas, K. R.
2013-05-01
We have conducted experimental and theoretical studies on the radiation forcing due to suspended aerosols in the nocturnal boundary layer. We present radiative, conductive and convective equilibrium profile for different bottom boundaries where calculated Rayleigh number is higher than the critical Rayleigh number in laboratory conditions. The temperature profile can be fitted using an exponential distribution of aerosols concentration field. We also present the vertical temperature profiles in a nocturnal boundary in the presence of fog in the field. Our results show that during the presence of fog in the atmosphere, the ground temperature is greater than the dew-point temperature. The temperature profiles before and after the formation of fog are also observed to be different.
Boundary Layer Effect on Behavior of Discrete Models.
Eliáš, Jan
2017-02-10
The paper studies systems of rigid bodies with randomly generated geometry interconnected by normal and tangential bonds. The stiffness of these bonds determines the macroscopic elastic modulus while the macroscopic Poisson's ratio of the system is determined solely by the normal/tangential stiffness ratio. Discrete models with no directional bias have the same probability of element orientation for any direction and therefore the same mechanical properties in a statistical sense at any point and direction. However, the layers of elements in the vicinity of the boundary exhibit biased orientation, preferring elements parallel with the boundary. As a consequence, when strain occurs in this direction, the boundary layer becomes stiffer than the interior for the normal/tangential stiffness ratio larger than one, and vice versa. Nonlinear constitutive laws are typically such that the straining of an element in shear results in higher strength and ductility than straining in tension. Since the boundary layer tends, due to the bias in the elemental orientation, to involve more tension than shear at the contacts, it also becomes weaker and less ductile. The paper documents these observations and compares them to the results of theoretical analysis.
The atmospheric boundary layer in the CSIRO global climate model: simulations versus observations
Garratt, J. R.; Rotstayn, L. D.; Krummel, P. B.
2002-07-01
A 5-year simulation of the atmospheric boundary layer in the CSIRO global climate model (GCM) is compared with detailed boundary-layer observations at six locations, two over the ocean and four over land. Field observations, in the form of surface fluxes and vertical profiles of wind, temperature and humidity, are generally available for each hour over periods of one month or more in a single year. GCM simulations are for specific months corresponding to the field observations, for each of five years. At three of the four land sites (two in Australia, one in south-eastern France), modelled rainfall was close to the observed climatological values, but was significantly in deficit at the fourth (Kansas, USA). Observed rainfall during the field expeditions was close to climatology at all four sites. At the Kansas site, modelled screen temperatures (Tsc), diurnal temperature amplitude and sensible heat flux (H) were significantly higher than observed, with modelled evaporation (E) much lower. At the other three land sites, there is excellent correspondence between the diurnal amplitude and phase and absolute values of each variable (Tsc, H, E). Mean monthly vertical profiles for specific times of the day show strong similarities: over land and ocean in vertical shape and absolute values of variables, and in the mixed-layer and nocturnal-inversion depths (over land) and the height of the elevated inversion or height of the cloud layer (over the sea). Of special interest is the presence climatologically of early morning humidity inversions related to dewfall and of nocturnal low-level jets; such features are found in the GCM simulations. The observed day-to-day variability in vertical structure is captured well in the model for most sites, including, over a whole month, the temperature range at all levels in the boundary layer, and the mix of shallow and deep mixed layers. Weaknesses or unrealistic structure include the following, (a) unrealistic model mixed-layer
Boundary layer studies related to fusion theory. Final report
International Nuclear Information System (INIS)
1981-01-01
The described work studied the boundary between closed and open field lines in EBT geometry, with emphasis on the microstability properties. These properties were established primarily for drift waves in the lower hybrid range of frequencies. The transport due to these modes was evaluated by a self-consistent treatment, using quasilinear models in a plasma diffusion code. The model was benchmarked against the EDT experimental results from ORNL and the sensitivity to transport model established. Viscosity was estimated to be negligible compared with anomalous transport. Drift wave turbulence gave a boundary layer size much more consistent with experiment than either collisional transport or Bohm diffusion
Edge Plasma Boundary Layer Generated By Kink Modes in Tokamaks
International Nuclear Information System (INIS)
Zakharov, L.E.
2010-01-01
This paper describes the structure of the electric current generated by external kink modes at the plasma edge using the ideally conducting plasma model. It is found that the edge current layer is created by both wall touching and free boundary kink modes. Near marginal stability, the total edge current has a universal expression as a result of partial compensation of the (delta)-functional surface current by the bulk current at the edge. The resolution of an apparent paradox with the pressure balance across the plasma boundary in the presence of the surface currents is provided.
Impurity production and transport in the boundary layer of tokamaks
International Nuclear Information System (INIS)
McCracken, G.M.
1987-01-01
The processes by which impurities are produced and enter the discharge are discussed. Emphasis is placed on sputtering at the limiter and an analytical global model is described which incorporates the self-stabilizing effects whch control the edge temperature. Predictions of the scaling of edge temperature and of total radiated power are compared with experimental data from JET and other tokamaks operating with limiters. Under many conditions the scaling of the edge conditions and of the radiated power is accurately predicted. Impurity transport in the boundary and the question of how to control the boundary layer is then discussed. The example of the Impurity Control Limiter on DITE is described. (author)
Bottom boundary layer forced by finite amplitude long and short surface waves motions
Elsafty, H.; Lynett, P.
2018-04-01
A multiple-scale perturbation approach is implemented to solve the Navier-Stokes equations while including bottom boundary layer effects under a single wave and under two interacting waves. In this approach, fluid velocities and the pressure field are decomposed into two components: a potential component and a rotational component. In this study, the two components are exist throughout the entire water column and each is scaled with appropriate length and time scales. A one-way coupling between the two components is implemented. The potential component is assumed to be known analytically or numerically a prior, and the rotational component is forced by the potential component. Through order of magnitude analysis, it is found that the leading-order coupling between the two components occurs through the vertical convective acceleration. It is shown that this coupling plays an important role in the bottom boundary layer behavior. Its effect on the results is discussed for different wave-forcing conditions: purely harmonic forcing and impurely harmonic forcing. The approach is then applied to derive the governing equations for the bottom boundary layer developed under two interacting wave motions. Both motions-the shorter and the longer wave-are decomposed into two components, potential and rotational, as it is done in the single wave. Test cases are presented wherein two different wave forcings are simulated: (1) two periodic oscillatory motions and (2) short waves interacting with a solitary wave. The analysis of the two periodic motions indicates that nonlinear effects in the rotational solution may be significant even though nonlinear effects are negligible in the potential forcing. The local differences in the rotational velocity due to the nonlinear vertical convection coupling term are found to be on the order of 30% of the maximum boundary layer velocity for the cases simulated in this paper. This difference is expected to increase with the increase in wave
Gunn, A.; Jerolmack, D. J.; Edmonds, D. A.; Ewing, R. C.; Wanker, M.; David, S. R.
2017-12-01
Aolian sand dunes grow to 100s or 1000s of meters in wavelength by sand saltation, which also produces dust plumes that feed cloud formation and may spread around the world. The relations among sediment transport, landscape dynamics and wind are typically observed at the limiting ends of the relevant range: highly resolved and localized ground observations of turbulence and relevant fluxes; or regional and synoptic-scale meteorology and satellite imagery. Between the geostrophic winds aloft and shearing stress on the Earth's surface is the boundary layer, whose stability and structure determines how momentum is transferred and ultimately entrains sediment. Although the literature on atmospheric boundary layer flows is mature, this understanding is rarely applied to aeolian landscape dynamics. Moreover, there are few vertically and time-resolved datasets of atmospheric boundary layer flows in desert sand seas, where buoyancy effects are most pronounced. Here we employ a ground-based upward-looking doppler lidar to examine atmospheric boundary layer flow at the upwind margin of the White Sands (New Mexico) dune field, providing continuous 3D wind velocity data from the surface to 300-m aloft over 70 days of the characteristically windy spring season. Data show highly resolved daily cyles of convective instabilty due to daytime heating and stable stratification due to nightime cooling which act to enhance or depress, respectively, the surface wind stresses for a given free-stream velocity. Our data implicate convective instability in driving strong saltation and dust emission, because enhanced mixing flattens the vertical velocity profile (raising surface wind speed) while upward advection helps to deliver dust to the high atmosphere. We also find evidence for Ekman spiralling, with a magnitude that depends on atmospheric stability. This spiralling gives rise to a deflection in the direction between geostrophic and surface winds, that is significant for the
Characteristics of nocturnal coastal boundary layer in Ahtopol based on averaged SODAR profiles
Barantiev, Damyan; Batchvarova, Ekaterina; Novitzky, Mikhail
2014-05-01
The ground-based remote sensing instruments allow studying the wind regime and the turbulent characteristics of the atmosphere with height, achieving new knowledge and solving practical problems, such as air quality assessments, mesoscale models evaluation with high resolution data, characterization of the exchange processes between the surface and the atmosphere, the climate comfort conditions and the risk for extreme events, etc. Very important parameter in such studies is the height of the atmospheric boundary layer. Acoustic remote sensing data of the coastal atmospheric boundary layer were explored based on over 4-years continuous measurements at the meteorological observatory of Ahtopol (Bulgarian Southern Black Sea Coast) under Bulgarian - Russian scientific agreement. Profiles of 12 parameters from a mid-range acoustic sounding instrument type SCINTEC MFAS are derived and averaged up to about 600 m according filtering based on wind direction (land or sea type of night fowls). From the whole investigated period of 1454 days with 10-minute resolution SODAR data 2296 profiles represented night marine air masses and 1975 profiles represented the night flow from land during the months May to September. Graphics of averaged profiles of 12 SODAR output parameters with different availability of data in height are analyzed for both cases. A marine boundary-layer height of about 300 m is identified in the profiles of standard deviation of vertical wind speed (σw), Turbulent Kinetic Energy (TKE) and eddy dissipation rate (EDR). A nocturnal boundary-layer height of about 420 m was identified from the profiles of the same parameters under flows from land condition. In addition, the Buoyancy Production (BP= σw3/z) profiles were calculated from the standard deviation of the vertical wind speed and the height z above ground.
Boundary layer turbulence in transitional and developed states
Park, George Ilhwan; Wallace, James M.; Wu, Xiaohua; Moin, Parviz
2012-03-01
Using the recent direct numerical simulations by Wu and Moin ["Transitional and turbulent boundary layer with heat transfer," Phys. Fluids 22, 85 (2010)] of a flat-plate boundary layer with a passively heated wall, statistical properties of the turbulence in transition at Reθ ≈ 300, from individual turbulent spots, and at Reθ ≈ 500, where the spots merge (distributions of the mean velocity, Reynolds stresses, kinetic energy production, and dissipation rates, enstrophy and its components) have been compared to these statistical properties for the developed boundary layer turbulence at Reθ = 1840. When the distributions in the transitional regions are conditionally averaged so as to exclude locations and times when the flow is not turbulent, they closely resemble the distributions in the developed turbulent state at the higher Reynolds number, especially in the buffer layer. Skin friction coefficients, determined in this conditional manner at the two Reynolds numbers in the transitional flow are, of course, much larger than when their values are obtained by including both turbulent and non-turbulent information there, and the conditional averaged values are consistent with the 1/7th power law approximation. An octant analysis based on the combinations of signs of the velocity and temperature fluctuations, u, v, and θ shows that the momentum and heat fluxes are predominantly of the mean gradient type in both the transitional and developed regions. The fluxes appear to be closely associated with vortices that transport momentum and heat toward and away from the wall in both regions of the flow. The results suggest that there may be little fundamental difference between the nonlinear processes involved in the formation of turbulent spots that appear in transition and those that sustain the turbulence when it is developed. They also support the view that the transport processes and the vortical structures that drive them in developed and transitional boundary
Acoustic Radiation From a Mach 14 Turbulent Boundary Layer
Zhang, Chao; Duan, Lian; Choudhari, Meelan M.
2016-01-01
Direct numerical simulations (DNS) are used to examine the turbulence statistics and the radiation field generated by a high-speed turbulent boundary layer with a nominal freestream Mach number of 14 and wall temperature of 0:18 times the recovery temperature. The flow conditions fall within the range of nozzle exit conditions of the Arnold Engineering Development Center (AEDC) Hypervelocity Tunnel No. 9 facility. The streamwise domain size is approximately 200 times the boundary-layer thickness at the inlet, with a useful range of Reynolds number corresponding to Re 450 ?? 650. Consistent with previous studies of turbulent boundary layer at high Mach numbers, the weak compressibility hypothesis for turbulent boundary layers remains applicable under this flow condition and the computational results confirm the validity of both the van Driest transformation and Morkovin's scaling. The Reynolds analogy is valid at the surface; the RMS of fluctuations in the surface pressure, wall shear stress, and heat flux is 24%, 53%, and 67% of the surface mean, respectively. The magnitude and dominant frequency of pressure fluctuations are found to vary dramatically within the inner layer (z/delta 0.< or approx. 0.08 or z+ < or approx. 50). The peak of the pre-multiplied frequency spectrum of the pressure fluctuation is f(delta)/U(sub infinity) approx. 2.1 at the surface and shifts to a lower frequency of f(delta)/U(sub infinity) approx. 0.7 in the free stream where the pressure signal is predominantly acoustic. The dominant frequency of the pressure spectrum shows a significant dependence on the freestream Mach number both at the wall and in the free stream.
Structure measurements in a synthetic turbulent boundary layer
Arakeri, Jaywant H.
1987-09-01
Extensive hot-wire measurements have been made to determine the structure of the large eddy in a synthejc turbulent boundary layer on a flat-plate model. The experiments were carried out in a wind tunnel at a nominal free-stream velocity of 12 m/s. The synthetic turbulent boundary layer had a hexagonal pattern of eddies and a ratio of streamwise scale to spanwise scale of 3.2:1. The measured celerity of the large eddy was 84.2 percent of the free-stream velocity. There was some loss of coherence, but very little distortion, as the eddies moved downstream. Several mean properties of the synthetic boundary layer were found to agree quite well with the mean properties of a natural turbulent boundary layer at the same Reynolds number. The large eddy is composed of a pair of primary counter-rotating vortices about five [...] long in the streamwise direction and about one [...] apart in the spanwise direction, where [...] is the mean boundary-layer thickness. The sense of the primary pair is such as to pump fluid away from the wall in the region between the vortices. A secondary pair of counter-rotating streamwise vortices, having a sense opposite to that of the primary pair, is observed outside of and slightly downstream from the primary vortices. Both pairs of vortices extend across the full thickness of the boundary layer and are inclined at a shallow angle to the surface of the flat plate. The data show that the mean vorticity vectors are not tangential to the large-eddy vortices. In fact, the streamwise and normal vorticity components that signal the presence of the eddy are of the same order of magnitude. Definite signatures are obtained in terms of the mean skin-friction coefficient and the mean wake parameter averaged at constant phase. Velocities induced by the vortices are partly responsible for entrainment of irrotational fluid, for transport of momentum, for generation of Reynolds stresses, and for maintenance of streamwise and normal vorticity in the outer
Fragmented coastal boundary layer induced by gap winds
Caldeira, Rui M. A.; Iglesias, Isabel; Sala, Iria; Vieira, Rui R.; Bastos, Luísa
2015-04-01
The oceanic impact of offshore-localized winds in the NW Iberian Peninsula was studied. Satellite and in situ observations showed the formation of plumes protruding offshore from the coast. To study the dynamics of such episodes tee Coupled-Ocean-Atmosphere-Wave- Sediment Transport Modeling System (COAWST) was used to reproduce the coastal conditions of the nortwestern Iberian Peninsula, allowing the concurrent representation of local winds, waves, currents, and rivers runoff. The use of coupled models is of outmost importance in order to accurately study the impact of the local winds on the coastal currents. The NW Iberian Peninsula has prominent capes, promontories and submarine canyons, which produce persistent hydrodynamic features. Thus far, the scientific literature shows that the western Iberian rivers produce a recurrent combined plume often denominated as the Western Iberian Buoyant Plume (WIBP) which increases the stratification of the water column and produces a vertical retention mechanism that keeps the biological material inshore. The WIBP extends northward along the coast (over the inner-shelf), and forms a front with the warmer and more saline surface (offshore) waters. However during episodes of strong offshore winds this coastal boundary layer is broken interrupting the WIBP. Coastal orography allows the formation of down-valley winds that produce coastal jets, promoting the offshore transport of pollutants, larvae and sediments. Acknowledgments: Acknowledgments: Numerical model solutions were calculated at CIIMARs HPC unit, acquired and maintained by FCT pluriannual funds (PesTC/Mar/LA0015/2013), and RAIA (0313-RAIA-1-E) and RAIA.co (0520-RAIACO-1-E) projects. The NICC (POCTI/CTA/49563/2002) project provided databases for this work. Rui Caldeira was supported by funds from the ECORISK project (NORTE-07-0124-FEDER-000054), co-financed by the North Portugal Regional Operational Programme (ON.2 - O Novo Norte), under the National Strategic Reference
Conference on Boundary and Interior Layers : Computational and Asymptotic Methods
Stynes, Martin; Zhang, Zhimin
2017-01-01
This volume collects papers associated with lectures that were presented at the BAIL 2016 conference, which was held from 14 to 19 August 2016 at Beijing Computational Science Research Center and Tsinghua University in Beijing, China. It showcases the variety and quality of current research into numerical and asymptotic methods for theoretical and practical problems whose solutions involve layer phenomena. The BAIL (Boundary And Interior Layers) conferences, held usually in even-numbered years, bring together mathematicians and engineers/physicists whose research involves layer phenomena, with the aim of promoting interaction between these often-separate disciplines. These layers appear as solutions of singularly perturbed differential equations of various types, and are common in physical problems, most notably in fluid dynamics. This book is of interest for current researchers from mathematics, engineering and physics whose work involves the accurate app roximation of solutions of singularly perturbed diffe...
Initializing a Mesoscale Boundary-Layer Model with Radiosonde Observations
Berri, Guillermo J.; Bertossa, Germán
2018-01-01
A mesoscale boundary-layer model is used to simulate low-level regional wind fields over the La Plata River of South America, a region characterized by a strong daily cycle of land-river surface-temperature contrast and low-level circulations of sea-land breeze type. The initial and boundary conditions are defined from a limited number of local observations and the upper boundary condition is taken from the only radiosonde observations available in the region. The study considers 14 different upper boundary conditions defined from the radiosonde data at standard levels, significant levels, level of the inversion base and interpolated levels at fixed heights, all of them within the first 1500 m. The period of analysis is 1994-2008 during which eight daily observations from 13 weather stations of the region are used to validate the 24-h surface-wind forecast. The model errors are defined as the root-mean-square of relative error in wind-direction frequency distribution and mean wind speed per wind sector. Wind-direction errors are greater than wind-speed errors and show significant dispersion among the different upper boundary conditions, not present in wind speed, revealing a sensitivity to the initialization method. The wind-direction errors show a well-defined daily cycle, not evident in wind speed, with the minimum at noon and the maximum at dusk, but no systematic deterioration with time. The errors grow with the height of the upper boundary condition level, in particular wind direction, and double the errors obtained when the upper boundary condition is defined from the lower levels. The conclusion is that defining the model upper boundary condition from radiosonde data closer to the ground minimizes the low-level wind-field errors throughout the region.
Bandgap tunability at single-layer molybdenum disulphide grain boundaries
Huang, Yu Li
2015-02-17
Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40±0.05 eV for single-layer, 2.10±0.05 eV for bilayer and 1.75±0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85±0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.
Compressible stability of growing boundary layers using parabolized stability equations
Chang, Chau-Lyan; Malik, Mujeeb R.; Erlebacher, Gordon; Hussaini, M. Y.
1991-01-01
The parabolized stability equation (PSE) approach is employed to study linear and nonlinear compressible stability with an eye to providing a capability for boundary-layer transition prediction in both 'quiet' and 'disturbed' environments. The governing compressible stability equations are solved by a rational parabolizing approximation in the streamwise direction. Nonparallel flow effects are studied for both the first- and second-mode disturbances. For oblique waves of the first-mode type, the departure from the parallel results is more pronounced as compared to that for the two-dimensional waves. Results for the Mach 4.5 case show that flow nonparallelism has more influence on the first mode than on the second. The disturbance growth rate is shown to be a strong function of the wall-normal distance due to either flow nonparallelism or nonlinear interactions. The subharmonic and fundamental types of breakdown are found to be similar to the ones in incompressible boundary layers.
Grain boundary layer behavior in ZnO/Si heterostructure
International Nuclear Information System (INIS)
Liu Bingce; Liu Cihui; Yi Bo
2010-01-01
The grain boundary layer behavior in ZnO/Si heterostucture is investigated. The current-voltage (I-V) curves, deep level transient spectra (DLTS) and capacitance-voltage (C-V) curves are measured. The transport currents of ZnO/Si heterojunction are dominated by grain boundary layer as high densities of interfacial states existed. The interesting phenomenon that the crossing of In I-V curves of ZnO/Si heterojunction at various measurement temperatures and the decrease of its effective barrier height with the decrement of temperature are in contradiction with the ideal heterojunction thermal emission model is observed. The details will be discussed in the following. (semiconductor physics)
Heat conduction boundary layers of condensed clumps in cooling flows
International Nuclear Information System (INIS)
Boehringer, H.; Fabian, A.C.
1989-01-01
The structure of heat conduction boundary layers of gaseous condensations embedded in the hot intergalactic gas in clusters of galaxies is investigated by means of steady, one-dimensional, hydrodynamic models. It is assumed that heat conduction is effective only on scales much smaller than the total region of the cooling flow. Models are calculated for an arbitrary scaling factor, accounting for the reduction in heat conduction efficiency compared to the classical Spitzer case. The results imply a lower limit to the size spectrum of the condensations. The enhancement of cooling in the ambient medium due to heat conduction losses is calculated for a range of clump parameters. The luminosity of several observable emission lines, the extreme ultraviolet (EUV) and soft X-ray emission spectrum, and the column density of some important ions are determined for the model boundary layers and compared with observations. (author)
The Boundary Layer Flows of a Rivlin-Ericksen Fluid
Sadeghy, K.; Khabazi, N.; Taghavi, S. M.
The present work deals with the two-dimensional incompressible, laminar, steady-state boundary layer equations. First, we determine a family of velocity distributions outside the boundary layer such that these problems may have similarity solutions. We study the Falkner-Skan flow of a viscoelastic fluid governed by second order model, as the Reynolds number Re→ ∞. We obtain an ordinary forth order differential equation to obtain the stream function, velocity profile and the stress. The stream function is then governed by a generalized Falkner-Skan equation. In comparison with Newtonian Falkner-Skan equation that has two coefficients this new one has four coefficients that two of them represent elastic properties of the fluid. The effects of the elastic parameter on the velocity filed have been discussed. As it is shown in the figure there is a good agreement between numerical results and previous special cases confirm the validity of the presented algorithm.
Vortex Formation During Unsteady Boundary-Layer Separation
Das, Debopam; Arakeri, Jaywant H.
1998-11-01
Unsteady laminar boundary-layer separation is invariably accompanied by the formation of vortices. The aim of the present work is to study the vortex formation mechanism(s). An adverse pressure gradient causing a separation can be decomposed into a spatial component ( spatial variation of the velocity external to the boundary layer ) and a temporal component ( temporal variation of the external velocity ). Experiments were conducted in a piston driven 2-D water channel, where the spatial component could be be contolled by geometry and the temporal component by the piston motion. We present results for three divergent channel geometries. The piston motion consists of three phases: constant acceleration from start, contant velocity, and constant deceleration to stop. Depending on the geometry and piston motion we observe different types of unsteady separation and vortex formation.
Turbulent boundary layer under the control of different schemes.
Qiao, Z X; Zhou, Y; Wu, Z
2017-06-01
This work explores experimentally the control of a turbulent boundary layer over a flat plate based on wall perturbation generated by piezo-ceramic actuators. Different schemes are investigated, including the feed-forward, the feedback, and the combined feed-forward and feedback strategies, with a view to suppressing the near-wall high-speed events and hence reducing skin friction drag. While the strategies may achieve a local maximum drag reduction slightly less than their counterpart of the open-loop control, the corresponding duty cycles are substantially reduced when compared with that of the open-loop control. The results suggest a good potential to cut down the input energy under these control strategies. The fluctuating velocity, spectra, Taylor microscale and mean energy dissipation are measured across the boundary layer with and without control and, based on the measurements, the flow mechanism behind the control is proposed.
Flight Experiment Verification of Shuttle Boundary Layer Transition Prediction Tool
Berry, Scott A.; Berger, Karen T.; Horvath, Thomas J.; Wood, William A.
2016-01-01
Boundary layer transition at hypersonic conditions is critical to the design of future high-speed aircraft and spacecraft. Accurate methods to predict transition would directly impact the aerothermodynamic environments used to size a hypersonic vehicle's thermal protection system. A transition prediction tool, based on wind tunnel derived discrete roughness correlations, was developed and implemented for the Space Shuttle return-to-flight program. This tool was also used to design a boundary layer transition flight experiment in order to assess correlation uncertainties, particularly with regard to high Mach-number transition and tunnel-to-flight scaling. A review is provided of the results obtained from the flight experiment in order to evaluate the transition prediction tool implemented for the Shuttle program.
Diffusive boundary layers at the bottom of gaps and cracks
Etzold, Merlin A.; Landel, Julien R.; Dalziel, Stuart B.
2017-11-01
This work is motivated by the chemical decontamination of droplets of chemical warfare agents trapped in the gaps and cracks found in most man-made objects. We consider axial laminar flow within gaps with both straight and angled walls. We study the diffusive mass transfer from a source (e.g. a droplet surface) located at the bottom of the gap. This problem is similar to boundary layers and Graetz-type problems (heat transfer in pipe flow) with the added complication of a non-uniform lateral concentration profile due to the lateral variation of the velocity profile. We present 3D solutions for the diffusive boundary layer and demonstrate that a 2D mean-field model, for which we calculate series and similarity solutions, captures the essential physics. We demonstrate the immediate practical relevance of our findings by comparing decontamination of a droplet located in a gap and on an exposed surface.
Boundary layer development on turbine airfoil suction surfaces
Sharma, O. P.; Wells, R. A.; Schlinker, R. H.; Bailey, D. A.
1981-01-01
The results of a study supported by NASA under the Energy Efficient Engine Program, conducted to investigate the development of boundary layers under the influence of velocity distributions that simulate the suction sides of two state-of-the-art turbine airfoils, are presented. One velocity distribution represented a forward loaded airfoil ('squared-off' design), while the other represented an aft loaded airfoil ('aft loaded' design). These velocity distributions were simulated in a low-speed, high-aspect-ratio wind tunnel specifically designed for boundary layer investigations. It is intended that the detailed data presented in this paper be used to develop improved turbulence model suitable for application to turbine airfoil design.
Stereoscopic PIV measurement of boundary layer affected by DBD actuator
Directory of Open Access Journals (Sweden)
Procházka Pavel
2016-01-01
Full Text Available The effect of ionic wind generated by plasma actuator on developed boundary layer inside a narrow channel was investigated recently. Since the main investigated plane was parallel to the channel axis, the description of flow field was not evaluated credibly. This paper is dealing with cross-section planes downstream the actuator measured via 3D time-resolved PIV. The actuator position is in spanwise or in streamwise orientation so that ionic wind is blown in the same direction as the main flow or in opposite direction or perpendicularly. The interaction between boundary layer and ionic wind is evaluated for three different velocities of main flow and several parameters of plasma actuation (steady and unsteady regime, frequency etc.. Statistical properties of the flow are shown as well as dynamical behaviour of arising longitudinal vortices are discussed via phase-locked measurement and decomposition method.
Abyssal Upwelling and Downwelling and the role of boundary layers
McDougall, T. J.; Ferrari, R. M.
2016-02-01
The bottom-intensified mixing activity arising from the interaction of internal tides with bottom topography implies that the dianeutral advection in the ocean interior is downwards, rather than upwards as is required by continuity. The upwelling of Bottom Water through density surfaces in the deep ocean is however possible because of the sloping nature of the sea floor. A budget study of the abyss (deeper than 2000m) will be described that shows that while the upwelling of Bottom Water might be 25 Sv, this is achieved by very strong upwelling in the bottom turbulent boundary layer (of thickness 50m) of 100 Sv and strong downwelling in the ocean interior of 75 Sv. This downwelling occurs within 10 degrees of longitude of the continental boundaries. This near-boundary confined strong upwelling and downwelling clearly has implications for the Stommel-Arons circulation.
Spectral Gap Energy Transfer in Atmospheric Boundary Layer
Bhushan, S.; Walters, K.; Barros, A. P.; Nogueira, M.
