Статті в журналах з теми "Transport de turbulence"
Щоб переглянути інші типи публікацій з цієї теми, перейдіть за посиланням: Transport de turbulence.
Оформте джерело за APA, MLA, Chicago, Harvard та іншими стилями
Ознайомтеся з топ-50 статей у журналах для дослідження на тему "Transport de turbulence".
Біля кожної праці в переліку літератури доступна кнопка «Додати до бібліографії». Скористайтеся нею – і ми автоматично оформимо бібліографічне посилання на обрану працю в потрібному вам стилі цитування: APA, MLA, «Гарвард», «Чикаго», «Ванкувер» тощо.
Також ви можете завантажити повний текст наукової публікації у форматі «.pdf» та прочитати онлайн анотацію до роботи, якщо відповідні параметри наявні в метаданих.
Переглядайте статті в журналах для різних дисциплін та оформлюйте правильно вашу бібліографію.
1
Souza, José Francisco Almeida de, José Luiz Lima de Azevedo, Leopoldo Rota de Oliveira, Ivan Dias Soares, and Maurício Magalhães Mata. "TURBULENCE MODELING IN GEOPHYSICAL FLOWS – PART I – FIRST-ORDER TURBULENT CLOSURE MODELING." Revista Brasileira de Geofísica 32, no. 1 (March 1, 2014): 31. http://dx.doi.org/10.22564/rbgf.v32i1.395.
Анотація:
ABSTRACT. The usage of so-called turbulence closure models within hydrodynamic circulation models comes from the need to adequately describe vertical mixing processes. Even among the classical turbulence models; that is, those based on the Reynolds decomposition technique (Reynolds Averaged Navier-Stokes – RANS), there is a variety of approaches that can be followed for the modeling of turbulent flows (second moment) of momentum, heat, salinity, and other properties. Essentially, these approaches are divided into those which use the concept of turbulent viscosity/diffusivity in the modeling of the second moment, and those which do not use it. In this work we present and discuss the models that employ this concept, in which the viscosity can be considered constant or variable. In this latter scenario, besides those that use the concepts of mixture length, the models that use one or two differential transport equations for determining the viscosity are presented. The fact that two transport equations are used – one for the turbulent kinetic energy and the other for the turbulent length scale – make these latter ones the most complete turbulent closure models in this category. Keywords: turbulence modeling, turbulence models, first-order models, first-order turbulent closure. RESUMO. A descrição adequada dos processos de mistura vertical nos modelos de circulação hidrodinâmica é o objetivo dos chamados modelos de turbulência, os quais são acoplados aos primeiros. Mesmo entre os modelos clássicos de turbulência, isto é, aqueles que se baseiam na técnica de decomposição de Reynolds (Reynolds Averaged Navier-Stokes – RANS), existe uma variedade de abordagens que podem ser seguidas na modelagem dos fluxos turbulentos (segundos momentos) de momentum, calor, salinidade e outras propriedades. Fundamentalmente estas abordagens dividem-se entre aquelas que utilizam o conceito de viscosidade/ difusividade turbulenta na modelagem dos segundos momentos, e aquelas que não o utilizam. Nesse trabalho são apresentados e discutidos os modelos que empregam este conceito, onde a viscosidade pode ser considerada constante ou variável. No caso variável, além daqueles que utilizam o conceito de comprimento de mistura, são ainda apresentados os modelos que utilizam uma ou duas equações diferenciais de transporte para a determinação da viscosidade. O fato de empregar duas equações de transporte, uma para a energia cinética turbulenta e outra para a escala de comprimento turbulento, fazem destes últimos os mais completos modelos de fechamento turbulento desta categoria. Palavras-chave: modelagem da turbulência, modelos de turbulência, modelos de primeira ordem, fechamento turbulento de primeira orde
2
Kawata, Takuya, and Takahiro Tsukahara. "Spectral Analysis on Transport Budgets of Turbulent Heat Fluxes in Plane Couette Turbulence." Energies 15, no. 14 (July 20, 2022): 5258. http://dx.doi.org/10.3390/en15145258.
Анотація:
In recent years, scale-by-scale energy transport in wall turbulence has been intensively studied, and the complex spatial and interscale transfer of turbulent energy has been investigated. As the enhancement of heat transfer is one of the most important aspects of turbulence from an engineering perspective, it is also important to study how turbulent heat fluxes are transported in space and in scale by nonlinear multi-scale interactions in wall turbulence as well as turbulent energy. In the present study, the spectral transport budgets of turbulent heat fluxes are investigated based on direct numerical simulation data of a turbulent plane Couette flow with a passive scalar heat transfer. The transport budgets of spanwise spectra of temperature fluctuation and velocity-temperature correlations are investigated in detail in comparison to those of the corresponding Reynolds stress spectra. The similarity and difference between those scale-by-scale transports are discussed, with a particular focus on the roles of interscale transport and spatial turbulent diffusion. As a result, it is found that the spectral transport of the temperature-related statistics is quite similar to those of the Reynolds stresses, and in particular, the inverse interscale transfer is commonly observed throughout the channel in both transport of the Reynolds shear stress and wall-normal turbulent heat flux.
3
Wang, B. B., G. P. Zank, L. Adhikari, and L. L. Zhao. "On the Conservation of Turbulence Energy in Turbulence Transport Models." Astrophysical Journal 928, no. 2 (April 1, 2022): 176. http://dx.doi.org/10.3847/1538-4357/ac596e.
Анотація:
Abstract Zank et al. developed models describing the transport of low-frequency incompressible and nearly incompressible turbulence in inhomogeneous flows. The formalism was based on expressing the fluctuating variables in terms of the Elsässar variables and then taking “moments” subject to various closure hypotheses. The turbulence transport models are different according to whether the plasma beta regime is large, of order unity, or small. Here, we show explicitly that the three sets of turbulence transport models admit a conservation representation that resembles the well-known WKB transport equation for Alfvén wave energy density after introducing appropriate definitions of the “pressure” associated with the turbulent fluctuations. This includes introducing a distinct turbulent pressure tensor for 3D incompressible turbulence (the large plasma beta limit) and pressure tensors for quasi-2D and slab turbulence (the plasma beta order-unity or small regimes) that generalize the form of the WKB pressure tensor. Various limits of the different turbulent pressure tensors are discussed. However, the analogy between the conservation form of the turbulence transport models and the WKB model is not close for multiple reasons, including that the turbulence models express fully nonlinear physical processes unlike the strictly linear WKB description. The analysis presented here both serves as a check on the validity and correctness of the turbulence transport models and also provides greater transparency of the energy dissipation term and the “turbulent pressure” in our models, which is important for many practical applications.
4
Takuto, Inaba, Nagata Kouji, Sakai Yasuhiko, Suzuki Hiroyuki, Terashima Osamu, and Suzuki Hiroki. "1065 PRODUCTION AND TRANSPORT OF TURBULENT KINETIC ENERGY IN FRACTAL-GENERATED TURBULENCE." Proceedings of the International Conference on Jets, Wakes and Separated Flows (ICJWSF) 2013.4 (2013): _1065–1_—_1065–4_. http://dx.doi.org/10.1299/jsmeicjwsf.2013.4._1065-1_.
5
Okiy, Karinate Valentine. "A Comparative Analysis of Turbulence Models Utilised for the Prediction of Turbulent Airflow through a Sudden Expansion." International Journal of Engineering Research in Africa 16 (June 2015): 64–78. http://dx.doi.org/10.4028/www.scientific.net/jera.16.64.
