In-silico experiments on characteristic time scale at a shear-free gas-liquid interface in fully developed turbulence

Energy Technology Data Exchange (ETDEWEB)

Nagaosa, Ryuichi [Research Center for Compact Chemical System (CCS), AIST, 4-2-1 Nigatake, Miyagino, Sendai 983-8551 (Japan); Handler, Robert A, E-mail: ryuichi.nagaosa@aist.go.jp [Department of Mechanical Engineering, Texas A and M University, College Station, TX 77843-3123 (United States)

2011-12-22

The purpose of this study is to model scalar transfer mechanisms in a fully developed turbulence for accurate predictions of the turbulent scalar flux across a shear-free gas-liquid interface. The concept of the surface-renewal approximation (Dankwerts, 1951) is introduced in this study to establish the predictive models for the interfacial scalar flux. Turbulent flow realizations obtained by a direct numerical simulation technique are employed to prepare details of three-dimensional information on turbulence in the region very close to the interface. Two characteristic time scales at the interface have been examined for exact prediction of the scalar transfer flux. One is the time scale which is reciprocal of the root-mean-square surface divergence, T{sub {gamma}} = ({gamma}{gamma}){sup -1/2}, where {gamma} is the surface divergence. The other time scale to be examined is T{sub S} = {Lambda}/V, where {Lambda} is the zero-correlation length of the surface divergence as the interfacial length scale, and V is the root-mean-square velocity fluctuation in the streamwise direction as the interfacial velocity scale. The results of this study suggests that T{sub {gamma}} is slightly unsatisfactory to correlate the turbulent scalar flux at the gas-liquid interface based on the surface-renewal approximation. It is also found that the proportionality constant appear to be 0.19, which is different with that observed in the laboratory experiments, 0.34 (Komori, Murakami, and Ueda, 1989). It is concluded that the time scale, T{sub {gamma}}, is considered a different kind of the time scale observed in the laboratory experiments. On the other hand, the present in-silico experiments indicate that T{sub s} predicts the turbulent scalar flux based on the surface-renewal approximation in a satisfactory manner. It is also elucidated that the proportionality constant for T{sub s} is approximately 0.36, which is very close to that found by the laboratory experiments. This fact shows

Turbulent/non-turbulent interfaces detected in DNS of incompressible turbulent boundary layers

Science.gov (United States)

Watanabe, T.; Zhang, X.; Nagata, K.

2018-03-01

The turbulent/non-turbulent interface (TNTI) detected in direct numerical simulations is studied for incompressible, temporally developing turbulent boundary layers at momentum thickness Reynolds number Reθ ≈ 2000. The outer edge of the TNTI layer is detected as an isosurface of the vorticity magnitude with the threshold determined with the dependence of the turbulent volume on a threshold level. The spanwise vorticity magnitude and passive scalar are shown to be good markers of turbulent fluids, where the conditional statistics on a distance from the outer edge of the TNTI layer are almost identical to the ones obtained with the vorticity magnitude. Significant differences are observed for the conditional statistics between the TNTI detected by the kinetic energy and vorticity magnitude. A widely used grid setting determined solely from the wall unit results in an insufficient resolution in a streamwise direction in the outer region, whose influence is found for the geometry of the TNTI and vorticity jump across the TNTI layer. The present results suggest that the grid spacing should be similar for the streamwise and spanwise directions. Comparison of the TNTI layer among different flows requires appropriate normalization of the conditional statistics. Reference quantities of the turbulence near the TNTI layer are obtained with the average of turbulent fluids in the intermittent region. The conditional statistics normalized by the reference turbulence characteristics show good quantitative agreement for the turbulent boundary layer and planar jet when they are plotted against the distance from the outer edge of the TNTI layer divided by the Kolmogorov scale defined for turbulent fluids in the intermittent region.

Turbulent Heat Transfer in Curved Pipe Flow

Science.gov (United States)

Kang, Changwoo; Yang, Kyung-Soo

2013-11-01

In the present investigation, turbulent heat transfer in fully-developed curved pipe flow with axially uniform wall heat flux has been numerically studied. The Reynolds numbers under consideration are Reτ = 210 (DNS) and 1,000 (LES) based on the mean friction velocity and the pipe radius, and the Prandtl number (Pr) is 0.71. For Reτ = 210 , the pipe curvature (κ) was fixed as 1/18.2, whereas three cases of κ (0.01, 0.05, 0.1) were computed in the case of Reτ = 1,000. The mean velocity, turbulent intensities and heat transfer rates obtained from the present calculations are in good agreement with the previous numerical and experimental results. To elucidate the secondary flow structures due to the pipe curvature, the mean quantities and rms fluctuations of the flow and temperature fields are presented on the pipe cross-sections, and compared with those of the straight pipe flow. To study turbulence structures and their influence on turbulent heat transfer, turbulence statistics including but not limited to skewness and flatness of velocity fluctuations, cross-correlation coefficients, an Octant analysis, and turbulence budgets are presented and discussed. Based on our results, we attempt to clarify the effects of Reynolds number and the pipe curvature on turbulent heat transfer. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0008457).

Effect of pool turbulence on direct contact condensation at a steam/water interface

International Nuclear Information System (INIS)

Jackson, J.D.; Zhao, C.L.; Doerffer, S.; Byrne, J.E.; Falaki, H.

2000-01-01

Measurements of direct contact condensation beat transfer have been made for the case where the process takes place at the horizontal interface between saturated steam and a pool of water in a vertical cylindrical test section. A submerged vertical jet of subcooled water was injected upwards on the axis to promote the condensation and water was withdrawn at the same rate from the bottom of the pool. In conjunction with the above study, measurements of the turbulent velocity fluctuations just below a free surface produced by the injection of a vertical submerged jet have been measured using hot film anemometry on an isothermal air-water test facility of similar geometry for similar flow conditions at ambient temperature. A correlation is proposed in terms of a Stanton number based on turbulent velocity fluctuation near the interface on the liquid-side. Our results are in good agreement with those of others for similar configurations when compared in terms of condensation Stanton number. (author)

Statistics of the turbulent/non-turbulent interface in a spatially developing mixing layer

KAUST Repository

Attili, Antonio; Cristancho, Juan; Bisetti, Fabrizio

2014-01-01

available in the literature, such as turbulent jets and wakes. In addition, an analysis of the passive scalar field in the vicinity of the interface is presented. It is shown that the scalar has a jump at the interface, even stronger than that observed

On the correlation of heat transfer in turbulent boundary layers subjected to free-stream turbulence

Energy Technology Data Exchange (ETDEWEB)

Barrett, M.J.; Hollingsworth, D.K.

1999-07-01

The turbulent flow of a fluid bounded by a heated surface is a wonderfully complex yet derisively mundane phenomenon. Despite its commonness in natural and man-made environments, the authors struggle to accurately predict its behavior in many simple situations. A complexity encountered in a number of flows is the presence of free-stream turbulence. A turbulent free-stream typically yields increased surface friction and heat transfer. Turbulent boundary layers with turbulent free-streams are encountered in gas-turbine engines, rocket nozzles, electronic-cooling passages, geophysical flows, and numerous other dynamic systems. Here, turbulent boundary layers were subjected to grid-generated free-stream turbulence to study the effects of length scale and intensity on heat transfer. The research focused on correlating heat transfer without the use of conventional boundary-layer Reynolds numbers. The boundary-layers studied ranged from 400 to 2,700 in momentum-thickness Reynolds number and from 450 to 1,900 in enthalpy-thickness Reynolds number. Free-stream turbulence intensities varied from 0.1 to 8.0%. The turbulent-to-viscous length-scale ratios presented are the smallest found in the heat-transfer literature; the ratios spanned from 100 to 1000. The turbulent-to-thermal ratios (using enthalpy thickness as the thermal scale) are also the smallest reported; the ratios ranged from 3.2 to 12.3. A length-scale dependence was identified in a Stanton number based on a near-wall streamwise velocity fluctuation. A new near-wall Stanton number was introduced; this parameter was regarded as a constant in a two-region boundary-layer model. The new model correlated heat-transfer to within 7%.

Transfer anisotropy in a turbulent plasma

International Nuclear Information System (INIS)

Bychenkov, V.Y.; Gradov, O.M.; Silin, V.P.

1982-01-01

We formulate a theory for transfer phenomena in a plasma with developed ion-sound turbulence. A transfer anisotropy effect caused by a temperature gradient is revealed. The corresponding fluxes transverse to the effective force vector (1) which generates the turbulence turn out to be considerably larger than the longitudinal fluxes in a plasma with a relatively small degree of nonisothermality. For a strongly nonisothermal plasma a suppression of the transverse fluxes takes place and corresponds to a growth of the thermal insulation of the current-carrying plasma filaments

Large Eddy Simulations of turbulent flows with heat transfer; Simulation des grandes echelles d'ecoulements turbulents avec transferts de chaleur

Energy Technology Data Exchange (ETDEWEB)

Chatelain, A.

2004-09-15

LES of turbulent flows with heat transfer was used within the framework of conjugate heat transfer problems. The objective of this work lies not only in identifying the various elements likely to impair temperature fluctuations estimations at the fluid/solid interface but also to introduce adequate wall modeling. The choice of a proper convection scheme for the transport of passive scalars led to the adoption of a high order upwind scheme with slope limiter. The use of classical wall models having shown some weaknesses as for the estimation of parietal temperature fluctuations, two new approaches are proposed and tested. The first one relies on a complete resolution of the Navier-Stokes equations on a refined grid close to the wall making it possible to rebuild the temperature fluctuations near the wall. The second one relies on the simultaneous and one dimensional resolution of a turbulent boundary layer equation and a variance transport equation near the wall. (author)

Physical modelling of interactions between interfaces and turbulence; Modelisation physique des interactions entre interfaces et turbulence

Energy Technology Data Exchange (ETDEWEB)

Toutant, A

2006-12-15

The complex interactions between interfaces and turbulence strongly impact the flow properties. Unfortunately, Direct Numerical Simulations (DNS) have to entail a number of degrees of freedom proportional to the third power of the Reynolds number to correctly describe the flow behaviour. This extremely hard constraint makes it impossible to use DNS for industrial applications. Our strategy consists in using and improving DNS method in order to develop the Interfaces and Sub-grid Scales concept. ISS is a two-phase equivalent to the single-phase Large Eddy Simulation (LES) concept. The challenge of ISS is to integrate the two-way coupling phenomenon into sub-grid models. Applying a space filter, we have exhibited correlations or sub-grid terms that require closures. We have shown that, in two-phase flows, the presence of a discontinuity leads to specific sub-grid terms. Comparing the maximum of the norm of the sub-grid terms with the maximum of the norm of the advection tensor, we have found that sub-grid terms related to interfacial forces and viscous effect are negligible. Consequently, in the momentum balance, only the sub-grid terms related to inertia have to be closed. Thanks to a priori tests performed on several DNS data, we demonstrate that the scale similarity hypothesis, reinterpreted near discontinuity, provides sub-grid models that take into account the two-way coupling phenomenon. These models correspond to the first step of our work. Indeed, in this step, interfaces are smooth and, interactions between interfaces and turbulence occur in a transition zone where each physical variable varies sharply but continuously. The next challenge has been to determine the jump conditions across the sharp equivalent interface corresponding to the sub-grid models of the transition zone. We have used the matched asymptotic expansion method to obtain the jump conditions. The first tests on the velocity of the sharp equivalent interface are very promising (author)

Transfer anisotropy effect in a turbulent plasma

International Nuclear Information System (INIS)

Bychenkov, V.Yu.; Gradov, O.M.; Silin, V.P.

1982-01-01

A theory is developed of transfer phenomena with pronounced ion-sound turbulence. A transfer anisotropy effect is observed which is due to the temperature gradient. The corresponding fluxes across the effective force vector generating the turbulence are found to be considerably greater than the longitudinal fluxes in a plasma with a comparatively low degree of nonisothermality. In a strongly nonisothermal plasma the suppression of transverse fluxes occurs, corresponding to the growth of thermal insulation of the current-carrying plasma filaments

Turbulent heat transfer as a control of platelet ice growth in supercooled under-ice ocean boundary layers

Science.gov (United States)

McPhee, Miles G.; Stevens, Craig L.; Smith, Inga J.; Robinson, Natalie J.

2016-04-01

Late winter measurements of turbulent quantities in tidally modulated flow under land-fast sea ice near the Erebus Glacier Tongue, McMurdo Sound, Antarctica, identified processes that influence growth at the interface of an ice surface in contact with supercooled seawater. The data show that turbulent heat exchange at the ocean-ice boundary is characterized by the product of friction velocity and (negative) water temperature departure from freezing, analogous to similar results for moderate melting rates in seawater above freezing. Platelet ice growth appears to increase the hydraulic roughness (drag) of fast ice compared with undeformed fast ice without platelets. Platelet growth in supercooled water under thick ice appears to be rate-limited by turbulent heat transfer and that this is a significant factor to be considered in mass transfer at the underside of ice shelves and sea ice in the vicinity of ice shelves.

Analysis of small scale turbulent structures and the effect of spatial scales on gas transfer

Science.gov (United States)

Schnieders, Jana; Garbe, Christoph

2014-05-01

The exchange of gases through the air-sea interface strongly depends on environmental conditions such as wind stress and waves which in turn generate near surface turbulence. Near surface turbulence is a main driver of surface divergence which has been shown to cause highly variable transfer rates on relatively small spatial scales. Due to the cool skin of the ocean, heat can be used as a tracer to detect areas of surface convergence and thus gather information about size and intensity of a turbulent process. We use infrared imagery to visualize near surface aqueous turbulence and determine the impact of turbulent scales on exchange rates. Through the high temporal and spatial resolution of these types of measurements spatial scales as well as surface dynamics can be captured. The surface heat pattern is formed by distinct structures on two scales - small-scale short lived structures termed fish scales and larger scale cold streaks that are consistent with the footprints of Langmuir Circulations. There are two key characteristics of the observed surface heat patterns: 1. The surface heat patterns show characteristic features of scales. 2. The structure of these patterns change with increasing wind stress and surface conditions. In [2] turbulent cell sizes have been shown to systematically decrease with increasing wind speed until a saturation at u* = 0.7 cm/s is reached. Results suggest a saturation in the tangential stress. Similar behaviour has been observed by [1] for gas transfer measurements at higher wind speeds. In this contribution a new model to estimate the heat flux is applied which is based on the measured turbulent cell size und surface velocities. This approach allows the direct comparison of the net effect on heat flux of eddies of different sizes and a comparison to gas transfer measurements. Linking transport models with thermographic measurements, transfer velocities can be computed. In this contribution, we will quantify the effect of small scale

Turbulent buoyant jets and plumes

CERN Document Server

Rodi, Wolfgang

The Science & Applications of Heat and Mass Transfer: Reports, Reviews, & Computer Programs, Volume 6: Turbulent Buoyant Jets and Plumes focuses on the formation, properties, characteristics, and reactions of turbulent jets and plumes. The selection first offers information on the mechanics of turbulent buoyant jets and plumes and turbulent buoyant jets in shallow fluid layers. Discussions focus on submerged buoyant jets into shallow fluid, horizontal surface or interface jets into shallow layers, fundamental considerations, and turbulent buoyant jets (forced plumes). The manuscript then exami

Scalar transport across the turbulent/non-turbulent interface in jets: Schmidt number effects

Science.gov (United States)

Silva, Tiago S.; B. da Silva, Carlos; Idmec Team

2016-11-01

The dynamics of a passive scalar field near a turbulent/non-turbulent interface (TNTI) is analysed through direct numerical simulations (DNS) of turbulent planar jets, with Reynolds numbers ranging from 142 <= Reλ <= 246 , and Schmidt numbers from 0 . 07 <= Sc <= 7 . The steepness of the scalar gradient, as observed from conditional profiles near the TNTI, increases with the Schmidt number. Conditional scalar gradient budgets show that for low and moderate Schmidt numbers a diffusive superlayer emerges at the TNTI, where the scalar gradient diffusion dominates, while the production is negligible. For low Schmidt numbers the growth of the turbulent front is commanded by the molecular diffusion, whereas the scalar gradient convection is negligible. The authors acknowledge the Laboratory for Advanced Computing at University of Coimbra for providing HPC, computing, consulting resources that have contributed to the research results reported within this paper. URL http://www.lca.uc.pt.

Geometrical properties of turbulent premixed flames and other corrugated interfaces

Science.gov (United States)

Thiesset, F.; Maurice, G.; Halter, F.; Mazellier, N.; Chauveau, C.; Gökalp, I.

2016-01-01

This study focuses on the geometrical properties of turbulent flame fronts and other interfaces. Toward that end, we use an original tool based on proper orthogonal decomposition (POD), which is applied to the interface spatial coordinates. The focus is mainly on the degree of roughness of the flame front, which is quantified through the scale dependence of its coverage arclength. POD is first validated by comparing with the caliper technique. Fractal characteristics are extracted in an unambiguous fashion using a parametric expression which appears to be impressively well suited for representing Richardson plots. Then it is shown that, for the range of Reynolds numbers investigated here, the scale-by-scale contribution to the arclength does not comply with scale similarity, irrespectively of the type of similarity which is invoked. The finite ratios between large and small scales, referred to as finite Reynolds number effects, are likely to explain this observation. In this context, the Reynolds number that ought to be achieved for a proper inertial range to be discernible, and for scale similarity to be likely to apply, is calculated. Fractal characteristics of flame folding are compared to available predictions. It is confirmed that the inner cutoff satisfactorily correlates with the Kolmogorov scale while the outer cutoff appears to be proportional to the integral length scale. However, the scaling for the fractal dimension is much less obvious. It is argued that much higher Reynolds numbers have to be reached for drawing firm statements about the evolution (or constancy) of the fractal dimension with respect to flame and flow parameters. Finally, a heuristic phenomenology of corrugated interfaces is highlighted. The degree of generality of the latter phenomenology is confirmed by comparing the folding of different interfaces including a turbulent-nonturbulent interface, a liquid jet destabilized by a surrounding air jet, a cavitating flow, and an isoscalar

Turbulence modeling and surface heat transfer in a stagnation flow region

Science.gov (United States)

Wang, C. R.; Yeh, F. C.

1987-01-01

Analysis for the turbulent flow field and the effect of freestream turbulence on the surface heat transfer rate of a stagnation flow is presented. The emphasis is on modeling and its augmentation of surface heat transfer rate. The flow field considered is the region near the forward stagnation point of a circular cylinder in a uniform turbulent mean flow.

Visualization of an air-water interface on superhydrophobic surfaces in turbulent channel flows

Science.gov (United States)

Kim, Hyunseok; Park, Hyungmin

2017-11-01

In the present study, three-dimensional deformation of air-water interface on superhydrophobic surfaces in turbulent channel flows at the Reynolds numbers of Re = 3000 and 10000 is measured with RICM (Reflection Interference Contrast Microscopy) technique. Two different types of roughness feature of circular hole and rectangular grate are considered, whose depth is 20 μm and diameter (or width) is varied between 20-200 μm. Since the air-water interface is always at de-pinned state at the considered condition, air-water interface shape and its sagging velocity is maintained to be almost constant as time goes one. In comparison with the previous results under the laminar flow, due to turbulent characteristics of the flow, sagging velocity is much faster. Based on the measured sagging profiles, a modified model to describe the air-water interface dynamics under turbulent flows is suggested. Supported by City of Seoul through Seoul Urban Data Science Laboratory Project (Grant No 0660-20170004) administered by SNU Big Data Institute.

Effects of Schmidt number on near-wall turbulent mass transfer in pipe flow

Energy Technology Data Exchange (ETDEWEB)

Kang, Chang Woo; Yang, Kyung Soo [Inha University, Incheon (Korea, Republic of)

2014-12-15

Large Eddy simulation (LES) of turbulent mass transfer in circular-pipe flow has been performed to investigate the characteristics of turbulent mass transfer in the near-wall region. We consider a fully-developed turbulent pipe flow with a constant wall concentration. The Reynolds number under consideration is Re{sub r} = 500 based on the friction velocity and the pipe radius, and the selected Schmidt numbers (Sc) are 0.71, 5, 10, 20 and 100. Dynamic subgrid-scale (SGS) models for the turbulent SGS stresses and turbulent mass fluxes were employed to close the governing equations. The current paper reports a comprehensive characterization of turbulent mass transfer in circular-pipe flow, focusing on its near-wall characteristics and Sc dependency. We start with mean fields by presenting mean velocity and concentration profiles, mean Sherwood numbers and mean mass transfer coefficients for the selected values of the parameters. After that, we present the characteristics of fluctuations including root-mean-square (rms) profiles of velocity, concentration, and mass transfer coefficient fluctuations. Turbulent mass fluxes and correlations between velocity and concentration fluctuations are also discussed. The near-wall behaviour of turbulent diffusivity and turbulent Schmidt number is shown, and other authors' correlations on their limiting behaviour towards the pipe wall are evaluated based on our LES results. The intermittent characteristics of turbulent mass transfer in pipe flow are depicted by probability density functions (pdf) of velocity and concentration fluctuations; joint pdfs between them are also presented. Instantaneous snapshots of velocity and concentration fluctuations are shown to supplement our discussion on the turbulence statistics. Finally, we report the results of octant analysis and budget calculation of concentration variance to clarify Sc-dependency of the correlation between near-wall turbulence structures and concentration fluctuation in

Effects of Schmidt number on near-wall turbulent mass transfer in pipe flow

International Nuclear Information System (INIS)

Kang, Chang Woo; Yang, Kyung Soo

2014-01-01

Large Eddy simulation (LES) of turbulent mass transfer in circular-pipe flow has been performed to investigate the characteristics of turbulent mass transfer in the near-wall region. We consider a fully-developed turbulent pipe flow with a constant wall concentration. The Reynolds number under consideration is Re r = 500 based on the friction velocity and the pipe radius, and the selected Schmidt numbers (Sc) are 0.71, 5, 10, 20 and 100. Dynamic subgrid-scale (SGS) models for the turbulent SGS stresses and turbulent mass fluxes were employed to close the governing equations. The current paper reports a comprehensive characterization of turbulent mass transfer in circular-pipe flow, focusing on its near-wall characteristics and Sc dependency. We start with mean fields by presenting mean velocity and concentration profiles, mean Sherwood numbers and mean mass transfer coefficients for the selected values of the parameters. After that, we present the characteristics of fluctuations including root-mean-square (rms) profiles of velocity, concentration, and mass transfer coefficient fluctuations. Turbulent mass fluxes and correlations between velocity and concentration fluctuations are also discussed. The near-wall behaviour of turbulent diffusivity and turbulent Schmidt number is shown, and other authors' correlations on their limiting behaviour towards the pipe wall are evaluated based on our LES results. The intermittent characteristics of turbulent mass transfer in pipe flow are depicted by probability density functions (pdf) of velocity and concentration fluctuations; joint pdfs between them are also presented. Instantaneous snapshots of velocity and concentration fluctuations are shown to supplement our discussion on the turbulence statistics. Finally, we report the results of octant analysis and budget calculation of concentration variance to clarify Sc-dependency of the correlation between near-wall turbulence structures and concentration fluctuation in the

Large eddy simulation study of turbulent kinetic energy and scalar variance budgets and turbulent/non-turbulent interface in planar jets

Science.gov (United States)

Watanabe, Tomoaki; Sakai, Yasuhiko; Nagata, Koji; Ito, Yasumasa

2016-04-01

Spatially developing planar jets with passive scalar transports are simulated for various Reynolds (Re = 2200, 7000, and 22 000) and Schmidt numbers (Sc = 1, 4, 16, 64, and 128) by the implicit large eddy simulation (ILES) using low-pass filtering as an implicit subgrid-scale model. The budgets of resolved turbulent kinetic energy k and scalar variance are explicitly evaluated from the ILES data except for the dissipation terms, which are obtained from the balance in the transport equations. The budgets of k and in the ILES agree well with the DNS and experiments for both high and low Re cases. The streamwise decay of the mean turbulent kinetic energy dissipation rate obeys the power low obtained by the scaling argument. The mechanical-to-scalar timescale ratio C ϕ is evaluated in the self-similar region. For the high Re case, C ϕ is close to the isotropic value (C ϕ = 2) near the jet centerline. However, when Re is not large, C ϕ is smaller than 2 and depends on the Schmidt number. The T/NT interface is also investigated by using the scalar isosurface. The velocity and scalar fields near the interface depend on the interface orientation for all Re. The velocity toward the interface is observed near the interface facing in the streamwise, cross-streamwise, and spanwise directions in the planar jet in the resolved velocity field.

Two-dimensional numerical modeling and solution of convection heat transfer in turbulent He II

Science.gov (United States)

Zhang, Burt X.; Karr, Gerald R.

1991-01-01

Numerical schemes are employed to investigate heat transfer in the turbulent flow of He II. FEM is used to solve a set of equations governing the heat transfer and hydrodynamics of He II in the turbulent regime. Numerical results are compared with available experimental data and interpreted in terms of conventional heat transfer parameters such as the Prandtl number, the Peclet number, and the Nusselt number. Within the prescribed Reynolds number domain, the Gorter-Mellink thermal counterflow mechanism becomes less significant, and He II acts like an ordinary fluid. The convection heat transfer characteristics of He II in the highly turbulent regime can be successfully described by using the conventional turbulence and heat transfer theories.

Numerical prediction of flow, heat transfer, turbulence and combustion

CERN Document Server

Spalding, D Brian; Pollard, Andrew; Singhal, Ashok K

1983-01-01

Numerical Prediction of Flow, Heat Transfer, Turbulence and Combustion: Selected Works of Professor D. Brian Spalding focuses on the many contributions of Professor Spalding on thermodynamics. This compilation of his works is done to honor the professor on the occasion of his 60th birthday. Relatively, the works contained in this book are selected to highlight the genius of Professor Spalding in this field of interest. The book presents various research on combustion, heat transfer, turbulence, and flows. His thinking on separated flows paved the way for the multi-dimensional modeling of turbu

Energy transfer in turbulence under rotation

Science.gov (United States)

Buzzicotti, Michele; Aluie, Hussein; Biferale, Luca; Linkmann, Moritz

2018-03-01

It is known that rapidly rotating turbulent flows are characterized by the emergence of simultaneous upscale and downscale energy transfer. Indeed, both numerics and experiments show the formation of large-scale anisotropic vortices together with the development of small-scale dissipative structures. However the organization of interactions leading to this complex dynamics remains unclear. Two different mechanisms are known to be able to transfer energy upscale in a turbulent flow. The first is characterized by two-dimensional interactions among triads lying on the two-dimensional, three-component (2D3C)/slow manifold, namely on the Fourier plane perpendicular to the rotation axis. The second mechanism is three-dimensional and consists of interactions between triads with the same sign of helicity (homochiral). Here, we present a detailed numerical study of rotating flows using a suite of high-Reynolds-number direct numerical simulations (DNS) within different parameter regimes to analyze both upscale and downscale cascade ranges. We find that the upscale cascade at wave numbers close to the forcing scale is generated by increasingly dominant homochiral interactions which couple the three-dimensional bulk and the 2D3C plane. This coupling produces an accumulation of energy in the 2D3C plane, which then transfers energy to smaller wave numbers thanks to the two-dimensional mechanism. In the forward cascade range, we find that the energy transfer is dominated by heterochiral triads and is dominated primarily by interaction within the fast manifold where kz≠0 . We further analyze the energy transfer in different regions in the real-space domain. In particular, we distinguish high-strain from high-vorticity regions and we uncover that while the mean transfer is produced inside regions of strain, the rare but extreme events of energy transfer occur primarily inside the large-scale column vortices.

Modeling of turbulent flows in porous media and at the interface with a free fluid medium; Modelisation des ecoulements turbulents dans les milieux poreux et a l'interface avec un milieu libre

Energy Technology Data Exchange (ETDEWEB)

Chandesris, M

2006-12-15

This work deals with the numerical simulation of turbulent flows in the whole nuclear reactor core, using multi-scale approaches. First, a macroscopic turbulence model is built, based on a porous media approach, to describe the flow in the fuel assemblies part of the nuclear core. Then, we study the jump conditions that have to be applied at a free fluid/porous interface. A thorough analytical study is carried out for laminar flows. This study allows to answer some fundamental questions about the physical meaning of the jump conditions, the values of the jump parameters and the location of the interface. Using these results, jump conditions for turbulent flows are proposed. The model is then applied to the simulation of a turbulent flow in a simplified model of a reactor core. (author)

Turbulent heat transfer for heating of water in a short vertical tube

International Nuclear Information System (INIS)

Hata, Koichi; Noda, Nobuaki

2008-01-01

The turbulent heat transfer coefficients for the flow velocities (u=4.0 to 21 m/s), the inlet liquid temperatures (T in =296.5 to 353.4 K), the inlet pressures (P in =810 to 1014 kPa) and the increasing heat inputs (Q 0 exp(t/τ), τ=10, 20 and 33.3 s) are systematically measured by an experimental water loop. The Platinum test tubes of test tube inner diameters (d=3, 6 and 9 mm), heated lengths (L=32.7 to 100 mm), ratios of heated length to inner diameter (L/d=5.51 to 33.3) and wall thickness (δ=0.3, 0.4 and 0.5 mm) with surface roughness (Ra=0.40 to 0.78 μm) are used in this work. The turbulent heat transfer data for Platinum test tubes were compared with the values calculated by other workers' correlations for the turbulent heat transfer. The influence of Reynolds number (Re), Prandtl number (Pr), Dynamic viscosity (μ) and L/d on the turbulent heat transfer is investigated into details and, the widely and precisely predictable correlation of the turbulent heat transfer for heating of water in a short vertical tube is given based on the experimental data. The correlation can describe the turbulent heat transfer coefficients obtained in this work for the wide range of the temperature difference between heater inner surface temperature and average bulk liquid temperature (ΔT L =5 to 140 K) with d=3, 6 and 9 mm, L=32.7 to 100 mm and u=4.0 to 21 m/s within ±15%, difference. (author)

Turbulent heat transfer for heating of water in a short vertical tube

International Nuclear Information System (INIS)

Hata, Koichi; Noda, Nobuaki

2007-01-01

The turbulent heat transfer coefficients for the flow velocities (u=4.0 to 21 m/s), the inlet liquid temperatures (T in =296.5 to 353.4 K), the inlet pressures (P in =810 to 1014 kPa) and the increasing heat inputs (Q 0 exp(t/τ), τ=10, 20 and 33.3 s) are systematically measured by the experimental water loop. The Platinum test tubes of test tube inner diameters (d=3, 6 and 9 mm), heated lengths (L=32.7 to 100 mm), ratios of heated length to inner diameter (L/d=5.51 to 33.3) and wall thicknesses (δ=0.3, 0.4 and 0.5 mm) with surface roughness (Ra=0.40 to 0.78 μm) are used in this work. The turbulent heat transfer data for Platinum test tubes were compared with the values calculated by other workers' correlations for the turbulent heat transfer. The influences of Reynolds number (Re), Prandtl number (Pr), Dynamic viscosity (μ) and L/d on the turbulent heat transfer are investigated into details and, the widely and precisely predictable correlation of the turbulent heat transfer for heating of water in a short vertical tube is given based on the experimental data. The correlation can describe the turbulent heat transfer coefficients obtained in this work for wide range of the temperature difference between heater inner surface temperature and average bulk liquid temperature (ΔT L =5 to 140 K) with d=3, 6 and 9 mm, L=32.7 to 100 mm and u=4.0 to 21 m/s within ±15% difference. (author)

A new energy transfer model for turbulent free shear flow

Science.gov (United States)

Liou, William W.-W.

1992-01-01

A new model for the energy transfer mechanism in the large-scale turbulent kinetic energy equation is proposed. An estimate of the characteristic length scale of the energy containing large structures is obtained from the wavelength associated with the structures predicted by a weakly nonlinear analysis for turbulent free shear flows. With the inclusion of the proposed energy transfer model, the weakly nonlinear wave models for the turbulent large-scale structures are self-contained and are likely to be independent flow geometries. The model is tested against a plane mixing layer. Reasonably good agreement is achieved. Finally, it is shown by using the Liapunov function method, the balance between the production and the drainage of the kinetic energy of the turbulent large-scale structures is asymptotically stable as their amplitude saturates. The saturation of the wave amplitude provides an alternative indicator for flow self-similarity.

Heat Transfer Enhancement in Turbulent Flows by Blocked Surfaces

Directory of Open Access Journals (Sweden)

Onur YEMENİCİ

2013-04-01

Full Text Available In this study, the heat transfer analyses over flat and blocked surfaces were carried out in turbulent flow under the influence of the block height. A constant-temperature hot wire anemometer was used to the velocity and turbulent intensity measurements, while temperature values were measured by copper-constantan thermocouples. The average Stanton numbers for block heights of 15 and 25 mm were higher than those of flat surface by %38 and %84, respectively. The results showed that the presence of the blocks increased the heat transfer and the enhancement rose with block heights

Flowing and heat transfer characteristics of turbulent flow in typical rod bundles at rolling motion

International Nuclear Information System (INIS)

Yan Binghuo; Yu Lei; Gu Hanyang

2011-01-01

The influence mechanism of rolling motion on the flowing and heat transfer characteristics of turbulent flow in typical four rod bundles was investigated with Fluent code. The flowing and heat transfer characteristics of turbulent flow in rod bundles can be affected by rolling motion. But the flowing similarity of turbulent flow in adiabatic and non-adiabatic can not be affected. If the rolling period is small, the radial additional force can make the parameter profiles, the turbulent flowing and heat transfer change greatly. At rolling motion, as the pitch to diameter ratio decreases, especially if it is less than 1.1, the flowing and heat transfer of turbulent flow at rolling motion change significantly. The variation of pitch to diameter ratio can change the profiles of secondary flow and turbulent kinetic energy in cross-section greatly. (authors)

Role of wall-attached structures in the interface of the quiescent core region in turbulent pipe flow

Science.gov (United States)

Yang, Jongmin; Hwang, Jinyul; Sung, Hyung Jin

2017-11-01

The effects of low- and high-speed structures on the interface of the quiescent core region are explored using direct numerical simulation data of turbulent pipe flow. The quiescent core region is a uniform momentum zone located at the center of the pipe flow, which contains the highest streamwise momentum with a low level of turbulence. The interface of the quiescent core region can be identified from the probability density function of the streamwise modal velocity. In the vicinity of the interface of the quiescent core region, the streamwise velocity changes abruptly. The abrupt jump in velocity causes an increase of the velocity gradient. The interface of the quiescent core region is similar to the laminar superlayer in turbulent/non-turbulent interface. The interface of the quiescent core region contains the low- and high-speed structures. They can be classified into wall-attached and detached structures depending on the distance between the structures and the wall. The influence of the detached structures accounted for most of the number of detected structures is negligible due to its small volume. Conversely, the wall-attached structures adjacent to the interface have a huge influence on the statistical amount of the interface, such as entrainment characteristics. This work was supported by the Creative Research Initiatives (No. 2017-013369) program of the National Research Foundation of Korea (MSIP).

DNS of fully developed turbulent heat transfer of a viscoelastic drag-reducing flow

Energy Technology Data Exchange (ETDEWEB)

Yu, Bo [Department of Oil and Gas Storage and Transportation Engineering, China University of Petroleum, Beijing 102249 (China); Kawaguchi, Yasuo [Department of Mechanical Engineering, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510 (Japan)

2005-10-01

A direct numerical simulation (DNS) of turbulent heat transfer in a channel flow with a Giesekus model was carried out to investigate turbulent heat transfer mechanism of a viscoelastic drag-reducing flow by additives. The configuration was a fully-developed turbulent channel flow with uniform heat flux imposed on both the walls. The temperature was considered as a passive scalar with the effect of buoyancy force neglected. The Reynolds number based on the friction velocity and half the channel height was 150. Statistical quantities such as root-mean-square temperature fluctuations, turbulent heat fluxes and turbulent Prandtl number were obtained and compared with those of a Newtonian fluid flow. Budget terms of the temperature variance and turbulent heat fluxes were also presented. (author)

Field observations of turbulent dissipation rate profiles immediately below the air-water interface

Science.gov (United States)

Wang, Binbin; Liao, Qian

2016-06-01

Near surface profiles of turbulence immediately below the air-water interface were measured with a free-floating Particle Image Velocimetry (PIV) system on Lake Michigan. The surface-following configuration allowed the system to measure the statistics of the aqueous-side turbulence in the topmost layer immediately below the water surface (z≈0˜15 cm, z points downward with 0 at the interface). Profiles of turbulent dissipation rate (ɛ) were investigated under a variety of wind and wave conditions. Various methods were applied to estimate the dissipation rate. Results suggest that these methods yield consistent dissipation rate profiles with reasonable scattering. In general, the dissipation rate decreases from the water surface following a power law relation in the top layer, ɛ˜z-0.7, i.e., the slope of the decrease was lower than that predicted by the wall turbulence theory, and the dissipation was considerably higher in the top layer for cases with higher wave ages. The measured dissipation rate profiles collapse when they were normalized with the wave speed, wave height, water-side friction velocity, and the wave age. This scaling suggests that the enhanced turbulence may be attributed to the additional source of turbulent kinetic energy (TKE) at the "skin layer" (likely due to micro-breaking), and its downward transport in the water column.

Energy Transfer and Dual Cascade in Kinetic Magnetized Plasma Turbulence

International Nuclear Information System (INIS)

Plunk, G. G.; Tatsuno, T.

2011-01-01

The question of how nonlinear interactions redistribute the energy of fluctuations across available degrees of freedom is of fundamental importance in the study of turbulence and transport in magnetized weakly collisional plasmas, ranging from space settings to fusion devices. In this Letter, we present a theory for the dual cascade found in such plasmas, which predicts a range of new behavior that distinguishes this cascade from that of neutral fluid turbulence. These phenomena are explained in terms of the constrained nature of spectral transfer in nonlinear gyrokinetics. Accompanying this theory are the first observations of these phenomena, obtained via direct numerical simulations using the gyrokinetic code AstroGK. The basic mechanisms that are found provide a framework for understanding the turbulent energy transfer that couples scales both locally and nonlocally.

Energy Transfer and Dual Cascade in Kinetic Magnetized Plasma Turbulence

Science.gov (United States)

Plunk, G. G.; Tatsuno, T.

2011-04-01

The question of how nonlinear interactions redistribute the energy of fluctuations across available degrees of freedom is of fundamental importance in the study of turbulence and transport in magnetized weakly collisional plasmas, ranging from space settings to fusion devices. In this Letter, we present a theory for the dual cascade found in such plasmas, which predicts a range of new behavior that distinguishes this cascade from that of neutral fluid turbulence. These phenomena are explained in terms of the constrained nature of spectral transfer in nonlinear gyrokinetics. Accompanying this theory are the first observations of these phenomena, obtained via direct numerical simulations using the gyrokinetic code AstroGK. The basic mechanisms that are found provide a framework for understanding the turbulent energy transfer that couples scales both locally and nonlocally.

Mass transfer from smooth alabaster surfaces in turbulent flows

Science.gov (United States)

Opdyke, Bradley N.; Gust, Giselher; Ledwell, James R.

1987-11-01

The mass transfer velocity for alabaster plates in smooth-wall turbulent flow is found to vary with the friction velocity according to an analytic solution of the advective diffusion equation. Deployment of alabaster plates on the sea floor can perhaps be used to estimate the viscous stress, and transfer velocities for other species.

Computation of turbulent flow and heat transfer in subassemblies

International Nuclear Information System (INIS)

Slagter, W.

1979-01-01

This research is carried out in order to provide information on the thermohydraulic behaviour of fast reactor subassemblies. The research work involves the development of versatile computation methods and the evaluation of combined theoretical and experimental work on fluid flow and heat transfer in fuel rod bundles. The computation method described here rests on the application of the distributed parameter approach. The conditions considered cover steady, turbulent flow and heat transfer of incompressible fluids in bundles of bare rods. Throughout 1978 main efforts were given to the development of the VITESSE program and to the validation of the hydrodynamic part of the code. In its present version the VITESSE program is applicable to predict the fully developed turbulent flow and heat transfer in the subchannels of a bundle with bare rods. In this paper the main features of the code are described as well as the present status of development

Morphing continuum analysis of energy transfer in compressible turbulence

Science.gov (United States)

Cheikh, Mohamad Ibrahim; Wonnell, Louis B.; Chen, James

2018-02-01

A shock-preserving finite volume solver with the generalized Lax-Friedrichs splitting flux for morphing continuum theory (MCT) is presented and verified. The numerical MCT solver is showcased in a supersonic turbulent flow with Mach 2.93 over an 8∘ compression ramp. The simulation results validated MCT with experiments as an alternative for modeling compressible turbulence. The required size of the smallest mesh cell for the MCT simulation is shown to be almost an order larger than that in a similar direct numerical simulation study. The comparison shows MCT is a much more computationally friendly theory than the classical Navier-Stokes equations. The dynamics of energy cascade at the length scale of individual eddies is illuminated through the subscale rotation introduced by MCT. In this regard, MCT provides a statistical averaging procedure for capturing energy transfer in compressible turbulence, not found in classical fluid theories. Analysis of the MCT results show the existence of a statistical coupling of the internal and translational kinetic energy fluctuations with the corresponding eddy rotational energy fluctuations, indicating a multiscale transfer of energy. In conclusion, MCT gives a new characterization of the energy cascade within compressible turbulence without the use of excessive computational resources.

Heat transfer and friction characteristics in steam cooled rectangular channels with rib turbulators

Energy Technology Data Exchange (ETDEWEB)

Gong, Jianying; Gao, Tieyu; Li, Guojun [Xi' an Jiaotong University, Xi' an (China)

2014-01-15

We studied the heat transfer and friction characteristics in steam-cooled rectangular channels with rib turbulators on W side or H side walls in the Reynolds number (Re) range of 10000-80000. Each of the test channels was welded by four stainless steel plates to simulate the actual geometry and heat transfer structure of blade/vane internal cooling passage. The length of the channel L was 1000 mm, the cross section of the channel was 40 mm X 80 mm, and the pitch-to-rib height ratio p/e was kept at 10. The channel blockage ratio (W/H) was 0.047. Results showed that the Nusselt number (Nu) distributions displayed different trends at the entrance region with the increase of Re for the rib turbulators on the W side walls. The heat transfer performance of the rib turbulators on the H side walls was about 24- 27% higher than that on the W side walls at the same pumping power. In addition, semi-empirical correlations for the two cases, rib turbulators on W side walls and rib turbulators on H side walls, were developed based on the heat transfer results, which could be used in the design of the internal cooling passage of new generation steam-cooled gas turbine blade/vane.

Modeling of turbulent flows in porous media and at the interface with a free fluid medium

International Nuclear Information System (INIS)

Chandesris, M.

2006-12-01

This work deals with the numerical simulation of turbulent flows in the whole nuclear reactor core, using multi-scale approaches. First, a macroscopic turbulence model is built, based on a porous media approach, to describe the flow in the fuel assemblies part of the nuclear core. Then, we study the jump conditions that have to be applied at a free fluid/porous interface. A thorough analytical study is carried out for laminar flows. This study allows to answer some fundamental questions about the physical meaning of the jump conditions, the values of the jump parameters and the location of the interface. Using these results, jump conditions for turbulent flows are proposed. The model is then applied to the simulation of a turbulent flow in a simplified model of a reactor core. (author)

Study of Variable Turbulent Prandtl Number Model for Heat Transfer to Supercritical Fluids in Vertical Tubes

Science.gov (United States)

Tian, Ran; Dai, Xiaoye; Wang, Dabiao; Shi, Lin

2018-06-01

In order to improve the prediction performance of the numerical simulations for heat transfer of supercritical pressure fluids, a variable turbulent Prandtl number (Prt) model for vertical upward flow at supercritical pressures was developed in this study. The effects of Prt on the numerical simulation were analyzed, especially for the heat transfer deterioration conditions. Based on the analyses, the turbulent Prandtl number was modeled as a function of the turbulent viscosity ratio and molecular Prandtl number. The model was evaluated using experimental heat transfer data of CO2, water and Freon. The wall temperatures, including the heat transfer deterioration cases, were more accurately predicted by this model than by traditional numerical calculations with a constant Prt. By analyzing the predicted results with and without the variable Prt model, it was found that the predicted velocity distribution and turbulent mixing characteristics with the variable Prt model are quite different from that predicted by a constant Prt. When heat transfer deterioration occurs, the radial velocity profile deviates from the log-law profile and the restrained turbulent mixing then leads to the deteriorated heat transfer.

Characteristics of turbulent heat transfer in an annulus at supercritical pressure

NARCIS (Netherlands)

Peeters, J.W.R.; Pecnik, R.; Rohde, M.; van der Hagen, T.H.J.J.; Boersma, B.J.

2017-01-01

Heat transfer to fluids at supercritical pressure is different from heat transfer at lower pressures due to strong variations of the thermophysical properties with the temperature. We present and analyze results of direct numerical simulations of heat transfer to turbulent CO2 at 8 MPa in an

An analytical wall-function for recirculating and impinging turbulent heat transfer

International Nuclear Information System (INIS)

Suga, K.; Ishibashi, Y.; Kuwata, Y.

2013-01-01

Highlights: ► Improvement of the analytical wall-function is proposed. ► Strain parameter dependency is introduced to the prescribed eddy viscosity profile of the analytical wall-function. ► The model performance is evaluated in turbulent pipe, channel, back-step, abrupt expansion pipe and plane impinging flows. ► Generally improved heat transfer is obtained in all the test cases with the standard k-e model. -- Abstract: The performance of the analytical wall-function (AWF) of Craft et al. [Craft, T.J., Gerasimov, A.V., Iacovides, H., Launder, B.E., 2002, Progress in the generalisation of wall-function treatments. Int. J. Heat Fluid Flow 23, 148–160.] is improved for predicting turbulent heat transfer in recirculating and impinging flows. Since constant parameters of the eddy viscosity formula were used to derive the AWF, the prediction accuracy of the original AWF tends to deteriorate in complex flows where those parameters need changing according to the local turbulence. To overcome such shortcomings, the present study introduces a functional behaviour on the strain parameter into the coefficient of the eddy viscosity of the AWF. The presently modified version of the AWF is validated in turbulent heat transfer of pipe flows, channel flows, back-step flows, pipe flows with abrupt expansion and plane impinging slot jets. The results confirm that the present modification successfully improves the performance of the original AWF for all the flows and heat transfer tested

The effect of turbulence-radiation interaction on radiative entropy generation and heat transfer

International Nuclear Information System (INIS)

Caldas, Miguel; Semiao, Viriato

2007-01-01

The analysis under the second law of thermodynamics is the gateway for optimisation in thermal equipments and systems. Through entropy minimisation techniques it is possible to increase the efficiency and overall performance of all kinds of thermal systems. Radiation, being the dominant mechanism of heat transfer in high-temperature systems, plays a determinant role in entropy generation within such equipments. Turbulence is also known to be a major player in the phenomenon of entropy generation. Therefore, turbulence-radiation interaction is expected to have a determinant effect on entropy generation. However, this is a subject that has not been dealt with so far, at least to the extent of the authors' knowledge. The present work attempts to fill that void, by studying the effect of turbulence-radiation interaction on entropy generation. All calculations are approached in such a way as to make them totally compatible with standard engineering methods for radiative heat transfer, namely the discrete ordinates method. It was found that turbulence-radiation interaction does not significantly change the spatial pattern of entropy generation, or heat transfer, but does change significantly their magnitude, in a way approximately proportional to the square of the intensity of turbulence

Application of turbulence modeling to predict surface heat transfer in stagnation flow region of circular cylinder

Science.gov (United States)

Wang, Chi R.; Yeh, Frederick C.

1987-01-01

A theoretical analysis and numerical calculations for the turbulent flow field and for the effect of free-stream turbulence on the surface heat transfer rate of a stagnation flow are presented. The emphasis is on the modeling of turbulence and its augmentation of surface heat transfer rate. The flow field considered is the region near the forward stagnation point of a circular cylinder in a uniform turbulent mean flow. The free stream is steady and incompressible with a Reynolds number of the order of 10 to the 5th power and turbulence intensity of less than 5 percent. For this analysis, the flow field is divided into three regions: (1) a uniform free-stream region where the turbulence is homogeneous and isotropic; (2) an external viscid flow region where the turbulence is distorted by the variation of the mean flow velocity; and, (3) an anisotropic turbulent boundary layer region over the cylinder surface. The turbulence modeling techniques used are the kappa-epsilon two-equation model in the external flow region and the time-averaged turbulence transport equation in the boundary layer region. The turbulence double correlations, the mean velocity, and the mean temperature within the boundary layer are solved numerically from the transport equations. The surface heat transfer rate is calculated as functions of the free-stream turbulence longitudinal microlength scale, the turbulence intensity, and the Reynolds number.

Nonlinear entropy transfer in ETG-TEM turbulence via TEM driven zonal flows

International Nuclear Information System (INIS)

Asahi, Yuuichi; Tsutsui, Hiroaki; Tsuji-Iio, Shunji; Ishizawa, Akihiro; Sugama, Hideo; Watanabe, Tomohiko

2015-01-01

Nonlinear interplay of the electron temperature gradient (ETG) modes and the trapped electron modes (TEMs) was investigated by means of gyrokinetic simulation. Focusing on the situation where both TEMs and ETG modes are linearly unstable, the effects of TEM-driven zonal flows on ETG turbulence were examined by means of entropy transfer analysis. In a statistically steady turbulence where the TEM driven zonal flows are dominant, it turned out that the zonal flows meditate the entropy transfer of the ETG modes from the low to high radial wavenumber regions. The successive entropy transfer broadens the potential fluctuation spectrum in the radial wavenumber direction. In contrast, in the situation where ETG modes are unstable but TEMs are stable, the pure ETG turbulence does not produce strong zonal flows, leading to a rather narrow spectrum in the radial wavenumber space and a higher transport level. (author)

Heat transfer, velocity-temperature correlation, and turbulent shear stress from Navier-Stokes computations of shock wave/turbulent boundary layer interaction flows

Science.gov (United States)

Wang, C. R.; Hingst, W. R.; Porro, A. R.

1991-01-01

The properties of 2-D shock wave/turbulent boundary layer interaction flows were calculated by using a compressible turbulent Navier-Stokes numerical computational code. Interaction flows caused by oblique shock wave impingement on the turbulent boundary layer flow were considered. The oblique shock waves were induced with shock generators at angles of attack less than 10 degs in supersonic flows. The surface temperatures were kept at near-adiabatic (ratio of wall static temperature to free stream total temperature) and cold wall (ratio of wall static temperature to free stream total temperature) conditions. The computational results were studied for the surface heat transfer, velocity temperature correlation, and turbulent shear stress in the interaction flow fields. Comparisons of the computational results with existing measurements indicated that (1) the surface heat transfer rates and surface pressures could be correlated with Holden's relationship, (2) the mean flow streamwise velocity components and static temperatures could be correlated with Crocco's relationship if flow separation did not occur, and (3) the Baldwin-Lomax turbulence model should be modified for turbulent shear stress computations in the interaction flows.

Conjugate heat transfer for turbulent flow in a thick walled plain pipe

Directory of Open Access Journals (Sweden)

Canli Eyub

2018-01-01

Full Text Available Laminar and turbulent flow have their own characteristics in respect of heat transfer in pipes. While conjugate heat transfer is a major concern for a thick walled pipe with laminar flow inside it, there are limited studies about a turbulent flow in a thick walled plain pipe considering the conjugate heat transfer. In order to conduct such a work by means of in-house developed code, it was desired to make a preliminary investigation with commercially available CFD codes. ANSYS CFD was selected as the tool since it has a positive reputation in the literature for reliability. Defined heat transfer problem was solved with SIMPLE and Coupled Schemes for pressure velocity coupling and results are presented accordingly.

Transitional and turbulent boundary layer with heat transfer

Science.gov (United States)

Wu, Xiaohua; Moin, Parviz

2010-08-01

We report on our direct numerical simulation of an incompressible, nominally zero-pressure-gradient flat-plate boundary layer from momentum thickness Reynolds number 80-1950. Heat transfer between the constant-temperature solid surface and the free-stream is also simulated with molecular Prandtl number Pr=1. Skin-friction coefficient and other boundary layer parameters follow the Blasius solutions prior to the onset of turbulent spots. Throughout the entire flat-plate, the ratio of Stanton number and skin-friction St/Cf deviates from the exact Reynolds analogy value of 0.5 by less than 1.5%. Mean velocity and Reynolds stresses agree with experimental data over an extended turbulent region downstream of transition. Normalized rms wall-pressure fluctuation increases gradually with the streamwise growth of the turbulent boundary layer. Wall shear stress fluctuation, τw,rms'+, on the other hand, remains constant at approximately 0.44 over the range, 800spots are tightly packed with numerous hairpin vortices. With the advection and merging of turbulent spots, these young isolated hairpin forests develop into the downstream turbulent region. Isosurfaces of temperature up to Reθ=1900 are found to display well-resolved signatures of hairpin vortices, which indicates the persistence of the hairpin forests.

A Dual-Plane PIV Study of Turbulent Heat Transfer Flows

Science.gov (United States)

Wernet, Mark P.; Wroblewski, Adam C.; Locke, Randy J.

2016-01-01

Thin film cooling is a widely used technique in turbomachinery and rocket propulsion applications, where cool injection air protects a surface from hot combustion gases. The injected air typically has a different velocity and temperature from the free stream combustion flow, yielding a flow field with high turbulence and large temperature differences. These thin film cooling flows provide a good test case for evaluating computational model prediction capabilities. The goal of this work is to provide a database of flow field measurements for validating computational flow prediction models applied to turbulent heat transfer flows. In this work we describe the application of a Dual-Plane Particle Image Velocimetry (PIV) technique in a thin film cooling wind tunnel facility where the injection air stream velocity and temperatures are varied in order to provide benchmark turbulent heat transfer flow field measurements. The Dual-Plane PIV data collected include all three components of velocity and all three components of vorticity, spanning the width of the tunnel at multiple axial measurement planes.

Sulphur dioxide removal by turbulent transfer over grass, snow, and water surfaces

Energy Technology Data Exchange (ETDEWEB)

Whelpdale, D M; Shaw, R W

1974-01-01

Vertical gradients of sulphur dioxide concentration have been measured over grass, snow, and water surfaces in order to assess the importance of these surfaces as SO/sub 2/ sinks. Concentrations were usually found to be lower near the surface indicating that removal occurs there. Vertical concentration gradients, normalized with repect to the concentration at 8 m, were generally greatest over water and least over snow, independent of meteorological conditions, suggesting that a water surface is the strongest SO/sub 2/ sink, with grass next, and snow weakest. The turbulent transfer of SO/sub 2/ to the interface is discussed in relation to stability of the lower atmosphere and physical and chemical properties of the surfaces. Using a bulk aerodynamic transfer approach similar to that for water vapour, values of SO/sub 2/ flux averaged over periods of from one to several hours were found to be of the order of 1 microgram/M/sup 2//S to the water and grass surfaces, and an order of magnitude smaller to the snow surface. Deposition velocities were found to be of the order of 1 cm/s.