2012-12-01
Experimental measurements of atmospheric turbulence energy spectra show E(k) ~ k-3 slopes at synoptic scales (~ 600 km - 2000 km) and k-5/3 slopes at the mesoscales (theory, it is expected that a strong backward energy cascade would develop at the synoptic scale, and that circulation would grow infinitely. To limit this backward transfer, energy arrest at macroscales must be introduced. The most commonly used turbulence models developed to mimic the above energy transfer include the energy backscatter model for 2D turbulence in the horizontal plane via Large Eddy Simulation (LES) models, dissipative URANS models in the vertical plane, and Ekman friction for the energy arrest. One of the controversial issues surrounding the atmospheric turbulence spectra is the explanation of the generation of the 2D and 3D spectra and transition between them, for energy injection at the synoptic scales. Lilly (1989) proposed that the existence of 2D and 3D spectra can only be explained by the presence of an additional energy injection in the meso-scale region. A second issue is related to the observations of dual peak spectra with small variance in meso-scale, suggesting that the energy transfer occurs across a spectral gap (Van Der Hoven, 1957). Several studies have confirmed the spectral gap for the meso-scale circulations, and have suggested that they are enhanced by smaller scale vertical convection rather than by the synoptic scales. Further, the widely accepted energy arrest mechanism by boundary layer friction is closely related to the spectral gap transfer. This study proposes an energy transfer mechanism for atmospheric turbulence with synoptic scale injection, wherein the generation of 2D and 3D spectra is explained using spectral gap energy transfer. The existence of the spectral gap energy transfer is validated by performing LES for the interaction of large scale circulation with a wall, and studying the evolution of the energy spectra both near to and far from the wall
Simulation of hypersonic shock wave - laminar boundary layer interactions
Kianvashrad, N.; Knight, D.
2017-06-01
The capability of the Navier-Stokes equations with a perfect gas model for simulation of hypersonic shock wave - laminar boundary layer interactions is assessed. The configuration is a hollow cylinder flare. The experimental data were obtained by Calspan-University of Buffalo (CUBRC) for total enthalpies ranging from 5.07 to 21.85 MJ/kg. Comparison of the computed and experimental surface pressure and heat transfer is performed and the computed §ow¦eld structure is analyzed.
Numerical solution of the resistive magnetohydrodynamic boundary-layer equations
International Nuclear Information System (INIS)
Glasser, A.H.; Jardin, S.C.; Tesauro, G.
1983-10-01
Three different techniques are presented for numerical solution of the equations governing the boundary layer of resistive magnetohydrodynamic tearing and interchange instabilities in toroidal geometry. Excellent agreement among these methods and with analytical results provides confidence in the correctness of the results. Solutions obtained in regimes where analytical medthods fail indicate a new scaling for the tearing mode as well as the existence of a new regime of stability
The curved kinetic boundary layer of active matter.
Yan, Wen; Brady, John F
2018-01-03
A body submerged in active matter feels the swim pressure through a kinetic accumulation boundary layer on its surface. The boundary layer results from a balance between translational diffusion and advective swimming and occurs on the microscopic length scale . Here , D T is the Brownian translational diffusivity, τ R is the reorientation time and l = U 0 τ R is the swimmer's run length, with U 0 the swim speed [Yan and Brady, J. Fluid. Mech., 2015, 785, R1]. In this work we analyze the swim pressure on arbitrary shaped bodies by including the effect of local shape curvature in the kinetic boundary layer. When δ ≪ L and l ≪ L, where L is the body size, the leading order effects of curvature on the swim pressure are found analytically to scale as J S λδ 2 /L, where J S is twice the (non-dimensional) mean curvature. Particle-tracking simulations and direct solutions to the Smoluchowski equation governing the probability distribution of the active particles show that λδ 2 /L is a universal scaling parameter not limited to the regime δ, l ≪ L. The net force exerted on the body by the swimmers is found to scale as F net /(n ∞ k s T s L 2 ) = f(λδ 2 /L), where f(x) is a dimensionless function that is quadratic when x ≪ 1 and linear when x ∼ 1. Here, k s T s = ζU 0 2 τ R /6 defines the 'activity' of the swimmers, with ζ the drag coefficient, and n ∞ is the uniform number density of swimmers far from the body. We discuss the connection of this boundary layer to continuum mechanical descriptions of active matter and briefly present how to include hydrodynamics into this purely kinetic study.
Dynamical structure of the turbulent boundary layer on rough surface
Czech Academy of Sciences Publication Activity Database
Uruba, Václav; Jonáš, Pavel; Hladík, Ondřej
2011-01-01
Roč. 11, č. 1 (2011), s. 603-604 ISSN 1617-7061 R&D Projects: GA ČR GA101/08/1112; GA ČR GAP101/10/1230 Institutional research plan: CEZ:AV0Z20760514 Keywords : turbulent boundary layer * rough wall * hairpin vortex Subject RIV: BK - Fluid Dynamics http://onlinelibrary.wiley.com/doi/10.1002/pamm.201110291/abstract
The double layers in the plasma sheet boundary layer during magnetic reconnection
Guo, J.; Yu, B.
2014-11-01
We studied the evolutions of double layers which appear after the magnetic reconnection through two-dimensional electromagnetic particle-in-cell simulation. The simulation results show that the double layers are formed in the plasma sheet boundary layer after magnetic reconnection. At first, the double layers which have unipolar structures are formed. And then the double layers turn into bipolar structures, which will couple with another new weak bipolar structure. Thus a new double layer or tripolar structure comes into being. The double layers found in our work are about several ten Debye lengths, which accords with the observation results. It is suggested that the electron beam formed during the magnetic reconnection is responsible for the production of the double layers.
Directory of Open Access Journals (Sweden)
H. Dadashazar
2018-02-01
Full Text Available This study uses airborne data from two field campaigns off the California coast to characterize aerosol size distribution characteristics in the entrainment interface layer (EIL, a thin and turbulent layer above marine stratocumulus cloud tops, which separates the stratocumulus-topped boundary layer (STBL from the free troposphere (FT. The vertical bounds of the EIL are defined in this work based on considerations of buoyancy and turbulence using thermodynamic and dynamic data. Aerosol number concentrations are examined from three different probes with varying particle diameter (Dp ranges: > 3 nm, > 10 nm, and 0.11–3.4 µm. Relative to the EIL and FT layers, the sub-cloud (SUB layer exhibited lower aerosol number concentrations and higher surface area concentrations. High particle number concentrations between 3 and 10 nm in the EIL are indicative of enhanced nucleation, assisted by high actinic fluxes, cool and moist air, and much lower surface area concentrations than the STBL. Slopes of number concentration versus altitude in the EIL were correlated with the particle number concentration difference between the SUB and lower FT layers. The EIL aerosol size distribution was influenced by varying degrees from STBL aerosol versus subsiding FT aerosol depending on the case examined. These results emphasize the important role of the EIL in influencing nucleation and aerosol–cloud–climate interactions.
Dadashazar, Hossein; Braun, Rachel A.; Crosbie, Ewan; Chuang, Patrick Y.; Woods, Roy K.; Jonsson, Haflidi H.; Sorooshian, Armin
2018-02-01
This study uses airborne data from two field campaigns off the California coast to characterize aerosol size distribution characteristics in the entrainment interface layer (EIL), a thin and turbulent layer above marine stratocumulus cloud tops, which separates the stratocumulus-topped boundary layer (STBL) from the free troposphere (FT). The vertical bounds of the EIL are defined in this work based on considerations of buoyancy and turbulence using thermodynamic and dynamic data. Aerosol number concentrations are examined from three different probes with varying particle diameter (Dp) ranges: > 3 nm, > 10 nm, and 0.11-3.4 µm. Relative to the EIL and FT layers, the sub-cloud (SUB) layer exhibited lower aerosol number concentrations and higher surface area concentrations. High particle number concentrations between 3 and 10 nm in the EIL are indicative of enhanced nucleation, assisted by high actinic fluxes, cool and moist air, and much lower surface area concentrations than the STBL. Slopes of number concentration versus altitude in the EIL were correlated with the particle number concentration difference between the SUB and lower FT layers. The EIL aerosol size distribution was influenced by varying degrees from STBL aerosol versus subsiding FT aerosol depending on the case examined. These results emphasize the important role of the EIL in influencing nucleation and aerosol-cloud-climate interactions.
Transitional and turbulent boundary layer with heat transfer
Wu, Xiaohua; Moin, Parviz
2010-08-01
We report on our direct numerical simulation of an incompressible, nominally zero-pressure-gradient flat-plate boundary layer from momentum thickness Reynolds number 80-1950. Heat transfer between the constant-temperature solid surface and the free-stream is also simulated with molecular Prandtl number Pr=1. Skin-friction coefficient and other boundary layer parameters follow the Blasius solutions prior to the onset of turbulent spots. Throughout the entire flat-plate, the ratio of Stanton number and skin-friction St/Cf deviates from the exact Reynolds analogy value of 0.5 by less than 1.5%. Mean velocity and Reynolds stresses agree with experimental data over an extended turbulent region downstream of transition. Normalized rms wall-pressure fluctuation increases gradually with the streamwise growth of the turbulent boundary layer. Wall shear stress fluctuation, τw,rms'+, on the other hand, remains constant at approximately 0.44 over the range, 800spots are tightly packed with numerous hairpin vortices. With the advection and merging of turbulent spots, these young isolated hairpin forests develop into the downstream turbulent region. Isosurfaces of temperature up to Reθ=1900 are found to display well-resolved signatures of hairpin vortices, which indicates the persistence of the hairpin forests.
Manipulation of Turbulent Boundary Layers Using Synthetic Jets
Berger, Zachary; Gomit, Guillaume; Lavoie, Philippe; Ganapathisubramani, Bharath
2015-11-01
This work focuses on the application of active flow control, in the form of synthetic jet actuators, of turbulent boundary layers. An array of 2 synthetic jets are oriented in the spanwise direction and located approximately 2.7 meters downstream from the leading edge of a flat plate. Actuation is applied perpendicular to the surface of the flat plate with varying blowing ratios and reduced frequencies (open-loop). Two-component large window particle image velocimetry (PIV) was performed at the University of Southampton, in the streamwise-wall-normal plane. Complementary stereo PIV measurements were performed at the University of Toronto Institute for Aerospace Studies (UTIAS), in the spanwise-wall-normal plane. The freestream Reynolds number is 3x104, based on the boundary layer thickness. The skin friction Reynolds number is 1,200 based on the skin friction velocity. The experiments at Southampton allow for the observation of the control effects as the flow propagates downstream. The experiments at UTIAS allow for the observation of the streamwise vorticity induced from the actuation. Overall the two experiments provide a 3D representation of the flow field with respect to actuation effects. The current work focuses on the comparison of the two experiments, as well as the effects of varying blowing ratios and reduced frequencies on the turbulent boundary layer. Funded Supported by Airbus.
Thermocapillary Bubble Migration: Thermal Boundary Layers for Large Marangoni Numbers
Balasubramaniam, R.; Subramanian, R. S.
1996-01-01
The migration of an isolated gas bubble in an immiscible liquid possessing a temperature gradient is analyzed in the absence of gravity. The driving force for the bubble motion is the shear stress at the interface which is a consequence of the temperature dependence of the surface tension. The analysis is performed under conditions for which the Marangoni number is large, i.e. energy is transferred predominantly by convection. Velocity fields in the limit of both small and large Reynolds numbers are used. The thermal problem is treated by standard boundary layer theory. The outer temperature field is obtained in the vicinity of the bubble. A similarity solution is obtained for the inner temperature field. For both small and large Reynolds numbers, the asymptotic values of the scaled migration velocity of the bubble in the limit of large Marangoni numbers are calculated. The results show that the migration velocity has the same scaling for both low and large Reynolds numbers, but with a different coefficient. Higher order thermal boundary layers are analyzed for the large Reynolds number flow field and the higher order corrections to the migration velocity are obtained. Results are also presented for the momentum boundary layer and the thermal wake behind the bubble, for large Reynolds number conditions.
Coupling of magnetopause-boundary layer to the polar ionosphere
International Nuclear Information System (INIS)
Wei, C.Q.; Lee, L.C.
1993-01-01
The authors develop a model which seeks to explain ultraviolet auroral images from the Viking satellite which show periodic bright regions which resemble open-quotes beadsclose quotes or open-quotes pearlsclose quotes aligned along the postnoon auroral oval. ULF geomagnetic pulsations observed in the cusp region are also addressed by this model. The model addresses plasma dynamics in the low-latitude boundary layer and interactions with the polar ionosphere by means of field-aligned current. The Kelvin-Helmholtz instability can develop in the presence of driven plasma flow, which can lead to the formation and growth of plasma vortices in the boundary layer. The finite conductivity of the earth ionosphere causes these vortices to decay. However regions of enhanced field-aligned power density in the postnoon auroral oval can be associated with field-aligned current filaments and boundary layer vortices. These structures may explain the observed bright spots. The authors also discuss the frequency spectrum and the polarization state of the pulsations
RANS Modeling of Benchmark Shockwave / Boundary Layer Interaction Experiments
Georgiadis, Nick; Vyas, Manan; Yoder, Dennis
2010-01-01
This presentation summarizes the computations of a set of shock wave / turbulent boundary layer interaction (SWTBLI) test cases using the Wind-US code, as part of the 2010 American Institute of Aeronautics and Astronautics (AIAA) shock / boundary layer interaction workshop. The experiments involve supersonic flows in wind tunnels with a shock generator that directs an oblique shock wave toward the boundary layer along one of the walls of the wind tunnel. The Wind-US calculations utilized structured grid computations performed in Reynolds-averaged Navier-Stokes mode. Three turbulence models were investigated: the Spalart-Allmaras one-equation model, the Menter Shear Stress Transport wavenumber-angular frequency two-equation model, and an explicit algebraic stress wavenumber-angular frequency formulation. Effects of grid resolution and upwinding scheme were also considered. The results from the CFD calculations are compared to particle image velocimetry (PIV) data from the experiments. As expected, turbulence model effects dominated the accuracy of the solutions with upwinding scheme selection indicating minimal effects.!
Cloud-Scale Numerical Modeling of the Arctic Boundary Layer
Krueger, Steven K.
1998-01-01
The interactions between sea ice, open ocean, atmospheric radiation, and clouds over the Arctic Ocean exert a strong influence on global climate. Uncertainties in the formulation of interactive air-sea-ice processes in global climate models (GCMs) result in large differences between the Arctic, and global, climates simulated by different models. Arctic stratus clouds are not well-simulated by GCMs, yet exert a strong influence on the surface energy budget of the Arctic. Leads (channels of open water in sea ice) have significant impacts on the large-scale budgets during the Arctic winter, when they contribute about 50 percent of the surface fluxes over the Arctic Ocean, but cover only 1 to 2 percent of its area. Convective plumes generated by wide leads may penetrate the surface inversion and produce condensate that spreads up to 250 km downwind of the lead, and may significantly affect the longwave radiative fluxes at the surface and thereby the sea ice thickness. The effects of leads and boundary layer clouds must be accurately represented in climate models to allow possible feedbacks between them and the sea ice thickness. The FIRE III Arctic boundary layer clouds field program, in conjunction with the SHEBA ice camp and the ARM North Slope of Alaska and Adjacent Arctic Ocean site, will offer an unprecedented opportunity to greatly improve our ability to parameterize the important effects of leads and boundary layer clouds in GCMs.
Reduction of horizontal wind speed in a boundary layer with obstacles
DEFF Research Database (Denmark)
Emeis, S.; Frandsen, S.
1993-01-01
The reduction of horizontal wind speed at hub height in an infinite cluster of wind turbines is computed from a balance between a loss of horizontal momentum due to the drag and replenishment from above by turbulent fluxes. This reduction is derived without assumptions concerning the vertical wind...... profile above or below hub height, only some basic assumptions on turbulent exchange have been made. Two applications of the result are presented, one considering wind turbines and one pressure drag on orographic obstacles in the atmospheric boundary layer. Both applications are basically governed...... by the same kind of momentum balance....
An observational study of the evolution of the atmospheric boundary-layer over Cabo Frio, Brazil
Directory of Open Access Journals (Sweden)
S. H. Franchito
2007-08-01
Full Text Available The effect of coastal upwelling on the evolution of the atmospheric boundary layer (ABL in Cabo Frio (Brazil is investigated. For this purpose, radiosounding data collected in two experiments made during the austral summer (upwelling case and austral winter (no upwelling case are analysed. The results show that during the austral summer, cold waters that crop up near the Cabo Frio coast favour the formation of an atmospheric stable layer, which persists during the upwelling episode. Due to the low SSTs, the descending branch of the sea-breeze circulation is located close to the coast, inhibiting the development of a mixed layer mainly during the day. At night, with the reduction of the land-sea thermal contrast the descending motion is weaker, allowing a vertical mixing. The stable ABL favours the formation of a low level jet, which may also contribute to the development of a nocturnal atmospheric mixed layer. During the austral winter, due to the higher SSTs observed near the coast, the ABL is less stable compared with that in the austral summer. Due to warming, a mixed layer is observed during the day. The observed vertical profiles of the zonal winds show that the easterlies at low levels are stronger in the austral summer, indicating that the upwelling modulates the sea-breeze signal, thus confirming model simulations.
International Nuclear Information System (INIS)
Jat, R.N.; Chaudhary, Santosh
2009-01-01
The flow of an electrically conducting fluid past a porous substrate attached to the flat plate with Beavers-Joseph boundary condition under the influence of a uniform transverse magnetic field has been studied. Taking suitable similar variables, the momentum equation is transformed to ordinary differential equation and solved by standard techniques. The energy equation is solved by considering two boundary layers, one in the porous substrate and the other above the porous substrate. The velocity and temperature distributions along with Nusselt number are discussed numerically and presented through graphs. (author)
Computational Study of Hypersonic Boundary Layer Stability on Cones
Gronvall, Joel Edwin
Due to the complex nature of boundary layer laminar-turbulent transition in hypersonic flows and the resultant effect on the design of re-entry vehicles, there remains considerable interest in developing a deeper understanding of the underlying physics. To that end, the use of experimental observations and computational analysis in a complementary manner will provide the greatest insights. It is the intent of this work to provide such an analysis for two ongoing experimental investigations. The first focuses on the hypersonic boundary layer transition experiments for a slender cone that are being conducted at JAXA's free-piston shock tunnel HIEST facility. Of particular interest are the measurements of disturbance frequencies associated with transition at high enthalpies. The computational analysis provided for these cases included two-dimensional CFD mean flow solutions for use in boundary layer stability analyses. The disturbances in the boundary layer were calculated using the linear parabolized stability equations. Estimates for transition locations, comparisons of measured disturbance frequencies and computed frequencies, and a determination of the type of disturbances present were made. It was found that for the cases where the disturbances were measured at locations where the flow was still laminar but nearly transitional, that the highly amplified disturbances showed reasonable agreement with the computations. Additionally, an investigation of the effects of finite-rate chemistry and vibrational excitation on flows over cones was conducted for a set of theoretical operational conditions at the HIEST facility. The second study focuses on transition in three-dimensional hypersonic boundary layers, and for this the cone at angle of attack experiments being conducted at the Boeing/AFOSR Mach-6 quiet tunnel at Purdue University were examined. Specifically, the effect of surface roughness on the development of the stationary crossflow instability are investigated
Near-wake flow structure downwind of a wind turbine in a turbulent boundary layer
Energy Technology Data Exchange (ETDEWEB)
Zhang, Wei; Markfort, Corey D. [University of Minnesota, Saint Anthony Falls Laboratory, Department of Civil Engineering, Minneapolis, MN (United States); Porte-Agel, Fernando [Ecole Polytechnique Federale de Lausanne (EPFL), ENAC-IIE-WIRE, Wind Engineering and Renewable Energy Laboratory (WIRE), Lausanne (Switzerland)
2012-05-15
Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm, occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure velocities in multiple vertical stream-wise planes (x-z) and vertical span-wise planes (y-z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development and
International Nuclear Information System (INIS)
Westin, K.J.A.; Boiko, A.V.; Klingmann, B.G.B.; Kozlov, V.V.; Alfredsson, P.H.
1993-12-01
The modification of the mean and fluctuating characteristics of a flat plate boundary layer subjected to nearly isotropic free stream turbulence (FST) is studied experimentally using hot-wire anemometry. The study is focussed on the region upstream of the transition onset, where the fluctuations inside the boundary layer are dominated by elongated flow structures which grow downstream both in amplitude and length. Their downstream development and scaling is investigated, and the results are compared to those obtained by previous authors. This allows some conclusions about the parameters which are relevant for the modelling of the transition process. The mechanisms underlying the transition process and the relative importance of the Tollmien-Schlichting wave instability in this flow are treated in an accompanying paper. 25 refs
Englberger, Antonia; Dörnbrack, Andreas
2018-03-01
The wake characteristics of a wind turbine for different regimes occurring throughout the diurnal cycle are investigated systematically by means of large-eddy simulation. Idealized diurnal cycle simulations of the atmospheric boundary layer are performed with the geophysical flow solver EULAG over both homogeneous and heterogeneous terrain. Under homogeneous conditions, the diurnal cycle significantly affects the low-level wind shear and atmospheric turbulence. A strong vertical wind shear and veering with height occur in the nocturnal stable boundary layer and in the morning boundary layer, whereas atmospheric turbulence is much larger in the convective boundary layer and in the evening boundary layer. The increased shear under heterogeneous conditions changes these wind characteristics, counteracting the formation of the night-time Ekman spiral. The convective, stable, evening, and morning regimes of the atmospheric boundary layer over a homogeneous surface as well as the convective and stable regimes over a heterogeneous surface are used to study the flow in a wind-turbine wake. Synchronized turbulent inflow data from the idealized atmospheric boundary-layer simulations with periodic horizontal boundary conditions are applied to the wind-turbine simulations with open streamwise boundary conditions. The resulting wake is strongly influenced by the stability of the atmosphere. In both cases, the flow in the wake recovers more rapidly under convective conditions during the day than under stable conditions at night. The simulated wakes produced for the night-time situation completely differ between heterogeneous and homogeneous surface conditions. The wake characteristics of the transitional periods are influenced by the flow regime prior to the transition. Furthermore, there are different wake deflections over the height of the rotor, which reflect the incoming wind direction.
Spatial structures in the heat budget of the Antarctic atmospheric boundary layer
Directory of Open Access Journals (Sweden)
W. J. van de Berg
2008-01-01
Full Text Available Output from the regional climate model RACMO2/ANT is used to calculate the heat budget of the Antarctic atmospheric boundary layer (ABL. The main feature of the wintertime Antarctic ABL is a persistent temperature deficit compared to the free atmosphere. The magnitude of this deficit is controlled by the heat budget. During winter, transport of heat towards the surface by turbulence and net longwave emission are the primary ABL cooling terms. These processes show horizontal spatial variability only on continental scales. Vertical and horizontal, i.e. along-slope, advection of heat are the main warming terms. Over regions with convex ice sheet topography, i.e. domes and ridges, warming by downward vertical advection is enhanced due to divergence of the ABL wind field. Horizontal advection balances excess warming caused by vertical advection, hence the temperature deficit in the ABL weakens over domes and ridges along the prevailing katabatic wind. Conversely, vertical advection is reduced in regions with concave topography, i.e. valleys, where the ABL temperature deficit enlarges along the katabatic wind. Along the coast, horizontal and vertical advection is governed by the inability of the large-scale circulation to adapt to small scale topographic features. Meso-scale topographic structures have thus a strong impact on the ABL winter temperature, besides latitude and surface elevation. During summer, this mechanism is much weaker, and the horizontal variability of ABL temperatures is smaller.
Interaction between plasma synthetic jet and subsonic turbulent boundary layer
Zong, Haohua; Kotsonis, Marios
2017-04-01
This paper experimentally investigates the interaction between a plasma synthetic jet (PSJ) and a subsonic turbulent boundary layer (TBL) using a hotwire anemometer and phase-locked particle imaging velocimetry. The PSJ is interacting with a fully developed turbulent boundary layer developing on the flat wall of a square wind tunnel section of 1.7 m length. The Reynolds number based on the freestream velocity (U∞ = 20 m/s) and the boundary layer thickness (δ99 = 34.5 mm) at the location of interaction is 44 400. A large-volume (1696 mm3) three-electrode plasma synthetic jet actuator (PSJA) with a round exit orifice (D = 2 mm) is adopted to produce high-speed (92 m/s) and short-duration (Tjet = 1 ms) pulsed jets. The exit velocity variation of the adopted PSJA in a crossflow is shown to remain almost identical to that in quiescent conditions. However, the flow structures emanating from the interaction between the PSJ and the TBL are significantly different from what were observed in quiescent conditions. In the midspan xy plane (z = 0 mm), the erupted jet body initially follows a wall-normal trajectory accompanied by the formation of a distinctive front vortex ring. After three convective time scales the jet bends to the crossflow, thus limiting the peak penetration depth to approximately 0.58δ99. Comparison of the normalized jet trajectories indicates that the penetration ability of the PSJ is less than steady jets with the same momentum flow velocity. Prior to the jet diminishing, a recirculation region is observed in the leeward side of the jet body, experiencing first an expansion and then a contraction in the area. In the cross-stream yz plane, the signature structure of jets in a crossflow, the counter-rotating vortex pair (CVP), transports high-momentum flow from the outer layer to the near-wall region, leading to a fuller velocity profile and a drop in the boundary layer shape factor (1.3 to 1.2). In contrast to steady jets, the CVP produced by the PSJ
Bonin, Timothy A.; Blumberg, William G.; Klein, Petra M.; Chilson, Phillip B.
2015-12-01
The nocturnal stable boundary layer (SBL) can generally be classified into the weakly stable boundary layer (wSBL) and very stable boundary layer (vSBL). Within the wSBL, turbulence is relatively continuous, whereas in the vSBL, turbulence is intermittent and not well characterized. Differentiating characteristics of each type of SBL are still unknown. Herein, thermodynamic and kinematic data collected by a suite of instruments in north central Oklahoma in autumn 2012 are analyzed to better understand both SBL regimes and their differentiating characteristics. Many low-level jets were observed during the experiment, as it took place near a climatological maximum. A threshold wind speed, above which bulk shear-generated turbulence develops, is found to exist up to 300 m. The threshold wind speed must also be exceeded at lower heights (down to the surface) in order for strong turbulence to develop. Composite profiles, which are normalized using low-level jet scaling, of potential temperature, wind speed, vertical velocity variance, and the third-order moment of vertical velocity (overline{w'^3}) are produced for weak and moderate/strong turbulence regimes, which exhibit features of the vSBL and wSBL, respectively. Within the wSBL, turbulence is generated at the surface and transported upward. In the vSBL, values of vertical velocity variance are small throughout the entire boundary layer, likely due to the fact that a strong surface inversion typically forms after sunset. The temperature profile tends to be approximately isothermal in the lowest portions of the wSBL, and it did not substantially change over the night. Within both types of SBL, stability in the residual layer tends to increase as the night progresses. It is thought that this stability increase is due to differential warm air advection, which frequently occurs in the southern Great Plains when southerly low-level jets and a typical north-south temperature gradient are present. Differential radiative
Yanase, Kazutaka; Saarenrinne, Pentti
2016-12-15
The boundary layers of rainbow trout, Oncorhynchus mykiss [0.231±0.016 m total body length (L) (mean±s.d.); N=6], swimming at 1.6±0.09 L s -1 (N=6) in an experimental flow channel (Reynolds number, Re=4×10 5 ) with medium turbulence (5.6% intensity) were examined using the particle image velocimetry technique. The tangential flow velocity distributions in the pectoral and pelvic surface regions (arc length from the rostrum, l x =71±8 mm, N=3, and l x =110±13 mm, N=4, respectively) were approximated by a laminar boundary layer model, the Falkner-Skan equation. The flow regime over the pectoral and pelvic surfaces was regarded as a laminar flow, which could create less skin-friction drag than would be the case with turbulent flow. Flow separation was postponed until vortex shedding occurred over the posterior surface (l x =163±22 mm, N=3). The ratio of the body-wave velocity to the swimming speed was in the order of 1.2. This was consistent with the condition of the boundary layer laminarization that had been confirmed earlier using a mechanical model. These findings suggest an energy-efficient swimming strategy for rainbow trout in a turbulent environment. © 2016. Published by The Company of Biologists Ltd.
Nowotarski, C. J.
