共查询到20条相似文献,搜索用时 15 毫秒
1.
Stable stratification turbulence, as a common phenomenon in atmospheric and oceanic flows, is an important mechanism for numerical prediction of such flows. In this paper the large eddy simulation is utilized for investigating stable stratification turbulence numerically. The paper is expected to provide correct statistical results in agreement with those measured in the atmosphere or ocean. The fully developed turbulence is obtained in the stable stratification fluid by large eddy simulation with different... 相似文献
2.
Large eddy simulation of city micro-atmospheric environment 总被引:1,自引:0,他引:1
Air quality is one of the important conditions for a better residence life in the populated urban area and it is closed related
to the micro-atmospheric environment. Atmospheric environment is controlled by air motion with multi-scales in the city, while
air flows in the residence area are of micro-scale atmospheric motion. This paper introduces a modern numerical simulation
method, i.e. large eddy simulation (LES), for studying micro-atmospheric flows in the city residence area. For the complex
flow features in the residence area, the proper application of LES is studied and various numerical methods are compared in
order to investigate their effects on the prediction accuracy of micro-atmospheric flows, for instance, roughness elements
and immersed boundary method for complex terrain, different subgrid models and so on. The wind field (including turbulence
properties) and contaminant dispersion are computed by the proposed method for a model and a realistic residence area, and
the numerical results are in good agreement with the experimental measurements.
Supported by the National Natural Science Foundation of China (Grant No. 10572073) and Foundation for Development of Science
and Technology in Macao (Grant No. 022/2006/A) 相似文献
3.
Amin Rasam Stefan Wallin Geert Brethouwer Arne V. Johansson 《Journal of Turbulence》2013,14(11):752-775
We analyse the performance of the explicit algebraic subgrid-scale (SGS) stress model (EASSM) in large eddy simulation (LES) of plane channel flow and the flow in a channel with streamwise periodic hill-shaped constrictions (periodic hill flow) which induce separation. The LESs are performed with the Code_Saturne which is an unstructured collocated finite volume solver with a second-order spatial discretisation suitable for LES of incompressible flow in complex geometries. At first, performance of the EASSM in LES of plane channel flow at two different resolutions using the Code_Saturne and a pseudo-spectral method is analysed. It is observed that the EASSM predictions of the mean velocity and Reynolds stresses are more accurate than the conventional dynamic Smagorinsky model (DSM). The results with the pseudo-spectral method were, in general, more accurate. In the second step, LES with the EASSM of flow separation in the periodic hill flow is compared to LES with the DSM, no SGS model and a highly resolved LES data using the DSM. Results show that the mean velocity profiles, the friction and pressure coefficients, the length and shape of the recirculation bubble, as well as the Reynolds stresses are considerably better predicted by the EASSM than the DSM and the no SGS model simulations. It was also observed that in some parts of the domain, the resolved strain-rate and SGS shear stress have the same sign. The DSM cannot produce a correct SGS stress in this case, in contrast to the EASSM. 相似文献
4.
The erosion process of a stably stratified light gas layer by a vertical turbulent fountain of denser fluid inside a generic containment – for which experimental reference data are available – is studied computationally using large eddy simulation (LES). In addition, various Reynolds averaged Navier–Stokes (RANS) models are applied aiming at a comparative assessment of different computational approaches for the considered case. With the LES methodology included into the present modelling study, a novelty to date is established for fountain-stratification interaction inside generic containments. The high Reynolds number RANS models applied in the framework of this study include both the realisable k–? eddy viscosity model (EVM) as well as the basic Reynolds stress model (RSM). Furthermore, we show that certain regimes of the present configuration can be predicted using an analytically derived scaling approach. Various data beyond the experimentally obtained ones are computationally provided in order to facilitate the calibration of less costly statistical turbulence models and lumped parameter codes, since the presently considered configuration is regarded to be a valuable small-scale equivalent for containment flow applications. 相似文献
5.