Anomalous roughness of turbulent interfaces with system size dependent local roughness exponent

International Nuclear Information System (INIS)

Balankin, Alexander S.; Matamoros, Daniel Morales

2005-01-01

In a system far from equilibrium the system size can play the role of control parameter that governs the spatiotemporal dynamics of the system. Accordingly, the kinetic roughness of interfaces in systems far from equilibrium may depend on the system size. To get an insight into this problem, we performed a detailed study of rough interfaces formed in paper combustion experiments. Using paper sheets of different width λ, we found that the turbulent flame fronts display anomalous multi-scaling characterized by non-universal global roughness exponent α and by the system size dependent spectrum of local roughness exponents, ζ q (λ)=ζ 1 (1)q -ω λ φ q =0.93q -0.15 . The structure factor of turbulent flame fronts also exhibits unconventional scaling dependence on λ. These results are expected to apply to a broad range of far from equilibrium systems when the kinetic energy fluctuations exceed a certain critical value.

Turbulent mixed buoyancy driven flow and heat transfer in lid driven enclosure

International Nuclear Information System (INIS)

Mishra, Ajay Kumar; Sharma, Anil Kumar

2015-01-01

Turbulent mixed buoyancy driven flow and heat transfer of air in lid driven rectangular enclosure has been investigated for Grashof number in the range of 10 8 to 10 11 and for Richardson number 0.1, 1 and 10. Steady two dimensional Reynolds-Averaged-Navier-Stokes equations and conservation equations of mass and energy, coupled with the Boussinesq approximation, are solved. The spatial derivatives in the equations are discretized using the finite-element method. The SIMPLE algorithm is used to resolve pressure-velocity coupling. Turbulence is modeled with the k-ω closure model with physical boundary conditions along with the Boussinesq approximation, for the flow and heat transfer. The predicted results are validated against benchmark solutions reported in literature. The results include stream lines and temperature fields are presented to understand flow and heat transfer characteristics. There is a marked reduction in mean Nusselt number (about 58%) as the Richardson number increases from 0.1 to 10 for the case of Ra=10 10 signifying the effect of reduction of top lid velocity resulting in reduction of turbulent mixing. (author)

Flow and heat transfer in laminar–turbulent transitional flow regime under rolling motion

International Nuclear Information System (INIS)

Yuan, Hongsheng; Tan, Sichao; Zhuang, Nailiang; Lan, Shu

2016-01-01

Highlights: • Flow and heat transfer experiment in transitional flow regime under rolling motion. • Increases of average friction factor and Nu were found. • Periodic breakdown of laminar flow contributes to the increase. • Nonlinear variation of pressure drop or Nu with Re also contributes to the increase. • Effect of critical Reynolds number shift was discussed. - Abstract: Flow and heat transfer characteristics under rolling motion are extremely important to thermohydraulic analysis of offshore nuclear reactors. An experimental study was conducted in a heated rectangular channel to investigate flow and heat transfer in laminar–turbulent transitional flow regime under rolling motion. The results showed that the average friction factor and Nusselt number are higher than that of the corresponding steady flow as the flow rate fluctuates in transitional flow regime. Larger relative flow rate fluctuation was observed under larger rolling amplitude or higher rolling frequency. In the same manner, larger increases of average friction factor and Nusselt number were achieved under larger rolling amplitude or higher rolling frequency. The increases were mainly caused by the flow rate fluctuation through periodic breakdown of laminar flow and development of turbulence in laminar–turbulent transitional flow regime. First, turbulence, which enhances the rate of momentum and energy exchange, occurs near the crest of flow rate wave even the flow is still in laminar flow regime according to the average Reynolds number. Second, as a result of rapid increases of the friction and heat transfer with Reynolds number in transitional flow regime, the increases of the friction and the heat transfer near the crest of flow rate wave are larger than the decreases of them near the trough of flow rate wave, which also contributes to increases of average friction and heat transfer. Additionally, the effect of critical Reynolds number shift under unsteady flow and heating

On the calculation of length scales for turbulent heat transfer correlation

Energy Technology Data Exchange (ETDEWEB)

Barrett, M.J.; Hollingsworth, D.K.

1999-07-01

Turbulence length scale calculation methods were critically reviewed for their usefulness in boundary layer heat transfer correlations. Merits and deficiencies in each calculation method were presented. A rigorous method for calculating an energy-based integral scale was introduced. The method uses the variance of the streamwise velocity and a measured dissipation spectrum to calculate the length scale. Advantages and disadvantages of the new method were discussed. A principal advantage is the capability to decisively calculate length scales in a low-Reynolds-number turbulent boundary layer. The calculation method was tested with data from grid-generated, free-shear-layer, and wall-bounded turbulence. In each case, the method proved successful. The length scale is well behaved in turbulent boundary layers with momentum thickness Reynolds numbers from 400 to 2,100 and in flows with turbulent Reynolds numbers as low as 90.

Density based topology optimization of turbulent flow heat transfer systems

DEFF Research Database (Denmark)

Dilgen, Sümer Bartug; Dilgen, Cetin Batur; Fuhrman, David R.

2018-01-01

The focus of this article is on topology optimization of heat sinks with turbulent forced convection. The goal is to demonstrate the extendibility, and the scalability of a previously developed fluid solver to coupled multi-physics and large 3D problems. The gradients of the objective and the con...... in the optimization process, while also demonstrating extension of the methodology to include coupling of heat transfer with turbulent flows.......The focus of this article is on topology optimization of heat sinks with turbulent forced convection. The goal is to demonstrate the extendibility, and the scalability of a previously developed fluid solver to coupled multi-physics and large 3D problems. The gradients of the objective...

Heat transfer enhancement in a turbulent natural convection boundary layer along a vertical flat plate

International Nuclear Information System (INIS)

Tsuji, Toshihiro; Kajitani, Tsuyoshi; Nishino, Tatsuhiko

2007-01-01

An experimental study on heat transfer enhancement for a turbulent natural convection boundary layer in air along a vertical flat plate has been performed by inserting a long flat plate in the spanwise direction (simple heat transfer promoter) and short flat plates aligned in the spanwise direction (split heat transfer promoter) with clearances into the near-wall region of the boundary layer. For a simple heat transfer promoter, the heat transfer coefficients increase by a peak value of approximately 37% in the downstream region of the promoter compared with those in the usual turbulent natural convection boundary layer. It is found from flow visualization and simultaneous measurements of the flow and thermal fields with hot- and cold-wires that such increase of heat transfer coefficients is mainly caused by the deflection of flows toward the outer region of the boundary layer and the invasion of low-temperature fluids from the outer region to the near-wall region with large-scale vortex motions riding out the promoter. However, heat transfer coefficients for a split heat transfer promoter exhibit an increase in peak value of approximately 60% in the downstream region of the promoter. Flow visualization and PIV measurements show that such remarkable heat transfer enhancement is attributed to longitudinal vortices generated by flows passing through the clearances of the promoter in addition to large-scale vortex motions riding out the promoter. Consequently, it is concluded that heat transfer enhancement of the turbulent natural convection boundary layer can be substantially achieved in a wide area of the turbulent natural convection boundary layer by employing multiple column split heat transfer promoters. It may be expected that the heat transfer enhancement in excess of approximately 40% can be accomplished by inserting such promoters

Direct Numerical Simulation of heat transfer in a turbulent flume

International Nuclear Information System (INIS)

Bergant, R.; Tiselj, I.

2001-01-01

Direct Numerical Simulation (DNS) can be used for the description of turbulent heat transfer in the fluid at low Reynolds numbers. DNS means precise solving of Navier-Stoke's equations without any extra turbulent models. DNS should be able to describe all relevant length scales and time scales in observed turbulent flow. The largest length scale is actually dimension of system and the smallest length and time scale is equal to Kolmogorov scale. In the present work simulations of fully developed turbulent velocity and temperature fields were performed in a turbulent flume (open channel) with pseudo-spectral approach at Reynolds number 2670 (friction Reynolds number 171) and constant Prandtl number 5.4, considering the fluid temperature as a passive scalar. Two ideal thermal boundary conditions were taken into account on the heated wall. The first one was an ideal isothermal boundary condition and the second one an ideal isoflux boundary condition. We observed different parameters like mean temperature and velocity, fluctuations of temperature and velocity, and auto-correlation functions.(author)

Energy transfer in compressible magnetohydrodynamic turbulence for isothermal self-gravitating fluids

Science.gov (United States)

Banerjee, Supratik; Kritsuk, Alexei G.

2018-02-01

Three-dimensional, compressible, magnetohydrodynamic turbulence of an isothermal, self-gravitating fluid is analyzed using two-point statistics in the asymptotic limit of large Reynolds numbers (both kinetic and magnetic). Following an alternative formulation proposed by Banerjee and Galtier [Phys. Rev. E 93, 033120 (2016), 10.1103/PhysRevE.93.033120; J. Phys. A: Math. Theor. 50, 015501 (2017), 10.1088/1751-8113/50/1/015501], an exact relation has been derived for the total energy transfer. This approach results in a simpler relation expressed entirely in terms of mixed second-order structure functions. The kinetic, thermodynamic, magnetic, and gravitational contributions to the energy transfer rate can be easily separated in the present form. By construction, the new formalism includes such additional effects as global rotation, the Hall term in the induction equation, etc. The analysis shows that solid-body rotation cannot alter the energy flux rate of compressible turbulence. However, the contribution of a uniform background magnetic field to the flux is shown to be nontrivial unlike in the incompressible case. Finally, the compressible, turbulent energy flux rate does not vanish completely due to simple alignments, which leads to a zero turbulent energy flux rate in the incompressible case.

Effects of polymer stresses on analogy between momentum and heat transfer in drag-reduced turbulent channel flow

Science.gov (United States)

Kim, Kyoungyoun; Sureshkumar, Radhakrishna

2018-03-01

The effects of polymer stresses on the analogy between momentum and heat transfer are examined by using a direct numerical simulation (DNS) of viscoelastic turbulent channel flows using a constant heat flux boundary condition. The Reynolds number based on the friction velocity and channel half height is 125, and the Prandtl number is 5. The polymer stress is modeled using the finitely extensible nonlinear elastic-Peterlin constitutive model, and low (15%), intermediate (34%), and high drag reduction (DR) (52%) cases are examined. The Colburn analogy is found to be inapplicable for viscoelastic turbulent flows, suggesting dissimilarity between the momentum and heat transfer at the macroscopic coefficient level. The mean temperature profile also shows behaviour different from the mean velocity profile in drag-reduced flows. In contrast to the dissimilarity in the mean profiles, the turbulent Prandtl number Prt predicted by the DNS is near unity. This implies that turbulent heat transfer is still analogous to turbulent momentum transfer in drag-reduced flows, as in Newtonian flow. An increase in DR is accompanied by an increase in the correlation coefficient ρuθ between the instantaneous fluctuations in the streamwise velocity u and temperature θ. The correlation coefficient between u' and wall-normal velocity fluctuations v', ρ-u v, exhibits a profile similar to that of ρ-θ v in drag-reduced and Newtonian flows. Finally, the budget analysis of the transport equations of turbulent heat flux shows a strong similarity between the turbulent momentum and heat transfer, which is consistent with the predictions of Prt near unity.

Mixing and Turbulence Statistics in an Inclined Interface Richtmyer-Meshkov Instability

Science.gov (United States)

Subramaniam, Akshay; Lele, Sanjiva

2016-11-01

The interaction of a Mach 1.55 shockwave with a nominally inclined interface is considered. Unlike the classical Richtmyer-Meshkov problem, the interface evolution is non-linear from early time and large highly correlated vortical structures are observed even after reshock. The simulations target the experiment of McFarland et al. (2014). Simulations are performed using the Miranda code (Cook et al., 2005) that uses high-order spectral-like numerics (Lele, 1992). Results from multiple grid resolutions up to 4 billion grid points establish grid convergence. Comparisons to the experiments show that the simulations adequately capture the physics of the problem. Analysis of the data from the simulations based on variable density turbulence equations in the Favre averaged form will be presented. Statistics of unclosed terms in the variable density RANS equations will also be presented and compared to standard closure models. It is observed that the Reynolds Stresses have a non-monotonic return to isotropy after reshock and that compressibility effects are important long after reshock. The effect of numerics are also quantified and presented. Computer time for this work was provided by NSF PRAC award "Multi-material turbulent mixing" on the Blue Waters system.

Mathematical Model of Transfer and Deposition of Finely Dispersed Particles in a Turbulent Flow of Emulsions and Suspensions

Science.gov (United States)

Laptev, A. G.; Basharov, M. M.

2018-05-01

The problem of modeling turbulent transfer of finely dispersed particles in liquids has been considered. An approach is used where the transport of particles is represented in the form of a variety of the diffusion process with the coefficient of turbulent transfer to the wall. Differential equations of transfer are written for different cases, and a solution of the cell model is obtained for calculating the efficiency of separation in a channel. Based on the theory of turbulent transfer of particles and of the boundary layer model, an expression has been obtained for calculating the rate of turbulent deposition of finely dispersed particles. The application of this expression in determining the efficiency of physical coagulation of emulsions in different channels and on the surface of chaotic packings is shown.

Anisotropic diffusion of volatile pollutants at air-water interface

Directory of Open Access Journals (Sweden)

Li-ping Chen

2013-04-01

Full Text Available The volatile pollutants that spill into natural waters cause water pollution. Air pollution arises from the water pollution because of volatilization. Mass exchange caused by turbulent fluctuation is stronger in the direction normal to the air-water interface than in other directions due to the large density difference between water and air. In order to explore the characteristics of anisotropic diffusion of the volatile pollutants at the air-water interface, the relationship between velocity gradient and mass transfer rate was established to calculate the turbulent mass diffusivity. A second-order accurate smooth transition differencing scheme (STDS was proposed to guarantee the boundedness for the flow and mass transfer at the air-water interface. Simulations and experiments were performed to study the trichloroethylene (C2HCl3 release. By comparing the anisotropic coupling diffusion model, isotropic coupling diffusion model, and non-coupling diffusion model, the features of the transport of volatile pollutants at the air-water interface were determined. The results show that the anisotropic coupling diffusion model is more accurate than the isotropic coupling diffusion model and non-coupling diffusion model. Mass transfer significantly increases with the increase of the air-water relative velocity at a low relative velocity. However, at a higher relative velocity, an increase in the relative velocity has no effect on mass transfer.

Heat transfer in initial region of a plane channel at different turbulence levels of inlet flow

International Nuclear Information System (INIS)

Sukomel, A.S.; Gutsev, D.F.; Velichko, V.I.

1976-01-01

Local heat transfer has been experimentally studied on the initial portion of the flat channel in the turbulent air flow. The channel measures 37.5 mm in height and 212.5 mm in width. The heat transfer measurements have been taken at inlet flow turbulence of epsilon 0 =0.7-0.8%. The charts are plotted showing variation of trannser with inlet and additional agitation of the flow. Critical values are found of the Reynolds number which are characteristic of the zones with various types of flow (laminar, transient and turbulent) at epsilon 0 =0.7-0.8%: Resub(crit 1) = 9.3x10sup(4), Resub(crit 2) = 2.9x10sup(5). With the increase of epsilon 0 up to 5% and above, the flow in the boundary layer becomes turbulent practically from the very beginning of the experimental portion. Considerable increase has been revealed of the heat transfer in this group of the experiments. At epsilon (>=) 5% the heat transfer grows up regularly

Electromagnetic control of mass transfer at liquid/liquid interfaces

International Nuclear Information System (INIS)

Saadi, B.

2006-04-01

Most metallurgical processes, such as steel refining or nuclear waste processing; the interfaces between two liquid phases are the regions of mass transfer. These transfers require the implementation of a means of stirring to accelerate the kinetics of the pollutants transfer between both phases. This thesis deals with the use of the electromagnetic forces to stir, without any material contact, the bath core and the interface in order to control or even increase the kinetic transfers. To achieve this, two complementary experimental installations were used. The first experiment allows the measurement of the Indium transfer, initially dissolved in mercury towards a covering electrolyte layer and the velocity field in mercury. The performed experiments, determine the topology of the fields flows speeds in the mercury bath, moreover the behaviour of the transfer kinetics versus the intensity of the magnetic field are established. This evolution is correlated with the dynamic behaviour of the mercury surface. The second installation allows the characterization of an element transfer (Pb, Zr or Ce) initially contained in a fluorinated salt towards an antimony matrix containing lithium. It appears that all transfers kinetics are very fast. The proposed experimental set-up is particularly efficient for Cerium transfer (limited by the interface) but does not present any action for Zirconium transfer. (author)

CFD Study of Deteriorated Turbulent Heat Transfer in Upward Flow

International Nuclear Information System (INIS)

Nietiadi, Yohanes Setiawan; Lee, Jeong Ik; Addad, Yacine

2014-01-01

DTHT regime can be induced by two effects: buoyancy and acceleration. Apart from these two deteriorating effects, another unique behavior of fluid in the DTHT regime is that the convective heat transfer rate will continue to deteriorate until it reaches certain point. The downstream of this point, is known as the recovery region, where the convective heat transfer rate returns back to the high values by recovering turbulence. We called this phenomena as re-turbulization.. The map of the DTHT regime can be seen from fig. 2, where the x-axis is the buoyancy parameter and y-axis is the acceleration parameter which is the agreed governing non-dimensional numbers among the researchers to illustrate the phenomena. The Buoyancy parameter is defind in Eq. (1) and the acceleration parameter is defined in Eq. (2), respectively. The threshold value for both effects to move from the forced turbulent heat transfer to the DTHT regime are found to be Bo* ≥ 2x10 -6 and Kv ≥ 2.5x10 -6 in the previous works. Bo * =Gr q /Re 3 '. 425 Pr 0 '. 8 (1). K v =4q + /Re (2). Many experiments and simulation have been done to investigate this phenomenon and the boundary of the regime. However, very limited number of experiment was conducted in the regime where buoyancy effect and acceleration effect are in the same order of magnitude and high enough to cause DTHT (mixed DTHT). Some important experimental researches that have been done in the gas DTHT regime is Lee et al. who investigated the heat transfer of gas flow in the range of buoyancy parameter from 3x10 -9 to 10 -5 and acceleration parameter span from 6x10 -8 to 5x10 -6 and presented the behavior of Nusselt number ratio from the experiment as fig. 3 and fig. 4. This paper will discuss a Computational Fluid Dynamics analysis on DTHT by assuming hypothetical boundary conditions especially on the mixed DTHT regime. It has been found that a gas cooled fast reactor has a tendency to operate in the Deteriorated Turbulent Heat

Local Entropy Production in Turbulent Shear Flows: A Tool for Evaluating Heat Transfer Performance

Institute of Scientific and Technical Information of China (English)

H. HERWIG; F. KOCK

2006-01-01

Performance evaluation of heat transfer devices can be based on the overall entropy production in these devices.In our study we therefore provide equations for the systematic and detailed determination of local entropy production due to dissipation of mechanical energy and due to heat conduction, both in turbulent flows. After turbulence modeling has been incorporated for the fluctuating parts the overall entropy production can be determined by integration with respect to the whole flow domain. Since, however, entropy production rates show very steep gradients close to the wall, numerical solutions are far more effective with wall functions for the entropy production terms. These wall functions are mandatory when high Reynolds number turbulence models are used. For turbulent flow in a pipe with an inserted twisted tape as heat transfer promoter it is shown that based on the overall entropy production rate a clear statement from a thermodynamic point of view is possible. For a certain range of twist strength there is a decrease in overall entropy production compared to the case without insert. Also, the optimum twist strength can be determined. This information is unavailable when only pressure drop and heat transfer data are given.

Semi-empirical model for heat transfer coefficient in liquid metal turbulent flow

International Nuclear Information System (INIS)

Fernandez y Fernandez, E.; Carajilescov, P.

1982-01-01

The heat transfer by forced convection in a metal liquid turbulent flow for circular ducts is analyzed. An analogy between the momentum and heat in the wall surface, is determined, aiming to determine an expression for heat transfer coefficient in function of the friction coefficient. (E.G.) [pt

Atmospheric turbulence affects wind turbine nacelle transfer functions

Directory of Open Access Journals (Sweden)

C. M. St. Martin

2017-06-01

Full Text Available Despite their potential as a valuable source of individual turbine power performance and turbine array energy production optimization information, nacelle-mounted anemometers have often been neglected because complex flows around the blades and nacelle interfere with their measurements. This work quantitatively explores the accuracy of and potential corrections to nacelle anemometer measurements to determine the degree to which they may be useful when corrected for these complex flows, particularly for calculating annual energy production (AEP in the absence of other meteorological data. Using upwind meteorological tower measurements along with nacelle-based measurements from a General Electric (GE 1.5sle model, we calculate empirical nacelle transfer functions (NTFs and explore how they are impacted by different atmospheric and turbulence parameters. This work provides guidelines for the use of NTFs for deriving useful wind measurements from nacelle-mounted anemometers. Corrections to the nacelle anemometer wind speed measurements can be made with NTFs and used to calculate an AEP that comes within 1 % of an AEP calculated with upwind measurements. We also calculate unique NTFs for different atmospheric conditions defined by temperature stratification as well as turbulence intensity, turbulence kinetic energy, and wind shear. During periods of low stability as defined by the Bulk Richardson number (RB, the nacelle-mounted anemometer underestimates the upwind wind speed more than during periods of high stability at some wind speed bins below rated speed, leading to a steeper NTF during periods of low stability. Similarly, during periods of high turbulence, the nacelle-mounted anemometer underestimates the upwind wind speed more than during periods of low turbulence at most wind bins between cut-in and rated wind speed. Based on these results, we suggest different NTFs be calculated for different regimes of atmospheric stability and turbulence

Transitional and turbulent flat-plate boundary layers with heat transfer

Science.gov (United States)

Wu, Xiaohua; Moin, Parviz

2010-11-01

We report on our direct numerical simulation of two incompressible, nominally zero-pressure-gradient flat-plate boundary layers from momentum thickness Reynolds number 80 to 1950. Heat transfer between the constant-temperature solid surface and the free-stream is also simulated with molecular Prandtl number=1. Throughout the entire flat-plate, the ratio of Stanton number and skin-friction St/Cfdeviates from the exact Reynolds analogy value of 0.5 by less than 1.5%. Turbulent Prandtl number t peaks at the wall. Preponderance of hairpin vortices is observed in both the transitional and turbulent regions of the boundary layers. In particular, the internal structure of merged turbulent spots is hairpin forest; the internal structure of infant turbulent spots is hairpin packet. Numerous hairpin vortices are readily detected in both the near-wall and outer regions of the boundary layers up to momentum thickness Reynolds number 1950. This suggests that the hairpin vortices in the turbulent region are not simply the aged hairpin forests convected from the upstream transitional region. Temperature iso-surfaces in the companion thermal boundary layers are found to be a useful tracer in identifying hairpin vortex structures.

DNS of Turbulent Flow and Heat Transfer in a Channel with Surface Mounted Cubes

NARCIS (Netherlands)

Verstappen, R.W.C.P.; Velde, R.M. van der; Veldman, A.E.P.

2000-01-01

The turbulent flow and heat transfer in a channel with surface mounted cubical obstacles forms a generic example of a problem that occurs in many engineering applications, for instance in the design of cooling devices. We have performed a numerical simulation of it without using any turbulence

DNS of turbulent flow and heat transfer in a channel with surface mounted cubes

NARCIS (Netherlands)

Verstappen, R.W.C.P.; Velde, R.M. van der; Veldman, A.E.P.

2000-01-01

The turbulent flow and heat transfer in a channel with surface mounted cubical obstacles forms a generic example of a problem that occurs in many engineering applications, for instance in the design of cooling devices. We have performed a numerical simulation of it without using any turbulence

An experimental investigation of turbulent flow heat transfer through ...

African Journals Online (AJOL)

An experimental investigation has been carried out to study the turbulent flow heat transfer and to determine the pressure drop characteristics of air, flowing through a tube with insert. An insert of special geometry is used inside the tube. The test section is electrically heated, and air is allowed to flow as the working fluid ...

Turbulent Mixing and Vertical Heat Transfer in the Surface Mixed Layer of the Arctic Ocean: Implication of a Cross-Pycnocline High-Temperature Anomaly

Science.gov (United States)

Kawaguchi, Yusuke; Takeda, Hiroki

2017-04-01

This study focuses on the mixing processes in the vicinity of surface mixed layer (SML) of the Arctic Ocean. Turbulence activity and vertical heat transfer are quantitatively characterized in the Northwind Abyssal Plain, based on the RV Mirai Arctic cruise, during the transition from late summer to early winter 2014. During the cruise, noticeable storm events were observed, which came over the ship's location and contributed to the deepening of the SML. According to the ship-based microstructure observation, within the SML, the strong wind events produced enhanced dissipation rates of turbulent kinetic energy in the order of magnitude of ɛ = 10-6-10-4W kg-1. On thermal variance dissipation rate, χ increases toward the base of SML, reaching O(10-7) K2 s-1, resulting in vertical heat flux of O(10) W m-2. During the occasional energetic mixing events, the near-surface warm water was transferred downward and penetrated through the SML base, creating a cross-pycnocline high-temperature anomaly (CPHTA) at approximately 20-30 m depth. Near CPHTA, the vertical heat flux was anomalously magnified to O(10-100) W m-2. Following the fixed-point observation, in the regions of marginal and thick ice zones, the SML heat content was monitored using an autonomous drifting buoy, UpTempO. During most of the ice-covered period, the ocean-to-ice turbulent heat flux was dominant, rather than the diapycnal heat transfer across the SML bottom interface.

Numerical prediction of turbulent heat transfer augmentation in an annular fuel channel with two-dimensional square ribs

International Nuclear Information System (INIS)

Takase, Kazuyuki

1996-01-01

The square-ribbed fuel rod for high temperature gas-cooled reactors was developed in order to enhance the turbulent heat transfer in comparison with the standard fuel rod. To evaluate the heat transfer performance of the square-ribbed fuel rod, the turbulent heat transfer coefficients in an annular fuel channel with repeated two-dimensional square ribs were analyzed numerically on a fully developed incompressible flow using the k - ε turbulence model and the two-dimensional axisymmetrical coordinate system. Numerical analyses were carried out for a range of Reynolds numbers from 3000 to 20000 and ratios of square-rib pitch to height of 10, 20 and 40, respectively. The predicted values of the heat transfer coefficients agreed within an error of 10% for the square-rib pitch to height ratio of 10, 20% for 20 and 25% for 40, respectively, with the heat transfer empirical correlations obtained from the experimental data. It was concluded by the present study that the effect of the heat transfer augmentation by square ribs could be predicted sufficiently by the present numerical simulations and also a part of its mechanism could be explained by means of the change in the turbulence kinematic energy distribution along the flow direction. (author)

Air-water transfer of hydrogen sulfide

DEFF Research Database (Denmark)

Yongsiri, C.; Vollertsen, J.; Rasmussen, M. R.

2004-01-01

The emissions process of hydrogen sulfide was studied to quantify air–water transfer of hydrogen sulfide in sewer networks. Hydrogen sulfide transfer across the air–water interface was investigated at different turbulence levels (expressed in terms of the Froude number) and pH using batch...... experiments. By means of the overall mass–transfer coefficient (KLa), the transfer coefficient of hydrogen sulfide (KLaH2S), referring to total sulfide, was correlated to that of oxygen (KLaO2) (i.e., the reaeration coefficient). Results demonstrate that both turbulence and pH in the water phase play...... a significant role for KLaH2S. An exponential expression is a suitable representation for the relationship between KLaH2S and the Froude number at all pH values studied (4.5 to 8.0). Because of the dissociation of hydrogen sulfide, KLaH2S increased with decreasing pH at a constant turbulence level. Relative...

Anisotropy and buoyancy in nuclear turbulent heat transfer - critical assessment and needs for modelling

International Nuclear Information System (INIS)

Groetzbach, G.

2007-12-01

Computational Fluid Dynamics (CFD) programs have a wide application field in reactor technique, like to diverse flow types which have to be considered in Accelerator Driven nuclear reactor Systems (ADS). This requires turbulence models for the momentum and heat transfer with very different capabilities. The physical demands on the models are elaborated for selected transport mechanisms, the status quo of the modelling is discussed, and it is investigated which capabilities are offered by the market dominating commercial CFD codes. One topic of the discussion is on the already earlier achieved knowledge on the distinct anisotropy of the turbulent momentum and heat transport near walls. It is shown that this is relevant in channel flows with inhomogeneous wall conditions. The related consequences for the turbulence modelling are discussed. The second topic is the turbulent heat transport in buoyancy influenced flows. The only turbulence model for heat transfer which is available in the large commercial CFD-codes is based on the Reynolds analogy. This means, it is required to prescribe suitable turbulent Prandtl number distributions. There exist many correlations for channel flows, but they are seldom used in practical applications. Here, a correlation is deduced for the local turbulent Prandtl number which accounts for many parameters, like wall distance, molecular Prandtl number of the fluid, wall roughness and local shear stress, thermal wall condition, etc. so that it can be applied to most ADS typical heat transporting channel flows. The spatial dependence is discussed. It is shown that it is essential for reliable temperature calculations to get accurate turbulent Prandtl numbers especially near walls. If thermal wall functions are applied, then the correlation for the turbulent Prandtl number has to be consistent with the wall functions to avoid unphysical discretisation dependences. In using Direct Numerical Simulation (DNS) data for horizontal fluid layers it

SIMULATION OF TURBULENT FLOW AND HEAT TRANSFER OVER A BACKWARD -FACING STEP WITH RIBS TURBULATORS

Directory of Open Access Journals (Sweden)

Khudheyer S Mushatet

2011-01-01

Full Text Available Simulation is presented for a backward facing step flow and heat transfer inside a channel with ribs turbulators. The problem was investigated for Reynolds numbers up to 32000. The effect of a step height, the number of ribs and the rib thickness on the flow and thermal field were investigated. The computed results are presented as streamlines counters, velocity vectors and graphs of Nusselt number and turbulent kinetic energy variation. A control volume method employing a staggered grid techniques was imposed to discretize the governing continuity, full Navier Stockes and energy equations. A computer program using a SIMPLE algorithm was developed to handle the considered problem. The effect of turbulence was modeled by using a k-є model with its wall function formulas. The obtained results show that the strength and size of the re-circulation zones behind the step are increased with the increase of contraction ratio(i.e. with the increase of a step height. The size of recirculation regions and the reattachment length after the ribs are decreased with increasing of the contraction ratio. Also the results show that the Reynolds number and contraction ratio have a significant effect on the variation of turbulent kinetic energy and Nusselt number

Large eddy simulation of rotating turbulent flows and heat transfer by the lattice Boltzmann method

Science.gov (United States)

Liou, Tong-Miin; Wang, Chun-Sheng

2018-01-01

Due to its advantage in parallel efficiency and wall treatment over conventional Navier-Stokes equation-based methods, the lattice Boltzmann method (LBM) has emerged as an efficient tool in simulating turbulent heat and fluid flows. To properly simulate the rotating turbulent flow and heat transfer, which plays a pivotal role in tremendous engineering devices such as gas turbines, wind turbines, centrifugal compressors, and rotary machines, the lattice Boltzmann equations must be reformulated in a rotating coordinate. In this study, a single-rotating reference frame (SRF) formulation of the Boltzmann equations is newly proposed combined with a subgrid scale model for the large eddy simulation of rotating turbulent flows and heat transfer. The subgrid scale closure is modeled by a shear-improved Smagorinsky model. Since the strain rates are also locally determined by the non-equilibrium part of the distribution function, the calculation process is entirely local. The pressure-driven turbulent channel flow with spanwise rotation and heat transfer is used for validating the approach. The Reynolds number characterized by the friction velocity and channel half height is fixed at 194, whereas the rotation number in terms of the friction velocity and channel height ranges from 0 to 3.0. A working fluid of air is chosen, which corresponds to a Prandtl number of 0.71. Calculated results are demonstrated in terms of mean velocity, Reynolds stress, root mean square (RMS) velocity fluctuations, mean temperature, RMS temperature fluctuations, and turbulent heat flux. Good agreement is found between the present LBM predictions and previous direct numerical simulation data obtained by solving the conventional Navier-Stokes equations, which confirms the capability of the proposed SRF LBM and subgrid scale relaxation time formulation for the computation of rotating turbulent flows and heat transfer.

CFD evaluation of turbulence model on heat transfer in 5 × 5 rod bundles

International Nuclear Information System (INIS)

Chao Yanmeng; Yang Lixin; Zhang Yuxiang; Pang Zhengzheng

2014-01-01

Different turbulence models may lead to different results when analyzing fuel assemblies using computational fluid dynamics (CFD) method. In this paper, a 5 × 5 rod bundle model was built to analyze the relationship between flow and heat transfer. The pressure drop and Nu were calculated using ANSYS CFX. Three factors evaluating swirling flow and cross-flow were used to analyze the inner relationship between flow field and heat transfer. The performances of various turbulence models, including eddy viscosity model and Reynold stress model, were evaluated. The comparison between numerical and similar experimental results indicates that Reynold stress model is more appropriate for modeling flow features and heat transfer in spacer grids discussed in this paper. (authors)

Turbulent heat transfer to longitudinal flow through a triangular array of circular rods

International Nuclear Information System (INIS)

Pfann, J.

1975-01-01

Temperature distribution and heat transfer to longitudinal turbulent, fully developed flow through triangular arrays of smooth circular rods are analysed for liquids with Prandtl number approximately 1 and << 1. Nusselt number is plotted versus pitch and turbulence for constant heat flow and for constant temperature on the rod surface, and the optimum pitch is determined. The influence of Prandtl number is analysed. (Auth.)

On the Statistical Properties of Turbulent Energy Transfer Rate in the Inner Heliosphere

Science.gov (United States)

Sorriso-Valvo, Luca; Carbone, Francesco; Perri, Silvia; Greco, Antonella; Marino, Raffaele; Bruno, Roberto

2018-01-01

The transfer of energy from large to small scales in solar wind turbulence is an important ingredient of the long-standing question of the mechanism of the interplanetary plasma heating. Previous studies have shown that magnetohydrodynamic (MHD) turbulence is statistically compatible with the observed solar wind heating as it expands in the heliosphere. However, in order to understand which processes contribute to the plasma heating, it is necessary to have a local description of the energy flux across scales. To this aim, it is customary to use indicators such as the magnetic field partial variance of increments (PVI), which is associated with the local, relative, scale-dependent magnetic energy. A more complete evaluation of the energy transfer should also include other terms, related to velocity and cross-helicity. This is achieved here by introducing a proxy for the local, scale-dependent turbulent energy transfer rate ɛ_{Δ t}(t), based on the third-order moment scaling law for MHD turbulence. Data from Helios 2 are used to determine the statistical properties of such a proxy in comparison with the magnetic and velocity fields PVI, and the correlation with local solar wind heating is computed. PVI and ɛ_{Δ t}(t) are generally well correlated; however, ɛ_{Δ t}(t) is a very sensitive proxy that can exhibit large amplitude values, both positive and negative, even for low amplitude peaks in the PVI. Furthermore, ɛ_{Δ t}(t) is very well correlated with local increases of the temperature when large amplitude bursts of energy transfer are localized, thus suggesting an important role played by this proxy in the study of plasma energy dissipation.

Influence of fluid-property variation on turbulent convective heat transfer in vertical annular CHANNEL FLOWS

International Nuclear Information System (INIS)

Joong Hun Bae; Jung Yul Yoo; Haecheon Choi

2005-01-01

Full text of publication follows: The influence of variable fluid property on turbulent convective heat transfer is investigated using direct numerical simulations. We consider thermally-developing flows of air and supercritical-pressure CO 2 in a vertical annular channel where the inner wall is heated with a constant heat flux and the outer wall is insulated. Turbulence statistics show that the heat and momentum transport characteristics of variable-property flows are significantly different from those of constant-property flows. The difference is mainly caused by the spatial and temporal variations of fluid density. The non-uniform density distribution causes fluid particles to be accelerated either by expansion or buoyancy force, while the temporal density fluctuations change the heat and momentum transfer via transport of turbulent mass flux, ρ'u' i . Both effects of the spatial and temporal variations of density are shown to be important in the analysis of turbulent convective heat transfer for supercritical-pressure fluids. For variable-property heated air flows, however, the effect of temporal density fluctuations can be neglected at low Mach number, which is in good accordance with the Morkovin's hypothesis. (authors)

Turbulent boundary layer heat transfer experiments: Convex curvature effects including introduction and recovery

Science.gov (United States)

Simon, T. W.; Moffat, R. J.; Johnston, J. P.; Kays, W. M.

1982-01-01

Measurements were made of the heat transfer rate through turbulent and transitional boundary layers on an isothermal, convexly curved wall and downstream flat plate. The effect of convex curvature on the fully turbulent boundary layer was a reduction of the local Stanton numbers 20% to 50% below those predicted for a flat wall under the same circumstances. The recovery of the heat transfer rates on the downstream flat wall was extremely slow. After 60 cm of recovery length, the Stanton number was still typically 15% to 20% below the flat wall predicted value. Various effects important in the modeling of curved flows were studied separately. These are: the effect of initial boundary layer thickness, the effect of freestream velocity, the effect of freestream acceleration, the effect of unheated starting length, and the effect of the maturity of the boundary layer. An existing curvature prediction model was tested against this broad heat transfer data base to determine where it could appropriately be used for heat transfer predictions.

Aerodynamics, heat and mass transfer in steam-aerosol turbulent flows in containment

Energy Technology Data Exchange (ETDEWEB)

Nigmatulin, B.I.; Pershukov, V.A.; Ris, V.V. [Research & Engineering Centre of Nuclear Plants Safety, Moscow (Russian Federation)] [and others

1995-09-01

In this report an analysis of aerodynamic and heat transfer processes at the blowdown of gas-dispersed mixture into the containment volume is presented. A few models for description of the volume averaged and local characteristics are analyzed. The mathematical model for description of the local characteristics of the turbulent gas-dispersed flows was developed. The calculation of aerodynamic, heat and mass transfer characteristics was based on the Navier-Stokes, energy and gas mass fractions conservation equations. For calculation of dynamics and deposition of the aerosols the original diffusion-inertia model is developed. The pulsating characteristics of the gaseous phase were calculated on the base (k-{xi}) model of turbulence with modification to account thermogravitational force action and influence of particle mass loading. The appropriate boundary conditions using the {open_quotes}near-wall function{close_quotes} approach was obtained. Testing of the mathematical models and boundary conditions has shown a good agreement between computation and data of comparison. The described mathematical models were applied to two- and three dimensional calculations of the turbulent flow in containment at the various stages of the accident.

A comprehensive spectral theory of zonal-mode dynamics in trapped electron mode turbulence

International Nuclear Information System (INIS)

Terry, P.W.; Gatto, R.; Baver, D.A.; Fernandez, E.

2005-01-01

A comprehensive, self-consistent theory for spectral dynamics in trapped electron mode (TEM) turbulence offers critical new understanding and insights into zonal-mode physics. This theory shows that 1) zonal mode structure, anisotropy, excitation, and temporal behavior arise at and from the interface of nonlinear advection and linear wave properties; 2) waves induce a marked spectral energy-transfer anisotropy that preferentially drives zonal modes relative to non zonal modes; 3) triplet correlations involving density (as opposed to those involving only flow) mediate the dominant energy transfer at long wavelengths; 4) energy transfer becomes inverse in the presence of wave anisotropy, where otherwise it is forward; 5) zonal-mode excitation is accompanied by excitation of a spectrum of damped eigenmodes, making zonal modes nonlinearly damped; and 6) the combination of anisotropic transfer to zonal modes and their nonlinear damping make this the dominant saturation mechanism for TEM turbulence. This accounts for the reduction of turbulence level by zonal modes, not zonal-flow ExB shearing. (author)

Transient turbulent heat transfer for heating of water in a short vertical tube

International Nuclear Information System (INIS)

Hata, Koichi; Kai, Naoto; Shirai, Yasuyuki; Masuzaki, Suguru

2011-01-01

The transient turbulent heat transfer coefficients in a short vertical Platinum test tube were systematically measured for the flow velocities (u=4.0 to 13.6 m/s), the inlet liquid temperatures (T in =296.93 to 304.81 K), the inlet pressures (P in =794.39 to 858.27 kPa) and the increasing heat inputs (Q 0 exp(t/τ), exponential periods, τ, of 18.6 ms to 25.7 s) by an experimental water loop comprised of a multistage canned-type circulation pump with high pump head. The Platinum test tubes of test tube inner diameters (d=3 and 6 mm), heated lengths (L=66.5 and 69.6 mm), effective lengths (L eff =56.7 and 59.2 mm), ratios of heated length to inner diameter (L/d=22.16 and 11.6), ratios of effective length to inner diameter (L eff /d=18.9 and 9.87) and wall thickness (δ=0.5 and 0.4 mm) with average surface roughness (Ra=0.40 and 0.45 μm) were used in this work. The surface heat fluxes between the two potential taps were given the difference between the heat generation rate per unit surface area and the rate of change of energy storage in the test tube obtained from the faired average temperature versus time curve. The heater inner surface temperature between the two potential taps was also obtained by solving the unsteady heat conduction equation in the test tube under the conditions of measured average temperature and heat generation rate per unit surface area of the test tube. The transient turbulent heat transfer data for Platinum test tubes were compared with the values calculated by authors' correlation for the steady state turbulent heat transfer. The influence of inner diameter (d), ratio of effective length to inner diameter (L eff /d), flow velocity (u) and exponential period (τ) on the transient turbulent heat transfer is investigated into details and the widely and precisely predictable correlation of the transient turbulent heat transfer for heating of water in a short vertical tube is given based on the experimental data and authors' studies for the

Transient turbulent heat transfer for heating of water in a short vertical tube

International Nuclear Information System (INIS)

Hata, Koichi; Kai, Naoto; Shirai, Yasuyuki; Masuzaki, Suguru

2011-01-01

The transient turbulent heat transfer coefficients in a short vertical Platinum test tube were systematically measured for the flow velocities (u=4.0 to 13.6 m/s), the inlet liquid temperatures (T in =296.93 to 304.81 K), the inlet pressures (P in =794.39 to 858.27 kPa) and the increasing heat inputs (Q 0 exp(t/τ), exponential periods, τ, of 18.6 ms to 25.7 s) by an experimental water loop comprised of a multistage canned-type circulation pump with high pump head. The Platinum test tubes of test tube inner diameters (d=3 and 6 mm), heated lengths (L=66.5 and 69.6 mm), effective lengths (L eff =56.7 and 59.2 mm), ratios of heated length to inner diameter (L/d=22.16 and 11.6), ratios of effective length to inner diameter (L eff /d=18.9 and 9.87) and wall thickness (δ=0.5 and 0.4 mm) with average surface roughness (Ra=0.40 and 0.45 μm) were used in this work. The surface heat fluxes between the two potential taps were given the difference between the heat generation rate per unit surface area and the rate of change of energy storage in the test tube obtained from the faired average temperature versus time curve. The heater inner surface temperature between the two potential taps was also obtained by solving the unsteady heat conduction equation in the test tube under the conditions of measured average temperature and heat generation rate per unit surface area of the test tube. The transient turbulent heat transfer data for Platinum test tubes were compared with the values calculated by authors' correlation for the steady state turbulent heat transfer. The influence of inner diameter (d), ratio of effective length to inner diameter (L eff /d), flow velocity (u) and exponential period (τ) on the transient turbulent heat transfer is investigated into details and the widely and precisely predictable correlation of the transient turbulent heat transfer for heating of water in a short vertical tube is given based on the experimental data and authors' studies for the

Transition Heat Transfer Modeling Based on the Characteristics of Turbulent Spots

Science.gov (United States)

Simon, Fred; Boyle, Robert

1998-01-01

While turbulence models are being developed which show promise for simulating the transition region on a turbine blade or vane, it is believed that the best approach with the greatest potential for practical use is the use of models which incorporate the physics of turbulent spots present in the transition region. This type of modeling results in the prediction of transition region intermittency which when incorporated in turbulence models give a good to excellent prediction of the transition region heat transfer. Some models are presented which show how turbulent spot characteristics and behavior can be employed to predict the effect of pressure gradient and Mach number on the transition region. The models predict the spot formation rate which is needed, in addition to the transition onset location, in the Narasimha concentrated breakdown intermittency equation. A simplified approach is taken for modeling turbulent spot growth and interaction in the transition region which utilizes the turbulent spot variables governing transition length and spot generation rate. The models are expressed in terms of spot spreading angle, dimensionless spot velocity, dimensionless spot area, disturbance frequency and Mach number. The models are used in conjunction with a computer code to predict the effects of pressure gradient and Mach number on the transition region and compared with VKI experimental turbine data.

Internal (Annular) and Compressible External (Flat Plate) Turbulent Flow Heat Transfer Correlations.

Energy Technology Data Exchange (ETDEWEB)

Dechant, Lawrence [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States); Smith, Justin [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

2016-01-01

Here we provide a discussion regarding the applicability of a family of traditional heat transfer correlation based models for several (unit level) heat transfer problems associated with flight heat transfer estimates and internal flow heat transfer associated with an experimental simulation design (Dobranich 2014). Variability between semi-empirical free-flight models suggests relative differences for heat transfer coefficients on the order of 10%, while the internal annular flow behavior is larger with differences on the order of 20%. We emphasize that these expressions are strictly valid only for the geometries they have been derived for e.g. the fully developed annular flow or simple external flow problems. Though, the application of flat plate skin friction estimate to cylindrical bodies is a traditional procedure to estimate skin friction and heat transfer, an over-prediction bias is often observed using these approximations for missile type bodies. As a correction for this over-estimate trend, we discuss a simple scaling reduction factor for flat plate turbulent skin friction and heat transfer solutions (correlations) applied to blunt bodies of revolution at zero angle of attack. The method estimates the ratio between axisymmetric and 2-d stagnation point heat transfer skin friction and Stanton number solution expressions for sub-turbulent Reynolds numbers %3C1x10 4 . This factor is assumed to also directly influence the flat plate results applied to the cylindrical portion of the flow and the flat plate correlations are modified by

BROWNIAN HEAT TRANSFER ENHANCEMENT IN THE TURBULENT REGIME

Directory of Open Access Journals (Sweden)

Suresh Chandrasekhar

2016-08-01

Full Text Available The paper presents convection heat transfer of a turbulent flow Al2O3/water nanofluid in a circular duct. The duct is a under constant and uniform heat flux. The paper computationally investigates the system’s thermal behavior in a wide range of Reynolds number and also volume concentration up to 6%. To obtain the nanofluid thermophysical properties, the Hamilton-Crosser model along with the Brownian motion effect are utilized. Then the thermal performance of the system with the nanofluid is compared to the conventional systems which use water as the working fluid. The results indicate that the use of nanofluid of 6% improves the heat transfer rate up to 36.8% with respect to pure water. Therefore, using the Al2O3/water nanofluid instead of water can be a great choice when better heat transfer is needed.

Interphasial energy transfer and particle dissipation in particle-laden wall turbulence

NARCIS (Netherlands)

Zhao, L.; Andersson, H.I.; Gillissen, J.J.J.

2013-01-01

Transfer of mechanical energy between solid spherical particles and a Newtonian carrier fluid has been explored in two-way coupled direct numerical simulations of turbulent channel flow. The inertial particles have been treated as individual point particles in a Lagrangian framework and their

Numerical prediction of augmented turbulent heat transfer in an annular fuel channel with repeated two-dimensional square ribs

International Nuclear Information System (INIS)

Takase, K.

1996-01-01

The square-ribbed fuel rod for high temperature gas-cooled reactors was designed and developed so as to enhance the turbulent heat transfer in comparison with the previous standard fuel rod. The turbulent heat transfer characteristics in an annular fuel channel with repeated two-dimensional square ribs were analysed numerically on a fully developed incompressible flow using the k-ε turbulence model and the two-dimensional axisymmetrical coordinate system. Numerical analyses were carried out under the conditions of Reynolds numbers from 3000 to 20000 and ratios of square-rib pitch to height of 10, 20 and 40 respectively. The predictions of the heat transfer coefficients agreed well within an error of 10% for the square-rib pitch to height ratio of 10, 20% for 20 and 25% for 40 respectively, with the heat transfer empirical correlations obtained from the experimental data due to the simulated square-ribbed fuel rods. Therefore it was found that the effect of heat transfer augmentation due to the square ribs could be predicted by the present numerical simulations and the mechanism could be explained by the change in the turbulence kinematic energy distribution along the flow direction. (orig.)

Turbulence Modeling and Computation of Turbine Aerodynamics and Heat Transfer

Science.gov (United States)

Lakshminarayana, B.; Luo, J.

1996-01-01

The objective of the present research is to develop improved turbulence models for the computation of complex flows through turbomachinery passages, including the effects of streamline curvature, heat transfer and secondary flows. Advanced turbulence models are crucial for accurate prediction of rocket engine flows, due to existance of very large extra strain rates, such as strong streamline curvature. Numerical simulation of the turbulent flows in strongly curved ducts, including two 180-deg ducts, one 90-deg duct and a strongly concave curved turbulent boundary layer have been carried out with Reynolds stress models (RSM) and algebraic Reynolds stress models (ARSM). An improved near-wall pressure-strain correlation has been developed for capturing the anisotropy of turbulence in the concave region. A comparative study of two modes of transition in gas turbine, the by-pass transition and the separation-induced transition, has been carried out with several representative low-Reynolds number (LRN) k-epsilon models. Effects of blade surface pressure gradient, freestream turbulence and Reynolds number on the blade boundary layer development, and particularly the inception of transition are examined in detail. The present study indicates that the turbine blade transition, in the presence of high freestream turbulence, is predicted well with LRN k-epsilon models employed. The three-dimensional Navier-Stokes procedure developed by the present authors has been used to compute the three-dimensional viscous flow through the turbine nozzle passage of a single stage turbine. A low Reynolds number k-epsilon model and a zonal k-epsilon/ARSM (algebraic Reynolds stress model) are utilized for turbulence closure. An assessment of the performance of the turbulence models has been carried out. The two models are found to provide similar predictions for the mean flow parameters, although slight improvement in the prediction of some secondary flow quantities has been obtained by the

The influence of streamwise vortices on turbulent heat transfer in rectangular ducts with various aspect ratios

International Nuclear Information System (INIS)

Choi, Hang Seok; Park, Tae Seon

2013-01-01

Highlights: ► With changing aspect ratio, the effect of secondary flows on the turbulent heat transfer is scrutinized by a LES. ► The conditional sampling technique of instantaneous near-wall streamwise vortices is developed. ► Clockwise and counter-clockwise rotating streamwise vortices are sampled and discussed with the wall heat transfer. ► The hot-sweep motions of CW and CCW vortices clearly appear with increasing aspect ratio. -- Abstract: The effect of aspect ratio of rectangular duct on the turbulent flow and heat transfer is very important for its engineering applications. But the turbulent thermal fields have not been fundamentally scrutinized in spite of its engineering significance especially for cooling device. Hence, in the present study, large eddy simulation is applied to the turbulent flow and heat transfer in rectangular ducts with varying aspect ratio. The turbulent statistics of the flow and thermal quantities are calculated and the characteristics of wall Nusselt number are investigated for each rectangular duct. Especially, to scrutinize near-wall streamwise vortices, a conditional sampling technique is developed and adopted. Clockwise and counter-clockwise rotating streamwise vortices are sampled and the probability density function of the vortex circulation Reynolds number and wall Nusselt number are calculated. From the results, the time-averaged secondary flow caused by instantaneous vortical motions has a great effect on the heat and momentum transport of the flow in the rectangular ducts. Hence, the wall Nusselt number is enhanced near the downwash flow region of the secondary flow. However, with increasing the aspect ratio, the effects of the hot-sweep flow of the clockwise and counter-clockwise rotating vortices become equally dominant near the wall normal bisector of the ducts. During time averaging process, these two counter-rotating vortices are canceled out each other diminishing a secondary flow but they still enhance the

Direct numerical simulation of turbulent concentric annular pipe flow Part 2: Heat transfer

International Nuclear Information System (INIS)

Chung, Seo Yoon; Sung, Hyung Jin

2003-01-01

A direct numerical simulation is performed for turbulent heat transfer in a concentric annulus at Re D h =8900 and Pr=0.71 for two radius ratios (R 1 /R 2 =0.1 and 0.5) and wall heat flux ratio q * =1.0. Main emphasis is placed on the transverse curvature effect on near-wall turbulent thermal structures. Near-wall turbulent thermal structures close to the inner and outer walls are scrutinized by computing the lower-order statistics. The fluctuating temperature variance and turbulent heat flux budgets are illustrated to confirm the results of the lower-order statistics. Probability density functions of the splat/anti-splat process are investigated to analyze the transverse curvature effect on the strong relationship between sweep and splat events. The present numerical results show that the turbulent thermal structures near the outer wall are more activated than those near the inner wall, which may be attributed to the different vortex regeneration processes between the inner and outer walls

Numerical Investigation of Turbulent Natural Convection Heat Transfer in an Internally-Heated Melt Pool and Metallic Layer

International Nuclear Information System (INIS)

Nourgaliev, R.R.; Dinh, A.T.; Dinh, T.N.; Sehgal, B.R.

1999-01-01

This paper presents results of numerical investigation of turbulent natural convection in an internally-heated oxidic pool, and in a metallic layer heated from below and cooled from top and sidewalls. Emphasis is placed upon applicability of the existing heat transfer correlations (obtained from simulant-material experiments) in assessments of a prototypic severe reactor accident. The objectives of this study are (i) to improve the current understanding of the physics of unstably stratified flows, and (ii) to reduce uncertainties associated with modeling and assessment of natural convection heat transfer in the above configuration. Prediction capabilities of different turbulence modeling approaches are first examined and discussed, based on extensive results of numerical investigations performed by present authors. Findings from numerical modeling of turbulent natural convection flow and heat transfer in melt pools and metallic layers are then described. (authors)

Investigating plume dynamics at the ocean-glacier interface with turbulence profiling and autonomous vessels

Science.gov (United States)

Jackson, R. H.; Nash, J. D.; Sutherland, D. A.; Amundson, J. M.; Kienholz, C.; Skyllingstad, E. D.; Motyka, R. J.