Анотація:
The turbulent airflow in a circular duct with sudden expansion was investigated utilizing three turbulence models. The turbulence models chosen are: the k-epsilon model, the shear stress transport model and the Reynolds-stress model. The performance of the models was investigated with respect to the flow parameter-recirculation length. The turbulent kinetic energy and velocity predictions were compared between the turbulence models and with experimental data, then interpreted on the basis of the recirculation length. From the results, the shear stress transport model predictions of recirculation length had the closest agreement with the experimental result compared to the other model. Likewise, the convergence rate for the shear stress transport model was reasonable compared to that of the Reynolds model which has the slowest convergence rate. In light of these findings, the shear stress transport model was discovered to be the most appropriate for the investigation of turbulent air flow in a circular duct with sudden expansion. Keywords: Turbulence, recirculation length, sudden expansion, Turbulence models.
6
Talon, Suzanne. "Rotational Transport Processes." Symposium - International Astronomical Union 215 (2004): 336–45. http://dx.doi.org/10.1017/s0074180900195841.
Анотація:
In this review, I discuss physical mechanisms leading to momentum and chemical transport in stars. Various instabilities leading to turbulence are discussed. I then present a self-consistent description of rotational mixing under the action of turbulence and meridional circulation in 1D models. Limitations of the model are discussed, both in terms of an extra mechanism for momentum transport in the Sun and solar-type stars (magnetic field and/or gravity waves) and in terms of our understanding of turbulent properties.
7
Kohli, Atul, and David G. Bogard. "Turbulent Transport in Film Cooling Flows." Journal of Heat Transfer 127, no. 5 (May 1, 2005): 513–20. http://dx.doi.org/10.1115/1.1865221.
Анотація:
This experimental study was performed on a single row of round holes with a 35° surface angle, representing film cooling geometry commonly used in turbine engines. Simultaneous velocity and temperature measurements were made using a cold-wire in conjunction with a LDV. The experimentally determined cross correlations provide a direct indication of the extent of turbulent transport of heat and momentum in the flow, which in turn governs dispersion of the film cooling jet. Actual engine environments have elevated mainstream turbulence levels that can severely reduce the cooling capability of film cooling jets. Clearly, the turbulent transport for very high mainstream turbulence is expected to be markedly different than that with low mainstream turbulence, and would improve our understanding of the mechanisms involved in the dispersion of film cooling jets. Experimental cross-correlation data was obtained for two vastly different freestream turbulence levels (0.6% and 20%) in this study. For this purpose, eddy diffusivities for momentum and heat transport were estimated from the measured data. These results will help develop new turbulence models and also explain why gradient diffusion based models do not give good predictions relative to experimental results.
8
Balbus, Steven A., and John F. Hawley. "Instability, Turbulence, and Enhanced Transport in Accretion Disks." International Astronomical Union Colloquium 163 (1997): 90–100. http://dx.doi.org/10.1017/s0252921100042536.
Анотація:
AbstractThe nature of MHD and hydrodynamical turbulence in accretion disks is discussed. Comparison is made with planar Couette flow, a classical system prone to nonlinear shear instability resulting in enhanced turbulent transport. Both Keplerian and non-Keplerian hydrodynamical disks are studied, and it is found that only constant angular momentum disks are unstable to nonlinear disturbances and develop enhanced turbulent transport. Convective instabilities do not lead to enhanced turbulent transport. Hydrodynamical Keplerian disks are quite stable to nonlinear disturbances. Several lines of argument are presented which all lead to this conclusion, but the key to disk turbulence is the interaction between the stress tensor and the mean flow gradients. The nature of this coupling is found to determine completely the stability properties of disks (hydrodynamics and magnetic), and the nature of turbulent transport. The weak field MHD instability, which is of great astrophysical importance, displays the same type of stress tensor – mean flow coupling that all classical local shear instabilities exhibit. Hydrodynamical Keplerian disks, on the other hand, do not. Accretion disk turbulence is MHD turbulence.
9
Giacomin, M., and P. Ricci. "Turbulent transport regimes in the tokamak boundary and operational limits." Physics of Plasmas 29, no. 6 (June 2022): 062303. http://dx.doi.org/10.1063/5.0090541.
Анотація:
Two-fluid, three-dimensional, flux-driven, global, electromagnetic turbulence simulations carried out by using the GBS (Global Braginskii Solver) code are used to identify the main parameters controlling turbulent transport in the tokamak boundary and to delineate an electromagnetic phase space of edge turbulence. Four turbulent transport regimes are identified: (i) a regime of fully developed turbulence appearing at intermediate values of collisionality and β, with turbulence driven by resistive ballooning modes, related to the L-mode operation of tokamaks, (ii) a regime of reduced turbulent transport at low collisionality and large heat source, with turbulence driven by drift-waves, related to a high-density H-mode regime, (iii) a regime of extremely large turbulent transport at high collisionality, which is associated with the crossing of the density limit, and (iv) a regime above the ideal ballooning limit at high β, with global modes affecting the dynamics of the entire confined region, which can be associated with the crossing of the β limit. The transition from the reduced to the developed turbulent transport regime is associated here with the H-mode density limit, and an analytical scaling law for maximum edge density achievable in H-mode is obtained. Analogously, analytical scaling laws for the crossing of the L-mode density and β limits are provided and compared to the results of GBS simulations.
10
Dong, G., and Z. Lin. "Role of wave-particle resonance in turbulent transport in toroidal plasmas." Plasma Physics and Controlled Fusion 64, no. 3 (January 21, 2022): 035005. http://dx.doi.org/10.1088/1361-6587/ac4275.
Анотація:
Abstract A clear understanding of wave-particle interaction and associated transport mechanisms of different particle species in the drift wave instabilities is important for accurate modeling and predictions of plasma confinement properties in tokamaks. In particular, the roles of linear resonance and nonlinear scattering in turbulent transport need to be delineated when constructing reduced transport models. First-principle, global gyrokinetic simulations find that electron particle and heat transport decreases to a very low level, while ion heat transport level has no dramatic change when wave-particle resonance is suppressed in the collisionless trapped electron mode (CTEM) turbulence. On the other hand, ion heat transport in the self-consistent ion temperature gradient (ITG) turbulence simulation is qualitatively similar to that in the test-particle simulation using the static ITG turbulence fields. These simulation results show that electron transport is primarily driven by the wave-particle resonance in the CTEM turbulence, and the ion transport is mostly driven by the nonlinear wave-particle scattering in both the CTEM and ITG turbulence.
11
Eidelman, A., T. Elperin, A. Kapusta, N. Kleeorin, A. Krein, and I. Rogachevskii. "Oscillating grids turbulence generator for turbulent transport studies." Nonlinear Processes in Geophysics 9, no. 3/4 (August 31, 2002): 201–5. http://dx.doi.org/10.5194/npg-9-201-2002.
Анотація:
Abstract. An oscillating grids turbulence generator was constructed for studies of two new effects associated with turbulent transport of particles, turbulent thermal diffusion and clustering instability. These effects result in formation of large-scale and small-scale inhomogeneities in the spatial distribution of particles. The advantage of this experimental set-up is the feasibility to study turbulent transport in mixtures with controllable composition and unlimited observation time. For flow measurements we used Particle Image Velocimetry with the adaptive multi-pass algorithm to determine a turbulent velocity field and its statistical characteristics. Instantaneous velocity vector maps, flow streamlines and probability density function of velocity field demonstrate properties of turbulence generated in the device.
12
Kestoras, M. D., and T. W. Simon. "Turbulence Measurements in a Heated, Concave Boundary Layer Under High-Free-Stream Turbulence Conditions." Journal of Turbomachinery 118, no. 1 (January 1, 1996): 172–80. http://dx.doi.org/10.1115/1.2836598.
