Instabilities and transition in boundary layers
Indian Academy of Sciences (India)
N Vinod; Rama Govindarajan
2005-03-01
Some recent developments in boundary layer instabilities and transition are reviewed. Background disturbance levels determine the instability mechanism that ultimately leads to turbulence. At low noise levels, the traditional Tollmien–Schlichting route is followed, while at high levels, a `by-pass' route is more likely. Our recent work shows that spot birth is related to the pattern of secondary instability in either route.
Application of Arnoldi method to boundary layer instability
Zhang, Yong-Ming; Luo, Ji-Sheng
2015-12-01
The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e., comparison with linear stability theory (LST) for two-dimensional (2D) disturbance on one-dimensional (1D) basic flow, comparison with LST for three-dimensional (3D) disturbance on 1D basic flow, and comparison with Floquet theory for 3D disturbance on 2D basic flow. Then it is applied to secondary instability analysis on the streaky boundary layer under spanwise-localized free-stream turbulence (FST). Three unstable modes are found, i.e., an inner mode at a high-speed center streak, a sinuous type outer mode at a low-speed center streak, and a sinuous type outer mode at low-speed side streaks. All these modes are much more unstable than Tollmien-Schlichting (TS) waves, implying the dominant contribution of secondary instability in bypass transition. The modes at strong center streak are more unstable than those at weak side streaks, so the center streak is ‘dangerous’ in secondary instability. Project supported by the National Natural Science Foundation of China (Grant Nos. 11202147, 11332007, 11172203, and 91216111) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120032120007).
Spatially Developing Secondary Instabilities in Compressible Swept Airfoil Boundary Layers
Li, Fei; Choudhari, Meelan M.
2011-01-01
Two-dimensional eigenvalue analysis is used on a massive scale to study spatial instabilities of compressible shear flows with two inhomogeneous directions. The main focus of the study is crossflow dominated swept-wing boundary layers although the methodology can also be applied to study other type of flows, such as the attachment-line flow. Certain unique aspects of formulating a spatial, two-dimensional eigenvalue problem for the secondary instability of finite amplitude crossflow vortices are discussed, namely, fixing the spatial growth direction unambiguously through a non-orthogonal formulation of the linearized disturbance equations. A primary test case used for parameter study corresponds to the low-speed, NLF-0415(b) airfoil configuration as tested in the ASU Unsteady Wind Tunnel, wherein a spanwise periodic array of roughness elements was placed near the leading edge in order to excite stationary crossflow modes with a specified fundamental wavelength. The two classes of flow conditions selected for this analysis include those for which the roughness array spacing corresponds to either the naturally dominant crossflow wavelength, or a subcritical wavelength that serves to reduce the growth of the naturally excited dominant crossflow modes. Numerical predictions are compared with the measured database, both as indirect validation for the spatial instability analysis and to provide a basis for comparison with a higher Reynolds number, supersonic swept-wing configuration. Application of the eigenvalue analysis to the supersonic configuration reveals that a broad spectrum of stationary crossflow modes can sustain sufficiently strong secondary instabilities as to potentially cause transition over this configuration. Implications of this finding for transition control in swept wing boundary layers are examined.
Radiative instabilities of atmospheric jets and boundary layers
International Nuclear Information System (INIS)
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)
Li, Fei; Choudhari, Meelan M.
2008-01-01
This paper reports on progress towards developing a spatial stability code for compressible shear flows with two inhomogeneous directions, such as crossflow dominated swept-wing boundary layers and attachment line flows. Certain unique aspects of formulating a spatial, two-dimensional eigenvalue problem for the secondary instability of finite amplitude crossflow vortices are discussed. A primary test case used for parameter study corresponds to the low-speed, NLF-0415(b) airfoil configuration as tested in the ASU Unsteady Wind Tunnel, wherein a spanwise periodic array of roughness elements was placed near the leading edge in order to excite stationary crossflow modes with a specified fundamental wavelength. The two classes of flow conditions selected for this analysis include those for which the roughness array spacing corresponds to either the naturally dominant crossflow wavelength, or a subcritical wavelength that serves to reduce the growth of the naturally excited dominant crossflow modes. Numerical predictions are compared with the measured database, both as indirect validation for the spatial instability analysis and to provide a basis for comparison with a higher Reynolds number, supersonic swept-wing configuration. Application of the eigenvalue analysis to the supersonic configuration reveals that a broad spectrum of stationary crossflow modes can sustain sufficiently strong secondary instabilities as to potentially cause transition over this configuration. Implications of this finding for transition control in swept wing boundary layers are examined. Finally, extension of the spatial stability analysis to supersonic attachment line flows is also considered.
Centralised versus Decentralised Active Control of Boundary Layer Instabilities
Dadfar, R; Bagheri, S; Henningson, D S
2014-01-01
We use linear control theory to construct an output feedback controller for the attenuation of small-amplitude Tollmien-Schlichting (TS) wavepackets in a flat-plate boundary layer.We distribute evenly in the spanwise direction up to 72 localized objects near the wall (18 disturbances sources, 18 actuators, 18 estimation sensors and 18 objective sensors). In a fully three-dimensional configuration,the interconnection between inputs and outputs becomes quickly unfeasible when the number of actuators and sensors increases in the spanswise direction. The objective of this work is to understand how an efficient controller may be designed by connecting only a subset of the actuators to sensors, thereby reducing the complexity of the controller, without comprising the efficiency. We find that using a semi-decentralized approach - where small control units consisting of 3 estimation sensors connected to 3 actuators are replicated 6 times along the spanwise direction - results only in a 11% reduction of control perfor...
Secondary Instability of Second Modes in Hypersonic Boundary Layers
Li, Fei; Choudhari, Meelan M.; Chang, Chau-Lyan; White, Jeffery A.
2012-01-01
Second mode disturbances dominate the primary instability stage of transition in a number of hypersonic flow configurations. The highest amplification rates of second mode disturbances are usually associated with 2D (or axisymmetric) perturbations and, therefore, a likely scenario for the onset of the three-dimensionality required for laminar-turbulent transition corresponds to the parametric amplification of 3D secondary instabilities in the presence of 2D, finite amplitude second mode disturbances. The secondary instability of second mode disturbances is studied for selected canonical flow configurations. The basic state for the secondary instability analysis is obtained by tracking the linear and nonlinear evolution of 2D, second mode disturbances using nonlinear parabolized stability equations. Unlike in previous studies, the selection of primary disturbances used for the secondary instability analysis was based on their potential relevance to transition in a low disturbance environment and the effects of nonlinearity on the evolution of primary disturbances was accounted for. Strongly nonlinear effects related to the self-interaction of second mode disturbances lead to an upstream shift in the upper branch neutral location. Secondary instability computations confirm the previously known dominance of subharmonic modes at relatively small primary amplitudes. However, for the Purdue Mach 6 compression cone configuration, it was shown that a strong fundamental secondary instability can exist for a range of initial amplitudes of the most amplified second mode disturbance, indicating that the exclusive focus on subharmonic modes in the previous applications of secondary instability theory to second mode primary instability may not have been fully justified.
Instability onset of the boundary layer on a rotating cylinder in a stratified fluid
Flor, Jan-Bert; Hirschberg, Lionel; Oostenrijk, Bart; van Heijst, Gertjan; Meige Team
2015-11-01
We consider the instability of the laminar shear layer on a circular cylinder that is impulsively set into rotation about its vertical axis with angular speed Ω. The outer wall of this large gap Taylor-Couette flow is at a radial distance of about 10 times the inner cylinder radius, and the gap is either filled with a homogeneous or linearly stratified fluid. In a homogeneous fluid, the thickness of the boundary layer on the cylinder, d, grows until it becomes centrifugally unstable with a wavelength that is determined by the boundary layer thickness d. In a linearly stratified fluid with stratification N, the flow instability is set by the Froude number F = Ω /N. For F>1 the onset of the centrifugal instability is well predicted by the Taylor-Görtler number and theory for homogenous fluids. When F vortex regime to a wave regime with a pure inertial wave in the boundary layer. The mechanism of instability is determined by parametric resonance and the generation of waves with subharmonic frequencies typical for Parametric Subharmonic Instability. The results are discussed in view of former results on stratified TC flow. Supported by LabEx Osug@2020 (Investissements d'avenir - ANR10LABX56).
Study of effect of a smooth hump on hypersonic boundary layer instability
Park, Donghun; Park, Seung O.
2016-05-01
Effect of a two-dimensional smooth hump on linear instability of hypersonic boundary layer is studied by using parabolized stability equations. Linear evolution of mode S over a hump is analyzed for Mach 4.5 and 5.92 flat plate and Mach 7.1 sharp cone boundary layers. Mean flow for stability analysis is obtained by solving the parabolized Navier-Stokes equations. Hump with height smaller than local boundary layer thickness is considered. The case of flat plate and sharp cone without the hump are also studied to provide comparable data. For flat plate boundary layers, destabilization and stabilization effect is confirmed for hump located at upstream and downstream of synchronization point, respectively. Results of parametric studies to examine the effect of hump height, location, etc., are also given. For sharp cone boundary layer, stabilization influence of hump is also identified for a specific range of frequency. Stabilization influence of hump on convective instability of mode S is found to be a possible cause of previous experimental observations of delaying transition in hypersonic boundary layers.
Secondary Instability of Stationary Crossflow Vortices in Mach 6 Boundary Layer Over a Circular Cone
Li, Fei; Choudhari, Meelan M.; Paredes-Gonzalez, Pedro; Duan, Lian
2015-01-01
Hypersonic boundary layer flows over a circular cone at moderate incidence can support strong crossflow instability. Due to more efficient excitation of stationary crossflow vortices by surface roughness, such boundary layer flows may transition to turbulence via rapid amplification of the high-frequency secondary instabilities of finite amplitude stationary crossflow vortices. The amplification characteristics of these secondary instabilities are investigated for crossflow vortices generated by an azimuthally periodic array of roughness elements over a 7-degree half-angle circular cone in a Mach 6 free stream. Depending on the local amplitude of the stationary crossflow mode, the most unstable secondary disturbances either originate from the second (i.e., Mack) mode instabilities of the unperturbed boundary layer or correspond to genuine secondary instabilities that reduce to stable disturbances at sufficiently small amplitudes of the stationary crossflow vortex. The predicted frequencies of dominant secondary disturbances are similar to those measured during wind tunnel experiments at Purdue University and the Technical University of Braunschweig, Germany.
Global instabilities and transient growth in Blasius boundary-layer flow over a compliant panel
Indian Academy of Sciences (India)
K Tsigklifis; A D Lucey
2015-05-01
We develop a hybrid of computational and theoretical approaches suited to study the fluid–structure interaction (FSI) of a compliant panel, flush between rigid upstream and downstream wall sections, with a Blasius boundary-layer flow. The ensuing linear-stability analysis is focused upon global instability and transient growth of disturbances. The flow solution is developed using a combination of vortex and source boundary-element sheets on a computational grid while the dynamics of a plate-spring compliant wall are couched in finite-difference form. The fully coupled FSI system is then written as an eigenvalue problem and the eigenvalues of the various flow- and wall-based instabilities are analysed. It is shown that coalescence or resonance of a structural eigenmode with either a flow-based Tollmien–Schlichting Wave (TSW) or wall-based travelling-wave flutter (TWF) modes can occur. This can render the nature of these well-known convective instabilities to become global for a finite compliant wall giving temporal growth of system disturbances. Finally, a non-modal analysis based on the linear superposition of the extracted temporal modes is presented. This reveals a high level of transient growth when the flow interacts with a compliant panel that has structural properties which render the FSI system prone to global instability. Thus, to design stable finite compliant panels for applications such as boundary-layer transition postponement, both global instabilities and transient growth must be taken into account.
On the Stability of Three-Dimensional Boundary Layers. Pt. 2; Secondary Instability
Janke, Erik; Balakumar, Ponnampalam
1999-01-01
The secondary instability of three-dimensional incompressible boundary layers is studied using Floquet theory. Starting from the equilibrium solutions that we obtained from the PSE computations documented in Part 1, we investigate the region where a purely stationary crossflow disturbance saturates for its secondary instability characteristics utilizing developed global and local eigenvalue solvers that are based on the Implicitly Restarted Arnoldi Method, and a Newton-Raphson technique, respectively. 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 routes by which high-frequency disturbances enter the boundary layer, as well as on gaining more information about threshold amplitudes for the growth of secondary disturbances.
Resonant Excitation of Boundary Layer Instability of DC Arc Plasma Jet by Current Modulation
Czech Academy of Sciences Publication Activity Database
Kopecký, Vladimír; Hrabovský, Milan
2011-01-01
Roč. 31, č. 6 (2011), s. 827-838. ISSN 0272-4324 R&D Projects: GA ČR GAP205/11/2070 Institutional research plan: CEZ:AV0Z20430508 Keywords : dc arc jet * plasma jet oscillations * boundary layer instability * frequency spectra Subject RIV: BL - Plasma and Gas Discharge Physics Impact factor: 1.602, year: 2011 http://www.springerlink.com/content/v160841757161758/
Multiple harmonic ULF waves in the plasma sheet boundary layer: Instability analysis
Denton, R. E.; Engebretson, M. J.; Keiling, A.; Walsh, A. P.; Gary, S. P.; DéCréAu, P. M. E.; Cattell, C. A.; RèMe, H.
2010-12-01
Multiple-harmonic electromagnetic waves in the ULF band have occasionally been observed in Earth's magnetosphere, both near the magnetic equator in the outer plasmasphere and in the plasma sheet boundary layer (PSBL) in Earth's magnetotail. Observations by the Cluster spacecraft of multiple-harmonic electromagnetic waves with fundamental frequency near the local proton cyclotron frequency, Ωcp, were recently reported in the plasma sheet boundary layer by Broughton et al. (2008). A companion paper surveys the entire magnetotail passage of Cluster during 2003, and reports 35 such events, all in the PSBL, and all associated with elevated fluxes of counterstreaming ions and electrons. In this study we use observed pitch angle distributions of ions and electrons during a wave event observed by Cluster on 9 September 2003 to perform an instability analysis. We use a semiautomatic procedure for developing model distributions composed of bi-Maxwellian components that minimizes the difference between modeled and observed distribution functions. Analysis of wave instability using the WHAMP electromagnetic plasma wave dispersion code and these model distributions reveals an instability near Ωcp and its harmonics. The observed and model ion distributions exhibit both beam-like and ring-like features which might lead to instability. Further instability analysis with simple beam-like and ring-like model distribution functions indicates that the instability is due to the ring-like feature. Our analysis indicates that this instability persists over an enormous range in the effective ion beta (based on a best fit for the observed distribution function using a single Maxwellian distribution), β', but that the character of the instability changes with β'. For β' of order unity (for instance, the observed case with β' ˜ 0.4), the instability is predominantly electromagnetic; the fluctuating magnetic field has components in both the perpendicular and parallel directions, but the
Unsteady Heat-Flux Measurements of Second-Mode Instability Waves in a Hypersonic Boundary Layer
Kergerise, Michael A.; Rufer, Shann J.
2016-01-01
In this paper we report on the application of the atomic layer thermopile (ALTP) heat- flux sensor to the measurement of laminar-to-turbulent transition in a hypersonic flat plate boundary layer. The centerline of the flat-plate model was instrumented with a streamwise array of ALTP sensors and the flat-plate model was exposed to a Mach 6 freestream over a range of unit Reynolds numbers. Here, we observed an unstable band of frequencies that are associated with second-mode instability waves in the laminar boundary layer that forms on the flat-plate surface. The measured frequencies, group velocities, phase speeds, and wavelengths of these instability waves are in agreement with data previously reported in the literature. Heat flux time series, and the Morlet-wavelet transforms of them, revealed the wave-packet nature of the second-mode instability waves. In addition, a laser-based radiative heating system was developed to measure the frequency response functions (FRF) of the ALTP sensors used in the wind tunnel test. These measurements were used to assess the stability of the sensor FRFs over time and to correct spectral estimates for any attenuation caused by the finite sensor bandwidth.
BUOYANCY INSTABILITY IN THE NATURAL CONVECTION BOUNDARY LAYER AROUND A VERTICAL HEATED FLAT PLATE
Institute of Scientific and Technical Information of China (English)
颜大椿; 张汉勋
2002-01-01
A systematic research on the buoyancy instability in the natural convection boundary layer was conducted, including the basic characteristics such as its spectral components, wave length and velocity, the location of its critical layer,and amplitude distributions of the triple independent eigenmodes with the linear instability theory, the growth rates of its temperature and velocity fluctuations and the corresponding neutral curves for the buoyancy eigenmode were also obtained.Results indicated that the neutral curve of the velocity fluctuation had a nose shape consistent with that obtained in the numerical calculation, but for the temperature fluctuation, a ring-like region could be measured at a lower Grashof number before the nose-shaped main portion of the neutral curve.
Optimal disturbances and instability in rotating non-parallel boundary layer flow
Yecko, P
2003-01-01
This work examines the stability of three-dimensional disturbances in rotating boundary layer flows, where the axis of rotation is either parallel or anti-parallel to that of the vorticity of the flow. Both Blasius and asymptotic suction layers are studied. The flows are non-parallel in that transverse velocity is taken into account in all disturbance calculations, but the effect of the transverse flow on the results is negligible. A Chebyshev collocation method is applied within a temporal stability framework to compute eigenvalues and maximum transient amplification factors. Anti-cyclonic rotation is eigenvalue destabilizing, as is known from results of the related cases of free shear and channel flows, introducing a region of instability characterized by modes of weak or no streamwise dependence. Disturbances that are three-dimensional are found to experience the greatest transient amplification under either cyclonic or anti-cyclonic rotation. In all cases examined here these optimal disturbances take the ...
Li, W.; Barros, A. P.
2008-05-01
Vertical profiles of wind, pressure, air temperature and humidity up to 500 m obtained from measurements by a tethersonde system were used in combination with upper level temperature and humidity soundings from Rapid Update Cycle (RUC), to calculate Convective Available Potential Energy (CAPE) in an unstable boundary layer. The surface fluxes of sensible and latent heat were also calculated based on turbulent similarity theory for the atmospheric surface layer. The measurements were performed during the Cloud and Land Surface Interaction Campaign (CLASIC) June 2007 that includes pre-storm and post-storm conditions for a record monthly rainfall in excess of 300 mm at the site. The daytime trajectories of the surface layer in the Relative-Humidity and Bowen Ratio phase-space are consistent with the rainfall and aridity attractors in previous studies, with strong decrease in the post-storm periods. The decrease of Bowen ratio was the result of a strong decrease in the magnitude of sensible heat fluxes. The latent heat fluxes in the post-storm environment were not significantly different from the pre-storm environment, which is explained by a significant decrease in the net radiation. High soil moisture and increased moisture in boundary layer in the post-storm environment led to sustained low-level instability and daily evening showers. The diurnal cycle of potential temperature and specific humidity during the duration of the field campaign with an emphasis on conditions before and after one major rainy event are also discussed in this study.
The Kelvin-Helmholtz instability of boundary-layer plasmas in the kinetic regime
Steinbusch, Benedikt; Gibbon, Paul; Sydora, Richard D.
2016-05-01
The dynamics of the Kelvin-Helmholtz instability are investigated in the kinetic, high-frequency regime with a novel, two-dimensional, mesh-free tree code. In contrast to earlier studies which focused on specially prepared equilibrium configurations in order to compare with fluid theory, a more naturally occurring plasma-vacuum boundary layer is considered here with relevance to both space plasma and linear plasma devices. Quantitative comparisons of the linear phase are made between the fluid and kinetic models. After establishing the validity of this technique via comparison to linear theory and conventional particle-in-cell simulation for classical benchmark problems, a quantitative analysis of the more complex magnetized plasma-vacuum layer is presented and discussed. It is found that in this scenario, the finite Larmor orbits of the ions result in significant departures from the effective shear velocity and width underlying the instability growth, leading to generally slower development and stronger nonlinear coupling between fast growing short-wavelength modes and longer wavelengths.
Mortensen, Clifton
2015-01-01
The current understanding of the effects of thermochemical nonequilibrium on hypersonic boundary-layer instability still contains uncertainties, and there has been little research into the effects of surface ablation, or two-dimensional roughness, on hypersonic boundary-layer instability. The objective of this work is to study the effects of thermochemical nonequilibrium on hypersonic boundary-layer instability. More specifically, two separate nonequilibrium flow configurations are studied: 1...
On the modulational instability of large amplitude waves in supersonic boundary layers
Hall, Philip; Papageorgiou, Demetrios T.
1995-01-01
The evolution of large amplitude Tollmien-Schlichting waves in a supersonic boundary layer is investigated. Disturbances which have their wavenumber and frequency slowly varying in time and space are described using a phase equation type of approach. Unlike the incompressible case we find that the initial bifurcation to a finite amplitude Tollmien-Schlichting wave is subcritical for most Mach numbers. In fact the bifurcation is only supercritical for a small range of Mach numbers and even then for only a finite range of wave propagation angles. The modulational instability of large amplitude wavetrains is considered and is shown to be governed by an equation similar to Burgers equation but with the viscous term replaced by a fractional derivative. A numerical investigation of the solution of this equation is described. It is shown that uniform wavetrains are unstable.
Directory of Open Access Journals (Sweden)
Syeda Khudeja Akbar
2015-08-01
Full Text Available The effect of chemical reaction on the linear stability of a viscoelastic fluid saturated horizontal densely-packed porous layer is investigated. The viscoelastic properties are given by Maxwell constitutive relations. The porous layer is cooled from the upper boundary while an adiabatic thermal boundary condition is imposed at the lower boundary. Linear stability analysis suggests that there is a competition between the processes of viscous relaxation and thermal diffusion that causes the first convective instability to be oscillatory rather than stationary. The effect of Deborah number, Darcy-Prandtl number, normalized porosity, and the Frank-Kamenetskii number on the stability of the system is investigated. Using a weighted residual method we calculate numerically the convective thresholds for both stationary and oscillatory instability. The effects of viscoelasticity and chemical reaction on the instability are emphasized. Some existing results are reproduced as the particular cases of the present study.
Loitsianskii. L. G.
1956-01-01
The fundamental, practically the most important branch of the modern mechanics of a viscous fluid or a gas, is that branch which concerns itself with the study of the boundary layer. The presence of a boundary layer accounts for the origin of the resistance and lift force, the breakdown of the smooth flow about bodies, and other phenomena that are associated with the motion of a body in a real fluid. The concept of boundary layer was clearly formulated by the founder of aerodynamics, N. E. Joukowsky, in his well-known work "On the Form of Ships" published as early as 1890. In his book "Theoretical Foundations of Air Navigation," Joukowsky gave an account of the most important properties of the boundary layer and pointed out the part played by it in the production of the resistance of bodies to motion. The fundamental differential equations of the motion of a fluid in a laminar boundary layer were given by Prandtl in 1904; the first solutions of these equations date from 1907 to 1910. As regards the turbulent boundary layer, there does not exist even to this day any rigorous formulation of this problem because there is no closed system of equations for the turbulent motion of a fluid. Soviet scientists have done much toward developing a general theory of the boundary layer, and in that branch of the theory which is of greatest practical importance at the present time, namely the study of the boundary layer at large velocities of the body in a compressed gas, the efforts of the scientists of our country have borne fruit in the creation of a new theory which leaves far behind all that has been done previously in this direction. We shall herein enumerate the most important results by Soviet scientists in the development of the theory of the boundary layer.
Convective instability in a ferrofluid layer with temperature-modulated rigid boundaries
International Nuclear Information System (INIS)
Under terrestrial as well as gravity-free conditions, a time-periodic modulation in temperatures of two horizontal rigid planes containing an initially quiescent ferrofluid layer induces time-periodic oscillations in the fluid layer at the onset of instability. This results in a series of patterns of time-periodically oscillating magnetoconvective rolls, along the vertical. The onset of instability in the ferrofluid layer is either a harmonic response or a subharmonic response depending upon the modulation. The instability is found to be significantly affected by the application of magnetic field across the ferrofluid layer. Under modulation, subcritical instabilities are found to occur in the form of subharmonic response. Also, the onset of instability in the ferrofluid layer when it is driven solely by the magnetic forces alone is found to heavily depend upon the frequency of modulation, the effect being greatest for the low-frequency modulation and negligible for the case of high-frequency modulation. The gravity-free limit is also evaluated as a function of the magnetic susceptibility, under modulation. To carry out this extensive study, the classical Floquet theory is utilized.
Burnel, S.; Gougat, P.; Martin, F.
1981-01-01
The natural instabilities which propagate in the laminar boundary layer of a flat plate composed of intermittent wave trains are described. A spectral analysis determines the frequency range and gives a frequency and the harmonic 2 only if there is a wall deformation. This analysis provides the amplitude modulation spectrum of the instabilities. Plots of the evolution of power spectral density are compared with the numerical results obtained from the resolve of the Orr-Sommerfeld equation, while the harmonic is related to a micro-recirculating flow near the wall deformation.
Tang, Qing; Zhu, Yiding; Chen, Xi; Lee, Cunbiao
2015-06-01
Particle image velocimetry, PCB pressure sensors, and planar Rayleigh scattering are combined to study the development of second-mode instability in a Mach 6 flow over a flat plate with two-dimensional roughness. To the best of the authors' knowledge, this is the first time that the instantaneous velocity fields and flow structures of the second-mode instability waves passing through the roughness are shown experimentally. A two-dimensional transverse wall blowing is used to generate second-mode instability in the boundary layer and seeding tracer particles. The two-dimensional roughness is located upstream of the synchronization point between mode S and mode F. The experimental results showed that the amplitude of the second-mode instability will be greatly increased upstream of the roughness. Then it damps and recovers quickly in the vicinity downstream of the roughness. Further downstream, it acts as no-roughness case, which confirms Fong's numerical results [K. D. Fong, X. W. Wang, and X. L. Zhong, "Numerical simulation of roughness effect on the stability of a hypersonic boundary layer," Comput. Fluids 96, 350 (2014)]. It also has been observed that the strength of the amplification and damping effect depends on the height of the roughness.
Time-Resolved Visualization of Instability Waves in a Hypersonic Boundary Layer
Laurence, Stuart; Wagner, Alexander; Hannemann, Klaus; Wartemann, Viola; Lüdeke, Heinrich; TANNO, Hideyuki; ITOH, Katsuhiro
2012-01-01
LAMINAR-TURBULENT transition in hypersonic boundary layers remains a challenging subject. This is especially true of the hypervelocity regime, in which an intriguing phenomenon is the possible damping of second-mode disturbances by chemical and vibrational nonequilibrium processes. To generate flows with sufficiently high enthalpy to investigate such effects, the use of shock-tunnel facilities is necessary; furthermore, it is now generally accepted that direct measurements of the instabili...
International Nuclear Information System (INIS)
The behavior of small disturbances in a 3-D laminar boundary layer on a yawed cylinder was experimentally investigated. This setup simulates the flow around the leading edge of swept wings. Since multiple instability modes appear near the attachment-line region, a point-source disturbance was artificially introduced to separate these modes. Amplitude and phase distributions of the disturbances originating from the point source were measured using a hotwire probe near the attachment-line flow to test existing theoretical predictions. Hotwire measurements show that two instability modes definitely coexist and overlap in the middle portion of the wedge-shaped region developing downstream of the point source. Decomposition by 2-D fast Fourier transform (FFT) analysis enables us to separate one oblique wave from the other. One of the oblique waves belongs to the cross-flow instability mode, which travels to the attachment line and grows even at Reynolds numbers that are slightly lower than the critical Reynolds number for the attachment-line instability. The origin of the other mode is not identifiable, because it has peculiar characteristics different from both the streamline-curvature instability mode and the cross-flow instability mode. This mode decays in the downstream direction for all frequencies examined. By investigating the spatial characteristics of the small disturbance, the critical Reynolds number for cross-flow instability was successfully determined in the off-attachment-line region. The value, Rc = 543, was lower than the critical Reynolds number of Rc = 583 for the attachment-line flow. Furthermore, the critical frequency and wavenumber were in good agreement with existing predictions based on linear stability theory.
Viola, Francesco; Iungo, Giacomo Valerio; Camarri, Simone; Porté-Agel, Fernando; Gallaire, François
2014-11-01
In wind farms, the separation distance among wind turbines is mainly determined by the downstream recovery of wind turbine wakes, which affects in turn power production and fatigue loads of downstream turbines. Thus, the optimization of a wind farm relies on the understanding of the single wake dynamics and a better characterization of their interactions within the atmospheric boundary layer (ABL). This work is focused on the stability analysis of vorticity structures present in wind turbine wakes. In order to take into account the effects of a non-uniform incoming wind investing the turbine, a 3D local stability analysis is performed on the non-axisymmetric swirling wake prevailing at different downstream stations. Different wind shear and veer of the incoming wind can now be investigated, together with a 3D non-isotropic turbulent velocity field. This procedure enables to perform stability analysis of wind turbine wakes for wind conditions very similar to the ones experienced in reality. The present analysis is carried out on wind tunnel data acquired in the wake of a down-scaled three-bladed wind turbine. The Reynolds stresses are taken into account via eddy-viscosity models calibrated on the experimental data. Furthermore, the effect of an external perturbation in the wake flow is investigated through linear sensitivity. This analysis represents a preliminary step for control of wind turbine wakes, and optimization of wake interactions and power harvesting.
Analysis of Instabilities in Non-Axisymmetric Hypersonic Boundary Layers Over Cones
Li, Fei; Choudhari, Meelan M.; Chang, Chau-Lyan; White, Jeffery A.
2010-01-01
Hypersonic flows over circular cones constitute one of the most important generic configurations for fundamental aerodynamic and aerothermodynamic studies. In this paper, numerical computations are carried out for Mach 6 flows over a 7-degree half-angle cone with two different flow incidence angles and a compression cone with a large concave curvature. Instability wave and transition-related flow physics are investigated using a series of advanced stability methods ranging from conventional linear stability theory (LST) and a higher-fidelity linear and nonlinear parabolized stability equations (PSE), to the 2D eigenvalue analysis based on partial differential equations. Computed N factor distribution pertinent to various instability mechanisms over the cone surface provides initial assessments of possible transition fronts and a guide to corresponding disturbance characteristics such as frequency and azimuthal wave numbers. It is also shown that strong secondary instability that eventually leads to transition to turbulence can be simulated very efficiently using a combination of advanced stability methods described above.
Indian Academy of Sciences (India)
M Turkyilmazoglu
2009-12-01
Direct spatial resonance phenomenon occurring in the viscous incompressible boundary layer ﬂow due to a rotating-disk is investigated in this paper based on the linear stability theory. The possible effects of suction and injection are explored on the direct spatial resonance instability mechanism detected earlier in the case of zero-suction. This instability leads to an algebraic growth of disturbances while the ﬂow is yet in the laminar regime and this in turn, may initiate the non-linearity and transition, competing with the unboundedly growing time-amplified perturbations. In line with the physical intuition, results show that suction delays the onset of resonance instability by increasing the critical Reynolds number, whereas it is enhanced by the presence of injection. The critical parameter for direct spatial resonance instability always precedes the onset value for absolute instability mechanism, after a comparison with the previous work. Therefore, in the case of suction, the onset parameter is close to the transition value as determined from the earlier experimental observations. It is further examined the inviscid nature of both absolute as well as direct spatial resonance instabilities when suction or injection is applied through the disk, and is demonstrated that these instability mechanisms are not in any way an artifact of the parallel ﬂow approximation assumed during the linearization of viscous incompressible stability equations.
International Nuclear Information System (INIS)
The effects of lack of local thermal equilibrium between the solid phase and the fluid phase are taken into account for the convective stability analysis of a horizontal porous layer. The layer is bounded by a pair of plane parallel walls which are impermeable and such that the lower wall is subject to a uniform flux heating, while the upper wall is isothermal. The local thermal non-equilibrium is modelled through a two-temperature formulation of the energy exchange between the phases, resulting in a pair of local energy balance equations: one for each phase. Small-amplitude disturbances of the basic rest state are envisaged to test the stability. Then, the standard normal mode procedure is adopted to detect the onset conditions of convective rolls. Beyond the Darcy-Rayleigh number, playing the role of order parameter for the transition to instability, the relevant dimensionless parameters are the inter-phase heat transfer parameter and the thermal conductivity ratio. The disturbance governing equations, formulated as an eigenvalue problem, are solved numerically by a shooting method. Results are reported for the neutral stability curves and for the critical values for the onset of instability.
Celli, Michele; Barletta, Antonio; Storesletten, Leiv
2014-04-01
The effects of lack of local thermal equilibrium between the solid phase and the fluid phase are taken into account for the convective stability analysis of a horizontal porous layer. The layer is bounded by a pair of plane parallel walls which are impermeable and such that the lower wall is subject to a uniform flux heating, while the upper wall is isothermal. The local thermal non-equilibrium is modelled through a two-temperature formulation of the energy exchange between the phases, resulting in a pair of local energy balance equations: one for each phase. Small-amplitude disturbances of the basic rest state are envisaged to test the stability. Then, the standard normal mode procedure is adopted to detect the onset conditions of convective rolls. Beyond the Darcy-Rayleigh number, playing the role of order parameter for the transition to instability, the relevant dimensionless parameters are the inter-phase heat transfer parameter and the thermal conductivity ratio. The disturbance governing equations, formulated as an eigenvalue problem, are solved numerically by a shooting method. Results are reported for the neutral stability curves and for the critical values for the onset of instability.
Mortensen, Clifton Holden
The current understanding of the effects of thermochemical nonequilibrium on hypersonic boundary-layer instability still contains uncertainties, and there has been little research into the effects of surface ablation, or two-dimensional roughness, on hypersonic boundary-layer instability. The objective of this work is to study the effects of thermochemical nonequilibrium on hypersonic boundary-layer instability. More specifically, two separate nonequilibrium flow configurations are studied: 1) flows with graphite surface ablation, and 2) flows with isolated two-dimensional surface roughness. These two flow types are studied numerically and theoretically, using direct numerical simulation and linear stability theory, respectively. To study surface ablation, a new high-order shock-fitting method with thermochemical nonequilibrium and finite-rate chemistry boundary conditions for graphite ablation is developed and validated. The method is suitable for direct numerical simulation of boundary-layer transition in a hypersonic real-gas flow with graphite ablation. The new method is validated by comparison with three computational data sets and one set of experimental data. Also, a thermochemical nonequilibrium linear stability theory solver with a gas phase model that includes multiple carbon species, as well as a linearized surface graphite ablation model, is developed and validated. It is validated with previously published linear stability analysis and direct numerical simulation results. A high-order method for discretizing the linear stability equations is used which can easily include high-order boundary conditions. The developed codes are then used to study hypersonic boundary-layer instability for a 7 deg half angle blunt cone at Mach 15.99 and the Reentry F experiment at 100~kft. Multiple simulations are run with the same geometry and freestream conditions to help separate real gas, blowing, and carbon species effects on hypersonic boundary-layer instability. For
Influence of Ion Streaming Instabilities on Transport Near Plasma Boundaries
Baalrud, Scott D
2015-01-01
Plasma boundary layers are susceptible to electrostatic instabilities driven by ion flows in presheaths and, when present, these instabilities can influence transport. In plasmas with a single species of positive ion, ion-acoustic instabilities are expected under conditions of low pressure and large electron-to-ion temperature ratio ($T_e/T_i \\gg 1$). In plasmas with two species of positive ions, ion-ion two-stream instabilities can also be excited. The stability phase-space is characterized using the Penrose criterion and approximate linear dispersion relations. Predictions for how these instabilities affect ion and electron transport in presheaths, including rapid thermalization due to instability-enhanced collisions and an instability-enhanced ion-ion friction force, are also briefly reviewed. Recent experimental tests of these predictions are discussed along with research needs required for further validation. The calculated stability boundaries provide a guide to determine the experimental conditions at ...
Influence of ion streaming instabilities on transport near plasma boundaries
Baalrud, Scott D.
2016-04-01
Plasma boundary layers are susceptible to electrostatic instabilities driven by ion flows in presheaths and, when present, these instabilities can influence transport. In plasmas with a single species of positive ion, ion-acoustic instabilities are expected under conditions of low pressure and large electron-to-ion temperature ratio ({{T}e}/{{T}i}\\gg 1 ). In plasmas with two species of positive ions, ion-ion two-stream instabilities can also be excited. The stability phase-space is characterized using the Penrose criterion and approximate linear dispersion relations. Predictions for how these instabilities affect ion and electron transport in presheaths, including rapid thermalization due to instability-enhanced collisions and an instability-enhanced ion-ion friction force, are briefly reviewed. Recent experimental tests of these predictions are discussed along with research needs required for further validation. The calculated stability boundaries provide a guide to determine the experimental conditions at which these effects can be expected.
Boundary-layer linear stability theory
Mack, L. M.
1984-06-01
Most fluid flows are turbulent rather than laminar and the reason for this was studied. One of the earliest explanations was that laminar flow is unstable, and the linear instability theory was first developed to explore this possibility. A series of early papers by Rayleigh produced many notable results concerning the instability of inviscid flows, such as the discovery of inflectional instability. Viscosity was commonly thought to act only to stabilize the flow, and flows with convex velocity profiles appeared to be stable. The investigations that led to a viscous theory of boundary layer instability was reported. The earliest application of linear stability theory to transition prediction calculated the amplitude ratio of the most amplified frequency as a function of Reynolds number for a Blasius boundary layer, and found that this quantity had values between five and nine at the observed Ret. The experiment of Schubauer and Skramstad (1947) completely reversed the prevailing option and fully vindicated the Gottingen proponents of the theory. This experiment demonstrated the existence of instability waves in a boundary layer, their connection with transition, and the quantitative description of their behavior by the theory of Tollmien and Schlichting. It is generally accepted that flow parameters such as pressure gradient, suction and heat transfer qualitatively affect transition in the manner predicted by the linear theory, and in particular that a flow predicted to be stable by the theory should remain laminar. The linear theory, in the form of the e9, or N-factor is today in routine use in engineering studies of laminar flow. The stability theory to boundary layers with pressure gradients and suction was applied. The only large body of numerical results for exact boundary layer solutions before the advent of the computer age by calculating the stability characteristics of the Falkner-Skan family of velocity profiles are given. When the digital computer
Boundary-layer linear stability theory
Mack, L. M.
1984-01-01
Most fluid flows are turbulent rather than laminar and the reason for this was studied. One of the earliest explanations was that laminar flow is unstable, and the linear instability theory was first developed to explore this possibility. A series of early papers by Rayleigh produced many notable results concerning the instability of inviscid flows, such as the discovery of inflectional instability. Viscosity was commonly thought to act only to stabilize the flow, and flows with convex velocity profiles appeared to be stable. The investigations that led to a viscous theory of boundary layer instability was reported. The earliest application of linear stability theory to transition prediction calculated the amplitude ratio of the most amplified frequency as a function of Reynolds number for a Blasius boundary layer, and found that this quantity had values between five and nine at the observed Ret. The experiment of Schubauer and Skramstad (1947) completely reversed the prevailing option and fully vindicated the Gottingen proponents of the theory. This experiment demonstrated the existence of instability waves in a boundary layer, their connection with transition, and the quantitative description of their behavior by the theory of Tollmien and Schlichting. It is generally accepted that flow parameters such as pressure gradient, suction and heat transfer qualitatively affect transition in the manner predicted by the linear theory, and in particular that a flow predicted to be stable by the theory should remain laminar. The linear theory, in the form of the e9, or N-factor is today in routine use in engineering studies of laminar flow. The stability theory to boundary layers with pressure gradients and suction was applied. The only large body of numerical results for exact boundary layer solutions before the advent of the computer age by calculating the stability characteristics of the Falkner-Skan family of velocity profiles are given. When the digital computer
Analysis of turbulent boundary layers
Cebeci, Tuncer
2012-01-01
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
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
Modeling the urban boundary layer
Bergstrom, R. W., Jr.
1976-01-01
A summary and evaluation is given of the Workshop on Modeling the Urban Boundary Layer; held in Las Vegas on May 5, 1975. Edited summaries from each of the session chairpersons are also given. The sessions were: (1) formulation and solution techniques, (2) K-theory versus higher order closure, (3) surface heat and moisture balance, (4) initialization and boundary problems, (5) nocturnal boundary layer, and (6) verification of models.
Boundary Layer under Oscillatory Wave
Mohammad Bagus Adityawan; Hitoshi Tanaka
2011-01-01
Turbulence due to wave motion and propagation is a very important aspect in sediment transport modeling. The boundary layer characteristic during the process will highly influence the sediment transport mechanism at the bottom. 1D model approach has been widely used to assess the turbulent boundary layer. However, the need for a more detailed model leads to the development of a more sophisticated models. This study presents a 2D turbulent model using k-ω equation to approach the turbulent bou...
On hydrodynamic instability of the ozone layer
International Nuclear Information System (INIS)
We show that instability may be arisen when the large-scale waves propagate in the ozone layer of Earth's atmosphere. The instability criterion suitable both for the acoustic waves and for the Rossby waves is found. Moreover, the possibility of the spatially located dissipative Rossby vortical structures formation in this layer is established
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...
Electron temperature gradient driven instability in the tokamak boundary plasma
International Nuclear Information System (INIS)
A general method is developed for calculating boundary plasma fluctuations across a magnetic separatrix in a tokamak with a divertor or a limiter. The slab model, which assumes a periodic plasma in the edge reaching the divertor or limiter plate in the scrape-off layer(SOL), should provide a good estimate, if the radial extent of the fluctuation quantities across the separatrix to the edge is small compared to that given by finite particle banana orbit. The Laplace transform is used for solving the initial value problem. The electron temperature gradient(ETG) driven instability is found to grow like t-1/2eγmt
Shear-induced instabilities in layered liquids
Auernhammer, Günter K.; Brand, Helmut R.; Pleiner, Harald
2002-12-01
Motivated by the experimentally observed shear-induced destabilization and reorientation of smectic-A-like systems, we consider an extended formulation of smectic-A hydrodynamics. We include both, the smectic layering (via the layer displacement u and the layer normal pcirc) and the director ncirc of the underlying nematic order in our macroscopic hydrodynamic description and allow both directions to differ in nonequilibrium situations. In an homeotropically aligned sample the nematic director does couple to an applied simple shear, whereas the smectic layering stays unchanged. This difference leads to a finite (but usually small) angle between ncirc and pcirc, which we find to be equivalent to an effective dilatation of the layers. This effective dilatation leads, above a certain threshold, to an undulation instability of the layers. We generalize our earlier approach [G. K. Auernhammer, H. R. Brand, and H. Pleiner, Rheol. Acta 39, 215 (2000)] and include the cross couplings with the velocity field and the order parameters for orientational and positional order and show how the order parameters interact with the undulation instability. We explore the influence of various material parameters on the instability. Comparing our results to recent experiments and molecular dynamic simulations, we find a good qualitative agreement.
Boundary Layer Heights from CALIOP
Kuehn, R.; Ackerman, S. A.; Holz, R.; Roubert, L.
2012-12-01
This work is focused on the development of a planetary boundary layer (PBL) height retrieval algorithm for CALIOP and validation studies. Our current approach uses a wavelet covariance transform analysis technique to find the top of the boundary layer. We use the methodology similar to that found in Davis et. al. 2000, ours has been developed to work with the lower SNR data provided by CALIOP, and is intended to work autonomously. Concurrently developed with the CALIOP algorithm we will show results from a PBL height retrieval algorithm from profiles of potential temperature, these are derived from Aircraft Meteorological DAta Relay (AMDAR) observations. Results from 5 years of collocated AMDAR - CALIOP retrievals near O'Hare airport demonstrate good agreement between the CALIOP - AMDAR retrievals. In addition, because we are able to make daily retrievals from the AMDAR measurements, we are able to observe the seasonal and annual variation in the PBL height at airports that have sufficient instrumented-aircraft traffic. Also, a comparison has been done between the CALIOP retrievals and the NASA Langley airborne High Spectral Resolution Lidar (HSRL) PBL height retrievals acquired during the GoMACCS experiment. Results of this comparison, like the AMDAR comparison are favorable. Our current work also involves the analysis and verification of the CALIOP PBL height retrieval from the 6 year CALIOP global data set. Results from this analysis will also be presented.
DNS Study on Physics of Late Boundary Layer Transition
Liu, Chaoqun
2014-01-01
This paper serves as a review of our recent new DNS study on physics of late boundary layer transition. This includes mechanism of the large coherent vortex structure formation, small length scale generation and flow randomization. The widely spread concept vortex breakdown to turbulence,which was considered as the last stage of flow transition, is not observed and is found theoretically incorrect. The classical theory on boundary layer transition is challenged and we proposed a new theory with five steps, i.e. receptivity, linear instability, large vortex formation, small length scale generation, loss of symmetry and randomization to turbulence. We have also proposed a new theory about turbulence generation. The new theory shows that all small length scales (turbulence) are generated by shear layer instability which is produced by large vortex structure with multiple level vortex rings, multiple level sweeps and ejections, and multiple level negative and positive spikes near the laminar sub-layers.Therefore,...
Electron temperature gradient driven instability in the tokamak boundary plasma
Energy Technology Data Exchange (ETDEWEB)
Xu, X.Q.; Rosenbluth, M.N.; Diamond, P.H.
1992-12-15
A general method is developed for calculating boundary plasma fluctuations across a magnetic separatrix in a tokamak with a divertor or a limiter. The slab model, which assumes a periodic plasma in the edge reaching the divertor or limiter plate in the scrape-off layer(SOL), should provide a good estimate, if the radial extent of the fluctuation quantities across the separatrix to the edge is small compared to that given by finite particle banana orbit. The Laplace transform is used for solving the initial value problem. The electron temperature gradient(ETG) driven instability is found to grow like t{sup {minus}1/2}e{sup {gamma}mt}.
Stabilization of boundary layer streaks by plasma actuators
International Nuclear Information System (INIS)
A flow's transition from laminar to turbulent leads to increased levels of skin friction. In recent years, dielectric barrier discharge actuators have been shown to be able to delay the onset of turbulence in boundary layers. While the laminar to turbulent transition process can be initiated by several different instability mechanisms, so far, only stabilization of the Tollmien–Schlichting path to transition has received significant attention, leaving the stabilization of other transition paths using these actuators less explored. To fill that void, a bi-global stability analysis is used here to examine the stabilization of boundary layer streaks in a laminar boundary layer. These streaks, which are important to both transient and by-pass instability mechanisms, are damped by the addition of a flow-wise oriented plasma body force to the boundary layer. Depending on the magnitude of the plasma actuation, this damping can be up to 25% of the perturbation's kinetic energy. The damping mechanism appears to be due to highly localized effects in the immediate vicinity of the body force, and when examined using a linearized Reynolds-averaged Navier–Stokes energy balance, indicate negative production of the perturbation's kinetic energy. Parametric studies of the stabilization have also been performed, varying the magnitude of the plasma actuator's body force and the spanwise wavenumber of the actuation. Based on these parametric studies, the damping of the boundary layer streaks appears to be linear with respect to the total amount of body force applied to the flow. (paper)
Microgravity Effects on Plant Boundary Layers
Stutte, Gary; Monje, Oscar
2005-01-01
The goal of these series of experiment was to determine the effects of microgravity conditions on the developmental boundary layers in roots and leaves and to determine the effects of air flow on boundary layer development. It is hypothesized that microgravity induces larger boundary layers around plant organs because of the absence of buoyancy-driven convection. These larger boundary layers may affect normal metabolic function because they may reduce the fluxes of heat and metabolically active gases (e.g., oxygen, water vapor, and carbon dioxide. These experiments are to test whether there is a change in boundary layer associated with microgravity, quantify the change if it exists, and determine influence of air velocity on boundary layer thickness under different gravity conditions.
BUBBLE - an urban boundary layer meteorology project
DEFF Research Database (Denmark)
Rotach, M.W.; Vogt, R.; Bernhofer, C.;
2005-01-01
The Basel urban Boundary Layer Experiment (BUBBLE) was a year-long experimental effort to investigate in detail the boundary layer structure in the City of Basel, Switzerland. At several sites over different surface types (urban, sub-urban and rural reference) towers up to at least twice the main...
Cyclone separator having boundary layer turbulence control
International Nuclear Information System (INIS)
A cyclone separator including boundary layer turbulence control that is operable to prevent undue build-up of particulate material at selected critical areas on the separator walls, by selectively varying the fluid pressure at those areas to maintain the momentum of the vortex, thereby preventing particulate material from inducing turbulence in the boundary layer of the vortical fluid flow through the separator
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 ...
Magnetohydrodynamic cross-field boundary layer flow
Directory of Open Access Journals (Sweden)
D. B. Ingham
1982-01-01
Full Text Available The Blasius boundary layer on a flat plate in the presence of a constant ambient magnetic field is examined. A numerical integration of the MHD boundary layer equations from the leading edge is presented showing how the asymptotic solution described by Sears is approached.
Cyclone separator having boundary layer turbulence control
Krishna, Coimbatore R.; Milau, Julius S.
1985-01-01
A cyclone separator including boundary layer turbulence control that is operable to prevent undue build-up of particulate material at selected critical areas on the separator walls, by selectively varying the fluid pressure at those areas to maintain the momentum of the vortex, thereby preventing particulate material from inducing turbulence in the boundary layer of the vortical fluid flow through the separator.
