共查询到20条相似文献，搜索用时 15 毫秒
1.
Large eddy simulation and study of the urban boundary layer 总被引：7，自引：1，他引：6
2.
Large-Eddy Simulation of the Stable Atmospheric Boundary Layer using Dynamic Models with Different Averaging Schemes 总被引：5，自引：5，他引：0
Large-eddy simulation (LES) of a stable atmospheric boundary layer is performed using recently developed dynamic subgrid-scale
(SGS) models. These models not only calculate the Smagorinsky coefficient and SGS Prandtl number dynamically based on the
smallest resolved motions in the flow, they also allow for scale dependence of those coefficients. This dynamic calculation
requires statistical averaging for numerical stability. Here, we evaluate three commonly used averaging schemes in stable
atmospheric boundary-layer simulations: averaging over horizontal planes, over adjacent grid points, and following fluid particle
trajectories. Particular attention is focused on assessing the effect of the different averaging methods on resolved flow
statistics and SGS model coefficients. Our results indicate that averaging schemes that allow the coefficients to fluctuate
locally give results that are in better agreement with boundary-layer similarity theory and previous LES studies. Even among
models that are local, the averaging method is found to affect model coefficient probability density function distributions
and turbulent spectra of the resolved velocity and temperature fields. Overall, averaging along fluid pathlines is found to
produce the best combination of self consistent model coefficients, first- and second-order flow statistics and insensitivity
to grid resolution. 相似文献
3.
Large-eddy Simulation of Turbulent Flow across a Forest Edge. Part II: Momentum and Turbulent Kinetic Energy Budgets 总被引：1，自引：0，他引：1
Bai Yang Andrew P. Morse Roger H. Shaw Kyaw Tha Paw U 《Boundary-Layer Meteorology》2006,121(3):433-457
Momentum and turbulent kinetic energy (TKE) budgets across a forest edge have been investigated using large-eddy simulation (LES). Edge effects are observed in the rapid variation of a number of budget terms across this vegetation transition. The enhanced drag force at the forest edge is largely balanced by the pressure gradient force and by streamwise advection of upstream momentum, while vertical turbulent diffusion is relatively insignificant. For variance and TKE budgets, the most important processes at the forest edge are production due to the convergence (or divergence) of the mean flow, streamwise advection, pressure diffusion and enhanced dissipation by canopy drag. Turbulent diffusion, pressure redistribution and vertical shear production, which are characteristic processes in homogeneous canopy flow, are less important at the forest transition. We demonstrate that, in the equilibrated canopy flow, a substantial amount of TKE produced in the streamwise direction by the vertical shear of the mean flow is redistributed in the vertical direction by pressure fluctuations. This redistribution process occurs in the upper canopy layers. Part of the TKE in the vertical velocity component is transferred by turbulent and pressure diffusion to the lower canopy levels, where pressure redistribution takes place again and feeds TKE back to the streamwise direction. In this TKE cycle, the primary source terms are vertical shear production for streamwise velocity variance and pressure redistribution for vertical velocity variance. The evolution of these primary source terms downwind of the forest edge largely controls the adjustment rates of velocity variances. 相似文献
4.
Inanc Senocak Andrew S. Ackerman Michael P. Kirkpatrick David E. Stevens Nagi N. Mansour 《Boundary-Layer Meteorology》2007,124(3):405-424
In large-eddy simulations (LES) of the atmospheric boundary layer (ABL), near-surface models are often used to supplement
subgrid-scale (SGS) turbulent stresses when a major fraction of the energetic scales within the surface layer cannot be resolved
with the temporal and spatial resolution at hand. In this study, we investigate the performance of both dynamic and non-dynamic
eddy viscosity models coupled with near-surface models in simulations of a neutrally stratified ABL. Two near-surface models
that are commonly used in LES of the atmospheric boundary layer are considered. Additionally, a hybrid Reynolds- averaged/LES
eddy viscosity model is presented, which uses Prandtl’s mixing length model in the vicinity of the surface, and blends in
with the dynamic Smagorinsky model away from the surface. Present simulations show that significant portions of the modelled
turbulent stresses are generated by the near-surface models, and they play a dominant role in capturing the expected logarithmic
wind profile. Visualizations of the instantaneous vorticity field reveal that flow structures in the vicinity of the surface
depend on the choice of the near-surface model. Among the three near-surface models studied, the hybrid eddy viscosity model
gives the closest agreement with the logarithmic wind profile in the surface layer. It is also observed that high levels of
resolved turbulence stresses can be maintained with the so-called canopy stress model while producing good agreement with
the logarithmic wind profile. 相似文献
5.
Atmospheric flow over complex terrain, particularly recirculation flows, greatly influences wind-turbine siting, forest-fire behaviour, and trace-gas and pollutant dispersion. However, there is a large uncertainty in the simulation of flow over complex topography, which is attributable to the type of turbulence model, the subgrid-scale (SGS) turbulence parametrization, terrain-following coordinates, and numerical errors in finite-difference methods. Here, we upgrade the large-eddy simulation module within the Weather Research and Forecasting model by incorporating the immersed-boundary method into the module to improve simulations of the flow and recirculation over complex terrain. Simulations over the Bolund Hill indicate improved mean absolute speed-up errors with respect to previous studies, as well an improved simulation of the recirculation zone behind the escarpment of the hill. With regard to the SGS parametrization, the Lagrangian-averaged scale-dependent Smagorinsky model performs better than the classic Smagorinsky model in reproducing both velocity and turbulent kinetic energy. A finer grid resolution also improves the strength of the recirculation in flow simulations, with a higher horizontal grid resolution improving simulations just behind the escarpment, and a higher vertical grid resolution improving results on the lee side of the hill. Our modelling approach has broad applications for the simulation of atmospheric flows over complex topography. 相似文献
6.