2017-12-01
Though most strong to violent tornadoes are associated with supercell thunderstorms, quasi-linear convective systems (QLCSs) pose a risk of tornadoes, often at times and locations where supercell tornadoes are less common. Because QLCS low-level mesocyclones and tornado signatures tend to be less coherent, forecasting such tornadoes remains particularly difficult. The majority of simulations of such storms rely on horizontally homogeneous base states lacking resolved boundary layer turbulence and surface fluxes. Previous work has suggested that heterogeneities associated with boundary layer turbulence in the form of horizontal convective rolls can influence the evolution and characteristics of low-level mesocyclones in supercell thunderstorms. This study extends methods for generating boundary layer convection to idealized simulations of QLCSs. QLCS simulations with resolved boundary layer turbulence will be compared against a control simulation with a laminar boundary layer. Effects of turbulence, the resultant heterogeneity in the near-storm environment, and surface friction on bulk storm characteristics and the intensity, morphology, and evolution of low-level rotation will be presented. Although maximum surface vertical vorticity values are similar, when boundary layer turbulence is included, a greater number of miso- and meso-scale vortices develop along the QLCS gust front. The source of this vorticity is analyzed using Eulerian decomposition of vorticity tendency terms and trajectory analysis to delineate the relative importance of surface friction and baroclinicity in generating QLCS vortices. The role of anvil shading in suppressing boundary layer turbulence in the near-storm environment and subsequent effects on QLCS vortices will also be presented. Finally, implications of the results regarding inclusion of more realistic boundary layers in future idealized simulations of deep convection will be discussed.
On the Impact of Wind Farms on a Convective Atmospheric Boundary Layer
Lu, Hao; Porté-Agel, Fernando
2015-10-01
With the rapid growth in the number of wind turbines installed worldwide, a demand exists for a clear understanding of how wind farms modify land-atmosphere exchanges. Here, we conduct three-dimensional large-eddy simulations to investigate the impact of wind farms on a convective atmospheric boundary layer. Surface temperature and heat flux are determined using a surface thermal energy balance approach, coupled with the solution of a three-dimensional heat equation in the soil. We study several cases of aligned and staggered wind farms with different streamwise and spanwise spacings. The farms consist of Siemens SWT-2.3-93 wind turbines. Results reveal that, in the presence of wind turbines, the stability of the atmospheric boundary layer is modified, the boundary-layer height is increased, and the magnitude of the surface heat flux is slightly reduced. Results also show an increase in land-surface temperature, a slight reduction in the vertically-integrated temperature, and a heterogeneous spatial distribution of the surface heat flux.
Investigation of Gas Seeding for Planar Laser-Induced Fluorescence in Hypersonic Boundary Layers
Arisman, C. J.; Johansen, C. T.; Bathel, B. F.; Danehy, P. M.
2015-01-01
Numerical simulations of the gas-seeding strategies required for planar laser-induced fluorescence in a Mach 10 (approximately Mach 8.2 postshock) airflow were performed. The work was performed to understand and quantify the adverse effects associated with gas seeding and to assess various types of seed gas that could potentially be used in future experiments. In prior experiments, NO and NO2 were injected through a slot near the leading edge of a flatplate wedge model used in NASA Langley Research Center's 31 in. Mach 10 air tunnel facility. In this paper, nitric oxide, krypton, and iodine gases were simulated at various injection rates. Simulations showing the deflection of the velocity boundary layer for each of the cases are presented. Streamwise distributions of velocity and concentration boundary-layer thicknesses, as well as vertical distributions of velocity, temperature, and mass distributions, are presented for each of the cases. A comparison between simulated streamwise velocity profiles and experimentally obtained molecular tagging velocimetry profiles using a nitric oxide seeding strategy is performed to verify the influence of such a strategy on the boundary layer. The relative merits of the different seeding strategies are discussed. The results from a custom solver based on OpenFOAM version 2.2.1 are compared against results obtained from ANSYS® Fluent version 6.3.
A Case Study of Offshore Advection of Boundary Layer Rolls over a Stably Stratified Sea Surface
Directory of Open Access Journals (Sweden)
Nina Svensson
2017-01-01
Full Text Available Streaky structures of narrow (8-9 km high wind belts have been observed from SAR images above the Baltic Sea during stably stratified conditions with offshore winds from the southern parts of Sweden. Case studies using the WRF model and in situ aircraft observations indicate that the streaks originate from boundary layer rolls generated over the convective air above Swedish mainland, also supported by visual satellite images showing the typical signature cloud streets. The simulations indicate that the rolls are advected and maintained at least 30–80 km off the coast, in agreement with the streaks observed by the SAR images. During evening when the convective conditions over land diminish, the streaky structures over the sea are still seen in the horizontal wind field; however, the vertical component is close to zero. Thus advected feature from a land surface can affect the wind field considerably for long times and over large areas in coastal regions. Although boundary layer rolls are a well-studied feature, no previous study has presented results concerning their persistence during situations with advection to a strongly stratified boundary layer. Such conditions are commonly encountered during spring in coastal regions at high latitudes.
The effects of external conditions in turbulent boundary layers
Brzek, Brian G.
The effects of multiple external conditions on turbulent boundary layers were studied in detail. These external conditions include: surface roughness, upstream turbulence intensity, and pressure gradient. Furthermore, the combined effects of these conditions show the complicated nature of many realistic flow conditions. It was found that the effects of surface roughness are difficult to generalize, given the importance of so many parameters. These parameters include: roughness geometry, roughness regime, roughness height to boundary layer thickness, (k/delta), roughness parameter, ( k+), Reynolds number, and roughness function (Delta B+). A further complication, is the difficulty in computing the wall shear stress, tauw/rho. For the sand grain type roughness, the mean velocity and Reynolds stresses were studied in inner and outer variables, as well as, boundary layer parameters, anisotropy tensor, production term, and viscous stress and form drag contributions. To explore the effects of roughness and Reynolds number dependence in the boundary layer, a new experiment was carefully designed to properly capture the x-dependence of the single-point statistics. It was found that roughness destroys the viscous layer near the wall, thus, reducing the contribution of the viscous stress in the wall region. As a result, the contribution in the skin friction due to form drag increases, while the viscous stress decreases. This yields Reynolds number invariance in the skin friction, near-wall roughness parameters, and inner velocity profiles as k + increases into the fully rough regime. However, in the transitionally rough regime, (i.e., 5 component shows the largest influence of roughness, where the high peak near the wall was decreased and became nearly flat for the fully rough regime profiles. In addition, the Reynolds stresses in outer variables show self-similarity for fixed experimental conditions. However, as the roughness parameter, k +, increases, all Reynolds stress
National Aerospace Laboratory; 航空宇宙技術研究所
1996-01-01
The following topics were discussed: vortex shedding, laminar boundary layer measurement, vortex ring, turbulent flow measurement, high Reynolds number turbulence, pulsed flow, boundary layer instability, Ekman boundary layer, sound receptivity, Tollmien-Schlichting wave in supersonic boundary layer, flow field instability, turbulent flow pattern, vorticity distribution in shear flow, turbulence wedge, streamwise vortex mixing, thermal convection, oblique wave generation in boundary layer, in...
Four-parametric two-layer algebraic model of transition boundary layer at a planar plate
International Nuclear Information System (INIS)
Labusov, A.N.; Lapin, Yu.V.
1996-01-01
Consideration is given to four-parametric two-layer algebraic model of transition boundary layer on a plane plate, based on generalization of one-parametric algebraic Prandtl-Loitsjansky-Klauzer-3 model. The algebraic model uses Prandtl formulas for mixing path with Loitsjansky damping multiplier in the internal region and the relation for turbulent viscosity, based on universal scales of external region and named the Klauzer-3 formula. 12 refs., 10 figs
Stress Distribution in Layered Elastic Creeping Array with a Vertical Cylindrical Shaft
Directory of Open Access Journals (Sweden)
Bobyleva Tatiana
2017-01-01
Full Text Available Construction should be taking into account the influence of time factor on the stability of the structures. In the paper hereditary creep and homogenization theories are used to determine stresses in the layered elastic creeping array with a vertical shaft. Volterra correspondence principle was applied. As a result, the reduction of a time-dependent elastic creeping problem to a corresponding elastic problem became possible. The method proposes a way to determine average (effective elastic creeping properties and homogenized stress field from known properties of the layers’ components. Creep kernels are of a convolution type and are taken in the exponential form. The problem of heterogeneous elastic creeping environment is reduced to a problem of homogeneous transversely isotropic medium. Different boundary conditions on the cylindrical shaft’s surface were considered. An analytical solution was obtained. These explicit expressions can be useful for the necessary calculations in the construction practice.
Radiative instabilities of atmospheric jets and boundary layers
International Nuclear Information System (INIS)
Candelier, J.
2010-01-01
Complex flows occur in the atmosphere and they can be source of internal gravity waves. We focus here on the sources associated with radiative and shear (or Kelvin-Helmholtz) instabilities. Stability studies of shear layers in a stably stratified fluid concern mainly cases where shear and stratification are aligned along the same direction. In these cases, Miles (1961) and Howard (1961) found a necessary condition for stability based on the Richardson number: Ri ≥ 1/4. In this thesis, we show that this condition is not necessary when shear and stratification are not aligned: we demonstrate that a two-dimensional planar Bickley jet can be unstable for all Richardson numbers. Although the most unstable mode remains 2D, we show there exists an infinite family of 3D unstable modes exhibiting a radiative structure. A WKBJ theory is found to provide the main characteristics of these modes. We also study an inviscid and stratified boundary layer over an inclined wall with non-Boussinesq and compressible effects. We show that this flow is unstable as soon as the wall is not horizontal for all Froude numbers and that strongly stratified 3D perturbations behave exactly like compressible 2D perturbations. Applications of the results to the jet stream and the atmospheric boundary layer are proposed. (author) [fr
Dynamical Properties of Vortex Furrows in Transitioning Boundary Layers
Bernard, Peter
2011-11-01
A vortex filament simulation of the spatially growing transitional boundary layer reveals the presence of low speed streaks underlying furrow-like streamwise oriented folds in the surface vorticity layer (AIAA J. Vol. 48, 2010; Proc. ETC13, 2011). The putative hairpin vortices and packets widely observed in boundary layers are found to be an illusion created by assigning the status of structure to the visualized form of regions of rotational motion created by the vortex furrows. Thus, at best, hairpins roughly describe the shape taken by that part of the vorticity within the furrows that directly causes rotation while ignoring the ``invisible'' and considerable non-rotational part. The life history of the furrows is discussed here including a description of how they grow and the dynamics of the vorticity field within them. Long lived furrows represent ``factories'' within which initially spanwise vorticity progresses from arch to either one or two-lobed mushroom-like structures in a continuous stream. Furrows grow by this same process. At the heart of the furrow phenomenon is a self-reinforcing process by which streamwise vorticity begets more streamwise vorticity.
Estimates of the height of the boundary layer using SODAR and rawinsoundings in Amazonia
Energy Technology Data Exchange (ETDEWEB)
Fisch, G [Instituto de Aeronautica e Espaco (IAE/CTA), Sao Jose dos Campos, 12228-904 (Brazil); Santos, L A R dos [Instituto Nacional de Meteorologia (INMET), BrasIlia, 70680-900 (Brazil)], E-mail: gfisch@iae.cta.br, E-mail: landre@inmet.gov.br
2008-05-01
During the LBA campaign in Amazonia 2002, simultaneous measurements were made of the boundary layer using different instruments (rawinsoundings and SODAR). The profiles of potential temperature and humidity were used to estimates the height of the boundary layer using 3 different techniques. The SODAR's measurements did not capture the shallow morning boundary layer observed at the profiles.
Comments on deriving the equilibrium height of the stable boundary layer
Steeneveld, G.J.; Wiel, van de B.J.H.; Holtslag, A.A.M.
2007-01-01
Recently, the equilibrium height of the stable boundary layer received much attention in a series of papers by Zilitinkevich and co-workers. In these studies the stable boundary-layer height is derived in terms of inverse interpolation of different boundary-layer height scales, each representing a
Modelling the artic stable boundary layer and its coupling to the surface
Steeneveld, G.J.; Wiel, van de B.J.H.; Holtslag, A.A.M.
2006-01-01
The impact of coupling the atmosphere to the surface energy balance is examined for the stable boundary layer, as an extension of the first GABLS (GEWEX Atmospheric Boundary-Layer Study) one-dimensional model intercomparison. This coupling is of major importance for the stable boundary-layer
Interaction of a Boundary Layer with a Turbulent Wake
Piomelli, Ugo
2004-01-01
The objective of this grant was to study the transition mechanisms on a flat-plate boundary layer interacting with the wake of a bluff body. This is a simplified configuration presented and designed to exemplify the phenomena that occur in multi-element airfoils, in which the wake of an upstream element impinges on a downstream one. Some experimental data is available for this configuration at various Reynolds numbers. The first task carried out was the implementation and validation of the immersed-boundary method. This was achieved by performing calculations of the flow over a cylinder at low and moderate Reynolds numbers. The low-Reynolds number results are discussed, which is enclosed as Appendix A. The high-Reynolds number results are presented in a paper in preparation for the Journal of Fluid Mechanics. We performed calculations of the wake-boundary-layer interaction at two Reynolds numbers, Re approximately equal to 385 and 1155. The first case is discussed and a comparison of the two calculations is reported. The simulations indicate that at the lower Reynolds number the boundary layer is buffeted by the unsteady Karman vortex street shed by the cylinder. This is shown: long streaky structures appear in the boundary layer in correspondence of the three-dimensionalities in the rollers. The fluctuations, however, cannot be self-sustained due to the low Reynolds-number, and the flow does not reach a turbulent state within the computational domain. In contrast, in the higher Reynolds-number case, boundary-layer fluctuations persist after the wake has decayed (due, in part, to the higher values of the local Reynolds number Re achieved in this case); some evidence could be observed that a self-sustaining turbulence generation cycle was beginning to be established. A third simulation was subsequently carried out at a higher Reynolds number, Re=3900. This calculation gave results similar to those of the Re=l155 case. Turbulence was established at fairly low
The interaction of synthetic jets with turbulent boundary layers
Cui, Jing
In recent years, a promising approach to the control of wall bounded as well as free shear flows, using synthetic jet (oscillatory jet with zero-net-mass-flux) actuators, has received a great deal of attention. A variety of impressive flow control results have been achieved experimentally by many researchers including the vectoring of conventional propulsive jets, modification of aerodynamic characteristics of bluff bodies, control of lift and drag of airfoils, reduction of skin-friction of a flat plate boundary layer, enhanced mixing in circular jets, and control of external as well as internal flow separation and of cavity oscillations. More recently, attempts have been made to numerically simulate some of these flowfields. Numerically several of the above mentioned flow fields have been simulated primarily by employing the Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with a turbulence model and a limited few by Direct Numerical Simulation (DNS). In simulations, both the simplified boundary conditions at the exit of the jet as well as the details of the cavity and lip have been included. In this dissertation, I describe the results of simulations for several two- and three-dimensional flowfields dealing with the interaction of a synthetic jet with a turbulent boundary layer and control of separation. These simulations have been performed using the URANS equations in conjunction with either one- or a two-equation turbulence model. 2D simulations correspond to the experiments performed by Honohan at Georgia Tech. and 3D simulations correspond to the CFD validation test cases proposed in the NASA Langley Research Center Workshop---"CFD Validation of Synthetic Jets and Turbulent Separation Control" held at Williamsburg VA in March 2004. The sources of uncertainty due to grid resolution, time step, boundary conditions, turbulence modeling etc. have been examined during the computations. Extensive comparisons for various flow variables are made with the
Marzooqi, Mohamed Al; Basha, Ghouse; Ouarda, Taha B. M. J.; Armstrong, Peter; Molini, Annalisa
2014-05-01
Strong sensible heat fluxes and deep turbulent mixing - together with marked dustiness and a low substrate water content - represent a characteristic signature in the boundary layer over hot deserts, resulting in "thicker" mixing layers and peculiar optical properties. Beside these main features however, desert ABLs present extremely complex local structures that have been scarcely addressed in the literature, and whose understanding is essential in modeling processes such as the transport of dust and pollutants, and turbulent fluxes of momentum, heat and water vapor in hyper-arid regions. In this study, we analyze a continuous record of observations of the atmospheric boundary layer (ABL) height from a single lens LiDAR ceilometer operated at Masdar Institute Field Station (24.4oN, 54.6o E, Abu Dhabi, United Arab Emirates), starting March 2013. We compare different methods for the estimation of the ABL height from Ceilometer data such as, classic variance-, gradient-, log gradient- and second derivation-methods as well as recently developed techniques such as the Bayesian Method and Wavelet covariance transform. Our goal is to select the most suited technique for describing the climatology of the ABL in desert environments. Comparison of our results with radiosonde observations collected at the nearby airport of Abu Dhabi indicate that the WCT and the Bayesian method are the most suitable tools to accurately identify the ABL height in all weather conditions. These two methods are used for the definition of diurnal and seasonal climatologies of the boundary layer conditional to different atmospheric stability classes.
Airborne observations of newly formed boundary layer aerosol particles under cloudy conditions
Directory of Open Access Journals (Sweden)
B. Altstädter
2018-06-01
Full Text Available This study describes the appearance of ultrafine boundary layer aerosol particles under classical non-favourable conditions at the research site of TROPOS (Leibniz Institute for Tropospheric Research. Airborne measurements of meteorological and aerosol properties of the atmospheric boundary layer (ABL were repeatedly performed with the unmanned aerial system ALADINA (Application of Light-weight Aircraft for Detecting IN-situ Aerosol during three seasons between October 2013 and July 2015. More than 100 measurement flights were conducted on 23 different days with a total flight duration of 53 h. In 26 % of the cases, maxima of ultrafine particles were observed close to the inversion layer at altitudes between 400 and 600 m and the particles were rapidly mixed vertically and mainly transported downwards during short time intervals of cloud gaps. This study focuses on two measurement days affected by low-level stratocumulus clouds, but different wind directions (NE, SW and minimal concentrations (< 4.6 µg m−3 of SO2, as a common indicator for precursor gases at ground. Taken from vertical profiles, the onset of clouds led to a non-linearity of humidity that resulted in an increased turbulence at the local-scale and caused fast nucleation e.g., but in relation to rapid dilution of surrounding air, seen in sporadic clusters of ground data, so that ultrafine particles disappeared in the verticality. The typical banana shape of new particle formation (NPF and growth was not seen at ground and thus these days might not have been classified as NPF event days by pure surface studies.
Energy Technology Data Exchange (ETDEWEB)
Nazar, R.; Amin, N. [Department of Mathematics, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor (Malaysia); Pop, I. [Faculty of Mathematics, University of Cluj, R-3400 Cluj, CP 253 (Romania)
2004-02-01
The laminar mixed convection boundary-layer flow of a viscous and incompressible fluid past a horizontal circular cylinder, which is maintained at a constant heat flux and is placed in a stream flowing vertically upward has been theoretically studied in this paper. The solutions for the flow and heat transfer characteristics are evaluated numerically for different values of the mixed convection parameter {lambda} with the Prandtl number Pr = 1 and 7, respectively. It is found, as for the case of a heated or cooled cylinder, considered by Merkin [5], that assisting flow delays separation of the boundary-layer and can, if the assisting flow is strong enough, suppress it completely. The opposing flow, on the other side, brings the separation point nearer to the lower stagnation point and for sufficiently strong opposing flows there will not be a boundary-layer on the cylinder. (orig.)
Boundary layer polarization and voltage in the 14 MLT region
Lundin, R.; Yamauchi, M.; Woch, J.; Marklund, G.
1995-05-01
Viking midlatitude observations of ions and electrons in the postnoon auroral region show that field-aligned acceleration of electrons and ions with energies up to a few kiloelectron volts takes place. The characteristics of the upgoing ion beams and the local transverse electric field observed by Viking indicate that parallel ion acceleration is primarily due to a quasi-electrostatic field-aligned acceleration process below Viking altitudes, i.e., below 10,000-13,500 km. A good correlation is found between the maximum upgoing ion beam energy and the depth of the local potential well determined by the Viking electric field experiment within dayside 'ion inverted Vs.' The total transverse potential throughout the entire region near the ion inverted Vs. is generally much higher than the field-aligned potential and may reach well above 10 kV. However, the detailed mapping of the transverse potential out to the boundary layer, a fundamental issue which remains controversial, was not attempted here. An important finding in this study is the strong correlation between the maximum up going ion beam energy of dayside ion inverted Vs and the solar wind velocity. This suggests a direct coupling of the solar wind plasma dynamo/voltage generator to the region of field-aligned particle acceleration. The fact that the center of dayside ion inverted Vs coincide with convection reversals/flow stagnation and upward Birkeland currents on what appears to be closed field lines (Woch et al., 1993), suggests that field-aligned potential structures connect to the inner part of an MHD dyanmo in the low-latitude boundary layer. Thus the Viking observations substantiate the idea of a solar wind induced boundary layer polarization where negatively charged perturbations in the postnoon sector persistently develops along the magnetic field lines, establishing accelerating potential drops along the geomagnetic field lines in the 0.5-10 kV range.
Discussion of boundary-layer characteristics near the casing of an axial-flow compressor
Mager, Artur; Mahoney, John J; Budinger, Ray E
1951-01-01
Boundary-layer velocity profiles on the casing of an axial-flow compressor behind the guide vanes and rotor were measured and resolved into two components: along the streamline of the flow and perpendicular to it. Boundary-layer thickness and the deflection of the boundary layer at the wall were the generalizing parameters. By use of these results and the momentum-integral equations, the characteristics of boundary on the walls of axial-flow compressor are qualitatively discussed. Important parameters concerning secondary flow in the boundary layer appear to be turning of the flow and the product of boundary-layer thickness and streamline curvature outside the boundary layer. Two types of separation are shown to be possible in three dimensional boundary layer.
Effect of nose bluntness on boundary layer stability and transition
Malik, M. R.; Spall, R. E.; Chang, C.-L.
1990-01-01
The effect of nose bluntness on boundary layer instability is studied theoretically for a Mach 8 flow past a 7 degree semivertex cone. The basic flow is computed by solving the parabolized Navier-Stokes equations. Linear stability analysis of the basic flow reveals that, with small amount of bluntness, the critical Reynolds number for the onset of instability increases by an order of magnitude compared to the sharp cone value. The computed second mode frequencies are also in reasonable agreement with the experimental results. The results are used to explain the effect of unit Reynolds number on transition present in the quiet aeroballistic range data.
Fluid Mechanics and Heat Transfer in Transitional Boundary Layers
Wang, Ting
2007-01-01
Experiments have been performed to investigate the effects of elevated free-stream turbulence and streamwise acceleration on flow and thermal structures in transitional boundary layers. The free-stream turbulence ranges from 0.5 to 6.4% and the streamwise acceleration ranges from K = 0 to 0.8 x 10(exp -6). The onset of transition, transition length and the turbulent spot formation rate are determined. The statistical results and conditionally sampled results of th streamwise and cross-stream velocity fluctuations, temperature fluctuations, Reynolds stress and Reynolds heat fluxes are presented.
Boundary-layer effects on cold fronts at a coastline
Garratt, J. R.
1986-07-01
The present note discusses one physical mechanism which may contribute to cold air channelling, manifest as a frontal bulge on a surface-analysis chart, in the coastal region of Victoria in southeast Australia. This involves the modification of boundary-layer air in both offshore (prefrontal) and onshore (postfrontal) flow, and the effect on cross-frontal thermal contrast. The problem is discussed in terms of a north-south-oriented cold front behaving as an atmospheric gravity current, propagating along an east-west-oriented coastline, in the presence of a prefrontal offshore stream.
Dynamics of turbulent spots in transitional boundary layer
Czech Academy of Sciences Publication Activity Database
Hladík, Ondřej; Jonáš, Pavel; Uruba, Václav
2011-01-01
Roč. 318, č. 032028 (2011), s. 1-5 E-ISSN 1742-6596. [European turbulence conference /13./. Warsaw, 12.09.2011-15.09.2011] R&D Projects: GA ČR GA101/08/1112; GA ČR GAP101/10/1230 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer transition * hairpin vortex * calmed region Subject RIV: BK - Fluid Dynamics http://iopscience.iop.org/1742-6596/318/3/032028?fromSearchPage=true
Streaming effect of wall oscillation to boundary layer separation
Wu, X. H.; Wu, J. Z.; Wu, J. M.
1991-01-01
This paper presents a preliminary theoretical result on the time averaged streaming effect of local forcing excitation to the boundary layer separation from smooth surface. The problem is formulated as a periodic disturbance to a basic steady breakaway separating flow, for which the data are taken from a numerical triple-deck solution. The ratio of Strouhal number St and Reynolds number Re plays an important role, both being assumed sufficiently high. The analytical and numerical results show that this streaming effect is quite strong at proper values of St/Re exp 1/4, which may delay or even suppress the separation.
Numerical Simulation of Transition in Hypersonic Boundary Layers
2011-02-01
sile domes. AGARD Report CP 493. Advisory Group for Aerospace Research and Development. 273 Horvath, T. 2002 Boundary layer transition on slender...reference skin-friction coefficient cp , cv Specific heats at constant pressure and volume, respectively cph Phase speed in propagation direction e...y)) 73 and two-dimensional (W = 0): u = U (y) + u′ , (4.9a) v = v′ , (4.9b) w = w′ , (4.9c) p = 1 + p′ , (4.9d) T = T (y) + T ′ , (4.9e) ρ = 1 T (y
Asymptotically optimal unsaturated lattice cubature formulae with bounded boundary layer
Energy Technology Data Exchange (ETDEWEB)
Ramazanov, M D [Institute of Mathematics with Computing Centre, Ufa Science Centre, Russian Academy of Sciences, Ufa (Russian Federation)
2013-07-31
This paper describes a new algorithm for constructing lattice cubature formulae with bounded boundary layer. These formulae are unsaturated (in the sense of Babenko) both with respect to the order and in regard to the property of asymptotic optimality on W{sub 2}{sup m}-spaces, m element of (n/2,∞). Most of the results obtained apply also to W{sub 2}{sup μ}(R{sup n})-spaces with a hypoelliptic multiplier of smoothness μ. Bibliography: 6 titles.
Fluid-membrane tethers: minimal surfaces and elastic boundary layers.
Powers, Thomas R; Huber, Greg; Goldstein, Raymond E
2002-04-01
Thin cylindrical tethers are common lipid bilayer membrane structures, arising in situations ranging from micromanipulation experiments on artificial vesicles to the dynamic structure of the Golgi apparatus. We study the shape and formation of a tether in terms of the classical soap-film problem, which is applied to the case of a membrane disk under tension subject to a point force. A tether forms from the elastic boundary layer near the point of application of the force, for sufficiently large displacement. Analytic results for various aspects of the membrane shape are given.
Earth's magnetosphere formed by the low-latitude boundary layer
Heikkila, W J
2011-01-01
The author argues that, after five decades of debate about the interactive of solar wind with the magnetosphere, it is time to get back to basics. Starting with Newton's law, this book also examines Maxwell's equations and subsidiary equations such as continuity, constitutive relations and the Lorentz transformation; Helmholtz' theorem, and Poynting's theorem, among other methods for understanding this interaction. Includes chapters on prompt particle acceleration to high energies, plasma transfer event, and the low latitude boundary layer More than 200 figures illustrate the text Includes a color insert.
LES of the adverse-pressure gradient turbulent boundary layer
International Nuclear Information System (INIS)
Inoue, M.; Pullin, D.I.; Harun, Z.; Marusic, I.
2013-01-01
Highlights: • The adverse-pressure gradient turbulent boundary layer at high Re is studied. • Wall-model LES works well for nonequilibrium turbulent boundary layer. • Relationship of skin-friction to Re and Clauser pressure parameter is explored. • Self-similarity is observed in the velocity statistics over a wide range of Re. -- Abstract: We describe large-eddy simulations (LES) of the flat-plate turbulent boundary layer in the presence of an adverse pressure gradient. The stretched-vortex subgrid-scale model is used in the domain of the flow coupled to a wall model that explicitly accounts for the presence of a finite pressure gradient. The LES are designed to match recent experiments conducted at the University of Melbourne wind tunnel where a plate section with zero pressure gradient is followed by section with constant adverse pressure gradient. First, LES are described at Reynolds numbers based on the local free-stream velocity and the local momentum thickness in the range 6560–13,900 chosen to match the experimental conditions. This is followed by a discussion of further LES at Reynolds numbers at approximately 10 times and 100 times these values, which are well out of range of present day direct numerical simulation and wall-resolved LES. For the lower Reynolds number runs, mean velocity profiles, one-point turbulent statistics of the velocity fluctuations, skin friction and the Clauser and acceleration parameters along the streamwise, adverse pressure-gradient domain are compared to the experimental measurements. For the full range of LES, the relationship of the skin-friction coefficient, in the form of the ratio of the local free-stream velocity to the local friction velocity, to both Reynolds number and the Clauser parameter is explored. At large Reynolds numbers, a region of collapse is found that is well described by a simple log-like empirical relationship over two orders of magnitude. This is expected to be useful for constant adverse
Two-media boundary layer on a flat plate
Nikolay Ilyich Klyuev; Asgat Gatyatovich Gimadiev; Yuriy Alekseevich Kryukov
2014-01-01
The present paper provides a solution to the problem of a flow over a flat semi-infinite plate set at an angle to the horizon, and having a thin liquid film on its surface by external airflow. The film is formed by extrusion of liquid from the porous wall. The paper proposes a mathematical model of a two-media boundary layer flow. The main characteristics of the flow to a zero and a first approximation are determined. A drop of frictional stress is obtained.