Sensitivity of African easterly waves to boundary layer conditions
A. Lenouo; Mkankam Kamga, F.
2008-01-01
A linearized version of the quasi-geostrophic model (QGM) with an explicit Ekman layer and observed static stability parameter and profile of the African easterly jet (AEJ), is used to study the instability properties of the environment of the West African wave disturbances. It is found that the growth rate, the propagation velocity and the structure of the African easterly waves (AEW) can be well simulated. Two different lower boundary conditions are applied. One assumes a lack of vertical g...
LDV measurements of turbulent baroclinic boundary layers
Energy Technology Data Exchange (ETDEWEB)
Neuwald, P.; Reichenbach, H. [Fraunhofer-Institut fuer Kurzzeitdynamik - Ernst-Mach-Institut (EMI), Freiburg im Breisgau (Germany); Kuhl, A.L. [Lawrence Livermore National Lab., El Segundo, CA (United States)
1993-07-01
Described here are shock tube experiments of nonsteady, turbulent boundary layers with large density variations. A dense-gas layer was created by injecting Freon through the porous floor of the shock tube. As the shock front propagated along the layer, vorticity was created at the air-Freon interface by an inviscid, baroclinic mechanism. Shadow-schlieren photography was used to visualize the turbulent mixing in this baroclinic boundary layer. Laser-Doppler-Velocimetry (LDV) was used to measure the streamwise velocity histories at 14 heights. After transition, the boundary layer profiles may be approximated by a power-law function u {approximately} u{sup {alpha}} where {alpha} {approx_equal} 3/8. This value lies between the clean flat plate value ({alpha} = 1/7) and the dusty boundary layer value ({alpha} {approx_equal} 0.7), and is controlled by the gas density near the wall.
Plasma boundary layer and magnetopause layer of the earth's magnetosphere
International Nuclear Information System (INIS)
IMP 6 observations of the plasma boundary layer (PBL) and magnetopause layer (MPL) of the earth's magnetosphere indicate that plasma in the low-latitude portion of the PBL is supplied primarily by direct transport of magnetosheath plasma across the MPL and that this transport process is relatively widespread over the entire sunward magnetospheric boundary
On the Ekman instability at the core-mantle boundary
Chkhetiani, O. G.; Shalimov, S. L.
2016-05-01
It is shown that the instability of turbulent flows with Ekman velocity profiles in the vicinity of the core-mantle boundary leads to the formation of horizontally oriented circulating roll structures. The geophysical implications of the presence of such structures in the liquid core are explored, namely, the formation of the hot zones with enhanced conductivity and their influence on geomagnetic reversals.
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.
Stabilization of the hypersonic boundary layer by finite-amplitude streaks
Ren, Jie; Fu, Song; Hanifi, Ardeshir
2016-02-01
Stabilization of two-dimensional disturbances in hypersonic boundary layer flows by finite-amplitude streaks is investigated using nonlinear parabolized stability equations. The boundary-layer flows at Mach numbers 4.5 and 6.0 are studied in which both first and second modes are supported. The streaks considered here are driven either by the so-called optimal perturbations (Klebanoff-type) or the centrifugal instability (Görtler-type). When the streak amplitude is in an appropriate range, i.e., large enough to modulate the laminar boundary layer but low enough to not trigger secondary instability, both first and second modes can effectively be suppressed.
Boundary layer physics over snow and ice
Directory of Open Access Journals (Sweden)
P. S. Anderson
2008-07-01
Full Text Available Observations of the unique chemical environment over snow and ice in recent decades, particularly in the polar regions, have stimulated increasing interest in the boundary layer processes that mediate exchanges between the ice/snow interface and the atmosphere. This paper provides a review of the underlying concepts and examples from recent field studies in polar boundary layer meteorology, which will generally apply to atmospheric flow over snow and ice surfaces. It forms a companion paper to the chemistry review papers in this special issue of ACP that focus on processes linking halogens to the depletion of boundary layer ozone in coastal environments, mercury transport and deposition, snow photochemistry, and related snow physics. In this context, observational approaches, stable boundary layer behavior, the effects of a weak or absent diurnal cycle, and transport and mixing over the heterogeneous surfaces characteristic of coastal ocean environments are of particular relevance.
Numerical studies on laminar-turbulent transition in boundary layers
International Nuclear Information System (INIS)
Laminar-turbulent transition in flat-plate boundary layers is investigated by direct numerical solution of the full Navier-Stokes equations. Both forced transition (in parallel Blasius flow excited by a vibrating ribbon) and natural transition (in a decelerating boundary layer) are studied. In both cases, an initial state containing random noise is employed to eliminate bias in selecting unstable waves. In the simulations of ribbon-induced transition, close agreement with experiments (Saric et al. (1984)) is obtained for low-amplitude two-dimensional Tollmien-Schlichting waves-producing subharmonic breakdown (C- or H-type). For high amplitudes, a mixture of subharmonic and fundamental structures is observed. Clear-cut fundamental breakdown (K-type) is never obtained. In the simulation of the early stages of natural transition in a decelerating boundary layer, two-dimensional and/or slightly oblique waves initially grow due to the inflectional instability. When they become strong enough, they initiate a secondary instability leading to three dimensional distortion and Λ vortices, in good agreement with experiments (Gad-el-Hak et al. (1984)). The tips of the Λ vortices are rarely aligned with the flow direction, and that they appear locally in apace. A simple wave-interference model accounting for these features of natural transition has been developed. It suggests that multiple waves are active in the secondary instability, and that they are determined by unpredictable initial disturbances. The later stages of transition in a decelerating boundary layer were also studied with higher numerical resolution. The naturally-born Λ vortices undergo breakdown processes similar to those of ribbon-induced Λ vortices. Conversely, this justifies the conventional approach to study laminar-turbulent transition-the vibrating-ribbon technique
Characterization of internal boundary layer capacitors
International Nuclear Information System (INIS)
Internal boundary layer capacitors were characterized by scanning transmission electron microscopy and by microscale electrical measurements. Data are given for the chemical and physical characteristics of the individual grains and boundaries, and their associated electric and dielectric properties. Segregated internal boundary layers were identified with resistivities of 1012-1013 Ω-cm. Bulk apparent dielectric constants were 10,000-60,000. A model is proposed to explain the dielectric behavior in terms of an equivalent n-c-i-c-n representation of ceramic microstructure, which is substantiated by capacitance-voltage analysis
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.
DEFF Research Database (Denmark)
Chivaee, Hamid Sarlak; Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming
2012-01-01
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......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...
Li, Fei; Choudhari, Meelan; Chang, Chau-Lyan; White, Jeffery
2011-01-01
Computations are performed to study the boundary layer instability mechanisms pertaining to hypersonic flow over blunt capsules. For capsules with ablative heat shields, transition may be influenced both by out-gassing associated with surface pyrolysis and the resulting modification of surface geometry including the formation of micro-roughness. To isolate the effects of out-gassing, this paper examines the stability of canonical boundary layer flows over a smooth surface in the presence of gas injection into the boundary layer. For a slender cone, the effects of out-gassing on the predominantly second mode instability are found to be stabilizing. In contrast, for a blunt capsule flow dominated by first mode instability, out-gassing is shown to be destabilizing. Analogous destabilizing effects of outgassing are also noted for both stationary and traveling modes of crossflow instability over a blunt sphere-cone configuration at angle of attack.
Passive Control of Supersonic Rectangular Jets through Boundary Layer Swirl
Han, Sang Yeop; Taghavi, Ray R.; Farokhi, Saeed
2013-06-01
Mixing characteristics of under-expanded supersonic jets emerging from plane and notched rectangular nozzles are computationally studied using nozzle exit boundary layer swirl as a mean of passive flow control. The coupling of the rectangular jet instability modes, such as flapping, and the swirl is investigated. A three-dimensional unsteady Reynolds-Averaged Navier-Stokes (RANS) code with shock adaptive grids is utilized. For plane rectangular nozzle with boundary layer swirl, the flapping and spanwise oscillations are captured in the jet's small and large dimensions at twice the frequencies of the nozzles without swirl. A symmetrical oscillatory mode is also observed in the jet with double the frequency of spanwise oscillation mode. For the notched rectangular nozzle with boundary layer swirl, the flapping oscillation in the small jet dimension and the spanwise oscillation in the large jet dimension are observed at the same frequency as those without boundary layer swirl. The mass flow rates in jets at 11 and 8 nozzle heights downstream of the nozzles increased by nearly 25% and 41% for the plane and notched rectangular nozzles respectively, due to swirl. The axial gross thrust penalty due to induced swirl was 5.1% for the plane and 4.9% for the notched rectangular nozzle.
Problems of matter-antimatter boundary layers
International Nuclear Information System (INIS)
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 2x0 approximately 10/BT0sup(1/4) meters, where B is the magnetic field strength in MKS units and T0 the characteristic temperature of the low-energy components in the layer. (Auth.)
Leading-edge effects on boundary-layer receptivity
Gatski, Thomas B.; Kerschen, Edward J.
1990-01-01
Numerical calculations are presented for the incompressible flow over a parabolic cylinder. The computational domain extends from a region upstream of the body downstream to the region where the Blasius boundary-layer solution holds. A steady mean flow solution is computed and the results for the scaled surface vorticity, surface pressure and displacement thickness are compared to previous studies. The unsteady problem is then formulated as a perturbation solution starting with and evolving from the mean flow. The response to irrotational time harmonic pulsation of the free-stream is examined. Results for the initial development of the velocity profile and displacement thickness are presented. These calculations will be extended to later times to investigate the initiation of instability waves within the boundary-layer.
Boundary layer physics over snow and ice
Directory of Open Access Journals (Sweden)
P. S. Anderson
2007-06-01
Full Text Available A general understanding of the physics of advection and turbulent mixing within the near surface atmosphere assists the interpretation and predictive power of air chemistry theory. The theory of the physical processes involved in diffusion of trace gas reactants in the near surface atmosphere is still incomplete. Such boundary layer theory is least understood over snow and ice covered surfaces, due in part to the thermo-optical properties of the surface. Polar boundary layers have additional aspects to consider, due to the possibility of long periods without diurnal forcing and enhanced Coriolis effects.
This paper provides a review of present concepts in polar boundary layer meteorology, which will generally apply to atmospheric flow over snow and ice surfaces. It forms a companion paper to the chemistry review papers in this special issue of ACP.
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)
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.
Transition in Hypersonic Boundary Layers: Role of Dilatational Waves
Zhu, Yiding; Yuan, Huijing; Wu, Jiezhi; Chen, Shiyi; Lee, Cunbiao; Gad-el-Hak, Mohamed
2015-01-01
Transition and turbulence production in a hypersonic boundary layer is investigated in a Mach 6 quiet wind tunnel using Rayleigh-scattering visualization, fast-response pressure measurements, and particle image velocimetry. It is found that the second instability acoustic mode is the key modulator of the transition process. The second mode experiences a rapid growth and a very fast annihilation due to the effect of bulk viscosity. The second mode interacts strongly with the first vorticity mode to directly promote a fast growth of the latter and leads to immediate transition to turbulence.
Boundary-layer theory for blast waves
Kim, K. B.; Berger, S. A.; Kamel, M. M.; Korobeinikov, V. P.; Oppenheim, A. K.
1975-01-01
It is profitable to consider the blast wave as a flow field consisting of two regions: the outer, which retains the properties of the inviscid solution, and the inner, which is governed by flow equations including terms expressing the effects of heat transfer and, concomitantly, viscosity. The latter region thus plays the role of a boundary layer. Reported here is an analytical method developed for the study of such layers, based on the matched asymptotic expansion technique combined with patched solutions.
Thick diffusion limit boundary layer test problems
International Nuclear Information System (INIS)
We develop two simple test problems that quantify the behavior of computational transport solutions in the presence of boundary layers that are not resolved by the spatial grid. In particular we study the quantitative effects of 'contamination' terms that, according to previous asymptotic analyses, may have a detrimental effect on the solutions obtained by both discontinuous finite element (DFEM) and characteristic-method (CM) spatial discretizations, at least for boundary layers caused by azimuthally asymmetric incident intensities. Few numerical results have illustrated the effects of this contamination, and none have quantified it to our knowledge. Our test problems use leading-order analytic solutions that should be equal to zero in the problem interior, which means the observed interior solution is the error introduced by the contamination terms. Results from DFEM solutions demonstrate that the contamination terms can cause error propagation into the problem interior for both orthogonal and non-orthogonal grids, and that this error is much worse for non-orthogonal grids. This behavior is consistent with the predictions of previous analyses. We conclude that these boundary layer test problems and their variants are useful tools for the study of errors that are introduced by unresolved boundary layers in diffusive transport problems. (authors)
DYNAMICS OF A BOUNDARY LAYER SEPARATION
Czech Academy of Sciences Publication Activity Database
Uruba, Václav; Knob, Martin
2009-01-01
Roč. 16, č. 1 (2009), s. 29-38. ISSN 1802-1484 R&D Projects: GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * triple-deck theory * Time-Resolved PIV Subject RIV: BK - Fluid Dynamics
Analysis of Laminar Boundary Layer Equations
Directory of Open Access Journals (Sweden)
R. Yesman
2012-01-01
Full Text Available The paper proposes methodology for analysis and calculation of laminar fluid flow processes in a boundary layer.The presented dependences can be used for practical calculations while power carriers of various application are moving in the channels of heat and power devices.
Global stability analysis of axisymmetric boundary layers
Vinod, N
2016-01-01
This paper presents the linear global stability analysis of the incompressible axisymmetric boundary layer on a circular cylinder. The base flow is parallel to the axis of the cylinder at inlet. The pressure gradient is zero in the streamwise direction. The base flow velocity profile is fully non-parallel and non-similar in nature. The boundary layer grows continuously in the spatial directions. Linearized Navier-Stokes(LNS) equations are derived for the disturbance flow quantities in the cylindrical polar coordinates. The LNS equations along with homogeneous boundary conditions forms a generalized eigenvalues problem. Since the base flow is axisymmetric, the disturbances are periodic in azimuthal direction. Chebyshev spectral collocation method and Arnoldi's iterative algorithm is used for the solution of the general eigenvalues problem. The global temporal modes are computed for the range of Reynolds numbers and different azimuthal wave numbers. The largest imaginary part of the computed eigenmodes are nega...
Clear-air radar observations of the atmospheric boundary layer
Ince, Turker
2001-10-01
This dissertation presents the design and operation of a high-resolution frequency-modulated continuous-wave (FM- CW) radar system to study the structure and dynamics of clear-air turbulence in the atmospheric boundary layer (ABL). This sensitive radar can image the vertical structure of the ABL with both high spatial and temporal resolutions, and provide both qualitative information about the morphology of clear-air structures and quantitative information on the intensity of fluctuations in refractive-index of air. The principles of operation and the hardware and data acquisition characteristics of the radar are described in the dissertation. In October 1999, the radar participated in the Cooperative Atmosphere-Surface Exchange Study (CASES'99) Experiment to characterize the temporal structure and evolution of the boundary-layer features in both convective and stable conditions. The observed structures include clear-air convection, boundary layer evolution, gravity waves, Kelvin-Helmholtz instabilities, stably stratified layers, and clear-air turbulence. Many of the S-band radar images also show high- reflectivity returns from Rayleigh scatterers such as insects. An adaptive median filtering technique based on local statistics has, therefore, been developed to discriminate between Bragg and Rayleigh scattering in clear-air radar observations. The filter is tested on radar observations of clear air convection with comparison to two commonly used image processing techniques. The dissertation also examines the statistical mean of the radar-measured C2n for clear-air convection, and compares it with the theoretical predictions. The study also shows that the inversion height, local thickness of the inversion layer, and the height of the elevated atmospheric layers can be estimated from the radar reflectivity measurements. In addition, comparisons to the radiosonde-based height estimates are made. To examine the temporal and spatial structure of C2n , the dissertation
Numerical simulations of two-fluid boundary layers beneath free-stream turbulence
Jung, Seo Yoon; Zaki, Tamer
2011-11-01
In two-fluid boundary layers, a wall-film is sheared by an external stream with different density and viscosity. As a result, the flow becomes prone to both shear and interfacial instabilities. In this study, the evolution of two-fluid boundary layers beneath free-stream vortical forcing is investigated using DNS. The simulations employ a conservative level-set technique in conjunction with a ghost fluid approach in order to capture a sharp interface. The wall film is less viscous than the outer flow, and its thickness is 10 % of that of the boundary layer at the inlet. The choice of viscosity ratio influences the spatial development of disturbances within the boundary layer. The spatial growth of instabilities is examined into the non-linear regime, which includes the region of breakdown to turbulence. We demonstrate that, at moderate levels of free-stream turbulence intensities, appropriate choice of the viscosity ratio can yield considerable transition delay.
Convective Instabilities in Two Liquid Layers
McFadden, G. B.; Coriell, S. R.; Gurski, K. F.; Cotrell, D. L.
2007-01-01
We perform linear stability calculations for horizontal fluid bilayers, taking into account both buoyancy effects and thermocapillary effects in the presence of a vertical temperature gradient. To help understand the mechanisms driving the instability, we have performed both long-wavelength and short-wavelength analyses. The mechanism for the large wavelength instability is complicated, and the detailed form of the expansion is found to depend on the Crispation and Bond numbers. The system al...
Plasma boundary layer with active surface. Pt. 1
International Nuclear Information System (INIS)
The space-charge boundary layer between plasma and wall which is normally (almost) homogeneous may become instable and may decay into largely independent spots of plasma-induced unipolar-like discharges. In Tokamaks the existence of such highly inhomogeneous boundary plasmas often has been found by observation of arc tracks and of ''hot spots'' a.s.o. In this way wall erosion and production rates of plasma impurities will be enhanced, and several special phenomena of intense wall erosion (like ''carbon blooming'') may be traced back to such effects. In this paper the influence of electron emission from the wall (i.e. of an ''active'' surface) on the parameter of the space charge sheath is investigated, applying simple balance equations, as a first step towards an explanation of the transition from a homogeneous into an inhomogeneous boundary layer. Several variations of such models are calculated, using typical plasma parameters. Essential result is the dependence of the sheath potential and of the surface power density on the emission yield and on the net current density. Irrespective of the chosen constants the potential drop between plasma and wall turns out to become the higher the lower is the electron emission and the higher is the net current. Opposite is the dependence of the energy flux to the wall which, however, passes a minimum and increases rapidly again near the maximum net current jmax (with jmax∼jis(γ-1), where jis=ion saturation current, and γ=emission yield per ion). As a consequence, the wall loading is strongly enhanced as well in case of high negative net currents and intense electron emission, as near the maximum net current. This will be infavour of an instability of the boundary layer, resulting - with high probability - in the decay of the layer into plasma-induced arc spots. As a next step in this investigation of such plasma boundary layers a careful analysis of this transition is provided for, taking the specified conditions of the
Topography and instability of monolayers near domain boundaries
International Nuclear Information System (INIS)
We theoretically study the topography of a biphasic surfactant monolayer in the vicinity of domain boundaries. The differing elastic properties of the two phases generally lead to a nonflat topography of 'mesas,' where domains of one phase are elevated with respect to the other phase. The mesas are steep but low, having heights of up to 10 nm. As the monolayer is laterally compressed, the mesas develop overhangs and eventually become unstable at a surface tension of about K(δc0)2 (δc0 being the difference in spontaneous curvature and K a bending modulus). In addition, the boundary is found to undergo a topography-induced rippling instability upon compression, if its line tension is smaller than about Kδc0. The effect of diffuse boundaries on these features and the topographic behavior near a critical point are also examined. We discuss the relevance of our findings to several experimental observations related to surfactant monolayers: (i) small topographic features recently found near domain boundaries; (ii) folding behavior observed in mixed phospholipid monolayers and model lung surfactants; (iii) roughening of domain boundaries seen under lateral compression; (iv) the absence of biphasic structures in tensionless surfactant films
New insights into adverse pressure gradient boundary layers
George, William K.; Stanislas, Michel; Laval, Jean-Philippe
2010-11-01
In a recent paper Shah et al. 2010 (Proc. of the WALLTURB Meeting, 2009), Lille, FR, Springer, in press) documented a number of adverse pressure gradient flows (APG's), with and without wall curvature, where the turbulence intensity peak moved quite sharply away from the wall with increasing distance. They further suggested that this peak was triggered by the adverse pressure gradient and had its origin in an instability hidden in the turbulent boundary layer, developing soon after the change of sign of the pressure gradient. They then offered that this may explain the difficulties encountered up to now in finding a universal scaling for turbulent boundary layers. We build on these observations, and show that in fact there is clear evidence in the literature (in most experiments, both old and new) for such a development downstream of the imposition of an adverse pressure gradient. The exact nature of the evolution and the distance over which it occurs depends on the upstream boundary layer and the manner in which the APG is imposed. But far enough downstream the mean velocity profile in all cases becomes an inflectional point profile with the location of the inflection point corresponding quite closely to the observed peak in the streamwise turbulence intensity. This does not seem to have been previously noticed.
Active control of ionized boundary layers
Mendes, R V
1997-01-01
The challenging problems, in the field of control of chaos or of transition to chaos, lie in the domain of infinite-dimensional systems. Access to all variables being impossible in this case and the controlling action being limited to a few collective variables, it will not in general be possible to drive the whole system to the desired behaviour. A paradigmatic problem of this type is the control of the transition to turbulence in the boundary layer of fluid motion. By analysing a boundary layer flow for an ionized fluid near an airfoil, one concludes that active control of the transition amounts to the resolution of an generalized integro-differential eigenvalue problem. To cope with the required response times and phase accuracy, electromagnetic control, whenever possible, seems more appropriate than mechanical control by microactuators.
Magnetic activity in accretion disc boundary layers
Armitage, Philip J.
2002-03-01
We use three-dimensional magnetohydrodynamic simulations to study the structure of the boundary layer between an accretion disc and a non-rotating, unmagnetized star. Under the assumption that cooling is efficient, we obtain a narrow but highly variable transition region in which the radial velocity is only a small fraction of the sound speed. A large fraction of the energy dissipation occurs in high-density gas adjacent to the hydrostatic stellar envelope, and may therefore be reprocessed and largely hidden from view of the observer. As suggested by Pringle, the magnetic field energy in the boundary layer is strongly amplified by shear, and exceeds that in the disc by an order of magnitude. These fields may play a role in generating the magnetic activity, X-ray emission and outflows in disc systems where the accretion rate is high enough to overwhelm the stellar magnetosphere.
Analytic prediction for planar turbulent boundary layers
Chen, Xi
2016-01-01
Analytic predictions of mean velocity profile (MVP) and streamwise ($x$) development of related integral quantities are presented for flows in channel and turbulent boundary layer (TBL), based on a symmetry analysis of eddy length and total stress. Specific predictions are the friction velocity $u_\\tau$: ${ U_e/u_\\tau }\\approx 2.22\\ln Re_x+2.86-3.83\\ln(\\ln Re_x)$; the boundary layer thickness $\\delta_e$: $x/\\delta_e \\approx 7.27\\ln Re_x-5.18-12.52\\ln(\\ln Re_x)$; the momentum thickness Reynolds number: $Re_x/Re_\\theta=4.94[{(\\ln {{\\mathop{\\rm Re}\
DYNAMICS OF A BOUNDARY LAYER SEPARATION
Czech Academy of Sciences Publication Activity Database
Uruba, Václav
Budapest : University of Technology and Economics , 2009, s. 268-275. ISBN 978-963-420-985-0. [Conference on Modelling Fluid Flow CMFF'09. Budapest (HU), 09.09.2009-12.09.2009] R&D Projects: GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * dynamics * separation * POPs Subject RIV: BK - Fluid Dynamics
Numerical Simulation of the Atmospheric Boundary Layer
Czech Academy of Sciences Publication Activity Database
Bauer, Petr
Praha : Česká technika - nakladatelství ČVUT, 2006 - (Ambrož, P.; Masáková, Z.), s. 11-18 [Doktorandské dny 2006. Katedra matematiky FJFI ČVUT, Praha (CZ), 10.11.2006-24.11.2006] Institutional research plan: CEZ:AV0Z20760514 Keywords : atmospheric boundary layer * numerical simulation * finite element method Subject RIV: DI - Air Pollution ; Quality
Dynamical analysis of separated boundary layer flow
Czech Academy of Sciences Publication Activity Database
Uruba, Václav
Berlin : Technische Universität Berlin, 2009. s. 1-2 ISBN N. [Nonlinear Normal Modes, Dimension Reduction and Localization in Vibrating Systems. 27.09.2009-02.10.2009, Frascati (Rome)] R&D Projects: GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * separation * dynamics Subject RIV: BK - Fluid Dynamics
Submarine design optimization using boundary layer control
Christopher L Warren
1997-01-01
Several hull designs are studied with parametric based volume and area estimates to obtain preliminary hull forms. The volume and area study includes the effects of technologies which manifest themselves in the parametric study through stack length requirements. Subsequently, the hull forms are studied using a Reynolds Averaged Navier Stokes analysis coupled with a vortex lattice propeller design code. Optimization is done through boundary layer control analysis and through studies on the eff...
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.
Grey zone simulations of the morning convective boundary layer development
Efstathiou, G. A.; Beare, R. J.; Osborne, S.; Lock, A. P.
2016-05-01
Numerical simulations of two cases of morning boundary layer development are conducted to investigate the impact of grid resolution on mean profiles and turbulent kinetic energy (TKE) partitioning from the large eddy simulation (LES) to the mesoscale limit. Idealized LES, using the 3-D Smagorinsky scheme, is shown to be capable of reproducing the boundary layer evolution when compared against measurements. However, increasing grid spacing results in the damping of resolved TKE and the production of superadiabatic temperature profiles in the boundary layer. Turbulence initiation is significantly delayed, exhibiting an abrupt onset at intermediate resolutions. Two approaches, the bounding of vertical diffusion coefficient and the blending of the 3-D Smagorinsky with a nonlocal 1D scheme, are used to model subgrid diffusion at grey zone resolutions. Simulations are compared against the coarse-grained fields from the validated LES results for each case. Both methods exhibit particular strengths and weaknesses, indicating the compromise that needs to be made currently in high-resolution numerical weather prediction. The blending scheme is able to reproduce the adiabatic profiles although turbulence is underestimated in favor of the parametrized heat flux, and the spin-up of TKE remains delayed. In contrast, the bounding approach gives an evolution of TKE that follows the coarse-grained LES very well, relying on the resolved motions for the nonlocal heat flux. However, bounding gives unrealistic static instability in the early morning temperature profiles (similar to the 3-D Smagorinsky scheme) because model dynamics are unable to resolve TKE when the boundary layer is too shallow compared to the grid spacing.
Numerical Computations of Hypersonic Boundary-Layer over Surface Irregularities
Chang, Chau-Lyan; Choudhari, Meelan M.; Li, Fei
2010-01-01
Surface irregularities such as protuberances inside a hypersonic boundary layer may lead to premature transition on the vehicle surface. Early transition in turn causes large localized surface heating that could damage the thermal protection system. Experimental measurements as well as numerical computations aimed at building a knowledge base for transition Reynolds numbers with respect to different protuberance sizes and locations have been actively pursued in recent years. This paper computationally investigates the unsteady wake development behind large isolated cylindrical roughness elements and the scaled wind-tunnel model of the trip used in a recent flight measurement during the reentry of space shuttle Discovery. An unstructured mesh, compressible flow solver based on the space-time conservation element, solution element (CESE) method is used to perform time-accurate Navier-Stokes calculations for the flow past a roughness element under several wind-tunnel conditions. For a cylindrical roughness element with a height to the boundary-layer thickness ratio from 0.8 to 2.5, the wake flow is characterized by a mushroom-shaped centerline streak and horse-shoe vortices. While time-accurate solutions converged to a steady-state for a ratio of 0.8, strong flow unsteadiness is present for a ratio of 1.3 and 2.5. Instability waves marked by distinct disturbance frequencies were found in the latter two cases. Both the centerline streak and the horse-shoe vortices become unstable downstream. The oscillatory vortices eventually reach an early breakdown stage for the largest roughness element. Spectral analyses in conjunction with the computed root mean square variations suggest that the source of the unsteadiness and instability waves in the wake region may be traced back to possible absolute instability in the front-side separation region.
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)
Turing Instability in a Boundary-fed System
Setayeshgar, S
1998-01-01
The formation of localized structures in the chlorine dioxide-idodine-malonic acid (CDIMA) reaction-diffusion system is investigated numerically using a realistic model of this system. We analyze the one-dimensional patterns formed along the gradients imposed by boundary feeds, and study their linear stability to symmetry-breaking perturbations (Turing instability) in the plane transverse to these gradients. We establish that an often-invoked simple local linear analysis which neglects longitudinal diffusion is inappropriate for predicting the linear stability of these patterns. Using a fully nonuniform analysis, we investigate the structure of the patterns formed along the gradients and their stability to transverse Turing pattern formation as a function of the values of two control parameters: the malonic acid feed concentration and the size of the reactor in the dimension along the gradients. The results from this investigation are compared with existing experiments.
Modelling turbulent spots in swept boundary layers
International Nuclear Information System (INIS)
Highlights: • A linear perturbation method can capture the important flow features within a turbulent spot. • The horseshoe vortex in the perturbed velocity field is the dominant flow feature. • Sweep leads to skewing of the turbulent spot and calmed region. • The effects of pressure gradient are generally reduced by sweep. -- Abstract: A computational technique is presented for determining the fully 3-d viscid unsteady perturbation to a non-developing laminar swept boundary layer. For zero pressure gradient, unswept boundary layers, the perturbation method reveals a strongly three dimensional flow within the turbulent spot and its associated calmed region which is very similar to that observed in experiments and full DNS calculations. The perturbation method cannot predict turbulent motion but nevertheless provides a simple yet accurate means of studying and understanding the development of turbulent spot geometry. The most influential flow feature is the horseshoe vortex observed in the fluctuation velocity field, which is responsible for delivering the fluid found in the calmed region between its trailing legs. The upwards flow around the outer periphery of the vortex is also responsible for delivering low momentum fluid to the spot, but additional high momentum fluid also enters the spot from its rear through the downward sweeping motion of fluid between the vortex legs. The effect of an adverse streamwise pressure gradient is to increase the size of the spot and calmed region whereas a favourable pressure gradient has the opposite effect. When sweep is introduced to the boundary layer the spot is skewed for all non-zero pressure gradients, but the changes in size of the spot and calmed region due to pressure gradient are reduced. For favourable pressure gradients the skew increases monotonically with sweep, but this is not the case for adverse pressure gradients where the effect of sweep is more complex
Two Dimensional Boundary Layer Growth with Suction
Directory of Open Access Journals (Sweden)
Krishna Lal
1970-07-01
Full Text Available The boundary layer equations for the unsteady fluid flow with constant suction velocity have been worked out for the impulsive motion of a circular cylinder in the form V(t=A exp (Ct where A and C are certain constants. The stream function has been expanded in terms of some functions X/sub 0/(s where s is a function of y coordinate. The phase angles for various terms have been calculated, and variations shown graphically for large and small frequency of oscillations, where the oscillatory motion is obtained on replacing C by iw.
Stationary plasma-field equilibrium states in astropause boundary layers
International Nuclear Information System (INIS)
The transition layer between a stellar wind plasma and the surrounding regime of magnetized interstellar plasma, i.e. the astropause boundary layer has been investigated theoretically. For the description of the 'microscopic' structures a planar representation of the transition zone geometry is used. Here the plasma is taken to be dominated by instability-induced collective relaxation processes as, for example, modified two-stream instabilities, keeping the effective electron and proton temperatures close to each other. These are caused by strong couplings between the plasma constituents and the equilibrium wave field. This permits a quasi-hydrodynamic description of the plasma flow in a two-fluid approximation. For this case a system of differential equations is developed describing consistently the dynamical variables of the plasma and the magnetic and electric fields in the transition region. Integrals of this system are discussed and it is shown that it can be reduced to one ordinary differential equation. This equation is solved in terms of elliptic integrals and gives an implicit representation of magnetic and electric fields and the density. (author)
Randall, David A.; Abeles, James A.; Corsetti, Thomas G.
1985-04-01
The UCLA general circulation model (GCM) has been used to simulate the seasonally varying planetary boundary layer (PBL), as well as boundary-layer stratus and stratocumulus clouds. The PBL depth is a prognostic variable of the GCM, incorporated through the use of a vertical coordinate system in which the PBL is identified with the lowest model layer.Stratocumulus clouds are assumed to occur whenever the upper portion of the PBL becomes saturated, provided that the cloud-top entrainment instability does not occur. As indicated by Arakawa and Schubert, cumulus clouds are assumed to originate at the PBL top, and tend to make the PBL shallow by drawing on its mass.Results are presented from a three-year simulation, starting from a 31 December initial condition obtained from an earlier run with a different version of the model. The simulated seasonally varying climates of the boundary layer and free troposphere are realistic. The observed geographical and seasonal variations of stratocumulus cloudiness are fairly well simulated. The simulation of the stratocumulus clouds associated with wintertime cold-air outbreaks is particularly realistic. Examples are given of individual events. The positions of the subtropical marine stratocumulus regimes are realistically simulated, although their observed frequency of occurrence is seriously underpredicted. The observed summertime abundance of Arctic stratus clouds is also underpredicted.In the GCM results, the layer cloud instability appears to limit the extent of the marine subtropical stratocumulus regimes. The instability also frequently occurs in association with cumulus convection over land.Cumulus convection acts as a very significant sink of PBL mass throughout the tropics, and over the midlatitude continents in summer.Three experiments have been performed to investigate the sensitivity of the GCM results to aspects of the PBL and stratocumulus parameterizations. For all three experiments, the model was started from 1
Instability modes of a two-layer Newtonian plane Couette flow past a porous medium
Praveen Kumar, A. Ananth; Goyal, Himanshu; Banerjee, Tamal; Bandyopadhyay, Dipankar
2013-06-01
We explore the salient features of the different instability modes of a pressure-driven two-layer plane Couette flow confined between a moving wall and a Darcy-Brinkman porous layer. A linear stability analysis of the conservation laws leads to an Orr-Sommerfeld system, which is solved numerically with appropriate boundary conditions to identify the time and length scales of the instability modes. The study reveals that the movement of the confining wall together with the slippage at the porous-liquid interface originating from the flow inside the porous layer can stimulate a pair of finite-wave-number shear modes in addition to the long-wave interfacial mode of instability. The shear modes dominate the interfacial mode, especially when the frictional influence at the liquid layers is smaller due to the movement of the confining plate or due to the larger slippage at the porous-liquid interface. The shear modes are found to be present under all combinations of the viscosity μr and thickness hr ratios of the liquid layers. This is in stark contrast to the two-layer flow confined between nonporous plates where the interfacial (shear) mode is observed only when μr>hr2 (μrflows where the presence of a bounding porous layer or moving wall can expedite the intermixing of layers to improve the multiphase mixing, heat and mass transfer, and emulsification characteristics.
Cebeci, Tuncer
2005-01-01
This second edition of our book extends the modeling and calculation of boundary-layer flows to include compressible flows. The subjects cover laminar, transitional and turbulent boundary layers for two- and three-dimensional incompressible and compressible flows. The viscous-inviscid coupling between the boundary layer and the inviscid flow is also addressed. The book has a large number of homework problems.
Aerodynamic Heating in Hypersonic Boundary Layers:\\ Role of Dilatational Waves
Zhu, Yiding; Wu, Jiezhi; Chen, Shiyi; Lee, Cunbiao; Gad-el-Hak, Mohamed
2016-01-01
The evolution of multi-mode instabilities in a hypersonic boundary layer and their effects on aerodynamic heating are investigated. Experiments are conducted in a Mach 6 wind tunnel using Rayleigh-scattering flow visualization, fast-response pressure sensors, fluorescent temperature-sensitive paint (TSP), and particle image velocimetry (PIV). Calculations are also performed based on both parabolized stability equations (PSE) and direct numerical simulations (DNS). It is found that second-mode dilatational waves, accompanied by high-frequency alternating fluid compression and expansion, produce intense aerodynamic heating in a small region that rapidly heats the fluid passing through it. As a result, the surface temperature rapidly increases and results in an overshoot over the nominal transitional value. When the dilatation waves decay downstream, the surface temperature decreases gradually until transition is completed. A theoretical analysis is provided to interpret the temperature distribution affected by ...
Characteristics of turbulent spots in transitional boundary layers
Marxen, Olaf; Zaki, Tamer
2015-11-01
The laminar-turbulent transition process in a flat-plate boundary layer beneath free-stream turbulence takes place through the inception and spreading of confined patches of turbulence in an otherwise laminar flow. These patches, also referred to as turbulent spots, result from a secondary instability of the Klebanoff streaks in the pre-transitional region. The dynamics of turbulence in the spots are investigated by analyzing data sets obtained from direct numerical simulations. Conditionally-averaged and spot-ensemble-averaged statistics are evaluated and describe the flow in the intermittent transition zone. Both mean-flow and disturbance root mean square levels obtained from conditional averaging agree very well with results for fully turbulent flows, in particular near the wall and at high intermittency levels. At relatively low intermittency, the spatial inhomogeneity of turbulence within the spots is important, and is examined using ensemble averaging of turbulent patches that have comparable volume and a similar streamwise location.
On Hydromagnetic Stresses in Accretion Disk Boundary Layers
DEFF Research Database (Denmark)
Pessah, Martin Elias; Chan, Chi-kwan
2012-01-01
viscosity satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI) is inefficient in disk regions where, as...... angular frequencies that increase outward in the shearing-sheet framework. We isolate the modes that are unrelated to the standard MRI and provide analytic solutions for the long-term evolution of the resulting shearing MHD waves. We show that, although the energy density of these waves can be amplified...... significantly, their associated stresses oscillate around zero, rendering them an inefficient mechanism to transport significant angular momentum (inward). These findings are consistent with the results obtained in numerical simulations of MHD accretion disk boundary layers and challenge the standard assumption...
Boiko, Andrey V; Grek, Genrih R; Kozlov, Victor V
2012-01-01
Starting from fundamentals of classical stability theory, an overview is given of the transition phenomena in subsonic, wall-bounded shear flows. At first, the consideration focuses on elementary small-amplitude velocity perturbations of laminar shear layers, i.e. instability waves, in the simplest canonical configurations of a plane channel flow and a flat-plate boundary layer. Then the linear stability problem is expanded to include the effects of pressure gradients, flow curvature, boundary-layer separation, wall compliance, etc. related to applications. Beyond the amplification of instability waves is the non-modal growth of local stationary and non-stationary shear flow perturbations which are discussed as well. The volume continues with the key aspect of the transition process, that is, receptivity of convectively unstable shear layers to external perturbations, summarizing main paths of the excitation of laminar flow disturbances. The remainder of the book addresses the instability phenomena found at l...
Competing disturbance amplification mechanisms in two-fluid boundary layers
Saha, Sandeep; Page, Jacob; Zaki, Tamer
2015-11-01
The linear stability of boundary layers above a thin wall film of lower viscosity is analyzed. Appropriate choice of the film thickness and viscosity excludes the possibility of interfacial instabilities. Transient amplification of disturbances is therefore the relevant destabilizing influence, and can take place via three different mechanisms in the two-fluid configuration. Each is examined in detail by solving an initial value problem whose initial condition comprises a pair of appropriately chosen eigenmodes from the discrete, continuous and interface modes. Two regimes are driven by the lift-up mechanism: (i) The response to a streamwise vortex and (ii) the normal vorticity generated by a stable Tollmien-Schlichting wave. Both are damped due to the film. The third regime is associated with the wall-normal vorticity that is generated by the interface displacement. It can lead to appreciable streamwise velocity disturbances in the near-wall region at relatively low viscosity ratios. The results demonstrate that a wall film can stabilize the early linear stages of boundary-layer transition, and explain the observations from the recent nonlinear direct numerical simulations of this configuration by Jung & Zaki (J. Fluid Mech., vol 772, 2015, 330-360).
The Thermomagnetic Instability in Superconducting Films with Adjacent Metal Layer
Vestgården, J. I.; Galperin, Y. M.; Johansen, T. H.
2013-12-01
Dendritic flux avalanches is a frequently encountered consequence of the thermomagnetic instability in type-II superconducting films. The avalanches, which are potentially harmful for superconductor-based devices, can be suppressed by an adjacent normal metal layer, even when the two layers are not in thermal contact. The suppression of the avalanches in this case is due to so-called magnetic braking, caused by eddy currents generated in the metal layer by propagating magnetic flux. We develop a theory of magnetic braking by analyzing coupled electrodynamics and heat flow in a superconductor-normal metal bilayer. The equations are solved by linearization and by numerical simulation of the avalanche dynamics. We find that in an uncoated superconductor, even a uniform thermomagnetic instability can develop into a dendritic flux avalanche. The mechanism is that a small non-uniformity caused by the electromagnetic non-locality induces a flux-flow hot spot at a random position. The hot spot quickly develops into a finger, which at high speeds penetrates into the superconductor, forming a branching structure. Magnetic braking slows the avalanches, and if the normal metal conductivity is sufficiently high, it can suppress the formation of the dendritic structure. During avalanches, the braking by the normal metal layer prevents the temperature from exceeding the transition temperature of the superconductor. Analytical criteria for the instability threshold are developed using the linear stability analysis. The criteria are found to match quantitatively the instability onsets obtained in simulations.
Modelling of the Evolving Stable Boundary Layer
Sorbjan, Zbigniew
2014-06-01
A single-column model of the evolving stable boundary layer (SBL) is tested for self-similar properties of the flow and effects of ambient forcing. The turbulence closure of the model is diagnostic, based on the K-theory approach, with a semi-empirical form of the mixing length, and empirical stability functions of the Richardson number. The model results, expressed in terms of local similarity scales, are universal functions, satisfied in the entire SBL. Based on similarity expression, a realizability condition is derived for the minimum allowable turbulent heat flux in the SBL. Numerical experiments show that the development of "horse-shoe" shaped, fixed-elevation hodographs in the interior of the SBL around sunrise is controlled by effects imposed by surface thermal forcing.
Atmospheric boundary layer over steep surface waves
Troitskaya, Yuliya; Sergeev, Daniil A.; Druzhinin, Oleg; Kandaurov, Alexander A.; Ermakova, Olga S.; Ezhova, Ekaterina V.; Esau, Igor; Zilitinkevich, Sergej
2014-08-01
Turbulent air-sea interactions coupled with the surface wave dynamics remain a challenging problem. The needs to include this kind of interaction into the coupled environmental, weather and climate models motivate the development of a simplified approximation of the complex and strongly nonlinear interaction processes. This study proposes a quasi-linear model of wind-wave coupling. It formulates the approach and derives the model equations. The model is verified through a set of laboratory (direct measurements of an airflow by the particle image velocimetry (PIV) technique) and numerical (a direct numerical simulation (DNS) technique) experiments. The experiments support the central model assumption that the flow velocity field averaged over an ensemble of turbulent fluctuations is smooth and does not demonstrate flow separation from the crests of the waves. The proposed quasi-linear model correctly recovers the measured characteristics of the turbulent boundary layer over the waved water surface.
Geometric invariance of compressible turbulent boundary layers
Bi, Wei-Tao; Wu, Bin; She, Zhen-Su; Hussain, Fazle
2015-11-01
A symmetry based approach is applied to analyze the mean velocity and temperature fields of compressible, flat plate turbulent boundary layers (CTBL). A Reynolds stress length scale and a turbulent heat flux length scale are identified to possess the same defect scaling law in the CTBL bulk, which is solely owing to the constraint of the wall to the geometry of the wall-attached eddies, but invariant to compressibility and wall heat transfer. This invariance is called the geometric invariance of CTBL eddies and is likely the origin of the Mach number invariance of Morkovin's hypothesis, as well as the similarity of energy and momentum transports. A closure for the turbulent transport by using the invariant lengths is attainted to predict the mean velocity and temperature profiles in the CTBL bulk- superior to the van Driest transformation and the Reynolds analogy based relations for its sound physics and higher accuracy. Additionally, our approach offers a new understanding of turbulent Prandtl number.
Effect of slip boundary conditions on interfacial stability of two-layer viscous fluids under shear
Patlazhan, Stanislav
2015-01-01
The traditional approach in the study of hydrodynamic stability of stratified fluids includes the stick boundary conditions between layers. However, this rule may be violated in polymer systems and as a consequence various instabilities may arise. The main objective of this paper is to analyze theoretically the influence of slip boundary conditions on the hydrodynamic stability of the interface between two immiscible viscous layers subjected to simple shear flow. It is found that the growth rate of long-wave disturbances is fairly sensitive to the slip at the interface between layers as well as at the external boundary. These phenomena are shown to give different contributions to the stability of shear flow depending on viscosity, thickness, and density ratios of the layers. Particularly, the interfacial slip can increase the perturbation growth rate and lead to unstable flow. An important consequence of this effect is the violation of stability for sheared layers with equal viscosities and densities in a bro...
Solutal Marangoni instability in layered two-phase flow
Picardo, Jason; Radhakrishna, T. G.; Pushpavanam, S.
2015-11-01
In this work, the instability of layered two-phase flow caused by the presence of a surface-active solute is studied. The fluids are density matched to focus on surfactant effects. The fluids flow between two flat plates, which are maintained at different solute concentrations. This establishes a constant flux of soluble surfactant from one fluid to the other, in the base state. A linear stability analysis is carried out, supported by energy budget calculations. The flow is first analyzed in the creeping flow regime. Long wave as well as short wave Marangoni instabilities are identified, each with a distinct energy signature. The short wave instability manifests as two distinct modes, characterized by the importance of interfacial deformations or lack thereof. The primary instability switches between these different modes as parameters are varied. The effect of small but finite inertia on these solutal Marangoni modes is then examined. The effect of soluble surfactant on a finte inertia flow is also studied, with focus on the transition from the viscosity-induced instability to solutal Marangoni instability. This analysis is relevant to microfluidic applications, such as solvent extraction, in which mass transfer is carried out between stratified immiscible fluids.
Wave mediated angular momentum transport in astrophysical boundary layers
Hertfelder, Marius; Kley, Wilhelm
2015-07-01
Context. Disk accretion onto weakly magnetized stars leads to the formation of a boundary layer (BL) where the gas loses its excess kinetic energy and settles onto the star. There are still many open questions concerning the BL, for instance the transport of angular momentum (AM) or the vertical structure. Aims: It is the aim of this work to investigate the AM transport in the BL where the magneto-rotational instability (MRI) is not operating owing to the increasing angular velocity Ω(r) with radius. We will therefore search for an appropriate mechanism and examine its efficiency and implications. Methods: We perform 2D numerical hydrodynamical simulations in a cylindrical coordinate system (r,ϕ) for a thin, vertically integrated accretion disk around a young star. We employ a realistic equation of state and include both cooling from the disk surfaces and radiation transport in radial and azimuthal direction. The viscosity in the disk is treated by the α-model; in the BL there is no viscosity term included. Results: We find that our setup is unstable to the sonic instability which sets in shortly after the simulations have been started. Acoustic waves are generated and traverse the domain, developing weak shocks in the vicinity of the BL. Furthermore, the system undergoes recurrent outbursts where the activity in the disk increases strongly. The instability and the waves do not die out for over 2000 orbits. Conclusions: There is indeed a purely hydrodynamical mechanism that enables AM transport in the BL. It is efficient and wave mediated; however, this renders it a non-local transport method, which means that models of a effective local viscosity like the α-viscosity are probably not applicable in the BL. A variety of further implications of the non-local AM transport are discussed.
DNS of Laminar-Turbulent Transition in Swept-Wing Boundary Layers
Duan, L.; Choudhari, M.; Li, F.
2014-01-01
Direct numerical simulation (DNS) is performed to examine laminar to turbulent transition due to high-frequency secondary instability of stationary crossflow vortices in a subsonic swept-wing boundary layer for a realistic natural-laminar-flow airfoil configuration. The secondary instability is introduced via inflow forcing and the mode selected for forcing corresponds to the most amplified secondary instability mode that, in this case, derives a majority of its growth from energy production mechanisms associated with the wall-normal shear of the stationary basic state. An inlet boundary condition is carefully designed to allow for accurate injection of instability wave modes and minimize acoustic reflections at numerical boundaries. Nonlinear parabolized stability equation (PSE) predictions compare well with the DNS in terms of modal amplitudes and modal shape during the strongly nonlinear phase of the secondary instability mode. During the transition process, the skin friction coefficient rises rather rapidly and the wall-shear distribution shows a sawtooth pattern that is analogous to the previously documented surface flow visualizations of transition due to stationary crossflow instability. Fully turbulent features are observed in the downstream region of the flow.
A Cautionary Note on the Thermal Boundary Layer Similarity Scaling for the Turbulent Boundary Layer
Weyburne, David
2016-01-01
Wang and Castillo have developed empirical parameters for scaling the temperature profile of the turbulent boundary layer flowing over a heated wall in the paper X. Wang and L. Castillo, J. Turbul., 4, 1(2003). They presented experimental data plots that showed similarity type behavior when scaled with their new scaling parameters. However, what was actually plotted, and what actually showed similarity type behavior, was not the temperature profile but the defect profile formed by subtracting the temperature in the boundary layer from the temperature in the bulk flow. We show that if the same data and same scaling is replotted as just the scaled temperature profile, similarity is no longer prevalent. This failure to show both defect profile similarity and temperature profile similarity is indicative of false similarity. The nature of this false similarity problem is discussed in detail.