2017-12-01

The exchanges of heat and freshwater at tidewater glacier termini are modulated by small-scale turbulent processes. However, few observations have been obtained near the ocean-glacier interface, limiting our ability to quantify turbulent fluxes or test melt parameterizations in ocean-glacier models. Here, we explore the turbulent plume dynamics at LeConte Glacier, Alaska with three extensive field campaigns in May, August and September (2016-17). Two autonomous vessels collected repeat transects of velocity and water properties near the glacier, often within 20 m of the terminus. Concurrent shipboard surveying measured turbulence with a vertical microstructure profiler, along with water properties and velocity. These high-resolution surveys provide a 3D view of the circulation and allow us to quantify turbulent fluxes in the near-glacier region. We observe two regimes at the terminus: an energetic upwelling plume driven by subglacial discharge at a persistent location, and submarine melt-driven convection along other parts of the terminus. We trace the evolution of the subglacial discharge plume as it flows away from the glacier, from an initial stage of vigorous mixing to a more quiescent outflow downstream. Resolving these spatial patterns of upwelling and mixing near glaciers is a key step towards understanding submarine melt rates and glacial fjord circulation.

Direct numerical simulation of MHD heat transfer in high Reynolds number turbulent channel flows for Prandtl number of 25

International Nuclear Information System (INIS)

Yamamoto, Yoshinobu; Kunugi, Tomoaki

2015-01-01

Graphical abstract: - Highlights: • For the first time, the MHD heat transfer DNS database corresponding to the typical nondimensional parameters of the fusion blanket design using molten salt, were established. • MHD heat transfer correlation was proposed and about 20% of the heat transfer degradation was evaluated under the design conditions. • The contribution of the turbulent diffusion to heat transfer is increased drastically with increasing Hartmann number. - Abstract: The high-Prandtl number passive scalar transport of the turbulent channel flow imposed a wall-normal magnetic field is investigated through the large-scale direct numerical simulation (DNS). All essential turbulence scales of velocities and temperature are resolved by using 2048 × 870 × 1024 computational grid points in stream, vertical, and spanwise directions. The heat transfer phenomena for a Prandtl number of 25 were observed under the following flow conditions: the bulk Reynolds number of 14,000 and Hartman number of up to 28. These values were equivalent to the typical nondimensional parameters of the fusion blanket design proposed by Wong et al. As a result, a high-accuracy DNS database for the verification of magnetohydrodynamic turbulent heat transfer models was established for the first time, and it was confirmed that the heat transfer correlation for a Prandtl number of 5.25 proposed by Yamamoto and Kunugi was applicable to the Prandtl number of 25 used in this study

Direct numerical simulation of MHD heat transfer in high Reynolds number turbulent channel flows for Prandtl number of 25

Energy Technology Data Exchange (ETDEWEB)

Yamamoto, Yoshinobu, E-mail: yamamotoy@yamanashi.ac.jp [Department of Mechanical Systems Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu 400-8511 (Japan); Kunugi, Tomoaki [Department of Nuclear Engineering, Kyoto University Yoshida, Sakyo, Kyoto 606-8501 (Japan)

2015-01-15

Graphical abstract: - Highlights: • For the first time, the MHD heat transfer DNS database corresponding to the typical nondimensional parameters of the fusion blanket design using molten salt, were established. • MHD heat transfer correlation was proposed and about 20% of the heat transfer degradation was evaluated under the design conditions. • The contribution of the turbulent diffusion to heat transfer is increased drastically with increasing Hartmann number. - Abstract: The high-Prandtl number passive scalar transport of the turbulent channel flow imposed a wall-normal magnetic field is investigated through the large-scale direct numerical simulation (DNS). All essential turbulence scales of velocities and temperature are resolved by using 2048 × 870 × 1024 computational grid points in stream, vertical, and spanwise directions. The heat transfer phenomena for a Prandtl number of 25 were observed under the following flow conditions: the bulk Reynolds number of 14,000 and Hartman number of up to 28. These values were equivalent to the typical nondimensional parameters of the fusion blanket design proposed by Wong et al. As a result, a high-accuracy DNS database for the verification of magnetohydrodynamic turbulent heat transfer models was established for the first time, and it was confirmed that the heat transfer correlation for a Prandtl number of 5.25 proposed by Yamamoto and Kunugi was applicable to the Prandtl number of 25 used in this study.

Heat transfer by liquids in suspension in a turbulent gas stream (1960)

International Nuclear Information System (INIS)

Grison, E.; Commissariat a l'Energie Atomique, Saclay

1960-01-01

The introduction of a small volume of liquid into a turbulent gas stream used as cooling agent improves considerably the heat transfer coefficient of the gas. When the turbulent regime is established, one observes in a cylindrical tube two types of flow whether the liquid wets or does not wet the wall. In the first case, one gets on the wall an annular liquid film and droplets in suspension are in the gas stream. In the second case, a fog of droplets is formed without any liquid film on the wall. Experiments were performed with the following mixtures: water-hydrogen, water-nitrogen, ethanol-nitrogen (wetting liquids) introduced into a stainless steel tube of 4 mm ID, electrically heated on 320 mm of length. We varied the gas flow rate (Reynolds until 50000), the rate of the liquid flow rate to gas flow rate (until 15), the pressure (until 10 kg/cm 2 ), the temperature (until the boiling point) and the heat flux (until 250 W/cm 2 ). Two types of burnout were observed. A formula of correlation of the burnout heat flux is given. Making use of the analogy between mass transfer and heat transfer, a dimensionless formula of correlation of the local heat transfer coefficients is established. (author) [fr

Influence of various aspects of low Reynolds number k-ε turbulence models on predicting in-tube buoyancy affected heat transfer to supercritical pressure fluids

International Nuclear Information System (INIS)

Zhao, Chen-Ru; Zhang, Zhen; Jiang, Pei-Xue; Bo, Han-Liang

2017-01-01

Highlights: • Understanding of the mechanism of buoyancy effect on supercritical heat transfer. • Turbulence related parameters in upward and downward flows were compared. • Turbulent Prandtl number affected the prediction insignificantly. • Buoyancy production was insignificant compared with shear production. • Damping function had the greatest effect and is a priority for further modification. - Abstract: Heat transfer to supercritical pressure fluids was modeled for normal and buoyancy affected conditions using several low Reynolds number k-ε models, including the Launder and Sharma, Myong and Kasagi, and Abe, Kondoh and Nagano, with the predictions compared with experimental data. All three turbulence models accurately predicted the cases without heat transfer deterioration, but failed to accurately predict the cases with heat transfer deterioration although the general trends were captured, indicating that further improvements and modifications are needed for the low Reynolds number k-ε turbulence models to better predict buoyancy deteriorated heat transfer. Further investigations studied the influence of various aspects of the low Reynolds number k-ε turbulence models, including the turbulent Prandtl number, the buoyancy production of turbulent kinetic energy, and the damping function to provide guidelines for model development to more precisely predict buoyancy affected heat transfer. The results show that the turbulent Prandtl number and the buoyancy production of turbulent kinetic energy have little influence on the predictions for cases in this study, while new damping functions with carefully selected control parameters are needed in the low Reynolds number k-ε turbulence models to correctly predict the buoyancy effect for heat transfer simulations in various applications such as supercritical pressure steam generators (SPSGs) in the high temperature gas cooled reactor (HTR) and the supercritical pressure water reactor (SCWR).

Influence of various aspects of low Reynolds number k-ε turbulence models on predicting in-tube buoyancy affected heat transfer to supercritical pressure fluids

Energy Technology Data Exchange (ETDEWEB)

Zhao, Chen-Ru; Zhang, Zhen [Institute of Nuclear and New Energy Technology of Tsinghua University, Advanced Nuclear Energy Technology Cooperation Innovation Centre, Key Laboratory of Advanced Nuclear Engineering and Safety, Ministry of Education, Beijing 100084 (China); Jiang, Pei-Xue, E-mail: jiangpx@tsinghua.edu.cn [Beijing Key Laboratory of CO_2 Utilization and Reduction Technology/Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084 (China); Bo, Han-Liang [Institute of Nuclear and New Energy Technology of Tsinghua University, Advanced Nuclear Energy Technology Cooperation Innovation Centre, Key Laboratory of Advanced Nuclear Engineering and Safety, Ministry of Education, Beijing 100084 (China)

2017-03-15

Highlights: • Understanding of the mechanism of buoyancy effect on supercritical heat transfer. • Turbulence related parameters in upward and downward flows were compared. • Turbulent Prandtl number affected the prediction insignificantly. • Buoyancy production was insignificant compared with shear production. • Damping function had the greatest effect and is a priority for further modification. - Abstract: Heat transfer to supercritical pressure fluids was modeled for normal and buoyancy affected conditions using several low Reynolds number k-ε models, including the Launder and Sharma, Myong and Kasagi, and Abe, Kondoh and Nagano, with the predictions compared with experimental data. All three turbulence models accurately predicted the cases without heat transfer deterioration, but failed to accurately predict the cases with heat transfer deterioration although the general trends were captured, indicating that further improvements and modifications are needed for the low Reynolds number k-ε turbulence models to better predict buoyancy deteriorated heat transfer. Further investigations studied the influence of various aspects of the low Reynolds number k-ε turbulence models, including the turbulent Prandtl number, the buoyancy production of turbulent kinetic energy, and the damping function to provide guidelines for model development to more precisely predict buoyancy affected heat transfer. The results show that the turbulent Prandtl number and the buoyancy production of turbulent kinetic energy have little influence on the predictions for cases in this study, while new damping functions with carefully selected control parameters are needed in the low Reynolds number k-ε turbulence models to correctly predict the buoyancy effect for heat transfer simulations in various applications such as supercritical pressure steam generators (SPSGs) in the high temperature gas cooled reactor (HTR) and the supercritical pressure water reactor (SCWR).

Turbulence-induced heat transfer in PBMR core using LES and RANS

International Nuclear Information System (INIS)

Lee, Jung-Jae; Yoon, Su-Jong; Park, Goon-Cherl; Lee, Won-Jae

2007-01-01

This paper introduces the results of numerical simulations on flow fields and relevant heat transfer in the pebble bed reactor (PBR) core, since the coolant passes a highly complicated random flow path with a high Reynolds number, an appropriate treatment of the turbulence is required. A set of simple experiments for the flow over a circular cylinder with heat transfer was conducted to finally select the large eddy simulation (LES) and k-ω model among the considering Reynolds-averaged Navier-Stokes (RANS) models for PBR application. Using these models, the PBR cores, whose geometries were simplified to the body-centered cubical (BCC) and face-centered cubical (FCC) structures, were simulated. A larger pressure drop, a more random flow field, a higher vorticity magnitude and a higher temperature at the local hot spots on the pebble surface were found in the results of the LES than in those of RANS for both geometries. In cases of the LES, the flow structures were resolved up to the grid scales. Irregular distributions of the flow and local heat transfer were found in the BCC core, while relatively regular distributions for the FCC core. The turbulent nature of the coolant flow in the pebble core evidently affected the fuel surface temperature distribution. (author)

Heat transfer on liquid-liquid interface of molten-metal and water

International Nuclear Information System (INIS)

Tanaka, T.; Saito, Yasushi; Mishima, Kaichiro

2001-01-01

Molten-core pool had been formed in the lower-head of TMI-2 pressure vessel at the severe accident. The lower head, however, didn't receive any damage by reactor core cooling. Heat transfer at outside of the lower head and boiling heat transfer at liquid-liquid interface of molten-metal and water, however, are important for initial cooling process of the molten-core pool. The heat transfer experiments for the liquid-liquid interface of molten-metal and water are carried out over the range of natural convection to film boiling region. Phenomenon on the heat transfer experiments are visualized by using of high speed video camera. Wood's metal and U-alloy 78 are used as molten-metal. The test section of the experiments consists of a copper block with heater, wood's metal, and water. Three thermocouple probes are used for temperature measurement of water side and the molten-metal side. Stability of the liquid-liquid interface is depended on the wetness of container wall for molten metal and the temperature distribution of the interface. Entrainment phenomena of molten-metal occurs by a fluctuation of the interface after boiling on the container wall surface. The boiling curves obtained from the liquid-liquid interface experiments are agree with the nucleate boiling and the film boiling correlations of solid-liquid system. (Suetake, M.)

Analytical solutions of steady-state conjugate heat transfer in ducts with turbulent flow

International Nuclear Information System (INIS)

Cerqueira, Djane R.; Jian Su

2007-01-01

In this work, we present an approximate analytical solution of the steady-state conjugate heat transfer of turbulent forced convection in a circular pipe with wall axial heat conduction and external convective boundary conditions. Improved lumped differential approach based on two points Hermite approximation for integrals was applied to reduce the heat conduction equation in the solid into a second-order ordinary differential equation for the radially averaged solid temperature. The energy equation in the fluid was solved by applying the generalized integral transform technique (GITT). The Sturm-Lioville eigenproblem for fluid energy equation in the cylindrical coordinate system was solved by the sign-count method. The truncated system of N ordinary differential equations for transformed potentials of the fluid temperature and the second-order ordinary differential equation for radially averaged solid temperature formed a homogeneous system of N+2 ordinary differential equations, which was solved analytically. The effects of the fluid-solid thermal conductivity ratio on the Nusselt number, the average fluid and solid temperatures, and the fluid-solid interface temperature were investigated. (author)

Development and implementation of an electronic interface for complex clinical laboratory instruments without a vendor-provided data transfer interface

Directory of Open Access Journals (Sweden)

Gary E Blank

2011-01-01

Full Text Available Background: Clinical pathology laboratories increasingly use complex instruments that incorporate chromatographic separation, e.g. liquid chromatography, with mass detection for rapid identification and quantification of biochemicals, biomolecules, or pharmaceuticals. Electronic data management for these instruments through interfaces with laboratory information systems (LIS is not generally available from the instrument manufacturers or LIS vendors. Unavailability of a data management interface is a limiting factor in the use of these instruments in clinical laboratories where there is a demand for high-throughput assays with turn-around times that meet patient care needs. Materials and Methods: Professional society guidelines for design and transfer of data between instruments and LIS were used in the development and implementation of the interface. File transfer protocols and support utilities were written to facilitate transfer of information between the instruments and the LIS. An interface was created for liquid chromatography-tandem mass spectroscopy and inductively coupled plasma-mass spectroscopy instruments to manage data in the Sunquest® LIS. Results: Interface validation, implementation and data transfer fidelity as well as training of technologists for use of the interface was performed by the LIS group. The technologists were familiarized with the data verification process as a part of the data management protocol. The total time for the technologists for patient/control sample data entry, assay results data transfer, and results verification was reduced from approximately 20 s per sample to <1 s per sample. Sample identification, results data entry errors, and omissions were eliminated. There was electronic record of the technologist performing the assay runs and data management. Conclusions: Development of a data management interface for complex, chromatography instruments in clinical laboratories has resulted in rapid, accurate

Entrainment at a sediment concentration interface in turbulent channel flow

Science.gov (United States)

Salinas, Jorge; Shringarpure, Mrugesh; Cantero, Mariano; Balachandar, S.

2016-11-01

In this work we address the role of turbulence on entrainment at a sediment concentration interface. This process can be conceived as the entrainment of sediment-free fluid into the bottom sediment-laden flow, or alternatively, as the entrainment of sediment into the top sediment-free flow. We have performed direct numerical simulations for fixed Reynolds and Schmidt numbers while varying the values of Richardson number and particle settling velocity. The analysis performed shows that the ability of the flow to pick up a given sediment size decreases with the distance from the bottom, and thus only fine enough sediment particles are entrained across the sediment concentration interface. For these cases, the concentration profiles evolve to a final steady state in good agreement with the well-known Rouse profile. The approach towards the Rouse profile happens through a transient self-similar state. Detailed analysis of the three dimensional structure of the sediment concentration interface shows the mechanisms by which sediment particles are lifted up by tongues of sediment-laden fluid with positive correlation between vertical velocity and sediment concentration. Finally, the mixing ability of the flow is addressed by monitoring the center of mass of the sediment-laden layer. With the support of ExxonMobil, NSF, ANPCyT, CONICET.

Simple Ion Transfer at Liquid|Liquid Interfaces

Directory of Open Access Journals (Sweden)

L. J. Sanchez Vallejo

2012-01-01

Full Text Available The main aspects related to the charge transfer reactions occurring at the interface between two immiscible electrolyte solutions (ITIES are described. The particular topics to be discussed involve simple ion transfer. Focus is given on theoretical approaches, numerical simulations, and experimental methodologies. Concerning the theoretical procedures, different computational simulations related to simple ion transfer are reviewed. The main conclusions drawn from the most accepted models are described and analyzed in regard to their relevance for explaining different aspects of ion transfer. We describe numerical simulations implementing different approaches for solving the differential equations associated with the mass transport and charge transfer. These numerical simulations are correlated with selected experimental results; their usefulness in designing new experiments is summarized. Finally, many practical applications can be envisaged regarding the determination of physicochemical properties, electroanalysis, drug lipophilicity, and phase-transfer catalysis.

Interferometric study of mass transfer enchancement by turbulence promoters

International Nuclear Information System (INIS)

Hanson, K.J.

1979-04-01

The use of small obstacles to thin the downstream mass transfer boundary layer has been investigated with a traveling, dual-beam laser interferometer. Plots of boundary layer thickness as a function of the distance from the leading edge of the electrode were developed to study the effects of obstacle shape, the distance of the obstacle from the electrode surface, and Reynolds number for the purposes of determining the optimum conditions to achieve high mass transfer rates. Parameters which characterize the efficiency of the obstacles, the minimum boundary layer thickness in the wake, and the recovery distance downstream of each obstacle have been introduced to quantitatively describe the results. In addition, the effect of local turbulence near the obstacles on the deposit morphology has been described

CHARACTERIZATION OF CATALYTIC COMBUSTOR TURBULENCE AND ITS INFLUENCE ON VANE AND ENDWALL HEAT TRANSFER AND ENDWALL FILM COOLING

Energy Technology Data Exchange (ETDEWEB)

Forrest E. Ames

2002-10-01

Endwall heat transfer distributions taken in a large-scale low speed linear cascade facility are documented for mock catalytic and dry low NOx (DLN) combustion systems. Inlet turbulence levels range from about 1.0 percent for the mock Catalytic combustor condition to 14 percent for the mock dry low NOx combustor system. Stanton number contours are presented at both turbulence conditions for Reynolds numbers based on true chord length and exit conditions ranging from 500,000 to 2,000,000. Catalytic combustor endwall heat transfer shows the influence of the complex three-dimensional flow field, while the effects of individual vortex systems are less evident for the mock dry low NOx cases. Turbulence scales have been documented for both cases. Inlet boundary layers are relatively thin for the mock catalytic combustor case while inlet flow approximates a channel flow with high turbulence for the mock DLN combustor case. Inlet boundary layer parameters are presented across the inlet passage for the three Reynolds numbers and both the mock catalytic and DLN combustor inlet cases. Both midspan and 95 percent span pressure contours are included. This research provides a well-documented database taken across a range of Reynolds numbers and turbulence conditions for assessment of endwall heat transfer predictive capabilities.

Numerical simulations of heat transfer in an annular fuel channel with three-dimensional spacer ribs set up periodically under a fully developed turbulent flow

International Nuclear Information System (INIS)

Takase, Kazuyuki; Akino, Norio

1996-06-01

Thermal-hydraulic characteristics of an annular fuel channel with spacer ribs for high temperature gas-cooled reactors were analyzed numerically by three-dimensional heat transfer computations under a fully developed turbulent flow. The two-equations κ-ε turbulence model was applied to the present turbulent analysis. In particular, the κ-ε turbulence model constants and the turbulent Prandtl number were improved from the previous standard values proposed by Jones and Launder in order to obtain heat transfer predictions with higher accuracy. Consequently, heat transfer coefficients and friction factors in the spacer-ribbed fuel channel were predicted with sufficient accuracy in the range of Reynolds number exceeding 3000. It was clarified quantitatively from the present study that main mechanism for the heat transfer augmentation in the spacer-ribbed fuel channel was combined effects of the turbulence promoter effect by the spacer ribs and the velocity acceleration effect by a reduction in the channel cross-section. (author)

Voltammetry for the charge transfer at two immiscible electrolyte solutions interface

International Nuclear Information System (INIS)

Kihara, S.; Suzuki, M.; Maeda, K.; Ogura, K.; Matsui, M.; Yoshida, Z.

1989-01-01

The voltammetry for the charge transfer (VCT) at the interface of immicible solutions is a very powerful method for understanding the dynamic features of the charge transfer because of its unmatched advantage that the transfer energy and the number of charges transferred can be measured simultaneously and in situ. In the present paper, several novel systems for electron transfer are outlined, and the following topics are discussed based on results obtained by the current scan polarography at the solution dropping electrode developed as a technique for VCT: the relation between the half-wave potential in VCT for ion transfer and the characteristics of the ion transferred; the relation between the half-wave potential in VCT for electron transfer and the electrochemical nature of a redox couple added in water and that added in organic solution; and the ion transfer through a liquid membrane promoted by electron transfer. Observations are presented and discussion is made on the characteristics of ion transfer polarograms, those of electron transfer polarograms, and ion transfer promoted by electron transfer at a liquid/membrane interface. (N.K.)

Charts Adapted from Van Driest's Turbulent Flat-plate Theory for Determining Values of Turbulent Aerodynamic Friction and Heat-transfer Coefficients

Science.gov (United States)

Lee, Dorothy B; Faget, Maxime A

1956-01-01

A modified method of Van Driest's flat-plate theory for turbulent boundary layer has been found to simplify the calculation of local skin-friction coefficients which, in turn, have made it possible to obtain through Reynolds analogy theoretical turbulent heat-transfer coefficients in the form of Stanton number. A general formula is given and charts are presented from which the modified method can be solved for Mach numbers 1.0 to 12.0, temperature ratios 0.2 to 6.0, and Reynolds numbers 0.2 times 10 to the 6th power to 200 times 10 to the 6th power.

Electromagnetic control of mass transfer at liquid/liquid interfaces; Controle electromagnetique des transferts de masse aux interfaces liquide/liquide

Energy Technology Data Exchange (ETDEWEB)

Saadi, B

2006-04-15

Most metallurgical processes, such as steel refining or nuclear waste processing; the interfaces between two liquid phases are the regions of mass transfer. These transfers require the implementation of a means of stirring to accelerate the kinetics of the pollutants transfer between both phases. This thesis deals with the use of the electromagnetic forces to stir, without any material contact, the bath core and the interface in order to control or even increase the kinetic transfers. To achieve this, two complementary experimental installations were used. The first experiment allows the measurement of the Indium transfer, initially dissolved in mercury towards a covering electrolyte layer and the velocity field in mercury. The performed experiments, determine the topology of the fields flows speeds in the mercury bath, moreover the behaviour of the transfer kinetics versus the intensity of the magnetic field are established. This evolution is correlated with the dynamic behaviour of the mercury surface. The second installation allows the characterization of an element transfer (Pb, Zr or Ce) initially contained in a fluorinated salt towards an antimony matrix containing lithium. It appears that all transfers kinetics are very fast. The proposed experimental set-up is particularly efficient for Cerium transfer (limited by the interface) but does not present any action for Zirconium transfer. (author)

Turbulent transport across an interface between dry and humid air in a stratified environment

Science.gov (United States)

Gallana, Luca; de Santi, Francesca; di Savino, Silvio; Iovieno, Michele; Ricchiardone, Renzo; Tordella, Daniela

2014-11-01

The transport of energy and water vapor across a thin layer which separates two decaying isotropic turbulent flows with different kinetic energy and humidity is considered. The interface is placed in a shearless stratified environment in temporal decay. This system reproduces a few aspects of small scale turbulent transport across a dry air/moist air interface in an atmospheric like context. In our incompressible DNS at Reλ = 250 , Boussinesq's approximation is used for momentum and energy transport while the vapor is modeled as a passive scalar (Kumar, Schumacher & Shaw 2014). We investigated different stratification levels with an initial Fr between 0.8 and 8 in presence of a kinetic energy ratio equal to 7. As the buoyancy term becomes of the same order of the inertial ones, a spatial redistribution of kinetic energy, dissipation and vapor concentration is observed. This eventually leads to the onset of a well of kinetic energy in the low energy side of the mixing layer which blocks the entrainment of dry air. Results are discussed and compared with laboratory and numerical experiments. A posteriori estimates of the eventual compression/expansion of fluid particles inside the interfacial mixing layer are given (Nance & Durran 1994).

Numerical Study of Wavy Film Flow on Vertical Plate Using Different Turbulent Models

Energy Technology Data Exchange (ETDEWEB)

Min, June Kee [Pusan National University, Busan (Korea, Republic of); Park, Il Seouk [Kyungpook National University, Daegu (Korea, Republic of)

2014-05-15

Film flows applied to shell-and-tube heat exchangers in various industrial fields have been studied for a long time. One boundary of the film flow interfaces with a fixed wall, and the other boundary interfaces with a gaseous region. Thus, the flows become so unstable that wavy behaviors are generated on free surfaces as the film Reynolds number increases. First, high-amplitude solitary waves are detected in a low Reynolds number laminar region; then, the waves transit to a low-amplitude, high frequency ripple in a turbulent region. Film thickness is the most significant factor governing heat transfer. Since the wave accompanied in the film flow results in temporal and spatial variations in film thickness, it can be of importance for numerically predicting the film's wavy behavior. In this study, various turbulent models are applied for predicting low-amplitude ripple flows in turbulent regions. The results are compared with existing experimental results, and finally, the applied turbulent models are appraised in from the viewpoint of wavy behaviors.

Numerical Study of Wavy Film Flow on Vertical Plate Using Different Turbulent Models

International Nuclear Information System (INIS)

Min, June Kee; Park, Il Seouk

2014-01-01

Film flows applied to shell-and-tube heat exchangers in various industrial fields have been studied for a long time. One boundary of the film flow interfaces with a fixed wall, and the other boundary interfaces with a gaseous region. Thus, the flows become so unstable that wavy behaviors are generated on free surfaces as the film Reynolds number increases. First, high-amplitude solitary waves are detected in a low Reynolds number laminar region; then, the waves transit to a low-amplitude, high frequency ripple in a turbulent region. Film thickness is the most significant factor governing heat transfer. Since the wave accompanied in the film flow results in temporal and spatial variations in film thickness, it can be of importance for numerically predicting the film's wavy behavior. In this study, various turbulent models are applied for predicting low-amplitude ripple flows in turbulent regions. The results are compared with existing experimental results, and finally, the applied turbulent models are appraised in from the viewpoint of wavy behaviors

Effect of Reynolds number, turbulence level and periodic wake flow on heat transfer on low pressure turbine blades.

Science.gov (United States)

Suslov, D; Schulz, A; Wittig, S

2001-05-01

The development of effective cooling methods is of major importance for the design of new gas turbines blades. The conception of optimal cooling schemes requires a detailed knowledge of the heat transfer processes on the blade's surfaces. The thermal load of turbine blades is predominantly determined by convective heat transfer which is described by the local heat transfer coefficient. Heat transfer is closely related to the boundary layer development along the blade surface and hence depends on various flow conditions and geometrical parameters. Particularly Reynolds number, pressures gradient and turbulence level have great impact on the boundary layer development and the according heat transfer. Therefore, in the present study, the influence of Reynolds number, turbulence intensity, and periodic unsteady inflow on the local heat transfer of a typical low pressure turbine airfoil is experimentally examined in a plane cascade.

Numerical prediction on turbulent heat transfer of a spacer ribbed fuel rod for high temperature gas-cooled reactors

International Nuclear Information System (INIS)

Takase, Kazuyuki

1994-11-01

The turbulent heat transfer of a fuel rod with three-dimensional trapezoidal spacer ribs for high temperature gas-cooled reactors was analyzed numerically using the k-ε turbulence model, and investigated experimentally using a simulated fuel rod under the helium gas condition of a maximum outlet temperature of 1000degC and pressure of 4MPa. From the experimental results, it found that the turbulent heat transfer coefficients of the fuel rod were 18 to 80% higher than those of a concentric smooth annulus at a region of Reynolds number exceeding 2000. On the other hand, the predicted average Nusselt number of the fuel rod agreed well with the heat transfer correlation obtained from the experimental data within a relative error of 10% with Reynolds number of more than 5000. It was verified that the numerical analysis results had sufficient accuracy. Furthermore, the numerical prediction could clarify quantitatively the effects of the heat transfer augmentation by the spacer rib and the axial velocity increase due to a reduction in the annular channel cross-section. (author)

Turbulent transfer characteristics of radioiodine effluents from air to grass

Energy Technology Data Exchange (ETDEWEB)

Markee, E. H. [ARFRO, Environmental Science Services Administration, Idaho Falls, Idaho (United States)

1967-07-01

A total of 20 controlled field releases of radioiodine have been performed at the National Reactor Testing Station in Idaho as a portion of a program to study the transmission of gaseous radioiodine through the air-vegetation-cow-milk-human chain. Most of the releases were conducted over typical pasture grasses during different wind and stability conditions. Radioiodine adherence to grass and carbon plates was measured during most of the tests. Vertical air concentration profiles and turbulence parameters were measured to determine flux characteristics. Analysis of the data reveals the complex interdisciplinary nature of transfer of radioiodine from air to a natural surface. The data are in reasonable agreement with the deposition models of Sheppard and Chamberlain when corrections for the physical and biological receptiveness of the grass and grass density are made. The average ratios of momentum to mass flux were found to be 0.9 in stable conditions and 1.4 in unstable conditions. These ratios demonstrate the effect on mass flux in the lowest 4m by a surface that acts as a partial sink for gaseous effluents. This series of releases indicates the need for further research on the biological receptiveness of grass and turbulent transfer within a grass canopy. (author)

DNS, LES and RANS of turbulent heat transfer in boundary layer with suddenly changing wall thermal conditions

International Nuclear Information System (INIS)

Hattori, Hirofumi; Yamada, Shohei; Tanaka, Masahiro; Houra, Tomoya; Nagano, Yasutaka

2013-01-01

Highlights: • We study the turbulent boundary layer with heat transfer by DNS. • Turbulent boundary layers with suddenly changing wall thermal conditions are observed. • The detailed turbulent statistics and structures in turbulent thermal boundary layer are discussed. • Turbulence models in LES and RANS are evaluated using DNS results. • LES and RANS are almost in good agreement with DNS results. -- Abstract: The objectives of this study are to investigate a thermal field in a turbulent boundary layer with suddenly changing wall thermal conditions by means of direct numerical simulation (DNS), and to evaluate predictions of a turbulence model in such a thermal field, in which DNS of spatially developing boundary layers with heat transfer can be conducted using the generation of turbulent inflow data as a method. In this study, two types of wall thermal condition are investigated using DNS and predicted by large eddy simulation (LES) and Reynolds-averaged Navier–Stokes equation simulation (RANS). In the first case, the velocity boundary layer only develops in the entrance of simulation, and the flat plate is heated from the halfway point, i.e., the adiabatic wall condition is adopted in the entrance, and the entrance region of thermal field in turbulence is simulated. Then, the thermal boundary layer develops along a constant temperature wall followed by adiabatic wall. In the second case, velocity and thermal boundary layers simultaneously develop, and the wall thermal condition is changed from a constant temperature to an adiabatic wall in the downstream region. DNS results clearly show the statistics and structure of turbulent heat transfer in a constant temperature wall followed by an adiabatic wall. In the first case, the entrance region of thermal field in turbulence can be also observed. Thus, both the development and the entrance regions in thermal fields can be explored, and the effects upstream of the thermal field on the adiabatic region are

DNS of turbulent channel flow with conjugate heat transfer at Prandtl number 0.01

Energy Technology Data Exchange (ETDEWEB)

Tiselj, Iztok, E-mail: iztok.tiselj@ijs.si [' Jozef Stefan' Institute, Jamova 39, SI-1000 Ljubljana (Slovenia); Cizelj, Leon, E-mail: leon.cizelj@ijs.si [' Jozef Stefan' Institute, Jamova 39, SI-1000 Ljubljana (Slovenia)

2012-12-15

Highlights: Black-Right-Pointing-Pointer DNS database for turbulent channel flow at Prandtl number 0.01 and various Re{sub {tau}}. Black-Right-Pointing-Pointer Two ideal boundary condition analyzed: non-fluctuating and fluctuating temperature. Black-Right-Pointing-Pointer DNS database with conjugate heat transfer for liquid sodium-steel contact. Black-Right-Pointing-Pointer Penetration of the turbulent temperature fluctuations into the solid wall analyzed. - Abstract: Direct Numerical Simulation (DNS) of the fully developed velocity and temperature fields in a turbulent channel flow coupled with the unsteady conduction in the heated walls was carried out. Simulations were performed with passive scalar approximation at Prandtl number 0.01, which roughly corresponds to the Prandtl number of liquid sodium. DNSs were performed at friction Reynolds numbers 180, 395 and 590. The obtained statistical quantities like mean temperatures, profiles of the root-mean-square (RMS) temperature fluctuations for various thermal properties of wall and fluid, and various wall thicknesses were obtained from a pseudo-spectral channel-flow code. Even for the highest implemented Reynolds number the temperature profile in the fluid does not exhibit log-law region and the near-wall RMS temperature fluctuations show Reynolds number dependence. Conjugate heat transfer simulations of liquid sodium-steel system point to a relatively intensive penetration of turbulent temperature fluctuations into the heated wall. Database containing the results is available in a digital form.

DNS of turbulent channel flow with conjugate heat transfer at Prandtl number 0.01

International Nuclear Information System (INIS)

Tiselj, Iztok; Cizelj, Leon

2012-01-01

Highlights: ► DNS database for turbulent channel flow at Prandtl number 0.01 and various Re τ . ► Two ideal boundary condition analyzed: non-fluctuating and fluctuating temperature. ► DNS database with conjugate heat transfer for liquid sodium–steel contact. ► Penetration of the turbulent temperature fluctuations into the solid wall analyzed. - Abstract: Direct Numerical Simulation (DNS) of the fully developed velocity and temperature fields in a turbulent channel flow coupled with the unsteady conduction in the heated walls was carried out. Simulations were performed with passive scalar approximation at Prandtl number 0.01, which roughly corresponds to the Prandtl number of liquid sodium. DNSs were performed at friction Reynolds numbers 180, 395 and 590. The obtained statistical quantities like mean temperatures, profiles of the root-mean-square (RMS) temperature fluctuations for various thermal properties of wall and fluid, and various wall thicknesses were obtained from a pseudo-spectral channel-flow code. Even for the highest implemented Reynolds number the temperature profile in the fluid does not exhibit log-law region and the near-wall RMS temperature fluctuations show Reynolds number dependence. Conjugate heat transfer simulations of liquid sodium–steel system point to a relatively intensive penetration of turbulent temperature fluctuations into the heated wall. Database containing the results is available in a digital form.

Elementary heat transfer analysis

CERN Document Server

Whitaker, Stephen; Hartnett, James P

1976-01-01

Elementary Heat Transfer Analysis provides information pertinent to the fundamental aspects of the nature of transient heat conduction. This book presents a thorough understanding of the thermal energy equation and its application to boundary layer flows and confined and unconfined turbulent flows. Organized into nine chapters, this book begins with an overview of the use of heat transfer coefficients in formulating the flux condition at phase interface. This text then explains the specification as well as application of flux boundary conditions. Other chapters consider a derivation of the tra

Turbulent flow and heat transfer from a slot jet impinging on a moving plate

International Nuclear Information System (INIS)

Chattopadhyay, Himadri; Saha, Sujoy K.

2003-01-01

The flow field due to an impinging jet over a moving surface at a moderately high Reynolds number, emanating from a rectangular slot nozzle has been computed using the large eddy simulation technique. A dynamic subgrid-scale stress model has been used for the small scales of turbulence. The velocity of the impinging surface perpendicular to the jet velocity has been varied up to two times the jet velocity at the nozzle exit. Turbulence quantities such as kinetic energy, production rate of turbulent kinetic energy and the Reynolds stresses are calculated for different surface velocities. It has been observed that, while the turbulent kinetic energy increases with increasing velocity of the impinging surface, production rate of turbulence initially increases with increasing surface velocity and then comes down. By analyzing the components of turbulent production it was found that P 33 is the dominant term up to the surface velocity of one unit and when the surface velocity is two times the jet velocity at the nozzle exit, the major contribution to turbulence production comes from P 13 and partly from P 11 . Heat transfer from the plate initially increases with non-dimensional surface velocity up to 1.2 and then comes down

Nonconservative and reverse spectral transfer in Hasegawa-Mima turbulence

International Nuclear Information System (INIS)

Terry, P.W.; Newman, D.E.

1993-01-01

The dual cascade is generally represented as a conservative cascade of enstrophy to short wavelengths through an enstrophy similarity range and an inverse cascade of energy to long wavelengths through an energy similarity range. This picture, based on a proof due to Kraichnan [Phys. Fluids 10, 1417 (1967)], is found to be significantly modified for a spectra of finite extent. Dimensional arguments and direct measurement of spectral flow in Hasegawa-Mima turbulence indicate that for both the energy and enstrophy cascades, transfer of the conserved quantity is accompanied by a nonconservative transfer of the other quantity. The decrease of a given invariant (energy or enstrophy) in the nonconservative transfer in one similarity range is balanced by the increase of that quantity in the other similarity range, thus maintaining net invariance. The increase or decrease of a given invariant quantity in one similarity range depends on the injection scale and is consistent with that quantity being carried in a self-similar transfer of the other invariant quantity. This leads, in an inertial range of finite size, to some energy being carried to small scales and some enstrophy being carried to large scales

Numerical study of turbulent heat transfer along a heated rod in an annular cavity

International Nuclear Information System (INIS)

Batta, A.; Class, A.; Daubner, M.; Gnieser, S.; Stieglitz, R.

2008-01-01

Fundamental knowledge on the turbulent convective heat transfer from a rod into liquid metal is of crucial importance for the design of advanced liquid metal operated nuclear systems since a single rod is the basic element of a fuel rod assembly. Therefore, a numerical investigation of the heated rod experiment at KALLA (KArlsruhe Liquid metal LAboratory) has been performed. This experiment investigates the turbulent heat transfer from a heated rod placed concentrically within in a cylindrical tube in a developing flow of a heavy liquid metal (HLM, here Pb 45 Bi 55 Eutectic) at reactor typical power levels and dimensions. It is set up with thermocouples (TCs), a traversable Pitot tube and three thermocouple rakes consisting of numerous thermocouples (TCs). The concentricity is ensured by means of mechanical spacers placed axially equidistant. This article concentrates on the numerical investigation of the impact of the experimental instrumentation on the developing flow pattern and temperature field. In particular, the influence of spacers which distort the velocity profile as well of a potential contact of the spacer with the heated rod changing the heat conduction regime are considered numerically in this paper using the STAR-CD code. The turbulent flow simulation assumes axis-symmetry and uses the SST turbulence model. The simulation results exhibit a flow pattern that is substantially altered by spacers. Hence, the flow can not be considered to be axis-symmetric. This in turn yields that the convective heat transfer from the heated rod towards the spacer region is reduced leading to a temperature rise in spacer region, which represents the maximum value in this domain. As a consequence the entire three-dimensional test section must be modelled in order to correctly represent the physics and to allow an adequate interpretation of the experimental data. (orig.)

CFD modeling of heat transfer performance of MgO-water nanofluid under turbulent flow

OpenAIRE

Davarnejad, Reza; Jamshidzadeh, Maryam

2015-01-01

In this paper, Computational fluid dynamics (CFD) modeling of turbulent heat transfer behavior of Magnesium Oxide-water nanofluid in a circular tube was studied. The modeling was two dimensional under k–ε turbulence model. The base fluid was pure water and the volume fraction of nanoparticles in the base fluid was 0.0625%, 0.125%, 0.25%, 0.5% and 1%. The applied Reynolds number range was 3000–19000. Three individual models including single phase, Volume of Fluid (VOF) and mixture were used. T...

Charge transfer and injection barrier at the metal-organic interfaces

Science.gov (United States)

Yan, Li

2002-09-01

The metal-organic interface plays a critical role in determining the functionality and performance of many innovative organic based devices. It has attracted extensive research interests in recent years. This thesis presents investigations of the electronic structures of organic materials, such as tris-(8-hydroxyquinoline) aluminum (Alq3) and copper phthalocyanine (CuPc), during their interface formation with metals. The characterization is accomplished by X-ray and ultraviolet photoelectron spectroscopes (XPS and UPS) and inverse photoelectron spectroscopy (IPES). As discussed herein, both occupied and unoccupied electronic states at the interfaces are carefully examined in different aspects. In Chapter 4, the charge transfer and chemical reaction at various metal/Alq3 interfaces are investigated using XPS and UPS to study the electron injection into the Alga film. Electron transfer from the low work function metal and Al/LiF(CsF) bilayer to the Alga has been observed. The role of the dielectric and possible chemistry at the interface are discussed in comparison of the low work function metals. Further in Chapter 5, the origin of the metal-interface dipole and the estimation of charge injection barrier is explored using several organic materials. A thermodynamic equilibrium model is extended to explain the relation between the charge transfer process ad the interface dipole. Further, in Chapter 6 the combination of XPS, UPS and IPES detailed the evolution of both occupied and unoccupied energy states during the alkali metal doping. The energy gap modification in organic due to metal doping is observed directly for the spectra. Chapter 7 provides stability study of the organic thin films under x-ray and UV light. The results verify the usability of UPS and XPS for the organic materials used in the thesis. Chapter 7 also shows the secondary ion mass spectroscopy results of metal diffusion in organic thin films.

Large interface simulation in an averaged two-fluid code

International Nuclear Information System (INIS)

Henriques, A.

2006-01-01

Different ranges of size of interfaces and eddies are involved in multiphase flow phenomena. Classical formalisms focus on a specific range of size. This study presents a Large Interface Simulation (LIS) two-fluid compressible formalism taking into account different sizes of interfaces. As in the single-phase Large Eddy Simulation, a filtering process is used to point out Large Interface (LI) simulation and Small interface (SI) modelization. The LI surface tension force is modelled adapting the well-known CSF method. The modelling of SI transfer terms is done calling for classical closure laws of the averaged approach. To simulate accurately LI transfer terms, we develop a LI recognition algorithm based on a dimensionless criterion. The LIS model is applied in a classical averaged two-fluid code. The LI transfer terms modelling and the LI recognition are validated on analytical and experimental tests. A square base basin excited by a horizontal periodic movement is studied with the LIS model. The capability of the model is also shown on the case of the break-up of a bubble in a turbulent liquid flow. The break-up of a large bubble at a grid impact performed regime transition between two different scales of interface from LI to SI and from PI to LI. (author) [fr

Effect of turbulence models on predicting convective heat transfer to hydrocarbon fuel at supercritical pressure

Directory of Open Access Journals (Sweden)

Tao Zhi

2016-10-01

Full Text Available A variety of turbulence models were used to perform numerical simulations of heat transfer for hydrocarbon fuel flowing upward and downward through uniformly heated vertical pipes at supercritical pressure. Inlet temperatures varied from 373 K to 663 K, with heat flux ranging from 300 kW/m2 to 550 kW/m2. Comparative analyses between predicted and experimental results were used to evaluate the ability of turbulence models to respond to variable thermophysical properties of hydrocarbon fuel at supercritical pressure. It was found that the prediction performance of turbulence models is mainly determined by the damping function, which enables them to respond differently to local flow conditions. Although prediction accuracy for experimental results varied from condition to condition, the shear stress transport (SST and launder and sharma models performed better than all other models used in the study. For very small buoyancy-influenced runs, the thermal-induced acceleration due to variations in density lead to the impairment of heat transfer occurring in the vicinity of pseudo-critical points, and heat transfer was enhanced at higher temperatures through the combined action of four thermophysical properties: density, viscosity, thermal conductivity and specific heat. For very large buoyancy-influenced runs, the thermal-induced acceleration effect was over predicted by the LS and AB models.

A unique metal-semiconductor interface and resultant electron transfer phenomenon

OpenAIRE

Taft, S. L.

2012-01-01

An unusual electron transfer phenomenon has been identified from an n-type semiconductor to Schottky metal particles, the result of a unique metal semiconductor interface that results when the metal particles are grown from the semiconductor substrate. The unique interface acts as a one-way (rectifying) open gateway and was first identified in reduced rutile polycrystalline titanium dioxide (an n-type semiconductor) to Group VIII (noble) metal particles. The interface significantly affects th...

Planform structure and heat transfer in turbulent free convection over horizontal surfaces

Science.gov (United States)

Theerthan, S. Ananda; Arakeri, Jaywant H.

2000-04-01

This paper deals with turbulent free convection in a horizontal fluid layer above a heated surface. Experiments have been carried out on a heated surface to obtain and analyze the planform structure and the heat transfer under different conditions. Water is the working fluid and the range of flux Rayleigh numbers (Ra) covered is 3×107-2×1010. The different conditions correspond to Rayleigh-Bénard convection, convection with either the top water surface open to atmosphere or covered with an insulating plate, and with an imposed external flow on the heated boundary. Without the external flow the planform is one of randomly oriented line plumes. At large Rayleigh number Ra and small aspect ratio (AR), these line plumes seem to align along the diagonal, presumably due to a large scale flow. The side views show inclined dyelines, again indicating a large scale flow. When the external flow is imposed, the line plumes clearly align in the direction of external flow. The nondimensional average plume spacing, Raλ1/3, varies between 40 and 90. The heat transfer rate, for all the experiments conducted, represented as RaδT-1/3, where δT is the conduction layer thickness, varies only between 0.1-0.2, showing that in turbulent convection the heat transfer rates are similar under the different conditions.

Experimental analysis for heat transfer of nanofluid with wire coil turbulators in a concentric tube heat exchanger

Science.gov (United States)

Akyürek, Eda Feyza; Geliş, Kadir; Şahin, Bayram; Manay, Eyüphan

2018-06-01

Nanofluids are a novel class of heat transfer suspensions of metallic or nonmetallic nanopowders with a size of less than 100 nm in base fluids and they can increase heat transfer potential of the base fluids in various applications. In the last decade, nanofluids have become an intensive research topic because of their improved thermal properties and possible heat transfer applications. For comparison, an experiment using water as the working fluid in the heat exchanger without wire coils was also performed. Turbulent forced convection heat transfer and pressure drop characteristics of Al2O3-water nanofluids in a concentric tube heat exchanger with and without wire coil turbulators were experimentally investigated in this research. Experiments effected particle volume concentrations of 0.4-0.8 to 1.2-1.6 vol% in the Reynolds number range from 4000 to 20,000. Two turbulators with the pitches of 25 mm and 39 mm were used. The average Nusselt number increased with increasing the Reynolds number and particle concentrations. Moreover, the pressure drop of the Al2O3-water nanofluid showed nearly equal to that of pure water at the same Reynolds number range. As a result, nanofluids with lower particle concentrations did not show an important influence on pressure drop change. Nonetheless, when the wire coils used in the heat exchanger, it increased pressure drop as well as the heat transfer coefficient.

An h-adaptive finite element method for turbulent heat transfer

Energy Technology Data Exchange (ETDEWEB)

Carriington, David B [Los Alamos National Laboratory

2009-01-01

A two-equation turbulence closure model (k-{omega}) using an h-adaptive grid technique and finite element method (FEM) has been developed to simulate low Mach flow and heat transfer. These flows are applicable to many flows in engineering and environmental sciences. Of particular interest in the engineering modeling areas are: combustion, solidification, and heat exchanger design. Flows for indoor air quality modeling and atmospheric pollution transport are typical types of environmental flows modeled with this method. The numerical method is based on a hybrid finite element model using an equal-order projection process. The model includes thermal and species transport, localized mesh refinement (h-adaptive) and Petrov-Galerkin weighting for the stabilizing the advection. This work develops the continuum model of a two-equation turbulence closure method. The fractional step solution method is stated along with the h-adaptive grid method (Carrington and Pepper, 2002). Solutions are presented for 2d flow over a backward-facing step.

About the Role of the Bottleneck/Cork Interface on Oxygen Transfer.

Science.gov (United States)

Lagorce-Tachon, Aurélie; Karbowiak, Thomas; Paulin, Christian; Simon, Jean-Marc; Gougeon, Régis D; Bellat, Jean-Pierre

2016-09-07

The transfer of oxygen through a corked bottleneck was investigated using a manometric technique. First, the effect of cork compression on oxygen transfer was evaluated without considering the glass/cork interface. No significant effect of cork compression (at 23% strain, corresponding to the compression level of cork in a bottleneck for still wines) was noticeable on the effective diffusion coefficient of oxygen. The mean value of the effective diffusion coefficient is equal to 10(-8) m(2) s(-1), with a statistical distribution ranging from 10(-10) to 10(-7) m(2) s(-1), which is of the same order of magnitude as for the non-compressed cork. Then, oxygen transfer through cork compressed in a glass bottleneck was determined to assess the effect of the glass/cork interface. In the particular case of a gradient-imposed diffusion of oxygen through our model corked bottleneck system (dry cork without surface treatment; 200 and ∼0 hPa of oxygen on both sides of the sample), the mean effective diffusion coefficient is of 5 × 10(-7) m(2) s(-1), thus revealing the possible importance of the role of the glass/stopper interface in the oxygen transfer.

Analysis of turbulent natural convection heat transfer in a lower plenum during external cooling using the COSMO code

Energy Technology Data Exchange (ETDEWEB)

Noguchi, H. [Nuclear Power Engineering Corp., Tokyo (Japan); Sawatari, Y.; Imada, T. [Fuji Research Institute Corporation, Tokyo (Japan)

2000-11-01

The behavior of a large volumetrically heated melt pool is important to evaluate the feasibility of in-vessel retention by external flooding as an accident management. The COSMO (Coolability Simulation of Molten corium during severe accident) code has been developed at NUPEC to simulate turbulent natural convection heat transfer with internal heat source. The COSMO code solves thermal hydraulic conservation equations with turbulent model and can simulate melting and solidification process. The standard k-{epsilon} model has a limitation to describe the turbulent natural convection in the very high Rayleigh number condition (10{sup 16}-10{sup 17}) assumed to occur in a lower plenum of RPV during a severe accident. This limitation results from the assumption of an analogy of momentum and energy transfer phenomena in the standard model. In this paper the modified turbulent model in which the turbulent number is treated, as a function of the flux Richardson number derived from the experiment, has been incorporated and verified by using the BALI experiments. It was found that the prediction of averaged Nusselt number became better than that of the standard model. In order to extend the COSMO code to the actual scale analysis under the external flooding conditions, more realistic boundary condition derived from the experiments should be treated. In this work the CHF correlation from ULPU experiment or the heat transfer coefficient correlation from CYBL experiment have been applied. The preliminary analysis of an actual scale analysis has been carried out under the condition of the TMI-2 accident. (author)

Analysis of turbulent natural convection heat transfer in a lower plenum during external cooling using the COSMO code

International Nuclear Information System (INIS)

Noguchi, H.; Sawatari, Y.; Imada, T.

2000-01-01

The behavior of a large volumetrically heated melt pool is important to evaluate the feasibility of in-vessel retention by external flooding as an accident management. The COSMO (Coolability Simulation of Molten corium during severe accident) code has been developed at NUPEC to simulate turbulent natural convection heat transfer with internal heat source. The COSMO code solves thermal hydraulic conservation equations with turbulent model and can simulate melting and solidification process. The standard k-ε model has a limitation to describe the turbulent natural convection in the very high Rayleigh number condition (10 16 -10 17 ) assumed to occur in a lower plenum of RPV during a severe accident. This limitation results from the assumption of an analogy of momentum and energy transfer phenomena in the standard model. In this paper the modified turbulent model in which the turbulent number is treated, as a function of the flux Richardson number derived from the experiment, has been incorporated and verified by using the BALI experiments. It was found that the prediction of averaged Nusselt number became better than that of the standard model. In order to extend the COSMO code to the actual scale analysis under the external flooding conditions, more realistic boundary condition derived from the experiments should be treated. In this work the CHF correlation from ULPU experiment or the heat transfer coefficient correlation from CYBL experiment have been applied. The preliminary analysis of an actual scale analysis has been carried out under the condition of the TMI-2 accident. (author)

Energy transformation, transfer, and release dynamics in high speed turbulent flows

Science.gov (United States)

2017-03-01

Secondly, a new high -order (4 th -order) convective flux formulation was developed that uses the tabulated information, yet produces a fully consistent...Klippenstein 2012 Comprehensive H2/O2 Kinetic Model for High - Pressure Combustion. Int. J. Chem. Kinetics 44:444-474. Cabot, W.H., A.W. Cook, P.L. Miller, D.E...AFRL-AFOSR-VA-TR-2017-0054 Energy Transformation, Transfer, and Release Dynamics in High -Speed Turbulent Flows Paul Dimotakis CALIFORNIA INSTITUTE

Relaxation of an unsteady turbulent boundary layer on a flat plate in an expansion tube

Science.gov (United States)

Gurta, R. N.; Trimpi, R. L.

1974-01-01

An analysis is presented for the relaxation of a turbulent boundary layer on a semi-infinite flat plate after passage of a shock wave and a trailing driver gas-driven gas interface. The problem has special application to expansion-tube flows. The flow-governing equations have been transformed into the Crocco variables, and a time-similar solution is presented in terms of the dimensionless distance-time variable alpha and the dimensionless velocity variable beta. An eddy-viscosity model, similar to that of time-steady boundary layers, is applied to the inner and outer regions of the boundary layer. A turbulent Prandtl number equal to the molecular Prandtl number is used to relate the turbulent heat flux to the eddy viscosity. The numerical results, obtained by using the Gauss-Seidel line-relaxation method, indicate that a fully turbulent boundary layer relaxes faster to the final steady-state values of heat transfer and skin friction than a laminar boundary layer. The results also give a fairly good estimate of the local skin friction and heat transfer for near steady-flow conditions.

CFD modeling of heat transfer performance of MgO-water nanofluid under turbulent flow

Directory of Open Access Journals (Sweden)

Reza Davarnejad

2015-12-01

Full Text Available In this paper, Computational fluid dynamics (CFD modeling of turbulent heat transfer behavior of Magnesium Oxide-water nanofluid in a circular tube was studied. The modeling was two dimensional under k–ε turbulence model. The base fluid was pure water and the volume fraction of nanoparticles in the base fluid was 0.0625%, 0.125%, 0.25%, 0.5% and 1%. The applied Reynolds number range was 3000–19000. Three individual models including single phase, Volume of Fluid (VOF and mixture were used. The results showed that the simulated data were in good agreement with the experimental ones available in the literature. According to the experimental work (literature and simulation (this research, Nusselt number (Nu increased with increasing the volume fraction of nanofluid. However friction factor of nanofluid increased but its effect was ignorable compared with the Nu on heat transfer increment. It was concluded that two phase models were more accurate than the others for heat transfer prediction particularly in the higher volume fractions of nanoparticle. The average deviation from experimental data for single phase model was about 11% whereas it was around 2% for two phase models.