Анотація:
Turbulence measurements for both momentum and heat transfer are taken in a lowvelocity, turbulent boundary layer growing naturally over a concave wall. The experiments are conducted with negligible streamwise acceleration and a nominal freestream turbulence intensity of ∼8 percent. Comparisons are made with data taken in an earlier study in the same test facility but with a 0.6 percent free-stream turbulence intensity. Results show that elevated free-stream turbulence intensity enhances turbulence transport quantities like uv and vt in most of the boundary layer. In contrast to the low-turbulence cases, high levels of transport of momentum are measured outside the boundary layer. Stable, Go¨rtlerlike vortices, present in the flow under low-turbulence conditions, do not form when the free-stream turbulence intensity is elevated. Turbulent Prandtl numbers, Prt, within the log region of the boundary layer over the concave wall increase with streamwise distance to values as high as 1.2. Profiles of Prt suggest that the increase in momentum transport with increased free-stream turbulence intensity precedes the increase in heat transport. Distributions of near-wall mixing length for momentum remain unchanged on the concave wall when free-stream turbulence intensity is elevated. Both for this level of free-stream turbulence and for the lower level, mixing length distributions increase linearly with distance from the wall, following the standard slope. However, when free-stream turbulence intensity is elevated, this linear region extends farther into the boundary layer, indicating the emerging importance of larger eddies in the wake of the boundary layer with the high-turbulence free stream. Because these eddies are damped by the wall, the influence of the wall grows with eddy size.
13
Kukulka, Tobias. "Horizontal Transport of Buoyant Material by Turbulent Jets in the Upper Ocean." Journal of Physical Oceanography 50, no. 3 (March 2020): 827–43. http://dx.doi.org/10.1175/jpo-d-19-0276.1.
Анотація:
AbstractCurrents in the ocean surface boundary layer (OSBL) determine the horizontal transport of submerged buoyant material, such as pollutants, plankton, and bubbles. Commonly, the mean horizontal transport, that is, the transport that changes the horizontal position of the material’s center of mass, is assumed to be accomplished by horizontal mean currents. However, surface convergence zones due to OSBL turbulence organize both wind-driven horizontal currents and near-surface concentrated buoyant material. In such surface convergence zones, concentrations of buoyant material are enhanced (e.g., apparent as windrows) and collocate with increased horizontal turbulent currents, here referred to as turbulent jets. In turn, the correlation of turbulent jet flow and material concentrations leads to a net mean horizontal transport due to turbulent motion. To examine this turbulent jet transport, an idealized model is devised for a wind-driven flow that is perturbed by prescribed cellular flow structures with crosswind surface convergence zones. Model solutions of the jet flow and material concentrations reveal that turbulent jet transport is comparable to the transport by horizontal mean currents for sufficiently strong cellular flow and material buoyancy. To test this model, we also perform more realistic turbulence-resolving large-eddy simulations (LESs) of wind and wave-driven OSBL turbulence. LES results are consistent with many features of the idealized model and suggest that the commonly overlooked turbulent jet transport is about 20%–50% of the traditional transport by horizontal mean currents. Thus, turbulent jet transport should be taken into account for accurate transport models of buoyant material in the OSBL.
14
Kamm, R. D., E. T. Bullister, and C. Keramidas. "The Effect of a Turbulent Jet on Gas Transport During Oscillatory Flow." Journal of Biomechanical Engineering 108, no. 3 (August 1, 1986): 266–72. http://dx.doi.org/10.1115/1.3138613.
Анотація:
Axial mass transport due to the combined effects of flow oscillation and a turbulent jet was studied both experimentally and with a simple theoretical model. The experiments show that the distance over which turbulence enhances transport is greatly increased by flow oscillation, and is particularly sensitive to tidal volume. The jet flow rate and jet configuration are relatively less important. To analyze the results, the region influenced by the jet is divided into two zones: a near field in which the time-mean flow velocities are larger than the turbulent fluctuations, and a far field where the time-mean flow is essentially zero. In the far field, axial mass transport is increased due to the turbulence which decays in strength away from the jet. When oscillatory flow is superimposed upon the steady jet flow, the turbulence in the far field interacts with the flow oscillations to augment the transport of turbulence energy and of mass. This transport enhancement is modeled by introducing an effective axial diffusivity analogous to that used in laminar oscillatory flow.
15
Brethouwer, Geert. "Passive scalar transport in rotating turbulent channel flow." Journal of Fluid Mechanics 844 (April 4, 2018): 297–322. http://dx.doi.org/10.1017/jfm.2018.198.
Анотація:
Passive scalar transport in turbulent channel flow subject to spanwise system rotation is studied by direct numerical simulations. The Reynolds number $Re=U_{b}h/\unicode[STIX]{x1D708}$ is fixed at 20 000 and the rotation number $Ro=2\unicode[STIX]{x1D6FA}h/U_{b}$ is varied from 0 to 1.2, where $U_{b}$ is the bulk mean velocity, $h$ the half channel gap width and $\unicode[STIX]{x1D6FA}$ the rotation rate. The scalar is constant but different at the two walls, leading to steady scalar transport across the channel. The rotation causes an unstable channel side with relatively strong turbulence and turbulent scalar transport, and a stable channel side with relatively weak turbulence or laminar-like flow, weak turbulent scalar transport but large scalar fluctuations and steep mean scalar gradients. The distinct turbulent–laminar patterns observed at certain $Ro$ on the stable channel side induce similar patterns in the scalar field. The main conclusions of the study are that rotation reduces the similarity between the scalar and velocity field and that the Reynolds analogy for scalar-momentum transport does not hold for rotating turbulent channel flow. This is shown by a reduced correlation between velocity and scalar fluctuations, and a strongly reduced turbulent Prandtl number of less than 0.2 on the unstable channel side away from the wall at higher $Ro$. On the unstable channel side, scalar scales become larger than turbulence scales according to spectra and the turbulent scalar flux vector becomes more aligned with the mean scalar gradient owing to rotation. Budgets in the governing equations of the scalar energy and scalar fluxes are presented and discussed as well as other statistics relevant for turbulence modelling.
16
Lopes, José Fortes. "Using Different Classic Turbulence Closure Models to Assess Salt and Temperature Modelling in a Lagunar System: A Sensitivity Study." Journal of Marine Science and Engineering 10, no. 11 (November 14, 2022): 1750. http://dx.doi.org/10.3390/jmse10111750.
Анотація:
Turbulence modelling is an important issue when dealing with hydrodynamic and transport models for better simulation of the transport of dissolved or suspended substances in a body-water. It controls processes involving physical balances (salt and water temperature) and, therefore, the ecosystem equilibrium. The study arises from the need to model the turbulence more efficiently when dealing with extreme situations on the Ria de Aveiro (Portugal), a coastal lagoon shallow water system dominated by tidal transport. Because the turbulence model is coupled to the hydrodynamic and transport models, a correct estimation of the eddy viscosity is important in simulating the salt and the heat transports. The aim is to assess the performance of four turbulence schemes/models (k, k-ε, Smagorinsky’s, and k-ε/Smagorinsky’s (k-ε/Sma), where k is turbulent kinetic energy and ε the dissipation rate of the turbulent kinetic energy) associated to a coupled hydrodynamic and transport models to simulate the eddy viscosity, the salinity, and the temperature. Overall, the results point out that among the different models/schemes used, the is the one which provides a more realistic value of the eddy viscosity within the range (1–6) m2 s−1, but most probably (1–3) m2 s−1. The application of the sensitivity analysis to some non-universal k-ε/Sma parameters evidenced significant sensitivity for the eddy viscosity and the salinity and moderate sensitivity for the water temperature. A 100% adjustment of the parameter values relative to the reference, translated into variations within the range of (1, 4) m2 s−1, (0, 13) PSU, and (1, 2.20) °C, for the eddy viscosity, salinity, and water temperature, respectively.
17
Hughes, Kenneth G., James N. Moum, and Emily L. Shroyer. "Heat Transport through Diurnal Warm Layers." Journal of Physical Oceanography 50, no. 10 (October 1, 2020): 2885–905. http://dx.doi.org/10.1175/jpo-d-20-0079.1.