Solutal Marangoni instability in layered two-phase flows
Picardo, Jason R; Pushpavanam, S
2015-01-01
In this paper, the instability of layered two-phase flows caused by the presence of a soluble surfactant (or a surface active solute) is studied. The fluids have different viscosities, but are density matched to focus on Marangoni effects. The fluids flow between two flat plates, which are maintained at different solute concentrations. This establishes a constant flux of solute from one fluid to the other in the base state. A linear stability analysis is performed, using a combination of asymptotic and numerical methods. In the creeping flow regime, Marangoni stresses destabilize the flow, provided a concentration gradient is maintained across the fluids. One long wave and two short wave Marangoni instability modes arise, in different regions of parameter space. A well-defined condition for the long wave instability is determined in terms of the viscosity and thickness ratios of the fluids, and the direction of mass transfer. Energy budget calculations show that the Marangoni stresses that drive long and shor...
The Boundary Layer Interaction with Shock Wave and Expansion Fan
Institute of Scientific and Technical Information of China (English)
MaratA.Goldfeld; RomanV.Nestoulia; 等
2000-01-01
The results of experimental investigation of a turbulent boundary layer on compression and expansion surfaces are presented.They include the study of the shock wave and /or expansion fan action upon the boundary layer,boundary layer sepqartion and its relaxation.Complex events of paired interactions and the flow on compression convex-concave surfaces were studied.The posibility and conditions of the boundary layer relaminarization behind the expansion fan and its effect on the relaxation length are presented.Different model configurations for wide range conditions were investigated.Comparison of results for different interactions was carried out.
Simulation of Wind turbines in the atmospheric boundary layer
DEFF Research Database (Denmark)
Chivaee, Hamid Sarlak; Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming
Large eddy simulation of an arbitrary wind farm is studied in the neutral and thermally stratified atmospheric boundary Layer. Large eddy simulations of industrial flows usually requires full resolution of the flow near the wall and this is believed to be one of the main deficiencies of LES because...... in the boundary layer. In the current study, another approach has been implemented to simulate the flow in a fully developed wind farm boundary layer. The approach is based on Immersed Boundary Method and involves implementation of an arbitrary prescribed initial boundary layer. An initial boundary...... based on the turbine wakes and buoyancy contributions. The implemented method is capable of capturing the most important features of wakes of wind farms [2] while having the advantage of resolving the wall layer with a coarser grid than a typical required grid size for such problems. LES simulations are...
Characteristics of the boundary layer of magnetic clouds and a new definition of the cloud boundary
Institute of Scientific and Technical Information of China (English)
魏奉思; 刘睿; 范全林; 冯学尚
2003-01-01
Based on the analysis of the boundaries of 70 magnetic clouds from 1967 to 1998, and relatively complete spacecraft observations, it is indicated that the magnetic cloud boundaries are boundary layers formed through the interaction between the magnetic clouds and the ambient medium. Most of the outer boundaries of the layers, with relatively high proton temperature, density and plasma β, are magnetic reconnection boundaries, while the inner boundaries, with low proton temperature, proton density and plasma β, separate the main body of magnetic clouds, which has not been affected by the interaction, from the boundary layers. The average time scale of the front boundary layer is 1.7 h and that of the tail boundary layer 3.1 h. It is also found that the magnetic probability distribution function undergoes significant changes across the boundary layers. This new definition, supported by the preliminary numerical simulation in principle, could qualitatively explain the observations of interplanetary magnetic clouds, and could help resolve the controversy in identifying the boundaries of magnetic clouds. Our concept of the boundary layer may provide some understanding of what underlies the observations, and a fresh train of thought in the interplanetary dynamics research.
Green House Gases Flux Model in Boundary Layer
Nurgaliev, Ildus
Analytical dynamic model of the turbulent flux in the three-layer boundary system is presented. Turbulence is described as a presence of the non-zero vorticity. The generalized advection-diffusion-reaction equation is derived for an arbitrary number of components in the flux. The fluxes in the layers are objects for matching requirements on the boundaries between the layers. Different types of transport mechanisms are dominant on the different levels of the layers.
Alpha effect due to buoyancy instability of a magnetic layer
Chatterjee, Piyali; Mitra, Dhrubaditya; Rheinhardt, Matthias; Brandenburg, Axel
2010-01-01
A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation. We calculate the alpha effect due to helical motions caused by a disintegrating magnetic layer in a rotating density-stratified system with angular velocity Omega making an angle theta with the vertical. We also study the dependence of the alpha effect on theta and the strength of the ini...
Effects of Nose Bluntness on Hypersonic Boundary-Layer Receptivity and Stability Over Cones
Kara, Kursat; Balakumar, Ponnampalam; Kandil, Osama A.
2011-01-01
The receptivity to freestream acoustic disturbances and the stability properties of hypersonic boundary layers are numerically investigated for boundary-layer flows over a 5 straight cone at a freestream Mach number of 6.0. To compute the shock and the interaction of the shock with the instability waves, the Navier-Stokes equations in axisymmetric coordinates were solved. In the governing equations, inviscid and viscous flux vectors are discretized using a fifth-order accurate weighted-essentially-non-oscillatory scheme. A third-order accurate total-variation-diminishing Runge-Kutta scheme is employed for time integration. After the mean flow field is computed, disturbances are introduced at the upstream end of the computational domain. The appearance of instability waves near the nose region and the receptivity of the boundary layer with respect to slow mode acoustic waves are investigated. Computations confirm the stabilizing effect of nose bluntness and the role of the entropy layer in the delay of boundary-layer transition. The current solutions, compared with experimental observations and other computational results, exhibit good agreement.
Structure Identification Within a Transitioning Swept-Wing Boundary Layer
Chapman, Keith; Glauser, Mark
1996-01-01
Extensive measurements are made in a transitioning swept-wing boundary layer using hot-film, hot-wire and cross-wire anemometry. The crossflow-dominated flow contains stationary vortices that breakdown near mid-chord. The most amplified vortex wavelength is forced by the use of artificial roughness elements near the leading edge. Two-component velocity and spanwise surface shear-stress correlation measurements are made at two constant chord locations, before and after transition. Streamwise surface shear stresses are also measured through the entire transition region. Correlation techniques are used to identify stationary structures in the laminar regime and coherent structures in the turbulent regime. Basic techniques include observation of the spatial correlations and the spatially distributed auto-spectra. The primary and secondary instability mechanisms are identified in the spectra in all measured fields. The primary mechanism is seen to grow, cause transition and produce large-scale turbulence. The secondary mechanism grows through the entire transition region and produces the small-scale turbulence. Advanced techniques use Linear Stochastic Estimation (LSE) and Proper Orthogonal Decomposition (POD) to identify the spatio-temporal evolutions of structures in the boundary layer. LSE is used to estimate the instantaneous velocity fields using temporal data from just two spatial locations and the spatial correlations. Reference locations are selected using maximum RMS values to provide the best available estimates. POD is used to objectively determine modes characteristic of the measured flow based on energy. The stationary vortices are identified in the first laminar modes of each velocity component and shear component. Experimental evidence suggests that neighboring vortices interact and produce large coherent structures with spanwise periodicity at double the stationary vortex wavelength. An objective transition region detection method is developed using
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 J
Direct spatial resonance in the laminar boundary layer due to a rotating-disk
Indian Academy of Sciences (India)
M Turkyilmazoglu; J S B Gajjar
2000-12-01
Numerical treatment of the linear stability equations is undertaken to investigate the occurrence of direct spatial resonance events in the boundary layer flow due to a rotating-disk. A spectral solution of the eigenvalue problem indicates that algebraic growth of the perturbations shows up, prior to the amplification of exponentially growing instability waves. This phenomenon takes place while the flow is still in the laminar state and it also tends to persist further even if the non-parallelism is taken into account. As a result, there exists the high possibility of this instability mechanism giving rise to nonlinearity and transition, long before the unboundedly growing time-amplified waves.
Local boundary layer scales in turbulent Rayleigh-Benard convection
Scheel, Janet D
2014-01-01
We compute fully local boundary layer scales in three-dimensional turbulent Rayleigh-Benard convection. These scales are directly connected to the highly intermittent fluctuations of the fluxes of momentum and heat at the isothermal top and bottom walls and are statistically distributed around the corresponding mean thickness scales. The local boundary layer scales also reflect the strong spatial inhomogeneities of both boundary layers due to the large-scale, but complex and intermittent, circulation that builds up in closed convection cells. Similar to turbulent boundary layers, we define inner scales based on local shear stress which can be consistently extended to the classical viscous scales in bulk turbulence, e.g. the Kolmogorov scale, and outer scales based on slopes at the wall. We discuss the consequences of our generalization, in particular the scaling of our inner and outer boundary layer thicknesses and the resulting shear Reynolds number with respect to Rayleigh number. The mean outer thickness s...
Boundary Layer to a System of Viscous Hyperbolic Conservation Laws
Institute of Scientific and Technical Information of China (English)
2008-01-01
In this paper, we investigate the large-time behavior of solutions to the initial-boundary value problem for nxn hyperbolic system of conservation laws with artificial viscosity in the half line (0, ∞). We first show that a boundary layer exists if the corresponding hyperbolic part contains at least one characteristic field with negative propagation speed. We further show that such boundary layer is nonlinearly stable under small initial perturbation. The proofs are given by an elementary energy method.
Numerical simulation of tsunami-scale wave boundary layers
DEFF Research Database (Denmark)
Williams, Isaac A.; Fuhrman, David R.
2016-01-01
, boundary layer thickness, turbulence, and bed shear stresses induced are systematically monitored and parameterised, under both hydraulically smooth and roughbed conditions. The results generally support a notion that the boundary layers induced by tsunami-scalewaves are both current-like, due...... layer properties beneath wind-waves maintain reasonable accuracy when extrapolated to full tsunami scales. Boundary layers driven by actual field-measured tsunami signals are likewise simulated, stemming from both the 2004 Indian Ocean as well as the 2011 Tohoku events. These results are reconciled...
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.
Role of boundary layer processes on the mixed layer CO2-budget
D. Pino; Vilà-Guerau de Arellano, J.
2010-01-01
The diurnal and vertical variability of temperature, humidity and specially CO2 in the atmospheric boundary layer is studied by combining detailed observations taken at Cabauw (The Netherlands), Large-Eddy simulations (LES) and mixed layer theory. The research focus on the role played by the entrainment and other boundary layer driven processes on the distribution and diurnal evolution of CO2 in the boundary layer. The relative importance of this entrained air to ventilate CO2 will be analyze...
Role of residual layer and large-scale phenomena on the evolution of the boundary layer
Blay, E.; D. Pino; Vilà-Guerau de Arellano, J.; Boer; Coster, van, R.; I. Faloona; 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. Continuous measurements made by several remote sensing and in situ instruments in combination with radiosoundings, and measurements done by unmanned aerial vehicles and an aircraft probed the verti...
Analyses of structure of planetary boundary layer in ice camp over Arctic ocean
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
The vertical structure of Planetary boundary layer over Arctic floating ice is presented by using about 50 atmospheric profiles and relevant data sounded at an ice station over Arctic Ocean from 22 August to 3 September, 2003. It shows that the height of the convective boundary layer in day is greater than that of the stability boundary layer in night. The boundary layer can be described as vertical structures of stability, instability and multipling The interaction between relative warm and wet down draft air from up level and cool air of surface layer is significant, which causes stronger wind shear, temperature and humidity inversion with typical wind shear of 10 m/s/100 m, intensity of temperature inversion of 8 ℃/100 m. While the larger pack ice is broken by such process, new ice free area in the high latitudes of arctic ocean. The interactions between air/ice/water are enhanced. The fact helps to understanding characteristics of atmospheric boundary layer and its effect in Arctic floating ice region.
Vortex dynamics and shear layer instability in high intensity cyclotrons
Cerfon, Antoine J
2016-01-01
We show that the space charge dynamics of high intensity beams in the plane perpendicular to the magnetic field in cyclotrons is described by the two-dimensional Euler equations for an incompressible fluid. This analogy with fluid dynamics gives a unified and intuitive framework to explain the beam spiraling and beam break up behavior observed in experiments and in simulations. In particular, we demonstrate that beam break up is the result of a classical instability occurring in fluids subject to a sheared flow. We give scaling laws for the instability and predict the nonlinear evolution of beams subject to it. Our work suggests that cyclotrons may be uniquely suited for the experimental study of shear layers and vortex distributions that are not achievable in Penning-Malmberg traps.
Receptivity and Forced Response to Acoustic Disturbances in High-Speed Boundary Layers
Balakumar, P.; King, Rudolph A.; Chou, Amanda; Owens, Lewis R.; Kegerise, Michael A.
2016-01-01
Supersonic boundary-layer receptivity to freestream acoustic disturbances is investigated by solving the Navier-Stokes equations for Mach 3.5 flow over a sharp flat plate and a 7-deg half-angle cone. The freestream disturbances are generated from a wavy wall placed at the nozzle wall. The freestream acoustic disturbances radiated by the wavy wall are obtained by solving the linearized Euler equations. The results for the flat plate show that instability modes are generated at all the incident angles ranging from zero to highly oblique. However, the receptivity coefficient decreases by about 20 times when the incident angle increases from zero to a highly oblique angle of 68 degrees. The results for the cone show that no instability modes are generated when the acoustic disturbances impinge the cone obliquely. The results show that the perturbations generated inside the boundary layer by the acoustic disturbances are the response of the boundary layer to the external forcing. The amplitude of the forced disturbances inside the boundary layer are about 2.5 times larger than the incoming field for zero azimuthal wavenumber and they are about 1.5 times for large azimuthal wavenumbers.
Effects of Nose Bluntness on Stability of Hypersonic Boundary Layers over Blunt Cone
Kara, K.; Balakumar, P.; Kandil, O. A.
2007-01-01
Receptivity and stability of hypersonic boundary layers are numerically investigated for boundary layer flows over a 5-degree straight cone at a free-stream Mach number of 6.0. To compute the shock and the interaction of shock with the instability waves, we solve the Navier-Stokes equations in axisymmetric coordinates. The governing equations are solved 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. After the mean flow field is computed, disturbances are introduced at the upstream end of the computational domain. Generation of instability waves from leading edge region and receptivity of boundary layer to slow acoustic waves are investigated. Computations are performed for a cone with nose radii of 0.001, 0.05 and 0.10 inches that give Reynolds numbers based on the nose radii ranging from 650 to 130,000. The linear stability results showed that the bluntness has a strong stabilizing effect on the stability of axisymmetric boundary layers. The transition Reynolds number for a cone with the nose Reynolds number of 65,000 is increased by a factor of 1.82 compared to that for a sharp cone. The receptivity coefficient for a sharp cone is about 4.23 and it is very small, approx.10(exp -3), for large bluntness.
Boundary-layer control by electric fields: A feasibility study
Mendes, R. Vilela; Dente, J. A.
1997-01-01
A problem of great concern in aviation and submarine propulsion is the control of the boundary layer and, in particular, the methods to extend the laminar region as a means to decrease noise and fuel consumption. In this paper we study the flow of air along an airfoil when a layer of ionized gas and a longitudinal electric field are created in the boundary layer region. By deriving scaling solutions and more accurate numerical solutions we discuss the possibility of achieving significant boun...
Characterization of the Martian Convective Boundary Layer
Martínez, Germán; Valero Rodríguez, Francisco; Vázquez Martínez, Luis
2009-01-01
The authors have carried out an extensive characterization of the Martian mixed layer formed under convective conditions. The values of the mixed layer height, convective velocity scale, convective temperature scale, mean temperature standard deviation, mean horizontal and vertical velocity standard deviations, and mean turbulent viscous dissipation rate have been obtained during the strongest convective hours for the mixed layer. In addition, the existing database of the surface layer has be...
Wave mediated angular momentum transport in astrophysical boundary layers
Hertfelder, Marius
2015-01-01
Context. Disk accretion onto weakly magnetized stars leads to the formation of a boundary layer (BL) where the gas loses its excess kinetic energy and settles onto the star. There are still many open questions concerning the BL, for instance the transport of angular momentum (AM) or the vertical structure. Aims. It is the aim of this work to investigate the AM transport in the BL where the magneto-rotational instability (MRI) is not operating owing to the increasing angular velocity $\\Omega(r)$ with radius. We will therefore search for an appropriate mechanism and examine its efficiency and implications. Methods. We perform 2D numerical hydrodynamical simulations in a cylindrical coordinate system $(r, \\varphi)$ for a thin, vertically inte- grated accretion disk around a young star. We employ a realistic equation of state and include both cooling from the disk surfaces and radiation transport in radial and azimuthal direction. The viscosity in the disk is treated by the {\\alpha}-model; in the BL there is no v...
Sensitivity of African easterly waves to boundary layer conditions
Directory of Open Access Journals (Sweden)
A. Lenouo
2008-06-01
Full Text Available A linearized version of the quasi-geostrophic model (QGM with an explicit Ekman layer and observed static stability parameter and profile of the African easterly jet (AEJ, is used to study the instability properties of the environment of the West African wave disturbances. It is found that the growth rate, the propagation velocity and the structure of the African easterly waves (AEW can be well simulated. Two different lower boundary conditions are applied. One assumes a lack of vertical gradient of perturbation stream function and the other assumes zero wind perturbation at the surface. The first case gives more realistic results since in the absence of horizontal diffusion, growth rate, phase speed and period have values of 0.5 day^{−1}, 10.83 m s^{−1} and 3.1 day, respectively. The zero wind perturbation at the surface case leads to values of these parameters that are 50 percent lower. The analysis of the sensitivity to diffusion shows that the magnitude of the growth rate decreases with this parameter. Modelled total relative vorticity has its low level maximum around 900 hPa under no-slip, and 700 hPa under free slip condition.
Sensitivity of African easterly waves to boundary layer conditions
Energy Technology Data Exchange (ETDEWEB)
Lenouo, A. [Douala Univ. (Cameroon). Dept. of Physics; Mkankam Kamga, F. [Yaounde I Univ. (Cameroon). LEMAP, Dept. of Physics
2008-07-01
A linearized version of the quasi-geostrophic model (QGM) with an explicit Ekman layer and observed static stability parameter and profile of the African easterly jet (AEJ), is used to study the instability properties of the environment of the West African wave disturbances. It is found that the growth rate, the propagation velocity and the structure of the African easterly waves (AEW) can be well simulated. Two different lower boundary conditions are applied. One assumes a lack of vertical gradient of perturbation stream function and the other assumes zero wind perturbation at the surface. The first case gives more realistic results since in the absence of horizontal diffusion, growth rate, phase speed and period have values of 0.5 day{sup -1}, 10.83 m s{sup -1} and 3.1 day, respectively. The zero wind perturbation at the surface case leads to values of these parameters that are 50 percent lower. The analysis of the sensitivity to diffusion shows that the magnitude of the growth rate decreases with this parameter. Modelled total relative vorticity has its low level maximum around 900 hPa under no-slip, and 700 hPa under free slip condition. (orig.)
Near continuum boundary layer flows at a flat plate
Directory of Open Access Journals (Sweden)
Chunpei Cai
2015-05-01
Full Text Available The problem of boundary layer flows at a flat plate surface with velocity-slip and temperature-jump boundary conditions is analyzed. With the velocity slip conditions, there are multiple physical factors lumped together, and the boundary layer solutions significantly change their behaviors. The self-similarity in the solutions degenerates, however, the problem is still an ordinary differential equation which can be solved. Shooting methods are applied to solve the flowfield. The results include velocity and temperature for both the surface and flowfield. Unlike the traditional Blasius flat plate boundary layer solutions which are self-similar through all the plate boundary layer, the new solutions indicate that the front tip is actually a singularity point, especially at locations within one mean free path from the leading edge.
Alpha effect due to buoyancy instability of a magnetic layer
Chatterjee, Piyali; Rheinhardt, Matthias; Brandenburg, Axel
2010-01-01
A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation. We calculate the alpha effect due to helical motions caused by a disintegrating magnetic layer in a rotating density-stratified system with angular velocity Omega making an angle theta with the vertical. We also study the dependence of the alpha effect on theta and the strength of the initial magnetic field. We carry out three-dimensional hydromagnetic simulations in Cartesian geometry. A turbulent EMF due to the correlations of the small scale velocity and magnetic field is generated. We use the test-field method to calculate the transport coefficients of the inhomogeneous turbulence produced by the layer. We show that the growth rate of the instability and the twist of the magnetic field vary monotonically with the ratio of thermal conductivity to magnetic diffusivity. The resulting alpha effect is inho...
Alpha effect due to buoyancy instability of a magnetic layer
Chatterjee, P.; Mitra, D.; Rheinhardt, M.; Brandenburg, A.
2011-10-01
Context. A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation. Aims: We calculate the α effect due to helical motions caused by an unstable magnetic layer in a rotating density-stratified system with angular velocity Ω making an angle θ with the vertical. We also study the dependence of the α effect on θ and the strength of the initial magnetic field. Methods: We carry out three-dimensional hydromagnetic simulations in Cartesian geometry. A turbulent electromotive force (EMF) due to the correlations of the small scale velocity and magnetic field is generated. We use the test-field method to calculate the transport coefficients of the inhomogeneous turbulence produced by the layer. Results: We show that the growth rate of the instability and the twist of the magnetic field vary monotonically with the ratio of thermal conductivity to magnetic diffusivity. The resulting α effect is non-uniform and increases with the strength of the initial magnetic field. It is thus an example of an "anti-quenched" α effect. The α effect is also nonlocal, i.e. scale dependent, requiring around 8-16 Fourier modes to reconstruct the actual mean EMF based on the actual mean field.
Crosshatch roughness distortions on a hypersonic turbulent boundary layer
Peltier, S. J.; Humble, R. A.; Bowersox, R. D. W.
2016-04-01
The effects of periodic crosshatch roughness (k+ = 160) on a Mach 4.9 turbulent boundary layer (Reθ = 63 000) are examined using particle image velocimetry. The roughness elements generate a series of alternating shock and expansion waves, which span the entire boundary layer, causing significant (up to +50% and -30%) variations in the Reynolds shear stress field. Evidence of the hairpin vortex organization of incompressible flows is found in the comparative smooth-wall boundary layer case (Reθ = 47 000), and can be used to explain several observations regarding the rough-wall vortex organization. In general, the rough-wall boundary layer near-wall vortices no longer appear to be well-organized into streamwise-aligned packets that straddle relatively low-speed regions like their smooth-wall counterpart; instead, they lean farther away from the wall, become more spatially compact, and their populations become altered. In the lower half of the boundary layer, the net vortex swirling strength and outer-scaled Reynolds stresses increase relative to the smooth-wall case, and actually decrease in the outer half of the boundary layer, as ejection and entrainment processes are strengthened and weakened in these two regions, respectively. A spectral analysis of the data suggests a relative homogenizing of the most energetic scales near Λ = ˜ 0.5δ across the rough-wall boundary layer.
Direct Numerical Simulation of Transition in a Swept-Wing Boundary Layer
Duan, Lian; Choudhari, Meelan M.; Li, Fei
2013-01-01
Direct numerical simulation (DNS) is performed to examine laminar to turbulent transition due to high-frequency secondary instability of stationary crossflow vortices in a subsonic swept-wing boundary layer for a realistic natural-laminar-flow airfoil configuration. The secondary instability is introduced via inflow forcing derived from a two-dimensional, partial-differential-equation based eigenvalue computation; and the mode selected for forcing corresponds to the most amplified secondary instability mode which, in this case, derives a majority of its growth from energy production mechanisms associated with the wall-normal shear of the stationary basic state. Both the growth of the secondary instability wave and the resulting onset of laminar-turbulent transition are captured within the DNS computations. The growth of the secondary instability wave in the DNS solution compares well with linear secondary instability theory when the amplitude is small; the linear growth is followed by a region of reduced growth resulting from nonlinear effects before an explosive onset of laminar breakdown to turbulence. The peak fluctuations are concentrated near the boundary layer edge during the initial stage of transition, but rapidly propagates towards the surface during the process of laminar breakdown. Both time-averaged statistics and flow visualization based on the DNS reveal a sawtooth transition pattern that is analogous to previously documented surface flow visualizations of transition due to stationary crossflow instability. The memory of the stationary crossflow vortex is found to persist through the transition zone and well beyond the location of the maximum skin friction.
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.
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.
Stable Boundary Layer Education (STABLE) Final Campaign Summary
Energy Technology Data Exchange (ETDEWEB)
Turner, David D. [Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
2016-03-01
The properties of, and the processes that occur in, the nocturnal stable boundary layer are not well understood, making it difficult to represent adequately in numerical models. The nocturnal boundary layer often is characterized by a temperature inversion and, in the Southern Great Plains region, a low-level jet. To advance our understanding of the nocturnal stable boundary layer, high temporal and vertical resolution data on the temperature and wind properties are needed, along with both large-eddy simulation and cloud-resolving modeling.
Linear Stability of the boundary layer under a solitary wave
Verschaeve, Joris C. G.; Pedersen, Geir K.
2013-01-01
A theoretical and numerical analysis of the linear stability of the boundary layer flow under a solitary wave is presented. In the present work, the nonlinear boundary layer equations are solved. The result is compared to the linear boundary layer solution in Liu et al. (2007) reveal- ing that both profiles are disagreeing more than has been found before. A change of frame of reference has been used to allow for a classical linear stability analysis without the need to redefine the notion of ...
Coupled wake boundary layer model of wind-farms
Stevens, Richard J. A. M.; Gayme, Dennice F.; Meneveau, Charles
2014-01-01
We present and test the coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a wind-farm. The model couples the traditional, industry-standard wake model approach with a "top-down" model for the overall wind-farm boundary layer structure. This wake model captures the effect of turbine positioning, while the "top-down" portion of the model adds the interactions between the wind-turbine wakes and the atmospheric boundary layer. Each portion of the mode...
Reactive boundary layers in metallic rolling contacts
International Nuclear Information System (INIS)
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
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
Otto, A.; Fairfield, D. H.
2000-09-01
On March 24, 1995, the Geotail spacecraft observed large fluctuations of the magnetic field and plasma properties in the low-latitude boundary layer about 15 RE tailward of the dusk meridian. Although the magnetospheric and magnetosheath magnetic fields were strongly northward, the Bz component showed strong short-duration fluctuations in which Bz could even reach negative values. We have used two-dimensional magnetohydrodynamic simulations with magnetospheric and magnetosheath input parameters specifically chosen for this Geotail event to identify the processes which cause the observed boundary properties. It is shown that these fluctuations can be explained by the Kelvin-Helmholtz instability if the k vector of the instability has a component along the magnetic field direction. The simulation results show many of the characteristic properties of the Geotail observations. In particular, the quasi-periodic strong fluctuations are well explained by satellite crossings through the Kelvin-Helmholtz vortices. It is illustrated how the interior structure of the Kelvin-Helmholtz vortices leads to the rapid fluctuations in the Geotail observations. Our results suggest an average Kelvin-Helmholtz wavelength of about 5 RE, with a vortex size of close to 2 RE for an average repetition time of 2.5 min. The growth time for these waves implies a source region of about 10-16 RE upstream from the location of the Geotail spacecraft (i.e., near the dusk meridian). The results also indicate a considerable mass transport of magnetosheath material into the magnetosphere by magnetic reconnection in the Kelvin-Helmholtz vortices.
Instability of the ferrofluid layer on a magnetizable substrate in a perpendicular magnetic field
Zakinyan, Arthur; Mkrtchyan, Levon
2012-01-01
The paper presents an experimental study of the instability of a magnetic fluid layer of finite thickness covering a magnetizable metal plate exposed to a perpendicular magnetic field. The critical field strength and the instability wave number have been measured.
Internal instability as a possible failure mechanism for layered composites.
Guz, I A; Menshykova, M; Soutis, C
2016-07-13
This paper revisits a three-dimensional analytical approach to study internal instability in layered composites, when the behaviour of each component of the material is described by the three-dimensional equations of solid mechanics. It shows the development of a unified computational procedure for numerical realization of the three-dimensional analytical method as applied to various constitutive equations of the layers and fibres, and different loading schemes (uniaxial or biaxial loading). The paper also contains many examples of calculation of critical controlled parameters for particular composites as well as analysis of different buckling modes. The results of this method can be used as a benchmark for simplified models. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'. PMID:27242292
DNS of laminar-turbulent boundary layer transition induced by solid obstacles
Orlandi, Paolo; Bernardini, Matteo
2015-01-01
Results of numerical simulations obtained by a staggered finite difference scheme together with an efficient immersed boundary method are presented to understand the effects of the shape of three-dimensional obstacles on the transition of a boundary layer from a laminar to a turbulent regime. Fully resolved Direct Numerical Simulations (DNS), highlight that the closer to the obstacle the symmetry is disrupted the smaller is the transitional Reynolds number. It has been also found that the transition can not be related to the critical roughness Reynolds number used in the past. The simulations highlight the differences between wake and inflectional instabilities, proving that two-dimensional tripping devices are more efficient in promoting the transition. Simulations at high Reynolds number demonstrate that the reproduction of a real experiment with a solid obstacle at the inlet is an efficient tool to generate numerical data bases for understanding the physics of boundary layers. The quality of the numerical ...
Thermal Marangoni instability and magnetic pressure for a thin ferrofluid layer.
Hennenberg, Marcel; Slavtchev, Slavtcho; Weyssow, Boris
2009-04-01
We study the linear coupling between the Marangoni and Cowley-Rosensweig instabilities for a thin layer of ferrofluid subjected to a temperature gradient and a magnetic field. Both are perpendicular to the reference horizontal boundaries, one of which is a rigid plate, while the other is a free surface remaining flat as long as the magnetic field is smaller than the critical value of the onset of the static isothermal Cowley-Rosensweig instability. Our study considers at first a ferrofluid layer resting on the rigid border. In the stationary case, when heating is directed from the rigid side, a magnetic field, smaller than the Cowley-Rosensweig critical one, can induce a new pattern: the critical Marangoni number is much lower than in the nonmagnetic undeformable case, for a dimensionless wavenumber of O(square root Bo) less than 1.992, its Newtonian classical value. When heating from the gaseous phase, an oscillatory marginal case exists theoretically, but for unphysical conditions. We consider also the case when the ferrofluid is hanging down from the rigid side. Only the wavelength critical value of the Rayleigh-Taylor instability that separates a stable region from an unstable one changes. PMID:19426330
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) ...
On Cauchy conditions for asymmetric mixed convection boundary layer flows
Energy Technology Data Exchange (ETDEWEB)
Amaouche, Mustapha [Laboratoire de Physique Theorique, Universite de Bejaia (Algeria); Kessal, Mohand [Departement Transport et Equipement Petrolier, Faculte des Hydrocarbures et de la Chimie, Universite de Boumerdes, 35000, Boumerdes (Algeria)
2003-06-01
The fundamental question of how and where does an asymmetric mixed convection boundary layer flow around a heated horizontal circular cylinder begin to develop is raised. We first transform the classical boundary layer equations by using an integral method of Karman-Pohlhausen type and obtain two coupled equations governing the evolutions of the dynamic and thermal boundary layers. Because of its global character, the implemented method allows to bypass the difficulty of downstream-upstream interactions. Cauchy conditions characterizing the starting of the boundary layers are found; they are obtained in a surprisingly simple manner for the limiting cases corresponding to Pr=1, Pr{yields}0 and Pr{yields}{infinity}. Otherwise, these conditions can be found by using a prediction correction algorithm. Some numerical experiments are finally performed in order to illustrate the theory. (authors)
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......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...
The Generalized Energy Equation and Instability in the Two-layer Barotropic Vortex
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
The linear two-layer barotropic primitive equations in cylindrical coordinates are used to derive a generalized energy equation, which is subsequently applied to explain the instability of the spiral wave in the model. In the two-layer model, there are not only the generalized barotropic instability and the super highspeed instability, but also some other new instabilities, which fall into the range of the Kelvin-Helmholtz instability and the generalized baroclinic instability, when the upper and lower basic flows are different.They are perhaps the mechanisms of the generation of spiral cloud bands in tropical cyclones as well.
Numerical simulation of turbulent atmospheric boundary layer flows
Energy Technology Data Exchange (ETDEWEB)
Bennes, L.; Bodnar, T.; Kozel, K.; Sladek, I. [Czech Technical Univ., Prague (Czech Republic). Dept. of Technical Mathematics; Fraunie, P. [Universite Toulon et du Var, La Garde (France). Lab. de Sondages Electromagnetiques de l' Environment Terrestre
2001-07-01
The work deals with the numerical solution of viscous turbulent steady flows in the atmospheric boundary layer including pollution propagation. For its description we use two different mathematical models: - a model based on the Reynolds averaged Navier-Stokes equations for incompressible flows - a model based on a system of boundary layer equations. These systems are completed by two transport equations for the concentration of passive pollutants and the potential temperature in conservative form, respectively, and by an algebraic turbulence model. (orig.)
Theoretical investigation on shocklets in compressible boundary layers
Institute of Scientific and Technical Information of China (English)
袁湘江; 刘智勇; 沈洁; 李国良
2014-01-01
By the shock relationships, the wavy characteristics and the forming condi-tions of a shock wave are analyzed. The wavy characteristics of an Euler system are stud-ied theoretically. The present research focuses on the wavy characteristics of Tollmien-Schlichting (T-S) waves, the excitation conditions of shocklets in compressible boundary layers, and the viscous effect on shock. The possibility of existence of shocklets in the compressible boundary layer and the physical mechanism of formation are theoretically interpreted.
Tropical boundary layer equilibrium in the last ice age
Betts, Alan K.; Ridgway, W.
1992-01-01
A radiative-convective boundary layer model is used to assess the effect of changing sea surface temperature, pressure, wind speed, and the energy export from the tropics on the boundary layer equilibrium equivalent potential temperature. It remains difficult to reconcile the observations that during the last glacial maximum (18,000 yr BP) the snowline on the tropical mountains fell 950 m, while the tropical sea surface temperatures fell only 1-2 K.
Analyzing transient instability phenomena beyond the classical stability boundary
Ali, Mahmoud; Glavic, Mevludin; Buisson, Jean; Wehenkel, Louis; Ernst, Damien
2008-01-01
We consider power systems for which the amount of power produced by their individual power plants is small with respect to the total generation of the system, and analyze how the transient instability mechanisms of these systems change qualitatively when their size or the dispersion of their generators increases. Simulation results show that loss of synchronism will propagate more slowly and even stop propagating. Given the evolution of power systems towards more dispersed generation and geog...
Dynamic Boundary Layer Properties in Turbulent Thermal Convection
Xia, Ke-Qing; Har Cheung, Yin; Sun, Chao
2004-11-01
We report an experimental study on the properties of the velocity and temperature boundary layers in turbulent thermal convection in a rectangular-shaped box over a range of Rayleigh numbers and at a constant Prandtl number. Velocity components both parallel and perpendicular to the conducting plate are measured simultaneously using the PIV technique. Our results show that, for the given geometry of the cell, the velocity boundary layer at the conduction plate is of a Blasius type, i.e. the boundary layer thickness δv scales with the Reynolds number Re as δv ˜ Re-1/2. The measurement further reveals that, at the velocity boundary layer, the turbulent (Reynolds) shear tress becomes larger than the viscous shear stress when Ra reaches 1-2×10^10, indicating that the boundary layer becomes turbulent for Ra >10^10. The viscous dissipation rate calculated based on the measured velocity field shows that it is dominated by contribution from the bulk over that from the boundary layer.
Structure and Growth of the Marine Boundary Layer
Mccumber, M.
1984-01-01
LANDSAT visible imagery and a one-dimensional Lagrangian boundary layer model were used to hypothesize the nature and the development of the marine boundary layer during a winter episode of strong seaward cold air advection. Over-water heating and moistening of the cold, dry continental air is estimable from linear relations involving horizontal gradients of the near-surface air temperature and humidity. A line of enhanced convection paralleling the Atlantic U.S. coast from south of New York Bay to the vicinity of Virginia Beach, VA was attributed to stronger convergence at low levels. This feature was characterized as a mesoscale front. With the assistance of a three-dimensional mesoscale boundary layer model, initialized with data obtained from the MASEX, the marine boundary layer can be mapped over the entire Atlantic coastal domain and the evolution of the boundary layer can be studied as a function of different characteristics of important surface level forcings. The effects on boundary layer growth due to the magnitude and pattern of sea surface temperature, to the shape of the coastline, and to atmospheric conditions, such as the orientation of the prevailing wind are examined.
Dense gas boundary layer experiments: Visualization, pressure measurements, concentration evaluation
Energy Technology Data Exchange (ETDEWEB)
Reichenbach, H.; Neuwald, P. [Ernst-Mach-Institut, Freiburg (DE); Kuhl, A.L. [R and D Associates, Los Angeles, CA (United States)
1992-11-01
This technical report describes methods that were applied to investigate turbulent boundary layers generated by inviscid, baroclinic effects. The Cranz-Schardin 24-sparks camera was used to visualize the interactions of a planar shock wave with a Freon R12-layer. The shock propagates more slowly in the Freon layer than in air because of its smaller sound speed. This causes the shock front to be curved and to be reflected between the wall and the layer interface. As a consequence of the reflection process, a series of compression and expansion waves radiate from the layer. Large fluctuations in the streamwise velocity and in pressure develop for about 1 ms. These waves strongly perturb the interface shear layer, which rapidly transitions to a turbulent boundary flow. Pressure measurements showed that the fluctuations in the Freon layer reach a peak pressure 4 times higher than in the turbulent boundary flow. To characterize the preshock Freon boundary layer, concentration measurements were performed with a differential interferometry technique. The refraction index of Freon R12 is so high that Mach-Zehnder interferometry was not successful in these experiments. The evaluation of the concentration profile is described here in detail. Method and results of corresponding LDV measurements under the same conditions are presented in a different report, EMI Report T 9/92. The authors plan to continue the dense gas layer investigations with the gas combination helium/Freon.
Boundary Layer Flow Over a Moving Wavy Surface
Hendin, Gali; Toledo, Yaron
2016-04-01
Boundary Layer Flow Over a Moving Wavy Surface Gali Hendin(1), Yaron Toledo(1) January 13, 2016 (1)School of Mechanical Engineering, Tel-Aviv University, Israel Understanding the boundary layer flow over surface gravity waves is of great importance as various atmosphere-ocean processes are essentially coupled through these waves. Nevertheless, there are still significant gaps in our understanding of this complex flow behaviour. The present work investigates the fundamentals of the boundary layer air flow over progressive, small-amplitude waves. It aims to extend the well-known Blasius solution for a boundary layer over a flat plate to one over a moving wavy surface. The current analysis pro- claims the importance of the small curvature and the time-dependency as second order effects, with a meaningful impact on the similarity pattern in the first order. The air flow over the ocean surface is modelled using an outer, inviscid half-infinite flow, overlaying the viscous boundary layer above the wavy surface. The assumption of a uniform flow in the outer layer, used in former studies, is now replaced with a precise analytical solution of the potential flow over a moving wavy surface with a known celerity, wavelength and amplitude. This results in a conceptual change from former models as it shows that the pressure variations within the boundary layer cannot be neglected. In the boundary layer, time-dependent Navier-Stokes equations are formulated in a curvilinear, orthogonal coordinate system. The formulation is done in an elaborate way that presents additional, formerly neglected first-order effects, resulting from the time-varying coordinate system. The suggested time-dependent curvilinear orthogonal coordinate system introduces a platform that can also support the formulation of turbulent problems for any surface shape. In order to produce a self-similar Blasius-type solution, a small wave-steepness is assumed and a perturbation method is applied. Consequently, a
The effect of boundaries on the ion acoustic beam-plasma instability in experiment and simulation
Energy Technology Data Exchange (ETDEWEB)
Rapson, Christopher, E-mail: chris.rapson@ipp.mpg.de [Max Planck Institute for Plasma Physics, Boltzmannstr. 2, D-85748 Garching (Germany); Grulke, Olaf [Max Planck Institute for Plasma Physics, Wendelsteinstr. 1, 17491 Greifswald (Germany); Matyash, Konstantin [Institut für Physik, Ernst-Moritz-Arndt Universität, Domstr. 10a, 17489 Greifswald (Germany); Klinger, Thomas [Max Planck Institute for Plasma Physics, Wendelsteinstr. 1, 17491 Greifswald (Germany); Institut für Physik, Ernst-Moritz-Arndt Universität, Domstr. 10a, 17489 Greifswald (Germany)
2014-05-15
The ion acoustic beam-plasma instability is known to excite strong solitary waves near the Earth's bow shock. Using a double plasma experiment, tightly coupled with a 1-dimensional particle-in-cell simulation, the results presented here show that this instability is critically sensitive to the experimental conditions. Boundary effects, which do not have any counterpart in space or in most simulations, unavoidably excite parasitic instabilities. Potential fluctuations from these instabilities lead to an increase of the beam temperature which reduces the growth rate such that non-linear effects leading to solitary waves are less likely to be observed. Furthermore, the increased temperature modifies the range of beam velocities for which an ion acoustic beam plasma instability is observed.
Stability analysis of Boundary Layer in Poiseuille Flow Through A Modified Orr-Sommerfeld Equation
Monwanou, A V; Orou, J B Chabi; 10.5539/apr.v4n4p138
2013-01-01
For applications regarding transition prediction, wing design and control of boundary layers, the fundamental understanding of disturbance growth in the flat-plate boundary layer is an important issue. In the present work we investigate the stability of boundary layer in Poiseuille flow. We normalize pressure and time by inertial and viscous effects. The disturbances are taken to be periodic in the spanwise direction and time. We present a set of linear governing equations for the parabolic evolution of wavelike disturbances. Then, we derive modified Orr-Sommerfeld equations that can be applied in the layer. Contrary to what one might think, we find that Squire's theorem is not applicable for the boundary layer. We find also that normalization by inertial or viscous effects leads to the same order of stability or instability. For the 2D disturbances flow ($\\theta=0$), we found the same critical Reynolds number for our two normalizations. This value coincides with the one we know for neutral stability of the k...
THE INSTABILITY OF THE DIFFUSION-CONTROLLED GRAIN-BOUNDARY VOID IN STRESSED SOLID
Institute of Scientific and Technical Information of China (English)
王华; 李中华
2003-01-01
As atoms migrate along a void surface and grain-boundary, driven by various thermodynamic forces, the grain-boundary void changes its shape and volume. When the void changes its configuration, the free energy of the system also changes. In this article, the free energy is calculated for an evolving grain-boundary void filled with gas in a stressed solid. Then the instability conditions and the equilibrium shape of the void are determined as a function of the grain-boundary and surface energies, the void volume, the externally applied stresses, as well as the internal pressure built up by the gas filled in the void.
Three dimensional chaotic advection by mixed layer baroclinic instabilities
Mukiibi, Daniel; Serra, Nuno
2015-01-01
Three dimensional (3D) Finite Time Lyapunov Exponents (FTLEs) are computed from numerical simulations of a freely evolving mixed layer (ML) front in a zonal channel undergoing baroclinic instability. The 3D FTLEs show a complex structure, with features that are less defined than the two-dimensional (2D) FTLEs, suggesting that stirring is not confined to the edges of vortices and along filaments and posing significant consequences on mixing. The magnitude of the FTLEs is observed to be strongly determined by the vertical shear. A scaling law relating the local FTLEs and the nonlocal density contrast used to initialize the ML front is derived assuming thermal wind balance. The scaling law only converges to the values found from the simulations within the pycnocline, while it displays differences within the ML, where the instabilities show a large ageostrophic component. The probability distribution functions of 2D and 3D FTLEs are found to be non Gaussian at all depths. In the ML, the FTLEs wavenumber spectra d...
Plasma boundary layer and magnetopause layer of the earth's magnetosphere
Energy Technology Data Exchange (ETDEWEB)
Eastman, T.E.
1979-06-01
IMP 6 observations of the plasma boundary layer (PBL) and magnetopause layer (MPL) of the earth's magnetosphere indicate that plasma in the low-latitude portion of the PBL is supplied primarily by direct transport of magnetosheath plasma across the MPL and that this transport process is relatively widespread over the entire sunward magnetospheric boundary.
Beta limitation of matter-antimatter boundary layers
International Nuclear Information System (INIS)
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 102 seconds for boundary layers at ambiplasma particle densities in the range from 104 to 10-2 m-3, respectively. (author)
Khan, Basit A.; Stenchikov, Georgiy; Abualnaja, Yasser
2014-05-01
Thermodynamic structure of sea-breeze, its interaction with coastal mountains, desert plateau and desert convective boundary layer have been investigated in the middle region of the Red Sea around 25°N, at the Western coast of Saudi Arabia. Sea and land breeze is a common meteorological phenomenon in most of the coastal regions around the world. Sea-Breeze effects the local meteorology and cause changes in wind speed, direction, cloud cover and sometimes precipitation. The occurrence of sea-breeze, its intensity and landward propagation are important for wind energy resource assessment, load forecasting for existing wind farms, air pollution, marine and aviation applications. The thermally induced mesoscale circulation of sea breeze modifies the desert Planetary Boundary Layer (PBL) by forming Convective Internal Boundary Layer (CIBL), and propagates inland as a density current. The leading edge of the denser marine air rapidly moves inland undercutting the hot and dry desert air mass. The warm air lifts up along the frontal boundary and if contains enough moisture a band of clouds is formed along the sea breeze front (SBF). This study focuses on the thermodynamic structure of sea-breeze as it propagates over coastal rocky mountain range of Al-Sarawat, east of the Red Sea coast, and the desert plateau across the mountain range. Additional effects of topographical gaps such as Tokar gap on the dynamics of sea-land breezes have also been discussed. Interaction of SBF with the desert convective boundary layer provide extra lifting that could further enhance the convective instability along the frontal boundary. This study provides a detailed analysis of the thermodynamics of interaction of the SBF and convective internal boundary layer over the desert. Observational data from a buoy and meteorological stations have been utilized while The Advanced Research WRF (ARW) modeling system has been employed in real and 2D idealized configuration.
Marginally stable and turbulent boundary layers in low-curvature Taylor-Couette flow
Brauckmann, Hannes J
2016-01-01
Marginal stability arguments are used to describe the rotation-number dependence of torque in Taylor-Couette (TC) flow for radius ratios $\\eta \\geq 0.9$ and shear Reynolds number $Re_S=2\\times 10^4$. With an approximate representation of the mean profile by piecewise linear functions, characterized by the boundary-layer thicknesses at the inner and outer cylinder and the angular momentum in the center, profiles and torques are extracted from the requirement that the boundary layers represent marginally stable TC subsystems and that the torque at the inner and outer cylinder coincide. This model then explains the broad shoulder in the torque as a function of rotation number near $R_\\Omega\\approx 0.2$. For rotation numbers $R_\\Omega < 0.07$ the TC stability conditions predict boundary layers in which shear Reynolds numbers are very large. Assuming that the TC instability is bypassed by some shear instability, a second maximum in torque appears, in very good agreement with numerical simulations. The results s...
Wave generation and particle transport in the plasma sheet and boundary layer. Semiannual report
International Nuclear Information System (INIS)
The one and two ion beam instability was considered as a possible explanation for the observations of broadband electrostatic noise in the plasma sheet region of the geomagnetic tail. When only hot streaming plasma sheet boundary layer ions were present, no broadband waves were excited. Cold, streaming ionospheric ions can generate electrostatic broadband waves propagating in the slow beam-acoustic mode, but the growth rates of the waves were significantly enhanced when warm boundary layer ions were presented. (Both the slow and fast beam-acoustic modes can be excited, depending on the relative ion drift.) This mode predicted that the wave intensity of the broadband noise should peak in the plasma sheet boundary layer (PSBL). Observations of less intense electrostatic waves in the lobes and plasma sheet were likely a result of the absence of warm ion beams or large ion temperatures, respectively, which resulted in smaller growth rates. The model dependence of the ion beam instability has also been studied. For cold and warm ions streaming in the same direction, researchers found wave growth peaked for wave normal angles theta = 0 deg. and wave frequencies approx. 0.1 x the electron plasma frequency. However, for anti-parallel streaming cold and warm ions, wave growth peaks near theta = 90 deg. and wave frequencies were an order of magnitude smaller
Wave generation and particle transport in the plasma sheet and boundary layer
Dusenbery, P. B.
1987-01-01
The one and two ion beam instability was considered as a possible explanation for the observations of broadband electrostatic noise in the plasma sheet region of the geomagnetic tail. When only hot streaming plasma sheet boundary layer ions were present, no broadband waves were excited. Cold, streaming ionospheric ions can generate electrostatic broadband waves propagating in the slow beam-acoustic mode, but the growth rates of the waves were significantly enhanced when warm boundary layer ions were presented. (Both the slow and fast beam-acoustic modes can be excited, depending on the relative ion drift.) This mode predicted that the wave intensity of the broadband noise should peak in the plasma sheet boundary layer (PSBL). Observations of less intense electrostatic waves in the lobes and plasma sheet were likely a result of the absence of warm ion beams or large ion temperatures, respectively, which resulted in smaller growth rates. The model dependence of the ion beam instability has also been studied. For cold and warm ions streaming in the same direction, researchers found wave growth peaked for wave normal angles theta = 0 deg. and wave frequencies approx. 0.1 x the electron plasma frequency. However, for anti-parallel streaming cold and warm ions, wave growth peaks near theta = 90 deg. and wave frequencies were an order of magnitude smaller.
Convective instabilities in two superposed horizontal liquid layers heated laterally
Madruga, S; Lebon, G
2003-01-01
This work is devoted to the theoretical study of the stability of two superposed horizontal liquid layers bounded by two solid planes and subjected to a horizontal temperature gradient. The liquids are supposed to be immiscible with a nondeformable interface. The forces acting on the system are buoyancy and interfacial tension. Four different flow patterns and temperature profiles are found for the basic state. A linear perturbative analysis with respect to two and three dimensional perturbations reveals the existence of three kind of patterns. Depending on the relative height of both liquids several situations are predicted: either wave propagation from cold to the hot regions, or waves propagating in the opposite direction or still stationary longitudinal rolls. The behavior of three different pairs of liquids which have been used in experiments on bilayers under vertical gradient by other authors have been examined. The instability mechanisms are discussed and a qualitative interpretation of the different ...