Das, Subrata Kumar; Das, Siddarth Shankar; Saha, Korak; Murali Krishna, U. V.; Dani, K. K.
2018-04-01
Characteristics of Kelvin Helmholtz Instability (KHI) using Doppler wind lidar observation have rarely been reported during the Indian summer monsoon season. In this paper, we present a case study of KHI near planetary boundary layer using Doppler wind lidar and radiosonde measurements at Mahabubnagar, a tropical Indian station. The data was collected during the Integrated Ground Observation Campaign (June-October 2011) under the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment-2011. The continuous wind lidar observation during 10-16 August 2011 shows there is an increase in carrier-to-noise ratio values near planetary boundary layer from 03:00 to 11:00 LT on 13 August; reveals the formation of KHI. There is a strong power bursts pattern corresponding to high turbulence characteristics in the early half of the day. The KHI temporal evolution from initial to dissipating stage is observed with clear variation in the carrier-to-noise ratio values. The observed KHI billows are in the height between 600 and 1200 m and lasted for about 7.5 h. The vertical velocity from Doppler lidar measurement shows the presence of updrafts after breaking of KHI in the boundary layer. The presence of strong wind shear, high stability parameter, low Richardson number and high relative humidity during the enhanced carrier-to-noise ratio period indicates the ideal condition for the formation and persistence of this dynamic instability. A typical characteristic of trapped humidity above the KHI billows suggest the presence of strong inversion. A wavelet analysis of 3-dimensional wind components show dominant periodicity of 45-65 min and the periodicity in vertical wind is more prominent.
Interactions between the thermal internal boundary layer and sea breezes
Energy Technology Data Exchange (ETDEWEB)
Steyn, D.G. [The Univ. of British Columbia, Dept. of Geography, Atmospheric Science Programme, Vancouver (Canada)
1997-10-01
In the absence of complex terrain, strongly curved coastline or strongly varying mean wind direction, the Thermal Internal Boundary Layer (TIBL) has well known square root behaviour with inland fetch. Existing slab modeling approaches to this phenomenon indicate no inland fetch limit at which this behaviour must cease. It is obvious however that the TIBL cannot continue to grow in depth with increasing fetch, since the typical continental Mixed Layer Depths (MLD) of 1500 to 2000 m must be reached between 100 and 200 km from the shoreline. The anticyclonic conditions with attendant strong convection and light winds which drive the TIBL, also drive daytime Sea Breeze Circulations (SBC) in the coastal zone. The onshore winds driving mesoscale advection of cool air are at the core of TIBL mechanisms, and are invariably part of a SBC. It is to be expected that TIBL and SBC be intimately linked through common mechanisms, as well as external conditions. (au)
Estimation of evaporation from equilibrium diurnal boundary layer humidity
Salvucci, G.; Rigden, A. J.; Li, D.; Gentine, P.
2017-12-01
Simplified conceptual models of the convective boundary layer as a well mixed profile of potential temperature (theta) and specific humidity (q) impinging on an initially stably stratified linear potential temperature profile have a long history in atmospheric sciences. These one dimensional representations of complex mixing are useful for gaining insights into land-atmosphere interactions and for prediction when state of the art LES approaches are infeasible. As previously shown (e.g. Betts), if one neglects the role of q in bouyancy, the framework yields a unique relation between mixed layer Theta, mixed layer height (h), and cumulative sensible heat flux (SH) throughout the day. Similarly assuming an initially q profile yields a simple relation between q, h, and cumulative latent heat flux (LH). The diurnal dynamics of theta and q are strongly dependent on SH and the initial lapse rates of theta (gamma_thet) and q (gamma q). In the estimation method proposed here, we further constrain these relations with two more assumptions: 1) The specific humidity is the same at the start of the period of boundary layer growth and at the collapse; and 2) Once the mixed layer reaches the LCL, further drying occurs proportionally to the deardorff convective velocity scale (omega) multiplied by q. Assumption (1) is based on the idea that below the cloud layer, there are no sinks of moisture within the mixed layer (neglecting lateral humidity divergence). Thus the net mixing of dry air aloft with evaporation from the surface must balance. Inclusion of the simple model of moisture loss above the LCL into the bulk-CBL model allows definition of an equilibrium humidity (q) condition at which the diurnal cycle of q repeats (i.e. additions of q from surface balance entrainment of dry air from above). Surprisingly, this framework allows estimation of LH from q, theta, and estimated net radiation by solving for the value of Evaporative Fraction (EF) for which the diurnal cycle of q
International Nuclear Information System (INIS)
Schubert, J.F.
1977-01-01
Acoustic sounder measurements of a vertical profile of the abrupt transition from a laminar to a turbulent atmospheric boundary layer were compared with meteorological measurements made at 10 and 137 m on an instrumented tower. Sounder data show that conditions necessary for the onset of the momentum burst phenomenon exist sometime during a clear afternoon when heat flux changes sign and the planetary surface cools. Under these conditions, the lowest part of the atmospheric boundary layer becomes stable. Prior to this situation, the entire boundary layer is in turbulent motion from surface heating. The boundary layer is then an effective barrier for all fluxes, and as the maximum flux Richardson number is reached at some height close to but above the surface, turbulence is dampened and a laminar layer forms. The profile of this layer is recorded by the sounder. Surface temperature drops, a strong wind shear develops, and the Richardson number decreases below its critical value (Ri/sub cr/<0.25). Subsequently, the laminar layer is eroded by turbulence from above, and with a burst of momentum and heat, it eventually reaches the ground
Cooper, D. I.; Eichinger, W. E.; Ecke, R. E.; Kao, J. C. Y.; Reisner, J. M.; Tellier, L. L.
During the Combined Sensor Program (CSP) in March of 1996, the Los Alamos National Laboratory (LANL) fielded an advanced scanning Raman lidar. The lidar was part of a larger suite of micrometeorological sensors to quantify processes associated with the ocean-atmosphere interface, including intermittency and coherent atmospheric features in the “warm pool” of the Tropical Western Pacific (TWP) near Manus Island (2° S. lat, 147° E. long). Initial inspection of the data has revealed excellent information on the microscale vertical and horizontal spatial and temporal structure of the equatorial Marine Atmospheric Boundary Layer (MABL). The data from this experiment have added to the increasing body of measurements on surface layer convection and intermittency including, for the first time, the observation of microscale cellular convective structures such as hexagonal patterns associated with Rayleigh-Bénard cells.
The time development of the plasma-glass boundary layer in a T-tube
International Nuclear Information System (INIS)
Pavlov, M.; Djurovic, S.
1982-01-01
The refraction of a laser beam by a flat boundary layer between the plasma and the glass plate is analysed. A boundary layer with a constant gradient electron density is assumed. Results of the analysis for plasmas produced in a small T-tube show that the boundary layer thickness increases with time faster than linearly. This means that a relatively fast collapse due to cooling through the boundary layer happens at the second half of the reflected plasma life time, while the boundary layer is negligible thin during the first 2μs after the reflected shock front has passed the point of observation. (author)
Boundary-Layer Control: In Memory of Bill Reynolds
Kim, John
2004-11-01
Professor Bill Reynolds (1933-2004) inspired many students and colleagues with his never-ending curiosity and thought-provoking ideas. Bill's relentless energy, together with his hallmark can-do character and do-it-yourself attitude, led to many seminal contributions to mechanical engineering in general, and fluid mechanics in particular. He has left a lasting impact on many of us, especially for those who had the privilege of working closely with him. Some of my current work on boundary-layer control, the use of neural networks in particular, were inspired by many discussions with Bill. He was among the first to see the potential of control-theoretic approaches for flow control, which has become the main thrust of my current research. Without his continued encouragement, I would not have been deeply involved in this line of research; and perhaps, we would not have seen the current flurry of research activities in applying modern control theories to flow control. In memory of Bill Reynolds, who himself has contributed much to flow control, an analysis of boundary-layer control from a linear system perspective will be presented.
On boundary layer modelling using the ASTEC code
International Nuclear Information System (INIS)
Smith, B.L.
1991-07-01
The modelling of fluid boundary layers adjacent to non-slip, heated surface using the ASTEC code is described. The pricipal boundary layer characteristics are derived using simple dimensional arguments and these are developed into criteria for optimum placement of the computational mesh to achieve realistic simulation. In particular, the need for externally-imposed drag and heat transfer correlations as a function of the local mesh concentration is discussed in the context of both laminar and turbulent flow conditions. Special emphasis is placed in the latter case on the (k-ε) turbulence model, which is standard in the code. As far as possible, the analyses are pursued from first principles, so that no comprehensive knowledge of the history of the subject is required for the general ASTEC user to derive practical advice from the document. Some attention is paid to the use of heat transfer correlations for internal solid/fluid surfaces, whose treatment is not straightforward in ASTEC. It is shown that three formulations are possible to effect the heat transfer, called Explicit, Jacobian and Implicit. The particular advantages and disadvantages of each are discussed with regard to numerical stability and computational efficiency. (author) 18 figs., 1 tab., 39 refs
Experimental research on crossing shock wave boundary layer interactions
Settles, G. S.; Garrison, T. J.
1994-10-01
An experimental research effort of the Penn State Gas Dynamics Laboratory on the subject of crossing shock wave boundary layer interactions is reported. This three year study was supported by AFOSR Grant 89-0315. A variety of experimental techniques were employed to study the above phenomena including planar laser scattering flowfield visualization, kerosene lampblack surface flow visualization, laser-interferometer skin friction surveys, wall static pressure measurements, and flowfield five-hole probe surveys. For a model configuration producing two intersecting shock waves, measurements were made for a range of oblique shock strengths at freestream Mach numbers of 3.0 and 3.85. Additionally, measurements were made at Mach 3.85 for a configuration producing three intersecting waves. The combined experimental dataset was used to formulate the first detailed flowfield models of the crossing-shock and triple-shock wave/boundary layer interactions. The structure of these interactions was found to be similar over a broad range of interaction strengths and is dominated by a large, separated, viscous flow region.
Aeromechanics Analysis of a Boundary Layer Ingesting Fan
Bakhle, Milind A.; Reddy, T. S. R.; Herrick, Gregory P.; Shabbir, Aamir; Florea, Razvan V.
2013-01-01
Boundary layer ingesting propulsion systems have the potential to significantly reduce fuel burn but these systems must overcome the challe nges related to aeromechanics-fan flutter stability and forced response dynamic stresses. High-fidelity computational analysis of the fan a eromechanics is integral to the ongoing effort to design a boundary layer ingesting inlet and fan for fabrication and wind-tunnel test. A t hree-dimensional, time-accurate, Reynolds-averaged Navier Stokes computational fluid dynamics code is used to study aerothermodynamic and a eromechanical behavior of the fan in response to both clean and distorted inflows. The computational aeromechanics analyses performed in th is study show an intermediate design iteration of the fan to be flutter-free at the design conditions analyzed with both clean and distorte d in-flows. Dynamic stresses from forced response have been calculated for the design rotational speed. Additional work is ongoing to expan d the analyses to off-design conditions, and for on-resonance conditions.
Wintertime Boundary Layer Structure in the Grand Canyon.
Whiteman, C. David; Zhong, Shiyuan; Bian, Xindi
1999-08-01
Wintertime temperature profiles in the Grand Canyon exhibit a neutral to isothermal stratification during both daytime and nighttime, with only rare instances of actual temperature inversions. The canyon warms during daytime and cools during nighttime more or less uniformly through the canyon's entire depth. This weak stability and temperature structure evolution differ from other Rocky Mountain valleys, which develop strong nocturnal inversions and exhibit convective and stable boundary layers that grow upward from the valley floor. Mechanisms that may be responsible for the different behavior of the Grand Canyon are discussed, including the possibility that the canyon atmosphere is frequently mixed to near-neutral stratification when cold air drains into the top of the canyon from the nearby snow-covered Kaibab Plateau. Another feature of canyon temperature profiles is the sharp inversions that often form near the canyon rims. These are generally produced when warm air is advected over the canyon in advance of passing synoptic-scale ridges.Wintertime winds in the main canyon are not classical diurnal along-valley wind systems. Rather, they are driven along the canyon axis by the horizontal synoptic-scale pressure gradient that is superimposed along the canyon's axis by passing synoptic-scale weather disturbances. They may thus bring winds into the canyon from either end at any time of day.The implications of the observed canyon boundary layer structure for air pollution dispersion are discussed.
Nonlinear evolution of Mack modes in a hypersonic boundary layer
Chokani, Ndaona
2005-01-01
In hypersonic boundary layer flows the nonlinear disturbance evolution occurs relatively slowly over a very long length scale and has a profound effect on boundary layer transition. In the case of low-level freestream disturbances and negligible surface roughness, the transition is due to the modal growth of exponentially growing Mack modes that are destabilized by wall cooling. Cross-bicoherence measurements, derived from hot-wire data acquired in a quiet hypersonic tunnel, are used to identify and quantify phase-locked, quadratic sum and difference interactions involving the Mack modes. In the early stages of the nonlinear disturbance evolution, cross-bicoherence measurements indicate that the energy exchange between the Mack mode and the mean flow first occurs to broaden the sidebands; this is immediately followed by a sum interaction of the Mack mode to generate the first harmonic. In the next stages of the nonlinear disturbance evolution, there is a difference interaction of the first harmonic, which is also thought to contribute to the mean flow distortion. This difference interaction, in the latter stages, is also accompanied by a difference interaction between Mack mode and first harmonic, and a sum interaction, which forces the second harmonic. Analysis using the digital complex demodulation technique, shows that the low-frequency, phase-locked interaction that is identified in the cross bicoherence when the Mack mode and first harmonic have large amplitudes, arises due to the amplitude modulation of Mack mode and first harmonic.
Transition Delay in Hypersonic Boundary Layers via Optimal Perturbations
Paredes, Pedro; Choudhari, Meelan M.; Li, Fei
2016-01-01
The effect of nonlinear optimal streaks on disturbance growth in a Mach 6 axisymmetric flow over a 7deg half-angle cone is investigated in an e ort to expand the range of available techniques for transition control. Plane-marching parabolized stability equations are used to characterize the boundary layer instability in the presence of azimuthally periodic streaks. The streaks are observed to stabilize nominally planar Mack mode instabilities, although oblique Mack mode disturbances are destabilized. Experimentally measured transition onset in the absence of any streaks correlates with an amplification factor of N = 6 for the planar Mack modes. For high enough streak amplitudes, the transition threshold of N = 6 is not reached by the Mack mode instabilities within the length of the cone, but subharmonic first mode instabilities, which are destabilized by the presence of the streaks, reach N = 6 near the end of the cone. These results suggest a passive flow control strategy of using micro vortex generators to induce streaks that would delay transition in hypersonic boundary layers.
Space Shuttle Boundary Layer Transition Flight Experiment Ground Testing Overview
Berger, Karen T.; Anderson, Brian P.; Campbell, Charles H.
2014-01-01
In support of the Boundary Layer Transition (BLT) Flight Experiment (FE) Project in which a manufactured protuberance tile was installed on the port wing of Space Shuttle Orbiter Discovery for STS-119, STS- 128, STS-131 and STS-133 as well as Space Shuttle Orbiter Endeavour for STS-134, a significant ground test campaign was completed. The primary goals of the test campaign were to provide ground test data to support the planning and safety certification efforts required to fly the flight experiment as well as validation for the collected flight data. These test included Arcjet testing of the tile protuberance, aerothermal testing to determine the boundary layer transition behavior and resultant surface heating and planar laser induced fluorescence (PLIF) testing in order to gain a better understanding of the flow field characteristics associated with the flight experiment. This paper provides an overview of the BLT FE Project ground testing. High-level overviews of the facilities, models, test techniques and data are presented, along with a summary of the insights gained from each test.
International Nuclear Information System (INIS)
Molnary, Leslie de
1993-01-01
A two layer bulk model is used to simulate numerically the time and spatial evolution of concentration of radionuclides in the Planetary Boundary Layer (PBL) for convective and stable conditions. In this model, the closure hypothesis are based on the integrated version of the Turbulent Kinetics Energy (TKE) equation (Smeda,1979). This type of model was adopted here because it is numerically simple to be applied operationally in routine and emergency support systems of atmospheric releases at nuclear power plants, and the hypothesis of the efficiency of the vertical mixing seems to be physically reasonable to simulated PBL evolution for high wind conditions and stable conditions in Subtropical latitudes regions. In order to validate the model to the nuclear power plants of the Centro Experimental Aramar (CEA), located in Ipero, State of Sao Paulo, Brazil, numerical simulations were carried out with initial and boundary conditions based on vertical profiles of temperature and horizontal wind speed and direction obtained from tethered balloon soundings, synoptic charts at 850 hPa and surface observations. Comparisons between a 24 hour long numerical simulation and observations indicate that the model is capable of reproduce the diurnal evolution of temperature and horizontal wind during the convective regime. During stable conditions, the slab model was able to simulate the intensity of the surface inversion as a difference between the mixed layer and surface temperatures. The simulated mixed layer height matches with observations during the convective and stable regime. A daytime release of radionuclides was simulated for CEA region and the results indicated that the maximum relative concentration reaches a distance about 15 Km in 1 hour, varing from 100 times background at the moment of the release to 15 times the background. For night releases, the maximum concentration reaches the same distance in 45 minutes, varing from 100 to 30 times the background values
A stable boundary layer perspective on global temperature trends
International Nuclear Information System (INIS)
McNider, R T; Christy, J R; Biazar, A
2010-01-01
One of the most significant signals in the thermometer-observed temperature record since 1900 is the decrease in the diurnal temperature range over land, largely due to warming of the minimum temperatures. While some data sets have indicated this asymmetrical warming has been reduced since 1979, regional analyses (e.g. East Africa) indicate that the nocturnal warming continues at a pace greater than daytime temperatures. The cause for this night time warming in the observed temperatures has been attributed to a variety of causes. Climate models have in general not replicated the change in diurnal temperature range well. Here we would like to try to distinguish between warming in the nocturnal boundary layer due to a redistribution of heat and warming due to the accumulation of heat. The temperature at night at shelter height is a result of competition between thermal stability and mechanical shear. If stability wins then turbulence is suppressed and the cooling surface becomes cut-off from the warmer air aloft, which leads to sharp decay in surface air temperature. If shear wins, then turbulence is maintained and warmer air from aloft is continually mixed to the surface, which leads to significantly lower cooling rates and warmer temperatures. This warming occurs due to a redistribution of heat. As will be shown by techniques of nonlinear analysis the winner of the stability and shear contest can be very sensitive to changes in greenhouse gas forcing, surface roughness, cloudiness, and surface heat capacity (including soil moisture). Further, the minimum temperatures measured in the nocturnal boundary layer represent only a very shallow layer of the atmosphere which is usually only a few hundred meters thick. It is likely that the observed warming in minimum temperature, whether caused by additional greenhouse forcing or land use changes or other land surface dynamics, is reflecting a redistribution of heat by turbulence-not an accumulation of heat. Because minimum
The stably stratified internal boundary layer for steady and diurnally varying offshore flow
Garratt, J. R.
1987-03-01
A two-dimensional numerical mesoscale model is used to investigate the internal structure and growth of the stably stratified internal boundary layer (IBL) beneath warm, continental air flowing over a cooler sea. Two situations are studied — steady-state and diurnally varying offshore flow. In the steady-state case, vertical profiles of mean quantities and eddy diffusion coefficients ( K) within the IBL show small, but significant, changes with increasing distance from the coast. The top of the IBL is well defined, with large vertical gradients within the layer and a maximum in the coast-normal wind component near the top. Well away from the coast, turbulence, identified by non-zero K, decreases to insignificant levels near the top of the IBL; the IBL itself is characterised by a critical value of the layer-flux Richardson number equal to 0.18. The overall behaviour of the mean profiles is similar to that found in the horizontally homogeneous stable boundary layer over land. A simple physical model is used to relate the depth of the layer h to several relevant physical parameters viz., x, the distance from the coast and U, the large-scale wind (both normal to the coastline) and gδθ/θ, Δθ being the temperature difference between continental mixed-layer air and sea surface, θ is the mean potential temperature and g is the acceleration due to gravity. Excellent agreement with the numerical results is found, with h = 0.014 x 1/2 U ( gδθ/θ)-1/2. In the diurnally varying case, the mean profiles within the IBL show only small differences from the steady-state case, although diurnal variations, particularly in the wind maximum, are evident within a few hundred kilometres of the coast. A mesoscale circulation normal to the coast, and superimposed upon the mean offshore flow, develops seawards of the coastline with maximum vertical velocities about sunset, of depth about 2 km and horizontal scale ≈ 500 km. The circulation is related to the advection, and
Predictions and Verification of an Isotope Marine Boundary Layer Model
Feng, X.; Posmentier, E. S.; Sonder, L. J.; Fan, N.
2017-12-01
A one-dimensional (1D), steady state isotope marine boundary layer (IMBL) model is constructed. The model includes meteorologically important features absent in Craig and Gordon type models, namely height-dependent diffusion/mixing and convergence of subsiding external air. Kinetic isotopic fractionation results from this height-dependent diffusion which starts as pure molecular diffusion at the air-water interface and increases linearly with height due to turbulent mixing. The convergence permits dry, isotopically depleted air subsiding adjacent to the model column to mix into ambient air. In δD-δ18O space, the model results fill a quadrilateral, of which three sides represent 1) vapor in equilibrium with various sea surface temperatures (SSTs) (high d18O boundary of quadrilateral); 2) mixture of vapor in equilibrium with seawater and vapor in the subsiding air (lower boundary depleted in both D and 18O); and 3) vapor that has experienced the maximum possible kinetic fractionation (high δD upper boundary). The results can be plotted in d-excess vs. δ18O space, indicating that these processes all cause variations in d-excess of MBL vapor. In particular, due to relatively high d-excess in the descending air, mixing of this air into the MBL causes an increase in d-excess, even without kinetic isotope fractionation. The model is tested by comparison with seven datasets of marine vapor isotopic ratios, with excellent correspondence; >95% of observational data fall within the quadrilateral area predicted by the model. The distribution of observations also highlights the significant influence of vapor from the nearby converging descending air on isotopic variations in the MBL. At least three factors may explain the affect the isotopic composition of precipitation. The model can be applied to modern as well as paleo- climate conditions.
International Nuclear Information System (INIS)
Benisahnoune, Omar
1996-01-01
A numerical procedure of a turbulent boundary layer with free surface in melted zone of metals is developed to describe interaction between Marangoni convection and turbulence. This study takes into account the phenomena below: Near the surface, vertical motions are damped while stream wise and span wise motions are promoted. Considering a plane surface, the validity of this turbulent model is verified in comparison with experimental results and laminar models. (author) [fr
Energy Technology Data Exchange (ETDEWEB)
Nitis, T.; Moussiopoulos, N. [Aristotle Univ. Thessaloniki (Greece). Lab. of Heat Transfer and Environmental Engineering; Klaic, Z.B. [Univ. of Zagreb (Croatia). Andrija Mohorovicic Geophysical Inst., Faculty of Science; Kitsiou, D. [Univ. of the Aegean, Mytilene (Greece). Dept. of Marine Sciences
2004-07-01
The atmospheric boundary layer height is a fundamental parameter characterising the structure of the lower troposphere. The determination of this parameter is important in applications that range from meteorological modelling and forecasting to dispersion problems of atmospheric pollutants. Since substances emitted into the atmospheric boundary layer are dispersed horizontally and vertically through the action of turbulence, they are well-mixed over this layer that is widely known as ''mixing layer''. There are two basic approaches for the practical estimation of this height; the first approach suggests profile measurements, either in-situ or by remote sounding (sodar, clear-air radar, lidar) and the second one, the use of models with only a few measured parameters as input. As far as the second approach is concerned, the majority of the models use relatively crude estimates of the roughness length that is often based on constant values for land cover. Consequently, the model results are not quite accurate. The present work aims firstly to evaluate the effect of alternative calculations of the roughness length on the non-hydrostatic mesoscale model (MEMO) performance, based on the use of satellite data, and secondly, to estimate the mixing layer height and analyze its variability in relation to underlying topography and land use. Rijeka, a region with complex topography and several islands in its surroundings, offers the opportunity to examine the above mentioned relationships. The non-hydrostatic mesoscale model MEMO was applied under summertime anticyclonic weather conditions during two multi-day periods characterised by stagnant meteorological conditions. The results proved MEMO capable of simulating mesoscale wind flow reasonably well, however, the use of AVHRR satellite data for calculating the roughness length based on the calculation of the NDVI parameter, optimised the model performance and resulted to a more accurate determination of
Directory of Open Access Journals (Sweden)
M. Huang
2015-09-01
Full Text Available The planetary boundary layer (PBL is the lowest part of the atmosphere and where its character is directly affected by its contact with the underlying planetary surface. The PBL is responsible for vertical sub-grid-scale fluxes due to eddy transport in the whole atmospheric column. It determines the flux profiles within the well-mixed boundary layer and the more stable layer above. It thus provides an evolutionary model of atmospheric temperature, moisture (including clouds, and horizontal momentum in the entire atmospheric column. For such purposes, several PBL models have been proposed and employed in the weather research and forecasting (WRF model of which the Yonsei University (YSU scheme is one. To expedite weather research and prediction, we have put tremendous effort into developing an accelerated implementation of the entire WRF model using graphics processing unit (GPU massive parallel computing architecture whilst maintaining its accuracy as compared to its central processing unit (CPU-based implementation. This paper presents our efficient GPU-based design on a WRF YSU PBL scheme. Using one NVIDIA Tesla K40 GPU, the GPU-based YSU PBL scheme achieves a speedup of 193× with respect to its CPU counterpart running on one CPU core, whereas the speedup for one CPU socket (4 cores with respect to 1 CPU core is only 3.5×. We can even boost the speedup to 360× with respect to 1 CPU core as two K40 GPUs are applied.
Boundary layer stability and Arctic climate change: a feedback study using EC-Earth
Energy Technology Data Exchange (ETDEWEB)
Bintanja, R.; Linden, E.C. van der; Hazeleger, W. [Royal Netherlands Meteorological Institute (KNMI), De Bilt (Netherlands)
2012-12-15
Amplified Arctic warming is one of the key features of climate change. It is evident in observations as well as in climate model simulations. Usually referred to as Arctic amplification, it is generally recognized that the surface albedo feedback governs the response. However, a number of feedback mechanisms play a role in AA, of which those related to the prevalent near-surface inversion have received relatively little attention. Here we investigate the role of the near-surface thermal inversion, which is caused by radiative surface cooling in autumn and winter, on Arctic warming. We employ idealized climate change experiments using the climate model EC-Earth together with ERA-Interim reanalysis data to show that boundary-layer mixing governs the efficiency by which the surface warming signal is 'diluted' to higher levels. Reduced vertical mixing, as in the stably stratified inversion layer in Arctic winter, thus amplifies surface warming. Modelling results suggest that both shortwave - through the (seasonal) interaction with the sea ice feedback - and longwave feedbacks are affected by boundary-layer mixing, both in the Arctic and globally, with the effect on the shortwave feedback dominating. The amplifying effect will decrease, however, with climate warming because the surface inversion becomes progressively weaker. We estimate that the reduced Arctic inversion has slowed down global warming by about 5% over the past 2 decades, and we anticipate that it will continue to do so with ongoing Arctic warming. (orig.)
Structure and Optical Properties of the Atmospheric Boundary Layer over Dusty Hot Deserts
Chalermthai, B.; Al Marzooqi, M.; Basha, G.; Ouarda, T.; Armstrong, P.; Molini, A.