Large eddy simulation coupled with the linear eddy model (LEM) is employed for the simulation of n-heptane spray flames to investigate the low temperature ignition and combustion process in a constant-volume combustion vessel under diesel-engine relevant conditions. Parametric studies are performed to give a comprehensive understanding of the ignition processes. The non-reacting case is firstly carried out to validate the present model by comparing the predicted results with the experimental data from the Engine Combustion Network (ECN). Good agreements are observed in terms of liquid and vapour penetration length, as well as the mixture fraction distributions at different times and different axial locations. For the reacting cases, the flame index was introduced to distinguish between the premixed and non-premixed combustion. A reaction region (RR) parameter is used to investigate the ignition and combustion characteristics, and to distinguish the different combustion stages. Results show that the two-stage combustion process can be identified in spray flames, and different ignition positions in the mixture fraction versus RR space are well described at low and high initial ambient temperatures. At an initial condition of 850 K, the first-stage ignition is initiated at the fuel-lean region, followed by the reactions in fuel-rich regions. Then high-temperature reaction occurs mainly at the places with mixture concentration around stoichiometric mixture fraction. While at an initial temperature of 1000 K, the first-stage ignition occurs at the fuel-rich region first, then it moves towards fuel-richer region. Afterwards, the high-temperature reactions move back to the stoichiometric mixture fraction region. For all of the initial temperatures considered, high-temperature ignition kernels are initiated at the regions richer than stoichiometric mixture fraction. By increasing the initial ambient temperature, the high-temperature ignition kernels move towards richer mixture regions. And after the spray flames gets quasi-steady, most heat is released at the stoichiometric mixture fraction regions. In addition, combustion mode analysis based on key intermediate species illustrates three-mode combustion processes in diesel spray flames. 相似文献
6.
This paper investigates the effective eddy viscosity inferred from direct numerical simulations of decaying stratified and non-stratified turbulence. It is shown that stratification affects the horizontal eddy viscosity dramatically, by increasing non-local energy transfer between large and small horizontal scales. This non-local horizontal energy transfer is around 20% of the local horizontal energy transfer at the cutoff wavenumber kc = 40. The non-local horizontal energy transfer occurs at large vertical wavenumbers, which may be larger than the buoyancy wavenumber kb = N/urms, where N is the buoyancy frequency and urms is the root-mean-square velocity. By increasing the value of the test cutoff wavenumber kc from large scales to the dissipation range, the non-local horizontal eddy viscosity decreases and the local eddy viscosity is dominant. Overall, the presence of stratification can significantly change the features of subgrid-scale (SGS) motions. Current SGS models should, therefore, be modified for use in large-eddy simulation of stratified turbulence. 相似文献
7.
Large eddy simulations (LESs) of turbulent horizontal buoyant jets are carried out using a high-order numerical method and Sigma subgrid-scale (SGS) eddy-viscosity model, for a number of different Reynolds (Re) and Richardson (Ri) numbers. Simulations at previous experimental flow conditions (Re = 3200, 24, 000 and Ri = 0, 0.01) are carried out first, and the results are found to be qualitatively and quantitatively similar to the experimental results, thus validating the numerical methodology. The effect of varying Ri (values 2×10?4, 0.001, 0.005, and 0.01) and Re (3200 and 24, 000) is studied next. The presence of stable stratification on one side and unstable stratification on the other side of the jet centreline leads to an asymmetric development of horizontal buoyant jets. It is found that this asymmetry, the total radial spread and the vertical deflection are significantly affected by Ri, while Re affects only the radial asymmetry. The need for developing improved integral models, accounting for this asymmetry, is pointed out. Turbulent production and dissipation rates are investigated, and are found to be symmetric in the horizontal plane, but asymmetric in the mid-vertical plane. A previously proposed model, for correlation between the vertical component of the fluctuating scalar flux vector and the vertical cross-correlation component of the Reynolds tensor, is modified based on the current LES results. Instantaneous scalar and velocity fields are analysed to reveal the structure of horizontal buoyant jets. Similar to the developed turbulent jet, the flow close to the nozzle too is found to be markedly different in the stable and unstable stratification regions. Persistent coherent vortex rings are found in the stable stratification region, while intermittent breakdown of vortex rings into small-scale structures is observed in the unstable stratification region. Similarities and differences between the flow structures in the horizontal buoyant jet configuration and those in the jet in crossflow configuration are discussed. Finally, a dynamic mode decomposition analysis is carried out, which indicates that the flow in the unstable stratification region is more energetic and prone to instabilities, as compared to the flow in the stable stratification region. 相似文献
8.
Large eddy simulations (LES) of a lattice Boltzmann magnetohydrodynamic (LB-MHD) model are performed for the unstable magnetized Kelvin–Helmholtz jet instability. This algorithm is an extension of Ansumali et al. [1] to MHD in which one performs first an expansion in the filter width on the kinetic equations followed by the usual low Knudsen number expansion. These two perturbation operations do not commute. Closure is achieved by invoking the physical constraint that subgrid effects occur at transport time scales. The simulations are in very good agreement with direct numerical simulations. 相似文献
9.