Sodium-ion transfer at the interface between ceramic and organic electrolytes

Energy Technology Data Exchange (ETDEWEB)

Sagane, Fumihiro; Abe, Takeshi; Ogumi, Zempachi [Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510 (Japan)

2010-11-01

Sodium-ion transfer through the interface between ceramic and organic electrolytes was studied by AC impedance spectroscopy. Na{sub 3}Zr{sub 1.88}Y{sub 0.12}Si{sub 2}PO{sub 12} (NASICON) and Na-{beta}''-alumina were used as ceramic electrolytes, and propylene carbonate (PC) and dimethyl sulfoxide (DMSO) containing 0.05 mol dm{sup -3} NaCF{sub 3}SO{sub 3} were used as organic electrolytes. The semi-circle ascribed to interfacial charge transfer resistance (R{sub ct}) was observed. The activation energies for sodium-ion transfer at the interface between ceramic and organic electrolytes were evaluated by the temperature dependency of R{sub ct}. As a result, the activation energies depended on the ceramic electrolytes but not on the solvents. These results suggest that sodium-ion transfer from ceramic to organic electrolytes should be responsible for the activation energies, which is contrary to the case in a lithium-ion transfer system. Based on these results, the mechanism of interfacial sodium-ion transfer was discussed. (author)

Exact relations for energy transfer in self-gravitating isothermal turbulence.

Science.gov (United States)

Banerjee, Supratik; Kritsuk, Alexei G

2017-11-01

Self-gravitating isothermal supersonic turbulence is analyzed in the asymptotic limit of large Reynolds numbers. Based on the inviscid invariance of total energy, an exact relation is derived for homogeneous (not necessarily isotropic) turbulence. A modified definition for the two-point energy correlation functions is used to comply with the requirement of detailed energy equipartition in the acoustic limit. In contrast to the previous relations (S. Galtier and S. Banerjee, Phys. Rev. Lett. 107, 134501 (2011)PRLTAO0031-900710.1103/PhysRevLett.107.134501; S. Banerjee and S. Galtier, Phys. Rev. E 87, 013019 (2013)PLEEE81539-375510.1103/PhysRevE.87.013019), the current exact relation shows that the pressure dilatation terms play practically no role in the energy cascade. Both the flux and source terms are written in terms of two-point differences. Sources enter the relation in a form of mixed second-order structure functions. Unlike the kinetic and thermodynamic potential energies, the gravitational contribution is absent from the flux term. An estimate shows that, for the isotropic case, the correlation between density and gravitational acceleration may play an important role in modifying the energy transfer in self-gravitating turbulence. The exact relation is also written in an alternative form in terms of two-point correlation functions, which is then used to describe scale-by-scale energy budget in spectral space.

Capturing coherent structures and turbulent interfaces in wake flows by means of the Organised Eddy Simulation, OES and by Tomo-PIV

International Nuclear Information System (INIS)

Deri, E; Braza, M; Cazin, S; Cid, E; Harran, G; Ouvrard, H; Hoarau, Y; Hunt, J

2011-01-01

The present study aims at a physical analysis of the coherent and chaotic vortex dynamics in the near wake around a flat plate at incidence, to provide new elements in respect of the flow physics turbulence modelling for high-Reynolds number flows around bodies. This constitutes nowadays a challenge in the aeronautics design. A special attention is paid to capture the thin shear layer interfaces downstream of the separation, responsible for aeroacoustics phenomena related to noise reduction and directly linked to an accurate prediction of the aerodynamic forces. The experimental investigation is carried out by means of tomographic PIV. The interaction of the most energetic coherent structures with the random turbulence is discussed. Furthermore, the POD analysis allowed evaluation of 3D phase averaged dynamics as well as the influence of higher modes associated with the finer-scale turbulence. The numerical study by means of the Organised Eddy Simulation, OES approach ensured a reduced turbulence diffusion that allowed development of the von Karman instability and of capturing of the thin shear-layer interfaces, by using appropriate criteria based on vorticity and dissipation rate of kinetic energy. A comparison between the experiments and the simulations concerning the coherent vortex pattern is carried out.

Large Eddy and Interface Simulation (LEIS) of liquid entrainment in turbulent stratified flow

International Nuclear Information System (INIS)

Gulati, S.; Buongiorno, J.; Lakehal, D.

2011-01-01

Dryout of the liquid film on the fuel rods in BWR fuel assemblies leads to an abrupt decrease in heat transfer coefficient and can result in fuel failure. The process of mechanical mass transfer from the continuous liquid field into the continuous vapor field along the liquid-vapor interface is called entrainment and is the dominant depletion mechanism for the liquid film in annular flow. Using interface tracking methods combined with a Large Eddy Simulation approach, implemented in the Computational Multi-Fluid Dynamics (CMFD) code TransAT®, we are studying entrainment phenomena in BWR fuel assemblies. In this paper we report on the CMFD simulation approaches and the current validation effort for the code. (author)

Transient heat transfer into superfluid helium under confined conditions

International Nuclear Information System (INIS)

Filippov, Yu.P.; Miklyaev, V.M.; Sergeev, I.A.

1988-01-01

Transient thermal processes at solid-HeII interface at input of step pulse of heat load was investigated. Particular attention is given to the study of influence of geometry of experimental specimen upon the heat transfer dynamics. Abrupt breakdown of highly efficient transfer modes caused by the developmet of superfluid turbulence under confined condition is revealed, and accompanying temperature shift is registered. Some characteristic parameters are selected, their dependence on experimental conditions is established

Numerical simulations of heat transfer distribution of a two-pass square channel with V-rib turbulator and bleed holes

Science.gov (United States)

Kumar, Sourabh; Amano, R. S.; Lucci, Jose Martinez

2013-08-01

The blade tip region in gas turbine encounters high thermal loads due to temperature difference and hence efforts for high durability and safe operations are essential. Improved and robust methods of cooling are required to downgrade heat transfer rate to turbine blades. The blade tip regions, which are exposed to high gas flow, suffers high local thermal load which are due to external tip leakage. Jet impingement, pin cooling etc. are techniques used for cooling blades. A more usual way is to use serpentine passage with 180-degree turn. In this study, numerical simulation of heat transfer distribution of a two-pass square channel with rib turbulators and bleed holes were done. Periodical rib turbulators and bleed holes were used in the channel. The ribs arrangement were 60 degree V rib, 60 degree inverted V ribs, combination of 60 degree V rib at inlet and 60 inverted V rib at outlet section and combination of Inverted V at inlet and V rib at the outlet. The results were numerically computed using Fluent with Reynolds number of 12,500 and 28,500. Turbulence models used for computations were k-ω-SST and RSM. Temperature based and shear stress based techniques were used for heat transfer distribution prediction. The results for 60 degree V rib, 60 degree inverted V ribs were compared with the experimental results for validation of the results obtained. Detailed distribution shows distinctive peaks in heat transfer around bleed holes and rib turbulator. Comparisons of the overall performance of the models with different orientation of rib turbulator are presented. It is found that due to the combination of 60 degree inverted V rib in inlet and 60 V rib in outlet with bleed holes provides better heat treatment. It is suggested that the use of rib turbulator with bleed holes provides suitable for augmenting blade cooling to achieve an optimal balance between thermal and mechanical design requirements.

An analytical wall-function for turbulent flows and heat transfer over rough walls

International Nuclear Information System (INIS)

Suga, K.; Craft, T.J.; Iacovides, H.

2006-01-01

This paper reports the development of a refined wall-function strategy for the modelling of turbulent forced convection heat transfer over smooth and rough surfaces. In order to include the effects of fine-grain surface roughness, the present study extends a more fundamental work by Craft et al. [Craft, T.J., Gerasimov, A.V., Iacovides, H., Launder, B.E., 2002. Progress in the generalisation of wall-function treatment. Int. J. Heat Fluid Flow 23, 148-160] on the development of advanced wall-functions of general applicability. The presently proposed model is validated through comparisons with data available for internal flows through channels and for external flows over flat and curved plates with both smooth and rough surfaces. Then, its further validation in separating flows over a sand dune and a sand-roughened ramp is discussed. The validation results suggest that the presently proposed form can be successfully applied to a wide range of attached and separated turbulent flows with heat transfer over smooth and fine-grain rough surfaces

Transfer equations for spectral densities of inhomogeneous MHD turbulence

International Nuclear Information System (INIS)

Tu, C.-Y.; Marsch, E.

1990-01-01

On the basis of the dynamic equations governing the evolution of magnetohydrodynamic fluctuations expressed in terms of Elsaesser variables and of their correlation functions derived by Marsch and Tu, a new set of equations is presented describing the evolutions of the energy spectrum e ± and of the residual energy spectra e R and e S of MHD turbulence in an inhomogeneous magnetofluid. The nonlinearities associated with triple correlations in these equations are analysed in detail and evaluated approximately. The resulting energy-transfer functions across wavenumber space are discussed. For e ± they are shown to be approximately energy-conserving if the gradients of the flow speed and density are weak. New cascading functions are heuristically determined by an appropriate dimensional analysis and plausible physical arguments, following the standard phenomenology of fluid turbulence. However, for e R the triple correlations do not correspond to an 'energy' conserving process, but also represent a nonlinear source term for e R . If this source term can be neglected, the spectrum equations are found to be closed. The problem of dealing with the nonlinear source terms remains to be solved in future investigations. (author)

WebFTS: File Transfer Web Interface for FTS3

CERN Multimedia

CERN. Geneva

2014-01-01

WebFTS is a web-delivered file transfer and management solution which allows users to invoke reliable, managed data transfers on distributed infrastructures. The fully open source solution offers a simple graphical interface through which the power of the FTS3 service can be accessed without the installation of any special grid tools. Created following simplicity and efficiency criteria, WebFTS allows the user to access and interact with multiple grid and cloud storage. The “transfer engine” used is FTS3, the service responsible for distributing the majority of LHC data across WLCG infrastructure. This provides WebFTS with reliable, multi-protocol, adaptively optimised data transfers.The talk will focus on the recent development which allows transfers from/to Dropbox and CERNBox (CERN ownCloud deployment)

Application of the Lion's integral to calculate heat transfer with the N2O4 turbulent flow in a tube

International Nuclear Information System (INIS)

Petrovich, V.Yu.; Tverkovkin, B.E.; Nesterenko, V.B.

1976-01-01

When carrying out engineering calculation of heat transfer in the case of turbulent flow of non-equilibrium reacting gas in a tube, it is necessary to dispose of criterion dependence to calculate Nusselt number. As a rule, dependences obtained by empirical methods are not widely adopted. It is proposed that the integral of Lion type be used for the heat transfer calculation in the form of which an expression for Nusselt number has been written under the conditions of turbulent flow with a non-equilibrium chemical reaction. On calculating turbulent fluctuations Millionshchikov two-layer model is used. A simple approximation of source-discharge of the mass of mixture components is suggested for the sake of simplification of Lion integral. The proposed theoretical dependences for the heat transfer calculation when chemical reactions are available substantially extend the field of application of Lion integral and may be used designing equipment with a chemically reacting coolant

Heat transfer across the interface between nanoscale solids and gas.

Science.gov (United States)

Cheng, Chun; Fan, Wen; Cao, Jinbo; Ryu, Sang-Gil; Ji, Jie; Grigoropoulos, Costas P; Wu, Junqiao

2011-12-27

When solid materials and devices scale down in size, heat transfer from the active region to the gas environment becomes increasingly significant. We show that the heat transfer coefficient across the solid-gas interface behaves very differently when the size of the solid is reduced to the nanoscale, such as that of a single nanowire. Unlike for macroscopic solids, the coefficient is strongly pressure dependent above ∼10 Torr, and at lower pressures it is much higher than predictions of the kinetic gas theory. The heat transfer coefficient was measured between a single, free-standing VO(2) nanowire and surrounding air using laser thermography, where the temperature distribution along the VO(2) nanowire was determined by imaging its domain structure of metal-insulator phase transition. The one-dimensional domain structure along the nanowire results from the balance between heat generation by the focused laser and heat dissipation to the substrate as well as to the surrounding gas, and thus serves as a nanoscale power-meter and thermometer. We quantified the heat loss rate across the nanowire-air interface, and found that it dominates over all other heat dissipation channels for small-diameter nanowires near ambient pressure. As the heat transfer across the solid-gas interface is nearly independent of the chemical identity of the solid, the results reveal a general scaling relationship for gaseous heat dissipation from nanostructures of all solid materials, which is applicable to nanoscale electronic and thermal devices exposed to gaseous environments.

The influence of heat transfer and the variations of the properties of the fluids in turbulent flow in tube

International Nuclear Information System (INIS)

Menon, G.J.; Sielwa, J.T.

1977-01-01

The study is presented of the effects of heat transfer and the variations of the properties of the fluids in turbulent flow in tube. One model for the turbulent Eddy viscosity and termal Eddy diffusivity developed by CEBECI; NA and HABIB was utilized. The theoretical results agree well with experimental results [pt

Molecular theory of mass transfer kinetics and dynamics at gas-water interface

International Nuclear Information System (INIS)

Morita, Akihiro; Garrett, Bruce C

2008-01-01

The mass transfer mechanism across gas-water interface is studied with molecular dynamics (MD) simulation. The MD results provide a robust and qualitatively consistent picture to previous studies about microscopic aspects of mass transfer, including interface structure, free energy profiles for the uptake, scattering dynamics and energy relaxation of impinging molecules. These MD results are quantitatively compared with experimental uptake measurements, and we find that the apparent inconsistency between MD and experiment could be partly resolved by precise decomposition of the observed kinetics into elemental steps. Remaining issues and future perspectives toward constructing a comprehensive multi-scale description of interfacial mass transfer are summarized.

An application of the modified turbulent model for analyzing supercritical heat transfer phenomena in a nuclear system

International Nuclear Information System (INIS)

Seo, Kyoung-Woo; Park, Cheon-Tae; Seo, Jae-Kwang; Kim, Moo-Hwan; Corradini, Michael L.

2007-01-01

For understanding the characteristic of a supercritical fluid heat transfer, we proposed a new parameter, a global Froude number (Fr), dependent on the heat and mass flux, to determine under what conditions the buoyancy effect is dominant and the reduction of the heat transfer rate. In the region of the global Fr>0.01, variable property effects, which may occur at a high heat flux, and buoyancy effects, which could occur at a low mass flux, make the existing standard turbulent model such as the standard wall function not suitably accurate to calculate the heat transfer in supercritical fluid, needed for a reactor thermal-hydraulics simulation and design. Therefore, the turbulence model, especially near the wall, the wall function for a momentum, applicable for a range of supercritical fluid conditions was modified. The modified models deal with a buoyancy, acceleration, and the variable property effect for supercritical conditions

Heat transfer in droplet-laden turbulent channel flow with phase transition in the presence of a thin film of water

NARCIS (Netherlands)

Bukhvostova, A.; Kuerten, J.G.M.; Geurts, B.J.; Grigoriadis, D.G.E.; Geurts, B.J.; Kuerten, H.; Fröhlich, J.; Armenio, V.

2018-01-01

In the field of multiphase systems droplet-laden channel flow presents a challenging topic not only because of how turbulent flow influences the mass and heat transfer properties of droplets but also how droplets modulate the flow. In this contribution we focus on droplet-laden turbulent channel

An eddy-viscosity treatment of the unsteady turbulent boundary layer on a flat plate in an expansion tube

Science.gov (United States)

Gupta, R. N.; Trimpi, R. L.

1974-01-01

An analysis is presented for the relaxation of a turbulent boundary layer on a semiinfinite flat plate after passage of a shock wave and a trailing driver gas-driven gas interface. The problem has special application to expansion tube flows. The flow-governing equations have been transformed into the Lamcrocco variables. The numerical results indicate that a fully turbulent boundary layer relaxes faster to the final steady-state values of heat transfer and skin-friction than a fully laminar boundary layer.

Fluid and mass transfer at subduction interfaces-The field metamorphic record

Science.gov (United States)

Bebout, Gray E.; Penniston-Dorland, Sarah C.

2016-01-01

The interface between subducting oceanic slabs and the hanging wall is a structurally and lithologically complex region. Chemically disparate lithologies (sedimentary, mafic and ultramafic rocks) and mechanical mixtures thereof show heterogeneous deformation. These lithologies are tectonically juxtaposed at mm to km scales, particularly in more intensely sheared regions (mélange zones, which act as fluid channelways). This juxtaposition, commonly in the presence of a mobile fluid phase, offers up huge potential for mass transfer and related metasomatic alteration. Fluids in this setting appear capable of transporting mass over scales of kms, along flow paths with widely varying geometries and P-T trajectories. Current models of arc magmatism require km-scale migration of fluids from the interface into mantle wedge magma source regions and implicit in these models is the transport of any fluids generated in the subducting slab along and ultimately through the subduction interface. Field and geochemical studies of high- and ultrahigh-pressure metamorphic rocks elucidate the sources and compositions of fluids in subduction interfaces and the interplay between deformation and fluid and mass transfer in this region. Recent geophysical studies of the subduction interface - its thickness, mineralogy, density, and H2O content - indicate that its rheology greatly influences the ways in which the subducting plate is coupled with the hanging wall. Field investigation of the magnitude and styles of fluid-rock interaction in metamorphic rocks representing "seismogenic zone" depths (and greater) yields insight regarding the roles of fluids and elevated fluid pore pressure in the weakening of plate interface rocks and the deformation leading to seismic events. From a geochemical perspective, the plate interface contributes to shaping the "slab signature" observed in studies of the composition of arc volcanic rocks. Understanding the production of fluids with hybridized chemical

Fluid mechanics of environmental interfaces

CERN Document Server

Gualtieri, Carlo

2012-01-01

Preface Preface of the first editionBiographies of the authors Part one - Preliminaries1. Environmental fluid mechanics: Current issues and future outlook B. Cushman-Roisin, C. Gualtieri & D.T. MihailovicPart two - Processes at atmospheric interfaces2. Point source atmospheric diffusionB. Rajkovic, I. Arsenic & Z. Grsic3. Air-sea interaction V. Djurdjevic & B. Rajkovic4. Modelling of flux exchanges between heterogeneous surfaces and atmosphere D.T. Mihailovic & D. Kapor5. Desert dust uptake-transport and deposition mechanisms - impacts of dust on radiation, clouds and precipitation G. Kallos, P. Katsafados & C. SpyrouPart three - Processes at water interfaces6. Gas-transfer at unsheared free-surfaces C. Gualtieri & G. Pulci Doria7. Advective diffusion of air bubbles in turbulent water flows H. Chanson8. Exchanges at the bed sediments-water column interface F.A. Bombardelli & P.A. Moreno9. Surface water and streambed sediment interaction: The hyporheic exchange D. Tonina10. Environm...

Atmospheric turbulence effects on the performance of the laser wireless power transfer system

Science.gov (United States)

Kapranov, V. V.; Matsak, I. S.; Tugaenko, V. Yu.; Blank, A. V.; Suhareva, N. A.

2017-02-01

Application of adaptive correction is necessary to control wandering of the laser beam in wireless power transfer (WPT) system. In this paper we describe experimental results of using different adaptive correction techniques for both weak and strong turbulence conditions. All experiments were performed over a 1.5 km near-horizontal atmospheric path. Some criteria for choosing parameters of adaptive correction are given.

Turbulent heat exchange between water and ice at an evolving ice-water interface

Science.gov (United States)

Ramudu, Eshwan; Hirsh, Benjamin Henry; Olson, Peter; Gnanadesikan, Anand

2016-07-01

We conduct laboratory experiments on the time evolution of an ice layer cooled from below and subjected to a turbulent shear flow of warm water from above. Our study is motivated by observations of warm water intrusion into the ocean cavity under Antarctic ice shelves, accelerating the melting of their basal surfaces. The strength of the applied turbulent shear flow in our experiments is represented in terms of its Reynolds number $\\textit{Re}$, which is varied over the range $2.0\\times10^3 \\le \\textit{Re} \\le 1.0\\times10^4$. Depending on the water temperature, partial transient melting of the ice occurs at the lower end of this range of $\\textit{Re}$ and complete transient melting of the ice occurs at the higher end. Following these episodes of transient melting, the ice reforms at a rate that is independent of $\\textit{Re}$. We fit our experimental measurements of ice thickness and temperature to a one-dimensional model for the evolution of the ice thickness in which the turbulent heat transfer is parameterized in terms of the friction velocity of the shear flow. The melting mechanism we investigate in our experiments can easily account for the basal melting rate of Pine Island Glacier ice shelf inferred from observations.

Graphene-ferromagnet interfaces: hybridization, magnetization and charge transfer.

Science.gov (United States)

Abtew, Tesfaye; Shih, Bi-Ching; Banerjee, Sarbajit; Zhang, Peihong

2013-03-07

Electronic and magnetic properties of graphene-ferromagnet interfaces are investigated using first-principles electronic structure methods in which a single layer graphene is adsorbed on Ni(111) and Co(111) surfaces. Due to the symmetry matching and orbital overlap, the hybridization between graphene pπ and Ni (or Co) d(z(2)) states is very strong. This pd hybridization, which is both spin and k dependent, greatly affects the electronic and magnetic properties of the interface, resulting in a significantly reduced (by about 20% for Ni and 10% for Co) local magnetic moment of the top ferromagnetic layer at the interface and an induced spin polarization on the graphene layer. The calculated induced magnetic moment on the graphene layer agrees well with a recent experiment. In addition, a substantial charge transfer across the graphene-ferromagnet interfaces is observed. We also investigate the effects of thickness of the ferromagnet slab on the calculated electronic and magnetic properties of the interface. The strength of the pd hybridization and the thickness-dependent interfacial properties may be exploited to design structures with desirable magnetic and transport properties for spintronic applications.

Proton Transfers at the Air-Water Interface

Science.gov (United States)

Mishra, Himanshu

Proton transfer reactions at the interface of water with hydrophobic media, such as air or lipids, are ubiquitous on our planet. These reactions orchestrate a host of vital phenomena in the environment including, for example, acidification of clouds, enzymatic catalysis, chemistries of aerosol and atmospheric gases, and bioenergetic transduction. Despite their importance, however, quantitative details underlying these interactions have remained unclear. Deeper insight into these interfacial reactions is also required in addressing challenges in green chemistry, improved water quality, self-assembly of materials, the next generation of micro-nanofluidics, adhesives, coatings, catalysts, and electrodes. This thesis describes experimental and theoretical investigation of proton transfer reactions at the air-water interface as a function of hydration gradients, electrochemical potential, and electrostatics. Since emerging insights hold at the lipid-water interface as well, this work is also expected to aid understanding of complex biological phenomena associated with proton migration across membranes. Based on our current understanding, it is known that the physicochemical properties of the gas-phase water are drastically different from those of bulk water. For example, the gas-phase hydronium ion, H3O +(g), can protonate most (non-alkane) organic species, whereas H 3O+(aq) can neutralize only relatively strong bases. Thus, to be able to understand and engineer water-hydrophobe interfaces, it is imperative to investigate this fluctuating region of molecular thickness wherein the 'function' of chemical species transitions from one phase to another via steep gradients in hydration, dielectric constant, and density. Aqueous interfaces are difficult to approach by current experimental techniques because designing experiments to specifically sample interfacial layers (applied quantum mechanics and molecular dynamics to simulate our experiments toward gaining insight at the

Modeling Europa's Ice-Ocean Interface

Science.gov (United States)

Elsenousy, A.; Vance, S.; Bills, B. G.

2014-12-01

This work focuses on modeling the ice-ocean interface on Jupiter's Moon (Europa); mainly from the standpoint of heat and salt transfer relationship with emphasis on the basal ice growth rate and its implications to Europa's tidal response. Modeling the heat and salt flux at Europa's ice/ocean interface is necessary to understand the dynamics of Europa's ocean and its interaction with the upper ice shell as well as the history of active turbulence at this area. To achieve this goal, we used McPhee et al., 2008 parameterizations on Earth's ice/ocean interface that was developed to meet Europa's ocean dynamics. We varied one parameter at a time to test its influence on both; "h" the basal ice growth rate and on "R" the double diffusion tendency strength. The double diffusion tendency "R" was calculated as the ratio between the interface heat exchange coefficient αh to the interface salt exchange coefficient αs. Our preliminary results showed a strong double diffusion tendency R ~200 at Europa's ice-ocean interface for plausible changes in the heat flux due to onset or elimination of a hydrothermal activity, suggesting supercooling and a strong tendency for forming frazil ice.

Numerical study on turbulent heat transfer and pressure drop of nanofluid in coiled tube-in-tube heat exchangers

International Nuclear Information System (INIS)

Aly, Wael I.A.

2014-01-01

Highlights: • The performance of helically coiled tube heat exchanger using nanofluid is modeled. • The 3D turbulent flow and conjugate heat transfer of CTITHE are solved using FVM. • The effects of nanoparticle concentration and curvature ratio are investigated. • The Gnielinski correlation for Nu for turbulent flow in helical tubes can be used for water-based Al 2 O 3 nanofluid. - Abstract: A computational fluid dynamics (CFD) study has been carried out to study the heat transfer and pressure drop characteristics of water-based Al 2 O 3 nanofluid flowing inside coiled tube-in-tube heat exchangers. The 3D realizable k–ε turbulent model with enhanced wall treatment was used. Temperature dependent thermophysical properties of nanofluid and water were used and heat exchangers were analyzed considering conjugate heat transfer from hot fluid in the inner-coiled tube to cold fluid in the annulus region. The overall performance of the tested heat exchangers was assessed based on the thermo-hydrodynamic performance index. Design parameters were in the range of; nanoparticles volume concentrations 0.5%, 1.0% and 2.0%, coil diameters 0.18, 0.24 and 0.30 m, inner tube and annulus sides flow rates from 2 to 5 LPM and 10 to 25 LPM, respectively. Nanofluid flows inside inner tube side or annular side. The results obtained showed a different behavior depending on the parameter selected for the comparison with the base fluid. Moreover, when compared at the same Re or Dn, the heat transfer coefficient increases by increasing the coil diameter and nanoparticles volume concentration. Also, the friction factor increases with the increase in curvature ratio and pressure drop penalty is negligible with increasing the nanoparticles volume concentration. Conventional correlations for predicting average heat transfer and friction factor in turbulent flow regime such as Gnielinski correlation and Mishra and Gupta correlation, respectively, for helical tubes are also valid for

Investigation of the influence of turbulence models on the prediction of heat transfer to low Prandtl number fluids

International Nuclear Information System (INIS)

Thiele, R.; Ma, W.; Anglart, H.

2011-01-01

Despite many advances in computational fluid dynamics (CFD), heat transfer modeling and validation of code for liquid metal flows needs to be improved. This contribution aims to provide validation of several turbulence models implemented in OpenFOAM. 6 different low Reynolds number and 3 high Reynolds number turbulence models have been validated against experimental data for 3 different Reynolds numbers. The results show that most models are able to predict the temperature profile tendencies and that especially the k-ω-SST by Menter has good predictive capabilities. However, all turbulence models show deteriorating capabilities with decreasing Reynolds numbers. (author)

Passive heat transfer in a turbulent channel flow simulation using large eddy simulation based on the lattice Boltzmann method framework

Energy Technology Data Exchange (ETDEWEB)

Wu Hong [National Key Laboratory of Science and Technology on Aero-Engine Aero-Thermodynamics, Beihang University, Beijing 100191 (China); Wang Jiao, E-mail: wangjiao@sjp.buaa.edu.cn [National Key Laboratory of Science and Technology on Aero-Engine Aero-Thermodynamics, Beihang University, Beijing 100191 (China); Tao Zhi [National Key Laboratory of Science and Technology on Aero-Engine Aero-Thermodynamics, Beihang University, Beijing 100191 (China)

2011-12-15

Highlights: Black-Right-Pointing-Pointer A double MRT-LBM is used to study heat transfer in turbulent channel flow. Black-Right-Pointing-Pointer Turbulent Pr is modeled by dynamic subgrid scale model. Black-Right-Pointing-Pointer Temperature gradients are calculated by the non-equilibrium temperature distribution moments. - Abstract: In this paper, a large eddy simulation based on the lattice Boltzmann framework is carried out to simulate the heat transfer in a turbulent channel flow, in which the temperature can be regarded as a passive scalar. A double multiple relaxation time (DMRT) thermal lattice Boltzmann model is employed. While applying DMRT, a multiple relaxation time D3Q19 model is used to simulate the flow field, and a multiple relaxation time D3Q7 model is used to simulate the temperature field. The dynamic subgrid stress model, in which the turbulent eddy viscosity and the turbulent Prandtl number are dynamically computed, is integrated to describe the subgrid effect. Not only the strain rate but also the temperature gradient is calculated locally by the non-equilibrium moments. The Reynolds number based on the shear velocity and channel half height is 180. The molecular Prandtl numbers are set to be 0.025 and 0.71. Statistical quantities, such as the average velocity, average temperature, Reynolds stress, root mean square (RMS) velocity fluctuations, RMS temperature and turbulent heat flux are obtained and compared with the available data. The results demonstrate great reliability of DMRT-LES in studying turbulence.

Turbulent flow in spiral tubes and effect of Prandtl number on a convective heat transfer

International Nuclear Information System (INIS)

Shistel', R.; Goss, Zh.

1976-01-01

Turbulent flow is analized of the fluid in the spiral tube with a pitch which is small enough as compared to the curvature radius. The effect of the curvature and the Prandtl number on the turbulent convection is studied. A description of three-dimensional model and its application for the spiral tubes is given. The example of heat convection in curved channels reveals the opportunity for employment of three-dimensional model to calculate the recirculating flows in complex-geometry channels, description of the turbulence field, and determination of the wall friction and heat transfer. The introduction of the wall functions into the numerical method affects adversely accuracy of calculations but ensures a considerable time saving and makes it possible to study the process in the first approximation. The example illustrates possible practical application of the calculation procedure

Integer Charge Transfer and Hybridization at an Organic Semiconductor/Conductive Oxide Interface

KAUST Repository

Gruenewald, Marco; Schirra, Laura K.; Winget, Paul; Kozlik, Michael; Ndione, Paul F.; Sigdel, Ajaya K.; Berry, Joseph J.; Forker, Roman; Bredas, Jean-Luc; Fritz, Torsten; Monti, Oliver L. A.

2015-01-01

with localized states (the shallow donors) in the substrate and charge back-donation, resulting in an effectively integer charge transfer across the interface. Charge transfer is thus not merely a question of locating the Fermi level above the PTCDA electron

Prediction of free turbulent mixing using a turbulent kinetic energy method

Science.gov (United States)

Harsha, P. T.

1973-01-01

Free turbulent mixing of two-dimensional and axisymmetric one- and two-stream flows is analyzed by a relatively simple turbulent kinetic energy method. This method incorporates a linear relationship between the turbulent shear and the turbulent kinetic energy and an algebraic relationship for the length scale appearing in the turbulent kinetic energy equation. Good results are obtained for a wide variety of flows. The technique is shown to be especially applicable to flows with heat and mass transfer, for which nonunity Prandtl and Schmidt numbers may be assumed.

Interaction of chemical reactions and radiant heat transfer with temperature turbulent pulsations and its effect on heat traner in high-temperature gas flows

International Nuclear Information System (INIS)

Petukhov, B.S.; Zal'tsman, I.G.; Shikov, V.K.

1980-01-01

Methods of taking account of mutual effect of chemical transformations, radiation and turbulence in the calculations of heat transfer in gas flows are considered. Exponential functions of medium parameters are used to describe chemical sources and optical properties of media. It is shown using as an example the dissociation reaction C 2 reversible 2C that the effect of temperature and composition pulsations on recombination rates is negligibly small. It is also shown on the example of turbulent flow of hot molecular gas in a flat channel with cold walls that at moderate temperatures the effect of temperature pulsations on heat radiation flow can be significant (30-40%). The calculational results also show that there is a region in a turbulent boundary layer where the radiation greatly affects the coefficient of turbulent heat transfer

Coherent Structures and Spectral Energy Transfer in Turbulent Plasma: A Space-Filter Approach

Science.gov (United States)

Camporeale, E.; Sorriso-Valvo, L.; Califano, F.; Retinò, A.

2018-03-01

Plasma turbulence at scales of the order of the ion inertial length is mediated by several mechanisms, including linear wave damping, magnetic reconnection, the formation and dissipation of thin current sheets, and stochastic heating. It is now understood that the presence of localized coherent structures enhances the dissipation channels and the kinetic features of the plasma. However, no formal way of quantifying the relationship between scale-to-scale energy transfer and the presence of spatial structures has been presented so far. In the Letter we quantify such a relationship analyzing the results of a two-dimensional high-resolution Hall magnetohydrodynamic simulation. In particular, we employ the technique of space filtering to derive a spectral energy flux term which defines, in any point of the computational domain, the signed flux of spectral energy across a given wave number. The characterization of coherent structures is performed by means of a traditional two-dimensional wavelet transformation. By studying the correlation between the spectral energy flux and the wavelet amplitude, we demonstrate the strong relationship between scale-to-scale transfer and coherent structures. Furthermore, by conditioning one quantity with respect to the other, we are able for the first time to quantify the inhomogeneity of the turbulence cascade induced by topological structures in the magnetic field. Taking into account the low space-filling factor of coherent structures (i.e., they cover a small portion of space), it emerges that 80% of the spectral energy transfer (both in the direct and inverse cascade directions) is localized in about 50% of space, and 50% of the energy transfer is localized in only 25% of space.

An analogy for evaporative heat transfer with wavy/stratified air-water flow in vertical counter-current flow conditions

International Nuclear Information System (INIS)

Kweon, H.; Park, K. C.

2001-01-01

An analogy for evaporative heat transfer with mass transfer was derived. From von-Karman analogy which has been applied between heat and momentum transfer in single phase turbulent flow, a modified Karman analogy was suggested at present paper. Nusselt number from this analogy showed good agreement with experimental results. Such a result shows that the analogy for a complex heat transfer mode between heat transfer and momentum transfer accompanying evaporation or condensation on the interface can be established

Direct and large eddy simulation of turbulent heat transfer at very low Prandtl number: Application to lead–bismuth flows

International Nuclear Information System (INIS)

Bricteux, L.; Duponcheel, M.; Winckelmans, G.; Tiselj, I.; Bartosiewicz, Y.

2012-01-01

Highlights: ► We perform direct and hybrid-large eddy simulations of high Reynolds and low Prandtl turbulent wall-bounded flows with heat transfer. ► We use a state-of-the-art numerical methods with low energy dissipation and low dispersion. ► We use recent multiscalesubgrid scale models. ► Important results concerning the establishment of near wall modeling strategy in RANS are provided. ► The turbulent Prandtl number that is predicted by our simulation is different than that proposed by some correlations of the literature. - Abstract: This paper deals with the issue of modeling convective turbulent heat transfer of a liquid metal with a Prandtl number down to 0.01, which is the order of magnitude of lead–bismuth eutectic in a liquid metal reactor. This work presents a DNS (direct numerical simulation) and a LES (large eddy simulation) of a channel flow at two different Reynolds numbers, and the results are analyzed in the frame of best practice guidelines for RANS (Reynolds averaged Navier–Stokes) computations used in industrial applications. They primarily show that the turbulent Prandtl number concept should be used with care and that even recent proposed correlations may not be sufficient.

Modeling charge transfer at organic donor-acceptor semiconductor interfaces

NARCIS (Netherlands)

Cakir, Deniz; Bokdam, Menno; de Jong, Machiel Pieter; Fahlman, M.; Brocks, G.

2012-01-01

We develop an integer charge transfer model for the potential steps observed at interfaces between donor and acceptor molecular semiconductors. The potential step can be expressed as the difference between the Fermi energy pinning levels of electrons on the acceptor material and holes on the donor

Developing a framework for integrating turbulence measurements and modeling of ecosystem-atmosphere interactions

Science.gov (United States)

Markfort, C. D.

2017-12-01

Aquatic ecosystems are integrators of nutrient and carbon from their watersheds. The effects of climate change in many cases will enhance the rate of these inputs and change the thermodynamics within aquatic environments. It is unclear the extent these changes will have on water quality and carbon assimilation, but the drivers of these processes will be determined by the complex interactions at the land-water and air-water interfaces. For example, flow over and beneath wind-driven surface waves generate turbulence that plays an important role in aquatic ecology and biogeochemistry, exchange of gases such as oxygen and carbon dioxide, and it is important for the transfer of energy and controlling evaporation. Energy transferred from the atmosphere promotes the generation and maintenance of waves. A fraction of the energy is transferred to the surface mixed layer through the generation of turbulence. Energy is also transferred back to the atmosphere by waves. There is a need to quantify the details of the coupled boundary layers of the air-water system to better understand how turbulence plays a role in the interactions. We have developed capabilities to conduct field and laboratory experiments using eddy covariance on tall-towers and rafts, UAS platforms integrated with remote sensing, and detailed wind-wave measurements with time-resolved PIV in a new boundary layer wind-wave tunnel. We will show measurements of the detailed structure of the air and water boundary layers under varying wind and wave conditions in the newly developed IIHR Boundary-Layer Wind-Wave Tunnel. The facility combines a 30-m long recirculating water channel with an open-return boundary layer wind tunnel. A thick turbulent boundary layer is developed in the 1 m high air channel, over the water surface, allowing for the study of boundary layer turbulence interacting with a wind-driven wave field. Results will help interpret remote sensing, energy budget measurements, and turbulence transport

Superfluid turbulence

International Nuclear Information System (INIS)

Donnelly, R.J.

1988-01-01

Most flows of fluids, in nature and in technology, are turbulent. Since much of the energy expended by machines and devices that involve fluid flows is spent in overcoming drag caused by turbulence, there is a strong motivation to understand the phenomena. Surprisingly, the peculiar, quantum-mechanical form of turbulence that can form in superfluid helium may turn out to be much simpler to understand that the classical turbulence that forms in normal fluids. It now seems that the study of superfluid turbulence may provide simplified model systems for studying some forms of classical turbulence. There are also practical motivations for studying superfluid turbulence. For example, superfuid helium is often used as a coolant in superconducting machinery. Superfluid turbulence is the primary impediment to the transfer of heat by superfluid helium; an understanding of the phenomena may make it possible to design more efficient methods of refrigeration for superconducting devices. 8 figs

Molecular Computational Investigation of Electron Transfer Kinetics across Cytochrome-Iron Oxide Interfaces

International Nuclear Information System (INIS)

Kerisit, Sebastien N.; Rosso, Kevin M.; Dupuis, Michel; Valiev, Marat

2007-01-01

The interface between electron transfer proteins such as cytochromes and solid phase mineral oxides is central to the activity of dissimilatory-metal reducing bacteria. A combination of potential-based molecular dynamics simulations and ab initio electronic structure calculations are used in the framework of Marcus' electron transfer theory to compute elementary electron transfer rates from a well-defined cytochrome model, namely the small tetraheme cytochrome (STC) from Shewanella oneidensis, to surfaces of the iron oxide mineral hematite (a-Fe2O3). Room temperature molecular dynamics simulations show that an isolated STC molecule favors surface attachment via direct contact of hemes I and IV at the poles of the elongated axis, with electron transfer distances as small as 9 Angstroms. The cytochrome remains attached to the mineral surface in the presence of water and shows limited surface diffusion at the interface. Ab initio electronic coupling matrix element (VAB) calculations of configurations excised from the molecular dynamics simulations reveal VAB values ranging from 1 to 20 cm-1, consistent with nonadiabaticity. Using these results, together with experimental data on the redox potential of hematite and hemes in relevant cytochromes and calculations of the reorganization energy from cluster models, we estimate the rate of electron transfer across this model interface to range from 1 to 1000 s-1 for the most exothermic driving force considered in this work, and from 0.01 to 20 s-1 for the most endothermic. This fairly large range of electron transfer rates highlights the sensitivity of the rate upon the electronic coupling matrix element, which is in turn dependent on the fluctuations of the heme configuration at the interface. We characterize this dependence using an idealized bis-imidazole heme to compute from first principles the VAB variation due to porphyrin ring orientation, electron transfer distance, and mineral surface termination. The electronic

Catalyst Interface Engineering for Improved 2D Film Lift-Off and Transfer

Science.gov (United States)

2016-01-01

The mechanisms by which chemical vapor deposited (CVD) graphene and hexagonal boron nitride (h-BN) films can be released from a growth catalyst, such as widely used copper (Cu) foil, are systematically explored as a basis for an improved lift-off transfer. We show how intercalation processes allow the local Cu oxidation at the interface followed by selective oxide dissolution, which gently releases the 2D material (2DM) film. Interfacial composition change and selective dissolution can thereby be achieved in a single step or split into two individual process steps. We demonstrate that this method is not only highly versatile but also yields graphene and h-BN films of high quality regarding surface contamination, layer coherence, defects, and electronic properties, without requiring additional post-transfer annealing. We highlight how such transfers rely on targeted corrosion at the catalyst interface and discuss this in context of the wider CVD growth and 2DM transfer literature, thereby fostering an improved general understanding of widely used transfer processes, which is essential to numerous other applications. PMID:27934130

Wave structure and transfer mechanisms at the interface of liquid films (a bibliographic synthesis)

International Nuclear Information System (INIS)

Spindler, Bertrand.

1978-10-01

The flow of a liquid film occurs in many industrial apparatuses. The waves which propagate at the film interface increase the momentum, mass and heat transfer rates of the system. The interface structure is studied; the different patterns of waves with their parameters (shape, amplitude, wavelength, celerity, frequency) and phenomena such as droplet entrainment are examined. An explanation is then given for the increase of transfer rates [fr

Instantaneous structure of a boundary layer subjected to free-stream turbulence

Science.gov (United States)

Hearst, R. Jason; de Silva, Charitha; Dogan, Eda; Ganapathisubramani, Bharathram

2017-11-01

A canonical turbulent boundary layer (TBL) has a distinct turbulent/non-turbulent interface (TNTI) separating the rotational wall-bounded fluid from the irrotational free-stream. If an intermittency profile is constructed separating the flow above and below the TNTI, this profile can be described by an error-function. Within the turbulent region, the flow is separated by interfaces that demarcate uniform momentum zones (UMZs). We observe that these characteristics of a TBL change if there is free-stream turbulence (FST). First, the entire flow is rotational, and thus a distinct TNTI does not exist. Nonetheless, it is possible to identify an interface that approximately separates the flow with mean zero vorticity from the distinctly wall-signed vorticity. This turbulent/turbulent interface is shown to be closer to the wall than the traditional TNTI, and the resulting intermittency profile is not an error-function. Also, UMZs appear to be masked by the free-stream perturbations. Despite these differences, a velocity field of a TBL with homogeneous, isotropic turbulence superimposed and weighted with the empirical intermittency profile, qualitatively reproduces the 1st and 2nd-order statistics. These findings suggest that a TBL subjected to FST may be described by a simple model. EPSRC, ERC, NSERC, Zonta International.

Convective heat transfer characteristics in the turbulent region of molten salt in concentric tube

International Nuclear Information System (INIS)

Chen, Y.S.; Wang, Y.; Zhang, J.H.; Yuan, X.F.; Tian, J.; Tang, Z.F.; Zhu, H.H.; Fu, Y.; Wang, N.X.

2016-01-01

In order to better understand the heat transfer behavior and characteristics of molten salt in heat exchanger, the convective heat transfer characteristics of molten salt in salt-to-oil concentric tube are studied. Overall heat transfer coefficients of the heat exchanger are calculated using Wilson plots. Heat transfer coefficients of tube side molten salt with the range of Reynolds number from 10,000 to 50,000 and the Prandtl number from 11 to 27 are evaluated invoking the calculated overall heat transfer coefficients. The effects of velocity and temperature on the convective heat transfer in the turbulent region of molten salt are studied by comparing with the traditional correlations. The results show that the heat transfer characteristics of molten salt are in line with the empirical heat transfer correlation; however, Dittus–Boelter, Gnielinski, Sieder–Tate and Hausen correlations all give a larger deviation for the experimental data. Finally, based on the experimental data and Sieder–Tate correlation, a modified heat transfer correlation is proposed and good agreement is observed between the experimental data and the modified correlation. The results will also provide an important reference for the design of the heat exchangers in the Thorium-based Molten Salt Reactor.

Heat and mass transfer in turbulent chemically nonequilibrium flow in the tube with boundary second kind conditions. The section with the stabilized heat and mass transfer

International Nuclear Information System (INIS)

Kritsuk, E.L.; Mishina, L.V.; Shegidevich, L.N.

1986-01-01

The hydrodynamically stabilized chemically nonequilibrium turbulent flow in a tube with the inert impermeable surface and constant specific heat flow on the wall is considered. The reversible homogeneous reaction of nitrogen dioxide dissociation 2NO 2 ↔ 2NO+O 2 takes place in the flow. Chemically equilibrium flow with homogeneous profile of temperature and concentration arrives into the channel inlet. After application of simplifying assumptions, the expressions for characteristics of heat and mass transfer have been written down, which are valid in the whole range of the flow parameter variation from frozen up to chemically equilibrium flow. An integral transformation method is suggested for a radial coordinate which allows a wall region to be extended, thereby essentially extending the step of integration. A solution in quadratures has been obtained for the heat and mass transfer problem in an inert fluid flow for the developed process section. The elimination method has been employed to solve the boundary-value second-kind problem for the function governing heat and mass transfer in a chemically nonequilibrium turbulent flow over the developed heat and mass transfer section. The results of calculations are presented

The role of the intense vorticity structures in the turbulent structure of the jet edge

Science.gov (United States)

Reis, Ricardo J. N.; da Silva, Carlos B.; Pereira, José C. F.

In free shear flows (jets, mixing layers and wakes) there is an highly contorted interface dividing the turbulent from the non-turbulent flow: the turbulent/non-turbulent (T/NT) interface. Across this interface important exchanges of mass, momentum and heat take place, in a process known as turbulent entrainment. Recently, the classical idea of the turbulent entrainment caused by engulfing [1] have been questioned, and it has been shown that the entrainment is mainly caused by small scale eddy motions (nibbling) [2, 3]). However, it is still argued that the entrainment rate is still largely governed by the large scale motions induced by the intense vorticity structures (IVS). The goal of the present work is to assess characterize the geometry and analyze the influence of these large scales structures in shaping the turbulent/nonturbulent interface.

Turbulence modeling for mass transfer enhancement by separation and reattachment with two-equation eddy-viscosity models

International Nuclear Information System (INIS)

Xiong Jinbiao; Koshizuka, Seiichi; Sakai, Mikio

2011-01-01

Highlights: → We selected and evaluated five two-equation eddy-viscosity turbulence models for modeling the separated and reattaching flow. → The behavior of the models in the simple flow is not consistent with that in the separated and reattaching flow. → The Abe-Kondoh-Nagano model is the best one among the selected model. → Application of the stress limiter and the Kato-Launder modification in the Abe-Kondoh-Nagano model helps to improve prediction of the peak mass transfer coefficient in the orifice flow. → The value of turbulent Schmidt number is investigated. - Abstract: The prediction of mass transfer rate is one of the key elements for estimation of the flow accelerated corrosion (FAC) rate. Three low Reynolds number (LRN) k-ε models (Lam-Bremhorst (LB), Abe-Kondoh-Nagano (AKN) and Hwang-Lin (HL)), one LRN k-ω (Wilcox, WX) model and the k-ω SST model are tested for the computation of the high Schmidt number mass transfer, especially in the flow through an orifice. The models are tested in the computation of three types of flow: (1) the fully developed pipe flow, (2) the flow over a backward facing step, (3) the flow through an orifice. The HL model shows a good performance in predicting mass transfer in the fully developed pipe flow but fails to give reliable prediction in the flow through an orifice. The WX model and the k-ω SST model underpredict the mass transfer rate in the flow types 1 and 3. The LB model underestimates the mass transfer in the flow type 1, but shows abnormal behavior at the reattaching point in type 3. Synthetically evaluating all the models in all the computed case, the AKN model is the best one; however, the prediction is still not satisfactory. In the evaluation in the flow over a backward facing step shows k-ω SST model shows superior performance. This is interpreted as an implication that the combination of the k-ε model and the stress limiter can improve the model behavior in the recirculation bubble. Both the

Integer charge transfer at the tetrakis(dimethylamino)ethylene/Au interface

DEFF Research Database (Denmark)

Lindell, L.; Unge, Mikael; Osikowicz, W.

2008-01-01

In organic-based electronics, interfacial properties have a profound impact on device performance. The lineup of energy levels is usually dependent on interface dipoles, which may arise from charge transfer reactions. In many applications, metal-organic junctions are prepared under ambient...

Tearing instabilities in turbulence

International Nuclear Information System (INIS)

Ishizawa, A.; Nakajima, N.

2009-01-01

Full text: Effects of micro-turbulence on tearing instabilities are investigated by numerically solving a reduced set of two-fluid equations. Micro-turbulence excites both large-scale and small-scale Fourier modes through energy transfer due to nonlinear mode coupling. The energy transfer to large scale mode does not directly excite tearing instability but it gives an initiation of tearing instability. When tearing instability starts to grow, the excited small scale mode plays an important role. The mixing of magnetic flux by micro-turbulence is the dominant factor of non-ideal MHD effect at the resonant surface and it gives rise to magnetic reconnection which causes tearing instability. Tearing instabilities were investigated against static equilibrium or flowing equilibrium so far. On the other hand, the recent progress of computer power allows us to investigate interactions between turbulence and coherent modes such as tearing instabilities in magnetically confined plasmas by means of direct numerical simulations. In order to investigate effects of turbulence on tearing instabilities we consider a situation that tearing mode is destabilized in a quasi-equilibrium including micro-turbulence. We choose an initial equilibrium that is unstable against kinetic ballooning modes and tearing instabilities. Tearing instabilities are current driven modes and thus they are unstable for large scale Fourier modes. On the other hand kinetic ballooning modes are unstable for poloidal Fourier modes that are characterized by ion Larmor radius. The energy of kinetic ballooning modes spreads over wave number space through nonlinear Fourier mode coupling. We present that micro-turbulence affects tearing instabilities in two different ways by three-dimensional numerical simulation of a reduced set of two-fluid equations. One is caused by energy transfer to large scale modes, the other is caused by energy transfer to small scale modes. The former is the excitation of initial

Atomic layer deposited oxide films as protective interface layers for integrated graphene transfer

Science.gov (United States)

Cabrero-Vilatela, A.; Alexander-Webber, J. A.; Sagade, A. A.; Aria, A. I.; Braeuninger-Weimer, P.; Martin, M.-B.; Weatherup, R. S.; Hofmann, S.

2017-12-01

The transfer of chemical vapour deposited graphene from its parent growth catalyst has become a bottleneck for many of its emerging applications. The sacrificial polymer layers that are typically deposited onto graphene for mechanical support during transfer are challenging to remove completely and hence leave graphene and subsequent device interfaces contaminated. Here, we report on the use of atomic layer deposited (ALD) oxide films as protective interface and support layers during graphene transfer. The method avoids any direct contact of the graphene with polymers and through the use of thicker ALD layers (≥100 nm), polymers can be eliminated from the transfer-process altogether. The ALD film can be kept as a functional device layer, facilitating integrated device manufacturing. We demonstrate back-gated field effect devices based on single-layer graphene transferred with a protective Al2O3 film onto SiO2 that show significantly reduced charge trap and residual carrier densities. We critically discuss the advantages and challenges of processing graphene/ALD bilayer structures.

Prediction of turbulent heat transfer with surface blowing using a non-linear algebraic heat flux model

International Nuclear Information System (INIS)

Bataille, F.; Younis, B.A.; Bellettre, J.; Lallemand, A.

2003-01-01

The paper reports on the prediction of the effects of blowing on the evolution of the thermal and velocity fields in a flat-plate turbulent boundary layer developing over a porous surface. Closure of the time-averaged equations governing the transport of momentum and thermal energy is achieved using a complete Reynolds-stress transport model for the turbulent stresses and a non-linear, algebraic and explicit model for the turbulent heat fluxes. The latter model accounts explicitly for the dependence of the turbulent heat fluxes on the gradients of mean velocity. Results are reported for the case of a heated boundary layer which is first developed into equilibrium over a smooth impervious wall before encountering a porous section through which cooler fluid is continuously injected. Comparisons are made with LDA measurements for an injection rate of 1%. The reduction of the wall shear stress with increase in injection rate is obtained in the calculations, and the computed rates of heat transfer between the hot flow and the wall are found to agree well with the published data

Forced convective melting at an evolving ice-water interface

Science.gov (United States)

Ramudu, Eshwan; Hirsh, Benjamin; Olson, Peter; Gnanadesikan, Anand

2015-11-01

The intrusion of warm Circumpolar Deep Water into the ocean cavity between the base of ice shelves and the sea bed in Antarctica causes melting at the ice shelves' basal surface, producing a turbulent melt plume. We conduct a series of laboratory experiments to investigate how the presence of forced convection (turbulent mixing) changes the delivery of heat to the ice-water interface. We also develop a theoretical model for the heat balance of the system that can be used to predict the change in ice thickness with time. In cases of turbulent mixing, the heat balance includes a term for turbulent heat transfer that depends on the friction velocity and an empirical coefficient. We obtain a new value for this coefficient by comparing the modeled ice thickness against measurements from a set of nine experiments covering one order of magnitude of Reynolds numbers. Our results are consistent with the altimetry-inferred melting rate under Antarctic ice shelves and can be used in climate models to predict their disintegration. This work was supported by NSF grant EAR-110371.