Анотація:
AbstractPenetration of solar radiation in the upper few meters of the ocean creates a near-surface, stratified diurnal warm layer. Wind stress accelerates a diurnal jet in this layer. Turbulence generated at the diurnal thermocline, where the shear of the diurnal jet is concentrated, redistributes heat downward via mixing. New measurements of temperature and turbulence from fast thermistors on a surface-following platform depict the details of this sequence in both time and depth. Temporally, the sequence at a fixed depth follows a counterclockwise path in logϵ–logN parameter space. This path also captures the evolution of buoyancy Reynolds number (a proxy for the anisotropy of the turbulence) and Ozmidov scale (a proxy for the outer vertical length scale of turbulence in the absence of the free surface). Vertically, the solar heat flux always leads to heating of fluid parcels in the upper few meters, whereas the turbulent heat flux divergence changes sign across the depth of maximum vertical temperature gradient, cooling fluid parcels above and heating fluid parcels below. In general, our measurements of fluid parcel heating or cooling rates of order 0.1°C h−1 are consistent with our estimates of heat flux divergence. In weak winds (<2 m s−1), sea surface temperature (SST) is controlled by the depth-dependent absorption of solar radiation. In stronger winds, turbulent mixing controls SST.
18
Volino, R. J., and T. W. Simon. "Boundary Layer Transition Under High Free-Stream Turbulence and Strong Acceleration Conditions: Part 2—Turbulent Transport Results." Journal of Heat Transfer 119, no. 3 (August 1, 1997): 427–32. http://dx.doi.org/10.1115/1.2824115.
Анотація:
Measurements from heated boundary layers along a concave-curved test wall subject to high (initially 8 percent) free-stream turbulence intensity and strong (K = (ν/U∞2 dU∞/dx, as high as 9 × 10−6) acceleration are presented and discussed. Conditions for the experiments were chosen to simulate those present on the downstream half of the pressure side of a gas turbine airfoil. Turbulence statistics, including the turbulent shear stress, the turbulent heat flux, and the turbulent Prandtl number are presented. The transition zone is of extended length in spite of the high free-stream turbulence level. Turbulence quantities are strongly suppressed below values in unaccelerated turbulent boundary layers. Turbulent transport quantities rise with the intermittency, as the boundary layer proceeds through transition. Octant analysis shows a similar eddy structure in the present flow as was observed in transitional flows under low free-stream turbulence conditions. To the authors’ knowledge, this is the first detailed documentation of a high-free-stream-turbulence boundary layer flow in such a strong acceleration field.
19
Gladskikh, Daria, Lev Ostrovsky, Yuliya Troitskaya, Irina Soustova, and Evgeny Mortikov. "Turbulent Transport in a Stratified Shear Flow." Journal of Marine Science and Engineering 11, no. 1 (January 6, 2023): 136. http://dx.doi.org/10.3390/jmse11010136.
Анотація:
Within the framework of the theory of unsteady turbulent flows in a stratified fluid, a new parameterization of the turbulent Prandtl number is proposed. The parameterization is included in the k-ε-closure and used within the three-dimensional model of thermohydrodynamics of an enclosed water body where density distribution includes pycnocline. This allows us to describe turbulence in a stratified shear flow without the restrictions associated with the gradient Richardson number and justify the choice of closure constants. Numerical experiments, where the downward penetration of turbulence was considered, confirm the advantage of the developed approach in describing the effects neglected in the classical closures.
20
Buice, C. U., and J. K. Eaton. "Turbulent Heat Transport in a Perturbed Channel Flow." Journal of Heat Transfer 121, no. 2 (May 1, 1999): 322–25. http://dx.doi.org/10.1115/1.2825983.
Анотація:
The recovering boundary layer downstream of a separation bubble is known to have a highly perturbed turbulence structure which creates difficulty for turbulence models. The present experiment addressed the effect of this perturbed structure on turbulent heat transport. The turbulent diffusion of heat downstream of a heated wire was measured in a perturbed channel flow and compared to that in a simple, fully developed channel flow. The turbulent diffusivity of heat was found to be more than 20 times larger in the perturbed flow. The turbulent Prandtl number increased to 1.7, showing that the turbulent eddy viscosity was affected even more strongly than the eddy thermal diffusivity. This result corroborates previous work showing that boundary layer disturbances generally have a stronger effect on the eddy viscosity, rendering prescribed turbulent Prandtl number models ineffective in perturbed flows.
21
Canuto, Vittorio M. "Mixing and Transport in Stars." Highlights of Astronomy 12 (2002): 295–97. http://dx.doi.org/10.1017/s1539299600013575.
Анотація:
Transport and mixing in stars is as important as it is difficult to quantify (Zahn 1992; Schatzman 1996; Maeder 1997; Pinsonneault 1997). A first difficulty is that both transport and mixing are dynamical processes which, given the low viscosities of stellar interiors, usually means that the flow is turbulent giving rise to technical difficulties for turbulence is still an incomplete chapter though recent studies have brought about considerable progress. A second difficulty is that turbulence is not self-sustaining and unless there is a source, dynamical mixing and transport will decay in time and eventually die out. Thus, the question: what is the source of turbulence, let alone how to describe it? In the convective zone, the source is the unstable stratification but the mixing there is so strong that one does not need a sophisticated theory to describe it. Strong turbulence is easier to describe than weak turbulence and yet the latter is when the problems become interesting and our descriptive power is less reliable. For example, below the solar CZ we don’t even know for sure the source of stirring, let alone how to describe it and yet, it is the region where we would like to be confident about models. The transport of Li is the best example of a mixing and transport that cannot be too strong or too weak (Schlattl and Weiss 1999). A third difficulty is the unstated assumption that “transport” (advection) and “mixing” (diffusion) have different origin.
22
Norscini, Claudia, Thomas Cartier-Michaud, Guilhem Dif-Pradalier, Xavier Garbet, Philippe Ghendrih, Virginie Grandgirard, and Yanick Sarazin. "Interface transport barriers in magnetized plasmas." Plasma Physics and Controlled Fusion 64, no. 5 (March 31, 2022): 055007. http://dx.doi.org/10.1088/1361-6587/ac5a07.
Анотація:
Abstract We address the formation of Interface Transport Barriers using a generic turbulent transport model, reduced to 2D, and used to investigate interchange turbulence in magnetized plasmas. The generation of a transport barrier at the edge-scrape off layer (SOL) plasma interface is governed by a zonation regime in the edge region with closed-field lines. The barrier is triggered by a gap in the turbulent spectrum between zero, the zonal flow wave vector, and the wave vector of the spectrum maximum. This gap is controlled by the energy injection wave vector of the interchange instability and the Rhine scale that bounds the inverse cascade. Increasing the magnitude of the turbulence drive at a given gap reinforces the transport barrier. In the interface transport barrier regime, edge relaxation bursts of turbulence regenerate the zonal flows that are eroded by damping processes such as collisions. The duration of the quiescent phase between the quasi-periodic relaxation events is then governed by the ion collision frequency. Such an interface transport barrier can play the role of a seed barrier prior to a full bifurcation to improved confinement.
23
Hawley, John F., and Steven A. Balbus. "MHD Turbulence in an Accretion Disk." Publications of the Astronomical Society of Australia 12, no. 2 (August 1995): 159–64. http://dx.doi.org/10.1017/s1323358000020208.
Анотація:
AbstractA long-standing problem in the theory of astrophysical accretion disks has been to determine the nature of the stress that transports orbital angular momentum outward. The discovery of a local MHD instability is strong evidence that transport occurs through turbulent Maxwell and Reynolds stresses. Using numerical simulations, we have demonstrated that a weak seed magnetic field in an accretion disk shear flow is unstable and leads to sustained MHD turbulence at dynamically important levels.
24
Kazbekov, Askar, Keishi Kumashiro, and Adam M. Steinberg. "Enstrophy transport in swirl combustion." Journal of Fluid Mechanics 876 (August 6, 2019): 715–32. http://dx.doi.org/10.1017/jfm.2019.551.