2014-12-01
Strong sensible heat fluxes and deep turbulent mixing - together with marked dustiness and a low substrate water content - represent a characteristic signature of the atmospheric boundary layer (ABL) over hot deserts, resulting in "thicker" mixing layers and peculiar optical properties. Beside these main common features however, desert boundary layers present extremely complex local structures that have been scarcely addressed in the literature, and whose understanding is essential in modeling processes such as transport and deposition of dust and pollutants, local wind fields, turbulent fluxes and their impacts on the sustainable development, human health and solar energy harvesting in these regions. In this study, we explore the potential of the joint usage of Lidar Ceilometer backscattering profiles and sun-photometer optical depth retrievals to quantitatively determine the vertical aerosol profile over dusty hot desert regions. Toward this goal, we analyze a continuous record of observations of the atmospheric boundary layer height from a single lens LiDAR ceilometer operated at Masdar Institute Field Station (24.4425N 54.6163E, Abu Dhabi, United Arab Emirates), starting March 2013, and the concurrent measurements of aerosol optical depth derived independently from the Masdar Institute AERONET sun-photometer. The main features of the desert ABL are obtained from the ceilometer range corrected backscattering profiles through bi-dimensional clustering technique we developed as a modification of the recently proposed single-profile clustering method, and therefore "directly" and "indirectly" calibrated to obtain a full diurnal cycle climatology of the aerosol optical depth and aerosol profiles. The challenges and the advantages of applying a similar methodology to the monitoring of aerosols and dust over hyper-arid regions are also discussed, together with the issues related to the sensitivity of commercial ceilometers to changes in the solar background.
Non-linear processes in the Earth atmosphere boundary layer
Grunskaya, Lubov; Valery, Isakevich; Dmitry, Rubay
2013-04-01
The work is connected with studying electromagnetic fields in the resonator Earth-Ionosphere. There is studied the interconnection of tide processes of geophysical and astrophysical origin with the Earth electromagnetic fields. On account of non-linear property of the resonator Earth-Ionosphere the tides (moon and astrophysical tides) in the electromagnetic Earth fields are kinds of polyharmonic nature. It is impossible to detect such non-linear processes with the help of the classical spectral analysis. Therefore to extract tide processes in the electromagnetic fields, the method of covariance matrix eigen vectors is used. Experimental investigations of electromagnetic fields in the atmosphere boundary layer are done at the distance spaced stations, situated on Vladimir State University test ground, at Main Geophysical Observatory (St. Petersburg), on Kamchatka pen., on Lake Baikal. In 2012 there was continued to operate the multichannel synchronic monitoring system of electrical and geomagnetic fields at the spaced apart stations: VSU physical experimental proving ground; the station of the Institute of Solar and Terrestrial Physics of Russian Academy of Science (RAS) at Lake Baikal; the station of the Institute of volcanology and seismology of RAS in Paratunka; the station in Obninsk on the base of the scientific and production society "Typhoon". Such investigations turned out to be possible after developing the method of scanning experimental signal of electromagnetic field into non- correlated components. There was used a method of the analysis of the eigen vectors ofthe time series covariance matrix for exposing influence of the moon tides on Ez. The method allows to distribute an experimental signal into non-correlated periodicities. The present method is effective just in the situation when energetical deposit because of possible influence of moon tides upon the electromagnetic fields is little. There have been developed and realized in program components
Boundary-layer height detection with a ceilometer at a coastal site in western Denmark
DEFF Research Database (Denmark)
Hannesdóttir, Ásta; Hansen, Aksel Walle
in atmospheric transport- and dispersion models. A new method of filtering clouds from the ceilometer data is presented. This allows for the inclusion of more than half of the data in the subsequent analysis, as the presence of clouds would otherwise complicate the boundary-layer height estimations. The boundary....... The boundary-layer height estimates are then used to analyse the daily evolution of the boundary layer and to perform monthly and annual frequency distributions of the boundary-layer height. For westerly winds bi-modal distributions are often found, which may be separated by different criteria, while...
The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence
Lothon, M.; Lohou, F.; Pino, D.; Vilà-Guerau De Arellano, J.; Hartogensis, O.K.; Boer, van de A.; Coster, de O.; Moene, A.F.; Steeneveld, G.J.
2014-01-01
Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary
Investigation of transition scenarios in boundary-layer flows
Energy Technology Data Exchange (ETDEWEB)
Stolte, A.
1999-11-01
Laminar-turbulent transition mechanisms triggered by crossflow instability in three-dimensional, accelerated boundary-layer flows are investigated using numerical methods of stability analysis. The investigations are based on the DLR swept plate experiment, where stationary and traveling crossflow modes can be selectively introduced into the flow field. Nonlinear instability analyses employing the parabolized stability equations (PSE) show that unique saturation amplitudes do neither exist for stationary crossflow vortices nor for traveling crossflow waves. This phenomenon is explained by means of a spatial bifurcation model. Using Floquet theory, temporal secondary instability analyses are then performed for the mean flow distorted by primary disturbances. In these analyses, secondary high-frequency disturbances with high growth rates are found. The location of these disturbances correlates well with regions of high shear in the primarily distorted flow field, especially on the back of the primary crossflow vortices. (orig.)
On the secondary instability of three-dimensional boundary layers
Energy Technology Data Exchange (ETDEWEB)
Janke, E. [DLR Deutsches Zentrum fuer Luft- und Raumfahrt e.V., Goettingen (Germany). Inst. fuer Stroemungsmechanik; Balakumar, P. [Department of Aerospace Engineering, Old Dominion University, Norfolk, VA 23529 (United States)
2000-09-01
One of the possible transition scenarios in three-dimensional boundary layers, the saturation of stationary crossflow vortices and their secondary instability to high-frequency disturbances, is studied using the parabolized stability equations (PSE) and Floquet theory. Starting from nonlinear PSE solutions, we investigate the region where a purely stationary crossflow disturbance saturates for its secondary instability characteristics utilizing global and local eigenvalue solvers that are based on the implicitly restarted Arnoldi method and a Newton-Raphson technique, respectively. Results are presented for swept Hiemenz flow and the DLR swept flat plate experiment. The main focuses of this study are on the existence of multiple roots in the eigenvalue spectrum that could explain experimental observations of time-dependent occurrences of an explosive growth of traveling disturbances, on the origin of high-frequency disturbances, as well as on gaining more information about threshold amplitudes of primary disturbances necessary for the growth of secondary disturbances. (orig.)
Small particle transport across turbulent nonisothermal boundary layers
Rosner, D. E.; Fernandez De La Mora, J.
1982-01-01
The interaction between turbulent diffusion, Brownian diffusion, and particle thermophoresis in the limit of vanishing particle inertial effects is quantitatively modeled for applications in gas turbines. The model is initiated with consideration of the particle phase mass conservation equation for a two-dimensional boundary layer, including the thermophoretic flux term directed toward the cold wall. A formalism of a turbulent flow near a flat plate in a heat transfer problem is adopted, and variable property effects are neglected. Attention is given to the limit of very large Schmidt numbers and the particle concentration depletion outside of the Brownian sublayer. It is concluded that, in the parameter range of interest, thermophoresis augments the high Schmidt number mass-transfer coefficient by a factor equal to the product of the outer sink and the thermophoretic suction.
Numerical treatment of the unsteady hydromagnetic thermal boundary layer problem
International Nuclear Information System (INIS)
Drymonitou, M.A.; Geroyannis, V.S.; Goudas, C.L.
1980-01-01
This paper presents a suitable numerical method for the treatment of the unsteady hydromagnetic thermal boundary layer problem for flows past an infinite porous flat plate, the motion of which is governed by a general time-dependent law, under the influence of a transverse externally set magnetic field. The normal velocity of suction/injection at the plate is also assumed to be time-dependent. The results obtained on the basis of numerical approximations seem to compare favourably with earlier results (Pande et al., 1976; Tokis, 1978). Analytical approximations are given for the cases of a plate (i) generally accelerated and (ii) harmonically oscillating. The direct numerical treatment is obviously advantageous since it allows handling of cases where the known methods for analytical approximations are not applicable. This problem is closely related to the motions and heat transfer occurring locally on the surfaces of stars. (orig.)
The large Reynolds number - Asymptotic theory of turbulent boundary layers.
Mellor, G. L.
1972-01-01
A self-consistent, asymptotic expansion of the one-point, mean turbulent equations of motion is obtained. Results such as the velocity defect law and the law of the wall evolve in a relatively rigorous manner, and a systematic ordering of the mean velocity boundary layer equations and their interaction with the main stream flow are obtained. The analysis is extended to the turbulent energy equation and to a treatment of the small scale equilibrium range of Kolmogoroff; in velocity correlation space the two-thirds power law is obtained. Thus, the two well-known 'laws' of turbulent flow are imbedded in an analysis which provides a great deal of other information.
Characterization of Alfvenic fluctuations in the magnetopause boundary layer
International Nuclear Information System (INIS)
Rezeau, L.; Morane, A.; Perraut, S.; Roux, A.; Schmidt, R.
1989-01-01
The European Space Agency GEOS 2 spacecraft happened to cross the magnetopause several times, at various local times. Intense electric and magnetic fluctuations, in the ultralow-frequency (ULF) range (0-10 Hz) have been detected during each such crossing, with a peak at the magnetopause and still large amplitudes in the adjacent magnetosheath and magnetopause boundary layer. By applying spectral analysis and correlations to the electric and magnetic fluctuations, and a minimum variance analysis to the magnetic fluctuations, the authors investigate the nature of these fluctuations which appear as short-lasting bursts in the spacecraft frame. Having reviewed possible interpretations, they show that the observed electric and magnetic signatures are consistent with small-scale (L ∼ ion Larmor radius) Alfvenic field-aligned structures passing by the spacecraft at high speed. It is suggested that these structures correspond to nonlinear Alfvenic structures
Turbulent thermal boundary layer on a permeable flat plate
International Nuclear Information System (INIS)
Vigdorovich, I. I.
2007-01-01
Scaling laws are established for the profiles of temperature, turbulent heat flux, rms temperature fluctuation, and wall heat transfer in the turbulent boundary layer on a flat plate with transpiration. In the case of blowing, the temperature distribution represented in scaling variables outside the viscous sublayer has a universal form known from experimental data for flows over impermeable flat plates. In the case of suction, the temperature distribution is described by a one-parameter family of curves. A universal law of heat transfer having the form of a generalized Reynolds analogy provides a basis for representation of the heat flux distributions corresponding to different Reynolds numbers and transpiration velocities in terms of a function of one variable. The results are obtained without invoking any special closure hypotheses
Buckling transition and boundary layer in non-Euclidean plates.
Efrati, Efi; Sharon, Eran; Kupferman, Raz
2009-07-01
Non-Euclidean plates are thin elastic bodies having no stress-free configuration, hence exhibiting residual stresses in the absence of external constraints. These bodies are endowed with a three-dimensional reference metric, which may not necessarily be immersible in physical space. Here, based on a recently developed theory for such bodies, we characterize the transition from flat to buckled equilibrium configurations at a critical value of the plate thickness. Depending on the reference metric, the buckling transition may be either continuous or discontinuous. In the infinitely thin plate limit, under the assumption that a limiting configuration exists, we show that the limit is a configuration that minimizes the bending content, among all configurations with zero stretching content (isometric immersions of the midsurface). For small but finite plate thickness, we show the formation of a boundary layer, whose size scales with the square root of the plate thickness and whose shape is determined by a balance between stretching and bending energies.
A preliminary assessment of the Titan planetary boundary layer
Allison, Michael
1992-01-01
Results of a preliminary assessment of the characteristic features of the Titan planetary boundary are addressed. These were derived from the combined application of a patched Ekman surface layer model and Rossby number similarity theory. Both these models together with Obukhov scaling, surface speed limits and saltation are discussed. A characteristic Akman depth of approximately 0.7 km is anticipated, with an eddy viscosity approximately equal to 1000 sq cm/s, an associated friction velocity approximately 0.01 m/s, and a surface wind typically smaller than 0.6 m/s. Actual values of these parameters probably vary by as much as a factor of two or three, in response to local temporal variations in surface roughness and stability. The saltation threshold for the windblown injection of approximately 50 micrometer particulates into the atmosphere is less than twice the nominal friction velocity, suggesting that dusty breezes might be an occassional feature of the Titan meteorology.
Combined core/boundary layer plasma transport simulations in tokamaks
International Nuclear Information System (INIS)
Prinja, A.K.; Schafer, R.F. Jr.; Conn, R.W.; Howe, H.C.
1987-01-01
Significant new numerical results are presented from self-consistent core and boundary or scrape-off layer plasma simulations with 3-D neutral transport calculations. For a symmetric belt limiter it is shown that, for plasma conditions considered here, the pump limiter collection efficiency increases from 11% to 18% of the core efflux as a result of local reionization of blade deflected neutrals. This hitherto unobserved effect causes a significant amplification of upstream ion flux entering the pump limiter. Results from coupling of an earlier developed two-zone edge plasma model ODESSA to the PROCTR core plasma simulation code indicates that intense recycling divertor operation may not be possible because of stagnation of upstream flow velocity. This results in a self-consistent reduction of density gradient in an intermediate region between the central plasma and separatrix, and a concomitant reduction of core-efflux. There is also evidence of increased recycling at the first wall. (orig.)
Rapid cycling of reactive nitrogen in the marine boundary layer.
Ye, Chunxiang; Zhou, Xianliang; Pu, Dennis; Stutz, Jochen; Festa, James; Spolaor, Max; Tsai, Catalina; Cantrell, Christopher; Mauldin, Roy L; Campos, Teresa; Weinheimer, Andrew; Hornbrook, Rebecca S; Apel, Eric C; Guenther, Alex; Kaser, Lisa; Yuan, Bin; Karl, Thomas; Haggerty, Julie; Hall, Samuel; Ullmann, Kirk; Smith, James N; Ortega, John; Knote, Christoph
2016-04-28
Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed. A 'renoxification' process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth's surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.
Directory of Open Access Journals (Sweden)
J. Song
2016-05-01
Full Text Available Urban land–atmosphere interactions can be captured by numerical modeling framework with coupled land surface and atmospheric processes, while the model performance depends largely on accurate input parameters. In this study, we use an advanced stochastic approach to quantify parameter uncertainty and model sensitivity of a coupled numerical framework for urban land–atmosphere interactions. It is found that the development of urban boundary layer is highly sensitive to surface characteristics of built terrains. Changes of both urban land use and geometry impose significant impact on the overlying urban boundary layer dynamics through modification on bottom boundary conditions, i.e., by altering surface energy partitioning and surface aerodynamic resistance, respectively. Hydrothermal properties of conventional and green roofs have different impacts on atmospheric dynamics due to different surface energy partitioning mechanisms. Urban geometry (represented by the canyon aspect ratio, however, has a significant nonlinear impact on boundary layer structure and temperature. Besides, managing rooftop roughness provides an alternative option to change the boundary layer thermal state through modification of the vertical turbulent transport. The sensitivity analysis deepens our insight into the fundamental physics of urban land–atmosphere interactions and provides useful guidance for urban planning under challenges of changing climate and continuous global urbanization.
The Stokes boundary layer for a thixotropic or antithixotropic fluid
McArdle, Catriona R.
2012-10-01
We present a mathematical investigation of the oscillatory boundary layer in a semi-infinite fluid bounded by an oscillating wall (the so-called \\'Stokes problem\\'), when the fluid has a thixotropic or antithixotropic rheology. We obtain asymptotic solutions in the limit of small-amplitude oscillations, and we use numerical integration to validate the asymptotic solutions and to explore the behaviour of the system for larger-amplitude oscillations. The solutions that we obtain differ significantly from the classical solution for a Newtonian fluid. In particular, for antithixotropic fluids the velocity reaches zero at a finite distance from the wall, in contrast to the exponential decay for a thixotropic or a Newtonian fluid.For small amplitudes of oscillation, three regimes of behaviour are possible: the structure parameter may take values defined instantaneously by the shear rate, or by a long-term average; or it may behave hysteretically. The regime boundaries depend on the precise specification of structure build-up and breakdown rates in the rheological model, illustrating the subtleties of complex fluid models in non-rheometric settings. For larger amplitudes of oscillation the dominant behaviour is hysteretic. We discuss in particular the relationship between the shear stress and the shear rate at the oscillating wall. © 2012 Elsevier B.V.
Effect of boundary layer thickness on the flow characteristics around a rectangular prism
International Nuclear Information System (INIS)
Ji, Ho Seong; Kim, Kyung Chun
2001-01-01
Effect of boundary layer thickness on the flow characteristics around a rectangular prism has been investigated by using a PIV(Particle Image Velocimetry) technique. Three different boundary layers (thick, medium and thin) were generated in the atmospheric boundary layer wind tunnel at Pusan National University. The thick boundary layer having 670mm thickness was generated by using spires and roughness elements. The medium thickness of boundary layer(δ=270mm) was the natural turbulent boundary layer at the test section with fully long developing length(18m). The thin boundary layer with 36.5mm thickness was generated by on a smooth panel elevated 70cm from the wind tunnel floor. The Reynolds number based on the free stream velocity and the height of the model was 7.9X10 3 . The mean velocity vector fields and turbulent kinetic energy distribution were measured and compared. The effect of boundary layer thickness is clearly observed not only in the length of separation bubble but also in the reattachment points. The thinner boundary layer thickness, the higher turbulent kinetic energy peak around the model roof. It is strongly recommended that the height ratio between model and approaching boundary layer thickness should be a major parameter
Energy Technology Data Exchange (ETDEWEB)
Colette, A
2005-12-15
Closing the tropospheric ozone budget requires a better understanding of the role of transport processes from the major reservoirs: the planetary boundary layer and the stratosphere. Case studies lead to the identification of mechanisms involved as well as their efficiency. However, their global impact on the budget must be addressed on a climatological basis. This manuscript is thus divided in two parts. First, we present case studies based on ozone LIDAR measurements performed during the ESCOMPTE campaign. This work consists in a data analysis investigation by means of a hybrid - Lagrangian study involving: global meteorological analyses, Lagrangian particle dispersion computation, and mesoscale, chemistry - transport, and Lagrangian photochemistry modeling. Our aim is to document the amount of observed ozone variability related to transport processes and, when appropriate, to infer the role of tropospheric photochemical production. Second, we propose a climatological analysis of the respective impact of transport from the boundary-layer and from the tropopause region on the tropospheric ozone budget. A multivariate analysis is presented and compared to a trajectography approach. Once validated, this algorithm is applied to the whole database of ozone profiles collected above Europe during the past 30 years in order to discuss the seasonal, geographical and temporal variability of transport processes as well as their impact on the tropospheric ozone budget. The variability of turbulent mixing and its impact on the persistence of tropospheric layers will also be discussed. (author)
Energy Technology Data Exchange (ETDEWEB)
Colette, A
2005-12-15
Closing the tropospheric ozone budget requires a better understanding of the role of transport processes from the major reservoirs: the planetary boundary layer and the stratosphere. Case studies lead to the identification of mechanisms involved as well as their efficiency. However, their global impact on the budget must be addressed on a climatological basis. This manuscript is thus divided in two parts. First, we present case studies based on ozone LIDAR measurements performed during the ESCOMPTE campaign. This work consists in a data analysis investigation by means of a hybrid - Lagrangian study involving: global meteorological analyses, Lagrangian particle dispersion computation, and mesoscale, chemistry - transport, and Lagrangian photochemistry modeling. Our aim is to document the amount of observed ozone variability related to transport processes and, when appropriate, to infer the role of tropospheric photochemical production. Second, we propose a climatological analysis of the respective impact of transport from the boundary-layer and from the tropopause region on the tropospheric ozone budget. A multivariate analysis is presented and compared to a trajectography approach. Once validated, this algorithm is applied to the whole database of ozone profiles collected above Europe during the past 30 years in order to discuss the seasonal, geographical and temporal variability of transport processes as well as their impact on the tropospheric ozone budget. The variability of turbulent mixing and its impact on the persistence of tropospheric layers will also be discussed. (author)
Analysis of the vertical penetration of a heated fluid layer in a solid, miscible bed
International Nuclear Information System (INIS)
Eck, G.
1982-03-01
The present study investigates the mass and heat transfer for the vertical penetration of a heated fluid layer in a solid, miscible bed using water-salt solutions (ZnBr 2 , NaBr) and polyethylenglycol 1500 (PEG) as simulation materials. The time depending spatial distribution of the molten material (PEG) has been measured for the first time with conductivity probes. The dependence of the downward heat flux on the density ratio rho*, i.e. the density of the fluid / the density of the molten solid, has been investigated with two different methods of heating, planar heating with a heat exchanger in a defined initial distance to the PEG-surface and electrolytical volume heating with a defined and timely constant power input. For 1 2 two layers have been observed in the fluid. This phenomenon is caused among other things by an anomality of the mixture density of the system salt solution-PEG. This process affects the downward heat flux so strongly, that it is impossible to transfer the results of such a system in this region of rho* to another system, for example to a corecatcher. The discrepancies between the measured heat fluxes and heat transfer coefficients of this study and that of other authors can be explained by the different construction of the planar heater, or by different boundary conditions in the case of volume heating. (orig.) [de
Quantification of topographic venting of boundary layer air to the free troposphere
Directory of Open Access Journals (Sweden)
S. Henne
2004-01-01
Full Text Available Net vertical air mass export by thermally driven flows from the atmospheric boundary layer (ABL to the free troposphere (FT above deep Alpine valleys was investigated. The vertical export of pollutants above mountainous terrain is presently poorly represented in global chemistry transport models (GCTMs and needs to be quantified. Air mass budgets were calculated using aircraft observations obtained in deep Alpine valleys. The results show that on average 3 times the valley air mass is exported vertically per day under fair weather conditions. During daytime the type of valleys investigated in this study can act as an efficient 'air pump' that transports pollutants upward. The slope wind system within the valley plays an important role in redistributing pollutants. Nitrogen oxide emissions in mountainous regions are efficiently injected into the FT. This could enhance their ozone (O3 production efficiency and thus influences tropospheric pollution budgets. Once lifted to the FT above the Alps pollutants are transported horizontally by the synoptic flow and are subject to European pollution export. Forward trajectory studies show that under fair weather conditions two major pathways for air masses above the Alps dominate. Air masses moving north are mixed throughout the whole tropospheric column and further transported eastward towards Asia. Air masses moving south descend within the subtropical high pressure system above the Mediterranean.
Toward low-cloud-permitting cloud superparameterization with explicit boundary layer turbulence
Parishani, Hossein; Pritchard, Michael S.; Bretherton, Christopher S.; Wyant, Matthew C.; Khairoutdinov, Marat
2017-07-01
Systematic biases in the representation of boundary layer (BL) clouds are a leading source of uncertainty in climate projections. A variation on superparameterization (SP) called "ultraparameterization" (UP) is developed, in which the grid spacing of the cloud-resolving models (CRMs) is fine enough (250 × 20 m) to explicitly capture the BL turbulence, associated clouds, and entrainment in a global climate model capable of multiyear simulations. UP is implemented within the Community Atmosphere Model using 2° resolution (˜14,000 embedded CRMs) with one-moment microphysics. By using a small domain and mean-state acceleration, UP is computationally feasible today and promising for exascale computers. Short-duration global UP hindcasts are compared with SP and satellite observations of top-of-atmosphere radiation and cloud vertical structure. The most encouraging improvement is a deeper BL and more realistic vertical structure of subtropical stratocumulus (Sc) clouds, due to stronger vertical eddy motions that promote entrainment. Results from 90 day integrations show climatological errors that are competitive with SP, with a significant improvement in the diurnal cycle of offshore Sc liquid water. Ongoing concerns with the current UP implementation include a dim bias for near-coastal Sc that also occurs less prominently in SP and a bright bias over tropical continental deep convection zones. Nevertheless, UP makes global eddy-permitting simulation a feasible and interesting alternative to conventionally parameterized GCMs or SP-GCMs with turbulence parameterizations for studying BL cloud-climate and cloud-aerosol feedback.
Boundary layer structure observed by Shipborne Doppler Sodar in the Suez Canal zone
International Nuclear Information System (INIS)
Fiocco, G.; Mastrantonio, G.; Ricotta, A.
1980-01-01
Observations of the boundary layer with a monostatic Sodar and other instrumentation were carried out in the Suez canal zone in January and March 1979, from the Italian m.v. Salernum on its way to and from a GARP assignment. The Sodar was operated almost continuously throughout the passages. In addition to the intensity records, an off-line Doppler analysis involving the use of the fast Fourier transform of the digitized Sodar echoes has provided the vertical component w of the velocity. In general, because of efficient filtering and of the good quality of the data, a vertical profile of w has been determined for each transmitted pulse. During a prolonged stay in the Bitter lakes vertical profiles of temperature and humidity were also obtained by deploying a tethered balloon. Large temporal and spatial temperature contrasts exist in the canal zone due to the presence of the desert and of large and small bodies of water. The ensuring phenomenology during the time of observation was quite varied. (author)
Boundary layer structure observed by Shipborne Doppler Sonar in the Suez Canal zone
Energy Technology Data Exchange (ETDEWEB)
Fiocco, G. (Rome Univ. (Italy). Ist. di Fisica); Mastrantonio, G.; Ricotta, A. (Consiglio Nazionale delle Ricerche, Frascati (Italy). Lab. per il Plasma nello Spazio)
Observations of the boundary layer with a monostatic Sonar and other instrumentation were carried out in the Suez canal zone in January and March 1979, from the Italian m.v. Salernum on its way to and from a GARP assignment. The Sonar was operated almost continuously throughout the passages. In addition to the intensity records, an off-line Doppler analysis involving the use of the fast Fourier transform of the digitized Sonar echoes has provided the vertical component w of the velocity. In general, because of efficient filtering and of the good quality of the data, a vertical profile of w has been determined for each transmitted pulse. During a prolonged stay in the Bitter lakes vertical profiles of temperature and humidity were also obtained by deploying a tethered balloon. Large temporal and spatial temperature contrasts exist in the canal zone due to the presence of the desert and of large and small bodies of water. The ensuring phenomenology during the time of observation was quite varied.
Studies of planetary boundary layer by infrared thermal imagery
Energy Technology Data Exchange (ETDEWEB)
Albina, Bogdan; Dimitriu, Dan Gheorghe, E-mail: dimitriu@uaic.ro; Gurlui, Silviu Octavian, E-mail: dimitriu@uaic.ro [Alexandru Ioan Cuza University of Iasi, Faculty of Physics, Atmosphere Optics, Spectroscopy and Lasers Laboratory, 11 Carol I Blvd., 700506 Iasi (Romania); Cazacu, Marius Mihai [Alexandru Ioan Cuza University of Iasi, Faculty of Physics, Atmosphere Optics, Spectroscopy and Lasers Laboratory, 11 Carol I Blvd., 700506 Iasi, Romania and Department of Physics, Gheorghe Asachi Technical University of Iasi, 59A Mangeron Blvd., 700 (Romania); Timofte, Adrian [Alexandru Ioan Cuza University of Iasi, Faculty of Physics, Atmosphere Optics, Spectroscopy and Lasers Laboratory, 11 Carol I Blvd., 700506 Iasi, Romania and National Meteorological Administration, Regional Forecast Center Bacau, 1 Cuza Voda Str., 60 (Romania)
2014-11-24
The IR camera is a relatively novel device for remote sensing of atmospheric thermal processes from the Planetary Boundary Layer (PBL) based on measurements of the infrared radiation. Infrared radiation is energy radiated by the motion of atoms and molecules on the surface of aerosols, when their temperature is more than absolute zero. The IR camera measures directly the intensity of radiation emitted by aerosols which is converted by an imaging sensor into an electric signal, resulting a thermal image. Every image pixel that corresponds to a specific radiance is pre-processed to identify the brightness temperature. The thermal infrared imaging radiometer used in this study, NicAir, is a precision radiometer developed by Prata et al. The device was calibrated for the temperature range of 270–320 K and using a calibration table along with image processing software, important information about variations in temperature can be extracted from acquired IR images. The PBL is the lowest layer of the troposphere where the atmosphere interacts with the ground surfaces. The importance of PBL lies in the fact that it provides a finite but varying volume in which pollutants can disperse. The aim of this paper is to analyze the PBL altitude and thickness variations over Iasi region using the IR imaging camera as well as its behavior from day to night and thermal processes occurring in PBL.
Complementary aspects on matter-antimatter boundary layers
International Nuclear Information System (INIS)
Lehnert, B.
1990-05-01
This paper gives some complementary aspects on the problems of the matter-antimatter metagalaxy model and its cellular structure, as being proposed by Klein and Alfven. A previously outlined one-dimensional model of a magnetized matter-antimatter boundary layer is updated and extended, by introducing amended nuclear annihilation data, and by making improved approximations of the layer structure and its dependence on relevant parameters. The critical beta value obtained from this model leads to critical plasma densities which are not high enough to become reconcilable with a cellular matter-antimatter structure within the volume of a galaxy. Additional investigations are required on the questions whether the obtained beta limit would still apply to cells of the size of a galaxy, and whether large modification of this limit could result from further refinement of the theory and from the transition to a three-dimensional model. Attention is called to the wide area of further research on ambiplasma physics, and on a three-dimensional cell structure with associated problems of equilibrium and stability. In particular, the high-energy ambiplasma component has to be further analysed in terms of kinetic theory, on account of the large Larmor radii of the corresponding electrons and positrons
Uncertainties in the CO2 buget associated to boundary layer dynamics and CO2-advection
Kaikkonen, J.P.; Pino, D.; Vilà-Guerau de Arellano, J.