Electrosynthesis of Copper-Tetracyanoquinodimethane Based on the Coupling Charge Transfer across Water/1,2-Dichloroethane Interface

International Nuclear Information System (INIS)

Huang, Li; Li, Pei; Pamphile, Ndagijimana; Tian, Zhong-Qun; Zhan, Dongping

2014-01-01

Graphical abstract: - Highlights: • Organic semiconductor CuTCNQ is synthesized through electrochemistry of liquid/liquid interface. • A coupling charge transfer (CCT) mechanism is proposed for organic electrosynthesis. • The obtained CuTCNQ has good electrochemical and electronic properties. - Abstract: The organic salt Copper-Tetracyanoquinodimethane (CuTCNQ) is an important semiconductor used in electronics for field-effect transistors, switches and memory devices. Here we present a novel electrosynthetic method of CuTCNQ microneedles based on the coupling charge transfer across water/1,2-dichloroethane (W/1,2-DCE) interface. A HOPG electrode is covered by a small volume of 1,2-DCE solution, which is further covered by an aqueous solution to construct the W/1,2-DCE interface. When TCNQ in 1,2-DCE phase is reduced on HOPG, Cu 2+ in the aqueous solution will transfer across the W/1,2-DCE interface in order to maintain the electric neutrality. Therein CuTCNQ microneedles are formed which have good solid-state electrochemical and electronic properties. This coupling charge transfer mechanism is valuable and broadens the applications of liquid/liquid interface in organic electrosynthesis

Power transfer capability assessment of transmission interfaces with SVC and load shedding systems

Energy Technology Data Exchange (ETDEWEB)

Pavlovsky, V. [DMCC-Engineering, Kiev (Ukraine). Inst. of Electrodynamics; Dolzhenitsa, Y. [DMCC Engineering, Kiev (Ukraine); Ushapovskiy, K. [National Power Co. Ukrenergo, Kiev (Ukraine)

2009-07-01

As a result of deregulation in the power industry, energy trade and markets are pushing transmission system operators to operate their systems closer to the edge of the power transfer capability. Voltage instability and inadequate reactive power support of generators is a key factor in most major outages around the world. The ideal way to control power systems is to avoid emergencies by reliable planning and secure operation of power systems. Therefore, the accurate calculation of the power transfer capability of transmission interfaces is an important task on the planning and operation stages. This paper discussed the issue of transfer capability assessment and monitoring for interfaces with static var compensator (SVC) and load shedding schemes. It also proposed a special measure, a distance to voltage instability point, to monitor transfer capability on-line. The distance may be observed by measurement of SVC output. The paper considered the problem of optimal SVC size selection and a new approach was proposed based on P-V curves analysis. The paper discussed the problem formulation and proposed approach. A case was also presented in order to demonstrate the proposed approach on the IPS Ukraine-Crimea interface. It was concluded that the proposed approach allows the optimal rating of SVC for increasing transfers capability of transmission corridors. 12 refs., 9 figs.

Unsteady coupling of Navier-Stokes and radiative heat transfer solvers applied to an anisothermal multicomponent turbulent channel flow

International Nuclear Information System (INIS)

Amaya, J.; Cabrit, O.; Poitou, D.; Cuenot, B.; El Hafi, M.

2010-01-01

Direct numerical simulations (DNS) of an anisothermal reacting turbulent channel flow with and without radiative source terms have been performed to study the influence of the radiative heat transfer on the optically non-homogeneous boundary layer structure. A methodology for the study of the emitting/absorbing turbulent boundary layer (TBL) is presented. Details on the coupling strategy and the parallelization techniques are exposed. An analysis of the first order statistics is then carried out. It is shown that, in the studied configuration, the global structure of the thermal boundary layer is not significantly modified by radiation. However, the radiative transfer mechanism is not negligible and contributes to the heat losses at the walls. The classical law-of-the-wall for temperature can thus be improved for RANS/LES simulations taking into account the radiative contribution.

Density functional theory for the description of charge-transfer processes at TTF/TCNQ interfaces

KAUST Repository

Van Regemorter, Tanguy; Guillaume, Maxime; Sini, Gjergji; Sears, John S.; Geskin, Victor; Bré das, Jean-Luc; Beljonne, David; Cornil, Jé rô me

2012-01-01

In the field of organic electronics, a central issue is to assess how the frontier electronic levels of two adjacent organic layers align with respect to one another at the interface. This alignment can be driven by the presence of a partial charge transfer and the formation of an interface dipole; it plays a key role for instance in determining the rates of exciton dissociation or exciton formation in organic solar cells or light-emitting diodes, respectively. Reliably modeling the processes taking place at these interfaces remains a challenge for the computational chemistry community. Here, we review our recent theoretical work on the influence of the choice of density functional theory (DFT) methodology on the description of the charge-transfer character in the ground state of TTF/ TCNQ model complexes and interfaces. Starting with the electronic properties of the isolated TTF and TCNQ molecules and then considering the charge transfer and resulting interface dipole in TTF/TCNQ donor-acceptor stacks and bilayers, we examine the impact of the choice of DFT functional in describing the interfacial electronic structure. Finally, we employ computations based on periodic boundary conditions to highlight the impact of depolarization effects on the interfacial dipole moment. © Springer-Verlag 2012.

Density functional theory for the description of charge-transfer processes at TTF/TCNQ interfaces

KAUST Repository

Van Regemorter, Tanguy

2012-09-15

In the field of organic electronics, a central issue is to assess how the frontier electronic levels of two adjacent organic layers align with respect to one another at the interface. This alignment can be driven by the presence of a partial charge transfer and the formation of an interface dipole; it plays a key role for instance in determining the rates of exciton dissociation or exciton formation in organic solar cells or light-emitting diodes, respectively. Reliably modeling the processes taking place at these interfaces remains a challenge for the computational chemistry community. Here, we review our recent theoretical work on the influence of the choice of density functional theory (DFT) methodology on the description of the charge-transfer character in the ground state of TTF/ TCNQ model complexes and interfaces. Starting with the electronic properties of the isolated TTF and TCNQ molecules and then considering the charge transfer and resulting interface dipole in TTF/TCNQ donor-acceptor stacks and bilayers, we examine the impact of the choice of DFT functional in describing the interfacial electronic structure. Finally, we employ computations based on periodic boundary conditions to highlight the impact of depolarization effects on the interfacial dipole moment. © Springer-Verlag 2012.

Turbulence model for melt pool natural convection heat transfer

International Nuclear Information System (INIS)

Kelkar, K.M.; Patankar, S.V.

1994-01-01

Under severe reactor accident scenarios, pools of molten core material may form in the reactor core or in the hemispherically shaped lower plenum of the reactor vessel. Such molten pools are internally heated due to the radioactive decay heat that gives rise to buoyant flows in the molten pool. The flow in such pools is strongly influenced by the turbulent mixing because the expected Rayleigh numbers under accidents scenarios are very high. The variation of the local heat flux over the boundaries of the molten pools are important in determining the subsequent melt progression behavior. This study reports results of an ongoing effort towards providing a well validated mathematical model for the prediction of buoyant flow and heat transfer in internally heated pool under conditions expected in severe accident scenarios

Turbulence convective heat transfer for cooling the photovoltaic cells

Science.gov (United States)

Arianmehr, Iman

Solar PV (photovoltaic) is a rapidly advancing renewable energy technology which converts sunlight directly into electricity. One of the outstanding challenges of the current PV technology is the reduction in its conversion efficiency with increasing PV panel temperature, which is closely associated with the increase in solar intensity and the ambient temperature surrounding the PV panels. To more effectively capture the available energy when the sun is most intense, significant efforts have been invested in active and passive cooling research over the last few years. While integrated cooling systems can lead to the highest total efficiencies, they are usually neither the most feasible nor the most cost effective solutions. This work examines some simple passive means of manipulating the prevailing wind turbulence to enhance convective heat transfer over a heated plate in a wind tunnel.

Multiple mechanisms generate a universal scaling with dissipation for the air-water gas transfer velocity

Science.gov (United States)

Katul, Gabriel; Liu, Heping

2017-02-01

A large corpus of field and laboratory experiments support the finding that the water side transfer velocity kL of sparingly soluble gases near air-water interfaces scales as kL˜(νɛ)1/4, where ν is the kinematic water viscosity and ɛ is the mean turbulent kinetic energy dissipation rate. Originally predicted from surface renewal theory, this scaling appears to hold for marine and coastal systems and across many environmental conditions. It is shown that multiple approaches to representing the effects of turbulence on kL lead to this expression when the Kolmogorov microscale is assumed to be the most efficient transporting eddy near the interface. The approaches considered range from simplified surface renewal schemes with distinct models for renewal durations, scaling and dimensional considerations, and a new structure function approach derived using analogies between scalar and momentum transfer. The work offers a new perspective as to why the aforementioned 1/4 scaling is robust.

Entropy Generation Analysis and Performance Evaluation of Turbulent Forced Convective Heat Transfer to Nanofluids

Directory of Open Access Journals (Sweden)

Yu Ji

2017-03-01

Full Text Available The entropy generation analysis of fully turbulent convective heat transfer to nanofluids in a circular tube is investigated numerically using the Reynolds Averaged Navier–Stokes (RANS model. The nanofluids with particle concentration of 0%, 1%, 2%, 4% and 6% are treated as single phases of effective properties. The uniform heat flux is enforced at the tube wall. To confirm the validity of the numerical approach, the results have been compared with empirical correlations and analytical formula. The self-similarity profiles of local entropy generation are also studied, in which the peak values of entropy generation by direct dissipation, turbulent dissipation, mean temperature gradients and fluctuating temperature gradients for different Reynolds number as well as different particle concentration are observed. In addition, the effects of Reynolds number, volume fraction of nanoparticles and heat flux on total entropy generation and Bejan number are discussed. In the results, the intersection points of total entropy generation for water and four nanofluids are observed, when the entropy generation decrease before the intersection and increase after the intersection as the particle concentration increases. Finally, by definition of Ep, which combines the first law and second law of thermodynamics and attributed to evaluate the real performance of heat transfer processes, the optimal Reynolds number Reop corresponding to the best performance and the advisable Reynolds number Read providing the appropriate Reynolds number range for nanofluids in convective heat transfer can be determined.

User Interface Technology Transfer to NASA's Virtual Wind Tunnel System

Science.gov (United States)

vanDam, Andries

1998-01-01

Funded by NASA grants for four years, the Brown Computer Graphics Group has developed novel 3D user interfaces for desktop and immersive scientific visualization applications. This past grant period supported the design and development of a software library, the 3D Widget Library, which supports the construction and run-time management of 3D widgets. The 3D Widget Library is a mechanism for transferring user interface technology from the Brown Graphics Group to the Virtual Wind Tunnel system at NASA Ames as well as the public domain.

Turbulence Model

DEFF Research Database (Denmark)

Nielsen, Mogens Peter; Shui, Wan; Johansson, Jens

2011-01-01

term with stresses depending linearly on the strain rates. This term takes into account the transfer of linear momentum from one part of the fluid to another. Besides there is another term, which takes into account the transfer of angular momentum. Thus the model implies a new definition of turbulence...

Turbulent Heat Transfer of a Finned Plate in a Duct as Tip Clearance Changes

Energy Technology Data Exchange (ETDEWEB)

Park, Hae-Kyun; Chung, Bum-Jin [Kyung Hee University, Yongin (Korea, Republic of)

2015-05-15

Fins are employed to enhance the cooling performance of a system. There are a number of studies relevant to geometry of fins. Meanwhile, the studies relevant to tip clearance, have not performed enough, which is the distance between the tips of the fins and the wall. We investigated the optimal tip clearance, which maximizes the heat transfers by experimental and numerical analyses with wider range of Re{sub Dh} than the previous studies. Turbulent heat transfers of a finned plate were measured. For an extended range of tip clearance and Re{sub Dh} than other studies. A joint experimental and numerical analyses was performed to measure heat transfers. Mass transfer experiments using electroplating system was used and FLUENT 6.3.26 was used for the calculation. For the narrow tip clearances below 5 mm, were investigated by numerical method only. The bypass flow to the tip clearance region contributes to heat transfer area at the tip clearance region and does not contributes that of the fin region. Thus, the optimal tip clearance was founded and it exists vicinity of 0.2 mm.

Charge Transfer and Catalysis at the Metal Support Interface

Energy Technology Data Exchange (ETDEWEB)

Baker, Lawrence Robert [Univ. of California, Berkeley, CA (United States)

2012-07-31

Kinetic, electronic, and spectroscopic characterization of model Pt–support systems are used to demonstrate the relationship between charge transfer and catalytic activity and selectivity. The results show that charge flow controls the activity and selectivity of supported metal catalysts. This dissertation builds on extensive existing knowledge of metal–support interactions in heterogeneous catalysis. The results show the prominent role of charge transfer at catalytic interfaces to determine catalytic activity and selectivity. Further, this research demonstrates the possibility of selectively driving catalytic chemistry by controlling charge flow and presents solid-state devices and doped supports as novel methods for obtaining electronic control over catalytic reaction kinetics.

Design of operator interfaces for ''bumpless'' transfers between operator behaviors

International Nuclear Information System (INIS)

Lindsay, R.W.; Brown-VanHoozer, S.A.

1993-01-01

Advances in the science and art of man-machine interface design have taken major strides forward for interface design practitioners with the advent of the computer. one concern still extant, however, is the need for design of interfaces that minimize confusion when an operator is required to shift from the different levels of cognitive control of skill, rule, and knowledge-based behaviors, (e.g., if an operator is following a set of procedures and a procedural error is noted by the operator, the behavior may, of necessity, shift from rule-based to a knowledge-based behavior). Shifting of the cognitive control levels requires that the information to be displayed to the operator should be designed so that a ''bumpless'' transfer can be made between the behavioral modes, thus reducing the possibility of error. This paper introduces a way to design human interfaces so that skill, rule, and knowledge-based behaviors are supported and provides for the necessary interchanges between behavioral types

Interface charge transfer process in ZnO:Mn/ZnS nanocomposites

Energy Technology Data Exchange (ETDEWEB)

Stefan, M.; Toloman, D., E-mail: dana.toloman@itim-cj.ro; Popa, A. [National Institute for R & D of Isotopic and Molecular Technology (Romania); Mesaros, A. [Technical University of Cluj-Napoca, Superconductivity, Spintronics and Surface Science Center – C4S (Romania); Vasile, O. R. [University “Politehnica” from Bucharest, Faculty of Applied Chemistry and Material Science (Romania); Leostean, C.; Pana, O. [National Institute for R & D of Isotopic and Molecular Technology (Romania)

2016-03-15

ZnO:Mn/ZnS nanocomposites were prepared by seed-mediated growth of ZnS QDs onto the preformed ZnO:Mn nanoparticles. The formation of the nanocomposite structure has been evidenced by XRD, HRTEM, and XPS. The architecture of the nanocomposite with outer ZnS QDs around ZnO:Mn cores is sustained by the sulfur and oxygen depth profiles resulted from XPS. When the two components are brought together, the band gap of ZnS component decreases while that of ZnO:Mn increases. It is the result of interface charge transfer from ZnO:Mn to ZnS QDs. Here ZnO:Mn valence states are extended through the interface into unoccupied gap states of ZnS. The energy band setup is modified from a type II into a type I band alignment. The process is accompanied by enhancement of composite UV emission of PL spectra as compared to its counterparts. The charge transfer from valence band also determines the increase of the core-polarization effect of sshell electrons at Mn{sup 2+} nucleus, thus determining the increase of the hyperfine field through the reduction of the covalency degree of Zn(Mn)–O bonds. The quantum confinement in ZnS QDs promotes the ferromagnetic coupling of singly occupied states due to Zn vacancies determining a superparamagnetic behavior of the ensemble. When the nanocomposites are formed, due to interface charge transfer effects, an increased number of filled cation vacancies in ZnS QDs develop, thus disrupting the pre-existing ferromagnetic coupling between spins resulting in a significant reduction of the overall saturation magnetization. The possibility to modulate nanocomposite properties by controlling the interface interactions may be foreseen in these types of materials.

Linear flow dynamics near a T/NT interface

Science.gov (United States)

Teixeira, Miguel; Silva, Carlos

2011-11-01

The characteristics of a suddenly-inserted T/NT interface separating a homogeneous and isotropic shear-free turbulence region from a non-turbulent flow region are investigated using rapid distortion theory (RDT), taking full account of viscous effects. Profiles of the velocity variances, TKE, viscous dissipation rate, turbulence length scales, and pressure statistics are derived, showing very good agreement with DNS. The normalized inviscid flow statistics at the T/NT interface do not depend on the form of the assumed TKE spectrum. In the non-turbulent region, where the flow is irrotational (except within a thin viscous boundary layer), the dissipation rate decays as z-6, where z is distance from the T/NT interface. The mean pressure exhibits a decrease towards the turbulence due to the associated velocity fluctuations, consistent with the generation of a mean entrainment velocity. The vorticity variance and dissipation rate display large maxima at the T/NT interface due to the existing inviscid discontinuities of the tangential velocity, and these maxima are quantitatively related to the thickness of the viscous boundary layer (VBL). At equilibrium, RDT suggests that the thickness of the T/NT interface scales on the Kolmogorov microscale. We acknowledge the financial support of FCT under Project PTDC/EME-MFE/099636/2008.

Heat transfer through turbulent boundary layers - The effects of introduction of and recovery from convex curvature

Science.gov (United States)

Simon, T. W.; Moffat, R. J.

1979-01-01

Measurements have been made of the heat transfer through a turbulent boundary layer on a convexly curved isothermal wall and on a flat plate following the curved section. Data were taken for one free-stream velocity and two different ratios of boundary layer thickness to radius of curvature delta/R = 0.051 and delta/R = 0.077. Only small differences were observed in the distribution of heat transfer rates for the two boundary layer thicknesses tested, although differences were noted in the temperature distributions within the boundary layer

Statistics of surface divergence and their relation to air-water gas transfer velocity

Science.gov (United States)

Asher, William E.; Liang, Hanzhuang; Zappa, Christopher J.; Loewen, Mark R.; Mukto, Moniz A.; Litchendorf, Trina M.; Jessup, Andrew T.

2012-05-01

Air-sea gas fluxes are generally defined in terms of the air/water concentration difference of the gas and the gas transfer velocity,kL. Because it is difficult to measure kLin the ocean, it is often parameterized using more easily measured physical properties. Surface divergence theory suggests that infrared (IR) images of the water surface, which contain information concerning the movement of water very near the air-water interface, might be used to estimatekL. Therefore, a series of experiments testing whether IR imagery could provide a convenient means for estimating the surface divergence applicable to air-sea exchange were conducted in a synthetic jet array tank embedded in a wind tunnel. Gas transfer velocities were measured as a function of wind stress and mechanically generated turbulence; laser-induced fluorescence was used to measure the concentration of carbon dioxide in the top 300 μm of the water surface; IR imagery was used to measure the spatial and temporal distribution of the aqueous skin temperature; and particle image velocimetry was used to measure turbulence at a depth of 1 cm below the air-water interface. It is shown that an estimate of the surface divergence for both wind-shear driven turbulence and mechanically generated turbulence can be derived from the surface skin temperature. The estimates derived from the IR images are compared to velocity field divergences measured by the PIV and to independent estimates of the divergence made using the laser-induced fluorescence data. Divergence is shown to scale withkLvalues measured using gaseous tracers as predicted by conceptual models for both wind-driven and mechanically generated turbulence.

Integer Charge Transfer and Hybridization at an Organic Semiconductor/Conductive Oxide Interface

KAUST Repository

Gruenewald, Marco

2015-02-11

We investigate the prototypical hybrid interface formed between PTCDA and conductive n-doped ZnO films by means of complementary optical and electronic spectroscopic techniques. We demonstrate that shallow donors in the vicinity of the ZnO surface cause an integer charge transfer to PTCDA, which is clearly restricted to the first monolayer. By means of DFT calculations, we show that the experimental signatures of the anionic PTCDA species can be understood in terms of strong hybridization with localized states (the shallow donors) in the substrate and charge back-donation, resulting in an effectively integer charge transfer across the interface. Charge transfer is thus not merely a question of locating the Fermi level above the PTCDA electron-transport level but requires rather an atomistic understanding of the interfacial interactions. The study reveals that defect sites and dopants can have a significant influence on the specifics of interfacial coupling and thus on carrier injection or extraction.

Turbulent heat transfer in a coolant channel of a pressurized water reactor (PWR) core

International Nuclear Information System (INIS)

Kumar, Sanjeev; Saha, Arun K.; Munshi, Prabhat

2016-01-01

Exact predictions in nuclear reactors are more crucial, because of the safety aspects. It necessitates the appropriate modeling of heat transfer phenomena in the reactors core. A two-dimensional thermal-hydraulics model is used to study the detailed analysis of the coolant region of a fuel pin. Governing equations are solved using Marker and Cell (MAC) method. Standard wall functions k-ε turbulence model is incorporated to consider the turbulent behaviour of the flow field. Validation of the code and a few results for a typical PWR running at normal operating conditions reported earlier. There were some discrepancies in the old calculations. These discrepancies have been resolved and updated results are presented in this work. 2D thermal-hydraulics model results have been compared with the 1D thermal-hydraulics model results and conclusions have been drawn. (author)

Heat transfer by liquids in suspension in a turbulent gas stream (1960); Transfert de chaleur par liquides entraines dans un ecoulement gazeux turbulent (1960)

Energy Technology Data Exchange (ETDEWEB)

Grison, E [Commissariat a l' Energie Atomique, Lab. de Physique-Chimie et basses temperatures, Grenoble (France).Centre d' Etudes Nucleaires; Commissariat a l' Energie Atomique, Saclay (France).Centre d' Etudes Nucleaires

1960-07-01

The introduction of a small volume of liquid into a turbulent gas stream used as cooling agent improves considerably the heat transfer coefficient of the gas. When the turbulent regime is established, one observes in a cylindrical tube two types of flow whether the liquid wets or does not wet the wall. In the first case, one gets on the wall an annular liquid film and droplets in suspension are in the gas stream. In the second case, a fog of droplets is formed without any liquid film on the wall. Experiments were performed with the following mixtures: water-hydrogen, water-nitrogen, ethanol-nitrogen (wetting liquids) introduced into a stainless steel tube of 4 mm ID, electrically heated on 320 mm of length. We varied the gas flow rate (Reynolds until 50000), the rate of the liquid flow rate to gas flow rate (until 15), the pressure (until 10 kg/cm{sup 2}), the temperature (until the boiling point) and the heat flux (until 250 W/cm{sup 2}). Two types of burnout were observed. A formula of correlation of the burnout heat flux is given. Making use of the analogy between mass transfer and heat transfer, a dimensionless formula of correlation of the local heat transfer coefficients is established. (author) [French] L'introduction d'un faible volume de liquide dans un ecoulement gazeux turbulent utilise comme fluide refrigerant permet une amelioration considerable des coefficients d'echanges thermiques que l'on aurait si le gaz etait employe seul (nous avons obtenu un facteur d'amelioration superieur a 10). En regime turbulent etabli, on observe dans un tube deux modes d'ecoulements selon que le liquide mouille ou ne mouille pas la paroi. Dans le premier cas, on obtient sur la paroi un film annulaire liquide et des gouttelettes en suspension dans le coeur gazeux. Dans le deuxieme cas, il se forme un veritable brouillard sans film liquide sur la paroi. Les etudes experimentales ont ete effectuees avec les melanges eau-hydrogene, eau-azote, ethanol-azote (liquides

Measurement and modeling of interface heat transfer coefficients

International Nuclear Information System (INIS)

Rollett, A.D.; Lewis, H.D.; Dunn, P.S.

1985-01-01

The results of preliminary work on the modeling and measurement of the heat transfer coefficients of metal/mold interfaces is reported. The system investigated is the casting of uranium in graphite molds. The motivation for the work is primarily to improve the accuracy of process modeling of prototype mold designs at the Los Alamos Foundry. The evolution in design of a suitable mold for unidirectional solidification is described, illustrating the value of simulating mold designs prior to use. Experiment indicated a heat transfer coefficient of 2 kW/m 2 /K both with and without superheat. It was possible to distinguish between solidification due to the mold and that due to radiative heat loss. This permitted an experimental estimate of the emissivity, epsilon = 0.2, of the solidified metal

PDF methods for turbulent reactive flows

Science.gov (United States)

Hsu, Andrew T.

1995-01-01

Viewgraphs are presented on computation of turbulent combustion, governing equations, closure problem, PDF modeling of turbulent reactive flows, validation cases, current projects, and collaboration with industry and technology transfer.

Electrochemical ion transfer across liquid/liquid interfaces confined within solid-state micropore arrays--simulations and experiments.

Science.gov (United States)

Strutwolf, Jörg; Scanlon, Micheál D; Arrigan, Damien W M

2009-01-01

Miniaturised liquid/liquid interfaces provide benefits for bioanalytical detection with electrochemical methods. In this work, microporous silicon membranes which can be used for interface miniaturisation were characterized by simulations and experiments. The microporous membranes possessed hexagonal arrays of pores with radii between 10 and 25 microm, a pore depth of 100 microm and pore centre-to-centre separations between 99 and 986 microm. Cyclic voltammetry was used to monitor ion transfer across arrays of micro-interfaces between two immiscible electrolyte solutions (microITIES) formed at these membranes, with the organic phase present as an organogel. The results were compared to computational simulations taking into account mass transport by diffusion and encompassing diffusion to recessed interfaces and overlapped diffusion zones. The simulation and experimental data were both consistent with the situation where the location of the liquid/liquid (l/l) interface was on the aqueous side of the silicon membrane and the pores were filled with the organic phase. While the current for the forward potential scan (transfer of the ion from the aqueous phase to the organic phase) was strongly dependent on the location of the l/l interface, the current peak during the reverse scan (transfer of the ion from the organic phase to the aqueous phase) was influenced by the ratio of the transferring ion's diffusion coefficients in both phases. The diffusion coefficient of the transferring ion in the gelified organic phase was ca. nine times smaller than in the aqueous phase. Asymmetric cyclic voltammogram shapes were caused by the combined effect of non-symmetrical diffusion (spherical and linear) and by the inequality of the diffusion coefficient in both phases. Overlapping diffusion zones were responsible for the observation of current peaks instead of steady-state currents during the forward scan. The characterisation of the diffusion behaviour is an important requirement

Effect of interfacial turbulence and accommodation coefficient on CFD predictions of pressurization and pressure control in cryogenic storage tank

Science.gov (United States)

Kassemi, Mohammad; Kartuzova, Olga

2016-03-01

Pressurization and pressure control in cryogenic storage tanks are to a large extent affected by heat and mass transport across the liquid-vapor interface. These mechanisms are, in turn, controlled by the kinetics of the phase change process and the dynamics of the turbulent recirculating flows in the liquid and vapor phases. In this paper, the effects of accommodation coefficient and interfacial turbulence on tank pressurization and pressure control simulations are examined. Comparison between numerical predictions and ground-based measurements in two large liquid hydrogen tank experiments, performed in the K-site facility at NASA Glenn Research Center (GRC) and the Multi-purpose Hydrogen Test Bed (MHTB) facility at NASA Marshall Space Flight Center (MSFC), are used to show the impact of accommodation coefficient and interfacial and vapor phase turbulence on evolution of pressure and temperatures in the cryogenic storage tanks. In particular, the self-pressurization comparisons indicate that: (1) numerical predictions are essentially independent of the magnitude of the accommodation coefficient; and (2) surprisingly, laminar models sometimes provide results that are in better agreement with experimental self-pressurization rates, even in parametric ranges where the bulk flow is deemed fully turbulent. In this light, shortcomings of the present CFD models, especially, numerical treatments of interfacial mass transfer and turbulence, as coupled to the Volume-of-Fluid (VOF) interface capturing scheme, are underscored and discussed.

Material transfer mechanisms between aluminum and fluorinated carbon interfaces

Energy Technology Data Exchange (ETDEWEB)

Sen, F.G. [NSERC/General Motors of Canada Industrial Research Chair, Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, N9B 3P4 (Canada); Qi, Y. [Chemical Sciences and Materials Systems Laboratory, General Motors R and D Center, 30500 Mound Road, Warren, MI 48090-9055 (United States); Alpas, A.T., E-mail: aalpas@uwindsor.ca [NSERC/General Motors of Canada Industrial Research Chair, Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, N9B 3P4 (Canada)

2011-04-15

First-principles calculations and sliding contact experiments were conducted to elucidate material transfer mechanisms between aluminum and fluorinated carbon (diamond, diamond-like carbon (DLC)) surfaces. An interface model that examined interactions between Al (1 1 1) and F-terminated diamond (1 1 1) surfaces revealed that F atoms would transfer to the Al surface in increasing quantities with an increase in the contact pressure, and this F transfer would lead to the formation of a stable AlF{sub 3} compound at the Al surface. The presence of AlF{sub 3} on the transfer layers formed at the Al counterface placed in sliding contact against DLC containing 3 at.% F was confirmed by both X-ray photoelectron spectroscopy and cross-sectional focussed-ion beam transmission electron microscopy analyses. The coefficient of friction (COF) of the DLC coating was high initially due to deformation and wear of Al counterface, but formation of -OH and -H passivated C-rich transfer layers on Al reduced the COF to a low steady-state value of 0.20. The repulsive forces generated between the two F-passivated surfaces further decreased the COF to 0.14.

Material transfer mechanisms between aluminum and fluorinated carbon interfaces

International Nuclear Information System (INIS)

Sen, F.G.; Qi, Y.; Alpas, A.T.

2011-01-01

First-principles calculations and sliding contact experiments were conducted to elucidate material transfer mechanisms between aluminum and fluorinated carbon (diamond, diamond-like carbon (DLC)) surfaces. An interface model that examined interactions between Al (1 1 1) and F-terminated diamond (1 1 1) surfaces revealed that F atoms would transfer to the Al surface in increasing quantities with an increase in the contact pressure, and this F transfer would lead to the formation of a stable AlF 3 compound at the Al surface. The presence of AlF 3 on the transfer layers formed at the Al counterface placed in sliding contact against DLC containing 3 at.% F was confirmed by both X-ray photoelectron spectroscopy and cross-sectional focussed-ion beam transmission electron microscopy analyses. The coefficient of friction (COF) of the DLC coating was high initially due to deformation and wear of Al counterface, but formation of -OH and -H passivated C-rich transfer layers on Al reduced the COF to a low steady-state value of 0.20. The repulsive forces generated between the two F-passivated surfaces further decreased the COF to 0.14.

Modeling Radiative Heat Transfer and Turbulence-Radiation Interactions in Engines

Energy Technology Data Exchange (ETDEWEB)

Paul, Chandan [Pennsylvania State Univ., University Park, PA (United States); Sircar, Arpan [Pennsylvania State Univ., University Park, PA (United States); Ferreyro-Fernandez, Sebastian [Pennsylvania State Univ., University Park, PA (United States); Imren, Abdurrahman [Pennsylvania State Univ., University Park, PA (United States); Haworth, Daniel C [Pennsylvania State Univ., University Park, PA (United States); Roy, Somesh P [Marquette University (United States); Ge, Wenjun [University of California Merced (United States); Modest, Michael F [University of California Merced (United States)

2017-04-26

Detailed radiation modelling in piston engines has received relatively little attention to date. Recently, it is being revisited in light of current trends towards higher operating pressures and higher levels of exhaust-gas recirculation, both of which enhance molecular gas radiation. Advanced high-efficiency engines also are expected to function closer to the limits of stable operation, where even small perturbations to the energy balance can have a large influence on system behavior. Here several different spectral radiation property models and radiative transfer equation (RTE) solvers have been implemented in an OpenFOAM-based engine CFD code, and simulations have been performed for a full-load (peak pressure ~200 bar) heavy-duty diesel engine. Differences in computed temperature fields, NO and soot levels, and wall heat transfer rates are shown for different combinations of spectral models and RTE solvers. The relative importance of molecular gas radiation versus soot radiation is examined. And the influence of turbulence-radiation interactions is determined by comparing results obtained using local mean values of composition and temperature to compute radiative emission and absorption with those obtained using a particle-based transported probability density function method.

Effects of flow depth and wall roughness on turbulence in compound channels

International Nuclear Information System (INIS)

Prinos, P.; Townsend, R.; Tavoularis, S.

1985-01-01

Current methods for estimating discharge in compound channels often lead to large errors. The error is largely due to momentum transfer mechanism (MTM) generated in the junction regions of the flow field (between adjacent deep and shallow zones). The MTM adversely affects system conveyance, particularly when the velocity differential between the deep and shallow zones is large. Improved prediction methods, therefore, will necessarily reflect the MTM's presence and its effect on the compound flow field. The mechanism's influence on system hydraulics is best examined by analysing the related turbulence characteristics in the junction zones of the compound section. Townsend reported increased turbulence levels in the junction region between a main channel and its shallower flood plain zone and Elsawy, McKee and McKeogh found that observed normal turbulent stresses in a similar region were of the same order of magnitude as the apparent shear stress on the junction's vertical interface plane. The objective of the present study is to measure turbulent stresses in the junction region of a symmetrical compound open channel and examine their dependence on relative depth and relative boundary roughness. Further details of this phase of the larger study are presented elsewhere. (author)

Prediction of transpiration effects on heat and mass transfer by different turbulence models

International Nuclear Information System (INIS)

Bucci, M.; Sharabi, M.; Ambrosini, W.; Forgione, N.; Oriolo, F.; He, S.

2008-01-01

The paper reports the results of a study related to transpirating flows, stimulated by the interest that these phenomena, occurring in the presence of simultaneous heat and mass transfer, have for nuclear reactor applications. The work includes a summary and the follow-up of previous experimental and numerical investigations on filmwise condensation and falling film evaporation and of a recent review of different forms of the heat and mass transfer analogy. The particular objective here pursued is to compare transpiration effects as predicted by different turbulence models with classical suction and blowing multipliers based on stagnant layer theories, in the attempt to clarify their quantitative implications on the predicted mass transfer rates. A commercial and an in-house CFD code have been adopted for evaluating the heat and mass transfer rates occurring over a flat plate exposed to an air-vapour stream, with uniform bulk steam mass fraction and temperature boundary conditions at the wall. This simple configuration was purposely selected since it is a simplified representation of the test section of an experimental facility presently in operation at the University of Pisa. This allows a direct comparison between the heat and mass transfer coefficients predicted by CFD models and classical correlations for Nusselt and Sherwood numbers

Analysis of turbulent heat and momentum transfer in a transitionally rough turbulent boundary layer

Science.gov (United States)

Doosttalab, Ali; Dharmarathne, Suranga; Tutkun, Murat; Adrian, Ronald; Castillo, Luciano

2016-11-01

A zero-pressure-gradient (ZPG) turbulent boundary layer over a transitionally rough surface is studied using direct numerical simulation (DNS). The rough surface is modeled as 24-grit sandpaper which corresponds to k+ 11 , where k+ is roughness height. Reynolds number based on momentum thickness is approximately 2400. The walls are isothermal and turbulent flow Prandtl number is 0.71. We simulate temperature as passive scalar. We compute the inner product of net turbulent force (d (u1ui) / dxi) and net turbulent heat flux (d (ui θ / dxi)) in order to investigate (i) the correlation between these vectorial quantities, (II) size of the projection of these fields on each other and (IIi) alignment of momentum and hear flux. The inner product in rough case results in larger projection and better alignment. In addition, our study on the vortices shows that surface roughness promotes production of vortical structures which affects the thermal transport near the wall.

Study of instantaneous unsteady heat transfer in a rapid compression-expansion machine using zero dimensional k- ε turbulence model

International Nuclear Information System (INIS)

Bakhshan, Y.; Karim, G. A.; Mansouri, S. H.

2003-01-01

In this investigation, the instantaneous unsteady heat transfer within a pneumatically driven rapid compression-expansion machine that offers simple, well-controlled and known boundary conditions was studied. Values of the instantaneous apparent overall heat flux from the cylinder gas to the wall surfaces were calculated using a thermodynamics analysis of the experimentally measured pressure and volume temporal development. Corresponding heat flux values were also calculated through the application of a zero-dimensional k- ε turbulence model the characteristic velocity is a contribution of turbulence kinetic energy, mean kinetic energy of charged air into cylinder and piston motion for the calculation of Reynolds, Nusselt and Prandtl numbers. Comparison of the zero-dimensional k- ε turbulence model prediction with experimental data shows good agreement for all compression ratios

Role of coherence and delocalization in photo-induced electron transfer at organic interfaces

Science.gov (United States)

Abramavicius, V.; Pranculis, V.; Melianas, A.; Inganäs, O.; Gulbinas, V.; Abramavicius, D.

2016-09-01

Photo-induced charge transfer at molecular heterojunctions has gained particular interest due to the development of organic solar cells (OSC) based on blends of electron donating and accepting materials. While charge transfer between donor and acceptor molecules can be described by Marcus theory, additional carrier delocalization and coherent propagation might play the dominant role. Here, we describe ultrafast charge separation at the interface of a conjugated polymer and an aggregate of the fullerene derivative PCBM using the stochastic Schrödinger equation (SSE) and reveal the complex time evolution of electron transfer, mediated by electronic coherence and delocalization. By fitting the model to ultrafast charge separation experiments, we estimate the extent of electron delocalization and establish the transition from coherent electron propagation to incoherent hopping. Our results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces.

Basal melting driven by turbulent thermal convection

Science.gov (United States)

Rabbanipour Esfahani, Babak; Hirata, Silvia C.; Berti, Stefano; Calzavarini, Enrico

2018-05-01

Melting and, conversely, solidification processes in the presence of convection are key to many geophysical problems. An essential question related to these phenomena concerns the estimation of the (time-evolving) melting rate, which is tightly connected to the turbulent convective dynamics in the bulk of the melt fluid and the heat transfer at the liquid-solid interface. In this work, we consider a convective-melting model, constructed as a generalization of the Rayleigh-Bénard system, accounting for the basal melting of a solid. As the change of phase proceeds, a fluid layer grows at the heated bottom of the system and eventually reaches a turbulent convection state. By means of extensive lattice-Boltzmann numerical simulations employing an enthalpy formulation of the governing equations, we explore the model dynamics in two- and three-dimensional configurations. The focus of the analysis is on the scaling of global quantities like the heat flux and the kinetic energy with the Rayleigh number, as well as on the interface morphology and the effects of space dimensionality. Independently of dimensionality, we find that the convective-melting system behavior shares strong resemblances with that of the Rayleigh-Bénard one, and that the heat flux is only weakly enhanced with respect to that case. Such similarities are understood, at least to some extent, considering the resulting slow motion of the melting front (with respect to the turbulent fluid velocity fluctuations) and its generally little roughness (compared to the height of the fluid layer). Varying the Stefan number, accounting for the thermodynamical properties of the material, also seems to have only a mild effect, which implies the possibility of extrapolating results in numerically delicate low-Stefan setups from more convenient high-Stefan ones. Finally, we discuss the implications of our findings for the geophysically relevant problem of modeling Arctic ice melt ponds.

Two-phase heat and mass transfer in turbulent parallel and countercurrent flows of liquid film and gas

International Nuclear Information System (INIS)

Kholpanov, L.P.; Babak, T.B.; Babak, V.N.; Malyusov, V.A.; Zhavoronkov, N.M.; AN SSSR, Moscow. Inst. Obshchej i Neorganicheskoj Khimii)

1980-01-01

To determine the ways of intensification of heat and mass transfer processes, the direct flow and counterflow heat and mass transfer is analytically investigated during the turbulent flow of a liquid and gas film on the basis of solving the energy equation for liquid and gas film, i.e. the two-phase film heat transfer is investigated from the position of a conjugate task. The analysis of the two-phase heat transfer has shown that it is necessary to know the position of each point in a plane before using this or that formula. Depending on its position on this plane, the heat transfer process will be determined by one or two phases only. It is found, that in the case of a single-phase heat transfer the temperature on the surface remains stable over the channel length. In the case of a two-phase heat transfer it can significantly change over the channel length [ru

Alfvénic turbulence in solar wind originating near coronal hole boundaries: heavy-ion effects?

Directory of Open Access Journals (Sweden)

B. Bavassano

2006-03-01

Full Text Available The mid-latitude phases of the Ulysses mission offer an excellent opportunity to investigate the solar wind originating near the coronal hole boundaries. Here we report on Alfvénic turbulence features, revealing a relevant presence of in-situ generated fluctuations, observed during the wind rarefaction phase that charaterizes the transition from fast to slow wind. Heavy-ion composition and magnetic field measurements indicate a strict time correspondence of the locally generated fluctuations with 1 the crossing of the interface between fast and slow wind and 2 the presence of strongly underwound magnetic field lines (with respect to the Parker spiral. Recent studies suggest that such underwound magnetic configurations correspond to fast wind magnetic lines that, due to footpoint motions at the Sun, have their inner leg transferred to slow wind and are stretched out by the velocity gradient. If this is a valid scenario, the existence of a magnetic connection across the fast-slow wind interface is a condition that, given the different state of the two kinds of wind, may favour the development of processes acting as local sources of turbulence. We suggest that heavy-ion effects could be responsible of the observed turbulence features.

Geometric and energetic considerations of surface fluctuations during ion transfer across the water-immiscible organic liquid interface

Energy Technology Data Exchange (ETDEWEB)

Karnes, John J.; Benjamin, Ilan, E-mail: benjamin@chemistry.ucsc.edu [Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064 (United States)

2016-07-07

Molecular dynamics simulations and umbrella sampling free energy calculations are used to examine the thermodynamics, energetics, and structural fluctuations that accompany the transfer of a small hydrophilic ion (Cl{sup −}) across the water/nitrobenzene interface. By examining several constrained interface structures, we isolate the energetic costs of interfacial deformation and co-transfer of hydration waters during the ion transfer. The process is monitored using both energy-based solvation coordinates and a geometric coordinate recently introduced by Morita and co-workers to describe surface fluctuations. Our simulations show that these coordinates provide a complimentary description of the water surface fluctuations during the transfer and are necessary for elucidating the mechanism of the ion transfer.

Single-crystal charge transfer interfaces for efficient photonic devices (Conference Presentation)

Science.gov (United States)

Alves, Helena; Pinto, Rui M.; Maçôas, Ermelinda M. S.; Baleizão, Carlos; Santos, Isabel C.

2016-09-01

Organic semiconductors have unique optical, mechanical and electronic properties that can be combined with customized chemical functionality. In the crystalline form, determinant features for electronic applications such as molecular purity, the charge mobility or the exciton diffusion length, reveal a superior performance when compared with materials in a more disordered form. Combining crystals of two different conjugated materials as even enable a new 2D electronic system. However, the use of organic single crystals in devices is still limited to a few applications, such as field-effect transistors. In 2013, we presented the first system composed of single-crystal charge transfer interfaces presenting photoconductivity behaviour. The system composed of rubrene and TCNQ has a responsivity reaching 1 A/W, corresponding to an external quantum efficiency of nearly 100%. A similar approach, with a hybrid structure of a PCBM film and rubrene single crystal also presents high responsivity and the possibility to extract excitons generated in acceptor materials. This strategy led to an extended action towards the near IR. By adequate material design and structural organisation of perylediimides, we demonstrate that is possible to improve exciton diffusion efficiency. More recently, we have successfully used the concept of charge transfer interfaces in phototransistors. These results open the possibility of using organic single-crystal interfaces in photonic applications.

Turbulence modelling; Modelisation de la turbulence isotherme

Energy Technology Data Exchange (ETDEWEB)

Laurence, D. [Electricite de France (EDF), Direction des Etudes et Recherches, 92 - Clamart (France)

1997-12-31

This paper is an introduction course in modelling turbulent thermohydraulics, aimed at computational fluid dynamics users. No specific knowledge other than the Navier Stokes equations is required beforehand. Chapter I (which those who are not beginners can skip) provides basic ideas on turbulence physics and is taken up in a textbook prepared by the teaching team of the ENPC (Benque, Viollet). Chapter II describes turbulent viscosity type modelling and the 2k-{epsilon} two equations model. It provides details of the channel flow case and the boundary conditions. Chapter III describes the `standard` (R{sub ij}-{epsilon}) Reynolds tensions transport model and introduces more recent models called `feasible`. A second paper deals with heat transfer and the effects of gravity, and returns to the Reynolds stress transport model. (author). 37 refs.

Numerical Simulation of Turbulent Fluid Flow and Heat Transfer in a Ribbed Rotating Two-Pass Square Duct

Directory of Open Access Journals (Sweden)

Liou Tong-Miin

2005-01-01

Full Text Available The local turbulent fluid flow and heat transfer in a rotating two-pass square duct with 19 pairs of in-line 90 ∘ ribs have been investigated computationally. A Reynolds-averaged Navier-Stokes equation (RANS with a two-layer k − ϵ turbulence model was solved. The in-line 90 ∘ ribs were arranged on the leading and trailing walls with rib height-to-hydraulic diameter ratio and pitch-to-height ratio of 0.136 and 10, respectively. The Reynolds number, based on duct hydraulic diameter and bulk mean velocity, was fixed at 1.0 × 10 4 whereas the rotational number varied from 0 to 0.2 . Results are validated with previous measured velocity field and heat transfer coefficient distributions. The validation shows that the effect of rotation on the passage-averaged Nusselt number ratio can be predicted reasonably well; nevertheless, the transverse mean velocity and, in turn, the distribution of regional-averaged Nusselt number ratio are markedly underpredicted in the regions toward which the Coriolis force is directed. Further CFD studies are needed.

Radiative heat transfer in a heat generating and turbulently convecting fluid layer

International Nuclear Information System (INIS)

Cheung, F.B.; Chan, S.H.; Chawla, T.C.; Cho, D.H.

1980-01-01

The coupled problem of radiative transport and turbulent natural convection in a volumetrically heated, horizontal gray fluid medium, bounded from above by a rigid, isothermal wall and below by a rigid, adiabatic wall, is investigated analytically. An approximate method based upon the boundary layer approach is employed to obtain the dependence of heat transfer at the upper wall on the principal parameters of the problem, which, for moderate Prandtl number, are the Rayleigh number, Ra, the optical thickness, KL, and the conduction-radiation coupling parameter, N. Also obtained in this study is the behaviour of the thermal boundary layer at the upper wall. At large kL, the contribution of thermal radiation to heat transfer in the layer is found to be negligible for N > 10, moderate for N approximately 1, and overwhelming for N < 0.1. However, at small kL, thermal radiation is found to be important only for N < 0.01. While a higher level of turbulence results in a thinner boundary layer, a larger effect of radiation is found to result in a thicker one. Thus, in the presence of strong thermal radiation, a much larger value of Ra is required for the boundary layer approach to remain valid. Under severe radiation conditions, no boundary layer flow regime is found to exist even at very high Rayleigh numbers. Accordingly, the ranges of applicability of the present results are determined and the approximate method justified. In particular, the validity of the present analysis is tested in three limiting cases, ie those of kL → infinity, N → infinity, and Ra → infinity, and is further confirmed by comparison with the numerical solution (author)

Efficient Data Transfer Rate and Speed of Secured Ethernet Interface System

Science.gov (United States)

Ghanti, Shaila

2016-01-01

Embedded systems are extensively used in home automation systems, small office systems, vehicle communication systems, and health service systems. The services provided by these systems are available on the Internet and these services need to be protected. Security features like IP filtering, UDP protection, or TCP protection need to be implemented depending on the specific application used by the device. Every device on the Internet must have network interface. This paper proposes the design of the embedded Secured Ethernet Interface System to protect the service available on the Internet against the SYN flood attack. In this experimental study, Secured Ethernet Interface System is customized to protect the web service against the SYN flood attack. Secured Ethernet Interface System is implemented on ALTERA Stratix IV FPGA as a system on chip and uses the modified SYN flood attack protection method. The experimental results using Secured Ethernet Interface System indicate increase in number of genuine clients getting service from the server, considerable improvement in the data transfer rate, and better response time during the SYN flood attack. PMID:28116350

Efficient Data Transfer Rate and Speed of Secured Ethernet Interface System.

Science.gov (United States)

Ghanti, Shaila; Naik, G M

2016-01-01

Embedded systems are extensively used in home automation systems, small office systems, vehicle communication systems, and health service systems. The services provided by these systems are available on the Internet and these services need to be protected. Security features like IP filtering, UDP protection, or TCP protection need to be implemented depending on the specific application used by the device. Every device on the Internet must have network interface. This paper proposes the design of the embedded Secured Ethernet Interface System to protect the service available on the Internet against the SYN flood attack. In this experimental study, Secured Ethernet Interface System is customized to protect the web service against the SYN flood attack. Secured Ethernet Interface System is implemented on ALTERA Stratix IV FPGA as a system on chip and uses the modified SYN flood attack protection method. The experimental results using Secured Ethernet Interface System indicate increase in number of genuine clients getting service from the server, considerable improvement in the data transfer rate, and better response time during the SYN flood attack.

Application of two-equation turbulence models to turbulent gas flow heated by a high heat flux

International Nuclear Information System (INIS)

Kawamura, Hiroshi

1978-01-01

Heat transfer in heated turbulent gas flow is analyzed using two-equation turbulence models. Four kinds of two-equation models are examined; that is, k-epsilon model by Jones-Launder, k-w model by Wilcox-Traci, k-kL model by Rotta, k-ω model by Saffman-Wilcox. The results are compared with more than ten experiments by seven authors. The k-kL model proposed originally by Rotta and modified by the present author is found to give relatively the best results. It well predicts the decrease in the heat transfer coefficient found in the heated turbulent gas flow; however, it fails to predict the laminarization due to a strong heating. (author)

A review of recent developments on turbulent entrainment in stratified flows

International Nuclear Information System (INIS)

Cotel, Aline J

2010-01-01

Stratified interfaces are present in many geophysical flow situations, and transport across such an interface is an essential factor for correctly evaluating the physical processes taking place at many spatial and temporal scales in such flows. In order to accurately evaluate vertical and lateral transport occurring when a turbulent flow impinges on a stratified interface, the turbulent entrainment and vorticity generation mechanisms near the interface must be understood and quantified. Laboratory experiments were performed for three flow configurations: a vertical thermal, a sloping gravity current and a vertical turbulent jet with various tilt angles and precession speeds. All three flows impinged on an interface separating a two-layer stably stratified environment. The entrainment rate is quantified for each flow using laser-induced fluorescence and compared to predictions of Cotel and Breidenthal (1997 Appl. Sci. Res. 57 349-66). The possible applications of transport across stratified interfaces include the contribution of hydrothermal plumes to the global ocean energy budget, turbidity currents on the ocean floor, the design of lake de-stratification systems, modeling gas leaks from storage reservoirs, weather forecasting and global climate change.

Turbulent heat transfer studies in annulus with inner cylinder rotation

International Nuclear Information System (INIS)

Kuzay, T.M.; Scott, C.J.

1977-01-01

Experimental investigations of turbulent heat transfer are made in a large-gap annulus with both rotating and nonrotating inner cylinder. The vertical annular channel has an electrically heated outer wall; the inner wall i thermally and electrically insulated. The axial air flow is allowed to develop before rotation and heating are imparted. The resulting temperature fields are investigated using thermocouple probes located near the channel exit. The wall heat flux, wall axial temperature development, and radial temperature profiles are measured. For each axial Reynolds number, three heat flux rates are used. Excellent correlation is established between rotational and nonrotational Nusselt number. The proper correlation parameter is a physical quantity characterizing the flow helix. This parameter is the inverse of the ratio of axial travel of the flow helix in terms of hydraulic diameter, per half revolution of the spinning wall

Numerical analysis of turbulent flow and heat transfer in a square sectioned U-bend duct by elliptic-blending second moment closure

International Nuclear Information System (INIS)

Shin, Jong Keun; Choi, Young Don; An, Jeong Soo

2007-01-01

A second moment turbulence closure using the elliptic-blending equation is introduced to analyze the turbulence and heat transfer in a square sectioned U-bend duct flow. The turbulent heat flux model based on the elliptic concept satisfies the near-wall balance between viscous diffusion, viscous dissipation and temperature-pressure gradient correlation, and also has the characteristics of approaching its respective conventional high Reynolds number model far away from the wall. Also, the traditional GGDH heat flux model is compared with the present elliptic concept-based heat flux model. The turbulent heat flux models are closely linked to the elliptic blending second moment closure which is used for the prediction of Reynolds stresses. The predicted results show their reasonable agreement with experimental data for a square sectioned U-bend duct flow field adopted in the present study

Finite element modeling of the neuron-electrode interface: stimulus transfer and geometry

NARCIS (Netherlands)

Buitenweg, Jan R.; Rutten, Wim; Marani, Enrico

1999-01-01

The relation between stimulus transfer and the geometry of the neuron-electrode interface can not be determined properly using electrical equivalent circuits, since current that flows from the sealing gap through the neuronal membrane is difficult to model in these circuits. Therefore, finite

Dust Tolerant Commodity Transfer Interface Mechanisms for Planetary Surfaces

Science.gov (United States)

Townsend, Ivan I.; Mueller, Robert P.; Tamasy, Gabor J.

2014-01-01

Regolith is present on most planetary surfaces such as Earth's moon, Mars, and Asteroids. If human crews and robotic machinery are to operate on these regolith covered surfaces, they must face the consequences of interacting with regolith fines which consist of particles below 100 microns in diameter down to as small as submicron scale particles. Such fine dust will intrude into mechanisms and interfaces causing a variety of problems such as contamination of clean fluid lines, jamming of mechanisms and damaging connector seals and couplings. Since multiple elements must be assembled in space for system level functionality, it will be inevitable that interfaces will be necessary for structural connections, and to pass commodities such as cryogenic liquid propellants, purge and buffer gases, water, breathing air, pressurizing gases, heat exchange fluids, power and data. When fine regolith dust is present in the environment it can be lofted into interfaces where it can compromise the utility of the interface by preventing the connections from being successfully mated, or by inducing fluid leaks or degradation of power and data transmission. A dust tolerant, hand held "quick-disconnect" cryogenic fluids connector housing has been developed at NASA KSC which can be used by astronaut crews to connect flex lines that will transfer propellants and other useful fluids to the end user. In addition, a dust tolerant, automated, cryogenic fluid, multiple connector, power and data interface mechanism prototype has been developed, fabricated and demonstrated by NASA at Kennedy Space Center (KSC). The design and operation of these prototypes are explained and discussed.

Electromotive force in strongly compressible magnetohydrodynamic turbulence

Science.gov (United States)

Yokoi, N.

2017-12-01

Variable density fluid turbulence is ubiquitous in geo-fluids, not to mention in astrophysics. Depending on the source of density variation, variable density fluid turbulence may be divided into two categories: the weak compressible (entropy mode) turbulence for slow flow and the strong compressible (acoustic mode) turbulence for fast flow. In the strong compressible turbulence, the pressure fluctuation induces a strong density fluctuation ρ ', which is represented by the density variance ( denotes the ensemble average). The turbulent effect on the large-scale magnetic-field B induction is represented by the turbulent electromotive force (EMF) (u': velocity fluctuation, b': magnetic-field fluctuation). In the usual treatment in the dynamo theory, the expression for the EMF has been obtained in the framework of incompressible or weak compressible turbulence, where only the variation of the mean density , if any, is taken into account. We see from the equation of the density fluctuation ρ', the density variance is generated by the large mean density variation ∂ coupled with the turbulent mass flux . This means that in the region where the mean density steeply changes, the density variance effect becomes relevant for the magnetic field evolution. This situation is typically the case for phenomena associated with shocks and compositional discontinuities. With the aid of the analytical theory of inhomogeneous compressible magnetohydrodynamic (MHD) turbulence, the expression for the turbulent electromotive force is investigated. It is shown that, among others, an obliqueness (misalignment) between the mean density gradient ∂ and the mean magnetic field B may contribute to the EMF as ≈χ B×∂ with the turbulent transport coefficient χ proportional to the density variance (χ ). This density variance effect is expected to strongly affect the EMF near the interface, and changes the transport properties of turbulence. In the case of an interface under the MHD slow

A high speed, selective multi-ADC to computer data transfer interface, for nuclear physics experiments

International Nuclear Information System (INIS)

Arctaedius, T.; Ekstroem, R.E.