Анотація:
The contributions of vortex stretching, dilatation, baroclinic torque and viscous diffusion to Reynolds-averaged enstrophy transport in turbulent swirl flames were experimentally measured using tomographic particle image velocimetry and $\text{CH}_{2}\text{O}$ planar laser induced fluorescence at jet Reynolds numbers of 26 000–51 000. The mean baroclinic torque was determined by subtracting the other terms in the enstrophy transport equation from the mean Lagrangian derivative. Enstrophy production from baroclinic torque was found to be significant relative to the other transport terms across all conditions studies. This result contrasts with direct numerical simulations of flames in homogeneous isotropic turbulence, which show a decreasing relative significance of baroclinic torque with increasing turbulence intensity (e.g. Bobbitt, Lapointe & Blanquart, Phys. Fluids, vol. 28 (1), 2016, 015101). Hence, the significance of baroclinic enstrophy production in flames is not determined entirely by the local turbulence and flame properties, but also depends on the configuration-specific pressure field.
25
Trotta, Domenico, Francesco Valentini, David Burgess, and Sergio Servidio. "Phase space transport in the interaction between shocks and plasma turbulence." Proceedings of the National Academy of Sciences 118, no. 21 (May 18, 2021): e2026764118. http://dx.doi.org/10.1073/pnas.2026764118.
Анотація:
The interaction of collisionless shocks with fully developed plasma turbulence is numerically investigated. Hybrid kinetic simulations, where a turbulent jet is slammed against an oblique shock, are employed to address the role of upstream turbulence on plasma transport. A technique, using coarse graining of the Vlasov equation, is proposed, showing that the particle transport strongly depends on upstream turbulence properties, such as strength and coherency. These results might be relevant for the understanding of acceleration and heating processes in space plasmas.
26
Ames, F. E., and L. A. Dvorak. "Turbulent Transport in Pin Fin Arrays: Experimental Data and Predictions." Journal of Turbomachinery 128, no. 1 (February 1, 2005): 71–81. http://dx.doi.org/10.1115/1.2098792.
Анотація:
The objective of this research has been to experimentally investigate the fluid dynamics of pin fin arrays in order to clarify the physics of heat transfer enhancement and uncover problems in conventional turbulence models. The fluid dynamics of a staggered pin fin array has been studied using hot wire anemometry with both single- and x-wire probes at array Reynolds numbers of 3000, 10,000, and 30,000. Velocity distributions off the endwall and pin surface have been acquired and analyzed to investigate turbulent transport in pin fin arrays. Well resolved 3D calculations have been performed using a commercial code with conventional two-equation turbulence models. Predictive comparisons have been made with fluid dynamic data. In early rows where turbulence is low, the strength of shedding increases dramatically with increasing Reynolds numbers. The laminar velocity profiles off the surface of pins show evidence of unsteady separation in early rows. In row three and beyond, laminar boundary layers off pins are quite similar. Velocity profiles off endwalls are strongly affected by the proximity of pins and turbulent transport. At the low Reynolds numbers, the turbulent transport and acceleration keep boundary layers thin. Endwall boundary layers at higher Reynolds numbers exhibit very high levels of skin friction enhancement. Well-resolved 3D steady calculations were made with several two-equation turbulence models and compared with experimental fluid mechanic and heat transfer data. The quality of the predictive comparison was substantially affected by the turbulence model and near-wall methodology.
27
Galassi, Davide, Guido Ciraolo, Patrick Tamain, Hugo Bufferand, Philippe Ghendrih, Nicolas Nace, and Eric Serre. "Tokamak Edge Plasma Turbulence Interaction with Magnetic X-Point in 3D Global Simulations." Fluids 4, no. 1 (March 15, 2019): 50. http://dx.doi.org/10.3390/fluids4010050.
Анотація:
Turbulence in the edge plasma of a tokamak is a key actor in the determination of the confinement properties. The divertor configuration seems to be beneficial for confinement, suggesting an effect on turbulence of the particular magnetic geometry introduced by the X-point. Simulations with the 3D fluid turbulence code TOKAM3X are performed here to evaluate the impact of a diverted configuration on turbulence in the edge plasma, in an isothermal framework. The presence of the X-point is found, locally, to affect both the shape of turbulent structures and the amplitude of fluctuations, in qualitative agreement with recent experimental observations. In particular, a quiescent region is found in the divertor scrape-off layer (SOL), close to the separatrix. Globally, a mild transport barrier spontaneously forms in the closed flux surfaces region near the separatrix, differently from simulations in limiter configuration. The effect of turbulence-driven Reynolds stress on the formation of the barrier is found to be weak by dedicated simulations, while turbulence damping around the X-point seems to globally reduce turbulent transport on the whole flux surface. The magnetic shear is thus pointed out as a possible element that contributes to the formation of edge transport barriers.
28
de Lemos, Marcelo J. S., and Maximilian S. Mesquita. "Comparison of Four Thermo-Mechanical Models for Simulating Reactive Flow in Porous Materials." Defect and Diffusion Forum 297-301 (April 2010): 1493–501. http://dx.doi.org/10.4028/www.scientific.net/ddf.297-301.1493.
Анотація:
The objective of this paper is to present numerical simulations of combustion of an air/methane mixture in porous materials using a model that considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. Four different thermo-mechanical models are compared, namely Laminar, Laminar with Radiation Transport, Turbulent, Turbulent with Radiation Transport. Combustion is modeled via a unique simple closure. Preliminary testing results indicate that a substantially different temperature distribution is obtained depending on the model used. In addition, for high excess air peak gas temperature are reduced.
29
Kawamura, Hiroshi, and Yoshitsune Kurihara. "Modelling of turbulent scalar transport in homogeneous turbulence." International Journal of Heat and Mass Transfer 43, no. 11 (June 2000): 1935–45. http://dx.doi.org/10.1016/s0017-9310(99)00272-0.
30
Iyer, Kartik P., Janet D. Scheel, Jörg Schumacher, and Katepalli R. Sreenivasan. "Classical 1/3 scaling of convection holds up to Ra = 1015." Proceedings of the National Academy of Sciences 117, no. 14 (March 25, 2020): 7594–98. http://dx.doi.org/10.1073/pnas.1922794117.
Анотація:
The global transport of heat and momentum in turbulent convection is constrained by thin thermal and viscous boundary layers at the heated and cooled boundaries of the system. This bottleneck is thought to be lifted once the boundary layers themselves become fully turbulent at very high values of the Rayleigh numberRa—the dimensionless parameter that describes the vigor of convective turbulence. Laboratory experiments in cylindrical cells forRa≳1012have reported different outcomes on the putative heat transport law. Here we show, by direct numerical simulations of three-dimensional turbulent Rayleigh–Bénard convection flows in a slender cylindrical cell of aspect ratio1/10, that the Nusselt number—the dimensionless measure of heat transport—follows the classical power law ofNu=(0.0525±0.006)×Ra0.331±0.002up toRa=1015. Intermittent fluctuations in the wall stress, a blueprint of turbulence in the vicinity of the boundaries, manifest at allRaconsidered here, increasing with increasingRa, and suggest that an abrupt transition of the boundary layer to turbulence does not take place.
31
Chhiber, Rohit, Arcadi V. Usmanov, William H. Matthaeus, and Melvyn L. Goldstein. "Large-scale Structure and Turbulence Transport in the Inner Solar Wind: Comparison of Parker Solar Probe’s First Five Orbits with a Global 3D Reynolds-averaged MHD Model." Astrophysical Journal 923, no. 1 (December 1, 2021): 89. http://dx.doi.org/10.3847/1538-4357/ac1ac7.