Instability of thermocapillary liquid layers for Oldroyd-B fluid
Hu, Kai-Xin; He, Meng; Chen, Qi-Sheng
2016-03-01
The linear stability analysis of Oldroyd-B fluid for thermocapillary liquid layers is carried out. Results are presented for linear flow and return flow with Prandtl numbers of 0.02 and 100. Three kinds of instabilities are found: oblique wave, streamwise wave, and spanwise stationary mode, whose properties are all significantly affected by elasticity. For the first, the critical Marangoni number increases with elasticity. For the second, the work done by perturbation stress fluctuates in vertical direction. The last becomes the preferred mode when the elasticity is high enough and its perturbation energy comes from the Marangoni force caused by perturbation temperature while dissipates by perturbation stress. Their mechanisms are discussed and the comparisons are made with Newtonian fluid.
Boundary-layer control by electric fields A feasibility study
Mendes, R V
1998-01-01
A problem of great concern in aviation and submarine propulsion is the control of the boundary layer and, in particular, the methods to extend the laminar region as a means to decrease noise and fuel consumption. In this paper we study the flow of air along an airfoil when a layer of ionized gas and a longitudinal electric field are created in the boundary layer region. By deriving scaling solutions and more accurate numerical solutions we discuss the possibility of achieving significant boundary layer control for realistic physical parameters. Practical design formulas and criteria are obtained. We also discuss the perspectives for active control of the laminar-to-turbulent transition fluctuations by electromagnetic field modulation.
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
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......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...... 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...
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.
Effects of upper disc boundary conditions on the linear Rossby wave instability
Lin, Min-Kai
2012-01-01
The linear Rossby wave instability (RWI) in global, 3D polytropic discs is revisited with a much simpler numerical method than that previously employed by the author. The governing partial differential equation is solved with finite differences in the radial direction and spectral collocation in the vertical direction. RWI modes are calculated subject to different upper disc boundary conditions. These include free surface, solid boundaries and variable vertical domain size. Boundary conditions that oppose vertical motion increase the instability growth rate by a few per cent. The magnitude of vertical flow throughout the fluid column can be affected but the overall flow pattern is qualitatively unchanged. Numerical results support the notion that the RWI is intrinsically two dimensional. This implies that inconsistent upper disc boundary conditions, such as vanishing enthalpy perturbation, may inhibit the RWI in 3D.
Nature, theory and modelling of geophysical convective planetary boundary layers
Zilitinkevich, Sergej
2015-04-01
Geophysical convective planetary boundary layers (CPBLs) are still poorly reproduced in oceanographic, hydrological and meteorological models. Besides the mean flow and usual shear-generated turbulence, CPBLs involve two types of motion disregarded in conventional theories: 'anarchy turbulence' comprised of the buoyancy-driven plumes, merging to form larger plumes instead of breaking down, as postulated in conventional theory (Zilitinkevich, 1973), large-scale organised structures fed by the potential energy of unstable stratification through inverse energy transfer in convective turbulence (and performing non-local transports irrespective of mean gradients of transporting properties). C-PBLs are strongly mixed and go on growing as long as the boundary layer remains unstable. Penetration of the mixed layer into the weakly turbulent, stably stratified free flow causes turbulent transports through the CPBL outer boundary. The proposed theory, taking into account the above listed features of CPBL, is based on the following recent developments: prognostic CPBL-depth equation in combination with diagnostic algorithm for turbulence fluxes at the CPBL inner and outer boundaries (Zilitinkevich, 1991, 2012, 2013; Zilitinkevich et al., 2006, 2012), deterministic model of self-organised convective structures combined with statistical turbulence-closure model of turbulence in the CPBL core (Zilitinkevich, 2013). It is demonstrated that the overall vertical transports are performed mostly by turbulence in the surface layer and entrainment layer (at the CPBL inner and outer boundaries) and mostly by organised structures in the CPBL core (Hellsten and Zilitinkevich, 2013). Principal difference between structural and turbulent mixing plays an important role in a number of practical problems: transport and dispersion of admixtures, microphysics of fogs and clouds, etc. The surface-layer turbulence in atmospheric and marine CPBLs is strongly enhanced by the velocity shears in
Highly buoyant bent-over plumes in a boundary layer
Tohidi, Ali; Kaye, Nigel B.
2016-04-01
Highly buoyant plumes, such as wildfire plumes, in low to moderate wind speeds have initial trajectories that are steeper than many industrial waste plumes. They will rise further into the atmosphere before bending significantly. In such cases the plume's trajectory will be influenced by the vertical variation in horizontal velocity of the atmospheric boundary layer. This paper examined the behavior of a plume in an unstratified environment with a power-law ambient velocity profile. Examination of previously published experimental measurements of plume trajectory show that inclusion of the boundary layer velocity profile in the plume model often provides better predictions of the plume trajectory compared to algebraic expressions developed for uniform flow plumes. However, there are many cases in which uniform velocity profile algebraic expressions are as good as boundary layer models. It is shown that it is only important to model the role of the atmospheric boundary layer velocity profile in cases where either the momentum length (square root of source momentum flux divided by the reference wind speed) or buoyancy length (buoyancy flux divided by the reference wind speed cubed) is significantly greater than the plume release height within the boundary layer. This criteria is rarely met with industrial waste plumes, but it is important in modeling wildfire plumes.
Bending Boundary Layers in Laminated-Composite Circular Cylindrical Shells
Nemeth, Michael P.; Smeltzer, Stanley S., III
2000-01-01
A study of the attenuation of bending boundary layers in balanced and unbalanced, symmetrically and unsymmetrically laminated cylindrical shells is presented for nine contemporary material systems. The analysis is based on the linear Sanders-Koiter shell equations and specializations to the Love-Kirchhoff shell equations and Donnell's equations are included. Two nondimensional parameters are identified that characterize the effects of laminate orthotropy and anisotropy on the bending boundary-layer decay length in a very general manner. A substantial number of structural design technology results are presented for a wide range of laminated-composite cylinders. For all laminates considered, the results show that the differences between results obtained with the Sanders-Koiter shell equations, the Love-Kirchhoff shell equations, and Donnell's equations are negligible. The results also show that the effect of anisotropy in the form of coupling between pure bending and twisting has a negligible effect on the size of the bending boundary-layer decay length of the balanced, symmetrically laminated cylinders considered. Moreover, the results show that coupling between the various types of shell anisotropies has a negligible effect on the calculation of the bending boundary-layer decay length in most cases. The results also show that, in some cases, neglecting the shell anisotropy results in underestimating the bending boundary-layer decay length and, in other cases, results in an overestimation.
Turbulent boundary-layer structure of flows over freshwater biofilms
Walker, J. M.; Sargison, J. E.; Henderson, A. D.
2013-12-01
The structure of the turbulent boundary-layer for flows over freshwater biofilms dominated by the diatom Tabellaria flocculosa was investigated. Biofilms were grown on large test plates under flow conditions in an Australian hydropower canal for periods up to 12 months. Velocity-profile measurements were obtained using LDV in a recirculating water tunnel for biofouled, smooth and artificially sandgrain roughened surfaces over a momentum thickness Reynolds number range of 3,000-8,000. Significant increases in skin friction coefficient of up to 160 % were measured over smooth-wall values. The effective roughnesses of the biofilms, k s, were significantly higher than their physical roughness measured using novel photogrammetry techniques and consisted of the physical roughness and a component due to the vibration of the biofilm mat. The biofilms displayed a k-type roughness function, and a logarithmic relationship was found between the roughness function and roughness Reynolds number based on the maximum peak-to-valley height of the biofilm, R t. The structure of the boundary layer adhered to Townsend's wall-similarity hypothesis even though the scale separation between the effective roughness height and the boundary-layer thickness was small. The biofouled velocity-defect profiles collapsed with smooth and sandgrain profiles in the outer region of the boundary layer. The Reynolds stresses and quadrant analysis also collapsed in the outer region of the boundary layer.
The inner core thermodynamics of the tropical cyclone boundary layer
Williams, Gabriel J.
2016-02-01
Although considerable progress has been made in understanding the inner-core dynamics of the tropical cyclone boundary layer (TCBL), our knowledge of the inner-core thermodynamics of the TCBL remains limited. In this study, the inner-core budgets of potential temperature (θ ), specific humidity (q), and reversible equivalent potential temperature (θ _e ) are examined using a high-resolution multilevel boundary layer model. The potential temperature budgets show that the heat energy is dominated by latent heat release in the eyewall, evaporative cooling along the outer edge of the eyewall, and upward surface fluxes of sensible and latent heat from the underlying warm ocean. It is shown that the vertical θ advection overcompensates the sum of radial advective warming from the boundary layer outflow jet and latent heating for the development of cooling in the eyewall within the TCBL. The moisture budgets show the dominant upward transport of moisture in the eyewall updrafts, partly by the boundary-layer outflow jet from the bottom eye region, so that the eyewall remains nearly saturated. The θ _e budgets reveal that the TCBL is maintained thermodynamically by the upward surface flux of higher-θ _e air from the underlying warm ocean, the radial transport of low-θ _e air from the outer regions of the TCBL, and the dry adiabatic cooling associated by eyewall updrafts. These results underscore the significance of vertical motion and the location of the boundary layer outflow jet in maintaining the inner core thermal structure of the TCBL.
Boundary Layer Transition in the Leading Edge Region of a Swept Cylinder in High Speed Flow
Coleman, Colin P.
1998-01-01
Experiments were conducted on a 76 degree swept cylinder to establish the behavior of the attachment line transition process in a low-disturbance level, Mach number 1.6 flow. For a near adiabatic wall condition, the attachment-line boundary layer remained laminar up to the highest attainable Reynolds number. The attachment-line boundary layer transition under the influence of trip wires depended on wind tunnel disturbance level, and a transition onset condition for this flow is established. Internal heating raised the surface temperature of the attachment line to induce boundary layer instabilities. This was demonstrated experimentally for the first time and the frequencies of the most amplified disturbances were determined over a range of temperature settings. Results were in excellent agreement to those predicted by a linear stability code, and provide the first experimental verification of theory. Transition onset along the heated attachment line at an R-bar of 800 under quiet tunnel conditions was found to correlate with an N factor of 13.2. Increased tunnel disturbance levels caused the transition onset to occur at lower cylinder surface temperatures and was found to correlate with an approximate N factor of 1 1.9, so demonstrating that the attachment-line boundary layer is receptive to increases in the tunnel disturbance level.
The effect of a shear boundary layer on the stability of a capillary jet
Ganan-Calvo, Alfonso; Montanero, Jose M.; Herrada, Miguel A.
2014-11-01
The generic stabilization effect of a shear boundary layer over the free surface of a capillary jet is here studied from analytical (asymptotic), numerical and experimental approaches. In first place, we show the consistency of the proposed asymptotic analysis by a linear stability (numerical) analysis of the Navier-Stokes equations for a finite boundary layer thickness. We show how the convective-to-absolute instability transition departs drastically from the flat velocity profile case as the axial coordinate becomes closer to the origin of the boundary layer development. For large enough axial distances from that origin, Rayleigh's dispersion relation is recovered. A collection of experimental observations is analyzed from the perspective provided by these results. We propose a systematic framework to the dynamics of capillary jets issued from a nozzle, either by direct injection into a quiescent atmosphere or in a co-flow (e.g. gas flow-focused jets), which exhibit peculiarities now definitely attributable in first order to the formation of shear boundary layers. Partial support from the Ministry of Economy and Competitiveness, Junta de Extremadura, and Junta de Andalucia (Spain) through Grant Nos. DPI2010-21103, GR10047, P08-TEP-04128, and TEP-7465, respectively, is gratefully acknowledged.
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.
Experimental observation of electrical instability of droplets on dielectric layer
International Nuclear Information System (INIS)
Polydimethylsiloxane (PDMS) has become the most widely used silicon-based organic polymer in bio-MEMS/NEMS devices. However, the inherent hydrophobic nature of PDMS hinders its wide applications in bio-MEMS/NEMS for efficient transport of liquids. Electrowetting is a useful tool to reduce the apparent contact angle of partially wetting conductive liquids and has been utilized widely in bio-MEMS/NEMS. Our experimental results show that the thin PDMS membranes exhibit good properties in electrowetting-on-dielectric. The electrical instability phenomenon of droplets was observed in our experiment. The sessile droplet lying on the PDMS membrane will lose its stability with the touch of the wire electrode to make the apparent contact angle change suddenly larger than 35 deg. Contact mode can protect the dielectric layer from electrical breakdown effectively. Electrical breakdown process of dielectric layer was recorded by a high speed camera. It is found experimentally that a PDMS membrane of 4.8 μm thick will not be destroyed due to the electric breakdown even at 800 V in the contact mode. (fast track communication)
Angular momentum transport in accretion disk boundary layers around weakly magnetized stars
DEFF Research Database (Denmark)
Pessah, M.E.; Chan, C.-K.
2013-01-01
The standard model for turbulent shear viscosity in accretion disks is based on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. This implies that the turbulent stress must be negative and thus transport angular momentum inwards, in...... the boundary layer where the accretion disk meets the surface of a weakly magnetized star. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability...... (MRI) is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. Motivated by the need of a deeper understanding of the behavior of an MHD fluid in a differentially rotating background that deviates from a Keplerian profile, we study the dynamics...
On the Asymptotic Approach to Thermosolutal Convection in Heated Slow Reactive Boundary Layer Flows
Directory of Open Access Journals (Sweden)
Stanford Shateyi
2008-01-01
Full Text Available The study sought to investigate thermosolutal convection and stability of two dimensional disturbances imposed on a heated boundary layer flow over a semi-infinite horizontal plate composed of a chemical species using a self-consistent asymptotic method. The chemical species reacts as it diffuses into the nearby fluid causing density stratification and inducing a buoyancy force. The existence of significant temperature gradients near the plate surface results in additional buoyancy and decrease in viscosity. We derive the linear neutral results by analyzing asymptotically the multideck structure of the perturbed flow in the limit of large Reynolds numbers. The study shows that for small Damkohler numbers, increasing buoyancy has a destabilizing effect on the upper branch Tollmien-Schlichting (TS instability waves. Similarly, increasing the Damkohler numbers (which corresponds to increasing the reaction rate has a destabilizing effect on the TS wave modes. However, for small Damkohler numbers, negative buoyancy stabilizes the boundary layer flow.
Behaviour of tracer diffusion in simple atmospheric boundary layer models
Directory of Open Access Journals (Sweden)
P. S. Anderson
2007-10-01
Full Text Available 1-D profiles and time series from an idealised atmospheric boundary layer model are presented, which show agreement with boundary layer measurements of polar NO_{x}. Diffusion models are increasingly being used as the framework for studying tropospheric air chemistry dynamics. Models based on standard boundary layer diffusivity profiles have an intrinsic behaviour that is not necessarily intuitive, due to the variation of turbulent diffusivity with height. The simple model presented captures the essence of the evolution of a trace gas released at the surface, and thereby provides both a programming and a conceptual tool in the analysis of observed trace gas evolution. A time scale inherent in the model can be tuned by fitting model time series to observations. This scale is then applicable to the more physically simple but chemically complex zeroth order or box models of chemical interactions.
Particle motion inside Ekman and Bödewadt boundary layers
Duran Matute, Matias; van der Linden, Steven; van Heijst, Gertjan
2014-11-01
We present results from both laboratory experiments and numerical simulations of the motion of heavy particles inside Ekman and Bödewadt boundary layers. The particles are initially at rest on the bottom of a rotating cylinder filled with water and with its axis parallel to the axis of rotation. The particles are set into motion by suddenly diminishing the rotation rate and the subsequent creation of a swirl flow with the boundary layer above the bottom plate. We consider both spherical and non-spherical particles with their size of the same order as the boundary layer thickness. It was found that the particle trajectories define a clear logarithmic spiral with its shape depending on the different parameters of the problem. Numerical simulations show good agreement with experiments and help explain the motion of the particles. This research is funded by NWO (the Netherlands) through the VENI Grant 863.13.022.
Theoretical skin-friction law in a turbulent boundary layer
International Nuclear Information System (INIS)
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 cfmax=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
Large Eddy Simulation of the ventilated wave boundary layer
DEFF Research Database (Denmark)
Lohmann, Iris P.; Fredsøe, Jørgen; Sumer, B. Mutlu; Christensen, Erik Damgaard
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...... overall (local) grid size. The results indicate that the large eddies develop in the resolved scale, corresponding to fluid with an effective viscosity decided by the sum of the kinematic and subgrid viscosity. Regarding case 2, the results are qualitatively in accordance with experimental findings....... Injection generally 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...
Localized travelling waves in the asymptotic suction boundary layer
Kreilos, Tobias; Schneider, Tobias M
2016-01-01
We present two spanwise-localized travelling wave solutions in the asymptotic suction boundary layer, obtained by continuation of solutions of plane Couette flow. One of the solutions has the vortical structures located close to the wall, similar to spanwise-localized edge states previously found for this system. The vortical structures of the second solution are located in the free stream far above the laminar boundary layer and are supported by a secondary shear gradient that is created by a large-scale low-speed streak. The dynamically relevant eigenmodes of this solution are concentrated in the free stream, and the departure into turbulence from this solution evolves in the free stream towards the walls. For invariant solutions in free-stream turbulence, this solution thus shows that that the source of energy of the vortical structures can be a dynamical structure of the solution itself, instead of the laminar boundary layer.
Bypass transition and spot nucleation in boundary layers
Kreilos, Tobias; Schlatter, Philipp; Duguet, Yohann; Henningson, Dan S; Eckhardt, Bruno
2016-01-01
The spatio-temporal aspects of the transition to turbulence are considered in the case of a boundary layer flow developing above a flat plate exposed to free-stream turbulence. Combining results on the receptivity to free-stream turbulence with the nonlinear concept of a transition threshold, a physically motivated model suggests a spatial distribution of spot nucleation events. To describe the evolution of turbulent spots a probabilistic cellular automaton is introduced, with all parameters directly fitted from numerical simulations of the boundary layer. The nucleation rates are then combined with the cellular automaton model, yielding excellent quantitative agreement with the statistical characteristics for different free-stream turbulence levels. We thus show how the recent theoretical progress on transitional wall-bounded flows can be extended to the much wider class of spatially developing boundary-layer flows.
Boundary layer for non-newtonian fluids on curved surfaces
International Nuclear Information System (INIS)
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)
Wind Tunnel Simulation of the Atmospheric Boundary Layer
Hohman, Tristen; Smits, Alexander; Martinelli, Luigi
2013-11-01
To simulate the interaction of large Vertical Axis Wind Turbines (VAWT) with the Atmospheric Boundary Layer (ABL) in the laboratory, we implement a variant of Counihan's technique [Counihan 1969] in which a combination of a castellated barrier, elliptical vortex generators, and floor roughness elements is used to create an artificial ABL profile in a standard closed loop wind tunnel. To examine the development and formation of the artificial ABL hotwire and SPIV measurements were taken at various downstream locations with changes in wall roughness, wall type, and vortex generator arrangements. It was found possible to generate a boundary layer at Reθ ~106 , with a mean velocity that followed the 1/7 power law of a neutral ABL over rural terrain and longitudinal turbulence intensities and power spectra that compare well with the data obtained for high Reynolds number flat plate turbulent boundary layers [Hultmark et al. 2010]. Supported by Hopewell Wind Power Ltd., and the Princeton Grand Challenges Program.
Coupled vs. decoupled boundary layers in VOCALS-REx
Directory of Open Access Journals (Sweden)
C. R. Jones
2011-03-01
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 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 150m 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 “well-mixed 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.
Numerical Modeling of the Evolving Stable Boundary Layer
Sorbjan, Z.
2013-12-01
A single-column model of the evolving stable boundary layer is tested for the consistency of turbulence parameterization, self-similar properties of the flow, and effects of ambient forcing. The turbulence closure of the model is based on the K-theory approach, with stability functions based on empirical data, and a semi-empirical form of the mixing length. The model has one internal, governing stability parameter, the Richardson number Ri, which dynamically adjusts to the boundary conditions and to external forcing. Model results, expressed in terms of local similarity scales, are universal functions of the Richardson number, i.e. they are satisfied in the entire stable boundary layer, for all instants of time, and all kinds of external forcing. Based on similarity expression, a realizability condition is derived for the minimum turbulent heat flux in the stable boundary layer. Numerical experiments show that the development of 'horse-shoe' shaped, 'fixed-elevation' wind hodographs in the interior of the stable boundary layer are solely caused by effects imposed by surface thermal forcing, and are not related to the inertial oscillation mechanism.
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. .
Lagrangian analysis of the laminar flat plate boundary layer
Gabr, Mohammad
2016-01-01
The leading edge flow properties has been a singularity to the Blasius laminar boundary layer equations, by applying the Lagrangian approach the leading edge velocity profiles of the laminar boundary layer over a flat plate are studied. Experimental observations as well as the theoretical analysis show an exact Gaussian distribution curve as the original starting profile of the laminar flow. Comparisons between the Blasius solution and the Gaussian curve solution are carried out providing a new insight into the physics of the laminar flow.
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
periods of analysis, that under both barotropic and baroclinic conditions, the model predicts the gradient and geostrophic wind well, explaining for a particular case an 'unusual' backing of the wind. The observed conditions at the surface, on the other hand, explain the differences in wind veering. The......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...... simulated winds underpredict the turning of the wind and the boundary-layer winds in general....
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.
Oscillations of the Boundary Layer and High-frequency QPOs
Directory of Open Access Journals (Sweden)
Blinova A. A.
2014-01-01
Full Text Available We observed persistent high-frequency oscillations of the boundary layer near an accreting, weakly-magnetized star in global 3D MHD simulations. The tilted dipole magnetic field is not strong enough to open a gap between the star and the disk. Instead, it forms a highly-wrapped azimuthal field near the surface of the star which slows down rotation of the disk matter, while a small tilt of the field excites oscillations of the boundary layer with a frequency below the Keplerian frequency. This mechanism may be responsible for the high-frequency oscillations in accreting neutron stars, white dwarfs and classical T Tauri stars.
Energy Technology Data Exchange (ETDEWEB)
Gauglitz, Phillip A.; Wells, Beric E.; Buchmiller, William C.; Rassat, Scot D.
2013-03-21
In Hanford underground waste storage tanks, a typical waste configuration is settled beds of waste particles beneath liquid layers. The settled beds are typically composed of layers, and these layers can have different physical and chemical properties. One postulated configuration within the settled bed is a less-dense layer beneath a more-dense layer. The different densities can be a result of different gas retention in the layers or different degrees of settling and compaction in the layers. This configuration can experience a Rayleigh-Taylor (RT) instability where the less dense lower layer rises into the upper layer. Previous studies of gas retention and release have not considered potential buoyant motion within a settle bed of solids. The purpose of this report is to provide a review of RT instabilities, discuss predictions of RT behavior for sediment layers, and summarize preliminary experimental observations of RT instabilities in simulant experiments.
Numerical simulation of quasi-streamwise hairpin-like vortex generation in turbulent boundary layer
Institute of Scientific and Technical Information of China (English)
ZHANG Nan; LU Li-peng; DUAN Zhen-zhen; YUAN Xiang-jiang
2008-01-01
A mechanism for generation of near wall quasi-streamwise hairpin-like vortex (QHV) and secondary quasi-streamwise vortices (SQV) is presented. The conceptual model of resonant triad in the theory of hydrodynamic instability and direct numerical simulation of a turbulent boundary layer were applied to reveal the formation of QHV and SQV. The generation procedures and the characteristics of the vortex structures are obtained, which share some similarities with previous numerical simulations. The research using resonant triad conceptual model and numerical simulation provides a possibility for investigating and controling the vortex structures, which play a dominant role in the evolution of coherent structures in the near-wall region.
Interaction between surface and atmosphere in a convective boundary layer /
Garai, Anirban
2013-01-01
Solar heating of the surface causes the near surface air to warm up and with sufficient buoyancy it ascends through the atmosphere as surface-layer plumes and thermals. The cold fluid from the upper part of the boundary layer descends as downdrafts. The downdrafts and thermals form streamwise roll vortices. All these turbulent coherent structures are important because they contribute most of the momentum and heat transport. While these structures have been studied in depth, their imprint on t...
Edge states for the turbulence transition in the asymptotic suction boundary layer
Kreilos, Tobias; Schneider, Tobias M; Eckhardt, Bruno
2013-01-01
We demonstrate the existence of an exact invariant solution to the Navier-Stokes equations for the asymptotic suction boundary layer. The identified periodic orbit with a very long period of several thousand advective time units is found as a local dynamical attractor embedded in the stability boundary between laminar and turbulent dynamics. Its dynamics captures both the interplay of downstream oriented vortex pairs and streaks observed in numerous shear flows as well as the energetic bursting that is characteristic for boundary layers. By embedding the flow into a family of flows that interpolates between plane Couette flow and the boundary layer we demonstrate that the periodic orbit emerges in a saddle-node infinite-period (SNIPER) bifurcation of two symmetry-related travelling wave solutions of plane Couette flow. Physically, the long period is due to a slow streak instability which leads to a violent breakup of a streak associated with the bursting and the reformation of the streak at a different spanwi...
Yao, Chengwei; Pal Arya, S.; Davis, Jerry; Main, Charles E.
Numerical solutions of the diffusion equation of Peronospora tabacina spores from a finite-area source over flat terrain in the evolving convective boundary layer are presented. Temporal variations in the release of spores, atmospheric stability, wind speed, and eddy diffusivity are considered. The model also includes the vertical variations of wind and eddy diffusivity. The model results indicate that ground level concentrations decrease with time as wind speed and eddy diffusivity increase in the evolving convective boundary layer. The loss of P. tabacina spores due to deposition at the surface also decrease with increasing instability and wind speed. Deposition is found to be particularly important close to the source area.
Boundary-Layer Wind Structure in a Landfalling Tropical Cyclone
Institute of Scientific and Technical Information of China (English)
无
2006-01-01
In this study, a slab boundary layer model with a constant depth is used to analyze the boundary-layer wind structure in a landfalling tropical cyclone. Asymmetry is found in both the tangential and radial components of horizontal wind in the tropical cyclone boundary layer at landfall. For a steady tropical cyclone on a straight coastline at landfall, the magnitude of the radial component is greater in the offshoreflow side and the tangential component is greater over the sea, slightly offshore, therefore the greater total wind speed occurs in the offshore-flow side over the sea. The budget analysis suggests that: (1) a greater surface friction over land produces a greater inflow and the nonlinear effect advects the maximum inflow downstream, and (2) a smaller surface friction over the sea makes the decrease of the tangential wind component less than that over land. Moreover, the boundary layer wind structures in a tropical cyclone are related to the locations of the tropical cyclone relative to the coastline due to the different surface frictions. During tropical cyclone landfall, the impact of rough terrain on the cyclone increases, so the magnitude of the radial component of wind speed increases in the offshore-flow side and the tangential component outside the radius of maximum wind speed decreases gradually.
Two Phases of Coherent Structure Motions in Turbulent Boundary Layer
Institute of Scientific and Technical Information of China (English)
LIU Jian-Hua; JIANG Nan
2007-01-01
Two phases of coherent structure motion are acquired after obtaining conditional phase-averaged waveforms for longitudinal velocity of coherent structures in turbulent boundary layer based on Harr wavelet transfer. The correspondences of the two phases to the two processes (i.e. ejection and sweep) during a burst are determined.
A parametric study of adverse pressure gradient turbulent boundary layers
International Nuclear Information System (INIS)
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.
Full-Scale Spectrum of Boundary-Layer Winds
DEFF Research Database (Denmark)
Larsén, Xiaoli Guo; Larsen, Søren Ejling; Lundtang Petersen, Erik
2016-01-01
Extensive mean meteorological data and high frequency sonic anemometer data from two sites in Denmark, one coastal onshore and one offshore, have been used to study the full-scale spectrum of boundary-layer winds, over frequencies f from about 1 yr−1 to10 Hz. 10-min cup anemometer data are used t...
Boundary Layer on a Moving Wall with Suction and Injection
Institute of Scientific and Technical Information of China (English)
Anuar Ishak; Roslinda Nazar; Ioan Pop
2007-01-01
@@ We investigate the boundary-layer flow on a moving permeable plate parallel to a moving stream. The governing equations are solved numerically by a finite-difference method. Dual solutions are found to exist when the plate and the free stream move in the opposite directions.
DNS of compressible turbulent boundary layer around a sharp cone
Institute of Scientific and Technical Information of China (English)
2008-01-01
Direct numerical simulation of the turbulent boundary layer over a sharp cone with 20° cone angle (or 10° half-cone angle) is performed by using the mixed seventh- order up-wind biased finite difference scheme and sixth-order central difference scheme. The free stream Mach number is 0.7 and free stream unit Reynolds number is 250000/inch. The characteristics of transition and turbulence of the sharp cone boundary layer are compared with those of the flat plate boundary layer. Statistics of fully developed turbulent flow agree well with the experimental and theoretical data for the turbulent flat-plate boundary layer flow. The near wall streak-like structure is shown and the average space between streaks (normalized by the local wall unit) keeps approximately invariable at different streamwise locations. The turbulent energy equation in the cylindrical coordinate is given and turbulent en-ergy budget is studied. The computed results show that the effect of circumferen-tial curvature on turbulence characteristics is not obvious.
DNS of compressible turbulent boundary layer around a sharp cone
Institute of Scientific and Technical Information of China (English)
LI XinLiang; FU DeXun; MA YanWen
2008-01-01
Direct numerical simulation of the turbulent boundary layer over a sharp cone with 20° cone angle (or 10° half-cone angle) is performed by using the mixed seventh-order up-wind biased finite difference scheme and sixth-order central difference scheme.The free stream Mach number is 0.7 and free stream unit Reynolds number is 250000/inch.The characteristics of transition and turbulence of the sharp cone boundary layer are compared with those of the flat plate boundary layer,Statistics of fully developed turbulent flow agree well with the experimental and theoretical data for the turbulent flat-plate boundary layer flow.The near wall streak-like structure is shown and the average space between streaks (normalized by the local wall unit) keeps approximately invariable at different streamwise locations,The turbulent energy equation in the cylindrical coordinate is given and turbulent en-ergy budget is studied.The computed results show that the effect of circumferen-tial curvature on turbulence characteristics is not obvious.
On the growth of turbulent regions in laminar boundary layers
Gad-El-hak, M.; Riley, J. J.; Blackwelder, R. F.
1981-01-01
Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a turbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a turbulent region in a laminar boundary layer. This mechanism, termed growth by destabilization, appears to be a result of the turbulence destabilizing the unstable laminar boundary layer in its vicinity. To further understand the growth mechanisms, the turbulence in the spot is modulated using drag-reducing additives and salinity stratification.
On the Effects of Surface Roughness on Boundary Layer Transition
Choudhari, Meelan M.; Li, Fei; Chang, Chau-Lyan; Edwards, Jack
2009-01-01
Surface roughness can influence laminar-turbulent transition in many different ways. This paper outlines selected analyses performed at the NASA Langley Research Center, ranging in speed from subsonic to hypersonic Mach numbers and highlighting the beneficial as well as adverse roles of the surface roughness in technological applications. The first theme pertains to boundary-layer tripping on the forebody of a hypersonic airbreathing configuration via a spanwise periodic array of trip elements, with the goal of understanding the physical mechanisms underlying roughness-induced transition in a high-speed boundary layer. The effect of an isolated, finite amplitude roughness element on a supersonic boundary layer is considered next. The other set of flow configurations examined herein corresponds to roughness based laminar flow control in subsonic and supersonic swept wing boundary layers. A common theme to all of the above configurations is the need to apply higher fidelity, physics based techniques to develop reliable predictions of roughness effects on laminar-turbulent transition.
CISM Course on Recent Advances in Boundary Layer Theory
1998-01-01
Recent advances in boundary-layer theory have shown how modern analytical and computational techniques can and should be combined to deepen the understanding of high Reynolds number flows and to design effective calculation strategies. This is the unifying theme of the present volume which addresses laminar as well as turbulent flows.
The collapse of turbulence in the atmospheric boundary layer
Energy Technology Data Exchange (ETDEWEB)
Van de Wiel, B J H; Clercx, H J H [Department of Physics, Eindhoven University of Technology (Netherlands); Moene, A F [Department of Meteorology and Air Quality, Wageningen University and Research Centre (Netherlands); Jonker, H J J, E-mail: b.j.h.v.d.wiel@tue.nl [Department of Multi-scale Pysics, Delft University of Technology (Netherlands)
2011-12-22
A well-known phenomenon in the atmospheric boundary layer is the fact that winds may become very weak in the evening after a clear sunny day. In these quiet conditions usually hardly any turbulence is present. Consequently this type of boundary layer is referred to as the quasi-laminar boundary layer. In spite of its relevance, the appearance of laminar boundary layers is poorly understood and forms a long standing problem in meteorological research. Here we investigate an analogue problem in the form of a stably stratified channel flow. The flow is studied with a simplified atmospheric model as well as with Direct Numerical Simulations. Both models show remarkably similar behaviour with respect to the mean variables such as temperature and wind speed. The similarity between both models opens new way for understanding and predicting the laminarization process. Mathematical analysis on the simplified model shows that relaminarization can be understood from the existence of a definite limit in the maximum sustainable heat flux under stably stratified conditions. This fascinating aspect will be elaborated in future work.
Linear Stability of the boundary layer under a solitary wave
Verschaeve, Joris C G
2013-01-01
A theoretical and numerical analysis of the linear stability of the boundary layer flow under a solitary wave is presented. In the present work, the nonlinear boundary layer equations are solved. The result is compared to the linear boundary layer solution in Liu et al. (2007) reveal- ing that both profiles are disagreeing more than has been found before. A change of frame of reference has been used to allow for a classical linear stability analysis without the need to redefine the notion of stability for this otherwise unsteady flow. For the linear stability the Orr-Sommerfeld equation and the parabolic stability equation were used. The results are compared to key results of inviscid stability theory and validated by means of a direct numerical simulation using a Legendre-Galerkin spectral ele- ment Navier-Stokes solver. Special care has been taken to ensure that the numerical results are valid. Linear stability predicts that the boundary layer flow is unstable for the entire parameter range considered, conf...
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...
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...
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...
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...
Turbulent spots detection during boundary layer by-pass transition
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel; Elsner, W.; Mazur, Oton; Uruba, Václav; Wysocki, M.
-, č. 80 (2009), s. 16-19. ISSN N R&D Projects: GA AV ČR(CZ) IAA200760614; GA MŠk MEB050810 Institutional research plan: CEZ:AV0Z20760514 Keywords : turbulent spot * boundary layer * by-pass transition * turbulent spot detection Subject RIV: BK - Fluid Dynamics
Spatially developing turbulent boundary layer on a flat plate
Lee, J H; Hutchins, N; Monty, J P
2012-01-01
This fluid dynamics video submitted to the Gallery of Fluid motion shows a turbulent boundary layer developing under a 5 metre-long flat plate towed through water. A stationary imaging system provides a unique view of the developing boundary layer as it would form over the hull of a ship or fuselage of an aircraft. The towed plate permits visualisation of the zero-pressure-gradient turbulent boundary layer as it develops from the trip to a high Reynolds number state ($Re_\\tau \\approx 3000$). An evolving large-scale coherent structure will appear almost stationary in this frame of reference. The visualisations provide an unique view of the evolution of fundamental processes in the boundary layer (such as interfacial bulging, entrainment, vortical motions, etc.). In the more traditional laboratory frame of reference, in which fluid passes over a stationary body, it is difficult to observe the full evolution and lifetime of turbulent coherent structures. An equivalent experiment in a wind/water-tunnel would requ...
Gravity-Driven Instability in a Liquid Film Overlying an Inhomogeneous Porous Layer
Institute of Scientific and Technical Information of China (English)
ZHAO Si-Cheng; LIU Qiu-Sheng; NGUYEN-THI Henri; BILLIA Bernard
2011-01-01
@@ A new model consisting of a liquid film overlying a saturated and inhomogeneous porous layer is investigated.We concentrate on effects of inhomogeneity on transition of instability modes.Influences of the averaged porosity and the gradient of porosity distribution on the instability behaviors of a liquid-porous layer system are emphasized.The average permeability of the porous layer is a key factor to determine the penetration of convection in the system.%A new model consisting of a liquid film overlying a saturated and inhomogeneous porous layer is investigated. We concentrate on effects of inhomogeneity on transition of instability modes. Influences of the averaged porosity and the gradient of porosity distribution on the instability behaviors of a liquid-porous layer system are emphasized.The average permeability of the porous layer is a key factor to determine the penetration of convection in the system.
Minimum Wind Dynamic Soaring Trajectories under Boundary Layer Thickness Limits
Bousquet, Gabriel; Triantafyllou, Michael; Slotine, Jean-Jacques
2015-11-01
Dynamic soaring is the flight technique where a glider, either avian or manmade, extracts its propulsive energy from the non-uniformity of horizontal winds. Albatrosses have been recorded to fly an impressive 5000 km/week at no energy cost of their own. In the sharp boundary layer limit, we show that the popular image, where the glider travels in a succession of half turns, is suboptimal for travel speed, airspeed, and soaring ability. Instead, we show that the strategy that maximizes the three criteria simultaneously is a succession of infinitely small arc-circles connecting transitions between the calm and windy layers. The model is consistent with the recordings of albatross flight patterns. This lowers the required wind speed for dynamic soaring by over 50% compared to previous beliefs. In the thick boundary layer limit, energetic considerations allow us to predict a minimum wind gradient necessary for sustained soaring consistent with numerical models.
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
Tetervin, Neal; Lin, Chia Chiao
1951-01-01
A general integral form of the boundary-layer equation, valid for either laminar or turbulent incompressible boundary-layer flow, is derived. By using the experimental finding that all velocity profiles of the turbulent boundary layer form essentially a single-parameter family, the general equation is changed to an equation for the space rate of change of the velocity-profile shape parameter. The lack of precise knowledge concerning the surface shear and the distribution of the shearing stress across turbulent boundary layers prevented the attainment of a reliable method for calculating the behavior of turbulent boundary layers.
BWR parametric sensitivity effect of regional mode instability on stability boundary
International Nuclear Information System (INIS)
Both in-phase (core wide mode) instability and out-of-phase (regional mode) instability are of great concerns in BWR stability issues. Normally, decay ratios for regional mode oscillations are much less than those under core wide conditions. However, under certain observation mode, the regional mode instability has the phenomenon of power increasing in one half of the core and at the same time, it decrease in the other half, so it looks like that the average power remains essentially constant. This research presents a study of fractional change of decay ratio to evaluate parametric effects of regional mode instability on reload core design power/flow stability boundary for the Chinshan Nuclear Power Plant Unit 2 Cycle 21 (BWR4). Making use of LAPUR5.2 and SIMULATE-3 codes, we have established a methodology to conduct such out-of-phase stability analysis. Many important parameters, such as system pressure, core flow rates, moderator void fraction, fuel physical and geometrical properties, have strong influences on regional mode stability. Current investigations have shown that at some operation points along the stability boundary, certain parameters present more sensitive characteristics. (author)
Edge Plasma Boundary Layer Generated By Kink Modes in Tokamaks
International Nuclear Information System (INIS)
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.
Studies of stability of blade cascade suction surface boundary layer
Institute of Scientific and Technical Information of China (English)
DONG Xue-zhi; YAN Pei-gang; HAN Wan-jin
2007-01-01
Compressible boundary layers stability on blade cascade suction surface was discussed by wind tunnel experiment and numerical solution. Three dimensional disturbance wave Parabolized Stability Equations(PSE) of orthogonal Curvilinear Coordinates in compressible flow was deducted. The surface pressure of blade in wind tunnel experiment was measured. The Falkner-Skan equation was solved under the boundary conditions of experiment result, and velocity, pressure and temperature of average flow were obtained. Substituted this result for discretization of the PSE Eigenvalue Problem, the stability problem can be solved.
Two-phase boundary layer prediction in upward boiling flow
International Nuclear Information System (INIS)
In the present work, the numerical modelling of the two-phase turbulent boundary layer in upward boiling flow was investigated. First, non-dimensional liquid velocity and temperature profiles in the two-phase boundary layer were validated on the one-dimensional section of a pipe with prescribed radial void fraction profiles. Simulations were performed on a fine grid with a commercial code CFX-5 using the k-ω turbulence model. A significant deviation of results from the analytical single-phase and two-phase wall functions from the literature was observed. Second, a wall boiling model in a vertical heated pipe was simulated (CFX-5) on the coarse grid. Here the prediction of the two-phase thermal boudary layer was compared to the experimental data, k-ω calculation on the fine grid and against the singlephase analytical wall function. Again a major deviation against single-phase temperature wall function was obtained. Presented analyses suggest that the existing analytical velocity and temperature wall functions cannot be valid for the boiling boundary layer with the high void fraction on the wall. (author)
DNS Study for the origin of the flow Randomization in Late Boundary Layer Transition
Thapa, Manoj; Liu, Chaoqun
2014-01-01
This paper is devoted to the investigation of the origin and mechanism of randomization in late boundary layer transition over a flat plate without pressure gradient. The flow randomization is a crucial phase before flow transition to the turbulent state. According to existing literatures, the randomization was caused by the big background noises and non-periodic spanwise boundary conditions. It was assumed that the large ring structure is affected by background noises first, and then the change of large ring structure affects the small length scales quickly, which directly leads to randomization and formation of turbulence. However, by careful analysis of our high order DNS results, we believe that the internal instability of multiple ring cycles structure is the main reason. What we observed is that randomization begins when the third cycle overlaps the first and second cycles. A significant asymmetric phenomenon is originated from the second cycle in the middle of both streamwise and spanwise directions. M...
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.
Institute of Scientific and Technical Information of China (English)
WANG Liang; FU Song
2009-01-01
Based on Reynolds-averaged Navier-Stokes approach, a laminar-turbulence transition model is proposed in this study that takes into account the effects of different instability modes associated with the variations in Mach numbers of compressible boundary layer flows. The model is based on k-ω-γ three-equation eddy-viscosity concept with k representing the fluctuating kinetic energy, ωthe specific dissipation rate and the intermittency factor γ.The particular features of the model are that: 1) k includes the non-turbulent, as well as turbulent fluctuations; 2) a transport equation for the intermittency factor γis proposed here with a source term set to trigger the transition onset; 3) through the introduction of a new length scale normal to wall, the present model employs the local variables only avoiding the use of the integral parameters, like the boundary layer thickness δ,which are often cost-ineffective with the modern CFD (Computational Fluid Dynamics) methods; 4) in the fully turbulent region, the model retreats to the well-known k-ωSST (Shear Stress Transport) model. This model is validated with a number of available experiments on boundary layer transitions including the incompressible, supersonic and hypersonic flows past flat plates, straight/flared cones at zero incidences, etc. It is demonstrated that the present model can be successfully applied to the engineering calculations of a variety of aerodynamic flow transition.
Institute of Scientific and Technical Information of China (English)
无
2009-01-01
Based on Reynolds-averaged Navier-Stokes approach,a laminar-turbulence transition model is proposed in this study that takes into account the effects of different instability modes associated with the variations in Mach numbers of compressible boundary layer flows.The model is based on k-ω-γ three-equation eddy-viscosity concept with k representing the fluctuating kinetic energy,ωthe specific dissipation rate and the intermittency factorγ.The particular features of the model are that:1)k includes the non-turbulent,as well as turbulent fluctuations;2)a transport equation for the intermittency factorγis proposed here with a source term set to trigger the transition onset;3)through the introduction of a new length scale normal to wall,the present model employs the local variables only avoiding the use of the integral parameters,like the boundary layer thicknessδ,which are often cost-ineffective with the modern CFD(Computational Fluid Dynamics)methods;4)in the fully turbulent region,the model retreats to the well-known k-ωSST(Shear Stress Transport)model.This model is validated with a number of available experiments on boundary layer transitions including the incompressible,supersonic and hypersonic flows past flat plates,straight/flared cones at zero incidences,etc.It is demonstrated that the present model can be successfully applied to the engineering calculations of a variety of aerodynamic flow transition.
Effect of boundary layer thickness on secondary structures in a short inlet curved duct
International Nuclear Information System (INIS)
Highlights: • Studied the flow field of a short inlet curved duct. • Flow field is asymmetric due to two opposing flow mechanisms. • Manipulation of the incoming boundary layers modified the secondary flow structures, resulting in a symmetric flow field. - Abstract: The flow pattern in short inlet ducts with aggressive curvature has been shown to lead, in some cases, to an asymmetric flow field at the aerodynamic interface plane. In the present work, a two-dimensional honeycomb mesh was added upstream of the curved duct to create a pressure drop across it, and therefore to an increased velocity deficit in the boundary layer. This velocity deficit led to a stronger streamwise separation, overcoming the instability that can result in an asymmetric flow field at the aerodynamic interface plane. Experiments were conducted at Mach numbers of M = 0.2, 0.44 and 0.58 in an expanding aggressive duct with rectangle to a square cross section with area ratio of 1.27. Steady and unsteady pressure measurements, together with Particle Image Velocimetry (PIV), were used to explore the effect of the honeycomb on the symmetry of the flow field. The effect of inserting a honeycomb was tested by increasing its height from 0 to 2.2 times the boundary layer thickness of the baseline flow upstream of the curve. Using the honeycomb, flow symmetry was achieved for the specific geometrical configuration tested with a negligible decrease of the pressure recovery
International Nuclear Information System (INIS)
Ionospheric depletions, produced by release of attachment chemicals into the ionosphere, were widely investigated and taken as a potential technique for the artificial modification of space weather. In this work, we reported the experimental evidence of spontaneously generated electromagnetic fluctuations in the boundary layer of laboratory-created ionospheric depletions. These depletions were produced by releasing attachment chemicals into the ambient plasmas. Electron density gradients and sheared flows arose in the boundary layer between the ambient and the negative ions plasmas. These generated electromagnetic fluctuations with fundamental frequency f0 = 70 kHz lie in the lower hybrid frequency range, and the mode propagates with angles smaller than 90° (0.3π–0.4π) relative to the magnetic field. Our results revealed that these observed structures were most likely due to electromagnetic components of the electron-ion hybrid instability. This research demonstrates that electromagnetic fluctuations also can be excited during active release experiments, which should be considered as an essential ingredient in the boundary layer processes of ionospheric depletions
The effect of non-Newtonian viscosity on the stability of the Blasius boundary layer
Griffiths, P. T.; Gallagher, M. T.; Stephen, S. O.
2016-07-01
We consider, for the first time, the stability of the non-Newtonian boundary layer flow over a flat plate. Shear-thinning and shear-thickening flows are modelled using a Carreau constitutive viscosity relationship. The boundary layer equations are solved in a self-similar fashion. A linear asymptotic stability analysis, that concerns the lower-branch structure of the neutral curve, is presented in the limit of large Reynolds number. It is shown that the lower-branch mode is destabilised and stabilised for shear-thinning and shear-thickening fluids, respectively. Favourable agreement is obtained between these asymptotic predictions and numerical results obtained from an equivalent Orr-Sommerfeld type analysis. Our results indicate that an increase in shear-thinning has the effect of significantly reducing the value of the critical Reynolds number, this suggests that the onset of instability will be significantly advanced in this case. This postulation, that shear-thinning destabilises the boundary layer flow, is further supported by our calculations regarding the development of the streamwise eigenfunctions and the relative magnitude of the temporal growth rates.
Energy Technology Data Exchange (ETDEWEB)
Liu, Yu [CAS Key Laboratory of Geospace Environment, School of Earth and Space Science, University of Science and Technology of China, Hefei 230026 (China); Lei, Jiuhou, E-mail: leijh@ustc.edu.cn [CAS Key Laboratory of Geospace Environment, School of Earth and Space Science, University of Science and Technology of China, Hefei 230026 (China); Collaborative Innovation Center of Astronautical Science and Technology, Harbin 150001 (China); Cao, Jinxiang; Xu, Liang [CAS Key Laboratory of Geospace Environment, Modern Physics Department, University of Science and Technology of China, Hefei 230026 (China)
2016-01-15
Ionospheric depletions, produced by release of attachment chemicals into the ionosphere, were widely investigated and taken as a potential technique for the artificial modification of space weather. In this work, we reported the experimental evidence of spontaneously generated electromagnetic fluctuations in the boundary layer of laboratory-created ionospheric depletions. These depletions were produced by releasing attachment chemicals into the ambient plasmas. Electron density gradients and sheared flows arose in the boundary layer between the ambient and the negative ions plasmas. These generated electromagnetic fluctuations with fundamental frequency f{sub 0} = 70 kHz lie in the lower hybrid frequency range, and the mode propagates with angles smaller than 90° (0.3π–0.4π) relative to the magnetic field. Our results revealed that these observed structures were most likely due to electromagnetic components of the electron-ion hybrid instability. This research demonstrates that electromagnetic fluctuations also can be excited during active release experiments, which should be considered as an essential ingredient in the boundary layer processes of ionospheric depletions.