1986-08-01

A link connecting up to fifteen Analog to Digital Converters with a computer, through a Direct Memory Access interface, is described. The interface decides which of the connected ADC:s that participate in an event, and transfers the output-data from these to the computer, accompanied with a 2-byte word identifying the participating ADC:s. This data format can be recorded on tape without further transformations, and is easy to unfold at the off-line analysis. Data transfer is accomplished in less than a few microseconds, which is made possible by the use of high speed TTL circuits. (authors)

A Comparison of the Scalar and Vorticity Criterion defining the T/NT Interface

Science.gov (United States)

Boschung, Jonas; Hennig, Fabian; Peters, Norbert

2013-11-01

Free shear flows are characterized by a turbulent core region, a non-turbulent outer flow and a turbulent/non-turbulent interface separating the two zones. While there exist different approaches to identify this transitional region, the interface position is mostly defined to coincide with the isoscalar surfaces of either a passive scalar or the magnitude of the vorticity. Both criteria are examined and compared using a shear layer DNS.

Radiative heat transfer in turbulent combustion systems theory and applications

CERN Document Server

Modest, Michael F

2016-01-01

This introduction reviews why combustion and radiation are important, as well as the technical challenges posed by radiation. Emphasis is on interactions among turbulence, chemistry and radiation (turbulence-chemistry-radiation interactions – TCRI) in Reynolds-averaged and large-eddy simulations. Subsequent chapters cover: chemically reacting turbulent flows; radiation properties, Reynolds transport equation (RTE) solution methods, and TCRI; radiation effects in laminar flames; TCRI in turbulent flames; and high-pressure combustion systems. This Brief presents integrated approach that includes radiation at the outset, rather than as an afterthought. It stands as the most recent developments in physical modeling, numerical algorithms, and applications collected in one monograph.

A combined volume-of-fluid method and low-Mach-number approach for DNS of evaporating droplets in turbulence

Science.gov (United States)

Dodd, Michael; Ferrante, Antonino

2017-11-01

Our objective is to perform DNS of finite-size droplets that are evaporating in isotropic turbulence. This requires fully resolving the process of momentum, heat, and mass transfer between the droplets and surrounding gas. We developed a combined volume-of-fluid (VOF) method and low-Mach-number approach to simulate this flow. The two main novelties of the method are: (i) the VOF algorithm captures the motion of the liquid gas interface in the presence of mass transfer due to evaporation and condensation without requiring a projection step for the liquid velocity, and (ii) the low-Mach-number approach allows for local volume changes caused by phase change while the total volume of the liquid-gas system is constant. The method is verified against an analytical solution for a Stefan flow problem, and the D2 law is verified for a single droplet in quiescent gas. We also demonstrate the schemes robustness when performing DNS of an evaporating droplet in forced isotropic turbulence.

Properties of small-scale interfacial turbulence from a novel thermography based approach

Science.gov (United States)

Schnieders, Jana; Garbe, Christoph

2013-04-01

Oceans cover nearly two thirds of the earth's surface and exchange processes between the Atmosphere and the Ocean are of fundamental environmental importance. At the air-sea interface, complex interaction processes take place on a multitude of scales. Turbulence plays a key role in the coupling of momentum, heat and mass transfer [2]. Here we use high resolution infrared imagery to visualize near surface aqueous turbulence. Thermographic data is analized from a range of laboratory facilities and experimental conditions with wind speeds ranging from 1ms-1 to 7ms-1 and various surface conditions. The surface heat pattern is formed by distinct structures on two scales - small-scale short lived structures termed fish scales and larger scale cold streaks that are consistent with the footprints of Langmuir Circulations. There are two key characteristics of the observed surface heat patterns: (1) The surface heat patterns show characteristic features of scales. (2) The structure of these patterns change with increasing wind stress and surface conditions. We present a new image processing based approach to the analysis of the spacing of cold streaks based on a machine learning approach [4, 1] to classify the thermal footprints of near surface turbulence. Our random forest classifier is based on classical features in image processing such as gray value gradients and edge detecting features. The result is a pixel-wise classification of the surface heat pattern with a subsequent analysis of the streak spacing. This approach has been presented in [3] and can be applied to a wide range of experimental data. In spite of entirely different boundary conditions, the spacing of turbulent cells near the air-water interface seems to match the expected turbulent cell size for flow near a no-slip wall. The analysis of the spacing of cold streaks shows consistent behavior in a range of laboratory facilities when expressed as a function of water sided friction velocity, u*. The scales

Charge transfer effects of ions at the liquid water/vapor interface

Energy Technology Data Exchange (ETDEWEB)

Soniat, Marielle; Rick, Steven W., E-mail: srick@uno.edu [Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 (United States)

2014-05-14

Charge transfer (CT), the movement of small amounts of electron density between non-bonded pairs, has been suggested as a driving force for a variety of physical processes. Herein, we examine the effect of CT on ion adsorption to the water liquid-vapor interface. Using a CT force field for molecular dynamics, we construct a potential of mean force (PMF) for Na{sup +}, K{sup +}, Cl{sup −}, and I{sup −}. The PMFs were produced with respect to an average interface and an instantaneous interface. An analysis of the PMF relative to the instantaneous surface reveals that the area in which the anions experience a free energy minimum is quite narrow, and the cations feel a steeply repulsive free energy near the interface. CT is seen to have only minor effects on the overall free energy profiles. However, the long-ranged effects of ions are highlighted by the CT model. Due to CT, the water molecules at the surface become charged, even when the ion is over 15 Å away from the surface.

Direct numerical simulation of turbulent mixing in grid-generated turbulence

International Nuclear Information System (INIS)

Nagata, Kouji; Suzuki, Hiroki; Sakai, Yasuhiko; Kubo, Takashi; Hayase, Toshiyuki

2008-01-01

Turbulent mixing of passive scalar (heat) in grid-generated turbulence (GGT) is simulated by means of direct numerical simulation (DNS). A turbulence-generating grid, on which the velocity components are set to zero, is located downstream of the channel entrance, and it is numerically constructed on the staggered mesh arrangement using the immersed boundary method. The grid types constructed are: (a) square-mesh biplane grid, (b) square-mesh single-plane grid, (c) composite grid consisting of parallel square-bars and (d) fractal grid. Two fluids with different temperatures are provided separately in the upper and lower streams upstream of the turbulence-generating grids, generating the thermal mixing layer behind the grids. For the grid (a), simulations for two different Prandtl numbers of 0.71 and 7.1, corresponding to air and water flows, are conducted to investigate the effect of the Prandtl number. The results show that the typical grid turbulence and shearless mixing layer are generated downstream of the grids. The results of the scalar field show that a typical thermal mixing layer is generated as well, and the effects of the Prandtl numbers on turbulent heat transfer are observed.

Direct numerical simulation of turbulent mixing in grid-generated turbulence

Energy Technology Data Exchange (ETDEWEB)

Nagata, Kouji; Suzuki, Hiroki; Sakai, Yasuhiko; Kubo, Takashi [Department of Mechanical Science and Engineering, Nagoya University, Nagoya 464-8603 (Japan); Hayase, Toshiyuki [Institute of Fluid Science, Tohoku University, Sendai 980-8577 (Japan)], E-mail: nagata@nagoya-u.jp, E-mail: hsuzuki@nagoya-u.jp, E-mail: ysakai@mech.nagoya-u.ac.jp, E-mail: t-kubo@nagoya-u.jp, E-mail: hayase@ifs.tohoku.ac.jp

2008-12-15

Turbulent mixing of passive scalar (heat) in grid-generated turbulence (GGT) is simulated by means of direct numerical simulation (DNS). A turbulence-generating grid, on which the velocity components are set to zero, is located downstream of the channel entrance, and it is numerically constructed on the staggered mesh arrangement using the immersed boundary method. The grid types constructed are: (a) square-mesh biplane grid, (b) square-mesh single-plane grid, (c) composite grid consisting of parallel square-bars and (d) fractal grid. Two fluids with different temperatures are provided separately in the upper and lower streams upstream of the turbulence-generating grids, generating the thermal mixing layer behind the grids. For the grid (a), simulations for two different Prandtl numbers of 0.71 and 7.1, corresponding to air and water flows, are conducted to investigate the effect of the Prandtl number. The results show that the typical grid turbulence and shearless mixing layer are generated downstream of the grids. The results of the scalar field show that a typical thermal mixing layer is generated as well, and the effects of the Prandtl numbers on turbulent heat transfer are observed.

Nonlinear energy transfer and current sheet development in localized Alfvén wavepacket collisions in the strong turbulence limit

Science.gov (United States)

Verniero, J. L.; Howes, G. G.; Klein, K. G.

2018-02-01

In space and astrophysical plasmas, turbulence is responsible for transferring energy from large scales driven by violent events or instabilities, to smaller scales where turbulent energy is ultimately converted into plasma heat by dissipative mechanisms. The nonlinear interaction between counterpropagating Alfvén waves, denoted Alfvén wave collisions, drives this turbulent energy cascade, as recognized by early work with incompressible magnetohydrodynamic (MHD) equations. Recent work employing analytical calculations and nonlinear gyrokinetic simulations of Alfvén wave collisions in an idealized periodic initial state have demonstrated the key properties that strong Alfvén wave collisions mediate effectively the transfer of energy to smaller perpendicular scales and self-consistently generate current sheets. For the more realistic case of the collision between two initially separated Alfvén wavepackets, we use a nonlinear gyrokinetic simulation to show here that these key properties persist: strong Alfvén wavepacket collisions indeed facilitate the perpendicular cascade of energy and give rise to current sheets. Furthermore, the evolution shows that nonlinear interactions occur only while the wavepackets overlap, followed by a clean separation of the wavepackets with straight uniform magnetic fields and the cessation of nonlinear evolution in between collisions, even in the gyrokinetic simulation presented here which resolves dispersive and kinetic effects beyond the reach of the MHD theory.

Wall-resolved Large Eddy Simulations of turbulent heat transfer in a T-junction

Science.gov (United States)

Georgiou, Michail; Papalexandris, Miltiadis V.

2017-11-01

In this talk we report on wall-resolved Large Eddy Simulations of turbulent heat transfer between a cold crossflow and a hot incoming jet in a T-junction. Due to their high efficiency in mixing and heat transfer, T-junctions are encountered in numerous industrial applications. Our study is motivated by the need to assess phenomena related to thermal fatigue that are often encountered at their walls. We first describe the important features of the flow with emphasis on the shear layers that are formed at the entry of the jet and the recirculation regions. We also show results for first- and second-order statistics of the flow and compare our predictions with previous experimental data. Lastly, we present results from the spectral analysis of the temperature signal that we performed in order to assess the oscillating mechanisms that dominate the flow and the risk of thermal fatigue at the walls of the T-junction.

Modeling the liquid-liquid interface and the transfer of a solute by molecular dynamics simulation

International Nuclear Information System (INIS)

Hayoun, Marc

1990-11-01

Molecular Dynamics method and Lennard-Jones potential functions have been employed to model Liquid-Liquid Interfaces. The variation of the miscibilities between the two liquids is obtained by changing the interaction between the two atomic species. The resulting interfaces have a thickness of about three atomic diameters and are stable on the time scale of the simulation. They have been characterized by the density and pressure profiles. The interfacial tension has also been computed and is of the order of magnitude of experimental values. The diffusion process is anisotropic in the interfacial region: the transverse diffusion coefficient (parallelly to the interface) is higher than the normal one. A qualitative explanation of this behaviour is suggested by considering the pressure tensor. The second part of this work, performed by Molecular Dynamics in the canonical ensemble, is devoted to the kinetic study of the transfer of a solute through the interface. A model of a symmetric interface with an atomic solute has been used. The interaction potential between the solute and the solvents has been built in order to obtain an activation barrier to the transfer. We have computed the mean force exerted by the solvent on the solute as a function of its distance to the interface. The resulting mean force potential corresponds to a free energy difference. The height of the energy barrier involved is about 4 kT. The potential energy and entropy profiles have also been calculated and discussed. The diffusion coefficient of the solute has been computed by equilibrium and non-equilibrium methods. We deduced the friction coefficient of the solvent, which is essential to determine the Kramers transmission coefficient. This coefficient is compared to the one obtained by simulation. Finally, the solute transfer rate constant has been calculated. (author) [fr

Tidal influence on the sea-to-air transfer of CH4 in the coastal ocean

International Nuclear Information System (INIS)

Hahm, Doshik; Kim, Guebuem; Lee, Yong-Woo; Nam, Sungh-Yun; Kim, Kyung-Ryul; Kim, Kuh

2006-01-01

We obtained real-time monitoring data of water temperature, salinity, wind, current, CH 4 and other oceanographic parameters in a coastal bay in the southern sea of Korea from July 8 to August 15, 2003, using an environmental monitoring buoy. In general, the transfer velocity of environmental gases across the air-sea interface is obtained exclusively from empirical relationships with wind speeds. However, our monitoring data demonstrate that the agitation of the aqueous boundary layer is controlled significantly by tidal turbulence, similar to the control exercised by wind stress in the coastal ocean. The sea-to-air transfer of CH 4 is enhanced significantly during spring tide due to an increase in the gas transfer velocity and vertical CH 4 transport from bottom water to the surface layer. Thus, our unique time-series results imply that the sea-to-air transfer of gases, such as CH 4 , DMS, DMHg, N 2 O, CO 2 and 222 Rn, from highly enriched coastal bottom waters, is controlled not only by episodic wind events but also by regular tidal turbulence in the coastal ocean

Turbulence closure: turbulence, waves and the wave-turbulence transition – Part 1: Vanishing mean shear

Directory of Open Access Journals (Sweden)

H. Z. Baumert

2009-03-01

Full Text Available This paper extends a turbulence closure-like model for stably stratified flows into a new dynamic domain in which turbulence is generated by internal gravity waves rather than mean shear. The model turbulent kinetic energy (TKE, K balance, its first equation, incorporates a term for the energy transfer from internal waves to turbulence. This energy source is in addition to the traditional shear production. The second variable of the new two-equation model is the turbulent enstrophy (Ω. Compared to the traditional shear-only case, the Ω-equation is modified to account for the effect of the waves on the turbulence time and space scales. This modification is based on the assumption of a non-zero constant flux Richardson number in the limit of vanishing mean shear when turbulence is produced exclusively by internal waves. This paper is part 1 of a continuing theoretical development. It accounts for mean shear- and internal wave-driven mixing only in the two limits of mean shear and no waves and waves but no mean shear, respectively.

The new model reproduces the wave-turbulence transition analyzed by D'Asaro and Lien (2000b. At small energy density E of the internal wave field, the turbulent dissipation rate (ε scales like ε~E². This is what is observed in the deep sea. With increasing E, after the wave-turbulence transition has been passed, the scaling changes to ε~E¹. This is observed, for example, in the highly energetic tidal flow near a sill in Knight Inlet. The new model further exhibits a turbulent length scale proportional to the Ozmidov scale, as observed in the ocean, and predicts the ratio between the turbulent Thorpe and Ozmidov length scales well within the range observed in the ocean.

Comparison of energy fluxes at the land surface-atmosphere interface in an Alpine valley as simulated with different models

Directory of Open Access Journals (Sweden)

G. Grossi

2003-01-01

Full Text Available Within the framework of a research project coupling meteorological and hydrological models in mountainous areas a distributed Snow-Soil-Vegetation-Atmosphere Transfer model was developed and applied to simulate the energy fluxes at the land surface – atmosphere interface in an Alpine valley (Toce Valley - North Italy during selected flood events in the last decade. Energy fluxes simulated by the distributed energy transfer model were compared with those simulated by a limited area meteorological model for the event of June 1997 and the differences in the spatial and temporal distribution. The Snow/Soil-Vegetation-Atmosphere Transfer model was also applied to simulate the energy fluxes at the land surface-atmosphere interface for a single cell, assumed to be representative of the Siberia site (Toce Valley, where a micro-meteorological station was installed and operated for 2.5 months in autumn 1999. The Siberia site is very close to the Nosere site, where a standard meteorological station was measuring precipitation, air temperature and humidity, global and net radiation and wind speed during the same special observing period. Data recorded by the standard meteorological station were used to force the energy transfer model and simulate the point energy fluxes at the Siberia site, while turbulent fluxes observed at the Siberia site were used to derive the latent heat flux from the energy balance equation. Finally, the hourly evapotranspiration flux computed by this procedure was compared to the evapotranspiration flux simulated by the energy transfer model. Keywords: energy exchange processes, land surface-atmosphere interactions, turbulent fluxes

Direct numerical simulations of evaporating droplets in turbulence

Science.gov (United States)

Palmore, John; Desjardins, Olivier

2015-11-01

This work demonstrates direct numerical simulations of evaporating two phase flows, with applications to studying combustion in aircraft engines. Inside the engine, liquid fuel is injected into the combustion chamber where it atomizes into droplets and evaporates. Combustion occurs as the fuel vapor mixes with the surrounding flow of turbulent gas. Understanding combustion, therefore, requires studying evaporation in a turbulent flow and the resulting vapor distribution. We study the problem using a finite volume framework to solve the Navier-Stokes and scalar transport equations under a low-Mach assumption [Desjardins et al., J. Comp. Phys., 2008]. The liquid-gas interface is tracked using a conservative level-set method [Desjardins et al., J. Comp. Phys., 2008] which allows for a sharp reconstruction of the discontinuity across the interface. Special care is taken in the discretization of cells near the liquid-gas interface to ensure the stability and accuracy of the solution. Results are discussed for non-reacting simulations of liquid droplets evaporating into a turbulent field of inert gas.

Interfacial area and interfacial transfer in two-phase systems. DOE final report

Energy Technology Data Exchange (ETDEWEB)

Ishii, Mamoru; Hibiki, T.; Revankar, S.T.; Kim, S.; Le Corre, J.M.

2002-07-01

In the two-fluid model, the field equations are expressed by the six conservation equations consisting of mass, momentum and energy equations for each phase. The existence of the interfacial transfer terms is one of the most important characteristics of the two-fluid model formulation. The interfacial transfer terms are strongly related to the interfacial area concentration and to the local transfer mechanisms such as the degree of turbulence near interfaces. This study focuses on the development of a closure relation for the interfacial area concentration. A brief summary of several problems of the current closure relation for the interfacial area concentration and a new concept to overcome the problem are given.

Turbulent black holes.

Science.gov (United States)

Yang, Huan; Zimmerman, Aaron; Lehner, Luis

2015-02-27

We demonstrate that rapidly spinning black holes can display a new type of nonlinear parametric instability-which is triggered above a certain perturbation amplitude threshold-akin to the onset of turbulence, with possibly observable consequences. This instability transfers from higher temporal and azimuthal spatial frequencies to lower frequencies-a phenomenon reminiscent of the inverse cascade displayed by (2+1)-dimensional fluids. Our finding provides evidence for the onset of transitory turbulence in astrophysical black holes and predicts observable signatures in black hole binaries with high spins. Furthermore, it gives a gravitational description of this behavior which, through the fluid-gravity duality, can potentially shed new light on the remarkable phenomena of turbulence in fluids.

Zonal PANS: evaluation of different treatments of the RANS-LES interface

Science.gov (United States)

Davidson, L.

2016-03-01

The partially Reynolds-averaged Navier-Stokes (PANS) model can be used to simulate turbulent flows either as RANS, large eddy simulation (LES) or DNS. Its main parameter is fk whose physical meaning is the ratio of the modelled to the total turbulent kinetic energy. In RANS fk = 1, in DNS fk = 0 and in LES fk takes values between 0 and 1. Three different ways of prescribing fk are evaluated for decaying grid turbulence and fully developed channel flow: fk = 0.4, fk = k3/2tot/ɛ and, from its definition, fk = k/ktot where ktot is the sum of the modelled, k, and resolved, kres, turbulent kinetic energy. It is found that the fk = 0.4 gives the best results. In Girimaji and Wallin, a method was proposed to include the effect of the gradient of fk. This approach is used at RANS- LES interface in the present study. Four different interface models are evaluated in fully developed channel flow and embedded LES of channel flow: in both cases, PANS is used as a zonal model with fk = 1 in the unsteady RANS (URANS) region and fk = 0.4 in the LES region. In fully developed channel flow, the RANS- LES interface is parallel to the wall (horizontal) and in embedded LES, it is parallel to the inlet (vertical). The importance of the location of the horizontal interface in fully developed channel flow is also investigated. It is found that the location - and the choice of the treatment at the interface - may be critical at low Reynolds number or if the interface is placed too close to the wall. The reason is that the modelled turbulent shear stress at the interface is large and hence the relative strength of the resolved turbulence is small. In RANS, the turbulent viscosity - and consequently also the modelled Reynolds shear stress - is only weakly dependent on Reynolds number. It is found in the present work that it also applies in the URANS region.

An algebraic stress/flux model for two-phase turbulent flow

International Nuclear Information System (INIS)

Kumar, R.

1995-12-01

An algebraic stress model (ASM) for turbulent Reynolds stress and a flux model for turbulent heat flux are proposed for two-phase bubbly and slug flows. These mathematical models are derived from the two-phase transport equations for Reynolds stress and turbulent heat flux, and provide C μ , a turbulent constant which defines the level of eddy viscosity, as a function of the interfacial terms. These models also include the effect of heat transfer. When the interfacial drag terms and the interfacial momentum transfer terms are absent, the model reduces to a single-phase model used in the literature

Ion Transfer Voltammetry Associated with Two Polarizable Interfaces Within Water and Moderately Hydrophobic Ionic Liquid Systems

DEFF Research Database (Denmark)

Gan, Shiyu; Zhou, Min; Zhang, Jingdong

2013-01-01

An electrochemical system composed of two polarizable interfaces (the metallic electrode|water and water|ionic liquid interfaces), namely two‐polarized‐interface (TPI) technique, has been proposed to explore the ion transfer processes between water and moderately hydrophobic ionic liquids (W...... to an extremely narrow polarized potential window (ppw) caused by these moderately hydrophobic ionic components. In this article, we show that TPI technique has virtually eliminated the ppw limitation based on a controlling step of concentration polarization at the electrode|water interface. With the aid...

Modelling and simulation of turbulence and heat transfer in wall-bounded flows

NARCIS (Netherlands)

Popovac, M.

2006-01-01

At present it is widely accepted that there is no universal turbulence model, i.e. no turbulence model can give acceptably good predictions for all turbulent flows that are found in nature or engineering. Every turbulence model is based on certain assumptions, and hence it is aimed at certain type

Heat Transfer Coefficient at Cast-Mold Interface During Centrifugal Casting: Calculation of Air Gap

Science.gov (United States)

Bohacek, Jan; Kharicha, Abdellah; Ludwig, Andreas; Wu, Menghuai; Karimi-Sibaki, Ebrahim

2018-06-01

During centrifugal casting, the thermal resistance at the cast-mold interface represents a main blockage mechanism for heat transfer. In addition to the refractory coating, an air gap begins to form due to the shrinkage of the casting and the mold expansion, under the continuous influence of strong centrifugal forces. Here, the heat transfer coefficient at the cast-mold interface h has been determined from calculations of the air gap thickness d a based on a plane stress model taking into account thermoelastic stresses, centrifugal forces, plastic deformations, and a temperature-dependent Young's modulus. The numerical approach proposed here is rather novel and tries to offer an alternative to the empirical formulas usually used in numerical simulations for a description of a time-dependent heat transfer coefficient h. Several numerical tests were performed for different coating thicknesses d C, rotation rates Ω, and temperatures of solidus T sol. Results demonstrated that the scenario at the interface is unique for each set of parameters, hindering the possibility of employing empirical formulas without a preceding experiment being performed. Initial values of h are simply equivalent to the ratio of the coating thermal conductivity and its thickness ( 1000 Wm-2 K-1). Later, when the air gap is formed, h drops exponentially to values at least one order of magnitude smaller ( 100 Wm-2 K-1).

Leveraging anatomical information to improve transfer learning in brain-computer interfaces

Science.gov (United States)

Wronkiewicz, Mark; Larson, Eric; Lee, Adrian K. C.

2015-08-01

Objective. Brain-computer interfaces (BCIs) represent a technology with the potential to rehabilitate a range of traumatic and degenerative nervous system conditions but require a time-consuming training process to calibrate. An area of BCI research known as transfer learning is aimed at accelerating training by recycling previously recorded training data across sessions or subjects. Training data, however, is typically transferred from one electrode configuration to another without taking individual head anatomy or electrode positioning into account, which may underutilize the recycled data. Approach. We explore transfer learning with the use of source imaging, which estimates neural activity in the cortex. Transferring estimates of cortical activity, in contrast to scalp recordings, provides a way to compensate for variability in electrode positioning and head morphologies across subjects and sessions. Main results. Based on simulated and measured electroencephalography activity, we trained a classifier using data transferred exclusively from other subjects and achieved accuracies that were comparable to or surpassed a benchmark classifier (representative of a real-world BCI). Our results indicate that classification improvements depend on the number of trials transferred and the cortical region of interest. Significance. These findings suggest that cortical source-based transfer learning is a principled method to transfer data that improves BCI classification performance and provides a path to reduce BCI calibration time.

Study of mass transfer at the air-water interface by an isotopic method

International Nuclear Information System (INIS)

Merlivat, L.

1975-01-01

It is shown by analysing the hydrogen and oxygen stable isotopes distribution in liquid and water vapor, that the processes taking place on a very small scale near the liquid can be investigated. The effect of molecular mass transfer is directly obtained without having to perform difficult measurements in the air in the immediate vicinity of the water surface. Experiments are carried out in the air-water tunnel especially designed for the simulation of ocean atmosphere energy exchanges. The wind velocities vary from 0.7 to 7m/sec. The experimental results obtained do not support the classical Reynolds' analogy between momentum and mass transfer down to the interface and the theory proposed by Sheppard, but they are in agreement with Sverdrup's, Kitaigorodskiy and Volkov's and Brutsaert's theories, all of which involve a layer just above the air-water interface through which mass transfer is dominated by molecular diffusion. The thickness of this layer in the two first theories is shown to decrease with increasing wind velocity. Direct application of Brutsaert's theory for roughness Reynolds numbers smaller than one is in good agreement with the experimental data obtained [fr

Turbulent energy generated by accelerations and shocks

International Nuclear Information System (INIS)

Mikaelian, K.O.

1986-01-01

The turbulent energy generated at the interface between two fluids undergoing a constant acceleration or a shock is calculated. Assuming linear density profiles in the mixed region we find E/sub turbulent//E/sub directed/ = 2.3A 2 % (constant acceleration) and 9.3A 2 % (shock), where A is the Atwood number. Diffusion models predict somewhat less turbulent energy and a density profile with a tail extending into the lower density fluid. Eddy sizes are approximately 27% (constant acceleration) and 17% (shock) of the mixing depth into the heavier fluid. 6 refs., 3 figs

Charge transfer and redistribution at interfaces between metals and 2D materials

NARCIS (Netherlands)

Bokdam, Menno

2013-01-01

Large potential steps are observed at the interfaces between metals and novel 2D materials. They can lower the work function by more than 1 eV, even when the two parts are only weakly interacting. In this thesis the transfer and redistribution of electrons in metal|2D material heterostructures are

Etchant-free graphene transfer using facile intercalation of alkanethiol self-assembled molecules at graphene/metal interfaces.

Science.gov (United States)

Ohtomo, Manabu; Sekine, Yoshiaki; Wang, Shengnan; Hibino, Hiroki; Yamamoto, Hideki

2016-06-02

We report a novel etchant-free transfer method of graphene using the intercalation of alkanethiol self-assembled monolayers (SAMs) at the graphene/Cu interfaces. The early stage of intercalation proceeds through graphene grain boundaries or defects within a few seconds at room temperature until stable SAMs are formed after a few hours. The formation of SAMs releases the compressive strain of graphene induced by Cu substrates and make graphene slightly n-doped due to the formation of interface dipoles of the SAMs on metal surfaces. After SAM formation, the graphene is easily delaminated off from the metal substrates and transferred onto insulating substrates. The etchant-free process enables us to decrease the density of charged impurities and the magnitude of potential fluctuation in the transferred graphene, which suppress scattering of carriers. We also demonstrate the removal of alkanethiol SAMs and reuse the substrate. This method will dramatically reduce the cost of graphene transfer, which will benefit industrial applications such as of graphene transparent electrodes.

Numerical simulation of turbulent flow and heat transfer in a parallel channel. Verification of the field synergy principle

International Nuclear Information System (INIS)

Tian Wenxi; Su, G.H.; Qiu Suizheng; Jia Dounan

2004-01-01

The field synergy principle was proposed by Guo(1998) which is based on 2-D boundary laminar flow and it resulted from a second look at the mechanism of convective heat transfer. Numerical verification of this principle's validity for turbulent flow has been carried out by very few researchers, and mostly commercial software such as FLUENT, CFX etc. were used in their study. In this paper, numerical simulation of turbulent flow with recirculation was developed using SIMPLE algorithm with two-equation k-ε model. Extension of computational region method and wall function method were quoted to regulate the whole computational region geometrically. Given the inlet Reynold number keeps constant: 10000, by changing the height of the solid obstacle, simulation was conducted and the result showed that the wall heat flux decreased with the angle between the velocity vector and the temperature gradient. Thus it is validated that the field synergy principle based on 2-D boundary laminar flow can also be applied to complex turbulent flow even with recirculation. (author)

The Dynamics of Turbulent Scalar Mixing near the Edge of a Shear Layer

Science.gov (United States)

Taveira, R. M. R.; da Silva, C. B.; Pereira, J. C. F.

2011-12-01

In free shear flows a sharp and convoluted turbulent/nonturbulent (T/NT) interface separates the outer fluid region, where the flow is essentially irrotational, from the shear layer turbulent region. It was found recently that the entrainment mechanism is mainly caused by small scale ("nibbling") motions (Westerweel et al. (2005)). The dynamics of this interface is crucial to understand important exchanges of enstrophy and scalars that can be conceived as a three-stage process of entrainment, dispersion and diffusion (Dimotakis (2005)). A thorough understanding of scalar mixing and transport is of indisputable relevance to control turbulent combustion, propulsion and contaminant dispersion (Stanley et al. (2002)). The present work uses several DNS of turbulent jets at Reynolds number ranging from Reλ = 120 to Reλ = 160 (da Silva & Taveira (2010)) and a Schmidt number Sc = 0.7 to analyze the "scalar interface" and turbulent mixing of a passive scalar. Specifically, we employ conditional statistics, denoted by langlerangleI, in order to eliminate the intermittency that affects statistics close to the jet edge. The physical mechanisms behind scalar mixing near the T/NT interfaces, their scales and topology are investigated detail. Analysis of the instantaneous fields showed intense scalar gradient sheet-like structures along regions of persistent strain, in particular at the T/NT interface. The scalar gradient transport equation, at the jet edge, showed that almost all mixing mechanisms are taking place in a confined region, beyond which they become reduced to an almost in perfect balance between production and dissipation of scalar variance. At the T/NT interface transport mechanisms are the ones responsible for the growth in the scalar fluctuations to the entrained fluid, where convection plays a dominant role, smoothing scalar gradients inside the interface and boosting them as far as

The Dynamics of Turbulent Scalar Mixing near the Edge of a Shear Layer

International Nuclear Information System (INIS)

Taveira, R M R; Silva, C B da; Pereira, J C F

2011-01-01

In free shear flows a sharp and convoluted turbulent/nonturbulent (T/NT) interface separates the outer fluid region, where the flow is essentially irrotational, from the shear layer turbulent region. It was found recently that the entrainment mechanism is mainly caused by small scale ('nibbling') motions (Westerweel et al. (2005)). The dynamics of this interface is crucial to understand important exchanges of enstrophy and scalars that can be conceived as a three-stage process of entrainment, dispersion and diffusion (Dimotakis (2005)). A thorough understanding of scalar mixing and transport is of indisputable relevance to control turbulent combustion, propulsion and contaminant dispersion (Stanley et al. (2002)). The present work uses several DNS of turbulent jets at Reynolds number ranging from Re λ = 120 to Re λ = 160 (da Silva and Taveira (2010)) and a Schmidt number Sc = 0.7 to analyze the 'scalar interface' and turbulent mixing of a passive scalar. Specifically, we employ conditional statistics, denoted by I , in order to eliminate the intermittency that affects statistics close to the jet edge. The physical mechanisms behind scalar mixing near the T/NT interfaces, their scales and topology are investigated detail. Analysis of the instantaneous fields showed intense scalar gradient sheet-like structures along regions of persistent strain, in particular at the T/NT interface. The scalar gradient transport equation, at the jet edge, showed that almost all mixing mechanisms are taking place in a confined region, beyond which they become reduced to an almost in perfect balance between production and dissipation of scalar variance. At the T/NT interface transport mechanisms are the ones responsible for the growth in the scalar fluctuations to the entrained fluid, where convection plays a dominant role, smoothing scalar gradients inside the interface 0.1y I /λ to 1y I /λand boosting them as far as -2.5y I /η θ C .

Rational construction of multiple interfaces in ternary heterostructure for efficient spatial separation and transfer of photogenerated carriers in the application of photocatalytic hydrogen evolution

Science.gov (United States)

Shi, Jian-Wen; Ma, Dandan; Zou, Yajun; Fan, Zhaoyang; Shi, Jinwen; Cheng, Linhao; Ji, Xin; Niu, Chunming

2018-03-01

The design of efficient and stable photocatalyst plays a critical role in the photocatalytic hydrogen evolution from water splitting. Herein, we develop a novel ZnS/CdS/ZnO ternary heterostructure by the in-situ sulfuration of CdS/ZnO, which includes four contact interfaces: CdS-ZnS interface, ZnS-ZnO interface, CdS-ZnO interface and ZnS-CdS-ZnO ternary interface, forming three charge carrier-transfer modes (type-I, type-II and direct Z-scheme) through five carrier-transfer pathways. As a result, the separation and transfer of photoexcited electron-hole pairs are promoted significantly, resulting in a high hydrogen evolution rate of 44.70 mmol h-1 g-1, which is 2, 3.7 and 8 times higher than those of binary heterostructures, CdS/ZnO, CdS/ZnS and ZnS/ZnO, respectively, and 26.5, 280 and 298 times higher than those of single CdS, ZnO and ZnS, respectively. As a counterpart ternary heterostructure, CdS/ZnS/ZnO contains only two interfaces: CdS-ZnS interface and ZnS-ZnO interface, which form two charge carrier-transfer modes (type-I and type-II) through two carrier-transfer pathways, leading to its much lower hydrogen evolution rate (27.25 mmol h-1 g-1) than ZnS/CdS/ZnO ternary heterostructure. This work is relevant for understanding the charge-transfer pathways between multi-interfaces in multicomponent heterojunctions.

Numerical study of turbulent heat transfer from confined impinging jets using a pseudo-compressibility method

Energy Technology Data Exchange (ETDEWEB)

Rahman, M.; Rautaheimo, P.; Siikonen, T.

1997-12-31

A numerical investigation is carried out to predict the turbulent fluid flow and heat transfer characteristics of two-dimensional single and three impinging slot jets. Two low-Reynolds-number {kappa}-{epsilon} models, namely the classical model of Chien and the explicit algebraic stress model of Gatski and Speziale, are considered in the simulation. A cell-centered finite-volume scheme combined with an artificial compressibility approach is employed to solve the flow equations, using a diagonally dominant alternating direction implicit (DDADI) time integration method. A fully upwinded second order spatial differencing is adopted to approximate the convective terms. Roe`s damping term is used to calculate the flux on the cell face. A multigrid method is utilized for the acceleration of convergence. On average, the heat transfer coefficients predicted by both models show good agreement with the experimental results. (orig.) 17 refs.

Turbulent solutal convection and surface patterning in solid dissolution

International Nuclear Information System (INIS)

Sullivan, T.S.; Liu, Y.; Ecke, R.E.

1996-01-01

We describe experiments in which crystals of NaCl, KBr, and KCl are dissolved from below by aqueous solutions containing concentrations of the respective salts from zero concentration to near saturation. The solution near the solid-liquid interface is gravitationally unstable, producing turbulent hydrodynamic motion similar to thermal convection from a single surface cooled from above. The coupling of the fluid flow with the solid dissolution produces irregular patterns at the solid-liquid interface with a distribution of horizontal length scales. The dissolution mass flux and the pattern length scales are compared with a turbulent boundary layer model. Remarkable agreement is found, showing that the fluid motion controls both the dissolution rate and the interface patterning. copyright 1996 The American Physical Society

Direct numerical simulation of turbulent flows over superhydrophobic surfaces with gas pockets using linearized boundary conditions

Science.gov (United States)

Seo, Jongmin; Bose, Sanjeeb; Garcia-Mayoral, Ricardo; Mani, Ali

2012-11-01

Superhydrophobic surfaces are shown to be effective for surface drag reduction under laminar regime by both experiments and simulations (see for example, Ou and Rothstein, Phys. Fluids 17:103606, 2005). However, such drag reduction for fully developed turbulent flow maintaining the Cassie-Baxter state remains an open problem due to high shear rates and flow unsteadiness of turbulent boundary layer. Our work aims to develop an understanding of mechanisms leading to interface breaking and loss of gas pockets due to interactions with turbulent boundary layers. We take advantage of direct numerical simulation of turbulence with slip and no-slip patterned boundary conditions mimicking the superhydrophobic surface. In addition, we capture the dynamics of gas-water interface, by deriving a proper linearized boundary condition taking into account the surface tension of the interface and kinematic matching of interface deformation and normal velocity conditions on the wall. We will show results from our simulations predicting the dynamical behavior of gas pocket interfaces over a wide range of dimensionless surface tensions. Supported by the Office of Naval Research and the Kwanjeong Educational Scholarship Foundation.

Electron transfer reactions to probe the electrode/solution interface

Energy Technology Data Exchange (ETDEWEB)

Capitanio, F.; Guerrini, E.; Colombo, A.; Trasatti, S. [Milan Univ., Milan (Italy). Dept. of Physical Chemistry and Electrochemistry

2008-07-01

The reactions that occur at the interface between an electrode and an electrolyte were examined with particular reference to the interaction of different electrode surfaces with redox couples. A semi-integration or convolution technique was used to study the kinetics of electron transfer on different electrode materials with different hydrophilic behaviour, such as Boron-Doped-Diamond (BDD), Au and Pt. Standard reversible redox couples were also investigated, including (Fe3+/2+, Fe(CN)63-/4-, Ru(NH3)63+/2+, Co(NH3)63+/2+, Ir4+/3+, V4+/5+ and V3+/2+). The proposed method proved to be simple, straightforward and reliable since the obtained kinetic information was in good agreement with data in the literature. It was concluded that the kinetics of the electrode transfer reactions depend on the chemical nature of the redox couple and electrode material. The method should be further extended to irreversible couples and other electrode materials such as mixed oxide electrodes. 3 refs., 2 figs.

Bridging the Science-Management Divide: Moving from Unidirectional Knowledge Transfer to Knowledge Interfacing and Sharing

Directory of Open Access Journals (Sweden)

Dirk J. Roux

2006-06-01

Full Text Available Sustainable ecosystem management relies on a diverse and multi-faceted knowledge system in which techniques are continuously updated to reflect current understanding and needs. The challenge is to minimize delay as ideas flow from intent through scientific capability, and finally to implementation to achieve desired outcomes. The best way to do this is by setting the stage for the flow of knowledge between researchers, policy makers, and resource managers. The cultural differences between these groups magnify the challenge. This paper highlights the importance of the tacit dimension of knowledge, and how this renders the concept of knowledge transfer much less useful than the concepts of information transfer and technology transfer. Instead of knowledge transfer, we propose that "co-production" of knowledge through collaborative learning between "experts" and "users" is a more suitable approach to building a knowledge system for the sustainable management of ecosystems. This can be achieved through knowledge interfacing and sharing, but requires a shift from a view of knowledge as a "thing" that can be transferred to viewing knowledge as a "process of relating" that involves negotiation of meaning among partners. Lessons from informal communities of practice provide guidance on how to nurture and promote knowledge interfacing between science and management in R&D programs.

LES of stratified-wavy flows using novel near-interface treatment

Science.gov (United States)

Karnik, Aditya; Kahouadji, Lyes; Chergui, Jalel; Juric, Damir; Shin, Seungwon; Matar, Omar K.

2017-11-01

The pressure drop in horizontal stratified wavy flows is influenced by interfacial shear stress. The near-interface behavior of the lighter phase is akin to that near a moving wall. We employ a front-tracking code, Blue, to simulate and capture the near-interface behaviour of both phases. Blue uses a modified Smagorinsky LES model incorporating a novel near-interface treatment for the sub-grid viscosity, which is influenced by damping due to the wall-like interface, and enhancement of the turbulent kinetic energy (TKE) due to the interfacial waves. Simulations are carried out for both air-water and oil-water stratified configurations to demonstrate the applicability of the present method. The mean velocities and tangential Reynolds stresses are compared with experiments for both configurations. At the higher Re, the waves penetrate well into the buffer region of the boundary layer above the interface thus altering its dynamics. Previous attempts to capture the secondary structures associated with such flows using RANS or standard LES methodologies have been unsuccessful. The ability of the present method to reproduce these structures is due to the correct estimation of the near-interface TKE governing energy transfer from the normal to tangential directions. EPSRC, UK, MEMPHIS program Grant (EP/K003976/1), RAEng Research Chair (OKM).

Fencing data transfers in a parallel active messaging interface of a parallel computer

Science.gov (United States)

Blocksome, Michael A.; Mamidala, Amith R.

2015-06-02

Fencing data transfers in a parallel active messaging interface (`PAMI`) of a parallel computer, the PAMI including data communications endpoints, each endpoint including a specification of data communications parameters for a thread of execution on a compute node, including specifications of a client, a context, and a task; the compute nodes coupled for data communications through the PAMI and through data communications resources including at least one segment of shared random access memory; including initiating execution through the PAMI of an ordered sequence of active SEND instructions for SEND data transfers between two endpoints, effecting deterministic SEND data transfers through a segment of shared memory; and executing through the PAMI, with no FENCE accounting for SEND data transfers, an active FENCE instruction, the FENCE instruction completing execution only after completion of all SEND instructions initiated prior to execution of the FENCE instruction for SEND data transfers between the two endpoints.

A note on the standard electron transfer potential at the interface between two immiscible electrolyte solutions

Czech Academy of Sciences Publication Activity Database

Samec, Zdeněk

2009-01-01

Roč. 55, č. 2 (2009), s. 75-81 ISSN 0034-6691 R&D Projects: GA ČR(CZ) GA203/07/1257 Institutional research plan: CEZ:AV0Z40400503 Keywords : interface between two immiscible electrolyte solutions * interfacial electron transfer * standard electron trasfer potential * homogeneous electron transfer Subject RIV: CG - Electrochemistry

The dynamics of interacting nonlinearities governing long wavelength driftwave turbulence

International Nuclear Information System (INIS)

Newman, D.E.

1993-09-01

Because of the ubiquitous nature of turbulence and the vast array of different systems which have turbulent solutions, the study of turbulence is an area of active research. Much present day understanding of turbulence is rooted in the well established properties of homogeneous Navier-Stokes turbulence, which, due to its relative simplicity, allows for approximate analytic solutions. This work examines a group of turbulent systems with marked differences from Navier-Stokes turbulence, and attempts to quantify some of their properties. This group of systems represents a variety of drift wave fluctuations believed to be of fundamental importance in laboratory fusion devices. From extensive simulation of simple local fluid models of long wavelength drift wave turbulence in tokamaks, a reasonably complete picture of the basic properties of spectral transfer and saturation has emerged. These studies indicate that many conventional notions concerning directions of cascades, locality and isotropy of transfer, frequencies of fluctuations, and stationarity of saturation are not valid for moderate to long wavelengths. In particular, spectral energy transfer at long wavelengths is dominated by the E x B nonlinearity, which carries energy to short scale in a manner that is highly nonlocal and anisotropic. In marked contrast to the canonical self-similar cascade dynamics of Kolmogorov, energy is efficiently passed between modes separated by the entire spectrum range in a correlation time. At short wavelengths, transfer is dominated by the polarization drift nonlinearity. While the standard dual cascade applies in this subrange, it is found that finite spectrum size can produce cascades that are reverse directed and are nonconservative in enstrophy and energy similarity ranges. In regions where both nonlinearities are important, cross-coupling between the nolinearities gives rise to large no frequency shifts as well as changes in the spectral dynamics

Study of two-dimensional interchange turbulence

International Nuclear Information System (INIS)

Sugama, Hideo; Wakatani, Masahiro.

1990-04-01

An eddy viscosity model describing enstrophy transfer in two-dimensional turbulence is presented. This model is similar to that of Canuto et al. and provides an equation for the energy spectral function F(k) as a function of the energy input rate to the system per unit wavenumber, γ s (k). In the enstrophy-transfer inertial range, F(k)∝ k -3 is predicted by the model. The eddy viscosity model is applied to the interchange turbulence of a plasma in shearless magnetic field. Numerical simulation of the two-dimensional interchange turbulence demonstrates that the energy spectrum in the high wavenumber region is well described by this model. The turbulent transport driven by the interchange turbulence is expressed in terms of the Nusselt number Nu, the Rayleigh number Ra and Prantl number Pr in the same manner as that of thermal convection problem. When we use the linear growth rate for γ s (k), our theoretical model predicts that Nu ∝ (Ra·Pr) 1/2 for a constant background pressure gradient and Nu ∝ (Ra·Pr) 1/3 for a self-consistent background pressure profile with the stress-free slip boundary conditions. The latter agrees with our numerical result showing Nu ∝ Ra 1/3 . (author)

Contribution of the bubbles to gas transfer across the ocean-atmosphere interface

International Nuclear Information System (INIS)

Memery, Laurent

1983-05-01

A first theoretical approach to gas transfer by bubbles is undertaken. Certain parameters which are neglected by smooth air-water interface models are studied. It is found that transfer velocity increases when solubility decreases. Further, bubble overpressure leads to water supersaturation at equilibrium, this supersaturation being more significant for less soluble gases. Although the transfer velocity remains roughly constant for a variable concentration gradient far from equilibrium, its range of variation becomes infinite near equilibrium. Because the notion of transfer velocity is not useful near equilibrium, attention is turned directly to the flux itself: the flux is a linear function of the concentration gradient. At least for tracers the coefficients of this function are entirely defined by the physico-chemical properties of the gas and by the bubble distribution. The dissertation is divided in three parts: - a synthesis which sums up the main experimental and theoretical results of the study of the influence of the bubbles created by breaking waves on gas transfer, - an article published in 'Journal of Geophysical Research', - an article submitted to 'Tellus'. (author) [fr

Heat transfer to surface and gaps of RSI tile arrays in turbulent flow at Mach 10.3

Science.gov (United States)

Throckmorton, D. A.

1974-01-01

Heat transfer to gap walls and surface of a simulated reusable surface insulation (RSI) tile array are presented. The data were obtained in the thick, turbulent tunnel wall boundary layer of the Langley Continuous Flow Hypersonic Tunnel at a freestream Mach number of 10.3 and a freestream unit Reynolds number of one million. Pertinent test variables were: (1) tile array orientation (staggered and in-line), (2) gap width, (3) flow angularity, and (4) tile mismatch.

Investigations of effect of phase change mass transfer rate on cavitation process with homogeneous relaxation model

Energy Technology Data Exchange (ETDEWEB)

He, Zhixia; Zhang, Liang; Saha, Kaushik; Som, Sibendu; Duan, Lian; Wang, Qian

2017-12-01

The super high fuel injection pressure and micro size of nozzle orifice has been an important development trend for the fuel injection system. Accordingly, cavitation transient process, fuel compressibility, amount of noncondensable gas in the fuel and cavitation erosion have attracted more attention. Based on the fact of cavitation in itself is a kind of thermodynamic phase change process, this paper takes the perspective of the cavitation phase change mass transfer process to analyze above mentioned phenomenon. The two-phase cavitating turbulent flow simulations with VOF approach coupled with HRM cavitation model and U-RANS of standard k-ε turbulence model were performed for investigations of cavitation phase change mass transfer process. It is concluded the mass transfer time scale coefficient in the Homogenous Relaxation Model (HRM) representing mass transfer rate should tend to be as small as possible in a condition that ensured the solver stable. At very fast mass transfer rate, the phase change occurs at very thin interface between liquid and vapor phase and condensation occurs more focused and then will contribute predictably to a more serious cavitation erosion. Both the initial non-condensable gas in fuel and the fuel compressibility can accelerate the cavitation mass transfer process.

Gas mass transfer for stratified flows

International Nuclear Information System (INIS)

Duffey, R.B.; Hughes, E.D.

1995-01-01

We analyzed gas absorption and release in water bodies using existing surface renewal theory. We show a new relation between turbulent momentum and mass transfer from gas to water, including the effects of waves and wave roughness, by evaluating the equilibrium integral turbulent dissipation due to energy transfer to the water from the wind. Using Kolmogoroff turbulence arguments the gas transfer velocity, or mass transfer coefficient, is then naturally and straightforwardly obtained as a non-linear function of the wind speed drag coefficient and the square root of the molecular diffusion coefficient. In dimensionless form, the theory predicts the turbulent Sherwood number to be Sh t = (2/√π)Sc 1/2 , where Sh t is based on an integral dissipation length scale in the air. The theory confirms the observed nonlinear variation of the mass transfer coefficient as a function of the wind speed; gives the correct transition with turbulence-centered models for smooth surfaces at low speeds; and predicts experimental data from both laboratory and environmental measurements within the data scatter. The differences between the available laboratory and field data measurements are due to the large differences in the drag coefficient between wind tunnels and oceans. The results also imply that the effect of direct aeration due to bubble entrainment at wave breaking is no more than a 20% increase in the mass transfer for the highest speeds. The theory has importance to mass transfer in both the geo-physical and chemical engineering literature

Assessment of CFD Hypersonic Turbulent Heating Rates for Space Shuttle Orbiter

Science.gov (United States)

Wood, William A.; Oliver, A. Brandon

2011-01-01

Turbulent CFD codes are assessed for the prediction of convective heat transfer rates at turbulent, hypersonic conditions. Algebraic turbulence models are used within the DPLR and LAURA CFD codes. The benchmark heat transfer rates are derived from thermocouple measurements of the Space Shuttle orbiter Discovery windward tiles during the STS-119 and STS-128 entries. The thermocouples were located underneath the reaction-cured glass coating on the thermal protection tiles. Boundary layer transition flight experiments conducted during both of those entries promoted turbulent flow at unusually high Mach numbers, with the present analysis considering Mach 10{15. Similar prior comparisons of CFD predictions directly to the flight temperature measurements were unsatisfactory, showing diverging trends between prediction and measurement for Mach numbers greater than 11. In the prior work, surface temperatures and convective heat transfer rates had been assumed to be in radiative equilibrium. The present work employs a one-dimensional time-accurate conduction analysis to relate measured temperatures to surface heat transfer rates, removing heat soak lag from the flight data, in order to better assess the predictive accuracy of the numerical models. The turbulent CFD shows good agreement for turbulent fuselage flow up to Mach 13. But on the wing in the wake of the boundary layer trip, the inclusion of tile conduction effects does not explain the prior observed discrepancy in trends between simulation and experiment; the flight heat transfer measurements are roughly constant over Mach 11-15, versus an increasing trend with Mach number from the CFD.

Charge transfer mechanism for the formation of metallic states at the KTaO3/SrTiO3 interface

KAUST Repository

Nazir, Safdar

2011-03-29

The electronic and optical properties of the KTaO3/SrTiO3 heterointerface are analyzed by the full-potential linearized augmented plane-wave approach of density functional theory. Optimization of the atomic positions points at subordinate changes in the crystal structure and chemical bonding near the interface, which is due to a minimal lattice mismatch. The creation of metallic interface states thus is not affected by structural relaxation but can be explained by charge transfer between transition metal and oxygen atoms. It is to be expected that a charge transfer is likewise important for related interfaces such as LaAlO3/SrTiO3. The KTaO3/SrTiO3 system is ideal for disentangling the complex behavior of metallic interface states, since almost no structural relaxation takes place.

Characteristics of Turbulent Transfer during Episodes of Heavy Haze Pollution in Beijing in Winter 2016/17

Science.gov (United States)

Ren, Yan; Zheng, Shuwen; Wei, Wei; Wu, Bingui; Zhang, Hongsheng; Cai, Xuhui; Song, Yu

2018-02-01

We analyzed the structure and evolution of turbulent transfer and the wind profile in the atmospheric boundary layer in relation to aerosol concentrations during an episode of heavy haze pollution from 6 December 2016 to 9 January 2017. The turbulence data were recorded at Peking University's atmospheric science and environment observation station. The results showed a negative correlation between the wind speed and the PM2.5 concentration. The turbulence kinetic energy was large and showed obvious diurnal variations during unpolluted (clean) weather, but was small during episodes of heavy haze pollution. Under both clean and heavy haze conditions, the relation between the non-dimensional wind components and the stability parameter z/ L followed a 1/3 power law, but the normalized standard deviations of the wind speed were smaller during heavy pollution events than during clean periods under near-neutral conditions. Under unstable conditions, the normalized standard deviation of the potential temperature σ θ /| θ *| was related to z/ L, roughly following a -1/3 power law, and the ratio during pollution days was greater than that during clean days. The three-dimensional turbulence energy spectra satisfied a -2/3 power exponent rate in the high-frequency band. In the low-frequency band, the wind velocity spectrum curve was related to the stability parameters under clear conditions, but was not related to atmospheric stratification under polluted conditions. In the dissipation stage of the heavy pollution episode, the horizontal wind speed first started to increase at high altitudes and then gradually decreased at lower altitudes. The strong upward motion during this stage was an important dynamic factor in the dissipation of the heavy haze.

About the possible options for models of convective heat transfer in closed volumes with local heating source

Directory of Open Access Journals (Sweden)

Maksimov Vyacheslav I.