Анотація:
Abstract Simulation results from a global magnetohydrodynamic model of the solar corona and solar wind are compared with Parker Solar Probe (PSP) observations during its first five orbits. The fully three-dimensional model is based on Reynolds-averaged mean-flow equations coupled with turbulence-transport equations. The model includes the effects of electron heat conduction, Coulomb collisions, turbulent Reynolds stresses, and heating of protons and electrons via a turbulent cascade. Turbulence-transport equations for average turbulence energy, cross helicity, and correlation length are solved concurrently with the mean-flow equations. Boundary conditions at the coronal base are specified using solar synoptic magnetograms. Plasma, magnetic field, and turbulence parameters are calculated along the PSP trajectory. Data from the first five orbits are aggregated to obtain trends as a function of heliocentric distance. Comparison of simulation results with PSP data shows good agreement, especially for mean-flow parameters. Synthetic distributions of magnetic fluctuations are generated, constrained by the local rms turbulence amplitude given by the model. Properties of this computed turbulence are compared with PSP observations.
32
BRIGGS, DAVID A., JOEL H. FERZIGER, JEFFREY R. KOSEFF, and STEPHEN G. MONISMITH. "Turbulent mixing in a shear-free stably stratified two-layer fluid." Journal of Fluid Mechanics 354 (January 10, 1998): 175–208. http://dx.doi.org/10.1017/s0022112097007672.
Анотація:
Direct numerical simulation is used to examine turbulent mixing in a shear-free stably stratified fluid. Energy is continuously supplied to a small region to maintain a well-developed kinetic energy profile, as in an oscillating grid flow (Briggs et al. 1996; Hopfinger & Toly 1976; Nokes 1988). A microscale Reynolds number of 60 is maintained in the source region. The turbulence forms a well-mixed layer which diffuses from the source into the quiescent fluid below. Turbulence transport at the interface causes the mixed layer to grow under weakly stratified conditions. When the stratification is strong, large-scale turbulent transport is inactive and pressure transport becomes the principal mechanism for the growth of the turbulence layer. Down-gradient buoyancy flux is present in the large scales; however, far from the source, weak counter-gradient fluxes appear in the medium to small scales. The production of internal waves and counter-gradient fluxes rapidly reduces the mixing when the turbulent Froude number is lower than unity. When the stratification is weak, the turbulence is strong enough to break up the density interface and transport fluid parcels of different density over large vertical distances. As the stratification intensifies, turbulent eddies flatten against the interface creating anisotropy and internal waves. The dominant entrainment mechanism is then scouring. Mixing efficiency, defined as the ratio of buoyancy flux to available kinetic energy, exhibits a similar dependence on Froude number to other stratified flows (Holt et al. 1992; Lienhard & Van Atta 1990). However, using the anisotropy of the turbulence to define an alternative mixing efficiency and Froude number improves the correlation and allows local scaling.
33
Fu, S., P. G. Huang, B. E. Launder, and M. A. Leschziner. "A Comparison of Algebraic and Differential Second-Moment Closures for Axisymmetric Turbulent Shear Flows With and Without Swirl." Journal of Fluids Engineering 110, no. 2 (June 1, 1988): 216–21. http://dx.doi.org/10.1115/1.3243537.
Анотація:
Computations are reported for three axisymmetric turbulent jets, two of which are swirling and one containing swirl-induced recirculation, obtained with two models of turbulence: a differential second-moment (DSM) closure and an algebraic derivative thereof (ASM). The models are identical in respect of all turbulent processes except that, in the ASM scheme, stress transport is represented algebraically in terms of the transport of turbulence energy. The comparison of the results thus provides a direct test of how well the model of stress transport adopted in ASM schemes simulates that of the full second-moment closure. The comparison indicates that the ASM hypothesis seriously misrepresents the diffusive transport of the shear stress in nonswirling axisymmetric flows, while in the presence of swirl the defects extend to all stress components and are aggravated by a failure to account for influential (additive) swirl-related stress-transport terms in the algebraic modelling process. The principal conclusion thus drawn is that in free shear flows where transport effects are significant, it is advisable to adopt a full second-moment closure if turbulence modelling needs to proceed beyond the eddy-viscosity level.
34
Yao, Mao-Sung, and Ye Cheng. "Cloud Simulations in Response to Turbulence Parameterizations in the GISS Model E GCM." Journal of Climate 25, no. 14 (July 15, 2012): 4963–74. http://dx.doi.org/10.1175/jcli-d-11-00399.1.
Анотація:
Abstract The response of cloud simulations to turbulence parameterizations is studied systematically using the GISS general circulation model (GCM) E2 employed in the Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report (AR5). Without the turbulence parameterization, the relative humidity (RH) and the low cloud cover peak unrealistically close to the surface; with the dry convection or with only the local turbulence parameterization, these two quantities improve their vertical structures, but the vertical transport of water vapor is still weak in the planetary boundary layers (PBLs); with both local and nonlocal turbulence parameterizations, the RH and low cloud cover have better vertical structures in all latitudes due to more significant vertical transport of water vapor in the PBL. The study also compares the cloud and radiation climatologies obtained from an experiment using a newer version of turbulence parameterization being developed at GISS with those obtained from the AR5 version. This newer scheme differs from the AR5 version in computing nonlocal transports, turbulent length scale, and PBL height and shows significant improvements in cloud and radiation simulations, especially over the subtropical eastern oceans and the southern oceans. The diagnosed PBL heights appear to correlate well with the low cloud distribution over oceans. This suggests that a cloud-producing scheme needs to be constructed in a framework that also takes the turbulence into consideration.
35
Agudelo Rueda, Jeffersson A., Daniel Verscharen, Robert T. Wicks, Christopher J. Owen, Georgios Nicolaou, Kai Germaschewski, Andrew P. Walsh, Ioannis Zouganelis, and Santiago Vargas Domínguez. "Energy Transport during 3D Small-scale Reconnection Driven by Anisotropic Plasma Turbulence." Astrophysical Journal 938, no. 1 (October 1, 2022): 4. http://dx.doi.org/10.3847/1538-4357/ac8667.
Анотація:
Abstract Energy dissipation in collisionless plasmas is a long-standing fundamental physics problem. Although it is well known that magnetic reconnection and turbulence are coupled and transport energy from system-size scales to subproton scales, the details of the energy distribution and energy dissipation channels remain poorly understood. Especially, the energy transfer and transport associated with 3D small-scale reconnection that occurs as a consequence of a turbulent cascade is unknown. We use an explicit fully kinetic particle-in-cell code to simulate 3D small-scale magnetic reconnection events forming in anisotropic and decaying Alfvénic turbulence. We identify a highly dynamic and asymmetric reconnection event that involves two reconnecting flux ropes. We use a two-fluid approach based on the Boltzmann equation to study the spatial energy transfer associated with the reconnection event and compare the power density terms in the two-fluid energy equations with standard energy-based damping, heating, and dissipation proxies. Our findings suggest that the electron bulk flow transports thermal energy density more efficiently than kinetic energy density. Moreover, in our turbulent reconnection event, the energy density transfer is dominated by plasma compression. This is consistent with turbulent current sheets and turbulent reconnection events, but not with laminar reconnection.
36
Menter, F. R. "Eddy Viscosity Transport Equations and Their Relation to the k-ε Model". Journal of Fluids Engineering 119, № 4 (1 грудня 1997): 876–84. http://dx.doi.org/10.1115/1.2819511.
Анотація:
A formalism will be presented which allows transforming two-equation eddy viscosity turbulence models into one-equation models. The transformation is based on Bradshaw’s assumption that the turbulent shear stress is proportional to the turbulent kinetic energy. This assumption is supported by experimental evidence for a large number of boundary layer flows and has led to improved predictions when incorporated into two-equation models of turbulence. Based on it, a new one-equation turbulence model will be derived from the k-ε model. The model will be tested against the one-equation model of Baldwin and Barth, which is also derived from the k-ε model (plus additional assumptions) and against its parent two-equation model. It will be shown that the assumptions involved in the derivation of the Baldwin-Barth model cause significant problems at the edge of a turbulent layer.