2012-01-01
The relationship between boundary layer dynamics and carbon dioxide (CO2) budget in the convective boundary layer (CBL) is investigated by using mixed-layer theory. We derive a new set of analytical relations to quantify the uncertainties on the estimation of the bulk CO2 mixing ratio and the
Response of the Atmospheric Boundary Layer and Soil Layer to a High Altitude, Dense Aerosol Cover.
Garratt, J. R.; Pittock, A. B.; Walsh, K.
1990-01-01
The response of the atmospheric boundary layer to the appearance of a high-altitude smoke layer has been investigated in a mesoscale numerical model of the atmosphere. Emphasis is placed on the changes in mean boundary-layer structure and near-surface temperatures when smoke of absorption optical depth (AOD) in the, range 0 to 1 is introduced. Calculations have been made at 30°S, for different soil thermal properties and degrees of surface wetness, over a time period of several days during which major smoke-induced cooling occurs. The presence of smoke reduces the daytime mixed-layer depth and, for large enough values of AOD, results in a daytime surface inversion with large cooling confined to heights of less than a few hundred meters. Smoke-induced reductions in daytime soil and air temperatures of several degrees are typical, dependent critically upon soil wetness and smoke AOD. Locations near the coast experience reduced cooling whenever there is a significant onshore flow related to a sea breeze (this would also be the case with a large-scale onshore flow). The sea breeze itself disappears for large enough smoke AOD and, over sloping coastal terrain, a smoke-induced, offshore drainage flow may exist throughout the diurnal cycle.
Multi-scale model analysis of boundary layer ozone over East Asia
Directory of Open Access Journals (Sweden)
M. Lin
2009-05-01
Full Text Available This study employs the regional Community Multiscale Air Quality (CMAQ model to examine seasonal and diurnal variations of boundary layer ozone (O_{3} over East Asia. We evaluate the response of model simulations of boundary layer O_{3} to the choice of chemical mechanisms, meteorological fields, boundary conditions, and model resolutions. Data obtained from surface stations, aircraft measurements, and satellites are used to advance understanding of O_{3} chemistry and mechanisms over East Asia and evaluate how well the model represents the observed features. Satellite measurements and model simulations of summertime rainfall are used to assess the impact of the Asian monsoon on O_{3} production. Our results suggest that summertime O_{3} over Central Eastern China is highly sensitive to cloud cover and monsoonal rainfall over this region. Thus, accurate simulation of the East Asia summer monsoon is critical to model analysis of atmospheric chemistry over China. Examination of hourly summertime O_{3} mixing ratios from sites in Japan confirms the important role of diurnal boundary layer fluctuations in controlling ground-level O_{3}. By comparing five different model configurations with observations at six sites, the specific mechanisms responsible for model behavior are identified and discussed. In particular, vertical mixing, urban chemistry, and dry deposition depending on boundary layer height strongly affect model ability to capture observed behavior. Central Eastern China appears to be the most sensitive region in our study to the choice of chemical mechanisms. Evaluation with TRACE-P aircraft measurements reveals that neither the CB4 nor the SAPRC99 mechanisms consistently capture observed behavior of key photochemical oxidants in springtime. However, our analysis finds that SAPRC99 performs somewhat better in simulating mixing ratios of H_{2}O_{2} (hydrogen peroxide
Structure of a mushy layer at the inner core boundary
Deguen, R.; Huguet, L.; Bergman, M. I.; Labrosse, S.; Alboussiere, T.
2015-12-01
We present experimental results on the solidification of ammonium chloride from an aqueous solution, yielding a mushy zone, under hyper-gravity. A commercial centrifuge has been equipped with a slip-ring so that electric power, temperature and ultrasonic signals could be transmitted between the experimental setup and the laboratory. A Peltier element provides cooling at the bottom of the cell. Probes monitor the temperature along the height of the cell. Ultrasound measurements (2 to 6 MHz) is used to detect the position of the front of the mushy zone and to determine attenuation in the mush. A significant increase of solid fraction (or decrease of mushy layer thickness) and attenuation in the mush is observed as gravity is increased. Kinetic undercooling is significant in our experiments and has been included in a macroscopic mush model. The other ingredients of the model are conservation of energy and chemical species, along with heat/species transfer between the mush and the liquid phase: boundary-layer exchanges at the top of the mush and bulk convection within the mush (formation of chimneys). The outputs of the model compare well with our experiments. We have then run the model in a range of parameters suitable for the Earth's inner core, which has shown the role of bulk mush convection for the inner core and the reason why a solid fraction very close to unity should be expected. We have also run melting experiments: after crystallization of a mush, the liquid has been heated from above until the mush started to melt, while the bottom cold temperature was maintained. These melting experiments were motivated by the possible local melting at the inner core boundary that has been invoked to explain the formation of the anomalously slow F-layer at the bottom of the outer core or inner core hemispherical asymmetry. Oddly, the consequences of melting are an increase in solid fraction and a decrease in attenuation. It is hence possible that surface seismic velocity
Using Intel Xeon Phi to accelerate the WRF TEMF planetary boundary layer scheme
Mielikainen, Jarno; Huang, Bormin; Huang, Allen
2014-05-01
The Weather Research and Forecasting (WRF) model is designed for numerical weather prediction and atmospheric research. The WRF software infrastructure consists of several components such as dynamic solvers and physics schemes. Numerical models are used to resolve the large-scale flow. However, subgrid-scale parameterizations are for an estimation of small-scale properties (e.g., boundary layer turbulence and convection, clouds, radiation). Those have a significant influence on the resolved scale due to the complex nonlinear nature of the atmosphere. For the cloudy planetary boundary layer (PBL), it is fundamental to parameterize vertical turbulent fluxes and subgrid-scale condensation in a realistic manner. A parameterization based on the Total Energy - Mass Flux (TEMF) that unifies turbulence and moist convection components produces a better result that the other PBL schemes. For that reason, the TEMF scheme is chosen as the PBL scheme we optimized for Intel Many Integrated Core (MIC), which ushers in a new era of supercomputing speed, performance, and compatibility. It allows the developers to run code at trillions of calculations per second using the familiar programming model. In this paper, we present our optimization results for TEMF planetary boundary layer scheme. The optimizations that were performed were quite generic in nature. Those optimizations included vectorization of the code to utilize vector units inside each CPU. Furthermore, memory access was improved by scalarizing some of the intermediate arrays. The results show that the optimization improved MIC performance by 14.8x. Furthermore, the optimizations increased CPU performance by 2.6x compared to the original multi-threaded code on quad core Intel Xeon E5-2603 running at 1.8 GHz. Compared to the optimized code running on a single CPU socket the optimized MIC code is 6.2x faster.
Application of the Remotely Piloted Aircraft (RPA) 'MASC' in Atmospheric Boundary Layer Research
Wildmann, Norman; Bange, Jens
2014-05-01
The remotely piloted aircraft (RPA) MASC (Multipurpose Airborne Sensor Carrier) was developed at the University of Tübingen in cooperation with the University of Stuttgart, University of Applied Sciences Ostwestfalen-Lippe and 'ROKE-Modelle'. Its purpose is the investigation of thermodynamic processes in the atmospheric boundary layer (ABL), including observations of temperature, humidity and wind profiles, as well as the measurement of turbulent heat, moisture and momentum fluxes. The aircraft is electrically powered, has a maximum wingspan of 3.40 m and a total weight of 5-8 kg, depending on battery- and payload. The standard meteorological payload consists of temperature sensors, a humidity sensor, a flow probe, an inertial measurement unit and a GNSS. In normal operation, the aircraft is automatically controlled by the ROCS (Research Onboard Computer System) autopilot to be able to fly predefined paths at constant altitude and airspeed. Since 2010 the system has been tested and improved intensively. In September 2012 first comparative tests could successfully be performed at the Lindenberg observatory of Germany's National Meteorological Service (DWD). In 2013, several campaigns were done with the system, including fundamental boundary layer research, wind energy meteorology and assistive measurements to aerosol investigations. The results of a series of morning transition experiments in summer 2013 will be presented to demonstrate the capabilities of the measurement system. On several convective days between May and September, vertical soundings were done to record the evolution of the ABL in the early morning, from about one hour after sunrise, until noon. In between the soundings, flight legs of up to 1 km length were performed to measure turbulent statistics and fluxes at a constant altitude. With the help of surface flux measurements of a sonic anemometer, methods of similarity theory could be applied to the RPA flux measurements to compare them to
Sources and Transport of Aerosol above the Boundary Layer over the Mediterranean Basin
Roberts, Greg; Corrigan, Craig; Ritchie, John; Pont, Veronique; Claeys, Marine; Sciare, Jean; Mallet, Marc; Dulac, François; Mihalopoulos, Nikos
2015-04-01
The Mediterranean Region has been identified as sensitive to changes in the hydrological cycle, which could affect the water resources for millions of people by the turn of the century. However, prior to recent observations, most climate models have not accounted for the impacts of aerosol in this region. Past airborne studies have shown that aerosol sources from Europe and Africa are often transported throughout the lower troposphere; yet, because of their complex vertical distribution, it is a challenge to capture the variability and quantify the contribution of these sources to the radiative budget and precipitation processes. The PAEROS ChArMEx Mountain Experiment (PACMEx) complemented the regional activities by collecting aerosol data from atop a mountain on the island of Corsica, France in order to assess boundary layer / free troposphere atmospheric processes. In June/July 2013, PACMEx instruments were deployed at 2000 m.asl near the center of Corsica, France to complement ground-based aerosol observations at 550 m.asl on the northern peninsula, as well as airborne measurements. Comparisons between the peninsula site and the mountain site show similar general trends in aerosol properties; yet, differences in aerosol properties reveal the myriad transport mechanisms over the Mediterranean Basin. Using aerosol physicochemical data coupled with back trajectory analysis, different sources have been identified including Saharan dust transport, residual dust mixed with sea salt, anthropogenic emissions from Western Europe, and a period of biomass burning from Eastern Europe. Each period exhibits distinct signatures in the aerosol related to transport processes above and below the boundary layer. In addition, the total aerosol concentrations at the mountain site revealed a strong diurnal cycling the between the atmospheric boundary layer and the free troposphere, which is typical of mountain-top observations. PACMEx was funded by the National Science Foundation
Boundary Layer Transition During the Orion Exploration Flight Test 1 (EFT-1)
Kirk, Lindsay C.
2016-01-01
Boundary layer transition was observed in the thermocouple data on the windside backshell of the Orion reentry capsule. Sensors along the windside centerline, as well as off-centerline, indicated transition late in the flight at approximately Mach 4 conditions. Transition progressed as expected, beginning at the sensors closest to the forward bay cover (FBC) and moving towards the heatshield. Sensors placed in off-centerline locations did not follow streamlines, so the progression of transition observed in these sensors is less intuitive. Future analysis will include comparisons to pre-flight predictions and expected transitional behavior will be investigated. Sensors located within the centerline and off-centerline launch abort system (LAS) attach well cavities on the FBC also showed indications of boundary layer transition. The transition within the centerline cavity was observed in the temperature traces prior to transition onset on the sensors upstream of the cavity. Transition behavior within the off centerline LAS attach well cavity will also be investigated. Heatshield thermocouples were placed within Avcoat plugs to attempt to capture transitional behavior as well as better understand the aerothermal environments. Thermocouples were placed in stacks of two or five vertically within the plugs, but the temperature data obtained at the sensors closest to the surface did not immediately indicate transitional behavior. Efforts to use the in depth thermocouple temperatures to reconstruct the surface heat flux are ongoing and any results showing the onset of boundary layer transition obtained from those reconstructions will also be included in this paper. Transition on additional features of interest, including compression pad ramps, will be included if it becomes available.
Atmospheric boundary layers in storms: advanced theory and modelling applications
Directory of Open Access Journals (Sweden)
S. S. Zilitinkevich
2005-01-01
Full Text Available Turbulent planetary boundary layers (PBLs control the exchange processes between the atmosphere and the ocean/land. The key problems of PBL physics are to determine the PBL height, the momentum, energy and matter fluxes at the surface and the mean wind and scalar profiles throughout the layer in a range of regimes from stable and neutral to convective. Until present, the PBLs typical of stormy weather were always considered as neutrally stratified. Recent works have disclosed that such PBLs are in fact very strongly affected by the static stability of the free atmosphere and must be treated as factually stable (we call this type of the PBL "conventionally neutral" in contract to the "truly neutral" PBLs developed against the neutrally stratified free flow. It is common knowledge that basic features of PBLs exhibit a noticeable dependence on the free-flow static stability and baroclinicity. However, the concern of the traditional theory of neural and stable PBLs was almost without exception the barotropic nocturnal PBL, which develops at mid latitudes during a few hours in the night, on the background of a neutral or slightly stable residual layer. The latter separates this type of the PBL from the free atmosphere. It is not surprising that the nature of turbulence in such regimes is basically local and does not depend on the properties of the free atmosphere. Alternatively, long-lived neutral (in fact only conditionally neutral or stable PBLs, which have much more time to grow up, are placed immediately below the stably stratified free flow. Under these conditions, the turbulent transports of momentum and scalars even in the surface layer - far away from the PBL outer boundary - depend on the free-flow Brunt-Väisälä frequency, N. Furthermore, integral measures of the long-lived PBLs (their depths and the resistance law functions depend on N and also on the baroclinic shear, S. In the traditional PBL models both non-local parameters N and S
Turbulent Boundary Layer Over Geophysical-like Topographies
Chamorro, L. P.; Hamed, A. M.; Castillo, L.
2016-12-01
An experimental investigation of the flow and the turbulence structure over 2D and 3D large-scale wavy walls was performed using high-resolution planar particle image velocimetry in a refractive-index-matching (RIM) channel. Extensive measurements were performed to characterize the developing and developed flows. The 2D wall is described by a sinusoidal wave in the streamwise direction with amplitude to wavelength ratio a/λx = 0.05, while the 3D wall has an additional wave superimposed in the spanwise direction with a/λy = 0.1. The flow over these walls was characterized at Reynolds numbers of 4000 and 40000, based on the bulk velocity and the channel half height. The walls have an amplitude to boundary layer thickness ratio a/δ99 ≈ 0.1 and resemble large-scale and geophysical-like roughnesses found in rivers beds and natural terrain. Instantaneous velocity fields and time-averaged turbulence quantities reveal strong coupling between large-scale topography and the turbulence dynamics near the wall. Turbulence statistics for both walls show the presence of a well-structured shear layer past the roughness crests. Analysis of the turbulent kinetic energy production rate suggests that the shear layer is responsible for the majority of turbulence production across both walls. However, the 3D wall exhibits preferential spanwise flows that are thought to result in the multiple distinctive flow features for the 3D wall including comparatively reduced spanwise vorticity and decreased turbulence levels. Further insight on the effect of roughness three-dimensionality and Reynolds number is drawn in both the developed and developing regions through proper orthogonal decomposition (POD) and quadrant analysis.
Large artificially generated turbulent boundary layers for the study of atmospheric flows
International Nuclear Information System (INIS)
Guimaraes, Joao Henrique D.; Santos Junior, Sergio J.F. dos; Freire, Atila P. Silva; Jian, Su
1999-01-01
The present work discusses in detail the experimental conditions for the establishment of thick artificially generated turbulent boundary layer which can be classified as having the near characteristics of an atmospheric boundary layer. The paper describes the experimental arrangement, including the features of the designed wind tunnel and of the instrumentation. the boundary layer is made to develop over a surface fitted with wedge generators which are used to yield a very thick boundary layer. The flow conditions were validated against the following features: growth, structure, equilibrium and turbulent transport momentum. Results are presented for the following main flow variables: mean velocity, local skin-friction coefficient, boundary layer momentum thickness and the Clauser factor. The velocity boundary layer characteristics were shown to be in good agreement with the expected trend in view of the classical expressions found in literature. (author)
High-energy X-ray production in a boundary layer of an accreting neutron star
International Nuclear Information System (INIS)
Hanawa, Tomoyuki
1991-01-01
It is shown by Monte Carlo simulation that high-energy X-rays are produced through Compton scattering in a boundary layer of an accreting neutron star. The following is the mechanism for the high-energy X-ray production. An accreting neutron star has a boundary layer rotating rapidly on the surface. X-rays radiated from the star's surface are scattered in part in the boundary layer. Since the boundary layer rotates at a semirelativistic speed, the scattered X-ray energy is changed by the Compton effect. Some X-rays are scattered repeatedly between the neutron star and the boundary layer and become high-energy X-rays. This mechanism is a photon analog of the second-order Fermi acceleration of cosmic rays. When the boundary layer is semitransparent, high-energy X-rays are produced efficiently. 17 refs
The effect of the Asian Monsoon to the atmospheric boundary layer over the Tibetan Plateau
Li, Maoshan; Su, Zhongbo; Chen, Xuelong; Zheng, Donghai; Sun, Fanglin; Ma, Yaoming; Hu, Zeyong
2016-04-01
needed temporal and spatial coupling and means for validation of mesoscale model simulations (Zhong et al., 2009, 2011). When these time series are integrated into energy balance analyses methods (Su, 2002, 2005) with reanalysis data, plateau scale diurnal radiative and turbulence fluxes can be generated (Oku et al., 2005; Su et al., 2010) for the study of the boundary layer atmospheric structures at plateau scale. As such regional land-atmosphere feedbacks and atmospheric boundary layer structures can be studied. The quantification of the multi-scale atmospheric boundary layer and land surface processes over the heterogeneous underlying surface of the Tibetan Plateau is a challenging problem that remains unsettled despite many years of efforts. Using field observation, truth investigation, land surface process parameterization and meso-scale simulation, the dynamical and thermal uniform function of the atmospheric boundary layer and its effect to the atmospheric boundary layer will be analyzed in this research. Results The different characteristics of the Boundary layer with Asia monsoon season exchange over TP The height of atmospheric boundary layer was higher before monsoon season than it in summer. It was around 3-4 km above the ground in spring, while it was 1-2 km during monsoon season. It due to sensible heat flux was stronger in spring than it in summer. Using wavelet analysis method, we decomposed the wind include horizontal and vertical velocity from radiosounding observational data. The reason of high boundary layer height was disclosed. Compared to the observation, the output of model was underestimation during spring, while it was reasonable in summer monsoon. The effect of the Asian Monsoon to the precipitation on the TP Numerical simulation of climate on the TP was implemented for the whole year of 2008 using WRF-Noah model. The output of the WRF model is compared to TRMM data set for precipitation and ERA-interim land product for soil moisture. Modeled
A novel approach for quantifying the zero-plane displacement of rough-wall boundary layers
Ferreira, Manuel; Rodriguez-Lopez, Eduardo; Ganapathisubramani, Bharath; Aerodynamics; Flight Mechanics Team
2017-11-01
Indirect methods of wall shear stress (WSS) estimation are frequently used to characterise rough wall boundary-layer flows. The zero-plane displacement, hypothesised to be the vertical location where it acts, is often treated as a fitting parameter. However, it would be preferrable to measure both these quantities directly, especially for surfaces with large roughness elements where established scaling and similarity laws may not hold. In this talk we present a novel floating element balance that is able to measure not only the WSS but also the wall normal location at which it acts. While allowing compensation for mild static pressure gradients by means of a first-order analytical model. Its architecture is based on a parallel-shift linkage and it's fitted with custom built force transducers and a data acquisition system especially designed to achieve high Signal-to-Noise Ratios (SNR). The smooth-wall boundary-layer flow is used as benchmark to assess the accuracy of this balance. The values of skin friction coefficient show an agreement with hot-wire anemometry to within 2 % at a local Reynolds number Reθ = 4 ×103 up to 104. A rough surface of regularly distributed large elements is used to investigate the ability to infer the zero-plane displacement.
Hydrodynamic theory of convective transport across a dynamically stabilized diffuse boundary layer
International Nuclear Information System (INIS)
Gerhauser, H.
1983-09-01
The diffuse boundary layer between miscible liquids is subject to Rayleigh-Taylor instabilities if the heavy fluid is supported by the light one. The resulting rapid interchange of the liquids can be suppressed by enforcing vertical oscillations on the whole system. This dynamic stabilization is incomplete and produces some peculiar novel transport phenomena such as decay off the density profile into several steps, periodic peeling of density sheets of the boundary layer and the appearance of steady vortex flow. The theory presented in this paper identifies the basic mechanism as formation of convective cells leading to enhanced diffusion, and explains previous experimental results with water and ZnJ 2 -solutions. A nonlinear treatment of the stationary convective flow problem gives the saturation amplitude of the ground mode and provides an upper bound for the maximum convective transport. The hydrodynamic model can be used for visualizing similar transport processes in the plasma of toroidal confinement devices such as sawtooth oscillations in soft disruptions of tokamak discharges and anomalous diffusion by excitation of convective cells. The latter process is investigated here in some detail, leading to the result that the maximum possible transport is of the order of Bohm diffusion. (orig.)
Natarajan, Murali; Fairlie, T. Duncan; Dwyer Cianciolo, Alicia; Smith, Michael D.
2015-01-01
We use the mesoscale modeling capability of Mars Weather Research and Forecasting (MarsWRF) model to study the sensitivity of the simulated Martian lower atmosphere to differences in the parameterization of the planetary boundary layer (PBL). Characterization of the Martian atmosphere and realistic representation of processes such as mixing of tracers like dust depend on how well the model reproduces the evolution of the PBL structure. MarsWRF is based on the NCAR WRF model and it retains some of the PBL schemes available in the earth version. Published studies have examined the performance of different PBL schemes in NCAR WRF with the help of observations. Currently such assessments are not feasible for Martian atmospheric models due to lack of observations. It is of interest though to study the sensitivity of the model to PBL parameterization. Typically, for standard Martian atmospheric simulations, we have used the Medium Range Forecast (MRF) PBL scheme, which considers a correction term to the vertical gradients to incorporate nonlocal effects. For this study, we have also used two other parameterizations, a non-local closure scheme called Yonsei University (YSU) PBL scheme and a turbulent kinetic energy closure scheme called Mellor- Yamada-Janjic (MYJ) PBL scheme. We will present intercomparisons of the near surface temperature profiles, boundary layer heights, and wind obtained from the different simulations. We plan to use available temperature observations from Mini TES instrument onboard the rovers Spirit and Opportunity in evaluating the model results.
Field measurements of bottom boundary layer and suspend particle materials on Jyoban coast in Japan
International Nuclear Information System (INIS)
Yagi, Hiroshi; Sugimatsu, Kouichi; Nishi, Yoshihiro; Kawamata, Shigeru; Nakayama, Akiyoshi; Udagawa, Toru; Suzuki, Akira
2013-01-01
To understand the characteristics of the bottom boundary layer (BBL), movements of suspended particle material (SPM) and its related radionuclide transport on Jyoban coast, the continuous monitoring of bottom environments using the mooring system and the intensive field survey of BBL with FRA-TRIPOD were performed. The observation results have shown the fundamental characteristics of BBL (vertical distributions of velocities and bottom roughness, etc.) and bottom turbidity variations. The turbidity at the shallow water depth (30 m) was strongly influenced by waves and turbid water generated on rough wave conditions was transported by the coastal currents with the several days period. Turbidities at the deeper depths (80 m and 130 m) were affected by semidiurnal internal tides. (author)
A 1290 MHZ profiler with RASS for monitoring wind and temperature in the boundary layer
Energy Technology Data Exchange (ETDEWEB)
Engelbart, D. [Deutscher Wetterdienst, Lindenberg (Germany). Meteorol. Obs.; Steinhagen, H. [Deutscher Wetterdienst, Lindenberg (Germany). Meteorol. Obs.; Goersdorf, U. [Deutscher Wetterdienst, Lindenberg (Germany). Meteorol. Obs.; Lippmann, J. [Deutscher Wetterdienst, Lindenberg (Germany). Meteorol. Obs.; Neisser, J. [Deutscher Wetterdienst, Lindenberg (Germany). Meteorol. Obs.
1996-02-01
A boundary layer wind profiler with RASS is described operating at 1290 MHz in a quasi-operational mode at the Meteorological Observatory Lindenberg of the German Weather Service (DWD). It provides vertical profiles of wind and temperature from the lower atmosphere with a height resolution of 50 m to 400 m and a time resolution of about 1 to 60 minutes. For an estimation of the system reliability, the availability of the measurements for all different height levels is analyzed. With regard to the data quality, a comparison of wind profiler/RASS and rawinsonde data is presented based on 856 wind and 451 temperature profiles. It reveals reasonable conformity of both sounding systems. Finally, case studies are shown, demonstrating the system ability to analyze some characteristic phenomena in the lower troposphere, which are unresolved temporally and spatially by the routine rawinsonde network. (orig.)
Numerical Investigation of PLIF Gas Seeding for Hypersonic Boundary Layer Flows
Johanson, Craig T.; Danehy, Paul M.
2012-01-01
Numerical simulations of gas-seeding strategies required for planar laser-induced fluorescence (PLIF) in a Mach 10 air flow were performed. The work was performed to understand and quantify adverse effects associated with gas seeding and to compare different flow rates and different types of seed gas. The gas was injected through a slot near the leading edge of a flat plate wedge model used in NASA Langley Research Center's 31- Inch Mach 10 Air Tunnel facility. Nitric oxide, krypton, and iodine gases were simulated at various injection rates. Simulation results showing the deflection of the velocity field for each of the cases are presented. Streamwise distributions of velocity and concentration boundary layer thicknesses as well as vertical distributions of velocity, temperature, and mass distributions are presented for each of the cases. Relative merits of the different seeding strategies are discussed.
Atmospheric Boundary Layer temperature and humidity from new-generation Raman lidar
Froidevaux, Martin; Higgins, Chad; Simeonov, Valentin; Pardyjak, Eric R.; Parlange, Marc B.
2010-05-01
Mixing ratio and temperature data, obtained with EPFL Raman lidar during the TABLE-08 experiment are presented. The processing methods will be discussed along with fundamental physics. An independent calibration is performed at different distances along the laser beam, demonstrating that the multi-telescopes design of the lidar system is reliable for field application. The maximum achievable distance as a function of time and/or space averaging will also be discussed. During the TABLE-08 experiment, different type of lidar measurements have been obtained including: horizontal and vertical time series, as well as boundary layer "cuts", during day and night. The high resolution data, 1s in time and 1.25 m in space, are used to understand the response of the atmosphere to variations in surface variability.
Arakeri, V. H.
1980-04-01
Boundary layer flow visualization in water with surface heat transfer was carried out on a body of revolution which had the predicted possibility of laminar separation under isothermal conditions. Flow visualization was by in-line holographic technique. Boundary layer stabilization, including elimination of laminar separation, was observed to take place on surface heating. Conversely, boundary layer destabilization was observed on surface cooling. These findings are consistent with the theoretical predictions of Wazzan et al. (1970).
Analysis of Windward Side Hypersonic Boundary Layer Transition on Blunted Cones at Angle of Attack
2017-01-09
correlated with PSE/LST N-Factors. 15. SUBJECT TERMS boundary layer transition, hypersonic, ground test 16. SECURITY CLASSIFICATION OF: 17. LIMITATION ...Maccoll) solution e condition at boundary layer edge w condition at wall, viscous ∞ condition in freestream Conventions LST Linear Stability Theory PSE...STATES AIR FORCE AFRL-RQ-WP-TP-2017-0169 ANALYSIS OF WINDWARD SIDE HYPERSONIC BOUNDARY LAYER TRANSITION ON BLUNTED CONES AT ANGLE OF ATTACK Roger
Boundary layer friction of solvate ionic liquids as a function of potential.