A Thermal Plume Model for the Martian Convective Boundary Layer
Colaïtis, Arnaud; Hourdin, Frédéric; Rio, Catherine; Forget, François; Millour, Ehouarn
2013-01-01
The Martian Planetary Boundary Layer [PBL] is a crucial component of the Martian climate system. Global Climate Models [GCMs] and Mesoscale Models [MMs] lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the "thermal plume" model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single-column models. The aim of this physically-based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in Large-Eddy Simulations [LESs]. We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near-surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking ...
Direct numerical simulation of supersonic turbulent boundary layers
Guarini, Stephen
The objectives of this research were to develop a method by which the spatially developing compressible turbulent boundary layer could be simulated using a temporally developing numerical simulation and to study the physics of the compressible turbulent boundary layer. We take advantage of the technique developed by Spalart (1987, 1988) for the incompressible case. In this technique, it is recognized that the boundary layer exhibits slow growth in the streamwise direction, so the turbulence can be treated as approximately homogeneous in this direction. The slow growth is accounted for with a coordinate transformation and a multiple scale analysis. The result is a modified system of equations (Navier-Stokes plus some extra terms, which we call "slow growth terms") that are homogeneous in both the streamwise and spanwise directions and represent the state of the boundary layer at a given streamwise location (or, equivalently, a given thickness). The compressible Navier-Stokes equations are solved using a mixed Fourier and B-spline "spectral" method. The dependent variables are expanded in terms of a Fourier representation in the horizontal directions and a B-spline representation in the wall-normal direction. In the wall-normal direction non-reflecting boundary conditions are used at the freestream boundary, and zero-heat-flux no-slip boundary conditions are used at the wall. This combination of splines and Fourier methods produces a very accurate numerical method. Mixed implicit/explicit time discretization is used. Results are presented for a case with a Mach number of 2.5, and a Reynolds number, based on momentum integral thickness and wall viscosity, of Rsb{thetasp'} = 840. The results show that the van Driest transformed velocity satisfies the incompressible scalings and a narrow logarithmic region is obtained. The results for the turbulence intensities compare well with the incompressible simulations of Spalart. Pressure fluctuations are found to be higher than
Chefranov, Sergey G
2010-01-01
For Gagen-Poiseuille flow, we show that exponential instability (to extremely small, axially symmetric disturbances represented by Galerkin's approximation) is possible only if there exists conditionally periodic variability of the disturbances along the pipe axis when the threshold Reynolds number depends on the ratio of two longitudinal periods. Absolute minimum (for) is obtained that corresponds to the observed conditions of transition from the laminar resistance law to the turbulent one and Tollmien-Schlichting waves exciting in the boundary layer.
Infrared propagation in the air-sea boundary layer
Larsen, R.; Preedy, K. A.; Drake, G.
1990-03-01
Over the oceans and other large bodies of water the structure of the lowest layers of the atmosphere is often strongly modified by evaporation of water vapor from the water surface. At radio wavelengths this layer will usually be strongly refracting or ducting, and the layer is commonly known as the evaporation duct. However, the refractive index of air at infrared wavelengths differs from that at radio wavelengths, and the effects of the marine boundary layer on the propagation of infrared radiation are examined. Meteorological models of the air-sea boundary layer are used to compute vertical profiles of temperature and water-vapor pressure. From these are derived profiles of atmospheric refractive index at radio wavelengths and at infrared wavelengths in the window regions of low absorption. For duct propagation to occur it is necessary that the refractivity of air decreases rapidly with increasing height above the surface. At radio wavelengths this usually occurs when there is a strong lapse of water vapor pressure with increasing height. By contrast, at infrared wavelengths the refractive index is almost independent of water vapor pressure, and it is found that an infrared duct is formed only when there is a temperature inversion.
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.
CFD Modeling of Non-Neutral Atmospheric Boundary Layer Conditions
DEFF Research Database (Denmark)
Koblitz, Tilman
For wind resource assessment, the wind industry is increasingly relying on Computational Fluid Dynamics models that focus on modeling the airflow in a neutrally stratified surface-layer. Physical processes like the Coriolis force, buoyancy forces and heat transport, that are important to the...... atmospheric boundary-layer, are mostly ignored so far. In order to decrease the uncertainty of wind resource assessment, the present work focuses on atmospheric flows that include atmospheric stability and the Coriolis effect. Within the present work a RANS model framework is developed and implemented into...
An algorithm for detecting layer boundaries in sediments
Directory of Open Access Journals (Sweden)
K. Bube
2006-01-01
Full Text Available In this paper we present an algorithm based on wavelet multiscale decomposition, designed to detect lines of maximal gradients in horizontal direction within two-dimensional data sets. The algorithm is capable of identifying layer boundaries within sediment profiles, as demonstrated for artificial as well as two field data sets. Layers are detected with a good resolution within (i digital images of a deep sea sediment core (IODP-expedition 301, core 15H and (ii chemical concentration patterns of recent tidal sediments (North Sea.
Boundary Layer Turbulence Index: Progress and Recent Developments
Pryor, Kenneth L
2008-01-01
A boundary layer turbulence index (TIBL) product has been developed to assess the potential for turbulence in the lower troposphere, generated using RUC-2 numerical model data. The index algorithm approximates boundary layer turbulent kinetic energy by parameterizing vertical wind shear, responsible for mechanical production of TKE, and kinematic heat flux, parameterized by the vertical temperature lapse rate and responsible for buoyant production of TKE. Validation for the TIBL product has been conducted for selected nonconvective wind events during the 2008 winter season over the Idaho National Laboratory mesonet domain. This paper presents studies of four significant wind events between December 2007 and February 2008 over southeastern Idaho. Based on the favorable results highlighted from validation statistics and in the case studies, the RUC TIBL product has demonstrated operational utility in assessing turbulence hazards to low-flying aircraft and ground transportation, and in the assessment of wildfire...
Behaviour of tracer diffusion in simple atmospheric boundary layer models
Directory of Open Access Journals (Sweden)
P. S. Anderson
2006-12-01
Full Text Available 1-D profiles and time series from an idealised atmospheric boundary layer model are presented, which show agreement with measurements of polar photogenic NO and NO_{2}. Diffusion models are increasingly being used as the framework for studying tropospheric air chemistry dynamics. Models based on standard boundary layer diffusivity profiles have an intrinsic behaviour that is not necessarily intuitive, due to the variation of turbulent diffusivity with height. The relatively simple model provides both a programming and a conceptual tool in the analysis of observed trace gas evolution. A time scale inherent in the model can be tuned by fitting model time series to observations. This scale is then applicable to the more physically simple but chemically complex zeroth order or box models of chemical interactions.
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.
Construction of a Non-Equilibrium Thermal Boundary Layer Facility
Biles, Drummond; Ebadi, Alireza; Ma, Allen; White, Christopher
2015-11-01
A thermally conductive, electrically heated wall-plate forming the bottom wall of a wind tunnel has been constructed and validation tests have been performed. The wall-plate is a sectioned wall design, where each section is independently heated and controlled. Each section consists of an aluminum 6061 plate, an array of resistive heaters affixed to the bottom of the aluminum plate, and a calcium silicate holder used for thermal isolation. Embedded thermocouples in the aluminum plates are used to monitor the wall temperature and for feedback control of wall heating. The wall-plate is used to investigate thermal transport in both equilibrium and non-equilibrium boundary layers. The non-equilibrium boundary layer flow investigated is oscillatory flow produced by a rotor-stator mechanism placed downstream of the test section of the wind tunnel.
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.
Works on theory of flapping wing. [considering boundary layer
Golubev, V. V.
1980-01-01
It is shown mathematically that taking account of the boundary layer is the only way to develop a theory of flapping wings without violating the basic observations and mathematics of hydromechanics. A theory of thrust generation by flapping wings can be developed if the conventional downstream velocity discontinuity surface is replaced with the observed Karman type vortex streets behind a flapping wing. Experiments show that the direction of such vortices is the reverse of that of conventional Karman streets. The streets form by breakdown of the boundary layer. Detailed analysis of the movements of certain birds and insects during flight 'in place' is fully consistent with this theory of the lift, thrust and drag of flapping wings. Further directions for research into flight with flapping wings are indicated.
Electrical properties of boundary layers of fatty acids
Deryagin, B. V.; Snitkovskii, M. M.
1992-05-01
Nonlinear current-voltage and coulomb-voltage characteristics with a hysteresis loop, which is peculiar to ferroelectrics, were observed in the boundary layers of individual saturated organic acids and oleic acid which have a domain structure and also an anomalously high conductivity which corresponds, in its order of magnitude, to the lower conductivity limit for metals. These effects are related with the formation of a volume space charge and by the cording of the current (formation of conductivity channels). The electrical properties of the boundary layers change in relation to the thickness: for subcritical thicknesses Ohm's Law is obeyed but for larger thicknesses strong field effects are observed. The thickness at which the system changes into the nonconducting stage has meaning as a physical characteristic of the system.
Turbulence Scales Simulations in Atmospheric Boundary Layer Wind Tunnels
Elena-Carmen Teleman; Radu Silion; Elena Axinte; Radu Pescaru
2008-01-01
The simulation of the air flow over models in atmospheric boundary layer tunnels is a research domain based on advanced scientific technologies imposed by the necessity of studying the turbulent fluid movements in the proximity of the Earth’s surface. The experiment presented herein is developed in the wind tunnel from the Laboratory of Structural Aerodynamics of the Faculty of Civil Engineering and Building Services in Iassy. Measurements necessary for the determination of the turbulence sca...
Simulation of aerosol substance transfer in the atmospheric boundary layer
Lezhenin, A. A.; Raputa, V. F.; Shlychkov, V. Ð. ń.
2014-11-01
A model for the reconstruction of the surface concentration of a heavy non-homogeneous substance transfered in the atmosphere is proposed. The model is used to simulate the snow surface contamination by benzo(a)pyren in the vicinity of Power Station-3 in the city of Barnaul. The effects of wind rotation in the atmospheric boundary layer on the field of long-term aerosol substance are assessed.
Pressure gradient effect in natural convection boundary layers
Higuera Antón, Francisco; Liñán Martínez, Amable
1993-01-01
The high Grashof number laminar natural convection flow around the lower stagnation point of a symmetric bowl- shaped heated body is analyzed. A region is identified where the direct effect on the flow of the component of the buoyancy force tangential to the body surface is comparable to the indirect effect of the component normal to the surface, which acts through the gradient of the nonuniform pressure that it induces in the boundary layer. Analysis of this region provides a description ...
Iodine monoxide in the Western Pacific marine boundary layer
Directory of Open Access Journals (Sweden)
K. Großmann
2012-10-01
Full Text Available A latitudinal cross-section and vertical profiles of iodine monoxide (IO are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E through the Western Pacific to Townsville, Australia (19° S, 146° E in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S, IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH_{3}I, CH_{2}I_{2}, CH_{2}ClI, CH_{2}BrI are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved, if an additional sea-air flux of inorganic iodine (e.g. I_{2} is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.
Computation of 2D stratified flows in atmospheric boundary layer
Czech Academy of Sciences Publication Activity Database
Tauer, M.; Šimonek, J.; Kozel, Karel; Jaňour, Zbyněk
Praha : Ústav termomechaniky AV ČR, v. v. i., 2009 - (Jonáš, P.; Uruba, V.), s. 47-48 ISBN 978-80-87012-21-5. [Colloquium Fluid Dynamics 2009. Praha (CZ), 21.10.2009-23.10.2009] R&D Projects: GA ČR GA103/09/0977 Institutional research plan: CEZ:AV0Z20760514 Keywords : computation stratified flows * Navier-Stokes equations * atmospheric boundary layer Subject RIV: DG - Athmosphere Sciences, Meteorology
Numerical solution of 2D flows in atmospheric boundary layer
Czech Academy of Sciences Publication Activity Database
Šimonek, J.; Tauer, J.; Kozel, K.; Jaňour, Zbyněk; Příhoda, Jaromír
Praha : Ústav termomechaniky AV ČR, v. v. i., 2008 - (Jonáš, P.; Uruba, V.), s. 51-52 ISBN 978-80-87012-14-7. [Colloquium FLUID DYNAMICS 2008. Praha (CZ), 22.10.2008-24.10.2008] R&D Projects: GA AV ČR 1ET400760405 Institutional research plan: CEZ:AV0Z20760514 Keywords : numerical simulation * atmospheric boundary layer * stratified flow Subject RIV: BK - Fluid Dynamics
Flat Plate Boundary Layer Under Negative Pressure Gradient
Czech Academy of Sciences Publication Activity Database
Antoš, Pavel; Jonáš, Pavel; Procházka, Pavel P.; Skála, Vladislav
Pretoria, South Africa: HEFAT, 2015 - (Meyer, J.), s. 251-253 ISBN 978-1-77592-108-0. [International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics : HEFAT 2015 /11./. SKUKUZA (ZA), 20.07.2015-23.07.2015] R&D Projects: GA ČR GAP101/12/1271 Institutional support: RVO:61388998 Keywords : boundary layer in decelerating flow * adverse pressure gradient * hot-wire anemometry Subject RIV: BK - Fluid Dynamics
Ozone in the Atlantic Ocean marine boundary layer
Patrick Boylan; Detlev Helmig; Samuel Oltmans
2015-01-01
Abstract In situ atmospheric ozone measurements aboard the R/V Ronald H. Brown during the 2008 Gas-Ex and AMMA research cruises were compared with data from four island and coastal Global Atmospheric Watch stations in the Atlantic Ocean to examine ozone transport in the marine boundary layer (MBL). Ozone measurements made at Tudor Hill, Bermuda, were subjected to continental outflow from the east coast of the United States, which resulted in elevated ozone levels above 50 ppbv. Ozone measurem...
Aerodynamic Optimization and Boundary Layer Control On Sailplane Wing Sections
Czech Academy of Sciences Publication Activity Database
Popelka, Lukáš; Matějka, Milan; Součková, Natálie
Berlin: CEAS Council of European Aerospace Societies, 2007, s. 1763-1767. ISSN 0070-4083. [CEAS European Air and Space Conference /1./. Berlin (DE), 10.09.2007-13.09.2007] R&D Projects: GA MŠk(CZ) 1M06031; GA AV ČR IAA2076403; GA AV ČR(CZ) IAA200760614 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer control * sailplane wing section Subject RIV: BK - Fluid Dynamics
Defects and boundary layers in non-Euclidean plates
Gemmer, John
2012-01-01
We investigate the behaviour of non-Euclidean plates with constant negative Gaussian curvature using the F\\"oppl-von K\\'arm\\'an 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. We also investigate the scaling with thickness of boundary layers where the stretching energy is concentrated with decreasing thickness.
Glyoxal observations in the global marine boundary layer
Mahajan, Anoop S.; Prados-Roman, Cristina; Hay, Timothy D.; Lampel, Johannes; Pöhler, Denis; Groβmann, Katja; Tschritter, Jens; Frieß, Udo; Platt, Ulrich; Johnston, Paul; Kreher, Karin; Wittrock, Folkard; Burrows, John P; Plane, John M. C.; Saiz-Lopez, Alfonso
2014-01-01
Glyoxal is an important intermediate species formed by the oxidation of common biogenic and anthropogenic volatile organic compounds such as isoprene, toluene and acetylene. Although glyoxal has been shown to play an important role in urban and forested environments, its role in the open ocean environment is still not well understood, with only a few observations showing evidence for its presence in the open ocean marine boundary layer (MBL). In this study, we report observations of glyoxal f...
Vortical Structures in a Boundary Layer Separation Region
Czech Academy of Sciences Publication Activity Database
Uruba, Václav; Sedlák, K.
Plzeň : ZČU Plzeň, 2009 - (Žitek, P.; Milčák, P.; Krivánka, D.), s. 209-214 ISBN 978-80-7043-804-9. [Conference on Power System Engineering /8./. Plzeň (CZ), 18.06.2009] R&D Projects: GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : vortex * boundary layer * separation Subject RIV: BK - Fluid Dynamics
Ouwersloot, H.G.; Arellano, de J.V.G.
2013-01-01
In Ouwersloot and Vila-Guerau de Arellano (Boundary-Layer Meteorol. doi: 10. 1007/s10546-013-9816-z, 2013, this issue), the analytical solutions for the boundary-layer height and scalar evolutions are derived for the convective boundary layer, based on the prognostic equations of mixed-layer slab mo
Using UAV's to Measure the Urban Boundary Layer
Jacob, R. L.; Sankaran, R.; Beckman, P. H.
2015-12-01
The urban boundary layer is one of the most poorly studied regions of the atmospheric boundary layer. Since a majority of the world's population now lives in urban areas, it is becoming a more important region to measure and model. The combination of relatively low-cost unmanned aerial vehicles and low-cost sensors can together provide a new instrument for measuring urban and other boundary layers. We have mounted a new sensor and compute platform called Waggle on an off-the-shelf XR8 octo-copter from 3DRobotics. Waggle consists of multiple sensors for measuring pressure, temperature and humidity as well as trace gases such as carbon monoxide, nitrogen dioxide, sulfur dioxide and ozone. A single board computer running Linux included in Waggle on the UAV allows in-situ processing and data storage. Communication of the data is through WiFi or 3G and the Waggle software can save the data in case communication is lost during flight. The flight pattern is a deliberately simple vertical ascent and descent over a fixed location to provide vertical profiles and so flights can be confined to urban parks, industrial areas or the footprint of a single rooftop. We will present results from test flights in urban and rural areas in and around Chicago.
Turbulent Boundary Layers in the Vicinity of Separation
Indinger, Thomas; Buschmann, Matthias H.; Gad-El-Hak, Mohamed
2004-11-01
There has been some controversy regarding the behavior of the mean velocity profile of turbulent boundary layers approaching separation. While a number of experiments show that the logarithmic law is sustained even under strong adverse-pressure-gradient and non-equilibrium conditions, other experiments and DNS results reveal that the mean velocity profile breaks down in the vicinity of separation. Measurements at TU Dresden of a decelerated, fully developed turbulent boundary layer over a smooth flat plate in a closed water channel show that the classical log law no longer describes the mean velocity in the overlap region after a certain fraction of the flow travels in the upstream direction. This finding is consistent with the physical explanation advanced by Dengel & Fernholz (J. Fluid Mech. 212, 1990) that the log law failure is caused by the first occurrence of reverse flow. Analyzing adverse-pressure-gradient turbulent boundary layer data from three independent groups, we demonstrate that the log law can be restored by replacing y^+ with a new variable depending both on the wall-normal coordinate and the reverse-flow parameter \\chi_w. This finding is of importance in cases where other complexities such as surface roughness or structured walls (riblets, dimples, etc.) are involved and a universal profile in inner variables is desired.
Coupled wake boundary layer model of wind-farms
Stevens, Richard J A M; Meneveau, Charles
2014-01-01
We present and test a coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a wind-farm. The model couples the traditional, industry-standard wake expansion/superposition approach with a top-down model for the overall wind-farm boundary layer structure. The wake expansion/superposition model captures the effect of turbine positioning, while the top-down portion adds the interaction between the wind-turbine wakes and the atmospheric boundary layer. Each portion of the model requires specification of a parameter that is not known a-priori. For the wake model the wake expansion coefficient is required, while the top-down model requires an effective span-wise turbine spacing within which the model's momentum balance is relevant. The wake expansion coefficient is obtained by matching the predicted mean velocity at the turbine from both approaches, while the effective span-wise turbine spacing depends on turbine positioning and thus can be determined from the wake expansion...
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.
Turbulent thermal boundary layers subjected to severe acceleration
Araya, Guillermo; Castillo, Luciano
2013-11-01
Favorable turbulent boundary layers are flows of great importance in industry. Particularly, understanding the mechanisms of quasi-laminarization by means of a very strong favorable streamwise pressure gradient is indeed crucial in drag reduction and energy management applications. Furthermore, due to the low Reynolds numbers involved in the quasi-laminarization process, abundant experimental investigation can be found in the literature for the past few decades. However, several grey zones still remain unsolved, principally associated with the difficulties that experiments encounter as the boundary layer becomes smaller. In addition, little attention has been paid to the heat transfer in a quasi-laminarization process. In this investigation, DNS of spatially-developing turbulent thermal boundary layers with prescribed very strong favorable pressure gradients (K = 4 × 10-6) are performed. Realistic inflow conditions are prescribed based on the Dynamic Multi-scale Approach (DMA) [Araya et al. JFM, Vol. 670, pp. 581-605, 2011]. In this sense the flow carries the footprint of turbulence, particularly in the streamwise component of the Reynolds stresses.
Improving Subtropical Boundary Layer Cloudiness in the 2011 NCEP GFS
Energy Technology Data Exchange (ETDEWEB)
Fletcher, J. K.; Bretherton, Christopher S.; Xiao, Heng; Sun, Ruiyu N.; Han, J.
2014-09-23
The current operational version of National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) shows significant low cloud bias. These biases also appear in the Coupled Forecast System (CFS), which is developed from the GFS. These low cloud biases degrade seasonal and longer climate forecasts, particularly of short-wave cloud radiative forcing, and affect predicted sea surface temperature. Reducing this bias in the GFS will aid the development of future CFS versions and contributes to NCEP's goal of unified weather and climate modelling. Changes are made to the shallow convection and planetary boundary layer parameterisations to make them more consistent with current knowledge of these processes and to reduce the low cloud bias. These changes are tested in a single-column version of GFS and in global simulations with GFS coupled to a dynamical ocean model. In the single-column model, we focus on changing parameters that set the following: the strength of shallow cumulus lateral entrainment, the conversion of updraught liquid water to precipitation and grid-scale condensate, shallow cumulus cloud top, and the effect of shallow convection in stratocumulus environments. Results show that these changes improve the single-column simulations when compared to large eddy simulations, in particular through decreasing the precipitation efficiency of boundary layer clouds. These changes, combined with a few other model improvements, also reduce boundary layer cloud and albedo biases in global coupled simulations.
Energy Technology Data Exchange (ETDEWEB)
Hashim, I [Centre for Modelling and Data Analysis, School of Mathematical Sciences, Universiti Kebangsaan Malaysia, 43600 Bangi Selangor (Malaysia); Awang Kechil, S [Centre of Mathematical Studies, Universiti Teknologi MARA, 40450 Shah Alam Selangor (Malaysia)], E-mail: ishak_h@ukm.my
2009-08-01
Linear active control of proportional feedback is applied to suppress the Marangoni convection in a heated layer of variable-viscosity fluid under microgravity conditions. The analytical solutions for the stationary and oscillatory modes are presented. The effects of the viscosity parameter, controller gain, Prandtl number and Biot number on the linear stability thresholds are examined. It is demonstrated that small perturbations in the thermal boundary conditions through feedback control can delay the onset of steady instability. Large controller gains play a major role in the occurrence of overstability that leads to oscillatory convection.
Instability of Two-Layer Rayleigh-Bénard Convection with Interfacial Thermocapillary Effect
Institute of Scientific and Technical Information of China (English)
LIU Qiu-Sheng(刘秋生); ZHOU Bing-Hong(周炳红); NGUYEN THI Henri; HU Wen-Rui(胡文瑞)
2004-01-01
The linear instability analysis of the Rayleigh-Marangoni-Bénard convection in a two-layer system of silicon oil 10cS and fluorinert FC70 liquids are performed in a larger range of two-layer depth ratios Hr from 0.2 to 5.0 for different total depth H(≤)12 mm. Our results are different from the previous study on the Rayleigh-Bénard instability and show strong effects of thermocapillary force at the interface on the time-dependent oscillations arising from the onset of instability convection.
BWR parametric sensitivity effect of regional mode instability on stability boundary
International Nuclear Information System (INIS)
Both in-phase (core wide mode) instability and out-of-phase (regional mode) instability are of great concerns in BWR stability issues. Normally, decay ratios for regional mode oscillations are much less than those under core wide conditions. This research presents a study of fractional change of decay ratio to evaluate parametric effects of regional mode instability on reload core design power/flow stability boundary for the Chinshan Nuclear Power Plant Unit 2 Cycle 21 (BWR4). Making use of LAPUR5 and SIMULATE-3 codes, we have established a methodology to conduct such out-of-phase stability analysis. Many important parameters, such as system pressure, core flow rates, coolant inlet subcoolings, and moderator void fraction, fuel physical and geometrical properties, have strong influences on regional mode stability. Current investigations have shown that at some operation points along the stability boundary, certain parameters are sensitive enough so that the regional mode decay ratio could become greater than the core wide mode decay ratio. (author)
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...... ping-pong ball experiments to study the influence of packing pattern, packing density, number of layers and surface roughness of the roughness elements. The results show that the friction factor seems to be not extremely sensitive to these factors. The results also show that the friction factor for...... extremely sensitive to the packing pattern, the packing density, the number of layers and the surface roughness of the roughness elements. There exists a steady streaming near the bed in the direction of wave propagation, in agreement with the existing work. The present data indicate that the steady...
Grain-boundary layering transitions and phonon engineering
Rickman, J. M.; Harmer, M. P.; Chan, H. M.
2016-09-01
We employ semi-grand canonical Monte Carlo simulation to investigate layering transitions at grain boundaries in a prototypical binary alloy. We demonstrate the existence of such transitions among various interfacial states and examine the role of elastic fields in dictating state equilibria. The results of these studies are summarized in the form of diagrams that highlight interfacial state coexistence in this system. Finally, we examine the impact of layering transitions on the phononic properties of the system, as given by the specific heat and, by extension, the thermal conductivity. Thus, it is suggested that by inducing interfacial layering transitions via changes in temperature or pressure, one can thereby engineer thermodynamic and transport properties in materials.
On the instabilities of supersonic mixing layers - A high-Mach-number asymptotic theory
Balsa, Thomas F.; Goldstein, M. E.
1990-01-01
The stability of a family of tanh mixing layers is studied at large Mach numbers using perturbation methods. It is found that the eigenfunction develops a multilayered structure, and the eigenvalue is obtained by solving a simplified version of the Rayleigh equation (with homogeneous boundary conditions) in one of these layers which lies in either of the external streams. This analysis leads to a simple hypersonic similarity law which explains how spatial and temporal phase speeds and growth rates scale with Mach number and temperature ratio. Comparisons are made with numerical results, and it is found that this similarity law provides a good qualitative guide for the behavior of the instability at high Mach numbers. In addition to this asymptotic theory, some fully numerical results are also presented (with no limitation on the Mach number) in order to explain the origin of the hypersonic modes (through mode splitting) and to discuss the role of oblique modes over a very wide range of Mach number and temperature ratio.
Direct Numerical Simulation of Two Shock Wave/Turbulent Boundary Layer Interactions
Priebe, Stephan
Direct numerical simulations (DNSs) of two shock wave/turbulent boundary layer interactions (STBLIs) are presented in this thesis. The first interaction is a 24° compression ramp at Mach 2.9, and the second interaction is an 8° compression ramp at Mach 7.2. The large-scale low-frequency unsteadiness in the Mach 2.9 DNS is investigated with the aim of shedding some light on its physical origin. Previous experimental and computational works have linked the unsteadiness either to fluctuations in the incoming boundary layer or to a mechanism in the downstream separated flow. Consistent with experimental observations, the shock in the DNS is found to undergo streamwise oscillations, which are broadband and occur at frequencies that are about two orders of magnitude lower than the characteristic frequency of the energy-containing turbulent scales in the incoming boundary layer. Based on a coherence and phase analysis of signals at the wall and in the flow field, it is found that the low frequency shock unsteadiness is statistically linked to pulsations of the downstream separated flow. The statistical link with fluctuations in the upstream boundary layer is also investigated. A weak link is observed: the value of the low-frequency coherence with the upstream flow is found to lie just above the limit of statistical significance, which is determined by means of a Monte Carlo study. The dynamics of the downstream separated flow are characterized further based on low-pass filtered DNS fields. The results suggest that structural changes occur in the downstream separated flow during the low-frequency motions, including the breaking-up of the separation bubble, which is observed when the shock moves downstream. The structural changes are described based on the Cf distribution through the interaction, as well as the velocity and vorticity fields. The possible link between the low-frequency dynamics observed in the DNS and results from global instability theory is explored. It
Stability Analysis of Roughness Array Wake in a High-Speed Boundary Layer
Choudhari, Meelan; Li, Fei; Edwards, Jack
2009-01-01
Computations are performed to examine the effects of both an isolated and spanwise periodic array of trip elements on a high-speed laminar boundary layer, so as to identify the potential physical mechanisms underlying an earlier transition to turbulence as a result of the trip(s). In the context of a 0.333 scale model of the Hyper-X forebody configuration, the time accurate solution for an array of ramp shaped trips asymptotes to a stationary field at large times, indicating the likely absence of a strong absolute instability in the mildly separated flow due to the trips. A prominent feature of the wake flow behind the trip array corresponds to streamwise streaks that are further amplified in passing through the compression corner. Stability analysis of the streaks using a spatial, 2D eigenvalue approach reveals the potential for a strong convective instability that might explain the earlier onset of turbulence within the array wake. The dominant modes of streak instability are primarily sustained by the spanwise gradients associated with the streaks and lead to integrated logarithmic amplification factors (N factors) approaching 7 over the first ramp of the scaled Hyper-X forebody, and substantially higher over the second ramp. Additional computations are presented to shed further light on the effects of both trip geometry and the presence of a compression corner on the evolution of the streaks.
Defining the Entrainment Zone in Stratocumulus-topped Boundary Layers
Wang, Q.; Zhou, M.; Kalogiros, J. A.; Lenschow, D. H.; Dai, C.; Wang, S.
2010-12-01
The presence of an entrainment zone near the top of the stratocumulus-topped boundary layers has been identified by many early studies. However, the definition of the entrainment zone was rather vague. We have examined the fine vertical variations of cloud liquid water content, wind, temperature and humidity near the stratocumulus top and developed a new method to identify the entrainment zone objectively. Aircraft measurements from various field projects in stratocumulus-topped boundary layers are used, taking advantage of the fast sampling capability of many of the aircraft sensors. Because of the inhomogeneous mixing of two air masses with distinctively different thermodynamic properties, the magnitude of temperature perturbations within the entrainment zone is significantly larger than those above or below. This characteristics is used to define the upper and lower boundaries of the entrainment zone using a wavelet spectra analyses. The definition of the entrainment zone is further evaluated by the presence of a linear mixing line through mixing line analyses. Various other interfaces at the cloud top are also examined, including the cloud interface, temperature interface (inversion), and moisture interface. The heights of these interfaces are examined relative to the height of the entrainment zone. This study also systematically revealed the presence of turbulence above the local cloud top and/or above the entrainment zone. Wind shear near the cloud top is one possible source that generated local turbulence. Other potential sources of turbulence will also be discussed.
On the partially reacted boundary layer in rate sticks
Partom, Y.
2014-05-01
Using our temperature dependent reactive flow model (TDRR) to simulate detonation in a rate stick, we observe that a partially reacted layer (PRL) is formed near the boundary. We are not aware that such a PRL has been observed in tests, and this is why we regarded it in the past as a numerical artifact. Assuming that such an artefact may be caused by the finite rise time of the detonation shock, we showed in [1] how it can be eliminated by delaying the outward boundary motion for a length of time comparable with the shock rise time. Here we revisit the PRL problem. We first show that it is not a numerical artifact but a real phenomenon. We do this by repeating the reactive flow run with a finer mesh. By looking at the PRL structure, we see that doubling the resolution affects the PRL only slightly. We then conjecture that the PRL formation has to do with the finite duration of the reaction process (or the finite extent of the reaction zone). By the time the boundary rarefaction reaches a cell near the boundary, it may be only partially reacted, and its reaction may therefore be cut off. To establish our conjecture we show how the PRL structure changes with the reaction duration.
The Jovian boundary layer as formed by magnetic-anomaly effects
Dessler, A. J.
1979-01-01
A model is presented in which a plasma boundary layer of Jupiter is formed from plasma of internal origin. It is proposed that, unlike the Earth's boundary layer, which is thought to consist principally of solar wind plasma, Jupiter's boundary layer consists principally of sulphur and oxygen from the Io plasma torus, plus a small component of hydrogen from Jupiter's ionosphere. Fresh plasma is supplied to the boundary layer once each planetary rotation period by a convection pattern that rotates with Jupiter.
Large Eddy Simulation and Study of the Urban Boundary Layer
Institute of Scientific and Technical Information of China (English)
苗世光; 蒋维楣
2004-01-01
Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The modeling and analysis show that the LES model can simulate the planetary boundary layer (PBL) with a uniform underlying surface under various stratifications very well. Then, similar to the description of a forest canopy, the drag term on momentum and the production term of TKE by subgrid city buildings are introduced into the LES equations to account for the area-averaged effect of the subgrid urban canopy elements and to simulate the meteorological fields of the urban boundary layer (UBL). Numerical experiments and comparison analysis show that: (1) the result from the LES of the UBL with a proposed formula for the drag coefficient is consistent and comparable with that from wind tunnel experiments and an urban subdomain scale model; (2) due to the effect of urban buildings, the wind velocity near the canopy is decreased, turbulence is intensified, TKE, variance, and momentum flux are increased, the momentum and heat flux at the top of the PBL are increased, and the development of the PBL is quickened; (3) the height of the roughness sublayer (RS) of the actual city buildings is the maximum building height (1.5-3 times the mean building height), and a constant flux layer (CFL) exists in the lower part of the UBL.
A thermal plume model for the Martian convective boundary layer
Colaïtis, A.; Spiga, A.; Hourdin, F.; Rio, C.; Forget, F.; Millour, E.
2013-07-01
The Martian planetary boundary layer (PBL) is a crucial component of the Martian climate system. Global climate models (GCMs) and mesoscale models (MMs) lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the "thermal plume" model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single-column models. The aim of this physically based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in large-eddy simulations (LESs). We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near-surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking into account stability and turbulent gustiness to calculate surface-atmosphere fluxes. Those new parameterizations for the surface and mixed layers are validated against near-surface lander measurements. Using a thermal plume model moreover enables a first-order estimation of key turbulent quantities (e.g., PBL height and convective plume velocity) in Martian GCMs and MMs without having to run costly LESs.
Turbulent thermal boundary layers with temperature-dependent viscosity
International Nuclear Information System (INIS)
Highlights: • Turbulent thermal boundary layers with temperature-dependent viscosity are simulated. • Effect of temperature-dependent viscosity on the statistics of the scalar field. • An identity for the Stanton number is derived and analyzed. • Effect of temperature-dependent viscosity on the statistics of scalar transfer rate. • Modification of turbulent flow field leads to an enhanced scalar transfer rate. - Abstract: Direct numerical simulations (DNS) of turbulent boundary layers (TBLs) over isothermally heated walls were performed, and the influence of the wall-heating on the thermal boundary layers was investigated. The DNS adopt an empirical relation for the temperature-dependent viscosity of water. The Prandtl number therefore changes with temperature, while the Péclet number is constant. Two wall temperatures (Tw = 70 °C and 99 °C) were considered relative to T∞ = 30 °C, and a reference simulation of TBL with constant viscosity was also performed for comparison. In the variable viscosity flow, the mean and variance of the scalar, when normalized by the friction temperature deficit, decrease relative to the constant viscosity flow. A relation for the mean scalar which takes into account the variable viscosity is proposed. Appropriate scalings for the scalar fluctuations and the scalar flux are also introduced, and are shown to be applicable for both variable and constant viscosity flows. Due to the modification of the near-wall turbulence, the Stanton number and the Reynolds analogy factor are augmented by 10% and 44%, respectively, in the variable viscosity flow. An identity for the Stanton number is derived and shows that the mean wall-normal velocity and wall-normal scalar flux cause the increase in the heat transfer coefficient. Finally, the augmented near-wall velocity fluctuations lead to an increase of the wall-normal scalar flux, which contributes favorably to the enhanced heat transfer at the wall
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.
Indian Academy of Sciences (India)
A K Gupta; R G Shandil
2011-11-01
We utilize the reformulated equations of the classical theory, as derived by Banerjee et al.(J. Math. Anal. Appl. 175 (1993) 458), to establish mathematically, the existence of hydrodynamic instability in single diffusive bottom heavy systems, when considered in the more general framework of the boundary conditions of the type specified by Beavers and Joseph (J. Fluid Mech. 30 (1967) 197), in the parameter regime $T_0_2>1$, where $T_0$ and 2 being some properly chosen mean temperature and coefficient of specific heat (at constant volume) variation due to temperature variation respectively.
International Nuclear Information System (INIS)
We study one-loop corrections in scalar and gauge field theories on the non-commutative torus. For rational θ, Morita equivalence allows these theories to be reformulated in terms of ordinary theories on a commutative torus with twisted boundary conditions. UV/IR mixing does not lead to singularities, however there can be large corrections. In particular, gauge theories show tachyonic instabilities for some of the modes. We discuss their relevance to spontaneous ZNxZN symmetry breaking in the Morita dual SU(N) theory due to electric flux condensation. (author)
Surface Temperature and Surface-Layer Turbulence in a Convective Boundary Layer
Garai, A.; Pardyjak, E.; Steeneveld, G.J.; Kleissl, J.
2013-01-01
Previous laboratory and atmospheric experiments have shown that turbulence influences the surface temperature in a convective boundary layer. The main objective of this study is to examine land-atmosphere coupled heat transport mechanism for different stability conditions. High frequency infrared im
Investigation of turbulent spot production rate in boundary layer
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel; Elsner, W.; Mazur, Oton; Uruba, Václav; Wysocki, M.
Žilina : Žilinská univerzita, 2010, s. 1-6. ISBN 978-80-554-0189-8. [Aplikácia experimentálnych a numerických metód v mechanike tekutín a energetike. Bojnice (SK), 28.04.2010-30.04.2010] R&D Projects: GA AV ČR(CZ) IAA200760614 Institutional research plan: CEZ:AV0Z20760514 Keywords : turbulent spot * by- pas boundary layer transition * transitional intermittency * wavelet analysis Subject RIV: BK - Fluid Dynamics
Laboratory simulation of rotating atmospheric boundary layer flows over obstacles
International Nuclear Information System (INIS)
The present study fits in the frame of a research program concerning in general the dynamics of airflow in the atmospheric boundary layer and in particular the influence of terrestrial rotation on the movements of air masses interacting with natural extended obstacles (mountains). The experiment has been performed by the method of hydraulic simulation, using schematic models at reduced scale in a channel placed on a rotating platform. Only the case of a neutral atmosphere was considered; the wake of an obstacle with semi-circular section and the reciprocal interaction of two obstacles of this kind placed perpendicular to the flow were studied
Experiments on the active control of transitional boundary layers
Nelson, P. A.; Rioual, J.-L.; Fisher, M. J.
Experimental results are presented which demonstrate that the streamwise position of the transition region of a flat plate boundary layer can be actively controlled. The means of control is through the application of suction through the surface of the plate, a progressive increase in suction rate being capable of producing transition at progressively larger distances downstream from the plate leading edge. A simple digital feedback regulator based on an integral control law is shown to be most effective in regulating the position of transition, an error signal being derived from measurements of pressure fluctuations on the surface of the plate.
On Hairpin Vortices in a Transitional Boundary Layer
Czech Academy of Sciences Publication Activity Database
Hladík, Ondřej; Jonáš, Pavel; Uruba, Václav
Liberec : Technical University of Liberec, 2011 - (Vít, T.; Dančová, P.; Novotný, P.), s. 163-170 ISBN 978-80-7372-784-0. - (Vol. 2). [Experimental Fluid Mechanics 2011. Jičín (CZ), 22.11.2011-25.11.2011] R&D Projects: GA ČR GA101/08/1112; GA ČR GAP101/10/1230 Institutional research plan: CEZ:AV0Z20760514 Keywords : turbulence transition * boundary layer * hairpin vortex Subject RIV: BK - Fluid Dynamics http:// orion .kez.tul.cz/efm/
Calculation of Turbulent Boundary Layers Using the Dissipation Integral Method
Institute of Scientific and Technical Information of China (English)
MatthiasBuschmann
1999-01-01
This paper gives an introduction into the dissipation integral method.The general integral equations for the three-dimensional case are derved.It is found that for a practical calculation algorithm the integral monentum equation and the integral energy equation are msot useful.Using Two different sets of mean velocity profiles the hyperbolical character of a dissipation integral method is shown.Test cases for two-and three-dimensional boundary layers are analysed and discussed.The paper concludes with a discussion of the advantages and limits of dissipation integral methods.
Dynamics of Controlled Boundary Layer Separation on a Circular Cylinder
Czech Academy of Sciences Publication Activity Database
Uruba, Václav; Matějka, Milan; Procházka, Pavel
Praha : Ústav termomechaniky AV ČR, v. v. i., 2008 - (Jonáš, P.; Uruba, V.), s. 61-62 ISBN 978-80-87012-14-7. [Colloquium FLUID DYNAMICS 2008. Praha (CZ), 22.10.2008-24.10.2008] R&D Projects: GA AV ČR IAA2076403; GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * separation * control * synthetic jet Subject RIV: BK - Fluid Dynamics www.it.cas.cz/dt
Dynamics of controlled boundary layer separation on a circular cylinder
Czech Academy of Sciences Publication Activity Database
Uruba, Václav; Matějka, Milan
Ostritz - St.Marienthal : DLR Berlin, 2008 - (Hage, W.; Wassen, E.; Choi, K.), s. 1-2 [European Drag Reduction and Flow Control Meeting 2008. Ostritz - St.Marienthal (DE), 08.09.2008-11.09.2008] R&D Projects: GA AV ČR IAA2076403; GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * separation * dynamics Subject RIV: BK - Fluid Dynamics http://edrfcm2008.cfd.tu-berlin.de/
Wave phenomena in a high Reynolds number compressible boundary layer
Bayliss, A.; Maestrello, L.; Parikh, P.; Turkel, E.
1987-01-01
The behavior of spatially unstable waves in a high Reynolds number compressible laminar boundary layer is investigated by solution of the laminar two-dimensional compressible Navier-Stokes equations (solved to fourth-order accuracy) over a flat plate with a fluctuating disturbance generated at the inflow. A significant nonlinear distortion is produced, in qualitative agreement with experimental data. It is shown that increasing compressibility can significantly stabilize the flow over a flat plate, and that the mechanism of phase cancellation is a viable mechanism for the control of growing disturbances.
A wavenumber-frequency spectral model for atmospheric boundary layers
International Nuclear Information System (INIS)
Motivated by the need to characterize power fluctuations in wind farms, we study spatio-temporal correlations of a neutral atmospheric boundary layer in terms of the joint wavenumber-frequency spectrum of the streamwise velocity fluctuations. To this end, we perform a theoretical analysis of a simple advection model featuring the advection of small- scale velocity fluctuations by the mean flow and large-scale velocity fluctuations. The model is compared to data from large-eddy simulations (LES). We find that the model captures the trends observed in LES, specifically a Doppler shift of frequencies due to the mean flow as well as a Doppler broadening due to random sweeping effects
Numerical simulation of 3D flows in atmospheric boundary layer
Czech Academy of Sciences Publication Activity Database
Šimonek, Jiří; Kozel, K.; Jaňour, Zbyněk
Praha : Ústav termomechaniky AV ČR, v. v. i, 2012 - (Šimurda, D.; Kozel, K.), s. 93-96 ISBN 978-80-87012-40-6. [Topical Problems of Fluid Mechanics 2012 . Praha (CZ), 15.02. 2012 -17.02. 2012 ] R&D Projects: GA ČR GAP101/12/1271 Institutional research plan: CEZ:AV0Z20760514 Keywords : numerical solution * atmospheric boundary layer * Navier-Stokes equation s Subject RIV: DG - Athmosphere Sciences, Meteorology
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....
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.
Injection-induced turbulence in stagnation-point boundary layers
Park, C.
1984-02-01
A theory is developed for the stagnation point boundary layer with injection under the hypothesis that turbulence is produced at the wall by injection. From the existing experimental heat transfer rate data obtained in wind tunnels, the wall mixing length is deduced to be a product of a time constant and an injection velocity. The theory reproduces the observed increase in heat transfer rates at high injection rates. For graphite and carbon-carbon composite, the time constant is determined to be 0.0002 sec from the existing ablation data taken in an arc-jet tunnel and a balistic range.
Stationary plasma-field equilibrium states in astropause boundary layers. I - General theory
Fahr, H. J.; Neutsch, W.
1983-11-01
A theoretical investigation has been made of the transition layer between a stellar wind plasma and the surrounding regime of magnetized interstellar plasma, i.e., the astropause boundary layer. For the description of the 'microscopic' structures, a planar representation of the transition zone geometry is used. Here the plasma is taken to be dominated by instability-induced collective relaxation processes as, for example, modified two-stream instabilities, keeping the effective electron and proton temperatures close to each other. These are caused by strong couplings between the plasma constituents and the equilibrium wave field. This permits a quasi-hydrodynamic description of the plasma flow in a two-fluid approximation. For this case, a system of differential equations describing consistently the dynamical variables of the plasma and the magnetic and electric fields in the transition region is developed. Integrals of this system are discussed and it is shown that it can be reduced to one ordinary differential equation. This equation is solved in terms of elliptic integrals and gives an implicit representation of magnetic and electric fields and the density.
Alpha effect due to magnetic buoyancy instability of a horizontal magnetic layer
Chatterjee, Piyali
In this paper we study the hydromagnetic instability of a toroidal magnetic layer such as that thought to be located in the solar tachocline. The magnetic layer is located in a convectively stable layer and is subject to what is known as the magnetic buoyancy instability (MBI) and under suitable conditions breaks up into twisted and arching magnetic flux tubes. The MBI gives rise to an anti-quenched α effect which can be measured by using the sophisticated quasi-kinematic test field method. This paper aims at summarizing the main results of a much longer paper by Chatterjee et al. 2011, A&A (in press).
Observation of a Free-Shercliff-Layer Instability in Cylindrical Geometry
Roach, Austin; Gissinger, Christophe; Edlund, Eric; Sloboda, Peter; Goodman, Jeremy; Ji, Hantao; 10.1103/PhysRevLett.108.154502
2012-01-01
We report on observations of a free-Shercliff-layer instability in a Taylor-Couette experiment using a liquid metal over a wide range of Reynolds numbers, $Re\\sim 10^3-10^6$. The free Shercliff layer is formed by imposing a sufficiently strong axial magnetic field across a pair of differentially rotating axial endcap rings. This layer is destabilized by a hydrodynamic Kelvin-Helmholtz-type instability, characterized by velocity fluctuations in the $r-\\theta$ plane. The instability appears with an Elsasser number above unity, and saturates with an azimuthal mode number $m$ which increases with the Elsasser number. Measurements of the structure agree well with 2D global linear mode analyses and 3D global nonlinear simulations. These observations have implications for a range of rotating MHD systems in which similar shear layers may be produced.
Observation of a free-Shercliff-layer instability in cylindrical geometry.
Roach, Austin H; Spence, Erik J; Gissinger, Christophe; Edlund, Eric M; Sloboda, Peter; Goodman, Jeremy; Ji, Hantao
2012-04-13
We report on observations of a free-Shercliff-layer instability in a Taylor-Couette experiment using a liquid metal over a wide range of Reynolds numbers, Re∼10(3)-10(6). The free Shercliff layer is formed by imposing a sufficiently strong axial magnetic field across a pair of differentially rotating axial end cap rings. This layer is destabilized by a hydrodynamic Kelvin-Helmholtz-type instability, characterized by velocity fluctuations in the r-θ plane. The instability appears with an Elsasser number above unity, and saturates with an azimuthal mode number m which increases with the Elsasser number. Measurements of the structure agree well with 2D global linear mode analyses and 3D global nonlinear simulations. These observations have implications for a range of rotating MHD systems in which similar shear layers may be produced. PMID:22587259
Atmospheric Boundary Layer Characteristics during BOBMEX-Pilot Experiment
Indian Academy of Sciences (India)
G S Bhat; S Ameenulla; M Venkataramana; K Sengupta
2000-06-01
The atmospheric boundary layer characteristics observed during the BOBMEX-Pilot experiment are reported. Surface meteorological data were acquired continuously through an automatic weather monitoring system and manually every three hours. High resolution radiosondes were launched to obtain the vertical thermal structure of the atmosphere. The study area was convectively active, the SSTs were high, surface air was warm and moist, and the surface air moist static energy was among the highest observed over the tropical oceans. The mean sea air temperature difference was about 1.25°C and the sea skin temperature was cooler than bucket SST by 0.5°C. The atmospheric mixed layer was shallow, fluctuated in response to synoptic conditions from 100 m to 900 m with a mean around 500 m.
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)
Energy Technology Data Exchange (ETDEWEB)
Sauer, Helmut
2013-08-01
The author of the book under consideration reports on a calculation of areas of instability for laminar boundary layers at a delayed flow without gradients, at an accelerated incompressible (and compressible) flow as well as heated and cooled wall. The calculation is based on a non-linearized perturbation differential equation. Stability studies of wakes delivered vortex frequencies of the most energetic oscillations as characteristic frequencies. A boundary layer flow already deformed by planar flows is unstable with respect to spatial interference and fanned. Then the regions of instability of the laminar flow are greatly expanded. The calculated wavelengths of the longitudinal vortex can be compared with measured values in the boundary layer flows and wakes in order to explain the flow phenomena occurring in cascades.