The rotational motion and orientational distribution of ellipsoidal particles in turbulent flows are of significance in environmental and engineering applications. Whereas the translational motion of an ellipsoidal particle is controlled by the turbulent motions at large scales, its rotational motion is determined by the fluid velocity gradient tensor at small scales, which raises a challenge when predicting the rotational dispersion of ellipsoidal particles using large eddy simulation (LES) method due to the lack of subgrid scale (SGS) fluid motions. We report the effects of the SGS fluid motions on the orientational and rotational statistics, such as the alignment between the long axis of ellipsoidal particles and the vorticity, the mean rotational energy at various aspect ratios against those obtained with direct numerical simulation (DNS) and filtered DNS. The performances of a stochastic differential equation (SDE) model for the SGS velocity gradient seen by the particles and the approximate deconvolution method (ADM) for LES are investigated. It is found that the missing SGS fluid motions in LES flow fields have significant effects on the rotational statistics of ellipsoidal particles. Alignment between the particles and the vorticity is weakened; and the rotational energy of the particles is reduced in LES. The SGS-SDE model leads to a large error in predicting the alignment between the particles and the vorticity and over-predicts the rotational energy of rod-like particles. The ADM significantly improves the rotational energy prediction of particles in LES. 相似文献
10.
Kenji Yamamoto Tomoya Murota Teruyuki Okazaki Masayuki Taniguchi 《Proceedings of the Combustion Institute》2011,33(2):1771-1778
Large eddy simulation (LES) is applied to a pulverized coal jet flame ignited by a preheated gas flow. The simulation results are compared to experimental data obtained for the inlet stoichiometric ratios of 0.14, 0.22, and 0.36. An accurate and computationally inexpensive devolatilization model suitable for combustion simulation in LES is proposed and incorporated into the LES. The numerical results of gas temperature and coal burnout on the centerline show good agreement with the experimental data. Two kinds of lift-off heights are introduced to verify the combustion simulation. One is the height from the primary nozzle exit to the starting point of the growing flame region. The other is the height from the primary nozzle exit to the starting point of the continuous flame region. The calculated results of the two lift-off heights show good agreement with the experimental data. In contrast to LES, the standard k–ε model overestimates the lift-off heights because it calculates time-averaged temperature which does not contain information about local flame structure. The stoichiometric ratio in the gas phase at the starting point of the growing flame region is found to be independent of the inlet stoichiometric ratio in the range from 0.14 to 0.36. 相似文献
11.
M. Buzzicotti H. Aluie L. Biferale J. Brasseur C. Meneveau 《Journal of Turbulence》2018,19(2):167-197
The effects of different filtering strategies on the statistical properties of the resolved-to-subfilter scale (SFS) energy transfer are analysed in forced homogeneous and isotropic turbulence. We carry out a-priori analyses of the statistical characteristics of SFS energy transfer by filtering data obtained from direct numerical simulations with up to 20483 grid points as a function of the filter cutoff scale. In order to quantify the dependence of extreme events and anomalous scaling on the filter, we compare a sharp Fourier Galerkin projector, a Gaussian filter and a novel class of Galerkin projectors with non-sharp spectral filter profiles. Of interest is the importance of Galilean invariance and we confirm that local SFS energy transfer displays intermittency scaling in both skewness and flatness as a function of the cutoff scale. Furthermore, we quantify the robustness of scaling as a function of the filtering type. 相似文献
12.
The influences of the modification of turbulent coherent structures on temperature field and heat transfer in turbulent channel
flow are studied using large eddy simulation (LES) of compressible turbulent channel flows with spanwise wall oscillation
(SWO). The reliability of the LES on such problems is proved by the comparisons of the drag reduction data with those of other
researches. The high consistency of coherent velocity structures and temperature structures is found based on the analyses
of the turbulent flow field. When the coherent velocity structures are suppressed, the transportations of momentum and heat
are reduced simultaneously, demonstrating the same trend. This shows that the turbulent coherent structures have the same
effects on the transportations of momentum and heat. The averaged wall heat flux can be reduced with appropriate oscillating
parameters.
Supported by the Key Subjects of National Natural Science Foundation of China (Grant No. 10732090), the National Natural Science
Foundation of China (Grant No. 50476004), and the 111 Project (Grant No. B08009) 相似文献
13.
14.
WANG Tao BAI JingSong LI Ping & LIU Kun Institute of Fluid Physics China Academy of Engineering Physics Mianyang China 《中国科学:物理学 力学 天文学(英文版)》2010,(5)
A parallel algorithm and code MVFT(multi-viscous-fluid and turbulence) of large-eddy simulation(LES) is developed from our MVPPM(multi-viscous-fluid piecewise parabolic method),and performed to solve the multi compressible Navier-Stokes(N-S) equations.The effect of the unresolved subgrid-scale(SGS) motions on the large scales is represented by different SGS stress models in LES.A Richtmyer-Meshkov instability experiment of the evolution of a rectangular block of SF6,which occupies half of the height of the ... 相似文献
15.
16.