Li, Hua; Rutland, Mark W; Watanabe, Masayoshi; Atkin, Rob
2017-07-01
Atomic force microscopy (AFM) has been used to investigate the potential dependent boundary layer friction at solvate ionic liquid (SIL)-highly ordered pyrolytic graphite (HOPG) and SIL-Au(111) interfaces. Friction trace and retrace loops of lithium tetraglyme bis(trifluoromethylsulfonyl)amide (Li(G4) TFSI) at HOPG present clearer stick-slip events at negative potentials than at positive potentials, indicating that a Li + cation layer adsorbed to the HOPG lattice at negative potentials which enhances stick-slip events. The boundary layer friction data for Li(G4) TFSI shows that at HOPG, friction forces at all potentials are low. The TFSI - anion rich boundary layer at positive potentials is more lubricating than the Li + cation rich boundary layer at negative potentials. These results suggest that boundary layers at all potentials are smooth and energy is predominantly dissipated via stick-slip events. In contrast, friction at Au(111) for Li(G4) TFSI is significantly higher at positive potentials than at negative potentials, which is comparable to that at HOPG at the same potential. The similarity of boundary layer friction at negatively charged HOPG and Au(111) surfaces indicates that the boundary layer compositions are similar and rich in Li + cations for both surfaces at negative potentials. However, at Au(111), the TFSI - rich boundary layer is less lubricating than the Li + rich boundary layer, which implies that anion reorientations rather than stick-slip events are the predominant energy dissipation pathways. This is confirmed by the boundary friction of Li(G4) NO 3 at Au(111), which shows similar friction to Li(G4) TFSI at negative potentials due to the same cation rich boundary layer composition, but even higher friction at positive potentials, due to higher energy dissipation in the NO 3 - rich boundary layer.
Turbulent boundary layer noise : direct radiation at Mach number 0.5
Gloerfelt , Xavier; Berland , Julien
2013-01-01
International audience; Boundary layers constitute a fundamental source of aerodynamic noise. A turbulent boundary layer over a plane wall can provide an indirect contribution to the noise by exciting the structure, and a direct noise contribution. The latter part can play a significant role even if its intensity is very low, explaining why it is hardly measured unambiguously. In the present study, the aerodynamic noise generated by a spatially developing turbulent boundary layer is computed ...
Energy Technology Data Exchange (ETDEWEB)
Yang, L., E-mail: qiaoqin.yang@mail.usask.ca; Yang, Q.; Zhang, C.; Li, Y.S.
2013-12-31
A double-layered nanostructure consisting of a layer of vertically aligned Carbon Nanotubes (CNTs) and a layer of diamond beneath has been synthesized on silicon substrate by Hot Filament Chemical Vapor Deposition. The synthesis was achieved by first depositing a layer of diamond on silicon and then depositing a top layer of vertically aligned CNTs by applying a negative bias on the substrate holder. The growth of CNTs was catalyzed by a thin layer of spin-coated iron nitride. The surface morphology and structure of the CNTs/diamond double-layered structure were characterized by Scanning Electron Microscope, Energy Dispersive X-ray spectrum, and Raman Spectroscopy. Their field electron emission (FEE) properties were measured by KEITHLEY 237 high voltage measurement unit, showing much higher FEE current stability than single layered CNTs. - Highlights: • A new double-layered nanostructure consisting of a layer of vertically aligned CNTs and a layer of diamond beneath has been synthesized by hot filament chemical vapor deposition. • This double-layered structure exhibits superior field electron emission stability. • The improvement of emission stability is due to the combination of the unique properties of diamond and CNTs.
MHD Boundary Layer Slip Flow and Heat Transfer over a Flat Plate
International Nuclear Information System (INIS)
Bhattacharyya, Krishnendu; Mukhopadhyay, Swati; Layek, G. C.
2011-01-01
An analysis of magnetohydrodynamic (MHD) boundary layer flow and heat transfer over a flat plate with slip condition at the boundary is presented. A complete self-similar set of equations are obtained from the governing equations using similarity transformations and are solved by a shooting method. In the boundary slip condition no local similarity occurs. Velocity and temperature distributions within the boundary layer are presented. Our analysis reveals that the increase of magnetic and slip parameters reduce the boundary layer thickness and also enhance the heat transfer from the plate. (fundamental areas of phenomenology(including applications))
Rhenium Dichalcogenides: Layered Semiconductors with Two Vertical Orientations.
Hart, Lewis; Dale, Sara; Hoye, Sarah; Webb, James L; Wolverson, Daniel
2016-02-10
The rhenium and technetium diselenides and disulfides are van der Waals layered semiconductors in some respects similar to more well-known transition metal dichalcogenides (TMD) such as molybdenum sulfide. However, their symmetry is lower, consisting only of an inversion center, so that turning a layer upside-down (that is, applying a C2 rotation about an in-plane axis) is not a symmetry operation, but reverses the sign of the angle between the two nonequivalent in-plane crystallographic axes. A given layer thus can be placed on a substrate in two symmetrically nonequivalent (but energetically similar) ways. This has consequences for the exploitation of the anisotropic properties of these materials in TMD heterostructures and is expected to lead to a new source of domain structure in large-area layer growth. We produced few-layer ReS2 and ReSe2 samples with controlled "up" or "down" orientations by micromechanical cleavage and we show how polarized Raman microscopy can be used to distinguish these two orientations, thus establishing Raman as an essential tool for the characterization of large-area layers.
SCHLEGEL, FABRICE; WEE, DAEHYUN; MARZOUK, YOUSSEF M.; GHONIEM, AHMED F.
2011-01-01
generation along the channel wall, captures unsteady interactions between the wall boundary layer and the jet - in particular, the separation of the wall boundary layer and its transport into the interior. For comparison, we also implement a reduced boundary
Bubble and boundary layer behaviour in subcooled flow boiling
Energy Technology Data Exchange (ETDEWEB)
Maurus, Reinhold; Sattelmayer, Thomas [Lehrstuhl fuer Thermodynamik, Technische Universitaet Muenchen, 85747 Garching (Germany)
2006-03-15
Subcooled flow boiling is a commonly applied technique for achieving efficient heat transfer. In the study, an experimental investigation in the nucleate boiling regime was performed for water circulating in a closed loop at atmospheric pressure. The horizontal orientated test-section consists of a rectangular channel with a one side heated copper strip and good optical access. Various optical observation techniques were applied to study the bubble behaviour and the characteristics of the fluid phase. The bubble behaviour was recorded by the high-speed cinematography and by a digital high resolution camera. Automated image processing and analysis algorithms developed by the authors were applied for a wide range of mass flow rates and heat fluxes in order to extract characteristic length and time scales of the bubbly layer during the boiling process. Using this methodology, the bubbles were automatically analysed and the bubble size, bubble lifetime, waiting time between two cycles were evaluated. Due to the huge number of observed bubbles a statistical analysis was performed and distribution functions were derived. Using a two-dimensional cross-correlation algorithm, the averaged axial phase boundary velocity profile could be extracted. In addition, the fluid phase velocity profile was characterised by means of the particle image velocimetry (PIV) for the single phase flow as well as under subcooled flow boiling conditions. The results indicate that the bubbles increase the flow resistance. The impact on the flow exceeds by far the bubbly region and it depends on the magnitude of the boiling activity. Finally, the ratio of the averaged phase boundary velocity and of the averaged fluid velocity was evaluated for the bubbly region. (authors)
CFD simulation of neutral ABL flows; Atmospheric Boundary Layer
Energy Technology Data Exchange (ETDEWEB)
Xiaodong Zhang
2009-04-15
This work is to evaluate the CFD prediction of Atmospheric Boundary Layer flow field over different terrains employing Fluent 6.3 software. How accurate the simulation could achieve depend on following aspects: viscous model, wall functions, agreement of CFD model with inlet wind velocity profile and top boundary condition. Fluent employ wall function roughness modifications based on data from experiments with sand grain roughened pipes and channels, describe wall adjacent zone with Roughness Height (Ks) instead of Roughness Length (z{sub 0}). In a CFD simulation of ABL flow, the mean wind velocity profile is generally described with either a logarithmic equation by the presence of aerodynamic roughness length z{sub 0} or an exponential equation by the presence of exponent. As indicated by some former researchers, the disagreement between wall function model and ABL velocity profile description will result in some undesirable gradient along flow direction. There are some methods to improve the simulation model in literatures, some of them are discussed in this report, but none of those remedial methods are perfect to eliminate the streamwise gradients in mean wind speed and turbulence, as EllipSys3D could do. In this paper, a new near wall treatment function is designed, which, in some degree, can correct the horizontal gradients problem. Based on the corrected model constants and near wall treatment function, a simulation of Askervein Hill is carried out. The wind condition is neutrally stratified ABL and the measurements are best documented until now. Comparison with measured data shows that the CFD model can well predict the velocity field and relative turbulence kinetic energy field. Furthermore, a series of artificial complex terrains are designed, and some of the main simulation results are reported. (au)
Pollutant Plume Dispersion in the Atmospheric Boundary Layer over Idealized Urban Roughness
Wong, Colman C. C.; Liu, Chun-Ho
2013-05-01
The Gaussian model of plume dispersion is commonly used for pollutant concentration estimates. However, its major parameters, dispersion coefficients, barely account for terrain configuration and surface roughness. Large-scale roughness elements (e.g. buildings in urban areas) can substantially modify the ground features together with the pollutant transport in the atmospheric boundary layer over urban roughness (also known as the urban boundary layer, UBL). This study is thus conceived to investigate how urban roughness affects the flow structure and vertical dispersion coefficient in the UBL. Large-eddy simulation (LES) is carried out to examine the plume dispersion from a ground-level pollutant (area) source over idealized street canyons for cross flows in neutral stratification. A range of building-height-to-street-width (aspect) ratios, covering the regimes of skimming flow, wake interference, and isolated roughness, is employed to control the surface roughness. Apart from the widely used aerodynamic resistance or roughness function, the friction factor is another suitable parameter that measures the drag imposed by urban roughness quantitatively. Previous results from laboratory experiments and mathematical modelling also support the aforementioned approach for both two- and three-dimensional roughness elements. Comparing the UBL plume behaviour, the LES results show that the pollutant dispersion strongly depends on the friction factor. Empirical studies reveal that the vertical dispersion coefficient increases with increasing friction factor in the skimming flow regime (lower resistance) but is more uniform in the regimes of wake interference and isolated roughness (higher resistance). Hence, it is proposed that the friction factor and flow regimes could be adopted concurrently for pollutant concentration estimate in the UBL over urban street canyons of different roughness.
Characterization of the Boundary Layer Wind and Turbulence in the Gulf of Mexico
Pichugina, Y. L.; Banta, R. M.; Choukulkar, A.; Brewer, A.; Hardesty, R. M.; McCarty, B.; Marchbanks, R.
2014-12-01
A dataset of ship-borne Doppler lidar measurements taken in the Gulf of Mexico was analyzed to provide insight into marine boundary-layer (BL) features and wind-flow characteristics, as needed for offshore wind energy development. This dataset was obtained as part of the intensive Texas Air Quality Study in summer of 2006 (TexAQS06). During the project, the ship, the R/V Ronald H. Brown, cruised in tracks in the Gulf of Mexico along the Texas coast, in Galveston Bay, and in the Houston Ship Channel obtaining air chemistry and meteorological data, including vertical profile measurements of wind and temperature. The primary observing system used in this paper is NOAA/ESRL's High Resolution Doppler Lidar (HRDL), which features high-precision and high-resolution wind measurements and a motion compensation system to provide accurate wind data despite ship and wave motions. The boundary layer in this warm-water region was found to be weakly unstable typically to a depth of 300 m above the sea surface. HRDL data were analyzed to provide 15-min averaged profiles of wind flow properties (wind speed, direction, and turbulence) from the water surface up to 2.5 km at a vertical resolution of 15 m. The paper will present statistics and distributions of these parameters over a wide range of heights and under various atmospheric conditions. Detailed analysis of the BL features including LLJs, wind and directional ramps, and wind shear through the rotor level heights, along with examples of hub-height and equivalent wind will be presented. The paper will discuss the diurnal fluctuations of all quantities critical to wind energy and their variability along the Texas coast.
A comparative study of near-wall turbulence in high and low Reynolds number boundary layers
International Nuclear Information System (INIS)
Metzger, M.M.; Klewicki, J.C.
2001-01-01
The present study explores the effects of Reynolds number, over three orders of magnitude, in the viscous wall region of a turbulent boundary layer. Complementary experiments were conducted both in the boundary layer wind tunnel at the University of Utah and in the atmospheric surface layer which flows over the salt flats of the Great Salt Lake Desert in western Utah. The Reynolds numbers, based on momentum deficit thickness, of the two flows were R θ =2x10 3 and R θ ≅5x10 6 , respectively. High-resolution velocity measurements were obtained from a five-element vertical rake of hot-wires spanning the buffer region. In both the low and high R θ flows, the length of the hot-wires measured less than 6 viscous units. To facilitate reliable comparisons, both the laboratory and field experiments employed the same instrumentation and procedures. Data indicate that, even in the immediate vicinity of the surface, strong influences from low-frequency motions at high R θ produce noticeable Reynolds number differences in the streamwise velocity and velocity gradient statistics. In particular, the peak value in the root mean square streamwise velocity profile, when normalized by viscous scales, was found to exhibit a logarithmic dependence on Reynolds number. The mean streamwise velocity profile, on the other hand, appears to be essentially independent of Reynolds number. Spectra and spatial correlation data suggest that low-frequency motions at high Reynolds number engender intensified local convection velocities which affect the structure of both the velocity and velocity gradient fields. Implications for turbulent production mechanisms and coherent motions in the buffer layer are discussed
Experiments on aerosol-induced cooling in the nocturnal boundary layer
Sreenivas, K.; Singh, D. K.; Vk, P.; Mukund, V.; Subramanian, G.
2012-12-01
In the nocturnal boundary layer (NBL), under calm & clear-sky conditions, radiation is the principal mode of heat transfer & it determines the temperature distribution close to the ground. Radiative processes thus influence the surface energy budget, & play a decisive role in many micro-meteorological processes including the formation of radiation-fog & inversion layer. Here, we report hyper-cooling of air layers close to the ground that has a radiative origin. Resulting vertical temperature distribution has an anomalous profile with an elevated minimum few decimetres above the ground (known as Lifted Temperature Minimum; LTM). Even though the first observation of this type of profile dates back to 1930s, its origin has not been explained till recently. We report field experiments to elucidate effects of emissivity and other physical properties of the ground on the LTM profile. Field observations clearly indicate that LTM-profiles are observed as a rule in the lowest meter of the NBL. We also demonstrate that the air-layer near the ground, rather than the ground itself, leads the post sunset cooling. This fact changes the very nature of the sensible heat-flux boundary condition. A laboratory experimental setup has been developed that can reproduce LTM. Lab-experiments demonstrate that the high cooling rates observed in the field experiments arise from the presence of aerosols & the intensity of cooling is proportional to aerosol concentration (Fig-1). We have also captured penetrative convection cells in the field experiments (Fig-2). Results presented here thus help in parameterizing transport processes in the NBL.
Simon, T. W.; Moffat, R. J.
1979-01-01
Measurements have been made of the heat transfer through a turbulent boundary layer on a convexly curved isothermal wall and on a flat plate following the curved section. Data were taken for one free-stream velocity and two different ratios of boundary layer thickness to radius of curvature delta/R = 0.051 and delta/R = 0.077. Only small differences were observed in the distribution of heat transfer rates for the two boundary layer thicknesses tested, although differences were noted in the temperature distributions within the boundary layer
Lash, E. Lara; Schmisseur, John
2017-11-01
Pressure-sensitive paint has been used to evaluate the unsteady dynamics of transitional and turbulent shock wave-boundary layer interactions generated by a vertical cylinder on a flat plate in a Mach 2 freestream. The resulting shock structure consists of an inviscid bow shock that bifurcates into a separation shock and trailing shock. The primary features of interest are the separation shock and an upstream influence shock that is intermittently present in transitional boundary layer interactions, but not observed in turbulent interactions. The power spectral densities, frequency peaks, and normalized wall pressures are analyzed as the incoming boundary layer state changes from transitional to fully turbulent, comparing both centerline and outboard regions of the interaction. The present study compares the scales and frequencies of the dynamics of the separation shock structure in different boundary layer regimes. Synchronized high-speed Schlieren imaging provides quantitative statistical analyses as well as qualitative comparisons to the fast-response pressure sensitive paint measurements. Materials based on research supported by the U.S. Office of Naval Research under Award Number N00014-15-1-2269.
Reactive Halogens in the Marine Boundary Layer (RHaMBLe: the tropical North Atlantic experiments
Directory of Open Access Journals (Sweden)
J. D. Lee
2010-02-01
Full Text Available The NERC UK SOLAS-funded Reactive Halogens in the Marine Boundary Layer (RHaMBLe programme comprised three field experiments. This manuscript presents an overview of the measurements made within the two simultaneous remote experiments conducted in the tropical North Atlantic in May and June 2007. Measurements were made from two mobile and one ground-based platforms. The heavily instrumented cruise D319 on the RRS Discovery from Lisbon, Portugal to São Vicente, Cape Verde and back to Falmouth, UK was used to characterise the spatial distribution of boundary layer components likely to play a role in reactive halogen chemistry. Measurements onboard the ARSF Dornier aircraft were used to allow the observations to be interpreted in the context of their vertical distribution and to confirm the interpretation of atmospheric structure in the vicinity of the Cape Verde islands. Long-term ground-based measurements at the Cape Verde Atmospheric Observatory (CVAO on São Vicente were supplemented by long-term measurements of reactive halogen species and characterisation of additional trace gas and aerosol species during the intensive experimental period.
This paper presents a summary of the measurements made within the RHaMBLe remote experiments and discusses them in their meteorological and chemical context as determined from these three platforms and from additional meteorological analyses. Air always arrived at the CVAO from the North East with a range of air mass origins (European, Atlantic and North American continental. Trace gases were present at stable and fairly low concentrations with the exception of a slight increase in some anthropogenic components in air of North American origin, though NO_{x} mixing ratios during this period remained below 20 pptv (note the non-IUPAC adoption in this manuscript of pptv and ppbv, equivalent to pmol mol^{−1} and nmol mol^{−1} to reflect common practice. Consistency with
Three-dimensional boundary layer stability and transition
Malik, M. R.; Li, F.
1992-01-01
Nonparallel and nonlinear stability of a three-dimensional boundary layer, subject to crossflow instability, is investigated using parabolized stability equations (PSEs). Both traveling and stationary disturbances are considered and nonparallel effect on crossflow instability is found to be destabilizing. Our linear PSE results for stationary disturbances agree well with the results from direct solution of Navier-Stokes equations obtained by Spalart (1989). Nonlinear calculations have been carried out for stationary vortices and the computed wall vorticity pattern results in streamwise streaks which resemble remarkably well with the surface oil-flow visualizations in swept-wing experiments. Other features of the stationary vortex development (half-mushroom structure, inflected velocity profiles, vortex doubling, etc.) are also captured in our nonlinear calculations. Nonlinear interaction of the stationary amplitude of the stationary vortex is large as compared to the traveling mode, and the stationary vortex dominates most of the downstream development. When the two modes have the same initial amplitude, the traveling mode dominates the downstream development owing to its higher growth rate, and there is a tendency for the stationary mode to be suppressed. The effect of nonlinear wave development on the skin-friction coefficient is also computed.
Non-parallel stability of compressible boundary layers
Chang, Chau-Lyan; Malik, Mujeeb R.
1993-01-01
Linear and nonlinear stability of compressible growing boundary layers is studied using parabolized stability equations (PSE). Linear PSE calculations are performed for Mach 1.6 and 4.5 plate-plate flow, and the results are compared with the predictions of the multiple-scales approach. In general, the nonparallel effect appears to be less significant for oblique waves near the lower neutral branch but it progressively becomes important at higher Reynolds numbers near the upper branch. In contrast, the nonparallel effect is more pronounced near the lower branch for two-dimensional first-mode waves. The PSE and multiple-scales results agree for the first mode waves, but in the first-second mode transition region, the latter approach tends to break down. Comparison with the first (oblique) and second mode growth rate data from Kendall's (1967) experiment shows good agreement; however, the peak second mode growth rate is over-predicted. Similar conclusions are drawn for the second mode experiment of Stetson et al. (1983) for Mach 8 flow past a sharp cone. We conjecture that the lower experimental growth rate is due to nonlinear saturation and provide supporting calculations.
Plume meander and dispersion in a stable boundary layer
Hiscox, April L.; Miller, David R.; Nappo, Carmen J.
2010-11-01
Continuous lidar measurements of elevated plume dispersion and corresponding micrometeorology data are analyzed to establish the relationship between plume behavior and nocturnal boundary layer dynamics. Contrasting nights of data from the JORNADA field campaign in the New Mexico desert are analyzed. The aerosol lidar measurements were used to separate the plume diffusion (plume spread) from plume meander (displacement). Mutiresolution decomposition was used to separate the turbulence scale (90 s). Durations of turbulent kinetic energy stationarity and the wind steadiness were used to characterize the local scale and submesoscale turbulence. Plume meander, driven by submesoscale wind motions, was responsible for most of the total horizontal plume dispersion in weak and variable winds and strong stability. This proportion was reduced in high winds (i.e., >4 m s-1), weakly stable conditions but remained the dominant dispersion mechanism. The remainder of the plume dispersion in all cases was accounted for by internal spread of the plume, which is a small eddy diffusion process driven by turbulence. Turbulence stationarity and the wind steadiness are demonstrated to be closely related to plume diffusion and plume meander, respectively.
Thermal catastrophe in the plasma sheet boundary layer
International Nuclear Information System (INIS)
Smith, R.A.; Goertz, C.K.; Grossmann, W.
1986-01-01
This letter presents a first step towards a substorm model including particle heating and transport in the plasma sheet boundary layer (PSBL). The heating mechanism discussed is resonant absorption of Alfven waves. For some assumed MHD perturbation incident from the tail lobes onto the plasma sheet, the local heating rate in the PSBL has the form of a resonance function of the one-fluid plasma temperature. Balancing the local heating by convective transport of the heated plasma toward the central plasma sheet, and ''equation of state'' is found for the steady-state PSBL whose solution has the form of a mathematical catastrophe: at a critical value of a parameter containing the incident power flux, the local density, and the convection velocity, the equilibrium temperature jumps discontinuously. Associating this temperature increase with the abrupt onset of the substorm expansion phase, the catastrophe model indicates at least three ways in which the onset may be triggered. Several other consequences related to substorm dynamics are suggested by the simple catastrophe model
Subgrid-scale turbulence in shock-boundary layer flows
Jammalamadaka, Avinash; Jaberi, Farhad
2015-04-01
Data generated by direct numerical simulation (DNS) for a Mach 2.75 zero-pressure gradient turbulent boundary layer interacting with shocks of different intensities are used for a priori analysis of subgrid-scale (SGS) turbulence and various terms in the compressible filtered Navier-Stokes equations. The numerical method used for DNS is based on a hybrid scheme that uses a non-dissipative central scheme in the shock-free turbulent regions and a robust monotonicity-preserving scheme in the shock regions. The behavior of SGS stresses and their components, namely Leonard, Cross and Reynolds components, is examined in various regions of the flow for different shock intensities and filter widths. The backscatter in various regions of the flow is found to be significant only instantaneously, while the ensemble-averaged statistics indicate no significant backscatter. The budgets for the SGS kinetic energy equation are examined for a better understanding of shock-tubulence interactions at the subgrid level and also with the aim of providing useful information for one-equation LES models. A term-by-term analysis of SGS terms in the filtered total energy equation indicate that while each term in this equation is significant by itself, the net contribution by all of them is relatively small. This observation is consistent with our a posteriori analysis.
Self-organization in cathode boundary layer discharges in xenon
International Nuclear Information System (INIS)
Takano, Nobuhiko; Schoenbach, Karl H
2006-01-01
Self-organization of direct current xenon microdischarges in cathode boundary layer configuration has been studied for pressures in the range 30-140 Torr and for currents in the range 50 μA-1 mA. Side-on and end-on observations of the discharge have provided information on the structure and spatial arrangement of the plasma filaments. The regularly spaced filaments, which appear in the normal glow mode when the current is lowered, have a length which is determined by the cathode fall. It varies, dependent on pressure and current, between 50 and 70 μm. The minimum diameter is approximately 80 μm, as determined from the radiative emission in the visible. The filaments are sources of extensive excimer emission. Measurements of the cathode fall length have allowed us to determine the secondary emission coefficient for the discharge in the normal glow mode and to estimate the cathode fall voltage at the transition from normal glow mode to filamentary mode. It was found that the cathode fall voltage at this transition decreases, indicating the onset of additional electron gain processes at the cathode. The regular arrangement of the filaments, self-organization, is assumed to be due to Coulomb interactions between the positively charged cathode fall channels and positive space charges on the surface of the surrounding dielectric spacer. Calculations based on these assumptions showed good agreement with experimentally observed filament patterns
Effect of free-stream turbulence on boundary layer transition.
Goldstein, M E
2014-07-28
This paper is concerned with the transition to turbulence in flat plate boundary layers due to moderately high levels of free-stream turbulence. The turbulence is assumed to be generated by an (idealized) grid and matched asymptotic expansions are used to analyse the resulting flow over a finite thickness flat plate located in the downstream region. The characteristic Reynolds number Rλ based on the mesh size λ and free-stream velocity is assumed to be large, and the turbulence intensity ε is assumed to be small. The asymptotic flow structure is discussed for the generic case where the turbulence Reynolds number εRλ and the plate thickness and are held fixed (at O(1) and O(λ), respectively) in the limit as [Formula: see text] and ε→0. But various limiting cases are considered in order to explain the relevant transition mechanisms. It is argued that there are two types of streak-like structures that can play a role in the transition process: (i) those that appear in the downstream region and are generated by streamwise vorticity in upstream flow and (ii) those that are concentrated near the leading edge and are generated by plate normal vorticity in upstream flow. The former are relatively unaffected by leading edge geometry and are usually referred to as Klebanoff modes while the latter are strongly affected by leading edge geometry and are more streamwise vortex-like in appearance. © 2014 The Author(s) Published by the Royal Society. All rights reserved.
Boundary Layer Instabilities Generated by Freestream Laser Perturbations
Chou, Amanda; Schneider, Steven P.
2015-01-01
A controlled, laser-generated, freestream perturbation was created in the freestream of the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT). The freestream perturbation convected downstream in the Mach-6 wind tunnel to interact with a flared cone model. The geometry of the flared cone is a body of revolution bounded by a circular arc with a 3-meter radius. Fourteen PCB 132A31 pressure transducers were used to measure a wave packet generated in the cone boundary layer by the freestream perturbation. This wave packet grew large and became nonlinear before experiencing natural transition in quiet flow. Breakdown of this wave packet occurred when the amplitude of the pressure fluctuations was approximately 10% of the surface pressure for a nominally sharp nosetip. The initial amplitude of the second mode instability on the blunt flared cone is estimated to be on the order of 10 -6 times the freestream static pressure. The freestream laser-generated perturbation was positioned upstream of the model in three different configurations: on the centerline, offset from the centerline by 1.5 mm, and offset from the centerline by 3.0 mm. When the perturbation was offset from the centerline of a blunt flared cone, a larger wave packet was generated on the side toward which the perturbation was offset. The offset perturbation did not show as much of an effect on the wave packet on a sharp flared cone as it did on a blunt flared cone.
The influence of viscosity stratification on boundary-layer turbulence
Lee, Jin; Jung, Seo Yoon; Sung, Hyung Jin; Zaki, Tamer A.
2012-11-01
Direct numerical simulations of turbulent flows over isothermally-heated walls were performed to investigate the influence of viscosity stratification on boundary-layer turbulence and drag. The adopted model for temperature-dependent viscosity was typical of water. The free-stream temperature was set to 30°C, and two wall temperatures, 70°C and 99°C, were simulated. In the heated flows, the mean shear-rate is enhanced near the wall and reduced in the buffer region, which induces a reduction in turbulence production. On the other hand, the turbulence dissipation is enhanced near the wall, despite the the reduction in fluid viscosity. The higher dissipation is attributed to a decrease in the smallest length scales and near-wall fine-scale motions. The combined effect of the reduced production and enhanced dissipation leads to lower Reynolds shear stresses and, as a result, reduction of the skin-friction coefficient. Supported by the Engineering and Physical Sciences Research Council (Grant EP/F034997/1) and partially supported by the Erasmus Mundus Build on Euro-Asian Mobility (EM-BEAM) programme.
Investigation of particle lift off in a turbulent boundary layer
Barros, Diogo; Tee, Yi Hui; Morse, Nicholas; Hiltbrand, Ben; Longmire, Ellen
2017-11-01
Entrainment and suspension of particles within turbulent flows occur widely in environmental and industrial processes. Three-dimensional particle tracking experiments are thus conducted in a water channel to understand the interaction of finite-size particles with a turbulent boundary layer. A neutrally buoyant sphere made of wax and iron oxide is first held in place on the bounding surface by a magnet before being released and tracked. The sphere is marked with dots to monitor rotation as well as translation. By setting up two pairs of cameras in a stereoscopic configuration, the trajectories of the sphere are reconstructed and tracked over a distance of 4 to 6 δ. Sphere diameters ranging from 40 to 130 wall units, initial particle Reynolds numbers of 600 to 2000 and friction Reynolds numbers of 500 to 1800 are considered. For this parameter set, the particle typically lifts off from the wall after release before falling back toward the wall. Aspects of both particle rotation and translation will be discussed. Supported by NSF (CBET-1510154).