37
van den Berg, J. P., N. E. Engelbrecht, N. Wijsen, and R. D. Strauss. "On the Turbulent Reduction of Drifts for Solar Energetic Particles." Astrophysical Journal 922, no. 2 (November 30, 2021): 200. http://dx.doi.org/10.3847/1538-4357/ac2736.
Анотація:
Abstract Particle drifts perpendicular to the background magnetic field have been proposed by some authors as an explanation for the very efficient perpendicular transport of solar energetic particles (SEPs). This process, however, competes with perpendicular diffusion caused by magnetic turbulence, which can also disrupt the drift patterns and reduce the magnitude of drift effects. The latter phenomenon is well known in cosmic-ray studies, but not yet considered in SEP models. Additionally, SEP models that do not include drifts, especially for electrons, use turbulent drift reduction as a justification of this omission, without critically evaluating or testing this assumption. This article presents the first theoretical step for a theory of drift suppression in SEP transport. This is done by deriving the turbulence-dependent drift reduction function with a pitch-angle dependence, as is applicable for anisotropic particle distributions, and by investigating to what extent drifts will be reduced in the inner heliosphere for realistic turbulence conditions and different pitch-angle dependencies of the perpendicular diffusion coefficient. The influence of the derived turbulent drift reduction factors on the transport of SEPs are tested, using a state-of-the-art SEP transport code, for several expressions of theoretically derived perpendicular diffusion coefficients. It is found, for realistic turbulence conditions in the inner heliosphere, that cross-field diffusion will have the largest influence on the perpendicular transport of SEPs, as opposed to particle drifts.
38
Wilson, R. "Turbulent diffusivity in the free atmosphere inferred from MST radar measurements: a review." Annales Geophysicae 22, no. 11 (November 29, 2004): 3869–87. http://dx.doi.org/10.5194/angeo-22-3869-2004.
Анотація:
Abstract. The actual impact on vertical transport of small-scale turbulence in the free atmosphere is still a debated issue. Numerous estimates of an eddy diffusivity exist, clearly showing a lack of consensus. MST radars were, and continue to be, very useful for studying atmospheric turbulence, as radar measurements allow one to estimate the dissipation rates of energy (kinetic and potential) associated with turbulent events. The two commonly used methods for estimating the dissipation rates, from the backscattered power and from the Doppler width, are discussed. The inference methods of a local diffusivity (local meaning here "within" the turbulent patch) by using the dissipation rates are reviewed, with some of the uncertainty causes being stressed. Climatological results of turbulence diffusivity inferred from radar measurements are reviewed and compared. As revealed by high resolution MST radar measurements, atmospheric turbulence is intermittent in space and time. Recent theoretical works suggest that the effective diffusivity of such a patchy turbulence is related to statistical parameters describing the morphology of turbulent events: filling factor, lifetime and height of the patches. It thus appears that a statistical description of the turbulent patches' characteristics is required in order to evaluate and parameterize the actual impact of small-scale turbulence on transport of energy and materials. Clearly, MST radars could be an essential tool in that matter.
39
Volino, Ralph J., and Terrence W. Simon. "Spectral Measurements in Transitional Boundary Layers on a Concave Wall Under High and Low Free-Stream Turbulence Conditions." Journal of Turbomachinery 122, no. 3 (August 1, 1997): 450–57. http://dx.doi.org/10.1115/1.1303075.
Анотація:
The relationship between free-stream turbulence and boundary layer behavior has been investigated using spectral measurements. The power spectral densities of turbulence quantities in transitional and fully turbulent boundary layers were computed and compared to the power spectra of the same quantities measured in the free stream. Comparisons were made using the “transfer function.” The transfer function is the ratio of two spectra at each frequency in the spectra. Comparisons were done in flows with low (0.6 percent) and high (8 percent) free-stream turbulence intensities. Evidence was gathered that suggests that relatively low-frequency, large-scale eddies in the free stream buffet the boundary layer, causing boundary layer unsteadiness at the same low frequencies. These fluctuations are present in both transitional and fully turbulent boundary layers. They are seen under both high and low free-stream turbulence conditions, although they are stronger in the high-turbulence case. Examination of the turbulent shear stress suggests that the low-frequency fluctuations enhance transport in the boundary layer but they are not so effective in promoting eddy transport as are turbulent eddies produced and residing within the boundary layer. In the fully-turbulent boundary layer, higher-frequency fluctuations are added to the low-frequency unsteadiness. These higher-frequency fluctuations, not seen in the transitional boundary layer, are associated with turbulence production in the boundary layer and appear not to be directly related to free-stream unsteadiness. [S0889-504X(00)00403-7]
40
Liang, Bin. "Turbulence and Anomalous Transport." Journal of Modern Physics 14, no. 13 (2023): 1735–40. http://dx.doi.org/10.4236/jmp.2023.1413102.
41
VLAD, M., J. D. REUSS, F. SPINEANU, and J. H. MISGUICH. "Transport scaling and trapping." Journal of Plasma Physics 59, no. 4 (June 1998): 707–18. http://dx.doi.org/10.1017/s0022377898006631.
Анотація:
It is shown that particle trapping plays an important role in the appearance of anomalous or strange diffusion in turbulent fields, mainly by inducing a non-Gaussian statistics of the stochastic displacements. Trapping processes are responsible for the important deviation observed between the traditional prediction of Bohm scaling for diffusion in low-frequency turbulence and the numerically checked prediction by Isichenko. Also, several theoretical models are proposed or reviewed.
42
SMITH, K. S., G. BOCCALETTI, C. C. HENNING, I. MARINOV, C. Y. TAM, I. M. HELD, and G. K. VALLIS. "Turbulent diffusion in the geostrophic inverse cascade." Journal of Fluid Mechanics 469 (October 15, 2002): 13–48. http://dx.doi.org/10.1017/s0022112002001763.
Анотація:
Motivated in part by the problem of large-scale lateral turbulent heat transport in the Earth's atmosphere and oceans, and in part by the problem of turbulent transport itself, we seek to better understand the transport of a passive tracer advected by various types of fully developed two-dimensional turbulence. The types of turbulence considered correspond to various relationships between the streamfunction and the advected field. Each type of turbulence considered possesses two quadratic invariants and each can develop an inverse cascade. These cascades can be modified or halted, for example, by friction, a background vorticity gradient or a mean temperature gradient. We focus on three physically realizable cases: classical two-dimensional turbulence, surface quasi-geostrophic turbulence, and shallow-water quasi-geostrophic turbulence at scales large compared to the radius of deformation. In each model we assume that tracer variance is maintained by a large-scale mean tracer gradient while turbulent energy is produced at small scales via random forcing, and dissipated by linear drag. We predict the spectral shapes, eddy scales and equilibrated energies resulting from the inverse cascades, and use the expected velocity and length scales to predict integrated tracer fluxes.When linear drag halts the cascade, the resulting diffusivities are decreasing functions of the drag coefficient, but with different dependences for each case. When β is significant, we find a clear distinction between the tracer mixing scale, which depends on β but is nearly independent of drag, and the energy-containing (or jet) scale, set by a combination of the drag coefficient and β. Our predictions are tested via high- resolution spectral simulations. We find in all cases that the passive scalar is diffused down-gradient with a diffusion coefficient that is well-predicted from estimates of mixing length and velocity scale obtained from turbulence phenomenology.
43
Kawata, T., and T. Tsukahara. "Spectral analysis on dissimilarity between turbulent momentum and heat transfers in plane Couette turbulence." Physics of Fluids 34, no. 7 (July 2022): 075135. http://dx.doi.org/10.1063/5.0094659.