Linear and Nonlinear Evolution and Diffusion Layer Selection in Electrokinetic Instability
Demekhin, E A; Polyanskikh, S V
2011-01-01
In the present work fournontrivial stages of electrokinetic instability are identified by direct numerical simulation (DNS) of the full Nernst-Planck-Poisson-Stokes (NPPS) system: i) The stage of the influence of the initial conditions (milliseconds); ii) 1D self-similar evolution (milliseconds-seconds); iii) The primary instability of the self-similar solution (seconds); iv) The nonlinear stage with secondary instabilities. The self-similar character of evolution at intermediately large times is confirmed. Rubinstein and Zaltzman instability and noise-driven nonlinear evolution to over-limiting regimes in ion-exchange membranes are numerically simulated and compared with theoretical and experimental predictions. The primary instability which happens during this stage is found to arrest self-similar growth of the diffusion layer and specifies its characteristic length as was first experimentally predicted by Yossifon and Chang (PRL 101, 254501 (2008)). A novel principle for the characteristic wave number sele...
Drag-driven instability of a dust layer in a magnetized protoplanetary disc
Shadmehri, Mohsen; Rastegarzade, Gohar
2016-01-01
We study drag-driven instability in a protoplanetary disc consisting of a layer of single-sized dust particles which are coupled to the magnetized gas aerodynamically and the particle-to-gas feedback is included. We find a dispersion relation for axisymmetric linear disturbances and growth rate of the unstable modes are calculated numerically. While the secular gravitational instability in the absence of particle-to-gas feedback predicts the dust layer is unstable, magnetic fields significantly amplifies the instability if the Toomre parameter for the gas component is fixed. We also show that even a weak magnetic field is able to amplify the instability more or less irrespective of the dust-gas coupling.
Longitudinal instabilities affecting the moving critical layer laser-plasma ion accelerators
Sahai, Aakash Ajit
2014-01-01
In this work we analyze the longitudinal instabilities of propagating acceleration structures that are driven by a relativistically intense laser at the moving plasma critical layer [1]. These instabilities affect the energy-spectra of the accelerated ion-beams in propagating critical layer acceleration schemes [2][3]. Specifically, using analytical theory and PIC simulations we look into three fundamental physical processes and their interplay that are crucial to the understanding of energy spectral control by making the laser-plasma ion accelerators stable. The interacting processes are (i) Doppler-shifted ponderomotive bunching [1][4] (ii) potential quenching by beam-loading [2] and (iii) two-stream instabilities. These phenomenon have been observed in simulations analyzing these acceleration processes [5][6][7]. From the preliminary models and results we present in this work, we can infer measures by which these instabilities can be controlled [8] for improving the energy-spread of the beams.
Direct numerical simulation of turbulent thermal boundary layers
Kong, Hojin; Choi, Haecheon; Lee, Joon Sik
2000-10-01
In this paper, a method of generating realistic turbulent temperature fluctuations at a computational inlet is proposed and direct numerical simulations of turbulent thermal boundary layers developing on a flat plate with isothermal and isoflux wall boundary conditions are carried out. Governing equations are integrated using a fully implicit fractional-step method with 352×64×128 grids for the Reynolds number of 300, based on the free-stream velocity and the inlet momentum thickness, and the Prandtl number of 0.71. The computed Stanton numbers for the isothermal and isoflux walls are in good agreement with power-law relations without transient region from the inlet. The mean statistical quantities including root-mean-square temperature fluctuations, turbulent heat fluxes, turbulent Prandtl number, and skewness and flatness of temperature fluctuations agree well with existing experimental and numerical data. A quadrant analysis is performed to investigate the coherence between the velocity and temperature fluctuations. It is shown that the behavior of the wall-normal heat flux is similar to that of the Reynolds shear stress, indicating close correlation between the streamwise velocity and temperature. The effect of different thermal boundary conditions at the wall on the near-wall turbulence statistics is also discussed.
Geostrophic convective turbulence: The effect of boundary layers
Ostilla-Mónico, Rodolfo; Kunnen, Rudie P J; Verzicco, Roberto; Lohse, Detlef
2014-01-01
This Letter presents results of the first direct numerical simulations of rotating Rayleigh--B\\'enard convection in the so-called geostrophic regime, (hence very small Ekman numbers $\\mathcal{O}(10^{-7})$ and high Rayleigh numbers~$Ra=10^{10}$ and~$5\\cdot 10^{10}$), employing the \\emph{full} Navier--Stokes equations. In the geostrophic regime the criteria of very strong rotation and large supercriticality are met simultaneously, which is true for many geophysical and astrophysical flows. Until now, numerical approaches of this regime have been based on \\emph{reduced} versions of the Navier--Stokes equations (cf. Sprague \\emph{et al.} J. Fluid Mech., \\textbf{551}, 141 (2006)), omitting the effect of the viscous (Ekman) boundary layers. By using different velocity boundary conditions at the plates, we study the effect of these Ekman layers. We find that the formation of large-scale structures (Rubio \\emph{et al.} (Phys. Rev. Lett. \\textbf{112} (2014)), which indicates the presence of an inverse energy cascade, ...
Characteristics of turbulent boundary layer flow over algal biofilm
Murphy, Elizabeth; Barros, Julio; Schultz, Michael; Steppe, Cecily; Flack, Karen; Reidenbach, Matthew
2015-11-01
Algal biofilms are an important fouling community on ship hulls, with severe economic consequences due to drag-induced increases in fuel use and cleaning costs. Here, we characterize the boundary layer flow structure in turbulent flow over diatomaceous slime, a type of biofilm. Diatomaceous slime composed of three species of diatoms commonly found on ship hulls was grown on acrylic test plates under shear stress. The slime averages 1.6 mm in thickness and has a high density of streamers, which are flexible elongated growths with a length on the order of 1- 2 mm located at the top of the biofilm that interact with the flow. Fouled acrylic plates were placed in a water tunnel facility specialized for detailed turbulent boundary layer measurements. High resolution Particle Image Velocimetry (PIV) data are analyzed for mean velocity profile as well as local turbulent stresses and turbulent kinetic energy (TKE) production, dissipation and transport. Quadrant analysis is used to characterize the impact of the instantaneous events of Reynolds shear stress (RSS) in the flow. To investigate the coherence of the large-scale motion in the flow two-point correlation analysis is employed. Funding provided by the Office of Naval Research and the National Science Foundation.
Using GPS Radio Occultation to study polar boundary layer properties
Ganeshan, M.; Wu, D. L.
2015-12-01
The sensitivity of GPS RO refractivity to moisture and temperature variations in polar regions is explored using radiosonde observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment. A retrieval algorithm for the boundary layer inversion height and surface-based inversion (SBI) frequency is developed for dry atmospheric conditions (total precipitable water < 3.6 mm) that typically exist during polar winter, as well as in high-latitude, elevated regions such as eastern Antarctica and central Greenland. The algorithm is applied to the high-resolution refractivity profiles obtained over the polar Arctic region using the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) dataset for the period 2006-2013. The method is found useful for capturing the spatiotemporal variability in Arctic inversion properties. For the Arctic Ocean, the spatial patterns show a minimum inversion height (maximum SBI frequency) over the ice-covered Pacific sector similar to that observed in past studies. Monthly evolution of the inversion characteristics suggests a surface temperature control in the multi-year sea ice region, with the peak in SBI frequency occurring during the transition period from winter to spring. For central Greenland, the seasonal peak in SBI frequency occurs during winter. The diurnal variability in SBI frequency is forced mainly by solar heating, consistent with past observations. Despite some limitations, the RO refractivity profile is found quite useful for monitoring the Arctic boundary layer, and is able to capture the interannual variability of inversion characteristics.
Delaying natural transition of a boundary layer using smooth steps
Xu, Hui; Sherwin, Spencer J
2015-01-01
The boundary layer flow over a smooth forward-facing stepped plate is studied with particular emphasis on the delay of the transition to turbulence. The interaction between the Tollmien-Schlichting (T-S) waves and the base flow over a single/two forward facing smooth steps is conducted by linear analysis indicating the amplitude of the T-S waves are attenuated in the boundary layer over a single smooth plate. Furthermore, we show that two smooth forward facing steps give rise to a further reduction of the amplitude of the T-S waves. A direct numerical simulation (DNS) is performed for the two smooth forward steps correlating favourably with the linear analysis and showing that for the investigated parameters, the K-type transition is inhibited whereas the turbulence onset of the H-type transition is postponed albeit not suppressed. Transition is indeed delayed and drag reduced for both these transition scenarios suggesting smooth forward facing steps could be leveraged as a passive flow control strategy to de...
Optimizing EDMF parameterization for stratocumulus-topped boundary layer
Jones, C. R.; Bretherton, C. S.; Witek, M. L.; Suselj, K.
2014-12-01
We present progress in the development of an Eddy Diffusion / Mass Flux (EDMF) turbulence parameterization, with the goal of improving the representation of the cloudy boundary layer in NCEP's Global Forecast System (GFS), as part of a multi-institution Climate Process Team (CPT). Current GFS versions substantially under-predict cloud amount and cloud radiative impact over much of the globe, leading to large biases in the surface and top of atmosphere energy budgets. As part of the effort to correct these biases, the CPT is developing a new EDMF turbulence scheme for GFS, in which local turbulent mixing is represented by an eddy diffusion term while nonlocal shallow convection is represented by a mass flux term. The sum of both contributions provides the total turbulent flux. Our goal is for this scheme to more skillfully simulate cloud radiative properties without negatively impacting other measures of weather forecast skill. One particular challenge faced by an EDMF parameterization is to be able to handle stratocumulus regimes as well as shallow cumulus regimes. In order to isolate the behavior of the proposed EDMF parameterization and aid in its further development, we have implemented the scheme in a portable MATLAB single column model (SCM). We use this SCM framework to optimize the simulation of stratocumulus cloud top entrainment and boundary layer decoupling.
On boundary layer modelling using the ASTEC code
International Nuclear Information System (INIS)
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
Second Law Analysis of the Turbulent Flat Plate Boundary Layer
Directory of Open Access Journals (Sweden)
Dragos Isvoranu
2000-09-01
Full Text Available
Until now the second law analysis of turbulent flow relied only on the irreversibilities performed by the mean velocity and mean temperature gradients. Using the Reynolds decomposition of the volumetric entropy generation rate expression we found that the dissipation rates of both, turbulent kinetic energy and fluctuating temperature variance, also represent the irreversibilities of the flow. Applying the above results, the second law analysis of the turbulent boundary layer shows that the maximum values of the "mean motion irreversibilities" (generated by the mean velocity and mean temperature gradient are located at the wall, while the maximum values of the "turbulent irreversibilities" (performed by the dissipation rate of turbulent kinetic energy and fluctuating temperature variance are located in the buffer sublayer. As a consequence, for a given location on the plate, the integral values of the "mean motion irreversibilities" are approximately constant and the "turbulent irreversibilities" grow up with the boundary layer thickness.
Cross-Field Current Instabilities in Thin Ionization Layers and the Enhanced Aurora
Energy Technology Data Exchange (ETDEWEB)
Jay R. Johnson and Hideo Okuda
2008-05-20
Nearly half of the time, auroral displays exhibit thin, bright layers known as \\enhanced aurora." There is a substantial body of evidence that connects these displays with thin, dense, heavy ion layers in the E-region. Based on the spectral characteristics of the enhanced layers, it is believed that they result when wave-particle interaction heats ambient electrons to energies at or just above the 17 eV ionization energy of N2. While there are several possible instabilities that could produce suprathermal electrons in thin layers, there has been no clear theoretical investigation which examines in detail how wave instabilities in the thin ionization layers could develop and produce the suprathermal electrons. We examine instabilities which would occur in thin, dense, heavy ion layers using extensive analytical analysis combined with particle simulations. We analyze a cross field current instability that is found to be strongly unstable in the heavy ion layers. Electrostatic simulations show that substantial heating of the ambient electrons occurs with energization at or above the N2 ionization energy.
Cross-Field Current Instabilities in Thin Ionization Layers and the Enhanced Aurora
International Nuclear Information System (INIS)
Nearly half of the time, auroral displays exhibit thin, bright layers known as 'enhanced aurora'. There is a substantial body of evidence that connects these displays with thin, dense, heavy ion layers in the E-region. Based on the spectral characteristics of the enhanced layers, it is believed that they result when wave-particle interaction heats ambient electrons to energies at or just above the 17 eV ionization energy of N2. While there are several possible instabilities that could produce suprathermal electrons in thin layers, there has been no clear theoretical investigation which examines in detail how wave instabilities in the thin ionization layers could develop and produce the suprathermal electrons. We examine instabilities which would occur in thin, dense, heavy ion layers using extensive analytical analysis combined with particle simulations. We analyze a cross field current instability that is found to be strongly unstable in the heavy ion layers. Electrostatic simulations show that substantial heating of the ambient electrons occurs with energization at or above the N2 ionization energy.
Kink Band Instability and Propagation in Layered Structures
Wadee, M.A.; Hunt, G.W.; Peletier, M.A.
2003-01-01
A recent two-dimensional prototype model for the initiation of kink banding in compressed layered structures is extended to embrace the two propagation mechanisms of band broadening and band progression. As well as interlayer friction, overburden pressure and layer bending energy, the characteristic
Choudhari, Meelan M.; Tokugawa, Naoko; Li, Fei; Chang, Chau-Lyan; White, Jeffery A.; Ishikawa, Hiroaki; Ueda, Yoshine; Atobe, Takashi; Fujii, Keisuke
2012-01-01
Boundary layer transition over axisymmetric bodies at non-zero angle of attack in supersonic flow is numerically investigated as part of joint research between the National Aeronautics and Space Administration (NASA) and Japan Aerospace Exploration Agency (JAXA). Transition over four axisymmetric bodies (namely, Sears-Haack body, semi-Sears-Haack body, 5-degree straight cone and flared cone) with different axial pressure gradients has been studied at Mach 2 in order to understand the effect of axial pressure gradient on instability amplification along the leeward symmetry plane and in the region of nonzero crossflow away from it. Comparisons are made with measured transition data in Mach 2 facilities as well as with predicted and measured transition characteristics for a 5-degree straight cone in a Mach 3.5 low disturbance tunnel. Limitations of using linear stability correlations for predicting transition over axisymmetric bodies at angle of attack are pointed out.
Structuring of turbulence and its impact on basic features of Ekman boundary layers
Directory of Open Access Journals (Sweden)
I. Esau
2013-08-01
Full Text Available The turbulent Ekman boundary layer (EBL has been studied in a large number of theoretical, laboratory and modeling works since F. Nansen's observations during the Norwegian Polar Expedition 1893–1896. Nevertheless, the proposed analytical models, analysis of the EBL instabilities, and turbulence-resolving numerical simulations are not fully consistent. In particular, the role of turbulence self-organization into longitudinal roll vortices in the EBL and its dependence on the meridional component of the Coriolis force remain unclear. A new set of large-eddy simulations (LES are presented in this study. LES were performed for eight different latitudes (from 1° N to 90° N in the domain spanning 144 km in the meridional direction. Geostrophic winds from the west and from the east were used to drive the development of EBL turbulence. The emergence and growth of longitudinal rolls in the EBL was simulated. The simulated rolls are in good agreement with EBL stability analysis given in Dubos et al. (2008. The destruction of rolls in the westerly flow at low latitude was observed in simulations, which agrees well with the action of secondary instability on the rolls in the EBL. This study quantifies the effect of the meridional component of the Coriolis force and the effect of rolls in the EBL on the internal EBL parameters such as friction velocity, cross-isobaric angle, parameters of the EBL depth and resistance laws. A large impact of the roll development or destruction is found. The depth of the EBL in the westerly flow is about five times less than it is in the easterly flow at low latitudes. The EBL parameters, which depend on the depth, also exhibit large difference in these two types of the EBL. Thus, this study supports the need to include the horizontal component of the Coriolis force into theoretical constructions and parameterizations of the boundary layer in models.
Climatic impacts of the boundary layer circulation over Antarctica
International Nuclear Information System (INIS)
Prolonged periods of strong radiational cooling over the sloping ice fields of Antarctica produce cold, negatively buoyant air in the lowest layers of the atmosphere. This cooling generates a continental-scale, near-surface wind-field which is highly irregular. Cold air in the interior is channeled into narrow zones that enable the downstream coastal katabatic winds to become anomalously strong and persistent. This probably means that the boundary layer transport of air across the Antarctic coastline is concentrated in a small number of narrow regions, and that previous quantitative evaluations of the importance of this boundary layer circulation are likely to be substantially in error. From continuity considerations, the time-averaged outflow of cold surface air must be compensated by inflow aloft and sinking over the continent. This time-averaged meridional mass circulation plays a dominant role in the heat budget of the Antarctic atmosphere by adiabatic compression in the statically stable atmosphere. The tropospheric convergence and sinking motion also generate cyclonic vorticity which is comparable in magnitude to that arising from the temperature contrast between the ice sheet and the surrounding ocean. That is, the circumpolar vortex is centered over the East Antarctic ice sheet in pan because of the tropospheric mass convergence. The concentration of cold surface air transport from the ice sheet into narrow coastal zones has important consequences for sea ice formation and cyclonic development. Katabatic jets can force coastal polynyas where very active sea ice formation and associated brine rejection produce saline shelf water. This water mass is a component of Antarctic Bottom Water. Such water mass formation provides a way to couple climatic variations over the ice sheet to the deep ocean on relatively short time scales
Angular Momentum Transport in Accretion Disk Boundary Layers Around Weakly Magnetized Stars
Directory of Open Access Journals (Sweden)
Pessah Martin E.
2013-04-01
Full Text Available The standard model for turbulent shear viscosity in accretion disks is based on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. This implies that the turbulent stress must be negative and thus transport angular momentum inwards, in the boundary layer where the accretion disk meets the surface of a weakly magnetized star. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. Motivated by the need of a deeper understanding of the behavior of an MHD fluid in a differentially rotating background that deviates from a Keplerian profile, we study the dynamics of MHD waves in configurations that are stable to the standard MRI. Employing the shearing-sheet framework, we show that transient amplification of shearing MHD waves can generate magnetic energy without leading to a substantial generation of hydromagnetic stresses. While these results are in agreement with numerical simulations, they emphasize the need to better understand the mechanism for angular momentum transport in the inner disk regions on more solid grounds.
Hydrodynamic theory of convective transport across a dynamically stabilized diffuse boundary layer
International Nuclear Information System (INIS)
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 ZnJ2-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.)
Wagner, Alexander; Kuhn, Markus; Martinez Schramm, Jan; Hannemann, Klaus
2013-10-01
For the first time, the influence of ultrasonically absorptive carbon-carbon material on hypersonic laminar to turbulent boundary layer transition was investigated experimentally. A 7° half-angle blunted cone with a nose radius of 2.5 mm and a total length of 1,077 mm was tested at zero angle of attack in the High Enthalpy Shock Tunnel Göttingen of the German Aerospace Center (DLR) at Mach 7.5. One-third of the metallic model surface in circumferential direction was replaced by DLR in-house manufactured ultrasonically absorptive carbon-carbon material with random microstructure for passive transition control. The remaining model surface consisted of polished steel and served as reference surface. The model was equipped with coaxial thermocouples to determine the transition location by means of surface heat flux distribution. Flush-mounted piezoelectric fast-response pressure transducers were used to measure the pressure fluctuations in the boundary layer associated with second-mode instabilities. The free-stream unit Reynolds number was varied over a range of Re m = 1.5 × 106 m-1 to Re m = 6.4 × 106 m-1 at a stagnation enthalpy of h 0 ≈ 3.2 MJ/kg and a wall temperature ratio of T w/ T 0 ≈ 0.1. The present study revealed a clear damping of the second-mode instabilities and a delay of boundary layer transition along the ultrasonically absorptive carbon-carbon insert.
FOREWORD: International Conference on Planetary Boundary Layer and Climate Change
Djolov, G.; Esau, I.
2010-05-01
One of the greatest achievements of climate science has been the establisment of the concept of climate change on a multitude of time scales. The Earth's complex climate system does not allow a straightforward interpretation of dependences between the external parameter perturbation, internal stochastic system dynamics and the long-term system response. The latter is usually referred to as climate change in a narrow sense (IPCC, 2007). The focused international conference "Planetary Boundary Layers and Climate Change" has addressed only time scales and dynamical aspects of climate change with possible links to the turbulent processes in the Planetary Boundary Layer (PBL). Although limited, the conference topic is by no means singular. One should clearly understand that the PBL is the layer where 99% of biosphere and human activity are concentrated. The PBL is the layer where the energy fluxes, which are followed by changes in cryosphere and other known feedbacks, are maximized. At the same time, the PBL processes are of a naturally small scale. What is the averaged long-term effect of the small-scale processes on the long-term climate dynamics? Can this effect be recognized in existing long-term paleo-climate data records? Can it be modeled? What is the current status of our theoretical understanding of this effect? What is the sensitivity of the climate model projections to the representation of small-scale processes? Are there significant indirect effects, e.g. through transport of chemical components, of the PBL processes on climate? These and other linked questions have been addressed during the conference. The Earth's climate has changed many times during the planet's history, with events ranging from ice ages to long periods of warmth. Historically, natural factors such as the amount of energy released from the Sun, volcanic eruptions and changes in the Earth's orbit have affected the Earth's climate. Beginning late in the 18th century, human activities
Intermittent phenomena in the boiling two-phase boundary layer
International Nuclear Information System (INIS)
In order to investigate statistical properties of temperature fluctuation in a boiling two-phase boundary layer the corresponding intermittency functions, which describe liquid, vapour and interface region at an individual fixed point, have been defined. In water boiling on a horizontal surface the temperature fluctuation was measured with a microthermocouple and the signal was processed through the digital computer with the detector function specified for liquid, vapor and interface region. The results obtained confirm that the temperature fluctuation in the boiling two-phase layer can be divided into three parts corresponding to individual regions and that its statistical distribution depends on the properties of respective systems. It has also been shown that the temperature fluctuation in the interface region is determinative and corresponds to the temperature changes in the liquid layer surrounding vapor bubble growth. Amplitude distribution in the liquid region changes its form with the distance from the wall as a result of the change in intensity of turbulence at different distances. The probability density distribution in the vapor region shows very small amplitude fluctuation and is almost constant for all distances. (author)
Fichtl, G. H.
1973-01-01
The realistic simulation of flow in the atmospheric boundary layers at heights greater than two kilometers is discussed. Information concerning horizontally homogeneous and statistically stationary atmospheric boundary layer flows is presented. The problems related to the incorporation of the information into atmospheric wind simulation programs are analyzed. The information which the meteorologist must acquire in order to provide a basis for improving the simulation of atmospheric boundary flows is explained.
A Lagrangian Study of Southeast Pacific Boundary Layer Clouds
Painter, Gallia
concentration which extend far offshore into regions of normally very clean cloud. We use Lagrangian trajectories to investigate the source of the high droplet concentrations of the mesoscale "hooks", and evaluate whether boundary layer transport of coastal pollutants alone can account for their extent. We find that boundary layer trajectories past 85 W do not pass sufficiently close to the coastline to explain high aerosol concentrations offshore.
The viscous boundary layer at the free surface of a rotating baroclinic fluid
Hide, R.
2011-01-01
The properties of the viscous boundary layer at the free surface of a rotating baroclinic fluid are analyzed and compared with those of the well-known Ekman boundary layer at a rigid surface. Although the ageostrophic components of the flow in the free surface boundary layer are weaker than in the Ekman layer, there are problems of practical interest in which their effects are not negligible.DOI: 10.1111/j.2153-3490.1964.tb00188.x
Instabilities of flows and transition to turbulence
Sengupta, Tapan K
2012-01-01
Introduction to Instability and TransitionIntroductionWhat Is Instability?Temporal and Spatial InstabilitySome Instability MechanismsComputing Transitional and Turbulent FlowsFluid Dynamical EquationsSome Equilibrium Solutions of the Basic EquationBoundary Layer TheoryControl Volume Analysis of Boundary LayersNumerical Solution of the Thin Shear Layer (TSL) EquationLaminar Mixing LayerPlane Laminar JetIssues of Computing Space-Time Dependent FlowsWave Interaction: Group Velocity and Energy FluxIssues of Space-Time Scale Resolution of FlowsTemporal Scales in Turbulent FlowsComputing Time-Averag
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.
Turbulence Scales Simulations in Atmospheric Boundary Layer Wind Tunnels
Directory of Open Access Journals (Sweden)
Elena-Carmen Teleman
2008-01-01
Full Text Available The simulation of the air flow over models in atmospheric boundary layer tunnels is a research domain based on advanced scientific technologies imposed by the necessity of studying the turbulent fluid movements in the proximity of the Earth’s surface. The experiment presented herein is developed in the wind tunnel from the Laboratory of Structural Aerodynamics of the Faculty of Civil Engineering and Building Services in Iassy. Measurements necessary for the determination of the turbulence scales of the wind action in urban environment were conducted. The data obtained were processed and analyzed and interpreted with specific software. The results are used for a synthesis regarding the scales of turbulence of the model of flow and the actual accuracy of measurements. The paper presents some of the important elements of this synthesis.
Aeroelectric structures and turbulence in the atmospheric boundary layer
Directory of Open Access Journals (Sweden)
S. V. Anisimov
2013-10-01
Full Text Available Complex electrical measurements with the use of sodar data show that electric field pulsation analysis is useful for electrodynamics/turbulence monitoring under different conditions. In particular, the number of aeroelectric structures (AES generated per hour is a convenient measure of the turbulence intensity. During convectively unstable periods, as many as 5–10 AES form per hour. Under stable conditions, AES occasionally form as well, indicating the appearance of occasional mixing events reflected in the electric field perturbations. AES magnitudes under stable conditions are relatively small, except in special cases such as high humidity and fog. The analysis of electric field (EF spectra gives additional useful information on the parameters of the atmospheric boundary layer and its turbulence. A rather sharp change in the spectrum slope takes place in the vicinity of 0.02 Hz under stable conditions. The characteristic slope of the spectrum and its change are reproduced in a simple model of EF formation.
Compressible Turbulent Boundary Layers on a Strongly Heated Wall
Institute of Scientific and Technical Information of China (English)
无
1993-01-01
This paper concerns the theoretical and experimental modelling of the flat wall,highly heated,compressible turbulent boundary layer.Its final objective is to develop a numerical Navier-Stokes solver and to conclude on its capability to correctly represent complex aerothermic viscous flows near the wall.The paper presents a constructed numerical method with particular attention given to the turbulence modelling at low Reynolds number and comparisons with supersonic and transonic experimental data.For the transonic experiment,very high wall temperature(Tw=1100K)is realized.The method of this difficult experimental set up is discussed.The comparison between experimental and computational data conducts to the first conclusion and gives some indications for the future work.
A Qualitative Description of Boundary Layer Wind Speed Records
Kavasseri, R G; Nagarajan, Radhakrishnan
2006-01-01
The complexity of the atmosphere endows it with the property of turbulence by virtue of which, wind speed variations in the atmospheric boundary layer (ABL) exhibit highly irregular fluctuations that persist over a wide range of temporal and spatial scales. Despite the large and significant body of work on microscale turbulence, understanding the statistics of atmospheric wind speed variations has proved to be elusive and challenging. Knowledge about the nature of wind speed at ABL has far reaching impact on several fields of research such as meteorology, hydrology, agriculture, pollutant dispersion, and more importantly wind energy generation. In the present study, temporal wind speed records from twenty eight stations distributed through out the state of North Dakota (ND, USA), ($\\sim$ 70,000 square-miles) and spanning a period of nearly eight years are analyzed. We show that these records exhibit a characteristic broad multifractal spectrum irrespective of the geographical location and topography. The rapi...
Coherent vorticity extraction in turbulent boundary layers using orthogonal wavelets
Energy Technology Data Exchange (ETDEWEB)
Khujadze, George; Oberlack, Martin [Chair of Fluid Dynamics, Technische Universitaet Darmstadt (Germany); Yen, Romain Nguyen van [Institut fuer Mathematik, Freie Universitaet Berlin (Germany); Schneider, Kai [M2P2-CNRS and CMI, Universite de Provence, Marseille (France); Farge, Marie, E-mail: khujadze@fdy.tu-darmstadt.de [LMD-IPSL-CNRS, Ecole Normale Superieure, Paris (France)
2011-12-22
Turbulent boundary layer data computed by direct numerical simulation are analyzed using orthogonal anisotropic wavelets. The flow fields, originally given on a Chebychev grid, are first interpolated on a locally refined dyadic grid. Then, they are decomposed using a wavelet basis, which accounts for the anisotropy of the flow by using different scales in the wall-normal direction and in the planes parallel to the wall. Thus the vorticity field is decomposed into coherent and incoherent contributions using thresholding of the wavelet coefficients. It is shown that less than 1% of the coefficients retain the coherent structures of the flow, while the majority of the coefficients corresponds to a structureless, i.e., noise-like background flow. Scale-and direction-dependent statistics in wavelet space quantify the flow properties at different wall distances.
Combined core/boundary layer transport simulations in tokamaks
International Nuclear Information System (INIS)
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
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.
Unsteady Phenomena in Shock Wave/Boundary Layer Interaction
Dolling, D. S.
1993-01-01
A brief review is given of the unsteadiness of shock wave/turbulent boundary layer interaction. The focus is on interactions generated by swept and unswept compression ramps, by flares, steps and incident shock waves, by cylinders and blunt fins, and by glancing shock waves. The effects of Mach number, Reynolds number, and separated flow scale are discussed as are the physical causes of the unsteadiness. The implications that the unsteadiness has for interpreting time-average surface and flowfield data, and for comparisons of such experimental data with computation, is also briefly discussed. Finally, some suggestions for future work are given. It is clear that there are large gaps in the data base and that many aspects of such phenomena are poorly understood. Much work remains to be done.
Segregation in the Atmospheric Boundary Layer - A Discussion
Dlugi, Ralph; Berger, Martina; Zelger, Michael; Hofzumahaus, Andreas; Rohrer, Franz; Holland, Frank; Lu, Keding; Tsokankunku, Anywhere; Sörgel, Matthias; Kramm, Gerhard; Mölders, Nicole
2016-04-01
Segregation is a well known topic in technical chemistry and means an incomplete mixing of the reactants. Incomplete mixing reduces the rate of reaction which is of utmost importance in technical chemistry but has been payed less attention in atmospheric chemistry. Different observational and modelling studies on chemical reactions in the turbulent and convective atmospheric boundary layer are analysed for the influences of segregation in the systems NO ‑ NO2 ‑ O3 and OH + V OCs (with main focus on isoprene). Also some estimates on reactions like HO2 + NO (an important recycling mechanism for OH) will be given. Especially, different terms of the intensity of segregation IS (correlation coefficients, standard deviations of mixing ratios) are compared and are related to characteristics of the flow regimes, such as mixing conditions and Damköhler numbers. Also influences of fluctuations of actinic fluxes are discussed which influence the mostly photo chemically driven reactions that were investigated.
Numerical analysis of the turbulent natural convection boundary layer
International Nuclear Information System (INIS)
It is considered to be one of options of nuclear fuel cycle policies in Japan to store spent fuel before reprocessing. Then we have to evaluate of the thermal integrity for dry type cask storage system. But the turbulent natural convection boundary layer is a flow with relatively large fluctuations of velocity and temperature at low velocity, and measurements of turbulent quantities near the wall are especially difficult. So, the turbulent structure has not been elucidated. On the other hand, numerical analyses of natural convection using turbulence models have been developed. However, there are not the models which are suitable for prediction of natural convection exactly, so it's effective to analyze of direct numerical simulation (DNS). The propose of this study is to simulate (DNS) for buoyant flow as economical as possible. We calculate two different grid size to investigate to numerical accuracy. (author)
Calculation of transitional boundary layer under pressure gradient
International Nuclear Information System (INIS)
A modified κ-ε model is proposed for calculation of transitional boundary-layer flows under pressure gradient with high freestream turbulence intensity. In order to develop the model for this problem, the flow is divided into three regions; pre-transition region, transition region and fully turbulent region. The effect of pressure gradient is taken into account in a stream-wise intermittency factor, bridging the eddy-viscosities between in the pre-transition region and in the fully turbulent region. From intermittency data in various flows, Narashima's intermittency function, F(γ), has been found to be proportional to xn according to the extent of pressure gradient. Three empirical correlations of intermittency factor being analyzed, the best one was chosen to calculate three benchmark cases of bypass transition under pressure gradient. It was found that the variations of skin friction and shape factor as well as the profiles of mean velocity in the transition region were very satisfactorily predicted
Vertical pressure gradient and particle motions in wave boundary layers
DEFF Research Database (Denmark)
Jensen, Karsten Lindegård
is a function of phase. Therefore the particle will settle towards the end of each half period, and after flow reversal, when the turbulent intensity becomes large enough it can be suspended. If the particle is light enough it can be maintained in suspension, otherwise it will settle before it is....... This is in contrast to velocity fluctuations that are diffusive, so they can also contain residual turbulence from the previous half cycle until they are dissipated. Furthermore, the magnitude of the mean value of conditionally averaged vertical pressure gradient (for −∂p∗/∂x∗ 2 > 0) is compared to the...... submerged weight of sediment. This revels that the upward directed vertical pressure gradient on average has a magnitude that yields in a contribution to the force needed to overcome the submerged weight of the water-sediment mixture. Secondly particle motion in the oscillatory boundary layer is...
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
2016-04-01
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.
Shock Wave-Boundary Layer Interaction in Forced Shock Oscillations
Institute of Scientific and Technical Information of China (English)
Piotr Doerffer; Oskar Szulc; Franco Magagnato
2003-01-01
The flow in transonic diffusers as well as in supersonic air intakes becomes often unsteady due to shock wave boundary layer interaction. The oscillations may be induced by natural separation unsteadiness or may be forced by boundary conditions. Significant improvement of CFD tools, increase of computer resources as well as development of experimental methods have again.drawn the attention of researchers to this topic.To investigate the problem forced oscillations of transonic turbulent flow in asymmetric two-dimensional Laval nozzle were considered. A viscous, perfect gas flow, was numerically simulated using the Reynolds-averaged compressible Navier-Stokes solver SPARC, employing a two-equation, eddy viscosity, turbulence closure in the URANS approach.For time-dependent and stationary flow simulations, Mach numbers upstream of the shock between 1.2 and 1.4 were considered. Comparison of computed and experimental data for steady states generally gave acceptable agreement. In the case of forced oscillations, a harmonic pressure variation was prescribed at the exit plane resulting in shock wave motion. Excitation frequencies between 0 Hz and 1024 Hz were investigated at the same pressure amplitude.The main result of the work carried out is the relation between the amplitude of the shock wave motion and the excitation frequency in the investigated range. Increasing excitation frequency resulted in decreasing amplitude of the shock movement. At high frequencies a natural mode of shock oscillation (of small amplitude) was observed which is not sensitive to forced excitement.
Turbulence transition in the asymptotic suction boundary layer
Kreilos, Tobias; Schneider, Tobias M; Veble, Gregor; Duguet, Yohann; Schlatter, Philipp; Henningson, Dan S; Eckhardt, Bruno
2015-01-01
We study the transition to turbulence in the asymptotic suction boundary layer (ASBL) by direct numerical simulation. Tracking the motion of trajectories intermediate between laminar and turbulent states we can identify the invariant object inside the laminar-turbulent boundary, the edge state. In small domains, the flow behaves like a travelling wave over short time intervals. On longer times one notes that the energy shows strong bursts at regular time intervals. During the bursts the streak structure is lost, but it reforms, translated in the spanwise direction by half the domain size. Varying the suction velocity allows to embed the flow into a family of flows that interpolate between plane Couette flow and the ASBL. Near the plane Couette limit, the edge state is a travelling wave. Increasing the suction, the travelling wave and a symmetry-related copy of it undergo a saddle-node infinite-period (SNIPER) bifurcation that leads to bursting and discrete-symmetry shifts. In wider domains, the structures loc...
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.
Boundary-layer stability analysis of Langley Research Center 8-foot LFC experimental data
Berry, Scott; Dagenhart, J. Ray; Brooks, Cuyler W., Jr.; Harris, Charles D.
1987-01-01
An analytical study of linear-amplifying instabilities of a laminar boundary layer as found in the experimental data of the LaRC/8-foot laminar-flow control (LFC) experiment was completed and the results are presented. The LFC airfoil used for this experiment was a swept, supercritical design which removed suction air through spanwise slots. The amplification of small disturbances by linear processes on a swept surface such as this can be due to either Tollmien-Schlichting (TS) and/or crossflow (CF) mechanisms. This study consists of the examination of these two instabilities by both the commonly used incompressible (SALLY and MARIA) analysis and the more involved compressible (COSAL) analysis. A wide range of experimental test conditions with variations in Mach number, Reynolds number, and suction distributions were available for this study. Experimentally determined transition locations were found from thin-film techniques and were used to correlate the n-factors at transition for the range of test cases.
Evidence of tropospheric layering: interleaved stratospheric and planetary boundary layer intrusions
Directory of Open Access Journals (Sweden)
J. Brioude
2007-01-01
Full Text Available We present a case study of interleaving in the free troposphere of 4 layers of non-tropospheric origin, with emphasis on their residence time in the troposphere. Two layers are stratospheric intrusions at 4.7 and 2.2 km altitude with residence times of about 2 and 6.5 days, respectively. The two other layers at 7 and 3 km altitude were extracted from the maritime planetary boundary layer by warm conveyor belts associated with two extratropical lows and have residence times of about 2 and 5.75 days, respectively. The event took place over Frankfurt (Germany in February 2002 and was observed by a commercial airliner from the MOZAIC programme with measurements of ozone, carbon monoxide and water vapour. Origins and residence times in the troposphere of these layers are documented with a trajectory and particle dispersion model. The combination of forward and backward simulations of the Lagrangian model allows the period of time during which the residence time can be assessed to be longer, as shown by the capture of the stratospheric-origin signature of the lowest tropopause fold just about to be completely mixed above the planetary boundary layer. This case study is of interest for atmospheric chemistry because it emphasizes the importance of coherent airstreams that produce laminae in the free troposphere and that contribute to the average tropospheric ozone. The interleaving of these 4 layers also provides the conditions for a valuable case study for the validation of global chemistry transport models used to perform tropospheric ozone budgets.
Mesoscale (50-km) Boundary Layer Eddies in CASES-97
LeMone, M. A.; Grossman, R. L.; Yates, D.; Chen, F.; Ikeda, K.
2001-05-01
Boundery-layer eddies 50 km across are documented for the morning of 10 May 1997 during the Cooperative Atmosphere Surface Exchange Study (CASES-97). CASES-97 was held from 21 April to 21 May 1997, in the lower Walnut River Watershed in south central Kansas, to study the role of the heterogeneous surface in boundary-layer evolution. The eddies appear to be tied to terrain, with warm, upwelling air over the relatively high terrain that forms the eastern edge of the watershed, and downwelling air over the watershed. The winds on this day were 5 m/s out of the south, and there were strong horizontal contrasts in vegetation and surface fluxes, suggesting that surfact fluxes could also play a role. For comparison, we examine two other days for the presence of mesoscale eddies, 29 April (characterized by high horizontal heterogeneity of vegetation and 10 m/s southerlies), and 20 May (characterized by a uniformly green and moist surface with winds ENE at 7 m/s). 29 April had significant but rapidly-changing horizontal variability at scales greater than 10 km, but variability on 20 May was on scales less than 5 km. Estimates of the sensible heat budgets for the three days revealed a large residual for 10 May, the day with the mesoscale eddies. Calculation of the expected errors and reasonable corrections for bias errors and radiative heating did not account for the residual, leading to the hypothesis that the residual is associated with the mesoscale eddies.
Numerical simulations of coupled sea waves and boundary layer dynamics
Chalikov, D.
2009-04-01
Wind-wave dynamic and thermodynamic interaction belongs to one of the most important problems of geophysical fluid dynamics. At present this interaction in a parameterized form is taken into account for formulation of boundary conditions in atmospheric and oceanic models, weather forecast models, coupled ocean-atmosphere climate models and wave forecasting models. However, the accuracy of this parameterization is mostly unknown. The main difficulty in experimental and theoretical investigation of small-scale ocean-atmosphere interaction is the presence of a multi-mode (and, occasionally, non- single-valued) nonstationary interface. It makes impossible many types of measurements in close vicinity of the physical surface, and highly complicates construction of numerical models. Existing approaches on the wind-wave interaction problem are based on assumptions that a wave field can be represented as superposition of linear waves whilst the process of wind-wave interaction is a superposition of elementary processes. This assumption is acceptable only for very small amplitude waves due to: (1) wave surface cannot be represented as superposition of linear waves with random phases as a result of nonlinearity leading to formation of ‘bound' waves, focusing energy in physical space and wave breaking; (2) dynamic interactions of waves with the air (for example, long waves modify the local flow, which influences energy input into short waves, while short waves create local drag that affects the flow over large waves). In general, all waves "spring, burgeon and fall" in the environment provided by the entire spectrum; (3) energy input into waves of even moderate steepness is concentrated rather in physical space than in Fourier space. Hence, a Fourier image of the input is often not quite representative. The new approach to the problem is based on coupled 2-D modeling of waves and boundary layer in joint conformal surface-following coordinates. The wave model is based on full
International Nuclear Information System (INIS)
Transition to turbulence occurring in a flat-plate boundary-layer flow subjected to high levels of free-stream turbulence is considered. This scenario, denoted bypass transition, is characterised by the non-modal growth of streamwise elongated disturbances. These so-called streaks are regions of positive and negative streamwise velocity alternating in the spanwise direction inside the boundary layer. When they reach large enough amplitudes, breakdown into turbulent spots occurs via their secondary instability. In this work, the bypass-transition process is simulated using direct numerical simulations (DNS) and large-eddy simulations (LES). The ADM-RT subgrid-scale model turned out to be particularly suited for transitional flows after a thorough validation. Linear feedback control is applied in order to reduce the perturbation energy and consequently delay transition. This case represents therefore an extension of the linear approach (Chevalier, M., Hoepffner, J., Akervik, E., Henningson, D.S., 2007a. Linear feedback control and estimation applied to instabilities in spatially developing boundary layers. J. Fluid Mech. 588, 163-187, 167-187.) to flows characterised by strong nonlinearities. Control is applied by blowing and suction at the wall and it is both based on the full knowledge of the instantaneous velocity field (i.e. full information control) and on the velocity field estimated from wall measurements. The results show that the control is able to delay the growth of the streaks in the region where it is active, which implies a delay of the whole transition process. The flow field can be estimated from wall measurements alone: The structures occurring in the 'real' flow are reproduced correctly in the region where the measurements are taken. Downstream of this region the estimated field gradually diverges from the 'real' flow, revealing the importance of the continuous excitation of the boundary layer by the external free-stream turbulence. Control based on
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 inferr
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
Surface layer similarity in the nocturnal boundary layer: the application of Hilbert-Huang transform
Directory of Open Access Journals (Sweden)
J. Hong
2009-10-01
Full Text Available Turbulence statistics such as flux-variance relationship is critical information in measuring and modeling carbon, water, energy, and momentum exchanges at the biosphere-atmosphere interface. Using a recently proposed mathematical technique, the Hilbert-Huang transform (HHT, this study highlights its possibility to quantify impacts of non-turbulent flows on turbulence statistics in the stable surface layer. The HHT is suitable for the analysis of non-stationary and intermittent data and thus very useful for better understanding of the interplay of the surface layer similarity with complex nocturnal environment. Our analysis showed that the HHT can successfully sift non-turbulent components and be used as a tool to estimate the relationships between turbulence statistics and atmospheric stability in complex environment such as nocturnal stable boundary layer.
Surface layer similarity in the nocturnal boundary layer: the application of Hilbert-Huang transform
Directory of Open Access Journals (Sweden)
J. Hong
2010-04-01
Full Text Available Turbulence statistics such as flux-variance relationship are critical information in measuring and modeling ecosystem exchanges of carbon, water, energy, and momentum at the biosphere-atmosphere interface. Using a recently proposed mathematical technique, the Hilbert-Huang transform (HHT, this study highlights its possibility to quantify impacts of non-turbulent flows on turbulence statistics in the stable surface layer. The HHT is suitable for the analysis of non-stationary and intermittent data and thus very useful for better understanding the interplay of the surface layer similarity with complex nocturnal environment. Our analysis showed that the HHT can successfully sift non-turbulent components and be used as a tool to estimate the relationships between turbulence statistics and atmospheric stability in complex environments such as nocturnal stable boundary layer.
Fuel decomposition and boundary-layer combustion processes of hybrid rocket motors
Chiaverini, Martin J.; Harting, George C.; Lu, Yeu-Cherng; Kuo, Kenneth K.; Serin, Nadir; Johnson, David K.
1995-01-01
Using a high-pressure, two-dimensional hybrid motor, an experimental investigation was conducted on fundamental processes involved in hybrid rocket combustion. HTPB (Hydroxyl-terminated Polybutadiene) fuel cross-linked with diisocyanate was burned with GOX under various operating conditions. Large-amplitude pressure oscillations were encountered in earlier test runs. After identifying the source of instability and decoupling the GOX feed-line system and combustion chamber, the pressure oscillations were drastically reduced from +/-20% of the localized mean pressure to an acceptable range of +/-1.5% Embedded fine-wire thermocouples indicated that the surface temperature of the burning fuel was around 1000 K depending upon axial locations and operating conditions. Also, except near the leading-edge region, the subsurface thermal wave profiles in the upstream locations are thicker than those in the downstream locations since the solid-fuel regression rate, in general, increases with distance along the fuel slab. The recovered solid fuel slabs in the laminar portion of the boundary layer exhibited smooth surfaces, indicating the existence of a liquid melt layer on the burning fuel surface in the upstream region. After the transition section, which displayed distinct transverse striations, the surface roughness pattern became quite random and very pronounced in the downstream turbulent boundary-layer region. Both real-time X-ray radiography and ultrasonic pulse-echo techniques were used to determine the instantaneous web thickness burned and instantaneous solid-fuel regression rates over certain portions of the fuel slabs. Globally averaged and axially dependent but time-averaged regression rates were also obtained and presented.
MESSENGER Observations of the Dayside Low-Latitude Boundary Layer in Mercury's Magnetosphere
Liljeblad, E. I.; Karlsson, T.; Raines, J. M.; Slavin, J. A.; Kullen, A.; Sundberg, T.; Zurbuchen, T.
2015-12-01
Observations from MESSENGER's MAG and FIPS instruments during the first orbital year have resulted in the identification of 25 magnetopause crossings in Mercury's magnetosphere with significant low-latitude boundary layers (LLBLs). The large majority of these crossings are observed on the dawnside and for northward interplanetary magnetic field. The estimated LLBL thickness is 450±56 km, and increases with distance to noon. The Na+-group ion is sporadically present in 14 of the boundary layers, with an observed average number density of 22±11 % of the proton density. Furthermore, the average Na+-group gyroradii in the layers is 220±34 km, the same order of magnitude as the LLBL thickness. Magnetic shear, plasma β and reconnection rates have been estimated for the LLBL crossings, and compared to those of a control group (non-LLBL) of 61 distinct magnetopause crossings which show signs of nearly no plasma inside the magnetopause. The results indicate that reconnection is significantly slower, or even suppressed, for the LLBL crossings compared to the non-LLBL cases. Possible processes that form or impact the LLBL are discussed. Protons injected through the cusp or flank may be important for the formation of the LLBL. Furthermore, the opposite asymmetry in the Kelvin-Helmholtz instability (KHI) as compared to the LLBL, rules out the KHI as a dominant formation mechanism. However, the KHI and LLBL could be related to each other, either by the impact of sodium ions gyrating across the magnetopause, or by the LLBL preventing the growth of KH waves on the dawnside.
Nejati, Iman; Hardt, Steffen
2016-01-01
The coupled dynamics of two conjugated liquid layers of disparate thicknesses, which coat a solid substrate and are subjected to a transverse temperature gradient, is investigated. The upper liquid layer evolves under the short-wavelength (SW) B\\'enard-Marangoni (BM) instability, whereas the lower, much thinner film undergoes a shear-driven long-wavelength deformation. Although the lubricating film should reduce the viscous stresses acting on the up to one hundred times thicker upper layer by only 10%, it is found that the critical Marangoni number of marginal stability may be as low as if a stress-free boundary condition were applied at the bottom of the upper layer, i.e. much lower than the classical value of 79.6 known for a single film. Furthermore, it is experimentally verified that the deformation of the liquid-liquid interface, albeit small, has a non-negligible effect on the temperature distribution along the liquid-gas interface of the upper layer. This stabilizes the hexagonal pattern symmetry towar...
Uddin, Mohammed J.; Khan, Waqar A.; Ahmed I Ismail
2012-01-01
Steady two dimensional MHD laminar free convective boundary layer flows of an electrically conducting Newtonian nanofluid over a solid stationary vertical plate in a quiescent fluid taking into account the Newtonian heating boundary condition is investigated numerically. A magnetic field can be used to control the motion of an electrically conducting fluid in micro/nano scale systems used for transportation of fluid. The transport equations along with the boundary conditions are first convert...
Vortices in stratified protoplanetary disks. From baroclinic instability to vortex layers
Barge, P.; Richard, S.; Le Dizès, S.