Appraisal of boundary layer trips for landing gear testing
McCarthy, Philip; Feltham, Graham; Ekmekci, Alis
2013-11-01
Dynamic similarity during scaled model testing is difficult to maintain. Forced boundary layer transition via a surface protuberance is a common method used to address this issue, however few guidelines exist for the effective tripping of complex geometries, such as aircraft landing gears. To address this shortcoming, preliminary wind tunnel tests were performed at Re = 500,000. Surface transition visualisation and pressure measurements show that zigzag type trips of a given size and location are effective at promoting transition, thus preventing the formation of laminar separation bubbles and increasing the effective Reynolds number from the critical regime to the supercritical regime. Extension of these experiments to include three additional tripping methods (wires, roughness strips, CADCUT dots) in a range of sizes, at Reynolds number of 200,000 and below, have been performed in a recirculating water channel. Analysis of surface pressure measurements and time resolved PIV for each trip device, size and location has established a set of recommendations for successful use of tripping for future, low Reynolds number landing gear testing.
Observations of the atmospheric electric field during two case studies of boundary layer processes
International Nuclear Information System (INIS)
Piper, I M; Bennett, A J
2012-01-01
We present measurements of potential gradient (PG) with associated meteorological variables and cloud profiles for two examples of convective boundary layer processes. Aerosol acts as a tracer layer to show lofting of the convective boundary layer; the rising aerosol layer results in a decrease in PG. In foggy conditions, the PG is seen to increase during the fog and then reduce as the fog lifts, as expected. (letter)
A vertically integrated dynamic RAM-cell: Buried bit line memory cell with floating transfer layer
Mouthaan, A.J.; Vertregt, Maarten
1986-01-01
A charge injection device has been realized in which charge can be injected on to an MOS-capacitor from a buried layer via an isolated transfer layer. The cell is positioned vertically between word and bit line. LOCOS (local oxidation) is used to isolate the cells and (deep) ion implantation to
Clouds, Aerosol, and Precipitation in the Marine Boundary Layer: An ARM Mobile Facility Deployment
Wood, Robert; Wyant, Matthew; Bretherton, Christopher S.; Remillard, Jasmine; Kollias, Pavlos; Fletcher, Jennifer; Stemmler, Jayson; de Szoeke, Simone; Yuter, Sandra; Miller, Matthew;
2015-01-01
Capsule: A 21-month deployment to Graciosa Island in the northeastern Atlantic Ocean is providing an unprecedented record of the clouds, aerosols and meteorology in a poorly-sampled remote marine environment The Clouds, Aerosol, and Precipitation in the Marine Boundary Layer (CAP-MBL) deployment at Graciosa Island in the Azores generated a 21 month (April 2009- December 2010) comprehensive dataset documenting clouds, aerosols and precipitation using the Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF). The scientific aim of the deployment is to gain improved understanding of the interactions of clouds, aerosols and precipitation in the marine boundary layer. Graciosa Island straddles the boundary between the subtropics and midlatitudes in the Northeast Atlantic Ocean, and consequently experiences a great diversity of meteorological and cloudiness conditions. Low clouds are the dominant cloud type, with stratocumulus and cumulus occurring regularly. Approximately half of all clouds contained precipitation detectable as radar echoes below the cloud base. Radar and satellite observations show that clouds with tops from 1- 11 km contribute more or less equally to surface-measured precipitation at Graciosa. A wide range of aerosol conditions was sampled during the deployment consistent with the diversity of sources as indicated by back trajectory analysis. Preliminary findings suggest important two-way interactions between aerosols and clouds at Graciosa, with aerosols affecting light precipitation and cloud radiative properties while being controlled in part by precipitation scavenging. The data from at Graciosa are being compared with short-range forecasts made a variety of models. A pilot analysis with two climate and two weather forecast models shows that they reproduce the observed time-varying vertical structure of lower-tropospheric cloud fairly well, but the cloud-nucleating aerosol concentrations less well. The Graciosa site has been chosen to be a
Clouds, Aerosols, and Precipitation in the Marine Boundary Layer: An Arm Mobile Facility Deployment
Energy Technology Data Exchange (ETDEWEB)
Wood, Robert; Wyant, Matthew; Bretherton, Christopher S.; Rémillard, Jasmine; Kollias, Pavlos; Fletcher, Jennifer; Stemmler, Jayson; de Szoeke, Simone; Yuter, Sandra; Miller, Matthew; Mechem, David; Tselioudis, George; Chiu, J. Christine; Mann, Julian A. L.; O’Connor, Ewan J.; Hogan, Robin J.; Dong, Xiquan; Miller, Mark; Ghate, Virendra; Jefferson, Anne; Min, Qilong; Minnis, Patrick; Palikonda, Rabindra; Albrecht, Bruce; Luke, Ed; Hannay, Cecile; Lin, Yanluan
2015-03-01
The Clouds, Aerosol, and Precipitation in the Marine Boundary Layer (CAP-MBL) 38 deployment at Graciosa Island in the Azores generated a 21 month (April 2009-December 2010) 39 comprehensive dataset documenting clouds, aerosols and precipitation using the Atmospheric 40 Radiation Measurement (ARM) Mobile Facility (AMF). The scientific aim of the deployment is 41 to gain improved understanding of the interactions of clouds, aerosols and precipitation in the 42 marine boundary layer. 43 Graciosa Island straddles the boundary between the subtropics and midlatitudes in the 44 Northeast Atlantic Ocean, and consequently experiences a great diversity of meteorological and 45 cloudiness conditions. Low clouds are the dominant cloud type, with stratocumulus and cumulus 46 occurring regularly. Approximately half of all clouds contained precipitation detectable as radar 47 echoes below the cloud base. Radar and satellite observations show that clouds with tops from 1-48 11 km contribute more or less equally to surface-measured precipitation at Graciosa. A wide 49 range of aerosol conditions was sampled during the deployment consistent with the diversity of 50 sources as indicated by back trajectory analysis. Preliminary findings suggest important two-way 51 interactions between aerosols and clouds at Graciosa, with aerosols affecting light precipitation 52 and cloud radiative properties while being controlled in part by precipitation scavenging. 53 The data from at Graciosa are being compared with short-range forecasts made a variety 54 of models. A pilot analysis with two climate and two weather forecast models shows that they 55 reproduce the observed time-varying vertical structure of lower-tropospheric cloud fairly well, 56 but the cloud-nucleating aerosol concentrations less well. The Graciosa site has been chosen to 57 be a long-term ARM site that became operational in October 2013.
Directory of Open Access Journals (Sweden)
H. G. Ouwersloot
2012-10-01
Full Text Available We studied the atmospheric boundary layer (ABL dynamics and the impact on atmospheric chemistry during the HUMPPA-COPEC-2010 campaign. We used vertical profiles of potential temperature and specific moisture, obtained from 132 radio soundings, to determine the main boundary layer characteristics during the campaign. We propose a classification according to several main ABL prototypes. Further, we performed a case study of a single day, focusing on the convective boundary layer, to analyse the influence of the dynamics on the chemical evolution of the ABL. We used a mixed layer model, initialized and constrained by observations. In particular, we investigated the role of large scale atmospheric dynamics (subsidence and advection on the ABL development and the evolution of chemical species concentrations. We find that, if the large scale forcings are taken into account, the ABL dynamics are represented satisfactorily. Subsequently, we studied the impact of mixing with a residual layer aloft during the morning transition on atmospheric chemistry. The time evolution of NO_{x} and O_{3} concentrations, including morning peaks, can be explained and accurately simulated by incorporating the transition of the ABL dynamics from night to day. We demonstrate the importance of the ABL height evolution for the representation of atmospheric chemistry. Our findings underscore the need to couple the dynamics and chemistry at different spatial scales (from turbulence to mesoscale in chemistry-transport models and in the interpretation of observational data.
Typhoon 9707 observations with the MU radar and L-band boundary layer radar
Directory of Open Access Journals (Sweden)
M. Teshiba
2001-08-01
Full Text Available Typhoon 9707 (Opal was observed with the VHF-band Middle and Upper atmosphere (MU radar, an L-band boundary layer radar (BLR, and a vertical-pointing C-band meteorological radar at the Shigaraki MU Observatory in Shiga prefecture, Japan on 20 June 1997. The typhoon center passed about 80 km southeast from the radar site. Mesoscale precipitating clouds developed due to warm-moist airmass transport from the typhoon, and passed over the MU radar site with easterly or southeasterly winds. We primarily present the wind behaviour including the vertical component which a conventional meteorological Doppler radar cannot directly observe, and discuss the relationship between the wind behaviour of the typhoon and the precipitating system. To investigate the dynamic structure of the typhoon, the observed wind was divided into radial and tangential wind components under the assumption that the typhoon had an axi-symmetric structure. Altitude range of outflow ascended from 1–3 km to 2–10 km with increasing distance (within 80–260 km range from the typhoon center, and in-flow was observed above and below the outflow. Outflow and inflow were associated with updraft and downdraft, respectively. In the tangential wind, the maximum speed of counterclockwise winds was confirmed at 1–2 km altitudes. Based on the vertical velocity and the reflectivity obtained with the MU radar and the C-band meteorological radar, respectively, precipitating clouds, accompanied by the wind behaviour of the typhoon, were classified into stratiform and convective precipitating clouds. In the stratiform precipitating clouds, a vertical shear of radial wind and the maximum speed of counterclockwise wind were observed. There was a strong reflectivity layer called a ‘bright band’ around the 4.2 km altitude. We confirmed strong updrafts and down-drafts below and above it, respectively, and the existence of a relatively dry layer around the bright band level from radiosonde
Typhoon 9707 observations with the MU radar and L-band boundary layer radar
Directory of Open Access Journals (Sweden)
M. Teshiba
Full Text Available Typhoon 9707 (Opal was observed with the VHF-band Middle and Upper atmosphere (MU radar, an L-band boundary layer radar (BLR, and a vertical-pointing C-band meteorological radar at the Shigaraki MU Observatory in Shiga prefecture, Japan on 20 June 1997. The typhoon center passed about 80 km southeast from the radar site. Mesoscale precipitating clouds developed due to warm-moist airmass transport from the typhoon, and passed over the MU radar site with easterly or southeasterly winds. We primarily present the wind behaviour including the vertical component which a conventional meteorological Doppler radar cannot directly observe, and discuss the relationship between the wind behaviour of the typhoon and the precipitating system. To investigate the dynamic structure of the typhoon, the observed wind was divided into radial and tangential wind components under the assumption that the typhoon had an axi-symmetric structure. Altitude range of outflow ascended from 1–3 km to 2–10 km with increasing distance (within 80–260 km range from the typhoon center, and in-flow was observed above and below the outflow. Outflow and inflow were associated with updraft and downdraft, respectively. In the tangential wind, the maximum speed of counterclockwise winds was confirmed at 1–2 km altitudes. Based on the vertical velocity and the reflectivity obtained with the MU radar and the C-band meteorological radar, respectively, precipitating clouds, accompanied by the wind behaviour of the typhoon, were classified into stratiform and convective precipitating clouds. In the stratiform precipitating clouds, a vertical shear of radial wind and the maximum speed of counterclockwise wind were observed. There was a strong reflectivity layer called a ‘bright band’ around the 4.2 km altitude. We confirmed strong updrafts and down-drafts below and above it, respectively, and the existence of a relatively dry layer around the bright band level from radiosonde
Study of cross-spectra of velocity components and temperature series in a nocturnal boundary layer
Maqueda, Gregorio; Sastre, Mariano; Viñas, Carmen; Viana, Samuel; Yagüe, Carlos
2010-05-01
The main characteristic of the Planetary Boundary Layer is the turbulent flow that can be understood as the motions of many superimposed eddies with different scales, which are very irregular and produce mixing among the atmospheric properties. Spectral analysis is a widely used statistical tool to know the size of eddies into the flow. The Turbulent Kinetic Energy is split in fractions for each scale of eddy by mean the power spectrum of the wind velocity components. Also, the fluctuation of the other variables as temperature, humidity, gases concentrations or material particles presents in the atmosphere can be divided according to the importance of different scales in a similar way than the wind. A Cross-spectrum between two time series is used in meteorology to know their correlation in frequency space. Specially, coespectrum, or real part of cross-spectrum, amplitud and coherence give us many information about the low or high correlation between two variables in a particular frecuency or scale (Stull, 1988). In this work we have investigated cross-spectra of velocity components and temperature measured along the summer 2009 at the CIBA, Research Centre for the Lower Atmosphere, located in Valladolid province (Spain), which is on a quite flat terrain (Cuxart et al., 2000; Viana et al., 2009). In these experimental dataset, among other instrumentation, two sonic anemometers (20 Hz, sampling rate) at 1.5 m and 10 m height are available. Cross-spectra between variables of the two levels, specially, wind vertical component and sonic temperature, under stable stratification are studied in order to improve the knowledge of the proprieties of the momentum and heat fluxes near the ground in the PBL. Nevertheless, power spectral of horizontal components of the wind, at both levels, have been also analysed. The spectra and cross-spectra were performed by mean the Blackman-Tukey method, widely utilised in the time series studies (Blackman & Tukey, 1958) and, where it is
Directory of Open Access Journals (Sweden)
Kiran Bhaganagar
2014-09-01
Full Text Available Turbulence structure in the wake behind a full-scale horizontal-axis wind turbine under the influence of real-time atmospheric inflow conditions has been investigated using actuator-line-model based large-eddy-simulations. Precursor atmospheric boundary layer (ABL simulations have been performed to obtain mean and turbulence states of the atmosphere under stable stratification subjected to two different cooling rates. Wind turbine simulations have revealed that, in addition to wind shear and ABL turbulence, height-varying wind angle and low-level jets are ABL metrics that influence the structure of the turbine wake. Increasing stability results in shallower boundary layers with stronger wind shear, steeper vertical wind angle gradients, lower turbulence, and suppressed vertical motions. A turbulent mixing layer forms downstream of the wind turbines, the strength and size of which decreases with increasing stability. Height dependent wind angle and turbulence are the ABL metrics influencing the lateral wake expansion. Further, ABL metrics strongly impact the evolution of tip and root vortices formed behind the rotor. Two factors play an important role in wake meandering: tip vortex merging due to the mutual inductance form of instability and the corresponding instability of the turbulent mixing layer.
Energy Technology Data Exchange (ETDEWEB)
Heinze, Rieke; Raasch, Siegfried; Etling, Dieter [Hannover Univ. (Germany). Inst. fuer Meteorologie und Klimatologie
2012-06-15
Karman vortex streets generated in the wake of an idealized island are studied using large eddy simulation (LES). Simulations were carried out under conditions of a dry convective boundary layer, capped by an inversion below the island top. These conditions are more realistic compared to previous studies in which mesoscale models with a uniform stable stratification were used. Several properties of the vortex streets like the shedding period of the vortices and the distances between cyclonic and anti-cyclonic vortices were determined for various values of Froude number and surface heat flux. The main focus of the study was to identify the azimuthally averaged structure of fully developed single vortices, which is presented here for the first time. For this purpose a tracking mechanism was developed which allows to detect and to follow vortices automatically. Because the capping inversion is located below the obstacle top, the vortices extend throughout the whole depth of the mixed layer and their features are almost constant with height. They have a nearly upright vertical axis with a warm core, which is feeded by a convergent near-surface inflow of warm air. The vortex core is dominated by a continuous updraft in the order of 10 cm s{sup -1}, which is associated with a divergent outflow of air at the vortex' top. This flow divergence creates an additional increase in temperature due to a locally sinking inversion, which is probably responsible for the cloud-free eye of many observed vortices. An increase in the surface heat flux is causing a faster decay of the vortices due to stronger boundary layer turbulence. Other vortex features derived from the simulations are very similar to those from previous studies. (orig.)
International Nuclear Information System (INIS)
Gerhauser, H.
1980-02-01
Two superimposed miscible liquids are separated by a diffuse boundary layer providing a steady transition of density. If the heavy fluid is on top of the light one, Rayleigh-Taylor-instabilities develop and cause a rapid interchange and eventually an intermixing. This process can be subjected to dynamic stabilization by enforcing vertical oscillations upon the whole system. However, since only part of the unstable mode spectrum is completely stabilized, the remaining weakly unstable modes lead to turbulent transport processes through the boundary layer ('anomalous diffusion'), so that only a quasistationary equilibrium is achieved. In the present paper, previous experimental results on the dynamic stabilization of water superimposed by an aqueous ZnJ-solution are theoretically interpreted, and the observed spatial structure as well as the time development of the density profiles are explained. There exists an analogy between these phenomena and turbulent transport processes in tokamak discharges such as the sawtooth oscillations of internal disruptions. (orig.) [de
International Nuclear Information System (INIS)
Abramov, Alexey G; Smirnov, Evgueni M; Goryachev, Valery D
2014-01-01
Results of direct numerical simulations for time-developing air natural-convection boundary layer are presented. Computations have been performed assuming periodicity conditions in both the directions parallel to the vertical isothermal hot plate. The contribution is mainly focused on understanding of laminar–turbulent transition peculiarities in the case of perturbation action of external turbulence that is modeled by isotropic disturbances initially introduced into the computational domain. Special attention is paid to identification and analysis of evolving three-dimensional vortices that clearly manifest themselves through the whole stages of laminar–turbulent transition in the boundary layer. A comparison of computed profiles of mean velocity, mean temperature and fluctuation characteristics for turbulent regimes of convection with experimental data is performed as well. (paper)
Ghannam, Khaled
structure function of the longitudinal and vertical velocity components is examined using five experimental data sets that span the roughness sub-layer above vegetation canopies, the atmospheric surface-layer above a lake and a grass field, and an open channel experiment. The results indicate that close to the wall/surface, this scaling exists in the longitudinal velocity structure function only, with the vertical velocity counterpart exhibiting a much narrower extent of this range due to smaller separation of scales. Phenomenological aspects of the large-scale eddies show that the length scale formed by the friction velocity and energy dissipation acts as a dominant similarity length scale in collapsing experimental data at different heights, mainly due to the imbalance between local production and dissipation of turbulence kinetic energy. • Nonlocal heat transport in the convective atmospheric boundary-layer: Failure of the mean gradient-diffusion (K-theory) in the convective boundary-layer is explored. Using large eddy simulation runs for the atmospheric boundary layer spanning weakly to strongly convective conditions, a generic diagnostic framework that encodes the role of third-order moments in nonlocal heat transport is developed and tested. The premise is that these nonlocal effects are responsible for the inherent asymmetry in vertical transport, and hence the necessary non-Gaussian nature of the joint probability density function (JPDF) of vertical velocity and potential temperature must account for these effects. Conditional sampling (quadrant analysis) of this function and the imbalance between the flow mechanisms of ejections and sweeps are used to characterize this asymmetry, which is then linked to the third-order moments using a cumulant-discard method for the Gram-Charlier expansion of the JPDF. The connection between the ejection-sweep events and the third-order moments shows that the concepts of bottom-up/top-down diffusion, or updraft/downdraft models
de Szoeke, S. P.
2017-12-01
Averaged over the tropical marine boundary layer (BL), 130 W m-2 turbulent surface moist static energy (MSE) flux, 120 W m-2 of which is evaporation, is balanced by upward MSE flux at the BL top due to 1) incorporation of cold air by downdrafts from deep convective clouds, and 2) turbulent entrainment of dry air into the BL. Cold saturated downdraft air, and warm clear air entrained into the BL have distinct thermodynamic properties. This work observationally quantifies their respective MSE fluxes in the central Indian Ocean in 2011, under different convective conditions of the intraseasonal (40-90 day) Madden Julian oscillation (MJO). Under convectively suppressed conditions, entrainment and downdraft fluxes export equal shares (60 W m-2) of MSE from the BL. Downdraft fluxes are more variable, increasing for stronger convection. In the convectively active phase of the MJO, downdrafts export 90 W m-2 from the BL, compared to 40 W m-2 by entrainment. These processes that control the internal, latent (condensation), and MSE of the tropical marine atmospheric BL determine the parcel buoyancy and strength of tropical deep convection.
Flat Plate Boundary Layer Stimulation Using Trip Wires and Hama Strips
Peguero, Charles; Henoch, Charles; Hrubes, James; Fredette, Albert; Roberts, Raymond; Huyer, Stephen
2017-11-01
Water tunnel experiments on a flat plate at zero angle of attack were performed to investigate the effect of single roughness elements, i.e., trip wires and Hama strips, on the transition to turbulence. Boundary layer trips are traditionally used in scale model testing to force a boundary layer to transition from laminar to turbulent flow at a single location to aid in scaling of flow characteristics. Several investigations of trip wire effects exist in the literature, but there is a dearth of information regarding the influence of Hama strips on the flat plate boundary layer. The intent of this investigation is to better understand the effects of boundary layer trips, particularly Hama strips, and to investigate the pressure-induced drag of both styles of boundary layer trips. Untripped and tripped boundary layers along a flat plate at a range of flow speeds were characterized with multiple diagnostic measurements in the NUWC/Newport 12-inch water tunnel. A wide range of Hama strip and wire trip thicknesses were used. Measurements included dye flow visualization, direct skin friction and parasitic drag force, boundary layer profiles using LDV, wall shear stress fluctuations using hot film anemometry, and streamwise pressure gradients. Test results will be compared to the CFD and boundary layer model results as well as the existing body of work. Conclusions, resulting in guidance for application of Hama strips in model scale experiments and non-dimensional predictions of pressure drag will be presented.
Shooting method for solution of boundary-layer flows with massive blowing
Liu, T.-M.; Nachtsheim, P. R.
1973-01-01
A modified, bidirectional shooting method is presented for solving boundary-layer equations under conditions of massive blowing. Unlike the conventional shooting method, which is unstable when the blowing rate increases, the proposed method avoids the unstable direction and is capable of solving complex boundary-layer problems involving mass and energy balance on the surface.
UAV-borne coherent doppler lidar for marine atmospheric boundary layer observations
Wu, Songhua; Wang, Qichao; Liu, Bingyi; Liu, Jintao; Zhang, Kailin; Song, Xiaoquan
2018-04-01
A compact UAV-borne Coherent Doppler Lidar (UCDL) has been developed at the Ocean University of China for the observation of wind profile and boundary layer structure in Marine Atmospheric Boundary Layer (MABL). The design, specifications and motion-correction methodology of the UCDL are presented. Preliminary results of the first flight campaign in Hailing Island in December 2016 is discussed.
Direct numerical simulation of stable and unstable turbulent thermal boundary layers
International Nuclear Information System (INIS)
Hattori, Hirofumi; Houra, Tomoya; Nagano, Yasutaka
2007-01-01
This paper presents direct numerical simulations (DNS) of stable and unstable turbulent thermal boundary layers. Since a buoyancy-affected boundary layer is often encountered in an urban environmental space where stable and unstable stratifications exist, exploring a buoyancy-affected boundary layer is very important to know the transport phenomena of the flow in an urban space. Although actual observation may qualitatively provide the characteristics of these flows, the relevant quantitative turbulent quantities are very difficult to measure. Thus, in order to quantitatively investigate a buoyancy-affected boundary layer in detail, we have here carried out for the first time time- and space-developing DNS of slightly stable and unstable turbulent thermal boundary layers. The DNS results show the quantitative turbulent statistics and structures of stable and unstable thermal boundary layers, in which the characteristic transport phenomena of thermally stratified boundary layers are demonstrated by indicating the budgets of turbulent shear stress and turbulent heat flux. Even though the input of buoyant force is not large, the influence of buoyancy is clearly revealed in both stable and unstable turbulent boundary layers. In particular, it is found that both stable and unstable thermal stratifications caused by the weak buoyant force remarkably alter the structure of near-wall turbulence
On the Nature, Theory, and Modeling of Atmospheric Planetary Boundary Layers
DEFF Research Database (Denmark)
Baklanov, Alexander A.; Grisogono, Branko; Bornstein, Robert
2011-01-01
The gap between our modern understanding of planetary boundary layer physics and its decades-old representations in current operational atmospheric models is widening, which has stimulated this review of the current state of the art and an analysis of the immediate needs in boundary layer theory......, measurements, and modeling....
The boundary layers as the primary transport regions of the earth's magnetotail
Eastman, T. E.; Frank, L. A.; Huang, C. Y.
1985-01-01
A comprehensive survey of ISEE and IMP LEPEDEA plasma measurements in the earth's magnetotail reveals that the magnetospheric boundary layer and the plasma sheet boundary layer are the primary transport regions there. These plasma measurements also reveal various components of the plasma sheet, including the central plasma sheet and plasma sheet boundary layer. A significant new result reported here is that of cold- and hot-plasma components that are spatially co-present within the central plasma sheet. Such plasma components cannot be explained merely by temporal variations in spectra involving the entire plasma sheet. Contributions to a low temperature component of the plasma sheet enter directly from the boundary layer located along the magnetotail flanks. Field-aligned flows predominate within the plasma sheet boundary layer which is almost always present and is located near the high- and low-latitude border of the plasma sheet. The plasma sheet boundary layer comprises highly anisotropic ion distributions, including counter-streaming ion beams, that evolve into the hot, isotropic component of the plasma sheet. Tailward acceleration regions generate these ion beams with plasma input from the magnetospheric boundary layer. Antisunward-flowing ion beams, at E/q less than 1 kV and of ionospheric composition, are frequently observed in the plasma sheet boundary layer and in tail lobes. These ion beams are likely accelerated at low altitude over the polar cap and especially along auroral field lines.
Stochastic Theory of Turbulence Mixing by Finite Eddies in the Turbulent Boundary Layer
Dekker, H.; Leeuw, G. de; Maassen van den Brink, A.
1995-01-01
Turbulence mixing is treated by means of a novel formulation of nonlocal K-theory, involving sample paths and a stochastic hypothesis. The theory simplifies for mixing by exchange (strong-eddies) and is then applied to the boundary layer (involving scaling). This maps boundary layer turbulence onto
A class of backward free-convective boundary-layer similarity solutions
Kuiken, H.K.
1983-01-01
This paper presents a class of backward free-convective boundary-layer similarity solutions. It is shown that these boundary layers can be produced along slender downward-projecting slabs of prescribed thickness variation, which are infinitely long. It is pointed out that these solutions can be used
Quantized vortex pair production in 4He films as a boundary-layer problem
International Nuclear Information System (INIS)
McCauley, J.L. Jr.
1979-01-01
The author shows that the idea of a boundary layer for discrete vortices arises naturally from the equation of motion for the probability distribution of an interacting vortex pair. In contrast with classical hydrodynamics, this boundary layer is of statistical origin, and the method leads to a scaling law for the exact dissociation rate of a bound vortex pair. (Auth.)
DEFF Research Database (Denmark)
Lauros, J.; Sogachev, Andrey; Smolander, S.
2011-01-01
the atmospheric boundary layer during nucleation event days shows a highly dynamical picture, where particle formation is coupled with chemistry and turbulent transport. We have demonstrated the suitability of our turbulent mixing scheme in reproducing the most important characteristics of particle dynamics...... within the boundary layer. Deposition and particle flux simulations show that deposition affects noticeably only the smallest particles...
Effects of micro-ramps on a shock wave/turbulent boundary layer interaction
Blinde, P.L.; Humble, R.A.; Van Oudheusden, B.W.; Scarano, F.
2009-01-01
Stereoscopic particle image velocimetry is used to investigate the effects of micro-ramp sub-boundary layer vortex generators, on an incident shock wave/boundary layer interaction at Mach 1.84. Single- and double-row arrangements of micro-ramps are considered. The micro-ramps have a height of 20% of
Representing the atmospheric boundary layer in climate models of intermediate compexity
Ronda, R.J.; Haarsma, R.J.; Holtslag, A.A.M.
2003-01-01
In this study the role of atmospheric boundary layer schemes in climate models is investigated. Including a boundary layer scheme in an Earth system model of intermediate complexity (EMIC) produces only minor differences in the estimated global distribution of sensible and latent heat fluxes over
Efficient modelling of aerodynamic flows in the boundary layer for high performance computing
CSIR Research Space (South Africa)
Smith, L
2011-01-01
Full Text Available A unique technique to couple boundary-layer solutions with an inviscid solver is introduced. The boundary-layer solution is obtained using the two-integral method to solve displacement thickness with Newton’s method, at a fraction of the cost of a...