Анотація:
Nonlinear interactions between different scales in turbulence result in both interscale and spatial transport of turbulent energy, and their role in the turbulent heat transfer mechanism is also of practical importance from an engineering viewpoint. In this study, we investigate a turbulent plane Couette flow with passive-scalar heat transfer at the Prandtl number of 0.71 to discuss the similarity/difference between scale interactions in velocity and temperature fields. The constant-temperature-difference boundary condition is used so that the mean velocity and temperature profiles are similar, and then, the roles of interscale and spatial transports are compared for the spectral transport budgets of turbulent energies and temperature-related statistics. We show that turbulent heat transfer occurs at relatively small streamwise length scales compared to momentum transfer, although molecular diffusion is more significant in the temperature field as the Prandtl number is less than 1. Detailed analysis on the transport budgets of temperature-related spectra shows that scale interactions in the temperature field supply more energy to small scales than those in the velocity field. This significant temperature cascade causes more energetic temperature fluctuation at small scales, resulting in the spectral dissimilarity between turbulent heat and momentum transfers.
44
Madaliev, Murodil, Zokhidjon Abdulkhaev, Jamshidbek Otajonov, Khasanboy Kadyrov, Inomjan Bilolov, Sharabiddin Israilov, and Nurzoda Abdullajonov. "Comparison of numerical results of turbulence models for the problem of heat transfer in turbulent molasses." E3S Web of Conferences 508 (2024): 05007. http://dx.doi.org/10.1051/e3sconf/202450805007.
Анотація:
The study introduces Malikov's two-fluid methodology along with the RSM turbulence model for simulating turbulent heat transfer phenomena. It elucidates that temperature fluctuations within turbulent flows arise from temperature differentials between the respective fluids. Leveraging the two-fluid paradigm, the researchers develop a mathematical framework to characterize turbulent heat transfer dynamics. This resultant turbulence model is then applied to analyze heat propagation in turbulent flows around a flat plate and in scenarios involving submerged jets. To validate the model's efficacy, numerical outcomes are juxtaposed against established RSM turbulence models and experimental findings. The comparative analysis reveals that the two-fluid turbulent transport model aptly captures the thermodynamic behaviors inherent in turbulent flows with exceptional precision.
45
LLOR, ANTOINE, and PASCAL BAILLY. "A new turbulent two-field concept for modeling Rayleigh–Taylor, Richtmyer–Meshkov, and Kelvin–Helmholtz mixing layers." Laser and Particle Beams 21, no. 3 (July 2003): 311–15. http://dx.doi.org/10.1017/s0263034603213033.
Анотація:
An accurate turbulent mixing model for gravitationally induced instabilities with arbitrarily variable accelerations has been developed to capture the following physical aspects: (1) directed transport, (2) correct buoyancy forces, (3) turbulence diffusion, and (4) geometrical aspects. We present the two-structure two-fluid two-turbulence concept (2SFK), which consistently answers these requirements by identifying the large-scale transport structures in a statistical approach. An example of a 2SFK-based model is given and applied to the Rayleigh–Taylor case.
46
Pezzi, Oreste, Pasquale Blasi, and William H. Matthaeus. "Relativistic Particle Transport and Acceleration in Structured Plasma Turbulence." Astrophysical Journal 928, no. 1 (March 1, 2022): 25. http://dx.doi.org/10.3847/1538-4357/ac5332.
Анотація:
Abstract We discuss the phenomenon of energization of relativistic charged particles in three-dimensional incompressible MHD turbulence and the diffusive properties of the motion of the same particles. We show that the random electric field induced by turbulent plasma motion leads test particles moving in a simulated box to be accelerated in a stochastic way, a second-order Fermi process. A small fraction of these particles happen to be trapped in large-scale structures, most likely formed due to the interaction of islands in the turbulence. Such particles get accelerated exponentially, provided their pitch angle satisfies some conditions. We discuss at length the characterization of the accelerating structure and the physical processes responsible for rapid acceleration. We also comment on the applicability of the results to realistic astrophysical turbulence.
47
Wei, X. S., W. H. Wang, Z. Lin, G. J. Choi, S. Dettrick, C. Lau, P. F. Liu, and T. Tajima. "Effects of zonal flows on ion temperature gradient instability in the scrape-off layer of a field-reversed configuration." Nuclear Fusion 61, no. 12 (November 10, 2021): 126039. http://dx.doi.org/10.1088/1741-4326/ac3023.
Анотація:
Abstract Gyrokinetic simulations of long wavelength ion temperature gradient (ITG) turbulence in the scrape-off layer (SOL) of a field-reversed configuration (FRC) find that zonal flows are nonlinearly generated and are the dominant mechanism for the nonlinear saturation of the ITG instability. After the ITG saturation, zonal flows remain undamped and gradually suppress the turbulent transport to a very low level. In the simulations with collisions, collisional damping gradually reduces zonal flow amplitude to a lower level, which allows finite ITG turbulence intensity and ion heat transport in the SOL. The steady state turbulence intensity and ion heat transport are found to be proportional to the collision frequency. This favorable scaling suggests that minimizing collisions (e.g. increasing temperature, reducing impurity content, etc) and preserving toroidal symmetry could improve plasma confinement in the FRC.
48
Moss, R. W., and M. L. G. Oldfield. "Effect of Free-Stream Turbulence on Flat-Plate Heat Flux Signals: Spectra and Eddy Transport Velocities." Journal of Turbomachinery 118, no. 3 (July 1, 1996): 461–67. http://dx.doi.org/10.1115/1.2836691.
Анотація:
An experimental study of the eddy structure in a flat-plate turbulent boundary layer with significant levels of free-stream turbulence is presented. This is relevant to the enhancement of turbomachinery heat transfer by turbulence and should lead to more realistic CFD modeling. Previous measurements showed that Nusselt numbers may be increased by up to 35 percent, and that this increase depended on turbulence integral length scale as well as intensity. The new results described here provide an insight into the mechanism responsible. Thin film gages and hot wires were used to take simultaneous high-frequency measurements of fluctuating heat transfer rates to the flat plate and the fluctuating flow velocity in the free stream and boundary layer. Spectra and correlation analysis shows that the turbulent eddy structure of the boundary layer is dominated by the free-stream turbulence at intensities of 3 percent and above. Eddies in the boundary layer mimicked those in the free stream and convected at the free-stream velocity U, rather than the ∼0.8U characteristic of boundary layers. The main heat transfer enhancing mechanism is due to the penetration of free-stream turbulent eddies deep into the boundary layer, rather than enhancement of existing boundary layer turbulence.
49
Lee, T. W. "Origin of the Turbulence Structure in Wall-Bounded Flows, and Implications toward Computability." Fluids 6, no. 9 (September 17, 2021): 333. http://dx.doi.org/10.3390/fluids6090333.
Анотація:
Coordinate-transformed analysis of turbulence transport is developed, which leads to a symmetric set of gradient expressions for the Reynolds stress tensor components. In this perspective, the turbulence structure in wall-bounded flows is seen to arise from an interaction of a small number of intuitive dynamical terms: transport, pressure and viscous. Main features of the turbulent flow can be theoretically prescribed in this way and reconstructed for channel and boundary layer flows, with and without pressure gradients, as validated in comparison with available direct numerical simulation data. A succinct picture of turbulence structure and its origins emerges, reflective of the basic physics of momentum and energy balance if placed in a specific moving coordinate frame. An iterative algorithm produces an approximate solution for the mean velocity, and its implications toward computability of turbulent flows is discussed.
50
Balonishnikov, A. M., and J. V. Kruchkova. "Dissipation closure of fluid turbulence from energy spectrum." Journal of Physics: Conference Series 2697, no. 1 (February 1, 2024): 012009. http://dx.doi.org/10.1088/1742-6596/2697/1/012009.
Анотація:
Abstract Direct method of the turbulence closure consists of obtaining expression for small-scale velocity and inserting this expression in the Reynolds tensor and in the transport equations for turbulent energy, dissipation, eddy and so on, implementing some averaging. Previously such approaches were used for Burgers-Lundgren vortex. Here we used ‘naïve’ perturbation method for the transport equation for small-scale velocity incompressible fluid - the equation suggested by one of the authors earlier. It is assumed that the energy spectrum of turbulence is given.