The turbulence properties of a gas spray jet generated through the injection of a high-pressure atomising spray were studied in the context of the Euler–Lagrange formulation using a large eddy simulation (LES) approach. The study's main aim was to investigate nozzle flow effects on flow and turbulence statistics for a two-phase jet flow in the near and far fields of the spray. The study investigated the injection of an existing in-nozzle flow at the spray inlet. Simulation results were compared with measurements obtained for non-evaporating sprays under quasi-steady conditions, which, in practice, correspond to a long-duration injection. Both simulated high-injection pressure cases showed a good agreement with experimental data. A two-stage evolution of the spray was observed. The flow of the gas jet first developed along the spray cone, followed by the appearance of highly vortical structures around and within the spray. Cavitation in the nozzle flow resulted in an observed asymmetry of the generated gas spray jet: on the gas spray jet side of the nozzle (relative to the cavitation side), stronger jet spreading occurred, and droplets were smaller and slower. The spray jet typically exhibited an ellipsoidal cross-sectional shape. The paper provides a characterisation of the asymmetry that persisted for the duration of the spray evolution (at least for the distances considered here). 相似文献
17.
基于多组分混合物质量分数模型,采用色散最小耗散可控的高分辨率有限体积方法,数值模拟了弱激波冲击V形空气/SF_6界面后,界面不稳定性生成的旋涡与固体壁面作用问题.激波冲击V形界面之后,因斜压效应诱导涡量沉积在界面附近,形成沿界面规则排列的多个涡对结构.旋涡的诱导作用使界面不断变形和卷起,同时旋涡之间不断发生相互并对,诱导更多更小尺度的旋涡产生.旋涡诱导作用的叠加效应,使界面尖端处的初始涡对向上下壁面发展.随后,涡结构开始与壁面发生复杂的相互作用.旋涡与壁面作用后沿壁面加速,使得物质界面沿壁面伸展,随后,旋涡从壁面回弹,并诱导二次旋涡产生.旋涡与壁面相互作用的过程,能够明显加剧物质混合.本文从物质混合的角度研究了该过程的机理,分析了旋涡与壁面作用对物质混合的影响. 相似文献
18.
Large eddy simulations (LES) using a subgrid mixing and combustion model are carried out to study two bluff-body stabilized swirling non-premixed flames (SM1 and SMA2). The similarities and differences between the two flames are highlighted and discussed. Flow features, such as, the recirculation zone (RZ) size and the flame structure are captured accurately in both cases. The SM1 flame shows a toroidal RZ just behind the bluff body and a vortex breakdown bubble (VBB) downstream. In addition, a highly rotational non-recirculating region in-between the RZ and VBB is observed as well. On the other hand, the SMA2 shows a single elongated recirculation zone downstream the bluff body. Flame necking is observed downstream the bluff body for the SM1 flame but not for the SMA2 flame. The time-averaged velocity and temperature comparison also shows reasonable agreement. The study shows that the sensitivity of the flame structure to inflow conditions can be captured in the present LES without requiring any model changes. 相似文献
19.
We provide analytical and numerical results concerning multi-scale correlations between the resolved velocity field and the subgrid-scale (SGS) stress-tensor in large eddy simulations (LES). Following previous studies for Navier–Stokes equations, we derive the exact hierarchy of LES equations governing the spatio-temporal evolution of velocity structure functions of any order. The aim is to assess the influence of the subgrid model on the inertial range intermittency. We provide a series of predictions, within the multifractal theory, for the scaling of correlation involving the SGS stress and we compare them against numerical results from high-resolution Smagorinsky LES and from a-priori filtered data generated from direct numerical simulations (DNS). We find that LES data generally agree very well with filtered DNS results and with the multifractal prediction for all leading terms in the balance equations. Discrepancies are measured for some of the sub-leading terms involving cross-correlation between resolved velocity increments and the SGS tensor or the SGS energy transfer, suggesting that there must be room to improve the SGS modelisation to further extend the inertial range properties for any fixed LES resolution. 相似文献
20.
Constrained large-eddy simulation (CLES) method has been recently developed by Chen and his colleagues for simulating attached and detached wall-bounded turbulent flows. In CLES, the whole domain is simulated using large-eddy simulation (LES) while a Reynolds stress constraint is enforced on the subgrid-scale (SGS) stress model for near wall regions. In this paper, CLES is used to simulate the separated flow in a channel with streamwise-periodic constrictions at Re = 10,595. The results of CLES are compared with those of Reynolds-averaged Navier-Stokes (RANS) method, LES, detached eddy simulation (DES) and previous LES results by Breuer et al. and Ziefle et al. Although a coarse grid is used, our results from the present LES, DES and CLES do not show large deviations from the reference results using much finer grid resolution. The comparison also shows that CLES performs the best among different turbulence models tested, demonstrating that the CLES provides an excellent alternative model for separated flows. Furthermore, the cross-comparisons among different CLES implementations have been carried out. Our simulation results are in favor of using the constraint from algebraic RANS model or solving the RANS model equations in the whole domain with a length scale modification according to the idea from DES. 相似文献