2015-01-01

Full Text Available Results of mathematical modeling of convective heat transfer in air area surrounded on all sides enclosing structures, in the presence of heat source at the lower boundary of the media are presented. Solved the system of differential equations of unsteady Navier-Stokes equations with the appropriate initial and boundary conditions. The process of convective heat transfer is calculated using the models of turbulence Prandtl and Prandtl-Reichard. Takes into account the processes of heat exchange region considered with the environment. Is carried out the analysis of the dimensionless heat transfer coefficient at interfaces “air – enclosures”. The distributions average along the gas temperature range are obtained.

Design of twisted tape turbulator at different entrance angle for heat transfer enhancement in a solar heater

Directory of Open Access Journals (Sweden)

Suvanjan Bhattacharyya

2018-03-01

Full Text Available Numerical investigation of heat transfer characteristics in a tube fitted with inserted twisted tape swirl generator is performed. The twisted tapes are separately inserted from the tube wall. The configuration parameters include the, entrance angle (α and pitch (H. Investigations have been done in the range of α = 180°, 160° and 140° with Reynolds number varying between 100 and 20,000. In this paper, transition – SST model which can predict the transition of flow regime from laminar through intermittent to turbulent has been utilized for numerical simulations. The computational results are in good agreement with experimental data. The results show that higher entrance angle yields a higher heat transfer value. The using of single twist twisted tape supplies considerable increase on heat transfer and pressure drop when compared with the conventional twisted tapes. A large data set has been generated for heat transfer and thermal-hydraulic performance which is useful for the design of solar thermal heaters and heat exchangers.

Advanced k-epsilon modeling of heat transfer

Science.gov (United States)

Kwon, Okey; Ames, Forrest E.

1995-01-01

This report describes two approaches to low Reynolds-number k-epsilon turbulence modeling which formulate the eddy viscosity on the wall-normal component of turbulence and a length scale. The wall-normal component of turbulence is computed via integration of the energy spectrum based on the local dissipation rate and is bounded by the isotropic condition. The models account for the anisotropy of the dissipation and the reduced mixing length due to the high strain rates present in the near-wall region. The turbulent kinetic energy and its dissipation rate were computed from the k and epsilon transport equations of Durbin. The models were tested for a wide range of turbulent flows and proved to be superior to other k-epsilon models, especially for nonequilibrium anisotropic flows. For the prediction of airfoil heat transfer, the models included a set of empirical correlations for predicting laminar-turbulent transition and laminar heat transfer augmentation due to the presence of freestream turbulence. The predictions of surface heat transfer were generally satisfactory.

Strain transfer through film-substrate interface and surface curvature evolution during a tensile test

Science.gov (United States)

He, Wei; Han, Meidong; Goudeau, Philippe; Bourhis, Eric Le; Renault, Pierre-Olivier; Wang, Shibin; Li, Lin-an

2018-03-01

Uniaxial tensile tests on polyimide-supported thin metal films are performed to respectively study the macroscopic strain transfer through an interface and the surface curvature evolution. With a dual digital image correlation (DIC) system, the strains of the film and the substrate can be simultaneously measured in situ during the tensile test. For the true strains below 2% (far beyond the films' elastic limit), a complete longitudinal strain transfer is present irrespective of the film thickness, residual stresses and microstructure. By means of an optical surface profiler, the three-dimensional (3D) topography of film surface can be obtained during straining. As expected, the profile of the specimen center remains almost flat in the tensile direction. Nevertheless, a relatively significant curvature evolution (of the same order with the initial curvature induced by residual stresses) is observed along the transverse direction as a result of a Poisson's ratio mismatch between the film and the substrate. Furthermore, finite element method (FEM) has been performed to simulate the curvature evolution considering the geometric nonlinearity and the perfect strain transfer at the interface, which agrees well with the experimental results.

Development of model for studies on momentum transfer in electrochemical cells with entry region coil as turbulence promoter

Science.gov (United States)

Penta Rao, Tamarba; Rajendra Prasad, P.

2018-04-01

Entry region swirl promoters gain importance in industry because of its effectiveness in augmentation of mass and heat transfer augmentation. Design of equipment needs momentum transfer data along with mass or heat transfer data. Hence an experimental investigation was carried out with coaxially placed entry region spiral coil as turbulence promoters on momentum transfer in forced convection flow of electrolyte in circular conduits. Aqueous solution of sodium hydroxide and 0.01 M equimolal Ferri-ferro cyanide system was chosen for the study. The study covered parameters like effect of pitch of the coil, effect of length of the coil, diameter of the coil, diameter of the coil wire, diameter of the annular rod. The promoter is measured by limiting current technique using diffusion controlled electrochemical reactions. The study comprises of evaluation of momentum transfer rates at the outer wall of the electrochemical cell. Pressure drop measurements were also made to obtain the energy consumption pattern. Within the range of variables covered. The results are correlated by the momentum transfer similarity function. Momentum transfer coefficients were evaluated from measured limiting currents. Effect of each parameter was studied in terms of friction factor. A model was developed for momentum transfer. The experimental data on momentum transfer was modeled in terms of momentum transfer function and Reynolds number, geometric parameters.

Decaying and kicked turbulence in a shell model

DEFF Research Database (Denmark)

Hooghoudt, Jan Otto; Lohse, Detlef; Toschi, Federico

2001-01-01

Decaying and periodically kicked turbulence are analyzed within the Gledzer–Ohkitani–Yamada shell model, to allow for sufficiently large scaling regimes. Energy is transferred towards the small scales in intermittent bursts. Nevertheless, mean field arguments are sufficient to account for the ens......Decaying and periodically kicked turbulence are analyzed within the Gledzer–Ohkitani–Yamada shell model, to allow for sufficiently large scaling regimes. Energy is transferred towards the small scales in intermittent bursts. Nevertheless, mean field arguments are sufficient to account...

Augmented of turbulent heat transfer in an annular pipe with abrupt expansion

Directory of Open Access Journals (Sweden)

Togun Hussein

2016-01-01

Full Text Available This paper presents a study of heat transfer to turbulent air flow in the abrupt axisymmetric expansion of an annular pipe. The experimental investigations were performed in the Reynolds number range from 5000 to 30000, the heat flux varied from 1000 to 4000 W/m2, and the expansion ratio was maintained at D/d=1, 1.25, 1.67 and 2. The sudden expansion was created by changing the inner diameter of the entrance pipe to an annular passage. The outer diameter of the inner pipe and the inner diameter of the outer pipe are 2.5 and 10 cm, respectively, where both of the pipes are subjected to uniform heat flux. The distribution of the surface temperature of the test pipe and the local Nusselt number are presented in this investigation. Due to sudden expansion in the cross section of the annular pipe, a separation flow was created, which enhanced the heat transfer. The reduction of the surface temperature on the outer and inner pipes increased with the increase of the expansion ratio and the Reynolds number, and increased with the decrease of the heat flux to the annular pipe. The peak of the local Nusselt number was between 1.64 and 1.7 of the outer and inner pipes for Reynolds numbers varied from 5000 to 30000, and the increase of the local Nusselt number represented the augmentation of the heat transfer rate in the sudden expansion of the annular pipe. This research also showed a maximum heat transfer enhancement of 63-78% for the outer and inner pipes at an expansion ratio of D/d=2 at a Re=30000 and a heat flux of 4000W/m2.

The mechanism by which nonlinearity sustains turbulence in plane Couette flow

Science.gov (United States)

Nikolaidis, M.-A.; Farrell, B. F.; Ioannou, P. J.

2018-04-01

Turbulence in wall-bounded shear flow results from a synergistic interaction between linear non-normality and nonlinearity in which non-normal growth of a subset of perturbations configured to transfer energy from the externally forced component of the turbulent state to the perturbation component maintains the perturbation energy, while the subset of energy-transferring perturbations is replenished by nonlinearity. Although it is accepted that both linear non-normality mediated energy transfer from the forced component of the mean flow and nonlinear interactions among perturbations are required to maintain the turbulent state, the detailed physical mechanism by which these processes interact in maintaining turbulence has not been determined. In this work a statistical state dynamics based analysis is performed on turbulent Couette flow at R = 600 and a comparison to DNS is used to demonstrate that the perturbation component in Couette flow turbulence is replenished by a non-normality mediated parametric growth process in which the fluctuating streamwise mean flow has been adjusted to marginal Lyapunov stability. It is further shown that the alternative mechanism in which the subspace of non-normally growing perturbations is maintained directly by perturbation-perturbation nonlinearity does not contribute to maintaining the turbulent state. This work identifies parametric interaction between the fluctuating streamwise mean flow and the streamwise varying perturbations to be the mechanism of the nonlinear interaction maintaining the perturbation component of the turbulent state, and identifies the associated Lyapunov vectors with positive energetics as the structures of the perturbation subspace supporting the turbulence.

Experiential Interface Design for the Transference of Scientific Publications from University to SMEs

DEFF Research Database (Denmark)

Lykke, Marianne; Løkkegaard, Sarai; Jantzen, Christian

2017-01-01

to develop suggestions for the interface design of a generic knowledge pathway. The interface design is based on principles from experience design with the aim to draw attention to the scientific knowledge and make it desirable and understandable. The study used practice theory as its conceptual framework......Transferring knowledge between universities and industry is known to be problematic. The paper addresses the situation of small and medium-sized enterprises (SMEs) in order to understand their use of scientific knowledge. We use our understanding of SME situations and information practices...... to interface design. Generally, the SMEs look for knowledge in two situations: when they have a pressing problem to solve and when they want inspiration for new business opportunities. The SMEs consult many different channels and scientific knowledge from universities is not their first choice. SMEs see...

Turbulent mixed convection from a large, high temperature, vertical flat surface

International Nuclear Information System (INIS)

Evans, G.; Greif, R.; Siebers, D.; Tieszen, S.

2005-01-01

Turbulent mixed convection heat transfer at high temperatures and large length scales is an important and seldom studied phenomenon that can represent a significant part of the overall heat transfer in applications ranging from solar central receivers to objects in fires. This work is part of a study to validate turbulence models for predicting heat transfer to or from surfaces at large temperature differences and large length scales. Here, turbulent, three-dimensional, mixed convection heat transfer in air from a large (3m square) vertical flat surface at high temperatures is studied using two RANS turbulence models: a standard k-ε model and the v2-bar -f model. Predictions for three cases spanning the range of the experiment (Siebers, D.L., Schwind, R.G., Moffat, R.F., 1982. Experimental mixed convection from a large, vertical plate in a horizontal flow. Paper MC13, vol. 3, Proc. 7th Int. Heat Transfer Conf., Munich; Siebers, D.L., 1983. Experimental mixed convection heat transfer from a large, vertical surface in a horizontal flow. PhD thesis, Stanford University) from forced (GrH/ReL2=0.18) to mixed (GrH/ReL2=3.06) to natural (GrH/ReL2=∼) convection are compared with data. The results show a decrease in the heat transfer coefficient as GrH/ReL2 is increased from 0.18 to 3.06, for a free-stream velocity of 4.4m/s. In the natural convection case, the experimental heat transfer coefficient is approximately constant in the fully turbulent region, whereas the calculated heat transfer coefficients show a slight increase with height. For the three cases studied, the calculated and experimental heat transfer coefficients agree to within 5-35% over most of the surface with the v2-bar -f model results showing better agreement with the data. Calculated temperature and velocity profiles show good agreement with the data

On the inverse transfer of (non-)helical magnetic energy in a decaying magnetohydrodynamic turbulence

Science.gov (United States)

Park, Kiwan

2017-12-01

In our conventional understanding, large-scale magnetic fields are thought to originate from an inverse cascade in the presence of magnetic helicity, differential rotation or a magneto-rotational instability. However, as recent simulations have given strong indications that an inverse cascade (transfer) may occur even in the absence of magnetic helicity, the physical origin of this inverse cascade is still not fully understood. We here present two simulations of freely decaying helical and non-helical magnetohydrodynamic (MHD) turbulence. We verified the inverse transfer of helical and non-helical magnetic fields in both cases, but we found the underlying physical principles to be fundamentally different. In the former case, the helical magnetic component leads to an inverse cascade of magnetic energy. We derived a semi-analytic formula for the evolution of large-scale magnetic field using α coefficient and compared it with the simulation data. But in the latter case, the α effect, including other conventional dynamo theories, is not suitable to describe the inverse transfer of non-helical magnetic energy. To obtain a better understanding of the physics at work here, we introduced a 'field structure model' based on the magnetic induction equation in the presence of inhomogeneities. This model illustrates how the curl of the electromotive force leads to the build up of a large-scale magnetic field without the requirement of magnetic helicity. And we applied a quasi-normal approximation to the inverse transfer of magnetic energy.

Development of bubble-induced turbulence model for advanced two-fluid model

International Nuclear Information System (INIS)

Hosoi, Hideaki; Yoshida, Hiroyuki

2011-01-01

A two-fluid model can simulate two-phase flow by computational cost less than detailed two-phase flow simulation method such as interface tracking method. The two-fluid model is therefore useful for thermal hydraulic analysis in the large-scale domain such as rod bundles. However, since the two-fluid model includes a lot of constitutive equations verified by use of experimental results, it has problems that the result of analyses depends on accuracy of the constitutive equations. To solve these problems, an advanced two-fluid model has been developed by Japan Atomic Energy Agency. In this model, interface tracking method is combined with two-fluid model to accurately predict large interface structure behavior. Liquid clusters and bubbles larger than a computational cell are calculated using the interface tracking method, and those smaller than the cell are simulated by the two-fluid model. The constitutive equations to evaluate the effects of small bubbles or droplets on two-phase flow are also required in the advanced two-fluid model, just as with the conventional two-fluid model. However, the dependency of small bubbles and droplets on two-phase flow characteristics is relatively small, and fewer experimental results are required to verify the characteristics of large interface structures. Turbulent dispersion force model is one of the most important constitutive equations for the advanced two-fluid model. The turbulent dispersion force model has been developed by many researchers for the conventional two-fluid model. However, existing models implicitly include the effects of large bubbles and the deformation of bubbles, and are unfortunately not applicable to the advanced two-fluid model. In the previous study, the authors suggested the turbulent dispersion force model based on the analogy of Brownian motion. And the authors improved the turbulent dispersion force model in consideration of bubble-induced turbulence to improve the analysis results for small

Turbulent heavy liquid metal heat transfer along a heated rod in an annular cavity

International Nuclear Information System (INIS)

Lefhalm, C.-H.; Tak, N.-I.; Piecha, H.; Stieglitz, R.

2004-01-01

Heavy liquid metals (HLM) are considered as coolant and spallation material in accelerator driven systems (ADS), because of their good molecular heat conductivity. This property leads to a separation of the spatial extension of thermal and viscous boundary layers. Commercially available computational fluid dynamic codes (CFD) assume an analogy of momentum and energy transfer, which is problematic for liquid metals flow. Therefore, benchmark experiments are required, in order to validate codes or modify existing models used therein. Within this article an experimental and numerical study of a thermally developing turbulent lead bismuth (PbBi) flow along a uniformly heated rod in a circular tube is presented. Local temperatures and velocity distributions are measured using thermocouples and Pitot tubes. The data are compared to simulation results computed with the CFX code package. The measured velocity profiles coincide nearly perfect with the simulation results. However, discrepancies up to 7% between the measured and computed temperatures appear. A minor part of the deviations can be explained by the imperfect experimental set-up. But, the measured shape of the thermal boundary is different to the calculated one, indicating the inadequateness of the presently used models describing the turbulent heat transport within the thermal boundary layer

Numerical simulation of turbulent flow and heat transfer in parallel channel with an obstacle and verification of the field synergy principle

Energy Technology Data Exchange (ETDEWEB)

Tian, W.; Aye, M.; Qiu, S.; Jia, D. [Xi' an Jiaotong Univ., Dept. of Nuclear and Thermal Power Engineering, Xi' an (China)]. E-mail: wxtian_xjtu@163.com

2004-07-01

The field synergy principle was proposed by Guo Z. Y based on 2-D boundary laminar flow and it resulted from a second look at the mechanism of convective heat transfer. The objective of this paper is to numerically verify the applicability of this theory under turbulent flow or even with recirculating flow condition. (author)

Suppression of Phase Mixing in Drift-Kinetic Plasma Turbulence

Science.gov (United States)

Parker, J. T.; Dellar, P. J.; Schekochihin, A. A.; Highcock, E. G.

2017-12-01

The solar wind and interstellar medium are examples of strongly magnetised, weakly collisional, astrophysical plasmas. Their turbulent fluctuations are strongly anisotropic, with small amplitudes, and frequencies much lower than the Larmor frequency. This regime is described by gyrokinetic theory, a reduced five-dimensional kinetic system describing averages over Larmor orbits. A turbulent plasma may transfer free energy, a measure of fluctuation amplitudes, from injection at large scales, typically by an instability, to dissipation at small physical scales like a turbulent fluid. Alternatively, a turbulent plasma may form fine scale structures in velocity space via phase-mixing, the mechanism that leads to Landau damping in linear plasma theory. Macroscopic plasma properties like heat and momentum transport are affected by both mechanisms. While each is understood in isolation, their interaction is not. We study this interaction using a Hankel-Hermite velocity space representation of gyrokinetic theory. The Hankel transform interacts neatly with the Bessel functions that arise from averaging over Larmor orbits, so the perpendicular velocity space is decoupled for linearized problems. The Hermite transform expresses phase mixing as nearest-neighbor coupling between parallel velocity space scales represented by Hermite mode numbers. We use this representation to study transfer mechanisms in drift-kinetic plasma turbulence, the long wavelength limit of gyrokinetic theory. We show that phase space is divided into two regions, with one transfer mechanism dominating in each. Most energy is contained in the region where the fluid-like nonlinear cascade dominates. Moreover, in that region the nonlinear cascade interferes with phase mixing by exciting an "anti phase mixing" transfer of free energy from small to large velocity space scales. This cancels out the usual phase mixing, and renders the overall behavior fluid-like. These results profoundly change our understanding

A formal derivation of the local energy transfer (LET) theory of homogeneous turbulence

Science.gov (United States)

McComb, W. D.; Yoffe, S. R.

2017-09-01

A statistical closure of the Navier-Stokes hierarchy which leads to equations for the two-point, two-time covariance of the velocity field for stationary, homogeneous isotropic turbulence is presented. It is a generalisation of the self-consistent field method due to Edwards (1964) for the stationary, single-time velocity covariance. The probability distribution functional P≤ft[\\mathbf{u},t\\right] is obtained, in the form of a series, from the Liouville equation by means of a perturbation expansion about a Gaussian distribution, which is chosen to give the exact two-point, two-time covariance. The triple moment is calculated in terms of an ensemble-averaged infinitesimal velocity-field propagator, and shown to yield the Edwards result as a special case. The use of a Gaussian zero-order distribution has been found to justify the introduction of a fluctuation-response relation, which is in accord with modern dynamical theories. In a sense this work completes the analogy drawn by Edwards between turbulence and Brownian motion. Originally Edwards had shown that the noise input was determined by the correlation of the velocity field with the externally applied stirring forces but was unable to determine the system response. Now we find that the system response is determined by the correlation of the velocity field with internal quasi-entropic forces. This analysis is valid to all orders of perturbation theory, and allows the recovery of the local energy transfer (LET) theory, which had previously been derived by more heuristical methods. The LET theory is known to be in good agreement with experimental results. It is also unique among two-point statistical closures in displaying an acceptable (i.e. non-Markovian) relationship between the transfer spectrum and the system response, in accordance with experimental results. As a result of the latter property, it is compatible with the Kolmogorov (K41) spectral phenomenology. In memory of Professor Sir Sam Edwards F

Effect of Twisted-Tape Turbulators and Nanofluid on Heat Transfer in a Double Pipe Heat Exchanger

Directory of Open Access Journals (Sweden)

Heydar Maddah

2014-01-01

Full Text Available Heat transfer and overall heat transfer in a double pipe heat exchanger fitted with twisted-tape elements and titanium dioxide nanofluid were studied experimentally. The inner and outer diameters of the inner tube were 8 and 16 mm, respectively, and cold and hot water were used as working fluids in shell side and tube side. The twisted tapes were made from aluminum sheet with tape thickness (d of 1 mm, width (W of 5 mm, and length of 120 cm. Titanium dioxide nanoparticles with a diameter of 30 nm and a volume concentration of 0.01% (v/v were prepared. The effects of temperature, mass flow rate, and concentration of nanoparticles on the overall heat transfer coefficient, heat transfer changes in the turbulent flow regime Re≥2300, and counter current flow were investigated. When using twisted tape and nanofluid, heat transfer coefficient was about 10 to 25 percent higher than when they were not used. It was also observed that the heat transfer coefficient increases with operating temperature and mass flow rate. The experimental results also showed that 0.01% TiO2/water nanofluid with twisted tape has slightly higher friction factor and pressure drop when compared to 0.01% TiO2/water nanofluid without twisted tape. The empirical correlations proposed for friction factor are in good agreement with the experimental data.

Dissipation range turbulent cascades in plasmas

International Nuclear Information System (INIS)

Terry, P. W.; Almagri, A. F.; Forest, C. B.; Nornberg, M. D.; Rahbarnia, K.; Sarff, J. S.; Fiksel, G.; Hatch, D. R.; Jenko, F.; Prager, S. C.; Ren, Y.

2012-01-01

Dissipation range cascades in plasma turbulence are described and spectra are formulated from the scaled attenuation in wavenumber space of the spectral energy transfer rate. This yields spectra characterized by the product of a power law and exponential fall-off, applicable to all scales. Spectral indices of the power law and exponential fall-off depend on the scaling of the dissipation, the strength of the nonlinearity, and nonlocal effects when dissipation rates of multiple fluctuation fields are different. The theory is used to derive spectra for MHD turbulence with magnetic Prandtl number greater than unity, extending previous work. The theory is also applied to generic plasma turbulence by considering the spectrum from damping with arbitrary wavenumber scaling. The latter is relevant to ion temperature gradient turbulence modeled by gyrokinetics. The spectrum in this case has an exponential component that becomes weaker at small scale, giving a power law asymptotically. Results from the theory are compared to three very different types of turbulence. These include the magnetic plasma turbulence of the Madison Symmetric Torus, the MHD turbulence of liquid metal in the Madison Dynamo Experiment, and gyrokinetic simulation of ion temperature gradient turbulence.

Interface tracking simulations of bubbly flows in PWR relevant geometries

Energy Technology Data Exchange (ETDEWEB)

Fang, Jun, E-mail: jfang3@ncsu.edu [Department of Nuclear Engineering, North Carolina State University, Raleigh, NC 27695 (United States); Rasquin, Michel, E-mail: michel.rasquin@colorado.edu [Aerospace Engineering Department, University of Colorado, Boulder, CO 80309 (United States); Bolotnov, Igor A., E-mail: igor_bolotnov@ncsu.edu [Department of Nuclear Engineering, North Carolina State University, Raleigh, NC 27695 (United States)

2017-02-15

Highlights: • Simulations were performed for turbulent bubbly flows in PWR subchannel geometry. • Liquid turbulence is fully resolved by direct numerical simulation approach. • Bubble behavior is captured using level-set interface tracking method. • Time-averaged single- and two-phase turbulent flow statistical quantities are obtained. - Abstract: The advances in high performance computing (HPC) have allowed direct numerical simulation (DNS) approach coupled with interface tracking methods (ITM) to perform high fidelity simulations of turbulent bubbly flows in various complex geometries. In this work, we have chosen the geometry of the pressurized water reactor (PWR) core subchannel to perform a set of interface tracking simulations (ITS) with fully resolved liquid turbulence. The presented research utilizes a massively parallel finite-element based code, PHASTA, for the subchannel geometry simulations of bubbly flow turbulence. The main objective for this research is to demonstrate the ITS capabilities in gaining new insight into bubble/turbulence interactions and assisting the development of improved closure laws for multiphase computational fluid dynamics (M-CFD). Both single- and two-phase turbulent flows were studied within a single PWR subchannel. The analysis of numerical results includes the mean gas and liquid velocity profiles, void fraction distribution and turbulent kinetic energy profiles. Two sets of flow rates and bubble sizes were used in the simulations. The chosen flow rates corresponded to the Reynolds numbers of 29,079 and 80,775 based on channel hydraulic diameter (D{sub h}) and mean velocity. The finite element unstructured grids utilized for these simulations include 53.8 million and 1.11 billion elements, respectively. This has allowed to fully resolve all the turbulence scales and the deformable interfaces of individual bubbles. For the two-phase flow simulations, a 1% bubble volume fraction was used which resulted in 17 bubbles in

Direct numerical simulation and statistical analysis of turbulent convection in lead-bismuth

Energy Technology Data Exchange (ETDEWEB)

Otic, I.; Grotzbach, G. [Forschungszentrum Karlsruhe GmbH, Institut fuer Kern-und Energietechnik (Germany)

2003-07-01

Improved turbulent heat flux models are required to develop and analyze the reactor concept of an lead-bismuth cooled Accelerator-Driven-System. Because of specific properties of many liquid metals we have still no sensors for accurate measurements of the high frequency velocity fluctuations. So, the development of the turbulent heat transfer models which are required in our CFD (computational fluid dynamics) tools needs also data from direct numerical simulations of turbulent flows. We use new simulation results for the model problem of Rayleigh-Benard convection to show some peculiarities of the turbulent natural convection in lead-bismuth (Pr = 0.025). Simulations for this flow at sufficiently large turbulence levels became only recently feasible because this flow requires the resolution of very small velocity scales with the need for recording long-wave structures for the slow changes in the convective temperature field. The results are analyzed regarding the principle convection and heat transfer features. They are also used to perform statistical analysis to show that the currently available modeling is indeed not adequate for these fluids. Basing on the knowledge of the details of the statistical features of turbulence in this convection type and using the two-point correlation technique, a proposal for an improved statistical turbulence model is developed which is expected to account better for the peculiarities of the heat transfer in the turbulent convection in low Prandtl number fluids. (authors)

Effect of wind waves on air-sea gas exchange: proposal of an overall CO2 transfer velocity formula as a function of breaking-wave parameter

International Nuclear Information System (INIS)

Zhao, D.; Suzuki, Y.; Komori, S.

2003-01-01

A new formula for gas transfer velocity as a function of the breaking-wave parameter is proposed based on correlating gas transfer with whitecap coverage. The new formula for gas transfer across an air-sea interface depends not only on wind speed but also on wind-wave state. At the same wind speed, a higher gas transfer velocity will be obtained for a more developed wind-sea, which is represented by a smaller spectral peak frequency of wind waves. We suggest that the large uncertainties in the traditional relationship of gas transfer velocity with wind speed be ascribed to the neglect of the effect of wind waves. The breaking-wave parameter can be regarded as a Reynolds number that characterizes the intensity of turbulence associated with wind waves in the downward-bursting boundary layer (DBBL). DBBL provides an effective way to exchange gas across the air-sea interface, which might be related to the surface renewal

A turbulent mixing Reynolds stress model fitted to match linear interaction analysis predictions

International Nuclear Information System (INIS)

Griffond, J; Soulard, O; Souffland, D

2010-01-01

To predict the evolution of turbulent mixing zones developing in shock tube experiments with different gases, a turbulence model must be able to reliably evaluate the production due to the shock-turbulence interaction. In the limit of homogeneous weak turbulence, 'linear interaction analysis' (LIA) can be applied. This theory relies on Kovasznay's decomposition and allows the computation of waves transmitted or produced at the shock front. With assumptions about the composition of the upstream turbulent mixture, one can connect the second-order moments downstream from the shock front to those upstream through a transfer matrix, depending on shock strength. The purpose of this work is to provide a turbulence model that matches LIA results for the shock-turbulent mixture interaction. Reynolds stress models (RSMs) with additional equations for the density-velocity correlation and the density variance are considered here. The turbulent states upstream and downstream from the shock front calculated with these models can also be related through a transfer matrix, provided that the numerical implementation is based on a pseudo-pressure formulation. Then, the RSM should be modified in such a way that its transfer matrix matches the LIA one. Using the pseudo-pressure to introduce ad hoc production terms, we are able to obtain a close agreement between LIA and RSM matrices for any shock strength and thus improve the capabilities of the RSM.

Study of turbulent natural-circulation flow and low-Prandtl-number forced-convection flow

International Nuclear Information System (INIS)

Chung, K.S.; Thompson, D.H.

1980-01-01

Calculational methods and results are discussed for the coupled energy and momentum equations of turbulent natural circulation flow and low Prandtl number forced convection flow. The objective of this paper is to develop a calculational method for the study of the thermal-hydraulic behavior of coolant flowing in a liquid metal fast breeder reactor channel under natural circulation conditions. The two-equation turbulence model is used to evaluate the turbulent momentum transport property. Because the analogy between momentum transfer and heat transfer does not generally hold for low Prandtl number fluid and natural circulation flow conditions, the turbulent thermal conductivity is calculated independently using equations similar to the two-equation turbulence model. The numerical technique used in the calculation is the finite element method

Simulation analysis of air flow and turbulence statistics in a rib grit roughened duct.

Science.gov (United States)

Vogiatzis, I I; Denizopoulou, A C; Ntinas, G K; Fragos, V P

2014-01-01

The implementation of variable artificial roughness patterns on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the ducts of solar air heaters. Different geometries of roughness elements investigated have demonstrated the pivotal role that vortices and associated turbulence have on the heat transfer characteristics of solar air heater ducts by increasing the convective heat transfer coefficient. In this paper we investigate the two-dimensional, turbulent, unsteady flow around rectangular ribs of variable aspect ratios by directly solving the transient Navier-Stokes and continuity equations using the finite elements method. Flow characteristics and several aspects of turbulent flow are presented and discussed including velocity components and statistics of turbulence. The results reveal the impact that different rib lengths have on the computed mean quantities and turbulence statistics of the flow. The computed turbulence parameters show a clear tendency to diminish downstream with increasing rib length. Furthermore, the applied numerical method is capable of capturing small-scale flow structures resulting from the direct solution of Navier-Stokes and continuity equations.

Turbulent Combustion Modeling Advances, New Trends and Perspectives

CERN Document Server

Echekki, Tarek

2011-01-01

Turbulent combustion sits at the interface of two important nonlinear, multiscale phenomena: chemistry and turbulence. Its study is extremely timely in view of the need to develop new combustion technologies in order to address challenges associated with climate change, energy source uncertainty, and air pollution. Despite the fact that modeling of turbulent combustion is a subject that has been researched for a number of years, its complexity implies that key issues are still eluding, and a theoretical description that is accurate enough to make turbulent combustion models rigorous and quantitative for industrial use is still lacking. In this book, prominent experts review most of the available approaches in modeling turbulent combustion, with particular focus on the exploding increase in computational resources that has allowed the simulation of increasingly detailed phenomena. The relevant algorithms are presented, the theoretical methods are explained, and various application examples are given. The book ...

Modeling 3D conjugate heat and mass transfer for turbulent air drying of Chilean papaya in a direct contact dryer

Science.gov (United States)

Lemus-Mondaca, Roberto A.; Vega-Gálvez, Antonio; Zambra, Carlos E.; Moraga, Nelson O.

2017-01-01

A 3D model considering heat and mass transfer for food dehydration inside a direct contact dryer is studied. The k- ɛ model is used to describe turbulent air flow. The samples thermophysical properties as density, specific heat, and thermal conductivity are assumed to vary non-linearly with temperature. FVM, SIMPLE algorithm based on a FORTRAN code are used. Results unsteady velocity, temperature, moisture, kinetic energy and dissipation rate for the air flow are presented, whilst temperature and moisture values for the food also are presented. The validation procedure includes a comparison with experimental and numerical temperature and moisture content results obtained from experimental data, reaching a deviation 7-10 %. In addition, this turbulent k- ɛ model provided a better understanding of the transport phenomenon inside the dryer and sample.

Noise and ac impedance analysis of ion transfer kinetics at the micro liquid/liquid interface

Czech Academy of Sciences Publication Activity Database

Josypčuk, Oksana; Holub, Karel; Mareček, Vladimír

2015-01-01

Roč. 56, JUL 2015 (2015), s. 43-45 ISSN 1388-2481 R&D Projects: GA ČR GA13-04630S Institutional support: RVO:61388955 Keywords : noise analysis * liquid/liquid interface * ion transfer kinetics Subject RIV: CG - Electrochemistry Impact factor: 4.569, year: 2015

Entropy generation in turbulent mixed convection heat transfer to highly variable property pipe flow of supercritical fluids

International Nuclear Information System (INIS)

Mohseni, Mahdi; Bazargan, Majid

2014-01-01

Highlights: • The entropy generation in supercritical fluid flows has been numerically investigated. • The mechanisms of entropy generation are different near and away from the walls. • In the near wall region, the energy dissipation is the deciding parameter. • Away from the wall, the heat transfer is the effective factor in entropy generation. • The bulk Be number is greater in the liquid-like region than in vapor-like region. - Abstract: In this study, a two dimensional CFD code has been developed to investigate entropy generation in turbulent mixed convection heat transfer flow of supercritical fluids. Since the fluid properties vary significantly under supercritical conditions, the changes of entropy generation are large. The contribution of each of the mechanisms of entropy production (heat transfer and energy dissipation) is compared in different regions of the flow. The results show that the mechanisms of entropy generation act differently in the near wall region within the viscous sub-layer and in the region away from the wall. The effects of the wall heat flux on the entropy generation are also investigated

Radial transfer effects for poloidal rotation

Science.gov (United States)

Hallatschek, Klaus

2010-11-01

Radial transfer of energy or momentum is the principal agent responsible for radial structures of Geodesic Acoustic Modes (GAMs) or stationary Zonal Flows (ZF) generated by the turbulence. For the GAM, following a physical approach, it is possible to find useful expressions for the individual components of the Poynting flux or radial group velocity allowing predictions where a mathematical full analysis is unfeasible. Striking differences between up-down symmetric flux surfaces and asymmetric ones have been found. For divertor geometries, e.g., the direction of the propagation depends on the sign of the ion grad-B drift with respect to the X-point, reminiscent of a sensitive determinant of the H-mode threshold. In nonlocal turbulence computations it becomes obvious that the linear energy transfer terms can be completely overwhelmed by the action of the turbulence. In contrast, stationary ZFs are governed by the turbulent radial transfer of momentum. For sufficiently large systems, the Reynolds stress becomes a deterministic functional of the flows, which can be empirically determined from the stress response in computational turbulence studies. The functional allows predictions even on flow/turbulence states not readily obtainable from small amplitude noise, such as certain transport bifurcations or meta-stable states.

Heat-transfer resistance at solid-liquid interfaces: a tool for the detection of single-nucleotide polymorphisms in DNA.

Science.gov (United States)

van Grinsven, Bart; Vanden Bon, Natalie; Strauven, Hannelore; Grieten, Lars; Murib, Mohammed; Monroy, Kathia L Jiménez; Janssens, Stoffel D; Haenen, Ken; Schöning, Michael J; Vermeeren, Veronique; Ameloot, Marcel; Michiels, Luc; Thoelen, Ronald; De Ceuninck, Ward; Wagner, Patrick

2012-03-27

In this article, we report on the heat-transfer resistance at interfaces as a novel, denaturation-based method to detect single-nucleotide polymorphisms in DNA. We observed that a molecular brush of double-stranded DNA grafted onto synthetic diamond surfaces does not notably affect the heat-transfer resistance at the solid-to-liquid interface. In contrast to this, molecular brushes of single-stranded DNA cause, surprisingly, a substantially higher heat-transfer resistance and behave like a thermally insulating layer. This effect can be utilized to identify ds-DNA melting temperatures via the switching from low- to high heat-transfer resistance. The melting temperatures identified with this method for different DNA duplexes (29 base pairs without and with built-in mutations) correlate nicely with data calculated by modeling. The method is fast, label-free (without the need for fluorescent or radioactive markers), allows for repetitive measurements, and can also be extended toward array formats. Reference measurements by confocal fluorescence microscopy and impedance spectroscopy confirm that the switching of heat-transfer resistance upon denaturation is indeed related to the thermal on-chip denaturation of DNA. © 2012 American Chemical Society

Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Importance of Molecular Orientation

KAUST Repository

Ayzner, Alexander L.

2015-01-02

© 2014 American Chemical Society. Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C60 acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation.

Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Importance of Molecular Orientation

KAUST Repository

Ayzner, Alexander L.; Nordlund, Dennis; Kim, Do-Hwan; Bao, Zhenan; Toney, Michael F.

2015-01-01

© 2014 American Chemical Society. Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C60 acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation.

Shear flow generation and energetics in electromagnetic turbulence

DEFF Research Database (Denmark)

Naulin, V.; Kendl, A.; Garcia, O.E.

2005-01-01

acoustic mode (GAM) transfer in drift-Alfvén turbulence is investigated. By means of numerical computations the energy transfer into zonal flows owing to each of these effects is quantified. The importance of the three driving ingredients in electrostatic and electromagnetic turbulence for conditions...... relevant to the edge of fusion devices is revealed for a broad range of parameters. The Reynolds stress is found to provide a flow drive, while the electromagnetic Maxwell stress is in the cases considered a sink for the flow energy. In the limit of high plasma β, where electromagnetic effects and Alfvén...

Turbulent heat transfer and nanofluid flow in a protruded ribbed square passage

Science.gov (United States)

Kumar, Sunil; Kothiyal, Alok Darshan; Bisht, Mangal Singh; Kumar, Anil

In this article, turbulent heat transfer of nanofluid flow in square passage with protruded rib shape is numerically and experimentally studied over Reynolds number ranges of 4000-18000. Different nanoparticles (Al2O3, CuO, and ZnO), with different concentration (φ) range of 1-4% and different nanoparticle diameter (dnp) range of 30-45 nm are disperse in water (base fluid). Several parameters such as stream wise distance (Xs /dp) range of 1.4-2.6, span wise distance (Ys /dp) range of 1.4-2.6, ratio of protruded height to print diameter (ep /dp) range of 0.83-1.67 also studied to find the consequence on thermal and hydrodynamic characteristics. Simulations were carried out to obtain heat and fluid flow behaviour of smooth and ribbed square channel using commercial CFD software, ANSYS 15.0 (Fluent). Renormalization k - ε model was employed to assess the influence of protruded ribs on turbulent flow and velocity field. The outcome indicates that Al2O3 nanofluid has the highest value of average Nusselt number as compare to other nanofluids. The average Nusselt number increases as the concentration increases and it decreases as nanoparticle diameter increases. The thermal hydrodynamic performance parameter based on equal pumping power, average Nusselt number and average friction factor were found to be highest for Al2O3, φ = 0.04, dnp = 30 nm, Xs /dp = 1.8, Ys /dp = 1.8 and ep /dp = 1.0 . The numerical data are compared with the corresponding experimental data. Comparison between CFD and experimental analysis results showed that good agreement as the data fell within ±7.0% error band.

Development of a system code with CFD capability for analyzing turbulent mixed convection in gas-cooled reactors

International Nuclear Information System (INIS)

Kim, Hyeon Il

2010-02-01

In order to demonstrate the accuracy of predictions in a turbulent mixed convection regime in which both inertia and buoyancy force compete with each other, we found out that assessments done using a single-dimensional system code with a recently updated heat transfer package have shown that this approach cannot give a reasonable prediction of the wall temperature in a case involving strong heating, where the regime falls into turbulent mixed convection regime. It has been known that the main reason of this deficiency comes from the degraded heat transfer in turbulent mixed convection regime, which is below that of convective heat transfer during turbulent forced convection. We investigated two mechanisms that cause this deterioration in convective heat transfer influenced by buoyancy: (1) modification of turbulence, also known as the direct (structural) effect, through the buoyancy-induced production of turbulent kinetic energy: and (2) an indirect (external) effect that occurs through modification of the mean flow. We investigated the Launder-Sharma model of turbulence whether it can appropriately represent the mechanisms causing the degraded heat transfer in Computational Fluid Dynamics (CFD). We found out that this model can capture low Re effects such that a non-equilibrium turbulent boundary layer in turbulent mixed convection regime can be resolved. The model was verified and validated extensively initially with the commercial CFD code, Fluent with a user application package known as the User Defined Function (UDF). The results from this implementation were compared to a set of data that included (1) an experimental data commonly accepted as a standardized problem to verify a turbulent flow, (2) the results from a Direct Numerical Simulation (DNS) in a turbulent forced and mixed convection regime, (3) empirical correlations regarding the friction coefficient and the non-dimensional heat transfer coefficient, the Nusselt number for a turbulent forced

Broadband Prosthetic Interfaces: Combining Nerve Transfers and Implantable Multichannel EMG Technology to Decode Spinal Motor Neuron Activity

Directory of Open Access Journals (Sweden)

Konstantin D. Bergmeister

2017-07-01

Full Text Available Modern robotic hands/upper limbs may replace multiple degrees of freedom of extremity function. However, their intuitive use requires a high number of control signals, which current man-machine interfaces do not provide. Here, we discuss a broadband control interface that combines targeted muscle reinnervation, implantable multichannel electromyographic sensors, and advanced decoding to address the increasing capabilities of modern robotic limbs. With targeted muscle reinnervation, nerves that have lost their targets due to an amputation are surgically transferred to residual stump muscles to increase the number of intuitive prosthetic control signals. This surgery re-establishes a nerve-muscle connection that is used for sensing nerve activity with myoelectric interfaces. Moreover, the nerve transfer determines neurophysiological effects, such as muscular hyper-reinnervation and cortical reafferentation that can be exploited by the myoelectric interface. Modern implantable multichannel EMG sensors provide signals from which it is possible to disentangle the behavior of single motor neurons. Recent studies have shown that the neural drive to muscles can be decoded from these signals and thereby the user's intention can be reliably estimated. By combining these concepts in chronic implants and embedded electronics, we believe that it is in principle possible to establish a broadband man-machine interface, with specific applications in prosthesis control. This perspective illustrates this concept, based on combining advanced surgical techniques with recording hardware and processing algorithms. Here we describe the scientific evidence for this concept, current state of investigations, challenges, and alternative approaches to improve current prosthetic interfaces.

Turbulent Spot Characterization and the Modeling of Transitional Heat Transfer in Turbines

National Research Council Canada - National Science Library

LaGraff, John

2001-01-01

.... The purpose of this research is to obtain more information on the generation. propagation and coalescence of turbulent spots in a transitional boundary layer and examine the effects of free stream turbulence pressure gradient and cross flow...

Macro-scale turbulence modelling for flows in porous media

International Nuclear Information System (INIS)

Pinson, F.

2006-03-01

- This work deals with the macroscopic modeling of turbulence in porous media. It concerns heat exchangers, nuclear reactors as well as urban flows, etc. The objective of this study is to describe in an homogenized way, by the mean of a spatial average operator, turbulent flows in a solid matrix. In addition to this first operator, the use of a statistical average operator permits to handle the pseudo-aleatory character of turbulence. The successive application of both operators allows us to derive the balance equations of the kind of flows under study. Two major issues are then highlighted, the modeling of dispersion induced by the solid matrix and the turbulence modeling at a macroscopic scale (Reynolds tensor and turbulent dispersion). To this aim, we lean on the local modeling of turbulence and more precisely on the k - ε RANS models. The methodology of dispersion study, derived thanks to the volume averaging theory, is extended to turbulent flows. Its application includes the simulation, at a microscopic scale, of turbulent flows within a representative elementary volume of the porous media. Applied to channel flows, this analysis shows that even within the turbulent regime, dispersion remains one of the dominating phenomena within the macro-scale modeling framework. A two-scale analysis of the flow allows us to understand the dominating role of the drag force in the kinetic energy transfers between scales. Transfers between the mean part and the turbulent part of the flow are formally derived. This description significantly improves our understanding of the issue of macroscopic modeling of turbulence and leads us to define the sub-filter production and the wake dissipation. A f - f - w >f model is derived. It is based on three balance equations for the turbulent kinetic energy, the viscous dissipation and the wake dissipation. Furthermore, a dynamical predictor for the friction coefficient is proposed. This model is then successfully applied to the study of

Numerical study of heat and mass transfer during evaporation of a turbulent binary liquid film

Directory of Open Access Journals (Sweden)

Khalal Larbi

2015-01-01

Full Text Available This paper deals with a computational study for analysing heat and mass exchanges in the evaporation of a turbulent binary liquid film (water-ethanol and water-methanol along a vertical tube. The film is in co-current with the dry air and the tube wall is subjected to a uniform heat flux. The effect of gas-liquid phase coupling, variable thermophysical properties and film vaporization are considered in the analysis. The numerical method applied solves the coupled governing equations together with the boundary and interfacial conditions. The algebraic systems of equations obtained are solved using the Thomas algorithm. The results concern the effects of the inlet liquid Reynolds number and inlet film composition on the intensity of heat and mass transfer. In this study, results obtained show that heat transferred through the latent mode is more pronounced when the concentration of volatile components is higher in the liquid mixture .The comparisons of wall temperature and accumulated mass evaporation rate with the literature results are in good agreement.

A non-local shell model of hydrodynamic and magnetohydrodynamic turbulence

Energy Technology Data Exchange (ETDEWEB)

Plunian, F [Laboratoire de Geophysique Interne et Tectonophysique, CNRS, Universite Joseph Fourier, Maison des Geosciences, BP 53, 38041 Grenoble Cedex 9 (France); Stepanov, R [Institute of Continuous Media Mechanics, Korolyov 1, 614013 Perm (Russian Federation)

2007-08-15

We derive a new shell model of magnetohydrodynamic (MHD) turbulence in which the energy transfers are not necessarily local. Like the original MHD equations, the model conserves the total energy, magnetic helicity, cross-helicity and volume in phase space (Liouville's theorem) apart from the effects of external forcing, viscous dissipation and magnetic diffusion. The model of hydrodynamic (HD) turbulence is derived from the MHD model setting the magnetic field to zero. In that case the conserved quantities are the kinetic energy and the kinetic helicity. In addition to a statistically stationary state with a Kolmogorov spectrum, the HD model exhibits multiscaling. The anomalous scaling exponents are found to depend on a free parameter {alpha} that measures the non-locality degree of the model. In freely decaying turbulence, the infra-red spectrum also depends on {alpha}. Comparison with theory suggests using {alpha} = -5/2. In MHD turbulence, we investigate the fully developed turbulent dynamo for a wide range of magnetic Prandtl numbers in both kinematic and dynamic cases. Both local and non-local energy transfers are clearly identified.

Forward and inverse cascades in decaying two-dimensional electron magnetohydrodynamic turbulence

International Nuclear Information System (INIS)

Wareing, C. J.; Hollerbach, R.

2009-01-01

Electron magnetohydrodynamic (EMHD) turbulence in two dimensions is studied via high-resolution numerical simulations with a normal diffusivity. The resulting energy spectra asymptotically approach a k -5/2 law with increasing R B , the ratio of the nonlinear to linear time scales in the governing equation. No evidence is found of a dissipative cutoff, consistent with nonlocal spectral energy transfer. Dissipative cutoffs found in previous studies are explained as artificial effects of hyperdiffusivity. Relatively stationary structures are found to develop in time, rather than the variability found in ordinary or MHD turbulence. Further, EMHD turbulence displays scale-dependent anisotropy with reduced energy transfer in the direction parallel to the uniform background field, consistent with previous studies. Finally, the governing equation is found to yield an inverse cascade, at least partially transferring magnetic energy from small to large scales.

Investigation on the applicability of turbulent-Prandtl-number models for liquid lead-bismuth eutectic

Energy Technology Data Exchange (ETDEWEB)

Chen, Fei, E-mail: chenfei@iet.cn [Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190 (China); North China University of Water Resources and Electric Power, Zhengzhou, Henan 450011 (China); Huai, Xiulan, E-mail: hxl@iet.cn [Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190 (China); Cai, Jun, E-mail: caijun@iet.cn [Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190 (China); Li, Xunfeng, E-mail: lixunfeng@iet.cn [Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190 (China); Meng, Ruixue, E-mail: mengruixue@iet.cn [Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190 (China)

2013-04-15

Highlights: ► We examine the applicability of various Pr{sub t} models into the simulation of LBE flow. ► Reynolds analogy suitable for conventional fluids cannot accurately simulate the heat transfer characteristics of LBE flow. ► The different Pr{sub t} model should be selected for the different thermal boundary condition of LBE flow. -- Abstract: With the proposal of Accelerator Driven Sub-critical System (ADS) together with liquid lead-bismuth eutectic (LBE) as coolant for both reactor and spallation target, the use of accurate heat transfer correlation and reliable turbulent-Prandtl-number model of LBE in turbulent flows is essential when designing ADS components of primary loop and heat exchanger of secondary loop. Unlike conventional fluids, there is not an acknowledged turbulent-Prandtl-number model for LBE flows. This paper reviews and assesses the existing turbulent-Pandtl-number models and various heat transfer correlations in circular tubes. Computational fluid dynamics (CFD) analysis is employed to evaluate the applicability of various turbulent-Prandtl-number models for LBE in the circular tube under boundary conditions of constant heat flux and constant wall temperature. Based on the assessment of turbulent-Prandtl-number models, the reliable turbulent-Prandtl-number models are recommended for CFD applications to LBE flows under boundary conditions of constant heat flux and constant wall temperature. The present study indicates that turbulent Prandtl number has a significant difference in turbulent LBE flow between constant-heat-flux and constant-wall-temperature boundary conditions.

Numerical analysis of flow resistance and heat transfer in the transitional regime of pipe flow with twisted-tape turbulators

Science.gov (United States)

Rossi, R.; Cattani, L.; Mocerino, A.; Bozzoli, F.; Rainieri, S.; Caminati, R.; Pagliarini, G.

2017-11-01

In this paper, we present the numerical analysis of the fully developed ow and heat transfer in pipes equipped with twisted-tape inserts in the laminar to transitional flow regime. The flow Reynolds number ranges from 210 to 3100 based on the pipe diameter, whereas the Prandtl number of the working fluid, a 40% mixture of water and ethylene glycol, is about 45 at the average film temperature. The numerical study is carried out via Scale Adaptive Simulations (SAS) where the k-ω SST model is employed for turbulence modeling. Using SAS and low-dissipation discretization schemes, the present study shows that it is possible to capture the transition from the laminar regime to the pulsating or pseudo-laminar flow regime induced by the twisted-tape at low Reynolds numbers, as well as the transition to moderate turbulent regime at the higher, yet non-turbulent for smooth pipes, range of Reynolds numbers. Numerical results, validated against experiments performed in a dedicated test rig, show very good agreement with measured data and an increase of the friction factor and Nusselt number in the range of 4 to 7 times and 6 to 15 times, respectively, of the values for an empty pipe.

Suppression of material transfer at contacting surfaces: the effect of adsorbates on Al/TiN and Cu/diamond interfaces from first-principles calculations

Science.gov (United States)

Feldbauer, Gregor; Wolloch, Michael; Bedolla, Pedro O.; Redinger, Josef; Vernes, András; Mohn, Peter

2018-03-01

The effect of monolayers of oxygen (O) and hydrogen (H) on the possibility of material transfer at aluminium/titanium nitride (Al/TiN) and copper/diamond (Cu/Cdia) interfaces, respectively, were investigated within the framework of density functional theory (DFT). To this end the approach, contact, and subsequent separation of two atomically flat surfaces consisting of the aforementioned pairs of materials were simulated. These calculations were performed for the clean as well as oxygenated and hydrogenated Al and Cdia surfaces, respectively. Various contact configurations were considered by studying several lateral arrangements of the involved surfaces at the interface. Material transfer is typically possible at interfaces between the investigated clean surfaces; however, the addition of O to the Al and H to the Cdia surfaces was found to hinder material transfer. This passivation occurs because of a significant reduction of the adhesion energy at the examined interfaces, which can be explained by the distinct bonding situations.

Spectral Transfer Learning using Information Geometry for a User-Independent Brain-Computer Interface

OpenAIRE

Nicholas Roy Waytowich; Nicholas Roy Waytowich; Vernon Lawhern; Vernon Lawhern; Addison Bohannon; Addison Bohannon; Kenneth Ball; Brent Lance

2016-01-01

Recent advances in signal processing and machine learning techniques have enabled the application of Brain-Computer Interface (BCI) technologies to fields such as medicine, industry and recreation. However, BCIs still suffer from the requirement of frequent calibration sessions due to the intra- and inter- individual variability of brain-signals, which makes calibration suppression through transfer learning an area of increasing interest for the development of practical BCI systems. In this p...

Spectral Transfer Learning Using Information Geometry for a User-Independent Brain-Computer Interface

OpenAIRE

Waytowich, Nicholas R.; Lawhern, Vernon J.; Bohannon, Addison W.; Ball, Kenneth R.; Lance, Brent J.

2016-01-01

Recent advances in signal processing and machine learning techniques have enabled the application of Brain-Computer Interface (BCI) technologies to fields such as medicine, industry, and recreation; however, BCIs still suffer from the requirement of frequent calibration sessions due to the intra- and inter-individual variability of brain-signals, which makes calibration suppression through transfer learning an area of increasing interest for the development of practical BCI systems. In this p...

Effect of wall thermal conductivity on the heat transfer process in annular turbulent gas flow for constant wall temperature

International Nuclear Information System (INIS)

Groshev, A.I.; Anisimov, V.V.; Kashcheev, V.M.; Khudasko, V.V.; Yur'ev, Yu.S.

1987-01-01

The effect of wall material on convective heat transfer of turbulent gas flow in an annular tube with account of longitudinal diffusion both in the wall and in the liquid is studied numerically. The conjugated problem is solved for P r =0.7 (Re=10 4 -10 6 ). Based on numerical calculations it is stated that thermal conductivity of the wall and gas essentially affects the degree of preliminary heating of liquid in the range of a non-heated section

Considering fluctuation energy as a measure of gyrokinetic turbulence

International Nuclear Information System (INIS)

Plunk, G G; Tatsuno, T; Dorland, W

2012-01-01

In gyrokinetic theory, there are two quadratic measures of fluctuation energy, left invariant under nonlinear interactions, that constrain turbulence. In a recent work (Plunk and Tatsuno 2011 Phys. Rev. Lett. 106 165003) we reported on the novel consequences that this constraint has for the direction and locality of spectral energy transfer. This paper builds on that previous work. We provide a detailed analysis in support of the results of Plunk and Tatsuno (2011 Phys. Rev. Lett. 106 165003), but significantly broaden the scope and use additional methods to address the problem of energy transfer. The perspective taken here is that the fluctuation energies are not merely formal invariants of an idealized model (two-dimensional gyrokinetics (Plunk et al 2010 J. Fluid Mech. 664 407–35)) but also general measures of gyrokinetic turbulence, i.e. quantities that can be used to predict the behavior of turbulence. Although many questions remain open, this paper collects evidence in favor of this perspective by demonstrating in several contexts that constrained spectral energy transfer governs the dynamics. (paper)

A weakened cascade model for turbulence in astrophysical plasmas

International Nuclear Information System (INIS)

Howes, G. G.; TenBarge, J. M.; Dorland, W.