2016-08-01
Context. Large-scale vortices could play a key role in the evolution of protoplanetary disks, particularly in the dead-zone where no turbulence associated with magnetic field is expected. Their possible formation by the subcritical baroclinic instability is a complex issue because of the vertical structure of the disk and the elliptical instability. Aims: In 2D disks the baroclinic instability is studied as a function of the thermal transfer efficiency. In 3D disks we explore the importance of radial and vertical stratification on the processes of vortex formation and amplification. Methods: Numerical simulations are performed using a fully compressible hydrodynamical code based on a second-order finite volume method. We assume a perfect gas law in inviscid disk models in which heat transfer is due to either relaxation or diffusion. Results: In 2D, the baroclinic instability with thermal relaxation leads to the formation of large-scale vortices, which are unstable with respect to the elliptic instability. In the presence of heat diffusion, hollow vortices are formed which evolve into vortical structures with a turbulent core. In 3D, the disk stratification is found to be unstable in a finite layer which can include the mid-plane or not. When the unstable layer contains the mid-plane, the 3D baroclinic instability with thermal relaxation is found to develop first in the unstable layer as in 2D, producing large-scale vortices. These vortices are then stretched out in the stable layer, creating long-lived columnar vortical structures extending through the width of the disk. They are also found to be the source of internal vortex layers that develop across the whole disk along baroclinic critical layer surfaces, and form new vortices in the upper region of the disk. Conclusions: In 3D disks, vortices can survive for a very long time if the production of vorticity by the baroclinic amplification balances the destruction of vorticity by the elliptical instability
Institute of Scientific and Technical Information of China (English)
无
2010-01-01
A direct numerical simulation of the shock/turbulent boundary layer interaction flow in a supersonic 24-degree compression ramp is conducted with the free stream Mach number 2.9.The blow-and-suction disturbance in the upstream wall boundary is used to trigger the transition.Both the mean wall pressure and the velocity profiles agree with those of the experimental data,which validates the simulation.The turbulent kinetic energy budget in the separation region is analyzed.Results show that the turbulent production term increases fast in the separation region,while the turbulent dissipation term reaches its peak in the near-wall region.The turbulent transport term contributes to the balance of the turbulent conduction and turbulent dissipation.Based on the analysis of instantaneous pressure in the downstream region of the mean shock and that in the separation bubble,the authors suggest that the low frequency oscillation of the shock is not caused by the upstream turbulent disturbance,but rather the instability of separation bubble.
Energy Technology Data Exchange (ETDEWEB)
Ishak, Anuar; Nazar, Roslinda [School of Mathematical Sciences, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor (Malaysia); Pop, Ioan [Faculty of Mathematics, University of Cluj, R-3400 Cluj, CP 253 (Romania)
2008-07-01
The steady mixed convection boundary layer flow through a stable stratified medium adjacent to a vertical surface is investigated. The velocity outside the boundary layer and the surface temperature are assumed to vary linearly from the leading edge of the surface. The transformed ordinary differential equations are solved numerically by the Keller-box method. It is found that dual solutions exist, and the thermal stratification delays the boundary layer separation. (author)
Seasonality of mercury in the Atlantic marine boundary layer
Soerensen, Anne L.; Sunderland, Elsie; Skov, Henrik; Holmes, Christopher; Jacob, Daniel J.
2010-05-01
Around one third of the mercury emissions today are from primary anthropogenic sources, with the remaining two-thirds from secondary reemissions of earlier deposition and natural sources (AMAP/UNEP 2008). Mercury exchange at the air-sea interface is important for the global distribution of atmospheric mercury as parts of deposited mercury will reenter the atmosphere through evasion. The exchange at the air-sea interface also affects the amount of inorganic mercury in the ocean and thereby the conversion to the neuro-toxic methylmercury. Here we combine new cruise measurements in the atmospheric marine boundary layer (MBL) of the Atlantic Ocean (Northern Hemisphere) from the fall of 2006 and the spring of 2007 with existing data from cruises in the Atlantic Ocean since 1978. We observe from these data a seasonal cycle in Hg(0) concentrations in the Atlantic marine boundary later (MBL) that exhibits minimum concentrations during summer and high concentrations during fall to spring. These observations suggest a local, seasonally dependent Hg(0) source in the MBL that causes variability in concentrations above the open ocean. To further investigate controls on Hg(0) concentrations in the MBL, we developed an improved representation of oceanic air-sea exchange processes within the GEOS-Chem global 3-D biogeochemical mercury model. Specifically, we used new data on mercury redox reactions in the surface ocean as a function of biological and photochemical processes, and implemented new algorithms for mercury dynamics associated with suspended particles. Our coupled atmospheric-oceanic modeling results support the premise that oceanic evasion is a main driver controlling Hg(0) concentrations in the MBL. We also use the model to investigate what drivers the evasion across the air-sea interface on shorter timescales. This is done by tracking evasion rates and other model components on an hourly basis for chosen locations in the Atlantic Ocean.
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)
Marine boundary layer simulation and verification during BOBMEX-Pilot using NCMRWF model
Indian Academy of Sciences (India)
Swati Basu
2000-06-01
A global spectral model (T80L18) that is operational at NCMRWF is utilized to study the structure of the marine boundary layer over the Bay of Bengal during the BOBMEX-Pilot period. The vertical profiles of various meteorological parameters within the boundary layer are studied and verified against the available observations. The diurnal variation of various surface fields are also studied. The impact of non-local closure scheme for the boundary layer parameterisation is seen in simulation of the flow pattern as well as on the boundary layer structure over the oceanic region.
Synchrotron x-ray study of the smectic layer directional instability
Dierking; Glusen; Lagerwall; Ober
2000-02-01
We have investigated the phenomenon of field-induced smectic layer instability, as monitored by synchrotron x-ray scattering. This instability means that, upon application of time-asymmetric electric fields to chiral smectics, the layer direction seems to "rotate" locally around an axis given by the direction of the applied field. For moderate values of field amplitude and asymmetry, domains with a favored layer inclination grow at the expense of unfavored ones, while larger fields and asymmetries generally lead to a chaotic flow behavior. At moderate amplitudes, we have followed the process of the horizontal layer folding (or horizontal chevron domain formation) and the smectic C* layer reorientation of ferroelectric liquid crystals by applying symmetric and asymmetric wave forms, respectively, and performing time resolved x-ray measurements. The studies unambiguously show the formation of a horizontal (in-plane, i.e., in a plane parallel to the cell substrates) chevron domain structure from a nonoriented sample by application of a symmetric electric field of sufficient amplitude. It is then demonstrated that a transition from the horizontal chevron domain structure to an in-plane uniform smectic layer direction takes place on application of asymmetric electric wave forms. Reversal of the field asymmetry reverses the inclination direction and selects the other layer normal direction as the uniform end state. The in-plane smectic layer reorientation process is followed here as it evolves, and analyzed directly by means of x-ray scattering. PMID:11046442
Energy Technology Data Exchange (ETDEWEB)
Aifantis, K E [Lab of Mechanics and Materials, Aristotle University of Thessaloniki, 54124 Thessaloniki (Greece); Senger, J; Weygand, D [Institut fuer Zuverlaessigkeit von Bauteilen und Systemen (IZBS), Universitaet Karlsruhe (Thailand), 76131 Karlsruhe (Germany); Zaiser, M, E-mail: k.aifantis@mom.gen.auth.gr [Centre for Materials Science and Engineering, University of Edinburgh, The King' s Buildings, Sanderson Building, Edinburgh EH93JL (United Kingdom)
2009-07-15
Since the mid 80s various gradient plasticity models have been developed for obtaining the plastic response of materials at the micron- and submicron- scales. In particular, gradient terms have been proven to be crucial for understanding size effects in constrained plastic flow, which are related to the emergence of plasticity boundary layers near passive (plastically not deformable) boundaries. In spite of the success of gradient theories in modeling boundary layer formation, there remain unresolved issues concerning the physical interpretation of the internal length scale involved in the theoretical formulation. Physically, boundary layer formation is related to the piling up of dislocations against the boundaries. This phenomenon is investigated by performing discrete dislocation dynamics (DDD) simulations on a tri-crystal with plastically non-deforming grain boundaries. Strain distributions are derived from the DDD simulations and matched with the results of gradient plasticity calculations, in order to identify the internal length scale governing the boundary layer width.
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.
Turbulent boundary layer over a convergent and divergent superhydrophobic surface
Nadeem, Muhammad; Hwang, Jinyul; Sung, Hyung Jin
2015-11-01
Direct numerical simulation (DNS) of spatially developing turbulent boundary layer (TBL) over a convergent and divergent superhydrophobic surface (SHS) was performed. The convergent and divergent SHS was aligned in the streamwise direction. The SHS was modeled as a pattern of slip and no-slip surfaces. For comparison, DNS of TBL over a straight SHS was also carried out. The momentum thickness Reynolds number was varied from 800 to 1400. The gas fraction of the convergent and divergent SHS was the same as that of the straight SHS, keeping the slip area constant. The slip velocity in the convergent SHS was higher than that of the straight SHS. An optimal streamwise length of the convergent and divergent SHS was obtained. The convergent and divergent SHS gave more drag reduction than the straight SHS. The convergent and divergent SHS led to the modification of near wall-turbulent structures, resembling the narrowing and widening streaky structures near the wall. The convergent and divergent SHS had a relatively larger damping effect on near-wall turbulence than the straight SHS. These observations will be further analyzed statistically to demonstrate the effect of the convergent and divergent SHS on the interaction of inner and outer regions of TBL.
The decay of wake vortices in the convective boundary layer
Energy Technology Data Exchange (ETDEWEB)
Holzaepfel, F.; Gerz, T.; Frech, M.; Doernbrack, A.
2000-03-01
The decay of three wake vortex pairs of B-747 aircraft in a convectively driven atmospheric boundary layer is investigated by means of large-eddy simulations (LES). This situation is considered as being hazardous as the updraft velocities of a thermal may compensate the induced descent speed of the vortex pair resulting in vortices stalled in the flight path. The LES results, however, illustrate that (i) the primary rectilinear vortices are rapidly deformed on the scale of the alternating updraft and downdraft regions; (ii) parts of the vortices stay on flight level but are quickly eroded by the enhanced turbulence of an updraft; (iii) longest living sections of the vortices are found in regions of relatively calm downdraft flow which augments their descent. Strip theory calculations are used to illustrate the temporal and spatial development of lift and rolling moments experienced by a following medium weight class B-737 aircraft. Characteristics of the respective distributions are analysed. Initially, the maximum rolling moments slightly exceed the available roll control of the B-737. After 60 seconds the probability of rolling moments exceeding 50% of the roll control, a value which is considered as a threshold for acceptable rolling moments, has decreased to 1% of its initial probability. (orig.)
Bypass transition of the bottom boundary layer under solitary wave
Sadek, Mahmoud; Diamessis, Peter; Parras, Luis; Liu, Philip
2015-11-01
The transition to turbulence in the bottom boundary layer (BBL) flow driven by a soliton-like pressure gradient in an oscillating water tunnel (an approximation for the BBL under solitary waves) is investigated using hydrodynamic linear stability theory and DNS. As observed in the laboratory experiment by Sumer et al. (2010), two possible transition scenarios exist. The first scenario is associated with the classical transition resulting from the breakdown of the exponentially growing 2-D Tollmien-Schlichting waves. The alternative scenario; i.e., bypass transition; takes place through formation of localized turbulent spots. The investigation of the latter transition scenario is performed in two steps. The first step consists of reformulating the linear stability analysis in the non-modal framework for the purpose of finding the optimum disturbance characteristics which lead to the formation of those turbulent spots. In the second step, the computed optimum noise structure is inserted in the 3D DNS in order to induce the formation of the turbulent spots and effectively simulate the bypass transition observed experimentally.
Proper orthogonal decomposition of a decelerating turbulent boundary layer
Tutkun, Murat
2010-11-01
Our analysis is based only on streamwise component of velocity fluctuations since the data were simultaneously obtained using a hot-wire rake of 143 single wire probes. The experiment was carried out in the large wind tunnel of Laboratoire de M'ecanique de Lille whose test section is 20 m long, 2 m wide and 1 m high. A 2D bump was used to create converging-diverging flow inside the test section. The thickness of the boundary layer was 25 cm at the measurement location and Reynolds number based on momentum thickness, Reθ, was 17:100 for 10 m s-1 external free stream velocity measured before the bump. Eigenvalue distribution over POD modes shows that approximately 90% of turbulence kinetic energy due to streamwise fluctuations within the domain was captured by the first 5 POD modes. The first POD mode carried more than 45% of turbulence kinetic energy. Resulting eigenspectra are studied for different frequencies and spanwise Fourier indices in order to reduce the number of modes used in reconstructed velocity fields.
Plasma structures inside boundary layers of magnetic clouds
Institute of Scientific and Technical Information of China (English)
WEI Fengsi; FENG Xueshang; YANG Fang; ZHONG Dingkun
2004-01-01
We analyze the plasma structures for 50 magnetic cloud boundary layers (BLs) which were observed by the spacecraft WIND from February, 1995 to June 2003. Main discoveries are: (ⅰ) The BL is a non-pressure balanced structure, its total pressure, PT,L, (the thermal pressure, Pth,L, plus the magnetic pressure, PM,L) is generally less than the total pressure PT,S and PT,C of the front solar wind (SW) and the following magnetic clouds (MC), respectively. The rising of the Pth,L inside the BLs is often not enough to compensate the declining of PM,L; (ⅱ) The ratio of electron and proton temperatures, (Te/Tp)L, inside the BLs is offen less than (Te/Tp)s and (Te/Tp)c in the SW and the MC, respectively, because the heating of proton is more obvious than that of electron; and (ⅲ) The reversal jet is observed in 80% BLs investigated, in which the reversal jets from all of three directions (±Vx, ±Vy, ±Vz), were observed in ≈25% BLs. These basic characteristics could be associated with a possible magnetic reconnection process inside the BLs. The results above suggest that the cloud BL owns the plasma structures different from those in the SW and MC. It is a manifestation for the existing significant dynamic interaction between the magnetic cloud and the solar wind.
Ion beams in the plasma sheet boundary layer
Birn, J.; Hesse, M.; Runov, A.; Zhou, X.-Z.
2015-09-01
We explore characteristics of energetic particles in the plasma sheet boundary layer associated with dipolarization events, based on simulations and observations. The simulations use the electromagnetic fields of an MHD simulation of magnetotail reconnection and flow bursts as basis for test particle tracing. They are complemented by self-consistent fully electrodynamic particle-in-cell (PIC) simulations. The test particle simulations confirm that crescent-shaped earthward flowing ion velocity distributions with strong perpendicular anisotropy can be generated as a consequence of near-tail reconnection, associated with earthward flows and propagating magnetic field dipolarization fronts. Both PIC and test particle simulations show that the ion distribution in the outflow region close to the reconnection site also consist of a beam superposed on an undisturbed population, which, however, does not show strong perpendicular anisotropy. This suggests that the crescent shape is created by quasi-adiabatic deformation from ion motion along the magnetic field toward higher field strength. The simulation results compare favorably with "Time History of Events and Macroscale Interactions during Substorms" observations.
Iodine oxide in the global marine boundary layer
Directory of Open Access Journals (Sweden)
C. Prados-Roman
2014-08-01
Full Text Available Emitted mainly by the oceans, iodine is a halogen compound important for atmospheric chemistry due to its high ozone depletion potential and effect on the oxidizing capacity of the atmosphere. Here we present a comprehensive dataset of iodine oxide (IO measurements in the open marine boundary layer (MBL made during the Malaspina 2010 circumnavigation. Results show IO mixing ratios ranging from 0.4 to 1 pmol mol−1 and, complemented with additional field campaigns, this dataset confirms through observations the ubiquitous presence of reactive iodine chemistry in the global marine environment. We use a global model with organic (CH3I, CH2ICl, CH2I2 and CH2IBr and inorganic (HOI and I2 iodine ocean emissions to investigate the contribution of the different iodine source gases to the budget of IO in the global MBL. In agreement with previous estimates, our results indicate that, globally averaged, the abiotic precursors contribute about 75% to the iodine oxide budget. However, this work reveals a strong geographical pattern in the contribution of organic vs. inorganic precursors to reactive iodine in the global MBL.
Uranus evolution models with simple thermal boundary layers
Nettelmann, Nadine; Redmer, Ronald; Fortney, Jonathan J.; Hamel, Sebastien; Bethkenhagen, Mandy
2016-04-01
The strikingly low luminosity of Uranus imposes a long-standing challenge to our understanding of Ice Giant planets. Similar to the Earth, Uranus appears to evolve in equilibrium with the solar incident flux (Teq). Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. We argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary layer (TBL) and adjust it to reproduce the luminosity. Due to this TBL, the deep interior of the Uranus models are up to a factor 3 warmer than adiabatic models, necessitating the presence of rocks there with a possible I:R of 1 x solar. Furthermore, we also allow for an equilibrium evolution (Teff ~ Teq) that begun prior to the present day, which would therefore no longer constitute a "special time" in Uranus' evolution. Once Teff ~ Teq happens, a shallow, subadiabatic zone in the atmosphere begins to develop. Its depth is adjusted to meet the luminosity constraint. This work provides a simple foundation for future Ice Giant structure and evolution models, that can be improved by properly treating the heat and particle fluxes in the diffusive zones.
Evidence of reactive iodine chemistry in the Arctic boundary layer
Mahajan, Anoop S.; Shaw, Marvin; Oetjen, Hilke; Hornsby, Karen E.; Carpenter, Lucy J.; Kaleschke, Lars; Tian-Kunze, Xiangshan; Lee, James D.; Moller, Sarah J.; Edwards, Peter; Commane, Roisin; Ingham, Trevor; Heard, Dwayne E.; Plane, John M. C.
2010-10-01
Although it has recently been established that iodine plays an important role in the atmospheric chemistry of coastal Antarctica, where it occurs at levels which cause significant ozone (O3) depletion and changes in the atmospheric oxidising capacity, iodine oxides have not previously been observed conclusively in the Arctic boundary layer (BL). This paper describes differential optical absorption spectroscopy (DOAS) observations of iodine monoxide (IO), along with gas chromatographic measurements of iodocarbons, in the sub-Arctic environment at Kuujjuarapik, Hudson Bay, Canada. Episodes of elevated levels of IO (up to 3.4 ± 1.2 ppt) accompanied by a variety of iodocarbons were observed. Air mass back trajectories show that the observed iodine compounds originate from open water polynyas that form in the sea ice on Hudson Bay. A combination of long-path DOAS and multiaxis DOAS observations suggested that the IO is limited to about 100 m in height. The observations are interpreted using a one-dimensional model, which indicates that the iodocarbon sources from these exposed waters can account for the observed concentrations of IO. These levels of IO deplete O3 at rates comparable to bromine oxide (BrO) and, more importantly, strongly enhance the effect of bromine-catalyzed O3 depletion in the Arctic BL, an effect which has not been quantitatively considered hitherto. However, the measurements and modeling results indicate that the effects of iodine chemistry are on a much more localized scale than bromine chemistry in the Arctic environment.
Recommendations for Hypersonic Boundary Layer Transition Flight Testing
Berry, Scott A.; Kimmel, Roger; Reshotko, Eli
2011-01-01
Much has been learned about the physics underlying the transition process at supersonic and hypersonic speeds through years of analysis, experiment and computation. Generally, the application of this knowledge has been restricted to simple shapes like plates, cones and spherical bodies. However, flight reentry vehicles are in reality never simple. They typically are highly complex geometries flown at angle of attack so three-dimensional effects are very important, as are roughness effects due to surface features and/or ablation. This paper will review our present understanding of the physics of the transition process and look back at some of the recent flight test programs for their successes and failures. The goal of this paper is to develop rationale for new hypersonic boundary layer transition flight experiments. Motivations will be derived from both an inward look at what we believe constitutes a good flight test program as well as an outward review of the goals and objectives of some recent US based unclassified proposals and programs. As part of our recommendations, this paper will address the need for careful experimental work as per the guidelines enunciated years ago by the U.S. Transition Study Group. Following these guidelines is essential to obtaining reliable, usable data for allowing refinement of transition estimation techniques.
Coupling between roughness and freestream acceleration in turbulent boundary layers
Yuan, Junlin; Piomelli, Ugo
2015-11-01
To explain various rough-wall flow responses to different types of free-stream conditions previously observed, we carried out a direct numerical simulation of a spatially developing turbulent boundary layer with freestream acceleration. Unlike the equilibrium (self-similar) accelerating scenario, where a strong acceleration leads to complete laminarization and lower friction, in the present non-equilibrium case the friction coefficient increases with acceleration, due to the faster near-wall acceleration than that of the freestream. At the same time, roughness reduces the near-wall time scale of the turbulence, preventing the acceleration from linearly stretching the near-wall eddies and freezing the turbulence intensity as in the smooth case. In addition, acceleration leads to similar decrease of mean-velocity logarithmic slope on rough and smooth walls; this allows a clear definition of the roughness function in a local sense. Interestingly, this roughness function correlates with the roughness Reynolds number in the same way as in self-similar or non-accelerating flows. This study may also help develop benchmark cases for evaluating rough-wall treatments for industrial turbulence models.
Reactive chlorine chemistry in the boundary layer of coastal Antarctica
Zielcke, Johannes; Poehler, Denis; Friess, Udo; Hay, Tim; Eger, Philipp; Kreher, Karin; Platt, Ulrich
2015-04-01
A unique feature of the polar troposphere is the strong impact of halogen photochemistry, in which reactive halogen species are responsible for ozone depletion as well as the oxidation of elemental mercury and dimethyl sulphide. The source, however, as well as release and recycling mechanisms of these halogen species - for some species even abundances - are far from being completely known, especially of chlorine and iodine compounds. Here we present active long-path differential optical absorption spectroscopy (LP-DOAS) measurements conducted during austral spring 2012 at Ross Island, Antarctica, observing several species (BrO, O3, NO2, IO, ClO, OBrO, OClO, OIO, I2, CHOCHO, HCHO, HONO). For the first time, ClO was detected and quantified in the marine boundary layer of coastal Antarctica, with typical mixing ratios around 20 pptv and maxima around 50 pptv. Meteorological controls on the mixing ratio of ClO as well as the interplay with other halogen compounds will be discussed, such as the lack of observed OClO (< 1 pptv). The results seem to reflect previously in chamber studies observed dependences on ozone levels and solar irradiance.
NOx and NOy in the Tropical Marine Boundary Layer
Reed, Chris; Evans, Mathew J.; Lee, James D.; Carpenter, Lucy J.; Read, Katie A.; Mendes, Luis N.
2016-04-01
Nitrogen oxides (NOx=NO+NO2) and their reservoir species (NOy) play a central role in determining the chemistry of the troposphere. Although their concentrations are low (1-100 ppt) in regions such as the remote marine boundary layer, they have a profound impact on ozone production and the oxidizing capacity. There are very few observations of NOx and NOy in remote oceanic regions due to the technical challenges of measuring such low concentrations, and thus our understanding of this background chemistry is incomplete. Here we present long term measurements of NOx (2006-2015) and more recent measurements of speciated NOy (total peroxyacetyl nitrates, PANs; alkyl nitrates, ANs; nitric acid; and aerosol analogues) made at the Cape Verde Atmospheric Observatory (CVAO; 16° 51' N, 24° 52' W) located in the tropical Atlantic Ocean. We identify potential interferences in the NO2 and NOy measurements and methods to eliminate them. Diurnal and seasonal cycles are interpreted using a box model. We find a complex chemistry with interactions between organic and inorganic chemistry, between the aerosol and gas phase, and between the very local and large scales.
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.
Boundary layer parameterizations and long-range transport
International Nuclear Information System (INIS)
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.)
Numerical analysis and optimization of boundary layer suction on airfoils
Directory of Open Access Journals (Sweden)
Shi Yayun
2015-04-01
Full Text Available Numerical approach of hybrid laminar flow control (HLFC is investigated for the suction hole with a width between 0.5 mm and 7 mm. The accuracy of Menter and Langtry’s transition model applied for simulating the flow with boundary layer suction is validated. The experiment data are compared with the computational results. The solutions show that this transition model can predict the transition position with suction control accurately. A well designed laminar airfoil is selected in the present research. For suction control with a single hole, the physical mechanism of suction control, including the impact of suction coefficient and the width and position of the suction hole on control results, is analyzed. The single hole simulation results indicate that it is favorable for transition delay and drag reduction to increase the suction coefficient and set the hole position closer to the trailing edge properly. The modified radial basis function (RBF neural network and the modified differential evolution algorithm are used to optimize the design for suction control with three holes. The design variables are suction coefficient, hole width, hole position and hole spacing. The optimization target is to obtain the minimum drag coefficient. After optimization, the transition delay can be up to 17% and the aerodynamic drag coefficient can decrease by 12.1%.
Improvement of Turbine Performance by Streamwise Boundary Layer Fences
Directory of Open Access Journals (Sweden)
M Govardhan
2012-01-01
Full Text Available In the present investigations, effect of streamwise end wall fences on the performance improvement of a turbine is studied. The fences with heights of 12 mm, 16 mm were attached normal to the end wall and at a half pitch away from the blades. A miniaturized pressure probe was traversed at the exit of the cascade from midspan to the end wall at 26 locations covering more points in the end wall region. For each spanwise location, the probe was traversed in the pitchwise direction for more than 25 points covering one blade pitch. The boundary layer fence near the end wall remains effective in changing the path of pressure side of leg of horseshoe and weaken the cross flow. The overturn in flow has reduced near the end wall when fences are incorporated while outside end wall and in loss core region, it underturns slightly as result of reduction in secondary loss. The total loss is reduced by 15%, 25% for fences of height 12 mm, and 16 mm respectively. The corresponding change was obtained in the drag and lift coefficients.
Benthic boundary layer. IOS observational and modelling programme
International Nuclear Information System (INIS)
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 cm2 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)
Allaerts, Dries; Meyers, Johan
2014-06-01
In this study we consider large wind farms in a conventionally neutral atmospheric boundary layer. In large wind farms the energy extracted by the turbines is dominated by downward vertical turbulent transport of kinetic energy from the airflow above the farm. However, atmospheric boundary layers are almost always capped by an inversion layer which slows down the entrainment rate and counteracts boundary layer growth. In a suite of large eddy simulations the effect of the strength of the capping inversion on the boundary layer and on the performance of a large wind farm is investigated. For simulations with and without wind turbines the results indicate that the boundary layer growth is effectively limited by the capping inversion and that the entrainment rate depends strongly on the inversion strength. The power output of wind farms is shown to decrease for increasing inversions.
MHD Boundary Layer Slip Flow and Heat Transfer over a Flat Plate
Institute of Scientific and Technical Information of China (English)
Krishnendu Bhattacharyya; Swati Mukhopadhyay; G.C.Layek
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.%@@ 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.
Two-fluid Instability of Dust and Gas in the Dust Layer of a Protoplanetary Disk
Ishitsu, Naoki; Sekiya, Minoru
2009-01-01
Instabilities of the dust layer in a protoplanetary disk are investigated. It is known that the streaming instability develops and dust density concentration occurs in a situation where the initial dust density is uniform. This work considers the effect of initial dust density gradient vertical to the midplane. Dust and gas are treated as different fluids. Pressure of dust fluid is assumed to be zero. The gas friction time is assumed to be constant. Axisymmetric two-dimensional numerical simulation was performed using the spectral method. We found that an instability develops with a growth rate on the order of the Keplerian angular velocity even if the gas friction time multiplied by the Keplerian angular velocity is as small as 0.001. This instability is powered by two sources: (1) the vertical shear of the azimuthal velocity, and (2) the relative motion of dust and gas coupled with the dust density fluctuation due to advection. This instability diffuses dust by turbulent advection and the maximum dust densi...
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
Experimental study of the boundary layer over an airfoil in plunging motion
Institute of Scientific and Technical Information of China (English)
F. Rasi Marzabadi; M. R. Soltani
2012-01-01
This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions.It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer.The wind tunnel measurements involved surfacemounted hot-film sensors and boundary-layer rake.The experiments were conducted at Reynolds numbers of 0.42 × 106 to 0.84 × 106 and the reduced frequency was varied from 0.01 to 0.1 1.The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases.For the static tests,boundary layer transition occurred through a laminar separation bubble.By increasing the angle of attack,disturbances and the transition location moved toward the leading edge.For the dynamic tests,earlier transition occurred with increasing rather than decreasing effective angle of attack.The mean angle of attack and the oscillating parameters significantly affected the state of the boundary layer.By increasing the reduced frequency,the boundary layer transition was promoted to the upstroke portion of the equivalent angle of attack,but the quasi skin friction coefficient was decreased.
Experimental study of the boundary layer over an airfoil in plunging motion
Marzabadi, F. Rasi; Soltani, M. R.
2012-04-01
This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions. It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer. The wind tunnel measurements involved surfacemounted hot-film sensors and boundary-layer rake. The experiments were conducted at Reynolds numbers of 0.42×106 to 0.84 × 106 and the reduced frequency was varied from 0.01 to 0.11. The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases. For the static tests, boundary layer transition occurred through a laminar separation bubble. By increasing the angle of attack, disturbances and the transition location moved toward the leading edge. For the dynamic tests, earlier transition occurred with increasing rather than decreasing effective angle of attack. The mean angle of attack and the oscillating parameters significantly affected the state of the boundary layer. By increasing the reduced frequency, the boundary layer transition was promoted to the upstroke portion of the equivalent angle of attack, but the quasi skin friction coefficient was decreased.
Directory of Open Access Journals (Sweden)
E. L. McGrath-Spangler
2008-07-01
Full Text Available The response of atmospheric carbon dioxide to a given amount of surface flux is inversely proportional to the depth of the boundary layer. Overshooting thermals that entrain free tropospheric air down into the boundary layer modify the characteristics and depth of the lower layer through the insertion of energy and mass. This alters the surface energy budget by changing the Bowen ratio and thereby altering the vegetative response and the surface boundary conditions. Although overshooting thermals are important in the physical world, their effects are unresolved in most regional models. A parameterization to include the effects of boundary layer entrainment was introduced into a coupled ecosystem-atmosphere model (SiB-RAMS. The parameterization is based on a downward heat flux at the top of the boundary layer that is proportional to the heat flux at the surface. Results with the parameterization show that the boundary layer simulated is deeper, warmer, and drier than when the parameterization is turned off. These results alter the vegetative stress factors thereby changing the carbon flux from the surface. The combination of this and the deeper boundary layer change the concentration of carbon dioxide in the boundary layer.
A note on turbulent spots over a rough bed in wave boundary layers
DEFF Research Database (Denmark)
Carstensen, Stefan; Sumer, B. Mutlu; Fredsøe, Jørgen
2012-01-01
This study is a continuation of the investigation of turbulent spots in wave boundary layers over a smooth wall reported by Carstensen et al. [J. Fluid Mech. 646, 169–206 (2010)]. The present paper summarises the results of an experimental investigation of turbulent spots in wave boundary layers ...
Application of Viscothermal Wave Propagation Theory for Reduction of Boundary Layer Induced Noise
Wijnant, Y.H.; Hannink, M.H.C.; Boer, de A.
2003-01-01
Boundary layer induced noise, i.e. noise inside the aircraft resulting from the turbulent boundary layer enclosing the fuselage, is known to dominate air-cabin noise at cruise conditions. In this paper a method is described to design trim panels containing a large number of coupled tubes to effectiv
Janssen, R.H.H.; Vilà-Guerau de Arellano, J.; Ganzeveld, L.N.; Kabat, P.; Jimenez, J.L.; Farmer, D.K.; Heerwaarden, van C.C.; Mammarella, I.
2012-01-01
We study the combined effects of land surface conditions, atmospheric boundary layer dynamics and chemistry on the diurnal evolution of biogenic secondary organic aerosol in the atmospheric boundary layer, using a model that contains the essentials of all these components. First, we evaluate the mod
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...
Ozone in the Atlantic Ocean marine boundary layer
Directory of Open Access Journals (Sweden)
Patrick Boylan
2015-04-01
Full Text Available Abstract In situ atmospheric ozone measurements aboard the R/V Ronald H. Brown during the 2008 Gas-Ex and AMMA research cruises were compared with data from four island and coastal Global Atmospheric Watch stations in the Atlantic Ocean to examine ozone transport in the marine boundary layer (MBL. Ozone measurements made at Tudor Hill, Bermuda, were subjected to continental outflow from the east coast of the United States, which resulted in elevated ozone levels above 50 ppbv. Ozone measurements at Cape Verde, Republic of Cape Verde, approached 40 ppbv in springtime and were influenced by outflow from Northern Africa. At Ragged Point, Barbados, ozone levels were ∼ 21 ppbv; back trajectories showed the source region to be the middle of the Atlantic Ocean. Ozone measurements from Ushuaia, Argentina, indicated influence from the nearby city; however, the comparison of the daily maxima ozone mole fractions measured at Ushuaia and aboard the Gas-Ex cruise revealed that these were representative of background ozone in higher latitudes of the Southern Hemisphere. Diurnal ozone cycles in the shipborne data, frequently reaching 6–7 ppbv, were larger than most previous reports from coastal or island monitoring locations and simulations based on HOx photochemistry alone. However, these data show better agreement with recent ozone modeling that included ozone-halogen chemistry. The transport time between station and ship was estimated from HYSPLIT back trajectories, and the change of ozone mole fractions during transport in the MBL was estimated. Three comparisons showed declining ozone levels; in the subtropical and tropical North Atlantic Ocean the loss of ozone was < 1.5 ppbv day−1. Back trajectories at Ushuaia were too inconsistent to allow for this determination. Comparisons between ship and station measurements showed that ozone behavior and large-scale (∼ 1000 km multi-day transport features were well retained during transport in the MBL.
Institute of Scientific and Technical Information of China (English)
Yang Shaoqiong杨绍琼; Li Shan李山; Tian Haiping田海平; Wang Qingyi王清毅; Jiang Nan姜楠
2015-01-01
The time series of velocity vector fields and their statistics in the turbulent boundary layer(TBL)over riblets and smooth plate were measured by utilizing a time-resolved particle image velocimetry(TR-PIV)system. The mean velocity profiles of the TBL were compared in the case of 0.13 m/s(the riblets with dimensionless peak-to-peak spacing being approximately s+≈21)and 0.19 m/s( s+≈28)for these two kinds of plates, respectively. Two kinds of drag-reducing velocity profiles were illustrated and analyzed. Then the spatial topologies of the physical vorticity for the coherent spanwise structures were detected and extracted at the fourth scale by utilizing an improved quadrant splitting method(IQSM). Results revealed that nearly 6.17%, and 10.73%, of a drag reduction was separately achieved over the riblets surface. Besides, it was visualized that the drag-reduction was acquired by the riblets influencing the bursting ejection(Q2)and sweep(Q4)events of the coherent spanwise vortex structures, the Q4 events in particular. Based on such two drag-reducing cases of the riblets, lastly, a simplified Kelvin-Helmholtz-like linear instability model proposed initially by García-Mayoral and Jiménez(2011)has been dis-cussed. It is still difficult to establish with certainty whether the observed phenomena, the appearance of coherent spanwise structures found at around or below y+≈20 in both cases of s+≈21 and s+≈28 and their topological changes, were consequences or causes of the breakdown of the viscous regime. We prefer to suggest that the inter-actions between those structures and the riblets, which contain the coherent spanwise structures extending toward the wall and penetrating into the riblet grooves, are the root causes.
Ardema, M. D.; Yang, L.
1985-01-01
A method of solving the boundary-layer equations that arise in singular-perturbation analysis of flightpath optimization problems is presented. The method is based on Picard iterations of the integrated form of the equations and does not require iteration to find unknown boundary conditions. As an example, the method is used to develop a solution algorithm for the zero-order boundary-layer equations of the aircraft minimum-time-to-climb problem.
Chitta, Subhashini; Steinhoff, John
2015-11-01
This paper describes the use of Vorticity Confinement (VC) to efficiently treat complex blunt bodies with thin shed vortex sheets and attached boundary layers. Because these flows involve turbulence in the vortical regions, there is currently no ab initio method to treat them on current or foreseeable computers. In fact, in spite of years of turbulence modeling efforts (such as LES or RANS), serious flaws in aerodynamic design involving vortex shedding may still be left undetected until the expensive prototype or production stage. Our basic premise is that, for a class of real-world problems requiring simulating ensembles of flow conditions for overall accuracy, conventional turbulence models suffer cost constraints. For these reasons, VC is used to rapidly simulate many operating conditions, as is often done in expensive testing programs for flying prototypes, and in realistic simulations. To achieve dramatically lower computational cost, VC treats the entire flow in a uniform, coarse grid with solid surfaces ``immersed'' in the grid so that they can be quickly generated for many configurations with no requirement for adaptive or conforming fine grids. Also, the VC method has the efficiency of panel methods, but the generality and ease of use of Euler equation methods. We would like to thank Dr. Frank Caradonna for his suggestions and support.
Burguete, J; Mukolobwiez, N.; Daviaud, F.; Garnier, N.; Chiffaudel, A.
2001-01-01
We report experiments on buoyant-thermocapillary instabilities in differentially heated liquid layers. The results are obtained for a fluid of Prandtl number 10 in a rectangular geometry with different aspect ratios. Depending on the height of liquid and on the aspect ratios, the two-dimensional basic flow destabilizes into oblique traveling waves or longitudinal stationary rolls, respectively, for small and large fluid heights. Temperature measurements and space–time recordings r...
Slow Manifolds and Multiple Equilibria in Stratocumulus-Capped Boundary Layers
Directory of Open Access Journals (Sweden)
Junya Uchida
2010-12-01
Full Text Available In marine stratocumulus-capped boundary layers under strong inversions, the timescale for thermodynamic adjustment is roughly a day, much shorter than the multiday timescale for inversion height adjustment. Slow-manifold analysis is introduced to exploit this timescale separation when boundary layer air columns experience only slow changes in their boundary conditions. Its essence is that the thermodynamic structure of the boundary layer remains approximately slaved to its inversion height and the instantaneous boundary conditions; this slaved structure determines the entrainment rate and hence the slow evolution of the inversion height. Slow-manifold analysis is shown to apply to mixed-layer model and large-eddy simulations of an idealized nocturnal stratocumulus- capped boundary layer; simulations with different initial inversion heights collapse onto single relationships of cloud properties with inversion height. Depending on the initial inversion height, the simulations evolve toward a shallow thin-cloud boundary layer or a deep, well-mixed thick cloud boundary layer. In the large-eddy simulations, these evolutions occur on two separate slow manifolds (one of which becomes unstable if cloud droplet concentration is reduced. Applications to analysis of stratocumulus observations and to pockets of open cells and ship tracks are proposed.
Wavelength dependence of the linear growth rate of the Es layer instability
Directory of Open Access Journals (Sweden)
R. B. Cosgrove
2007-06-01
Full Text Available It has recently been shown, by computation of the linear growth rate, that midlatitude sporadic-E (Es layers are subject to a large scale electrodynamic instability. This instability is a logical candidate to explain certain frontal structuring events, and polarization electric fields, which have been observed in Es layers by ionosondes, by coherent scatter radars, and by rockets. However, the original growth rate derivation assumed an infinitely thin Es layer, and therefore did not address the short wavelength cutoff. Also, the same derivation ignored the effects of F region loading, which is a significant wavelength dependent effect. Herein is given a generalized derivation that remedies both these short comings, and thereby allows a computation of the wavelength dependence of the linear growth rate, as well as computations of various threshold conditions. The wavelength dependence of the linear growth rate is compared with observed periodicities, and the role of the zeroth order meridional wind is explored. A three-dimensional paper model is used to explain the instability geometry, which has been defined formally in previous works.
Turbulent Suspension Mechanics in Sediment-Laden Boundary Layers
Kiger, K.
2013-05-01
Accurate prediction of benthic sediment transport is a challenging problem due the two-phase nature of the flow near the mobile bed, as well as the large difference in scales between the meso-scale flow and smaller-scale structures interacting with the sediment bed. Of particular importance is the parameterization of the physics at the bottom boundary. This requires estimation of key quantities such as effective bed stress and sediment flux based on the on the outer regional-scale velocity field. An appropriate turbulence/sediment parameterization is needed to specify the correct bottom momentum and sediment flux. Prior work has shown the shortcoming of standard models to properly predict such behavior, which is speculated to result from the dominant role played by large-scale coherent structures in the generation of the bed morphology, suspension of particulates, and important particle-fluid coupling effects. The goal of the current work is to elucidate such relationships through a combination of direct simulation and laboratory-scale experiment, the latter of which will be the primary focus of this paper. Specifically, two-phase PIV is used to provide a novel quantitative description of both phases, allowing for a detailed examination of the flow behavior and particle-turbulence coupling. Experiments were conducted in both a steady, fully-developed turbulent channel flow and an oscillatory boundary layer in order to examine the fundamental behaviour of the suspension and particle coupling mechanisms. The turbulent channel flow measurements indicated an increase in the effective wall stress due to the presence of the sediment on the order of 7%. The sediment suspension was directly correlated with the ejection dynamics of prototypical hairpin structures, but were found to settle back towards the bed in a manner uncorrelated with the fluid structure. In contrast, the measurements of the oscillatory flow reveal it to be dominated by alternating streaming motions and
A Note on the bottom shear stress in oscillatory planetary boundary layer flow
Directory of Open Access Journals (Sweden)
Dag Myrhaug
1988-07-01
Full Text Available A simple analytical theory is presented, which describes the motion in a turbulent oscillatory planetary boundary layer near a rough seabed using a two-layer, time-invariant eddy viscosity model. The bottom shear stress is outlined, and comparison is made with Pingree and Griffiths' (1974 measurements of turbulent tidal planetary boundary layer flow on the continental shelf south-west of Lands End, England.
A Note on the bottom shear stress in oscillatory planetary boundary layer flow
Dag Myrhaug
1988-01-01
A simple analytical theory is presented, which describes the motion in a turbulent oscillatory planetary boundary layer near a rough seabed using a two-layer, time-invariant eddy viscosity model. The bottom shear stress is outlined, and comparison is made with Pingree and Griffiths' (1974) measurements of turbulent tidal planetary boundary layer flow on the continental shelf south-west of Lands End, England.
Jain, Aashish; Shankar, V
2007-10-01
The linear stability of viscoelastic (Oldroyd-B) film flow down an inclined plane lined with a deformable (neo-Hookean) solid layer is analyzed using low-wave-number asymptotic analysis and the Chebyshev-Tau spectral numerical method. The free surface of film flows of viscoelastic liquids, unlike that of their Newtonian counterparts, becomes unstable in flow down a rigid inclined surface even in the absence of fluid inertia, due to the elastic nature of the liquids. For film flow past a deformable solid, our low-wave-number asymptotic analysis reveals that the solid deformability has a stabilizing effect on the free-surface instability, and, remarkably, this prediction is insensitive to rheology of the liquid film, be it viscoelastic or Newtonian. Using the spectral numerical method, we demonstrate that the free-surface instability can be completely suppressed at all wave numbers when the solid becomes sufficiently deformable. For the case of pure polymeric liquids without any solvent, when the solid layer is made further deformable, both the free surface and the liquid-solid interface are destabilized at finite wave numbers. We also demonstrate a type of mode exchange phenomenon between the modes corresponding to the two interfaces. Importantly, our numerical results show that there is a sufficient range of shear modulus of the solid where both the modes are stable at all wave numbers. For polymer solutions described by the Oldroyd-B model, while the free-surface instability is suppressed by the deformable solid, a host of new unstable modes appear at finite Reynolds number and wave number because of the coupling between liquid flow and free shear waves in the solid. Our study thus demonstrates that the elastohydrodynamic coupling between liquid flow and solid deformation can be exploited either to suppress the free-surface instability (present otherwise in rigid inclines) in viscoelastic film flows, or to induce new instabilities that are absent in flow adjacent
Bunching Instability of Rotating Relativistic Electron Layers and Coherent Synchrotron Radiation
Schmekel, B S; Wasserman, I M; Schmekel, Bjoern S.; Lovelace, Richard V.E.; Wasserman, Ira M.
2004-01-01
We study the stability of a collisionless, relativistic, finite-strength, cylindrical layer of charged particles in free space by solving the linearized Vlasov-Maxwell equations and compute the power of the emitted electromagnetic waves. The layer is rotating in an external magnetic field parallel to the layer. This system is of interest to understanding the high brightness temperature of pulsars which cannot be explained by an incoherent radiation mechanism. Coherent synchrotron radiation has also been observed recently in bunch compressors used in particle accelerators. We consider equilibrium layers with a `thermal' energy spread and therefore a non-zero radial thickness. The particles interact with their retarded electromagnetic self-fields. The effect of the betatron oscillations is retained. A short azimuthal wavelength instability is found which causes a modulation of the charge and current densities. The growth rate is found to be an increasing function of the azimuthal wavenumber, a decreasing functi...
Total Solar Eclipses and Atmospheric Boundary Layer Response
Stoev, A.; Stoeva, P.; Kuzin, S.
2012-11-01
The effect of three total solar eclipses on meteorological parameters is discussed in the paper. Measurements were conducted at the village of Ravnets,General Toshevo municipality, Bulgaria, 1999,in Manavgat, near Antalya, Turkey, 2006 and in Tian Huang Ping, China, 2009. The observed decrease of the sky illumination (incoming solar radiation) during the eclipses was proportional to the percentage of solar coverage. The after eclipse sky illumination level is due to the effect of the natural change of the solar elevation angle. For the 1999 TSE it did not regain its pre eclipse value, it has exactly the same value for the 2006 TSE, and, It is three times larger than the pre eclipse value for the 2009 TSE. This fact can be easily explained by the Local Time of the maximum of the eclipses: LT 13:12, LT 12:58, and LT 09:34, respectively. Measurements showed significant changes in the surface air temperature. The minimum of the air temperature during the 2009 TSE (Tmin=4.5°C) was measured 6 min after the end of the total phase. This minimal temperature drop and larger time lag can be explained with the huge artificial lake near the place of observation, which minimizes the temperature response due to its larger heat capacity. During the 1999 TSE, minimal temperature (Tmin=6.4°C) is measured 7 min 30 s after the total phase, and for the 2006 TSE (Tmin=5°C) - 5 min. It is in accordance with the fact that the temperature minima at residential/commercial stations occurred in general, before the minima at stations in agricultural terrains. In 2006 we were at the yard of the hotel, and in 1999 in the countryside. The wind velocity drops during the total phase as a result of the cooling and stabilization of the atmospheric boundary layer. The wind direction during the total phase changes and the wind begins to blow in the same direction as the direction of motion of the lunar shadow on the earth. Cirrus and cirrostratus clouds were observed during the 2006 total solar
Simultaneous profiling of the Arctic Atmospheric Boundary Layer
Mayer, S.; Jonassen, M.; Reuder, J.
2009-09-01
The structure of the Arctic atmospheric boundary layer (AABL) and the heat and moisture fluxes between relatively warm water and cold air above non-sea-ice-covered water (such as fjords, leads and polynyas) are of great importance for the sensitive Arctic climate system (e.g. Andreas and Cash, 1999). So far, such processes are not sufficiently resolved in numerical weather prediction (NWP) and climate models (e.g. Tjernström et al., 2005). Especially for regions with complex topography as the Svalbard mountains and fjords the state and diurnal evolution of the AABL is not well known yet. Knowledge can be gained by novel and flexible measurement techniques such as the use of an unmanned aerial vehicle (UAV). An UAV can perform vertical profiles as well as horizontal surveys of the mean meteorological parameters: temperature, relative humidity, pressure and wind. A corresponding UAV, called Small Unmanned Meteorological Observer (SUMO), has been developed at the Geophysical Institute at the University of Bergen in cooperation with Müller Engineering (www.pfump.org) and the Paparazzi Project (http://paparazzi.enac.fr). SUMO has been used under Arctic conditions at Longyear airport, Spitsbergen in March/April 2009. Besides vertical profiles up to 1500 m and horizontal surveys at flight levels of 100 and 200 m, SUMO could measure vertical profiles for the first time simultaneously in a horizontal distance of 1 km; one over the ice and snow-covered land surface and the other one above the open water of Isfjorden. This has been the first step of future multiple UAV operations in so called "swarms” or "flocks”. With this, corresponding measurements of the diurnal evolution of the AABL can be achieved with minimum technical efforts and costs. In addition, the Advanced Research Weather Forecasting model (AR-WRF version 3.1) has been run in high resolution (grid size: 1 km). First results of a sensitivity study where ABL schemes have been tested and compared with
Assimilation of Thermodynamic and Dynamic Boundary Layer Profiler Data
Crowell, S.; Turner, D. D.; Otkin, J.
2012-12-01
In 2009, the National Research Council issued a report stating that a fundamental limitation to our understanding of mesoscale meteorological phenomena is the absence of adequate observations in the atmospheric boundary layer. In Otkin et al (2011) and Hartung et al (2011), an Observing Systems Simulation Experiment was described that concluded that the inclusion of thermodynamic retrievals from instruments like the Atmospheric Emitted Radiance Interferometer, together with wind observations from a Doppler lidar, could improve precipitation forecast skill scores using an ensemble Kalman filter (DART) together with the Weather Research and Forecasting Model (WRF). Here we discuss a second set of experiments in which the density of the proposed profiler network was doubled. Surprisingly, the results were only marginally better, and in some cases were degraded. This can be seen to be an effect of decreasing spread in the location of the strongest atmospheric gradients. An alternate set of experiments was performed with the 3D Variational framework, with the background error correlation length scales being tuned to match the EnKF localization as closely as possible. Interestingly, the 3DVar solutions exhibit qualitatively different responses to the assimilation of the observations than the EnKF solutions, with the placement and magnitude of the precipitation being improved, as determined by examining model precipitation on transects passing orthogonal to the front. A second case study will also be presented, in which we explore the relative importance of model error and observations for a springtime convective cased modeled on the May 24, 2011 tornado outbreak that passed through Texas, Oklahoma and Kansas. The sensitivity of convective processes to subgrid physics parameterizations can be seen to be a challenging problem for a data assimilation system, regardless of the quality of the observations being assimilated. Rather than using precipitation as the metric for
On the hydrodynamic stability of a particle-laden flow in growing flat plate boundary layer
Institute of Scientific and Technical Information of China (English)
XIE Ming-liang; LIN Jian-zhong; XING Fu-tang
2007-01-01
The parabolized stability equation (PSE) was derived to study the linear stability of particle-laden flow in growing Blasius boundary layer. The stability characteristics for various Stokes numbers and particle concentrations were analyzed after solving the equation numerically using the perturbation method and finite difference. The inclusion of the nonparallel terms produces a reduction in the values of the critical Reynolds number compared with the parallel flow. There is a critical value for the effect of Stokes number, and the critical Stokes number being about unit, and the most efficient instability suppression takes place when Stokes number is of order 10. But the presence of the nonparallel terms does not affect the role of the particles in gas. That is, the addition of fine particles (Stokes number is much smaller than 1) reduces the critical Reynolds number while the addition of coarse particles (Stokes number is much larger than 1) enhances it. Qualitatively the effect of nonparallel mean flow is the same as that for the case of plane parallel flows.