2011-01-01

A refined cascade model for kinetic turbulence in weakly collisional astrophysical plasmas is presented that includes both the transition between weak and strong turbulence and the effect of nonlocal interactions on the nonlinear transfer of energy. The model describes the transition between weak and strong MHD turbulence and the complementary transition from strong kinetic Alfven wave (KAW) turbulence to weak dissipating KAW turbulence, a new regime of weak turbulence in which the effects of shearing by large scale motions and kinetic dissipation play an important role. The inclusion of the effect of nonlocal motions on the nonlinear energy cascade rate in the dissipation range, specifically the shearing by large-scale motions, is proposed to explain the nearly power-law energy spectra observed in the dissipation range of both kinetic numerical simulations and solar wind observations.

Efficient charge generation by relaxed charge-transfer states at organic interfaces

KAUST Repository

Vandewal, Koen

2013-11-17

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy. © 2014 Macmillan Publishers Limited.

Efficient charge generation by relaxed charge-transfer states at organic interfaces

KAUST Repository

Vandewal, Koen; Albrecht, Steve N.; Hoke, Eric T.; Graham, Kenneth; Widmer, Johannes; Douglas, Jessica D.; Schubert, Marcel; Mateker, William R.; Bloking, Jason T.; Burkhard, George F.; Sellinger, Alan; Frechet, Jean; Amassian, Aram; Riede, Moritz Kilian; McGehee, Michael D.; Neher, Dieter; Salleo, Alberto

2013-01-01

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy. © 2014 Macmillan Publishers Limited.

Numerical investigation of turbulent fluid flow and heat transfer in complex ducts

Energy Technology Data Exchange (ETDEWEB)

Rokni, M.

1998-01-01

The need for a reliable and reasonable accurate turbulence model without specific convergence problem for calculating duct flows in industrial applications has become more evident. In this study a general computational method has been developed for calculating turbulent quantities in any arbitrary three dimensional duct. Four different turbulence models for predicting the turbulent Reynolds stresses namely; standard k-{epsilon} model, the non-linear-k-{epsilon} model of Speziale, an Explicit Algebraic Stress Model (EASM) and a full Reynolds Stress Model (RSM) are compared with each other. The advantages, disadvantages and accuracy of these models are discussed. The turbulent heat fluxes are modeled by the SED concept, the GGDH and the WET methods. The advantages of GGDH and WET compared to SED are discussed and the limitations of these models are clarified. The two-equation model of temperature invariance and its dissipation rate for calculating turbulent heat fluxes are also discussed. The low Reynolds number version of all the models are considered except for the RSM. At high Reynolds numbers the wall functions for both the temperature field and the flow field are applied. It has been shown that the standard k-{epsilon} model with the curvilinear transformation provides false secondary motions in general non-orthogonal ducts and can not be used for predicting the turbulent secondary motions in ducts. The numerical method is based on the finite volume technique with non-staggered grid arrangement. The SIMPLEC algorithm is used for pressure-velocity coupling. A modified SIP and TDMA solving methods are implemented for solving the equations. The van Leer, QUICK and hybrid schemes are applied for treating the convective terms. However, in order to achieve stability in the k and {epsilon} equations, the hybrid scheme is used for the convective terms in these equations. Periodic boundary conditions are imposed in the main flow direction for decreasing the number of

Scalable Quantum Information Transfer between Individual Nitrogen-Vacancy Centers by a Hybrid Quantum Interface

International Nuclear Information System (INIS)

Pei Pei; He-Fei Huang; Yan-Qing Guo; He-Shan Song

2016-01-01

We develop a design of a hybrid quantum interface for quantum information transfer (QIT), adopting a nanomechanical resonator as the intermedium, which is magnetically coupled with individual nitrogen-vacancy centers as the solid qubits, while capacitively coupled with a coplanar waveguide resonator as the quantum data bus. We describe the Hamiltonian of the model, and analytically demonstrate the QIT for both the resonant interaction and large detuning cases. The hybrid quantum interface allows for QIT between arbitrarily selected individual nitrogen-vacancy centers, and has advantages of the scalability and controllability. Our methods open an alternative perspective for implementing QIT, which is important during quantum storing or processing procedures in quantum computing. (paper)

Models for turbulent flows with variable density and combustion

International Nuclear Information System (INIS)

Jones, W.P.

1980-01-01

Models for transport processes and combustion in turbulent flows are outlined with emphasis on the situation where the fuel and air are injected separately. Attention is restricted to relatively simple flames. The flows investigated are high Reynolds number, single-phase, turbulent high-temperature flames in which radiative heat transfer can be considered negligible. Attention is given to the lower order closure models, algebraic stress and flux models, the k-epsilon turbulence model, the diffusion flame approximation, and finite rate reaction mechanisms

Mass-transfer studies in an electrochemical reactor with a small interelectrode gap

International Nuclear Information System (INIS)

Colli, A.N.; Toelzer, R.; Bergmann, M.E.H.; Bisang, J.M.

2013-01-01

Highlights: • Turbulence promoters increase from two to eight times the mass-transfer coefficients. • Turbulence promoters become more uniform the mass-transfer distribution. • Expanded plastics with an open structure are appropriate as turbulence promoters. -- Abstract: This paper reports the distribution of the local mass-transfer coefficient along the electrode length for an electrochemical reactor with parallel-plate electrodes and narrow interelectrode gaps of 1 and 2.2 mm, using the reduction of ferricyanide as a test reaction. The studies were performed at different flow rates, Reynolds numbers ranging from 370 to 3700, with the empty reactor and also the interelectrode gap was filled with two types of expanded plastics and a woven plastic mesh as turbulence promoters. The effect of both the interelectrode gap and the partial placing of the turbulence promoter along the electrode length on the mass-transfer behaviour was also analyzed. In all cases the pressure drop across the reactor was measured. A more uniform distribution of the local mass-transfer coefficient, ±15% related to its mean value, and an important increase of the mean mass-transfer coefficient, enhancement factor ranging from 2 to 8, were observed, depending on the type of turbulence promoter, the volumetric flow rate, and the interelectrode gap

Experimental and Computational Studies of Turbulent Mass Transfer in a Mixing Channel

DEFF Research Database (Denmark)

Hjertager, Lene Kristin; Hjertager, Bjørn H.; Solberg, Tron

2008-01-01

. Three different flow cases are studied. The 2D numerical predictions of the mixing channel show that none of the k- ε turbulence models tested is suitable for the flow cases studied here. The turbulent Schmidt number is reduced to obtain a better agreement between measured and predicted mean...

Sales-marketing interfaces within business-firms : Transitions and evolution

NARCIS (Netherlands)

Biemans, W.G.; Makovec Brencic, M.; Malshe, A.

2012-01-01

In these turbulent economic times sales-marketing interfaces in business firms are in the state of flux. Sales-marketing interface researchers have till date characterized marketing within business firms in terms of a marketing department rather than a marketing function thereby ignoring B2B firms

Experimental study of transition from laminar to turbulent flow in vertical narrow channel

International Nuclear Information System (INIS)

Wang Chang; Gao Puzhen; Wang Zhanwei; Tan Sichao

2012-01-01

Highlights: ► The effect of wall heating on the laminar to turbulent transition is experimentally studied. ► The flow characteristic demonstrates that heating leads to the delay of transition from laminar to turbulent regimes. ► The heat transfer characteristics also indicates that heating leads to the delay of flow regime transition. - Abstract: Experimental investigation of flow and heat transfer characteristics of a vertical narrow channel with uniform heat flux condition are conducted to analysis the effect of wall heating on the laminar to turbulent transition. The friction factor in the heating condition is compared with that in the adiabatic condition and the results show that wall heating leads to the delay of laminar to turbulent transition. In addition, the heat transfer characteristic indicates that the critical Reynolds number at the point of laminar flow breakdown increases with the increase of fluid temperature difference, and the local Nusselt number at the point of laminar breakdown increases with the increase of the inlet Reynolds number. The analyses of the flow and heat transfer characteristics both indicate that the heating has a stabilizing effect on the water flow at present experimental scale.

Sediment and plankton lift off recirculations in strong synthetic turbulence (KS)

Science.gov (United States)

Redondo, Jose M.; Castilla, Roberto; Sekula, Emil; Furmanek, Petr

2014-05-01

stratified flow. The properties of ensemble averages of the separation between two particles in a 2D turbulent flow were considered, and the KS approach was found to give satisfactory answers, with good comparison to experiment. We also compare structure and intermittency between KS and DNS. And experiments (Redondo 1988) The dynamical processes associated with the stably stratified atmospheric boundary layer or in the ocean thermocline are less well understood than those of its convective counterparts. This is due to its complexity, and the fact that buoyancy reduces entrainment across density interfaces. We present results on a numerical simulation of homogeneous and density stratified fluids and of comparable laboratory experiments where a sharp density interface generated by either salt concentration or heat, advances due to grid stirred turbulence Redondo (1988, 1990). The appearance of bursts of turbulence in very stable conditions due to breaking up of the internal waves, confers a sporadic character to the turbulence; these conditions of non-fully developed turbulence could explain this unusual behaviour of the scaling exponents. (Mahjoub et al. 1998, 20009 The structure functions show, in the inertial range, a potential law . The relation is concave in strong mixing situations (instability with fully developed turbulence), and convex in very stable situations (in which the breaking up of the interval waves confers a sporadic character to the turbulence).The multifractal model can not be used to represent situations of non-fully developed turbulence but the use of structure function analysis allows the investigation of intermittent and scale to scale energy transfer even in local non equilibrium flows. The relative diffusion of tracers is strongly dependent on the slope of the energy spectra which tends to Richardson's law also for very steep spectra. (Castilla et al. 2007) Local turbulence is used to establish the geometry of the turbulence mixing, changes in the

Experiments and CFD Modelling of Turbulent Mass Transfer in a Mixing Channel

DEFF Research Database (Denmark)

Hjertager Osenbroch, Lene Kristin; Hjertager, Bjørn H.; Solberg, Tron

2006-01-01

. Three different flow cases are studied. The 2D numerical predictions of the mixing channel show that none of the k-ε turbulence models tested is suitable for the flow cases studied here. The turbulent Schmidt number is reduced to obtain a better agreement between measured and predicted mean......Experiments are carried out for passive mixing in order to obtain local mean and turbulent velocities and concentrations. The mixing takes place in a square channel with two inlets separated by a block. A combined PIV/PLIF technique is used to obtain instantaneous velocity and concentration fields...... and fluctuating concentrations. The multi-peak presumed PDF mixing model is tested....

Turbulent flow and heat transfer in channels with combined rough and smooth surfaces

International Nuclear Information System (INIS)

Aytekin, A.

1978-01-01

A two-part experimental investigation is reported on the effects of transverse square rib roughening on fluid flow and heat transfer in channels with uniform and non-uniform boundary conditions. The first part of the experimental programme consisted of providing detailed measurements of mean and basic turbulent characteristics of fully developed flow in two rectangular ducts of aspect ratios 1.63 and 3.0. In each duct only one wall was roughened. In channels having low aspect ratios secondary flows play an important part in momentum transfer, and an interpretation of their effect on the measured Reynolds shear stress distribution has been attempted. In the second part of the experimental programme mean velocity and temperature profiles, friction factors and Stanton numbers were measured in an internally roughened pipe and annuli composed of a rough inner rod and either a smooth or a rough outer pipe. Heating was always applied on the outer surface. In all the geometries the mean velocities near the rough walls were found to be represented by logarithmic straight lines. The gradients of these lines were independent of Reynolds number but differed for various geometries. The mean temperature profiles, measured in the rough pipe and the fully rough annulus, showed that these could also be represented by logarithmic straight lines, but the slopes of these profiles were markedly different from those of the velocity profiles. (author)

Advances in fluid modeling and turbulence measurements

International Nuclear Information System (INIS)

Wada, Akira; Ninokata, Hisashi; Tanaka, Nobukazu

2002-01-01

The context of this book consists of four fields: Environmental Fluid Mechanics; Industrial Fluid Mechanics; Fundamentals of Fluid Mechanics; and Turbulence Measurements. Environmental Fluid Mechanics includes free surface flows in channels, rivers, seas, and estuaries. It also discusses wind engineering issues, ocean circulation model and dispersion problems in atmospheric, water and ground water environments. In Industrial Fluid Mechanics, fluid phenomena in energy exchanges, modeling of turbulent two- or multi-phase flows, swirling flows, flows in combustors, variable density flows and reacting flows, flows in turbo-machines, pumps and piping systems, and fluid-structure interaction are discussed. In Fundamentals of Fluid Mechanics, progress in modeling turbulent flows and heat/mass transfers, computational fluid dynamics/numerical techniques, parallel computing algorithms, applications of chaos/fractal theory in turbulence are reported. In Turbulence Measurements, experimental studies of turbulent flows, experimental and post-processing techniques, quantitative and qualitative flow visualization techniques are discussed. Separate abstracts were presented for 15 of the papers in this issue. The remaining 89 were considered outside the subject scope of INIS. (J.P.N.)

TURBULENT OXYGEN FLAMES IN TYPE Ia SUPERNOVAE

International Nuclear Information System (INIS)

Aspden, A. J.; Bell, J. B.; Woosley, S. E.

2011-01-01

In previous studies, we examined turbulence-flame interactions in carbon-burning thermonuclear flames in Type Ia supernovae. In this study, we consider turbulence-flame interactions in the trailing oxygen flames. The two aims of the paper are to examine the response of the inductive oxygen flame to intense levels of turbulence, and to explore the possibility of transition to detonation in the oxygen flame. Scaling arguments analogous to the carbon flames are presented and then compared against three-dimensional simulations for a range of Damkoehler numbers (Da 16 ) at a fixed Karlovitz number. The simulations suggest that turbulence does not significantly affect the oxygen flame when Da 16 16 >1, turbulence enhances heat transfer and drives the propagation of a flame that is narrower than the corresponding inductive flame would be. Furthermore, burning under these conditions appears to occur as part of a combined carbon-oxygen turbulent flame with complex compound structure. The simulations do not appear to support the possibility of a transition to detonation in the oxygen flame, but do not preclude it either.

Direct numerical simulation of fractal-generated turbulence

International Nuclear Information System (INIS)

Suzuki, H; Hasegawa, Y; Ushijima, T; Nagata, K; Sakai, Y; Hayase, T

2013-01-01

We simulate fractal-generated turbulence (Hurst and Vassilicos 2007 Phys. Fluids 19 035103)) by means of a direct numerical simulation and address its fundamental characteristics. We examine whether the fractal-generated turbulence in the upstream region has a nature similar to that of a wake. We propose an equation for predicting peak values of the velocity fluctuation intensity and devise a method for formulating the functional form of the quantity of interest by focusing on the time scale of decaying turbulence, and we examine those forms for the turbulent kinetic energy and rms of pressure fluctuation through this method. By using the method, both of these functional forms are found to be power-law functions in the downstream region, even though these profiles follow exponential functions around these peaks. In addition, decay exponents of these quantities are estimated. The integral length scales of velocity fluctuations for transverse as well as streamwise directions are essentially constant in the downstream direction. Decaying turbulence having both these characteristics conflicts with decaying turbulence described by the theory predicting exponential decay. We discuss a factor causing the difference by focusing on the functional form of the transfer function of homogeneous, isotropic turbulence. (paper)

Transition from weak wave turbulence regime to solitonic regime

Science.gov (United States)

Hassani, Roumaissa; Mordant, Nicolas

2017-11-01

The Weak Turbulence Theory (WTT) is a statistical theory describing the interaction of a large ensemble of random waves characterized by very different length scales. For both weak non-linearity and weak dispersion a different regime is predicted where solitons propagate while keeping their shape unchanged. The question under investigation here is which regime between weak turbulence or soliton gas does the system choose ? We report an experimental investigation of wave turbulence at the surface of finite depth water in the gravity-capillary range. We tune the wave dispersion and the level of nonlinearity by modifying the depth of water and the forcing respectively. We use space-time resolved profilometry to reconstruct the deformed surface of water. When decreasing the water depth, we observe a drastic transition between weak turbulence at the weakest forcing and a solitonic regime at stronger forcing. We characterize the transition between both states by studying their Fourier Spectra. We also study the efficiency of energy transfer in the weak turbulence regime. We report a loss of efficiency of angular transfer as the dispersion of the wave is reduced until the system bifurcates into the solitonic regime. This project has recieved funding from the European Research Council (ERC, Grant Agreement No. 647018-WATU).

The electrochemical transfer reactions and the structure of the iron|oxide layer|electrolyte interface

International Nuclear Information System (INIS)

Petrović, Željka; Metikoš-Huković, Mirjana; Babić, Ranko

2012-01-01

The thickness, barrier (protecting) and semiconducting properties of the potentiostatically formed oxide films on the pure iron electrode in an aqueous borate buffer solution were investigated by electrochemical quartz crystal nanobalance (EQCN), electrochemical impedance spectroscopy (EIS), and Mott–Schottky (MS) analysis. The thicknesses of the prepassive Fe(II)hydroxide layer (up to monolayer) nucleated on the bare iron surface and the passive Fe(II)/Fe(III) layer (up to 2 nm), deposited on the top of the first one, were determined using in situ gravimetry. Electronic properties of iron prepassive and passive films as well as ionic and electronic transfer reactions at the film|solution interface were discussed on the basis of a band structure model of the surface oxide film and the potential distribution at the interface. The anodic oxide film formation and cathodic decomposition are coupled processes and their reversible inter-conversion is mediated by the availability of free charge carriers on the electrode|solution interface. The structure of the reversible double layer at the iron oxide|solution interface was discussed based on the concept of the specific adsorption of the imidazolium cation on the negatively charged electrode surface at pH > pH pzc .

General correlations for pressure drop and heat transfer for single-phase turbulent flow in internally ribbed tubes

International Nuclear Information System (INIS)

Ravigururajan, T.S.; Bergles, A.E.

1985-01-01

General correlations for friction factors and heat transfer coefficients for single-phase turbulent flow in internally ribbed tubes are presented. Data from previous investigations are gathered for a wide range of tube parameters with e/d: 0.01 to 0.2; p/d: 0.1 to 7.0; α/90: 0.3 to 1.0, and flow parameters Re: 5000 to 250,000 and Pr: 0.66 to 37.6. The data were applied to a linear model to get normalized correlations that were then modified to fit tubes with extremely small parametric values. A shape function was included in the friction correlation to account for different rib profiles. The friction correlation predicts 96% of the data base to within +. 50% and 77% of the data base to within +. 20%. Corresponding figures for the heat transfer correlation are 99% and 69%. The present correlations are superior, for this extensive data base, to those presented by other investigators

Heat transfer in an asymmetrically heated duct, 2

International Nuclear Information System (INIS)

Satoh, Isao; Kurosaki, Yasuo

1986-01-01

The objective of this article is to study theoretically and experimentally the effects of nonuniform heating on turbulent heat transfer characteristics for flow in a horizontal rectangular duct ; a vertical side wall was uniformly heated, and the other wall were insulated. In our theoretical approach, the zero-equation model for turbulent eddy viscosity was employed. The effects of mesh size of finite difference on the calculation results were examined, and some refined compensation for wall temperatures and wall shear stresses by no use of fine mesh were proposed to reduce the calculation time. The heat transfer coefficients in thermally developing region for a nonuniformly heated duct obtained from numerical solutions are larger than the one for uniformly heated case. The buoyancy effects on heat transfer were evaluated. However, it was seen that the secondary flow due to buoyancy force was hardly expected to enhance heat transfer in a turbulent duct flow. Experiments were performed to measure the velocity and temperature profiles in a turbulent duct flow with a nonuniform heated wall. The experimental results were in good agreement with the theoretical ones. (author)

Why turbulence sustains in supercritically stratified free atmosphere?

Science.gov (United States)

Zilitinkevich, Sergej

2016-04-01

prevents degeneration of turbulence. The EFB theory has predicted that the critical Richardson number, Ric ˜ 0.25, characterising the hydrodynamic instability limit and the turbulent-laminar flow threshold at low Reynolds numbers, remains a principal threshold also in the very-high-Re turbulence; but here it separates the two turbulent regimes of dramatically different nature: Ri familiar "strong-mixing turbulence" typical of boundary-layer flows, wherein turbulent Prandtl number remaines practically constant: PrT ˜ 1 (the so-called "Reynolds analogy"); Ri > Ric: the newly revealed "wave-like turbulence" typical of the free atmosphere and deep ocean, wherein sharply increases with increasing Ri (asymptotically as PrT ≈ 5 Ri). This theoretical finding fits well with experimental evidence. Modellers long ago knew that turbulent heat transfer in the free atmosphere should be taken much weaker than the momentum transfer. The EFB theory gives authentic formulation for this rule and provides physically grounded method for modelling turbulence up to very stable startifications.

Transport coefficients for laminar and turbulent flow through a four-cusp channel

International Nuclear Information System (INIS)

Souza Dutra, A. de; Parise, J.A.R.; Souza Mendes, P.R. de.

1986-01-01

The heat transfer coefficients for laminar and turbulent flow in a four-cusp channel were determined. A numerical solution was developed for laminar flow an and experimental study for turbulent flow was carried out. Systematic variations of the Reynolds number were done in the range 900-30000. The results show that the heat transfer coefficients for the four-cusp channel are much lower than the coefficients for the circular tube. (author) [pt

Spectral Transfer Learning Using Information Geometry for a User-Independent Brain-Computer Interface.

Science.gov (United States)

Waytowich, Nicholas R; Lawhern, Vernon J; Bohannon, Addison W; Ball, Kenneth R; Lance, Brent J

2016-01-01

Recent advances in signal processing and machine learning techniques have enabled the application of Brain-Computer Interface (BCI) technologies to fields such as medicine, industry, and recreation; however, BCIs still suffer from the requirement of frequent calibration sessions due to the intra- and inter-individual variability of brain-signals, which makes calibration suppression through transfer learning an area of increasing interest for the development of practical BCI systems. In this paper, we present an unsupervised transfer method (spectral transfer using information geometry, STIG), which ranks and combines unlabeled predictions from an ensemble of information geometry classifiers built on data from individual training subjects. The STIG method is validated in both off-line and real-time feedback analysis during a rapid serial visual presentation task (RSVP). For detection of single-trial, event-related potentials (ERPs), the proposed method can significantly outperform existing calibration-free techniques as well as outperform traditional within-subject calibration techniques when limited data is available. This method demonstrates that unsupervised transfer learning for single-trial detection in ERP-based BCIs can be achieved without the requirement of costly training data, representing a step-forward in the overall goal of achieving a practical user-independent BCI system.

Mass Transfer Process by Magneto-convection at a Solid-liquid Interface in a Heterogeneous Vertical Magnetic Field

Science.gov (United States)

Sugiyama, Atsushi; Morisaki, Shigeyoshi; Aogaki, Ryoichi

2003-08-01

When an external magnetic field is vertically imposed on a solid-liquid interface, the mass transfer process of a solute dissolving from or depositing on the interface was theoretically examined. In a heterogeneous vertical magnetic field, a material receives a magnetic force in proportion to the product of the magnetic susceptibility, the magnetic flux density B and its gradient (dB/dz). As the reaction proceeds, a diffusion layer of the solute with changing susceptibility is formed at the interface because of the difference of the the magnetic susceptibility on the concentration of the solute. In the case of an unstable condition where the dimensionless number of magneto-convection S takes a positive value, the magnetic force is applied to the layer and induces numerous minute convection cells. The mass transfer of the solute is thus accelerated, so that it is predicted that the mass flux increases with the 1/3rd order of B(dB/dz) and the 4/3rd order of the concentration. The experiment was then performed by measuring the rate of the dissolution of copper sulfate pentahydrate crystal in water.

Meteorology Associated with Turbulence Encounters During NASA's Fall-2000 Flight Experiments

Science.gov (United States)

Hamilton, David W.; Proctor, Fred H.

2002-01-01

Initial flight experiments have been conducted to investigate convectively induced turbulence and to test technologies for its airborne detection. Turbulence encountered during the experiments is described with sources of data measured from in situ sensors, groundbased and airborne Doppler radars, and aircraft video. Turbulence measurements computed from the in situ system were quantified in terms of RMS normal loads (sigma(sub Delta n)), where 0.20 g is less than or equal to sigma(sub Delta n) is less than or equal to 0.30 g is considered moderate and sigma(sub Delta n) is greater than 0.30 g is severe. During two flights, 18 significant turbulence encounters (sigma(sub Delta) is greater than or equal to 0.20 g) occurred in the vicinity of deep convection; 14 moderate and 4 severe. In all cases, the encounters with turbulence occurred along the periphery of cumulus convection. These events were associated with relatively low values of radar reflectivity, i.e. RRF is less than 35 dBz, with most levels being below 20 dBz. The four cases of severe turbulence occurred in precipitation and were centered at the interface between a cumulus updraft turret and a downwind downdraft. Horizontal gradients of vertical velocity at this interface were found to be strongest on the downwind side of the cumulus turrets. Furthermore, the greatest loads to the aircraft occurred while flying along, not orthogonal to, the ambient environmental wind vector. During the two flights, no significant turbulence was encountered in the clear air (visual meteorological conditions), not even in the immediate vicinity of the deep convection.

Fencing direct memory access data transfers in a parallel active messaging interface of a parallel computer

Science.gov (United States)

Blocksome, Michael A.; Mamidala, Amith R.

2013-09-03

Fencing direct memory access (`DMA`) data transfers in a parallel active messaging interface (`PAMI`) of a parallel computer, the PAMI including data communications endpoints, each endpoint including specifications of a client, a context, and a task, the endpoints coupled for data communications through the PAMI and through DMA controllers operatively coupled to segments of shared random access memory through which the DMA controllers deliver data communications deterministically, including initiating execution through the PAMI of an ordered sequence of active DMA instructions for DMA data transfers between two endpoints, effecting deterministic DMA data transfers through a DMA controller and a segment of shared memory; and executing through the PAMI, with no FENCE accounting for DMA data transfers, an active FENCE instruction, the FENCE instruction completing execution only after completion of all DMA instructions initiated prior to execution of the FENCE instruction for DMA data transfers between the two endpoints.

Visualization of the interfacial turbulence associated with remarkable faradaic current amplification at a polarized water/1,2-dichloroethane interface

Czech Academy of Sciences Publication Activity Database

Trojánek, Antonín; Mareček, Vladimír; Samec, Zdeněk

2017-01-01

Roč. 80, JUL 2017 (2017), s. 1-4 ISSN 1388-2481 R&D Projects: GA ČR GA17-09980S Institutional support: RVO:61388955 Keywords : Water/1,2-dichloroethane interface * Ion transfer * Current amplification Subject RIV: CG - Electrochemistry OBOR OECD: Electrochemistry (dry cells, batteries, fuel cells, corrosion metals, electrolysis) Impact factor: 4.396, year: 2016

Reconciling estimates of the ratio of heat and salt fluxes at the ice-ocean interface

Science.gov (United States)

Keitzl, T.; Mellado, J. P.; Notz, D.

2016-12-01

The heat exchange between floating ice and the underlying ocean is determined by the interplay of diffusive fluxes directly at the ice-ocean interface and turbulent fluxes away from it. In this study, we examine this interplay through direct numerical simulations of free convection. Our results show that an estimation of the interface flux ratio based on direct measurements of the turbulent fluxes can be difficult because the flux ratio varies with depth. As an alternative, we present a consistent evaluation of the flux ratio based on the total heat and salt fluxes across the boundary layer. This approach allows us to reconcile previous estimates of the ice-ocean interface conditions. We find that the ratio of heat and salt fluxes directly at the interface is 83-100 rather than 33 as determined by previous turbulence measurements in the outer layer. This can cause errors in the estimated ice-ablation rate from field measurements of up to 40% if they are based on the three-equation formulation.

A generalized self-similar spectrum for decaying homogeneous and isotropic turbulence

Science.gov (United States)

Yang, Pingfan; Pumir, Alain; Xu, Haitao

2017-11-01

The spectrum of turbulence in dissipative and inertial range can be described by the celebrated Kolmogorov theory. However, there is no general solution of the spectrum in the large scales, especially for statistically unsteady turbulent flows. Here we propose a generalized self-similar form that contains two length-scales, the integral scale and the Kolmogorov scale, for decaying homogeneous and isotropic turbulence. With the help of the local spectral energy transfer hypothesis by Pao (Phys. Fluids, 1965), we derive and solve for the explicit form of the energy spectrum and the energy transfer function, from which the second- and third-order velocity structure functions can also be obtained. We check and verify our assumptions by direct numerical simulations (DNS), and our solutions of the velocity structure functions compare well with hot-wire measurements of high-Reynolds number wind-tunnel turbulence. Financial supports from NSFC under Grant Number 11672157, from the Alexander von Humboldt Foundation, and from the MPG are gratefully acknowledged.

Direct numerical simulation of heat transfer to CO2 at supercritical pressure in a vertical tube

International Nuclear Information System (INIS)

Bae, Joong-Hun; Yoo, Jung-Yul; Choi, Hae-Cheon

2003-01-01

In the present study, the turbulent heat transfer to CO 2 at supercritical pressure in a vertical tube is investigated using Direct Numerical Simulation (DNS), where no turbulence model is adopted. Heat transfer to the supercritical pressure fluids is characterized by rapid variation of thermodynamic/ thermo-physical properties in the fluids. This change in properties occurs within a very narrow range of temperature across the so-called pseudo-critical temperature, causing a peculiar behavior of heat transfer characteristics. The buoyancy effects associated with very large changes in density proved to play a major role in turbulent heat transfer to supercritical pressure fluids. Depending on the degree of buoyancy effects, turbulent heat transfer may increase or significantly decrease, resulting in a local hot spot along the wall. Based on the results of the present DNS study combined with theoretical considerations for turbulent mixed convection heat transfer, the basic mechanism of this local heat transfer deterioration is explained

Flow around turbulence promoters in parallel channel, 1

International Nuclear Information System (INIS)

Shiina, Yasuaki; Takizuka, Takakazu; Okamoto, Yoshizo

1982-01-01

Flow characteristics in relation to heat transfer characteristics in parallel channel with turbulence promoters were studied experimentally. Flow visualization experiments were made in paralle channel with one or two turbulence promoters for Reynolds number region of 100 lt = Resub(w) lt = 3,600. The vortex patterns behind one promoter were that a steady vortex was formed for low Reynolds number and vortex was shed for high Reynolds number,. For higher Reynolds number, it was observed that shedding vortex caused other vortices or disappeared itself randomly. The results indicate that the shedding vortices will augment heat transfer, whereas the steady vortex will give rise to deterioration in heat transfer. This inference agrees with the experimental results of Hishida et al. The results of two promoters experiment showed that the maximum performance of promoter would be attained at p/d -- 7. This agrees with the results formerly studied by other investigators. (author)

Investigation of mass transfer in swirling turbulent flames

International Nuclear Information System (INIS)

Sharaborin, D; Abdurakipov, S; Dulin, V

2016-01-01

The present paper reports on analysis of flow structure and turbulent transport in swirling flames. The particle image velocimetry and spontaneous Raman scattering techniques were used for the measurements of 2D velocity and density distributions. The focus was placed on comparison between low- and high-swirl flows. A pronounced bubble-type vortex breakdown with strong flow precession took place in the latter case. (paper)

Theoretical study of heat transfer with moving phase-change interface in thawing of frozen food

International Nuclear Information System (INIS)

Leung, M; Ching, W H; Leung, D Y C; Lam, G C K

2005-01-01

A theoretical solution was obtained for a transient phase-change heat transfer problem in thawing of frozen food. In the physical model, a sphere originally at a uniform temperature below the phase-change temperature is suddenly immersed in a fluid at a temperature above the phase-change temperature. As the body temperature increases, the phase-change interface will be first formed on the surface. Subsequently, the interface will absorb the latent heat and move towards the centre until the whole body undergoes complete phase change. In the mathematical formulation, the nonhomogeneous problem arises from the moving phase-change interface. The solution in terms of the time-dependent temperature field was obtained by use of Green's function. A one-step Newton-Raphson method was specially designed to solve for the position of the moving interface to satisfy the interface condition. The theoretical results were compared with numerical results generated by a finite difference model and experimental measurements collected from a cold water thawing process. As a good agreement was found, the theoretical solution developed in this study was verified numerically and experimentally. Besides thawing of frozen food, there are many other practical applications of the theoretical solution, such as food freezing, soil freezing/thawing, metal casting and bath quenching heat treatment, among others

Theoretical study of heat transfer with moving phase-change interface in thawing of frozen food

Science.gov (United States)

Leung, M.; Ching, W. H.; Leung, D. Y. C.; Lam, G. C. K.

2005-02-01

A theoretical solution was obtained for a transient phase-change heat transfer problem in thawing of frozen food. In the physical model, a sphere originally at a uniform temperature below the phase-change temperature is suddenly immersed in a fluid at a temperature above the phase-change temperature. As the body temperature increases, the phase-change interface will be first formed on the surface. Subsequently, the interface will absorb the latent heat and move towards the centre until the whole body undergoes complete phase change. In the mathematical formulation, the nonhomogeneous problem arises from the moving phase-change interface. The solution in terms of the time-dependent temperature field was obtained by use of Green's function. A one-step Newton-Raphson method was specially designed to solve for the position of the moving interface to satisfy the interface condition. The theoretical results were compared with numerical results generated by a finite difference model and experimental measurements collected from a cold water thawing process. As a good agreement was found, the theoretical solution developed in this study was verified numerically and experimentally. Besides thawing of frozen food, there are many other practical applications of the theoretical solution, such as food freezing, soil freezing/thawing, metal casting and bath quenching heat treatment, among others.

Progress in modeling hypersonic turbulent boundary layers

Science.gov (United States)

Zeman, Otto

1993-01-01

A good knowledge of the turbulence structure, wall heat transfer, and friction in turbulent boundary layers (TBL) at high speeds is required for the design of hypersonic air breathing airplanes and reentry space vehicles. This work reports on recent progress in the modeling of high speed TBL flows. The specific research goal described here is the development of a second order closure model for zero pressure gradient TBL's for the range of Mach numbers up to hypersonic speeds with arbitrary wall cooling requirements.

Transitional-turbulent spots and turbulent-turbulent spots in boundary layers.

Science.gov (United States)

Wu, Xiaohua; Moin, Parviz; Wallace, James M; Skarda, Jinhie; Lozano-Durán, Adrián; Hickey, Jean-Pierre

2017-07-03

Two observations drawn from a thoroughly validated direct numerical simulation of the canonical spatially developing, zero-pressure gradient, smooth, flat-plate boundary layer are presented here. The first is that, for bypass transition in the narrow sense defined herein, we found that the transitional-turbulent spot inception mechanism is analogous to the secondary instability of boundary-layer natural transition, namely a spanwise vortex filament becomes a [Formula: see text] vortex and then, a hairpin packet. Long streak meandering does occur but usually when a streak is infected by a nearby existing transitional-turbulent spot. Streak waviness and breakdown are, therefore, not the mechanisms for the inception of transitional-turbulent spots found here. Rather, they only facilitate the growth and spreading of existing transitional-turbulent spots. The second observation is the discovery, in the inner layer of the developed turbulent boundary layer, of what we call turbulent-turbulent spots. These turbulent-turbulent spots are dense concentrations of small-scale vortices with high swirling strength originating from hairpin packets. Although structurally quite similar to the transitional-turbulent spots, these turbulent-turbulent spots are generated locally in the fully turbulent environment, and they are persistent with a systematic variation of detection threshold level. They exert indentation, segmentation, and termination on the viscous sublayer streaks, and they coincide with local concentrations of high levels of Reynolds shear stress, enstrophy, and temperature fluctuations. The sublayer streaks seem to be passive and are often simply the rims of the indentation pockets arising from the turbulent-turbulent spots.

Effect of pressure on heat transfer coefficient at the metal/mold interface of A356 aluminum alloy

DEFF Research Database (Denmark)

Fardi Ilkhchy, A.; Jabbari, Masoud; Davami, P.

2012-01-01

The aim of this paper is to correlate interfacial heat transfer coefficient (IHTC) to applied external pressure, in which IHTC at the interface between A356 aluminum alloy and metallic mold during the solidification of casting under different pressures were obtained using the inverse heat...... conduction problem (IHCP) method. The method covers the expedient of comparing theoretical and experimental thermal histories. Temperature profiles obtained from thermocouples were used in a finite difference heat flow program to estimate the transient heat transfer coefficients. The new simple formula...... was presented for correlation between external pressure and heat transfer coefficient. Acceptable agreement with data in literature shows the accuracy of the proposed formula....

Synthesis of Stable Interfaces on SnO2 Surfaces for Charge-Transfer Applications

Science.gov (United States)

Benson, Michelle C.

The commercial market for solar harvesting devices as an alternative energy source requires them to be both low-cost and efficient to replace or reduce the dependence on fossil fuel burning. Over the last few decades there has been promising efforts towards improving solar devices by using abundant and non-toxic metal oxide nanomaterials. One particular metal oxide of interest has been SnO2 due to its high electron mobility, wide-band gap, and aqueous stability. However SnO2 based solar cells have yet to reach efficiency values of other metal oxides, like TiO2. The advancement of SnO2 based devices is dependent on many factors, including improved methods of surface functionalization that can yield stable interfaces. This work explores the use of a versatile functionalization method through the use of the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The CuAAC reaction is capable of producing electrochemically, photochemically, and electrocatalytically active surfaces on a variety of SnO2 materials. The resulting charge-transfer characteristics were investigated as well as an emphasis on understanding the stability of the resulting molecular linkage. We determined the CuAAC reaction is able to proceed through both azide-modified and alkyne-modified surfaces. The resulting charge-transfer properties showed that the molecular tether was capable of supporting charge separation at the interface. We also investigated the enhancement of electron injection upon the introduction of an ultra-thin ZrO2 coating on SnO2. Several complexes were used to fully understand the charge-transfer capabilities, including model systems of ferrocene and a ruthenium coordination complex, a ruthenium mononuclear water oxidation catalyst, and a commercial ruthenium based dye.

Simulation and measurement of enhanced turbulent heat transfer in a channel with periodic ribs on one principal wall

International Nuclear Information System (INIS)

Tongmin Liou; Jennjiang Hwang; Shihhui Chen

1993-01-01

This paper performs a numerical and experimental analysis to investigate the heat transfer and fluid flow behaviour in a rectangular channel flow with streamwise-periodic ribs mounted on one of the principal walls. The k --A PDM turbulence model together with a smoothed hybrid central/skew upstream difference scheme (SCSUDS) and the PISO pressure-velocity coupling algorithm was applied to solving the accelerated, separated and recirculating flows. The real-time holographic interferometry technique was adopted to measure the time-dependent temperature field in the ribbed duct. The predicted fluid flow and temperature field were tested by previous laser-Doppler velocimetry measurements and present holographic interferometry data, and reasonable agreement was achieved. By the examination of the local wall temperature distribution for the uniform wall heat flux (UHF) boundary condition the regions susceptible to the hot spots are identified. Moreover, the study provided the numerical solution to investigate the effect of geometry and flow parameters on the local as well as average heat transfer coefficients. The compact correlation of the average heat transfer coefficient was further developed and accounted for the rib height, rib spacing, and Reynolds number. (Author)

Macro-scale turbulence modelling for flows in porous media; Modelisation a l'echelle macroscopique d'un ecoulement turbulent au sein d'un milieu poreux

Energy Technology Data Exchange (ETDEWEB)

Pinson, F

2006-03-15

- This work deals with the macroscopic modeling of turbulence in porous media. It concerns heat exchangers, nuclear reactors as well as urban flows, etc. The objective of this study is to describe in an homogenized way, by the mean of a spatial average operator, turbulent flows in a solid matrix. In addition to this first operator, the use of a statistical average operator permits to handle the pseudo-aleatory character of turbulence. The successive application of both operators allows us to derive the balance equations of the kind of flows under study. Two major issues are then highlighted, the modeling of dispersion induced by the solid matrix and the turbulence modeling at a macroscopic scale (Reynolds tensor and turbulent dispersion). To this aim, we lean on the local modeling of turbulence and more precisely on the k - {epsilon} RANS models. The methodology of dispersion study, derived thanks to the volume averaging theory, is extended to turbulent flows. Its application includes the simulation, at a microscopic scale, of turbulent flows within a representative elementary volume of the porous media. Applied to channel flows, this analysis shows that even within the turbulent regime, dispersion remains one of the dominating phenomena within the macro-scale modeling framework. A two-scale analysis of the flow allows us to understand the dominating role of the drag force in the kinetic energy transfers between scales. Transfers between the mean part and the turbulent part of the flow are formally derived. This description significantly improves our understanding of the issue of macroscopic modeling of turbulence and leads us to define the sub-filter production and the wake dissipation. A f - <{epsilon}>f - <{epsilon}{sub w}>f model is derived. It is based on three balance equations for the turbulent kinetic energy, the viscous dissipation and the wake dissipation. Furthermore, a dynamical predictor for the friction coefficient is proposed. This model is then

SIMULATIONS OF THE KELVIN–HELMHOLTZ INSTABILITY DRIVEN BY CORONAL MASS EJECTIONS IN THE TURBULENT CORONA

Energy Technology Data Exchange (ETDEWEB)

Gómez, Daniel O.; DeLuca, Edward E. [Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138 (United States); Mininni, Pablo D. [Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires and Instituto de Física de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires (Argentina)

2016-02-20

Recent high-resolution Atmospheric Imaging Assembly/Solar Dynamics Observatory images show evidence of the development of the Kelvin–Helmholtz (KH) instability, as coronal mass ejections (CMEs) expand in the ambient corona. A large-scale magnetic field mostly tangential to the interface is inferred, both on the CME and on the background sides. However, the magnetic field component along the shear flow is not strong enough to quench the instability. There is also observational evidence that the ambient corona is in a turbulent regime, and therefore the criteria for the development of the instability are a priori expected to differ from the laminar case. To study the evolution of the KH instability with a turbulent background, we perform three-dimensional simulations of the incompressible magnetohydrodynamic equations. The instability is driven by a velocity profile tangential to the CME–corona interface, which we simulate through a hyperbolic tangent profile. The turbulent background is generated by the application of a stationary stirring force. We compute the instability growth rate for different values of the turbulence intensity, and find that the role of turbulence is to attenuate the growth. The fact that KH instability is observed sets an upper limit on the correlation length of the coronal background turbulence.

CFD analysis on heat transfer in low Prandtl number fluid flows

Energy Technology Data Exchange (ETDEWEB)

Borgohain, A.; Maheshwari, N.K.; Vijayan, P.K.; Sinha, R.K., E-mail: bananta@barc.gov.in [Bhabha Atomic Research Centre, Reactor Engineering Div., Trombay, Mumbai (India)

2011-07-01

Use of Computational Fluid Dynamics (CFD) code is helpful for designing liquid metal cooled nuclear reactor systems. Before using any CFD code proper evaluation of the code is essential for simulation of heat transfer in liquid metal flow. In this paper, a review of the literature on the correlations for liquid metal heat transfer is carried out and a comparison with experimental results is performed. CFD analysis is carried out using PHOENICS-3.6 code on heat transfer in molten Lead Bismuth Eutectic (LBE) flowing through tube. Turbulent flow analyses are carried out for the evaluation of the CFD code. The CFD results are compared with the available correlations. Assessment of various turbulence models and correlations for turbulent Prandtl number in the tube geometry are carried out. From the analysis it is found that, the CFD prediction can be improved with modified turbulent Prandtl number in the turbulence models. (author)

Spectral Transfer Learning using Information Geometry for a User-Independent Brain-Computer Interface

Directory of Open Access Journals (Sweden)

Nicholas Roy Waytowich

2016-09-01

Full Text Available Recent advances in signal processing and machine learning techniques have enabled the application of Brain-Computer Interface (BCI technologies to fields such as medicine, industry and recreation. However, BCIs still suffer from the requirement of frequent calibration sessions due to the intra- and inter- individual variability of brain-signals, which makes calibration suppression through transfer learning an area of increasing interest for the development of practical BCI systems. In this paper, we present an unsupervised transfer method (spectral transfer using information geometry, STIG, which ranks and combines unlabeled predictions from an ensemble of information geometry classifiers built on data from individual training subjects. The STIG method is validated in both offline and real-time feedback analysis during a rapid serial visual presentation task (RSVP. For detection of single-trial, event-related potentials (ERPs, the proposed method can significantly outperform existing calibration-free techniques as well as outperform traditional within-subject calibration techniques when limited data is available. This method demonstrates that unsupervised transfer learning for single-trial detection in ERP-based BCIs can be achieved without the requirement of costly training data, representing a step-forward in the overall goal of achieving a practical user-independent BCI system.

Carbon nanoparticle stabilised liquid|liquid micro-interfaces for electrochemically driven ion-transfer processes

International Nuclear Information System (INIS)

MacDonald, Stuart M.; Fletcher, Paul D.I.; Cui Zhenggang; Opallo, Marcin; Chen Jingyuan; Marken, Frank

2007-01-01

Stabilised liquid|liquid interfaces between an organic 4-(3-phenylpropyl)-pyridine (PPP) phase and an aqueous electrolyte phase are obtained in the presence of suitable nanoparticles. The use of nanoparticulate stabilisers (ca. 30 nm diameter laponite or 9-18 nm diameter carbon) in 'Pickering' emulsion systems allows stable organic microdroplets to be formed and these are readily deposited onto conventional tin-doped indium oxide (ITO) electrodes. In contrast to the electrically insulating laponite nanoparticles, conducting carbon nanoparticles are shown to effectively catalyse the simultaneous electron transfer and ion transfer process at triple phase boundary junctions. Anion transfer processes between the aqueous and organic phase are driven electrochemically at the extensive triple phase junction carbon nanoparticle|4-(3-phenylpropyl)-pyridine|aqueous electrolyte. The organic phase consists of a redox active reagent 5,10,15,20-tetraphenyl-21H,23H-porphinato manganese(III) (MnTPP + ), 5,10,15,20-tetraphenyl-21H,23H-porphinato iron(III) (FeTPP + ), or proto-porphyrinato-IX iron(III) (hemin) dissolved in 4-(3-phenylpropyl)-pyridine (PPP). The composition of the aqueous electrolyte phase determines the reversible potential for the Nernstian anion transfer process. The methodology is shown to be versatile and, in future, could be applied more generally in liquid|liquid electroanalysis

Effects of friction on forced two-dimensional Navier-Stokes turbulence.

Science.gov (United States)

Blackbourn, Luke A K; Tran, Chuong V

2011-10-01

Large-scale dissipation mechanisms have been routinely employed in numerical simulations of two-dimensional turbulence to absorb energy at large scales, presumably mimicking the quasisteady picture of Kraichnan in an unbounded fluid. Here, "side effects" of such a mechanism--mechanical friction--on the small-scale dynamics of forced two-dimensional Navier-Stokes turbulence are elaborated by both theoretical and numerical analysis. Given a positive friction coefficient α, viscous dissipation of enstrophy has been known to vanish in the inviscid limit ν→0. This effectively renders the scale-neutral friction the only mechanism responsible for enstrophy dissipation in that limit. The resulting dynamical picture is that the classical enstrophy inertial range becomes a dissipation range in which the dissipation of enstrophy by friction mainly occurs. For each α>0, there exists a critical viscosity ν(c), which depends on physical parameters, separating the regimes of predominant viscous and frictional dissipation of enstrophy. It is found that ν(c)=[η'(1/3)/(Ck(f)(2))]exp[-η'(1/3)/(Cα)], where η' is half the enstrophy injection rate, k(f) is the forcing wave number, and C is a nondimensional constant (the Kraichnan-Batchelor constant). The present results have important theoretical and practical implications. Apparently, mechanical friction is a poor choice in numerical attempts to address fundamental issues concerning the direct enstrophy transfer in two-dimensional Navier-Stokes turbulence. Furthermore, as relatively strong friction naturally occurs on the surfaces and at lateral boundaries of experimental fluids as well as at the interfaces of shallow layers in geophysical fluid models, the frictional effects discussed in this study are crucial in understanding the dynamics of these systems.

Effects of Force Fields on Interface Dynamics, in view of Two-Phase Heat Transfer Enhancement and Phase Management for Space Applications

Science.gov (United States)

Di Marco, P.; Saccone, G.

2017-11-01

On earth, gravity barely influences the dynamics of interfaces. For what concerns bubbles, buoyancy governs the dynamics of boiling mechanism and thus affects boiling heat transfer capacity. While, for droplets, the coupled effects of wettability and gravity affects interface exchanges. In space, in the lack of gravity, rules are changed and new phenomena come into play. The present work is aimed to study the effects of electric field on the shape and behaviour of bubbles and droplets in order to understand how to handle microgravity applications; in particular, the replacement of gravity with electric field and their coupled effects are evaluated. The experiments spread over different setups, gravity conditions, working fluids, interface conditions. Droplets and bubbles have been analysed with and without electric field, with and without (adiabatic) heat and mass transfer across the interface. Furthermore, the results of the 4 ESA Parabolic Flight Campaigns (PFC 58, 60, 64 & 66), for adiabatic bubbles, adiabatic droplets and evaporating droplets, will be summarized, discussed, and compared with the ground tests.

Turbulence spectra, transport, and E × B flows in helical plasmas

International Nuclear Information System (INIS)

Watanabe, T.-H.; Nunami, M.; Sugama, H.; Satake, S.; Matsuoka, S.; Ishizawa, A.; Tanaka, K.; Maeyama, Shinya

2012-11-01

Gyrokinetic simulation of ion temperature gradient turbulence and zonal flows for helical plasmas has been validated against the Large Helical Device experiments with high ion temperature, where a reduced modeling of ion heat transport is also considered. It is confirmed by the entropy transfer analysis that the turbulence spectrum elongated in the radial wavenumber space is associated with successive interactions with zonal flows. A novel multi-scale simulation for turbulence and zonal flows in poloidally-rotating helical plasmas has demonstrated strong zonal flow generation by turbulence, which implies that turbulent transport processes in non-axisymmetric systems are coupled to neoclassical transport through the macroscopic E × B flows determined by the ambipolarty condition for neoclassical particle fluxes. (author)

Pipe Flow and Wall Turbulence Using a Modified Navier-Stokes Equation

International Nuclear Information System (INIS)

Jirkovsky, L.; Muriel, A.

2012-01-01

We use a derived incompressible modified Navier-Stokes equation to model pipe flow and wall turbulence. We reproduce the observed flattened paraboloid velocity profiles of turbulence that cannot be obtained directly using standard incompressible Navier-Stokes equation. The solutions found are in harmony with multi-valued velocity fields as a definition of turbulence. Repeating the procedure for the flow of turbulent fluid between two parallel flat plates we find similar flattened velocity profiles. We extend the analysis to the turbulent flow along a single wall and compare the results with experimental data and the established controversial von Karman logarithmic law of the wall. (electromagnetism, optics, acoustics, heat transfer, classical mechanics, and fluid dynamics)

Transport equation for the time scale of a turbulent scalar field

International Nuclear Information System (INIS)

Kurbatskij, A.F.

1999-01-01

The two-parametric turbulence models cause serious difficulties by modeling the near-wall flows due to absence of the natural boundary condition on the wall for dissipation of the ε turbulence energy and the ε θ scalar field destruction. This difficulty may be overcome, if instead of the ε and ε θ , as the second parameter of the model, to apply the time scales of the turbulent dynamic and scalar fields. The equation of the scalar field is derived and numerical coefficients included therein, are determined from the simplest problems on the turbulent heat transfer [ru

SIGNATURES OF MRI-DRIVEN TURBULENCE IN PROTOPLANETARY DISKS: PREDICTIONS FOR ALMA OBSERVATIONS

Energy Technology Data Exchange (ETDEWEB)

Simon, Jacob B. [Department of Space Studies, Southwest Research Institute, Boulder, CO 80302 (United States); Hughes, A. Meredith; Flaherty, Kevin M. [Astronomy Department, Van Vleck Observatory, Wesleyan University, 96 Foss Hill Dr., Middletown, CT 06459 (United States); Bai, Xue-Ning [Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MS-51, Cambridge, MA 02138 (United States); Armitage, Philip J., E-mail: jbsimon.astro@gmail.com [JILA, University of Colorado and NIST, 440 UCB, Boulder, CO 80309-0440 (United States)

2015-08-01

Spatially resolved observations of molecular line emission have the potential to yield unique constraints on the nature of turbulence within protoplanetary disks. Using a combination of local non-ideal magnetohydrodynamics (MHD) simulations and radiative transfer calculations, tailored to properties of the disk around HD 163296, we assess the ability of ALMA to detect turbulence driven by the magnetorotational instability (MRI). Our local simulations show that the MRI produces small-scale turbulent velocity fluctuations that increase in strength with height above the mid-plane. For a set of simulations at different disk radii, we fit a Maxwell–Boltzmann distribution to the turbulent velocity and construct a turbulent broadening parameter as a function of radius and height. We input this broadening into radiative transfer calculations to quantify observational signatures of MRI-driven disk turbulence. We find that the ratio of the peak line flux to the flux at line center is a robust diagnostic of turbulence that is only mildly degenerate with systematic uncertainties in disk temperature. For the CO(3–2) line, which we expect to probe the most magnetically active slice of the disk column, variations in the predicted peak-to-trough ratio between our most and least turbulent models span a range of approximately 15%. Additional independent constraints can be derived from the morphology of spatially resolved line profiles, and we estimate the resolution required to detect turbulence on different spatial scales. We discuss the role of lower optical depth molecular tracers, which trace regions closer to the disk mid-plane where velocities in MRI-driven models are systematically lower.

Numerical simulations of turbulent heat transfer in a channel at Prandtl numbers higher than 100

International Nuclear Information System (INIS)

Bergant, R.; Tiselj, I.

2005-01-01

During the last years, many attempts have been made to extend turbulent heat transfer at low Prandtl numbers to high Prandtl numbers in the channel based on a very accurate pseudo-spectral code of direct numerical simulation (DNS). DNS describes all the length and time scales for velocity and temperature fields, which are different when Prandtl number is not equal to 1. DNS can be used at low Reynolds (Re τ =150. Very similar approach as for Pr=5.4 was done for numerical simulations at Pr=100 and Pr=200. Comparison was made with results of temperature fields performed on 9-times finer numerical grid, however without damping of the highest Fourier coefficients. The results of mean temperature profiles show no differences larger than statistical uncertainties (∼1%), while slightly larger differences are seen for temperature fluctuations. (author)