Sun, Jielun; Mahrt, Larry; Nappo, Carmen; Lenschow, Donald
2015-04-01
We investigate atmospheric internal gravity waves (IGWs): their generation and induction of global intermittent turbulence in the nocturnal stable atmospheric boundary layer based on the new concept of turbulence generation discussed in Sun et al. (2012). The IGWs are generated by air lifted by convergence forced by the colliding background flow and cold currents near the ground. The buoyancy-forced IGWs enhance wind speed at the wind-speed wave crests such that the bulk shear instability generates large coherent eddies, which augment local turbulent mixing and vertically redistribute momentum and heat. The periodically enhanced turbulent mixing, in turn, modifies the air temperature and flow oscillations of the original IGWs. These turbulence-forced oscillations (TFOs) resemble waves and coherently transport momentum and sensible heat. The observed momentum and sensible heat fluxes at the IGW frequency, which are either due to the buoyancy-forced IGWs themselves or by the TFOs, are larger than turbulent fluxes near the surface. The IGWs enhance not only the bulk shear at the wave crests, but also local shear over the wind speed troughs of the surface IGWs. Temporal and spatial variations of turbulent mixing as a result of this wave-induced turbulent mixing change the mean air flow and the shape of the IGWs.
Impacts of sea spray on the boundary layer structure of Typhoon Imbudo
Institute of Scientific and Technical Information of China (English)
TANG Jie; LI Weibiao; CHEN Shumin; WANG Lei
2013-01-01
High winds in a typhoon over the ocean can produce substantial amounts of spray in the lower part of the atmospheric boundary layer, which can modify the transfer of momentum, heat, and moisture across the air-sea interface. However, the consequent effects on the boundary layer structure and the evolution of the typhoon are largely unknown. The focus of this paper is on the role of sea spray on the storm intensity and the structure of the atmospheric boundary layer. The case study is Typhoon Imbudo in July 2003. The results show that sea spray tends to intensify storms by increasing the sea surface heat fluxes. Moreover, the effects of sea spray are mainly felt in boundary layer. Spray evaporation causes the atmospheric boundary layer to experience cooling and moistening. Sea spray can cause significant effects on the structure of boundary layer. The boundary-layer height over the eyewall area east to the center of Typhoon Imbudo was increased with a maximum up to about 550 m due to sea spray, which is closely related with the enhancements of the heat fluxes, upward motions, and horizontal winds in this region due to sea spray.
Multilevel Model of Planetary Boundary-layer Suitable for use with Mesoscale Dynamic Models
DEFF Research Database (Denmark)
Busch, N. E.; Chang, S. W.; Anthes, R. A.
1976-01-01
In this paper a simple model of the planetary boundary layer (PBL) is proposed. The surface layer is modeled according to established similarity theory. Above the surface layer a prognostic equation for the mixing length is introduced. The time-dependent mixing length is a function of the PBL...
Bathel, Brett F.; Johansen, Craig T.; Danehy, Paul M.; Inman, Jennifer A.; Jones, Stephen B.; Goyne, Christopher P.
2011-01-01
Measurements of instantaneous and mean streamwise velocity profiles in a hypersonic laminar boundary layer as well as a boundary layer undergoing laminar-to-turbulent transition were obtained over a 10-degree half-angle wedge model. A molecular tagging velocimetry technique consisting of a NO2 approaches?NO photo-dissociation reaction and two subsequent excitations of NO was used. The measurement of the transitional boundary layer velocity profiles was made downstream of a 1-mm tall, 4-mm diameter cylindrical trip along several lines lying within a streamwise measurement plane normal to the model surface and offset 6-mm from the model centerline. For laminar and transitional boundary layer measurements, the magnitudes of streamwise velocity fluctuations are compared. In the transitional boundary layer the fluctuations were, in general, 2-4 times larger than those in the laminar boundary layer. Of particular interest were fluctuations corresponding to a height of approximately 50% of the laminar boundary layer thickness having a magnitude of nearly 30% of the mean measured velocity. For comparison, the measured fluctuations in the laminar boundary layer were approximately 5% of the mean measured velocity at the same location. For the highest 10% signal-to-noise ratio data, average single-shot uncertainties using a 1 ?Es and 50 ?Es interframe delay were 115 m/s and 3 m/s, respectively. By averaging single-shot measurements of the transitional boundary layer, uncertainties in mean velocity as low as 39 m/s were obtained in the wind tunnel. The wall-normal and streamwise spatial resolutions were 0.14-mm (2 pixel) and 0.82-mm (11 pixels), respectively. These measurements were performed in the 31-inch Mach 10 Air Wind Tunnel at the NASA Langley Research Center.
Magnetohydrodynamic Boundary Layer Slip Flow and Heat Transfer of Power Law Fluid over a Flat Plate
Directory of Open Access Journals (Sweden)
Jacob Hirschhorn
2016-01-01
Full Text Available In this paper, we consider the magnetohydrodynamic (MHD boundary layer flow and heat transfer of power law fluid over a flat plate with slip boundary conditions. We use a similarity transformation to convert the governing nonlinear partial differential equations into a system of ordinary differential equations and solve the resulting system numerically using MATLAB’s boundary value solver, bvp4c, and the shooting method. We present velocity and temperature profiles within the boundary layer and demonstrate the effect of changing the magnetic parameter, Prandtl number, and slip parameters.
Faraday instability of a two-layer liquid film with a free upper surface
Pototsky, Andrey; Bestehorn, Michael
2016-06-01
We study the linear stability of a laterally extended flat two-layer liquid film under the influence of external vertical vibration. The first liquid layer rests on a vibrating solid plate and is overlaid by a second layer of immiscible fluid with deformable upper surface. Surface waves, excited as the result of the Faraday instability, can be characterized by a time-dependent relative amplitude of the displacements of the liquid-liquid and the liquid-gas interfaces. The in-phase displacements are associated with a zigzag (barotropic) mode and the antiphase displacement corresponds to the varicose thinning mode. We numerically determine the stability threshold in the vibrated two-layer film and compute the dispersion relation together with the decay rates of the surface waves in the absence of vibration. The in-phase and the antiphase displacements are strongly coupled in the vibrated system. The interplay between the Faraday and the Rayleigh-Taylor instabilities in the system with heavier fluid on top of a lighter fluid is analyzed.
3D microstructure modelling of coating layers including grain boundaries
Yashchuk, Ivan
2016-01-01
Nowadays, coatings have a significant role in increasing the lifetime of manufactured products. A coating layer applied to the surface of a product increases its corrosion and wear resistance. As with any other materials, coatings are subjected to damage phenomena. The damage of the coating layer usually happens because of delamination and crack propagation inside the coating layer. In order to know how to improve the coating resistance the fracture behavior is studied using finite element an...
A model for turbulent dissipation rate in a constant pressure boundary layer
Indian Academy of Sciences (India)
J DEY; P PHANI KUMAR
2016-04-01
Estimation of the turbulent dissipation rate in a boundary layer is a very involved process.Experimental determination of either the dissipation rate or the Taylor microscale, even in isotropic turbulence,which may occur in a portion of the turbulent boundary layer, is known to be a difficult task. For constant pressure boundary layers, a model for the turbulent dissipation rate is proposed here in terms of the local mean flow quantities. Comparable agreement between the estimated Taylor microscale and Kolmogorov length scale with other data in the logarithmic region suggests usefulness of this model in obtaining these quantitiesexperimentally
DEFF Research Database (Denmark)
Kirova, Hristina; Batchvarova, Ekaterina; Gryning, Sven-Erik;
2014-01-01
The boundary-layer processes in High Arctic area are studied based on consecutive radiosoundings and numerical simulations with Weather Research and Forecasting (WRF) model version 3.3.1 during a late winter period. The measurements consist of about 30 radiosondings performed every 12 hours in...... WRF were performed using Mellor – Yamada – Janjic scheme for planetary boundary processes with corresponding Monin – Obukhov (Janjic Eta) the surface layer scheme and the Noah land surface model. The variability of the correlation coefficient with height for all studied meteorological fields...... - 500 m. The modelled boundary-layer height is compared to its expert evaluation from measurements....
To definition of theory of boundary layer connected with motion on free liquid surface
International Nuclear Information System (INIS)
A modified theory of a boundary layer associated with a periodic capillary-gravitational motion on the free surface of an infinitely deep viscous liquid is proposed. The flow in the boundary layer is described in terms of a simplified (compared with the complete statement) model problem a solution to which correctly reflects the main features of an exact asymptotic solution: the rapid decay of the flow eddy part with depth of the liquid and insignificance of some terms appearing in the complete statement. The boundary layer thickness at which the discrepancy between the exact asymptotic solution and model solution is within a given margin is estimated
A Note Concerning the Turbulent Boundary Layer Drag at Large Reynolds Numbers
Barenblatt, G.I.; Chorin, A.J.; Prostokishin, V. M.
2000-01-01
A correlation is obtained for the drag coefficient $c '_f$ of the turbulent boundary layer as a function of the effective boundary layer Reynolds number $Re$ that we previously introduced. A comparison is performed also with another correlation for the drag coefficient as a function of the traditional Reynolds number $Re_{\\th}$, based on the momentum thickness of the boundary layer proposed recently by R.D.Watson, R.M.Hall and J.B.Anders (NASA Langley Research Center) on the basis of differen...
Integral method for the calculation of three-dimensional, laminar and turbulent boundary layers
Stock, H. W.
1978-01-01
The method for turbulent flows is a further development of an existing method; profile families with two parameters and a lag entrainment method replace the simple entrainment method and power profiles with one parameter. The method for laminar flows is a new development. Moment of momentum equations were used for the solution of the problem, the profile families were derived from similar solutions of boundary layer equations. Laminar and turbulent flows at the wings were calculated. The influence of wing tapering on the boundary layer development was shown. The turbulent boundary layer for a revolution ellipsoid is calculated for 0 deg and 10 deg incidence angles.
Boundary-layer effects in composite laminates: Free-edge stress singularities, part 6
Wanag, S. S.; Choi, I.
1981-01-01
A rigorous mathematical model was obtained for the boundary-layer free-edge stress singularity in angleplied and crossplied fiber composite laminates. The solution was obtained using a method consisting of complex-variable stress function potentials and eigenfunction expansions. The required order of the boundary-layer stress singularity is determined by solving the transcendental characteristic equation obtained from the homogeneous solution of the partial differential equations. Numerical results obtained show that the boundary-layer stress singularity depends only upon material elastic constants and fiber orientation of the adjacent plies. For angleplied and crossplied laminates the order of the singularity is weak in general.
The instability of water-mud interface in viscous two-layer flow with large viscosity contrast
Directory of Open Access Journals (Sweden)
Jiebin Liu
2014-01-01
Full Text Available The temporal instability of parallel viscous two-phase mixing layers is extended to current-fluid mud by considering a composite error function velocity profile. The influence of viscosity ratio, Reynolds number, and Froude number on the instability of the system are discussed and a new phenomenon never discussed is investigated based on our numerical results. It is shown that viscosity can enlarge the unstable wave number range, cause new instability modes, and certainly reduce the growth rate of Kelvin—Helmholtz (K—H instability.
International Nuclear Information System (INIS)
New Petrov-Galerkin formulations on the finite element methods for convection-diffusion problems with boundary layers are presented. Such formulations are based on a consistent new theory on discontinuous finite element methods. Existence and uniqueness of solutions for these problems in the new finite element spaces are demonstrated. Some numerical experiments shows how the new formulation operate and also their efficacy. (author)
Vortex Dynamics and Shear-Layer Instability in High-Intensity Cyclotrons
Cerfon, Antoine J.
2016-04-01
We show that the space-charge dynamics of high-intensity beams in the plane perpendicular to the magnetic field in cyclotrons is described by the two-dimensional Euler equations for an incompressible fluid. This analogy with fluid dynamics gives a unified and intuitive framework to explain the beam spiraling and beam breakup behavior observed in experiments and in simulations. Specifically, we demonstrate that beam breakup is the result of a classical instability occurring in fluids subject to a sheared flow. We give scaling laws for the instability and predict the nonlinear evolution of beams subject to it. Our work suggests that cyclotrons may be uniquely suited for the experimental study of shear layers and vortex distributions that are not achievable in Penning-Malmberg traps.
Assessment of boundary layer profiling formulas using tower, sodar and balloon data
Energy Technology Data Exchange (ETDEWEB)
Paine, R.J. [ENSR Consulting and Engineering, Inc., Acton, MA (United States); Kendall, S.B. [Phelps Dodge Corp., Phoenix, AZ (United States)
1994-12-31
The accuracy of an air quality dispersion model is largely dependent upon the availability of representative meteorological data for the simulation of plume rise, transport, and dispersion. In many cases where tall stacks and/or buoyant plumes are involved, the available meteorological measurements do not extend to plume height. Air quality models contend with these situations by either assuming no change of meteorological variables with elevation or by applying a profiling relationship based upon theoretical or empirical relationships. The latter treatment is employed in recently-developed models such as CTDMPLUS, and HPDM, and OML. In the well-mixed convective boundary layer, meteorological variables such as wind direction, wind speed, and turbulence do not vary substantially above the surface layer (about 0.1 z{sub i}, the mixed-layer height). Above the surface layer, behavior on an hourly average basis is fairly well parameterized by boundary-layer formulations. However, models are sensitive to the height of the convective boundary layer, z{sub i}, which affects the magnitude of the convective velocity scale, w., and is important for simulating plume trapping and plume penetration into the stable layer aloft. In the stable boundary layer, plumes are often released at heights above the stable boundary layer, the height of which is often hard to define. Models are sensitive to the manner in which wind direction, wind speed, temperature and turbulence are profiled with height in stable conditions.
Velocity Spectra in the Unstable Planetary Boundary Layer
DEFF Research Database (Denmark)
Højstrup, Jørgen
1982-01-01
Models for velocity spectra of all three components in the lower half of the unstable PBL are presented. The model spectra are written as a sum of two parts, nS(n) = A(fi, z/zi)w*2 + B(f, z/zi)u*02, a mixed layer part with a stability dependence, and a surface layer part without stability...
Energy Technology Data Exchange (ETDEWEB)
Liu, C.; Liu, Z. [Univ. of Colorado, Denver, CO (United States)
1994-12-31
A new multilevel technology was developed in this study which provides a successful numerical simulation for the whole process of flow transition in 3-D flat plate boundary layers, including linear growth, secondary instability, breakdown, and transition on a relatively coarse grid with low CPU cost. A fourth-order finite difference scheme on stretched and staggered grids, a fully implicit time-marching technique, a semi-coarsening multigrid based on the so-called approximate line-box relaxation, and a buffer domain for the outflow boundary conditions were all employed for high-order accuracy, good stability, and fast convergence. A new fine-coarse-fine grid mapping technique was developed to catch the large eddies and represent main roles of small eddies to keep the code running after the laminar flow breaks down. The computational results are in good agreement with linear stability theory, secondary instability theory, and some experiments. The computation also reproduced the K-type and C-type transition observed by laboratory experiments. The CPU cost for a typical case is around 2-9 CRAY-YMP hours.
Marine boundary-layer height estimated from the HIRLAM model
DEFF Research Database (Denmark)
Gryning, Sven-Erik; Batchvarova, E.
2002-01-01
-number estimates based on output from the operational numerical weather prediction model HIRLAM (a version of SMHI with a grid resolution of 22.5 km x 22.5 km). For southwesterly winds it was found that a relatively large island (Bornholm) lying 20 km upwind of the measuring site influences the boundary...
Review of magnetospheric boundary layer phenomena and relations to current theories
International Nuclear Information System (INIS)
Recent observations on the magnetopause and boundary layer are reviewed. A region with magnetosheath-like plasma is found in an entry layer inside the magnetopause, at least partly on closed field lines. There is no enhanced flow near the magnetopause, in contrast to what would be expected on the basis of reconnection theories. Inside the magnetopause there is a boundary layer, which must be polarized. Parallel electric fields and currents are involved, thus invalidating the mapping of the electric field along magnetic field lines. Access to the entry layer must be impulsive or diffusive in nature. (author)
Investigations of shock wave boundary layer interaction on suction side of compressor profile
International Nuclear Information System (INIS)
The shock wave boundary layer interaction on the suction side of transonic compressor blade is one of main objectives of TFAST project (Transition Location Effect on Shock Wave Boundary Layer Interaction). In order to look more closely into the flow structure on the suction side of a blade, a design of a generic test section in linear transonic wind tunnel was proposed. The test section which could reproduce flow structure, shock wave location, pressure distribution and boundary layer development similar to the obtained on a cascade profile is the main objective of the presented here design. The design of the proposed test section is very challenging, because of shock wave existence, its interaction with boundary layer and its influence on the 3-D flow structure in the test section.
An investigation of the effects of the propeller slipstream of a laminar wing boundary layer
Howard, R. M.; Miley, S. J.; Holmes, B. J.
1985-01-01
A research program is in progress to study the effects of the propeller slipstream on natural laminar flow. Flight and wind tunnel measurements of the wing boundary layer have been made using hot-film velocity sensor probes. The results show the boundary layer, at any given point, to alternate between laminar and turbulent states. This cyclic behavior is due to periodic external flow turbulence originating from the viscous wake of the propeller blades. Analytic studies show the cyclic laminar/turbulent boundary layer to result in a significantly lower wing section drag than a fully turbulent boundary layer. The application of natural laminar flow design philosophy yields drag reduction benefits in the slipstream affected regions of the airframe, as well as the unaffected regions.
Strong, Stuart L.; Meade, Andrew J., Jr.
1992-01-01
Preliminary results are presented of a finite element/finite difference method (semidiscrete Galerkin method) used to calculate compressible boundary layer flow about airfoils, in which the group finite element scheme is applied to the Dorodnitsyn formulation of the boundary layer equations. The semidiscrete Galerkin (SDG) method promises to be fast, accurate and computationally efficient. The SDG method can also be applied to any smoothly connected airfoil shape without modification and possesses the potential capability of calculating boundary layer solutions beyond flow separation. Results are presented for low speed laminar flow past a circular cylinder and past a NACA 0012 airfoil at zero angle of attack at a Mach number of 0.5. Also shown are results for compressible flow past a flat plate for a Mach number range of 0 to 10 and results for incompressible turbulent flow past a flat plate. All numerical solutions assume an attached boundary layer.
Marine boundary layer and turbulent fluxes over the Baltic Sea: Measurements and modelling
DEFF Research Database (Denmark)
Gryning, Sven-Erik; Batchvarova, E.
2002-01-01
Two weeks of measurements of the boundary-layer height over a small island (Christianso) in the Baltic Sea are discussed. The meteorological conditions are characterised by positive heat flux over the sea. The boundary-layer height was simulated with two models, a simple applied high-resolution (2...... km x 2 km) model, and the operational numerical weather prediction model HIRLAM (grid resolution of 22.5 km x 22.5 km). For southwesterly winds it was found that a relatively large island (Bornholm) lying 20-km upwind of the measuring site influences the boundary-layer height. In this situation the...... high-resolution simple applied model reproduces the characteristics of the boundary-layer height over the measuring site. Richardson-number based methods using data from simulations with the HIRLAM model fail, most likely because the island and the water fetch to the measuring site are about the size...
Two-phase gas bubble-liquid boundary layer flow along vertical and inclined surfaces
International Nuclear Information System (INIS)
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 450 to 1350 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
Space and Astrophysical Plasmas : Sun–Earth connection: Boundary layer waves and auroras
Indian Academy of Sciences (India)
G S Lakhina; B T Tsurutani; J K Arballo; C Galvan
2000-11-01
Boundary layers are the sites where energy and momentum are exchanged between two distinct plasmas. Boundary layers occurring in space plasmas can support a wide spectrum of plasma waves spanning a frequency range of a few mHz to 100 kHz and beyond. The main characteristics of the broadband plasma waves (with frequencies > 1 Hz) observed in the magnetopause, polar cap, and plasma sheet boundary layers are described. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactions with the waves can provide sufﬁcient precipitated energy ﬂux to the ionosphere to create the diffused auroral oval. The broadband plasma waves may also play an important role in the processes of local heating/acceleration of the boundary layer plasma.
On similarity and pseudo-similarity solutions of Falkner-Skan boundary layers
Guedda, Mohamed
2008-01-01
The present work deals with the two-dimensional incompressible,laminar, steady-state boundary layer equations. First, we determinea family of velocity distributions outside the boundary layer suchthat these problems may have similarity solutions. Then, we examenin detail new exact solutions, called Pseudo--similarity, where the external velocity varies inversely-linear with the distance along the surface $ (U_e(x) = U_\\infty x^{-1}). 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. Then, we examenin detail new exact solutions. The analysis shows that solutions exist only for a lateral suction. For specified conditions, we establish the existence of an infinite number of solutions, including monotonic solutions and solutions which oscillate an infinite number of times and tend to a certain limit. The properties o...
Kanatani, Kentaro
2008-01-01
We study an instability of thin liquid-vapor layers bounded by rigid parallel walls from both below and above. In this system, the interfacial instability is induced by lateral vapor pressure fluctuation, which is in turn attributed to the effect of phase change: evaporation occurs at the hotter portion of the interface and condensation at the colder one. The high vapor pressure drives the liquid away and the low one pulls it up. A set of equations describing the temporal evolution of the interface of the liquid-vapor layers is derived. This model neglects the effect of mass loss or gain at the interface and guarantees the mass conservation of the liquid layer. The result of linear stability analysis of the model shows that the presence of the pressure dependence of the local saturation temperature suppresses the growth of long-wave disturbances. We find the stability criterion, which suggests that only slight temperature gradients are sufficient to overcome the stabilizing gravitational effect for a water an...
Wagner, Johannes; Gohm, Alexander; Rotach, Mathias; Leukauf, Daniel; Posch, Christian
2014-05-01
The role of horizontal model grid resolution on the development of the daytime boundary layer over mountainous terrain is studied. A simple idealized valley topography with a cross-valley width of 20~km, a valley depth of 1.5~km and a constant surface heat flux forcing is used to generate upslope flows in a warming valley boundary layer. The goal of this study is to investigate differences in the upslope flow and boundary layer structure of the valley when its topography is either fully resolved, smoothed or not resolved by the numerical model. This is done by performing both large-eddy (LES) and kilometer-scale simulations with mesh sizes of 50, 1000, 2000, 4000, 5000 and 10000~m. In LES mode a valley inversion layer develops, which separates two vertically stacked circulation cells in an upper and lower boundary layer. These structures weaken with decreasing horizontal model grid resolution and change to a convective boundary layer similar to the one over an elevated flat plain when the valley is no longer resolved. Mean profiles of the LES run, which are obtained by horizontal averaging over the valley show a three-layer thermal structure and a secondary heat flux maximum at ridge height. Strong smoothing of the valley topography prevents the development of a valley inversion layer with stacked circulation cells and leads to higher valley temperatures due to smaller valley volumes. This investigation shows that a parameterization is needed in coarse resolution models to capture exchange processes over mountainous terrain.
Convection and Chemistry in the Atmospheric Boundary Layer
A. C. Petersen
1999-01-01
The earth’s troposphere is the lowest layer of the atmosphere and has a thickness of about 10 km. It is the layer that contains most of the mass (80%) of the atmosphere. All weather phenomena that we experience have their origin in the troposphere. It is the stage for some well-known environmental problems: climate change, ozone smog, and acidification. These problems are related to the trace amount of gases that are emitted into the troposphere from anthropogenic sources. Alth...
A simple method to determine evaporation duct height in the sea surface boundary layer
Musson-Genon, Luc; Gauthier, Sylvie; Bruth, Eric
1992-09-01
A formulation to determine the evaporation duct height in the sea surface boundary layer is presented. This formulation is based upon the theory of similarity of Monin Obukhov by using analytical solutions currently used in the field of numerical weather prediction. The proposed solution is simple, coherent with the surface boundary layer parameterization used in the Meteo France and European Centre for Medium-Range Weather Forecasts weather prediction models and gives good results when compared to more traditional methods.
Enhanced air pollution via aerosol-boundary layer feedback in China
Petäjä, T.; Järvi, L.; V.-M. Kerminen; Ding, A. J.; J. N. Sun; Nie, W.; Kujansuu, J.; A. Virkkula; Yang, X.; C. B. Fu; S. Zilitinkevich; Kulmala, M.
2016-01-01
Severe air pollution episodes have been frequent in China during the recent years. While high emissions are the primary reason for increasing pollutant concentrations, the ultimate cause for the most severe pollution episodes has remained unclear. Here we show that a high concentration of particulate matter (PM) will enhance the stability of an urban boundary layer, which in turn decreases the boundary layer height and consequently cause further increases in PM concentrations. We estimate the...
UK-ADMS - a new approach to modelling dispersion in the earth's atmospheric boundary layer
International Nuclear Information System (INIS)
The UK Atmospheric Dispersion Modelling System is described in considerable detail. The principle modules are dealt with. A key to the methodology is that vertical profiles of mean velocity, temperature and turbulence in the boundary layer depend on the relative values of the height above the ground, the height of the boundary layer, and a length scale determined by the friction velocity and the heat flux and air temperature at the surface. The models can be used at any location. (AB) (15 refs.)
Boundary Layer Measurements of the NACA0015 and Implications for Noise Modeling
Bertagnolio, Franck
2011-01-01
A NACA0015 airfoil section instrumented with an array of high frequency microphones flush-mounted beneath its surface was measured in the wind tunnel at LM Wind Power in Lunderskov. Various inflow speeds and angles of attack were investigated. In addition, a hot-wire device system was used to measure the velocity profiles and turbulence characteristics in the boundary layer near the trailing edge of the airfoil. The measured boundary layer data are presented in this report and compared with C...
Experiments on the wave train development in 3D boundary layer at Mach 2
International Nuclear Information System (INIS)
Stability experiments of controlled disturbances in 3D supersonic boundary layer on the thin swept wing at low unit Reynolds numbers are considered in the paper. The results of the linear evolution of stationary and traveling disturbances in supersonic boundary layer on swept wing at controlled conditions are presented. Wave characteristics of traveling disturbances are obtained. Stabilizing effect on the wave train evolution due to periodic micro roughnesses is demonstrated.
DNS and the theory of receptivity of a supersonic boundary layer to free-stream disturbances
International Nuclear Information System (INIS)
Direct numerical simulation (DNS) of receptivity of a boundary layer over flat plate is carried out. The free stream Mach number is equal to 6. The following two-dimensional disturbances are introduced into the free-stream flow: fast and slow acoustic waves, temperature spottiness. A theoretical model describing the excitation of unstable waves in the boundary layer is developed using the biorthogonal eigenfunction decomposition method. The DNS results agree with the theoretical predictions.
Turbulent Transport of 222-Rn and its Short-lived Daughters in Convective Boundary Layers
VINUESA JEAN; GALMARINI STEFANO
2006-01-01
222Rn is a natural radioactive compound with a half-life of 3.8 days. Because of its noble gas nature, it is a suitable tracer in studies of atmospheric boundary layers. Ground-based measurements and vertical distributions of 222Rn and its daughters have been extensively studied in the past, e.g., to characterize the turbulent properties of the atmospheric boundary layer, to perform regional and global circulation model benchmarking and to estimate regional surface flu...
Development of a laminar boundary layer model for curved wall jets
Valeriu DRĂGAN
2013-01-01
The paper addresses the issue of thin jets subjected to the Coandă effect and in particular the boundary layer modeling. An existing semi-empirical Coandă effect mathematical model is modified, with a more complex boundary layer model, in order to allow the estimative calculation of the detachment point and of other parameters such as friction coefficients, wall shear stress and the momentum and displacement integral thicknesses. The method used is analytical, based on the Rodman-Wood-Roberts...
Kok, H.J.M.; Voskuijl, M.; Van Tooren, M.J.L.
2010-01-01
The blended wing body aircraft is one of the promising contenders for the next generation large transport aircraft. This aircraft is particularly suitable for the use of boundary layer ingestion engines. Results published in literature suggest that it might be beneficial to have a large number of these engines (distributed propulsion). A conceptual design study is therefore performed to determine the potential benefits of boundary layer ingestion engines for a conventional number of engines i...
On similarity and pseudo-similarity solutions of Falkner-Skan boundary layers
Guedda, Mohamed; Hammouch, Zakia
2006-01-01
The present work deals with the two-dimensional incompressible,laminar, steady-state boundary layer equations. First, we determinea family of velocity distributions outside the boundary layer suchthat these problems may have similarity solutions. Then, we examenin detail new exact solutions, called Pseudo--similarity, where the external velocity varies inversely-linear with the distance along the surface $ (U_e(x) = U_\\infty x^{-1}). The present work deals with the two-dimensional incompressi...
Wind instability of a foam layer sandwiched between the atmosphere and the ocean
Shtemler, Yuri M; Mond, Michael
2007-01-01
Kelvin-Helmholtz instability of short gravity waves is examined in order to explain the recent findings of the decrease in momentum transfer from hurricane winds to sea waves. A three-fluid configuration of a foam layer between the atmosphere and the ocean is suggested to provide signifficant stabilization of the system and shifting the marginal critical wavelength to the shortwave part of the spectrum. It is conjectured that such stabilization leads to the observed drag reduction. The high contrasts in three fluid densities provide a universal mechanism for stabilizing surface perturbations.
Directory of Open Access Journals (Sweden)
A. Malvandi
2015-01-01
Full Text Available The objective of this paper is to consider both effects of slip and convective heat boundary conditions on steady two-dimensional boundary layer flow of a nanofluid over a stretching sheet in the presence of blowing/suction simultaneously. Flow meets the Navier's slip condition at the surface and Biot number is also used to consider the effects of convective heat transfer. The employed model for nanofluid includes two-component four-equation nonhomogeneous equilibrium model that incorporates the effects of nanoparticle migration owing to Brownian motion and thermophoresis. The basic partial boundary layer equations have been transformed into a two-point boundary value problem via similarity variables. Results for impermeable isothermal surface and also no-slip boundary condition were in best agreements with those existing in literatures. Effects of governing parameters such as Biot number (Bi, slip parameter (λ, thermophoresis (Nt, Prandtl number (Pr, Lewis number (Le, Brownian motion (Nb and blowing/suction (S on reduced Nusselt and Sherwood numbers are analyzed and discussed in details. The obtained results indicate that unlike heat transfer rate, concentration rate is very sensitive to all parameters among which Le, S and Pr are the most effective ones.
A boundary-layer cloud study using Southern Great Plains Cloud and radiation testbed (CART) data
Energy Technology Data Exchange (ETDEWEB)
Albrecht, B.; Mace, G.; Dong, X.; Syrett, W. [Pennsylvania State Univ., University Park, PA (United States)] [and others
1996-04-01
Boundary layer clouds-stratus and fairweather cumulus - are closely coupled involves the radiative impact of the clouds on the surface energy budget and the strong dependence of cloud formation and maintenance on the turbulent fluxes of heat and moisture in the boundary layer. The continuous data collection at the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site provides a unique opportunity to study components of the coupling processes associated with boundary layer clouds and to provide descriptions of cloud and boundary layer structure that can be used to test parameterizations used in climate models. But before the CART data can be used for process studies and parameterization testing, it is necessary to evaluate and validate data and to develop techniques for effectively combining the data to provide meaningful descriptions of cloud and boundary layer characteristics. In this study we use measurements made during an intensive observing period we consider a case where low-level stratus were observed at the site for about 18 hours. This case is being used to examine the temporal evolution of cloud base, cloud top, cloud liquid water content, surface radiative fluxes, and boundary layer structure. A method for inferring cloud microphysics from these parameters is currently being evaluated.
Belyaev, Mikhail A; Stone, James M
2012-01-01
The nature of angular momentum transport in the boundary layers of accretion disks has been one of the central and long-standing issues of accretion disk theory. In this work we demonstrate that acoustic waves excited by supersonic shear in the boundary layer serve as an efficient mechanism of mass, momentum and energy transport at the interface between the disk and the accreting object. We develop the theory of angular momentum transport by acoustic modes in the boundary layer, and support our findings with 3D hydrodynamical simulations, using an isothermal equation of state. Our first major result is the identification of three types of global modes in the boundary layer. We derive dispersion relations for each of these modes that accurately capture the pattern speeds observed in simulations to within a few percent. Second, we show that angular momentum transport in the boundary layer is intrinsically nonlocal, and is driven by radiation of angular momentum away from the boundary layer into both the star an...
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.
Receptivity of Boundary Layer over a Blunt Wedge due to Freestream Pulse Disturbances at Mach 6
Directory of Open Access Journals (Sweden)
Jianqiang Shi
2016-01-01
Full Text Available Direct numerical simulation (DNS of a hypersonic compressible flow over a blunt wedge with fast acoustic disturbances in freestream is performed. The receptivity characteristics of boundary layer to freestream pulse acoustic disturbances are numerically investigated at Mach 6, and the frequency effects of freestream pulse wave on boundary layer receptivity are discussed. Results show that there are several main disturbance mode clusters in boundary layer under acoustic pulse wave, and the number of main disturbance clusters decreases along the streamwise. As disturbance wave propagates from upstream to downstream direction, the component of the modes below fundamental frequency decreases, and the component of the modes above second harmonic components increases quickly in general. There are competition and disturbance energy transfer between different boundary layer modes. The nose boundary layer is dominated by the nearby mode of fundamental frequency. The number of the main disturbance mode clusters decreases as the freestream disturbance frequency increases. The frequency range with larger growth narrows along the streamwise. In general, the amplitudes of both fundamental mode and harmonics become larger with the decreasing of freestream disturbance frequency. High frequency freestream disturbance accelerates the decay of disturbance wave in downstream boundary layer.
Hydrodynamic structure of the boundary layers in a rotating cylindrical cavity with radial inflow
Herrmann-Priesnitz, Benjamín; Calderón-Muñoz, Williams R.; Salas, Eduardo A.; Vargas-Uscategui, Alejandro; Duarte-Mermoud, Manuel A.; Torres, Diego A.
2016-03-01
A flow model is formulated to investigate the hydrodynamic structure of the boundary layers of incompressible fluid in a rotating cylindrical cavity with steady radial inflow. The model considers mass and momentum transfer coupled between boundary layers and an inviscid core region. Dimensionless equations of motion are solved using integral methods and a space-marching technique. As the fluid moves radially inward, entraining boundary layers develop which can either meet or become non-entraining. Pressure and wall shear stress distributions, as well as velocity profiles predicted by the model, are compared to numerical simulations using the software OpenFOAM. Hydrodynamic structure of the boundary layers is governed by a Reynolds number, Re, a Rossby number, Ro, and the dimensionless radial velocity component at the periphery of the cavity, Uo. Results show that boundary layers merge for Re > 0.1, and boundary layers become predominantly non-entraining for low Ro, low Re, and high Uo. Results may contribute to improve the design of technology, such as heat exchange devices, and turbomachinery.
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.
The Modelling of Particle Resuspension in a Turbulent Boundary Layer
International Nuclear Information System (INIS)
lift and drag forces in turbulent boundary layers, the lift and drag we have con sidered a
The effects of curvature and viscosity on baroclinic instability: A two-layer model
Fowlis, W. W.; Arias, S.
1978-01-01
A linear stability analysis of a baroclinic zonal current contained between two parallel rigid boundaries is presented. Curvature is included by performing the analysis on a beta b-plane and viscosity by allowing for the effects of Ekman layers on the rigid boundaries. A two-layer model is used. This calculation was carried out to assist in the design of a spherical model of the general circulation of the earth's atmosphere for Spacelab. In the low-gravity environment on an orbiting vehicle, a dominant radial dielectric body force, analogous to planetary gravity, can be achieved over a volume of liquid held between two concentric spheres. The results show the Eady short wavelength cutoff, and long wavelength cutoffs due to Ekman damping and curvature.
One-dimensional, time dependent simulation of the planetary boundary layer over a 48-hour period
International Nuclear Information System (INIS)
Results of a one-dimensional, time dependent simulation of the planetary boundary layer are given. First, a description of the mathematical model used is given and its approximations are discussed. Then a description of the initial and boundary conditions used for the simulation is given. Results are discussed with respect to their agreement with observed data and their precision. It can be demonstrated that a simulation of the planetary boundary layer is possible with satisfactory precision. The incompleteness of observed data gives, however, problems with their use and thus introduces uncertainties into the simulation. As a consequence, the report tries to point to the inherent limitations of such a simulation. (Auth.)
Propeller slipstream/wing boundary layer effects at low Reynolds numbers
Miley, S. J.; Howard, R. M.; Holmes, B. J.
1985-01-01
The effects of propeller slipstream on the wing laminar boundary are being investigated. Hot-wire velocity sensor measurements have been performed in flight and in a wind tunnel. It is shown that the boundary layer cycles between a laminar state and a turbulent state at the propeller blade passage rate. The cyclic length of the turbulent state increases with decreasing laminar stability. Analyses of the time varying velocity profiles show the turbulent state to lie in a transition region between fully laminar and fully turbulent. The observed cyclic boundary layer has characteristics similar to relaminarizing flow and laminar flow with external turbulence.
Impedance of a rough conducting boundary in a layered medium
International Nuclear Information System (INIS)
The authors construct an operator for the equivalent impedance of a rough surface which bounds a layered medium, and discuss the physical approach by which it is generated. They give an example of the application of this operator in computing the attenuation of eigenmodes of an integrated optical waveguide with a rough substrate
Velocity Spectra in the Unstable Planetary Boundary Layer
DEFF Research Database (Denmark)
Højstrup, Jørgen
1982-01-01
dependence and with negligible influence of z/zi in B in the surface layer; A is independent of z/zi for the horizontal components. The model agrees very well with data for variances, peak frequencies and spectra from the Kansas and Minnesota experiments. Requirements for models of spectra in the upper half...
Volino, Ralph John
1995-01-01
Measurements from transitional, heated boundary layers along a concave-curved test wall are presented and discussed. A boundary layer subject to low free-stream turbulence intensity (FSTI), which contains stationary streamwise (Gortler) vortices, is documented. The low FSTI measurements are followed by measurements in boundary layers subject to high (initially 8%) free-stream turbulence intensity and moderate to strong (K = {nuover U_sp{infty} {2}}{dUinftyover dx} as high as 9times 10^{ -6}) acceleration. The high FSTI experiments are the main focus of the work. Conditions were chosen to simulate those present on the downstream half of the pressure side of a gas turbine airfoil. The high FSTI boundary layers undergo transition from a strongly disturbed non-turbulent state to a fully-turbulent state. Due to the stabilizing effect of strong acceleration, the transition zones are of extended length in spite of the high FSTI. Transitional values of skin friction coefficients and Stanton numbers drop below flat-plate, low FSTI, turbulent flow correlations, but remain well above laminar flow values. Mean velocity and temperature profiles exhibit clear changes in shape as the flow passes through transition. Turbulence statistics, including the turbulent shear stress, turbulent heat flux, and turbulent Prandtl number, are documented. Turbulent transport is strongly suppressed below values in unaccelerated turbulent boundary layers. A technique called "octant analysis" is introduced and applied to several cases from the literature as well as to data from the present study. Octant analysis shows a fundamental difference between transitional and fully-turbulent boundary layers. Transitional boundary layers are characterized by incomplete mixing compared to fully-turbulent boundary layers. Similar octant analysis results are observed in both low and high FSTI cases. Spectral analysis suggests that the non-turbulent zone of the high FSTI flow is dominated by large scale
International Nuclear Information System (INIS)
In present day devices, the external kink ideal MHD instability establishes hard operational boundaries for both the tokamak and the Reversed Field Pinch (RFP) configurations. An interesting feature of it is that its growth rate critically depends on the device passive boundary characteristics and this can slow it down to time scales accessible to modern real time feedback control systems, normally using external active coils as actuators. 3D passive structures and external fields play a key role in determining physics and control of this instability. This is in particular true for equilibria with multimodal unstable RWM spectra where modes can couple to specific 3D features of passive and active magnetic boundary. In the paper we will present recent data and simulations from RFX-mod, a medium size (R = 2 m, a = 0.459 m) device able to confine RFP and tokamak plasmas with currents up to 2 MA and 120 kA, respectively. Successful quantitative modeling of multimodal RWM control experiments performed using different actuator configurations will be presented and commented. (author)
Boundary layer and separation control on wings at low Reynolds numbers
Yang, Shanling
Results on boundary layer and separation control through acoustic excitation at low Re numbers are reported. The Eppler 387 profile is specifically chosen because of its pre-stall hysteresis and bi-stable state behavior in the transitional Re regime, which is a result of flow separation and reattachment. External acoustic forcing on the wing yields large improvements (more than 70%) in lift-to-drag ratio and flow reattachment at forcing frequencies that correlate with the measured anti-resonances in the wind tunnel. The optimum St/Re1/2 range for Re = 60,000 matches the proposed optimum range in the literature, but there is less agreement for Re = 40,000, which suggests that correct St scaling has not been determined. The correlation of aerodynamic improvements to wind tunnel resonances implies that external acoustic forcing is facility-dependent, which inhibits practical application. Therefore, internal acoustic excitation for the same wing profile is also pursued. Internal acoustic forcing is designed to be accomplished by embedding small speakers inside a custom-designed wing that contains many internal cavities and small holes in the suction surface. However, initial testing of this semi-porous wing model shows that the presence of the small holes in the suction surface completely transforms the aerodynamic performance by changing the mean chordwise separation location and causing an originally separated, low-lift state flow to reattach into a high-lift state. The aerodynamic improvements are not caused by the geometry of the small holes themselves, but rather by Helmholtz resonance that occurs in the cavities, which generate tones that closely match the intrinsic flow instabilities. Essentially, opening and closing holes in the suction surface of a wing, perhaps by digital control, can be used as a means of passive separation control. Given the similarity of wing-embedded pressure tap systems to Helmholtz resonators, particular attention must be given to the
Convective Instability of a Gravity Modulated Fluid Layer with Surface Tension Variation
Skarda, J. Raymond Lee
1998-01-01
Gravity modulation of an unbounded fluid layer with surface tension variations along its free surface is investigated. In parameter space of (wavenumber, Marangoni number) modulation has a destabilizing effect on the unmodulated neutral stability curve for large Prandtl number, Pr, and small modulation frequency, Omega, while a stabilizing effect is observed for small Pr and large Omega. As Omega yields infinity, the modulated neutral stability curves approach the unmodulated neutral stability curve. At certain values of Pr and L2 multiple minima are observed and the neutral stability curves become highly distorted. Closed regions of subharmonic instability are also observed. Alternating regions of synchronous and subharmonic instability separated by very thin stable regions are observed in (1/Omega,g(sub 1)) space for the singly diffusive cases. Quasiperiodic behavior in addition to the synchronous and subharmonic responses, are observed for the case of a double diffusive fluid layer. Minimum acceleration amplitudes were observed to closely correspond with a subharmonic response, Lambda(sub im) = Omega/2 .
The influence of thermal effects on the wind speed profile of the coastal marine boundary layer
DEFF Research Database (Denmark)
Lange, B.; Larsen, Søren Ejling; Højstrup, Jørgen;
2004-01-01
planetary boundary layer with an inversion lid offers a qualitative explanation for these findings. When warm air is advected over colder water, a capping inversion typically develops. The air below is constantly cooled by the water and gradually develops into a well-mixed layer with near...
PIV measurements of the bottom boundary layer under nonlinear surface waves
Henriquez, M.; Reniers, A. J H M; Ruessink, B. G.; Stive, M. J F
2014-01-01
Sediment in the nearshore is largely mobilized in the wave bottom boundary layer (wbbl) hereby emphasizing the importance of this relatively thin layer to nearshore morphology. This paper presents a laboratory experiment where hydrodynamic properties of the wbbl were quantified by measuring flow vel
Off-Body Boundary-Layer Measurement Techniques Development for Supersonic Low-Disturbance Flows
Owens, Lewis R.; Kegerise, Michael A.; Wilkinson, Stephen P.
2011-01-01
Investigations were performed to develop accurate boundary-layer measurement techniques in a Mach 3.5 laminar boundary layer on a 7 half-angle cone at 0 angle of attack. A discussion of the measurement challenges is presented as well as how each was addressed. A computational study was performed to minimize the probe aerodynamic interference effects resulting in improved pitot and hot-wire probe designs. Probe calibration and positioning processes were also developed with the goal of reducing the measurement uncertainties from 10% levels to less than 5% levels. Efforts were made to define the experimental boundary conditions for the cone flow so comparisons could be made with a set of companion computational simulations. The development status of the mean and dynamic boundary-layer flow measurements for a nominally sharp cone in a low-disturbance supersonic flow is presented.
Boundary Layer Ignition of Hydrogen-Air Mixtures in Supersonic Flows
Institute of Scientific and Technical Information of China (English)
无
1994-01-01
Due to viscous heating spontaneous ignition of a supersonic flow of premixed combustible gases can occur in boundary layers.This process is studied numerically for a hyedrogen-air mixture in the case of a laminar boundary layer over a flat plate.In a previous study the main structure of the reacting flow was given as well as a first mapping of the ignition conditions versus boundary conditions.In the present work computations are performed in order to further specify the controlling mechanisms and parameters of such a boundary layer ignition.We emphasize more precisely i) the elementary steps of the chemical process which efectively control the ignition ii) the unusual role played by the equivalence ratio of the mixture iii) the influence of the Soret effect (species transport due to temperature gradients).
Coherent structures in a zero-pressure-gradient and a strongly decelerated boundary layer
Simens, Mark P.; Gungor, Ayse G.; Maciel, Yvan
2016-04-01
Coherent structures in a strongly decelerated large-velocity-defect turbulent boundary layer (TBL) and a zero pressure gradient (ZPG) boundary layer are analysed by direct numerical simulation (DNS). The characteristics of the one-point velocity stastistics are also considered. The adverse pressure gradient (APG) TBL simulation is a new one carried out by the present authors. The APG TBL begins as a zero pressure gradient boundary layer, decelerates under a strong adverse pressure gradient, and separates near the end of the domain in the form of a very thin separation bubble. The one-point velocity statistics in the outer region of this large-defect boundary layer are compared to those of two other large-velocity-defect APG TBLs (one in dynamic equilibrium, the other in disequilibrium) and a mixing layer. In the upper half of the large-defect boundary layers, the velocity statistics are similar to those of the mixing layer. The dominant peaks of turbulence production and Reynolds stresses are located in the middle of the boundary layers. Three-dimensional spatial correlations of (u, u) and (u, v) show that coherence is lost in the streamwise and spanwise directions as the velocity defect increases. Near-wall streaks tend to disappear in the large-defect zone of the flow to be replaced by more disorganized u motions. Near-wall sweeps and ejections are also less numerous. In the outer region, the u structures tend to be shorter, less streaky, and more inclined with respect to the wall than in the ZPG TBL. The sweeps and ejections are generally bigger with respect to the boundary layer thickness in the large-defect boundary layer, even if the biggest structures are found in the ZPG TBL. Large sweeps and ejections that reach the wall region (wall-attached) are less streamwise elongated and they occupy less space than in the ZPG boundary layer. The distinction between wall-attached and wall-detached structures is not as pronounced in the large-defect TBL.
Uranus evolution models with simple thermal boundary layers
Nettelmann, N; Fortney, J J; Hamel, S; Yellamilli, S; Bethkenhagen, M; Redmer, R
2016-01-01
The strikingly low luminosity of Uranus (Teff ~ Teq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. Proceeding step by step, we confirm that adiabatic models yield cooling times that are too long, even when uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary and adjust it to reproduce the low luminosity. Due to this thermal boundary, the deep interior of the Uranus models are up to 2--3 warmer than adiabatic models, necessitating the presence of rocks in the deep interior with a possible I:R of $1\\tim...
On the Onsetof Thermal Instability in a Low Prandtl Number Nanofluid Layer in a Porous Medium
Directory of Open Access Journals (Sweden)
Ramesh Chand
2015-01-01
Full Text Available Thermal instability in a low Prandtl number nanofluid in a porous medium is investigated by using Galerkin weighted residuals method for free-free boundaries. For porous medium, Brinkman-Darcy modelis applied. The model used for the nanofluid describes the effects of Brownian motion and thermophoresis. Linear stability theory based upon normal mode analysis is employed to find the expression for stationary and oscillatory convection. The effects of Prandtl- number, Darcy number, Lewis number and modified diffusivity ratio on the stationary convection are investigated both analytically and graphically. The results indicated that the Prandtl and Darcy numbers have a destabilizing effect while the Lewis number and modified diffusivity ratio have a stabilizing effect for the stationary convection.