Particle image velocimetry (PIV) data obtained in a wind-tunnel model of a canopy boundary layer is used to examine the characteristics
of mean flow and turbulence. The vector spacing varies between 1.7 and 2.5 times the Kolmogorov scales. Conditional sampling
based on quadrants, i.e. based on the signs of velocity fluctuations, reveals fundamental differences in flow structure, especially
between sweep and ejection events, which dominate the flow. During sweeps, the downward flow generates a narrow, highly turbulent,
shear layer containing multiple small-scale vortices just below canopy height. During ejections, the upward flow expands this
shear layer and the associated small-scale flow structures to a broad region located above the canopy. Consequently, during
sweeps the turbulent kinetic energy (TKE), Reynolds stresses, as well as production and dissipation rates, have distinct narrow
peaks just below canopy height, whereas during ejections these variables have broad maxima well above the canopy. Three methods
to estimate the dissipation rate are compared, including spectral fits, measured subgrid-scale (SGS) energy fluxes at different
scales, and direct measurements of slightly underresolved instantaneous velocity gradients. The SGS energy flux is 40–60%
of the gradient-based (direct) estimates for filter sizes inside the inertial range, while decreasing with scale, as expected,
within the dissipation range. The spectral fits are within 5–30% of the direct estimates. The spectral fits exceed the direct
estimates near canopy height, but are lower well above and below canopy height. The dissipation rate below canopy height increases
with velocity magnitude, i.e. it has the highest values during sweep and quadrant 1 events, and is significantly lower during
ejection and quadrant 3 events. Well above the canopy, ejections are the most dissipative. Turbulent transport during sweep
events acts as a source below the narrow shear layer within the canopy and as a sink above it. Transport during ejection events
is a source only well above the canopy. The residual term in the TKE transport equation, representing mostly the effect of
pressure–velocity correlations, is substantial only within the canopy, and is dominated by sweeps. 相似文献
7.
Large-eddy simulation (LES) is used to simulate stably-stratified turbulent boundary-layer flow over a steep two-dimensional
hill. To parametrise the subgrid-scale (SGS) fluxes of heat and momentum, three different types of SGS models are tested:
(a) the Smagorinsky model, (b) the Lagrangian dynamic model, and (c) the scale-dependent Lagrangian dynamic model (Stoll and
Porté-Agel, Water Resour Res 2006, doi:). Simulation results obtained with the different models are compared with data from wind-tunnel experiments conducted at
the Environmental Flow Research Laboratory (EnFlo), University of Surrey, U.K. (Ross et al., Boundary-Layer Meteorol 113:427–459,
2004). It is found that, in this stably-stratified boundary-layer flow simulation, the scale-dependent Lagrangian dynamic model
is able to account for the scale dependence of the eddy-viscosity and eddy-diffusivity model coefficients associated with
flow anisotropy in flow regions with large mean shear and/or strong flow stratification. As a result, simulations using this
tuning-free model lead to turbulence statistics that are more realistic than those obtained with the other two models. 相似文献
8.
Bolund measurements were used for a blind comparison of microscale flow models. Fifty-seven models ranging from numerical
to physical were used, including large-eddy simulation (LES) models, Reynolds-averaged Navier–Stokes (RANS) models, and linearized
models, in addition to wind-tunnel and water-channel experiments. Many assumptions of linearized models were violated when
simulating the flow around Bolund. As expected, these models showed large errors. Expectations were higher for LES models.
However, of the submitted LES results, all had difficulties in applying the specified boundary conditions and all had large
speed-up errors. In contrast, the physical models both managed to apply undisturbed ‘free wind’ boundary conditions and achieve
good speed-up results. The most successful models were RANS with two-equation closures. These models gave the lowest errors
with respect to speed-up and turbulent kinetic energy (TKE) prediction. 相似文献
9.
Nesting Turbulence in an Offshore Convective Boundary Layer Using Large-Eddy Simulations 总被引：1，自引：1，他引：0
Domingo Muñoz-Esparza Branko Kosović Clara García-Sánchez Jeroen van Beeck 《Boundary-Layer Meteorology》2014,151(3):453-478
The applicability of the one-way nesting technique for numerical simulations of the heterogeneous atmospheric boundary layer using the large-eddy simulation (LES) framework of the Weather Research and Forecasting model is investigated. The focus of this study is on LES of offshore convective boundary layers. Simulations were carried out using two subgrid-scale models (linear and non-linear) with two different closures [diagnostic and prognostic subgrid-scale turbulent kinetic energy (TKE) equations]. We found that the non-linear backscatter and anisotropy model with a prognostic subgrid-scale TKE equation is capable of providing similar results when performing one-way nested LES to a stand-alone domain having the same grid resolution but using periodic lateral boundary conditions. A good agreement is obtained in terms of velocity shear and turbulent fluxes, while velocity variances are overestimated. A streamwise fetch of 14 km is needed following each domain transition in order for the solution to reach quasi-stationary results and for the velocity spectra to generate proper energy content at high wavelengths, however, a pile-up of energy is observed at the low-wavelength portion of the spectrum on the first nested domain. The inclusion of a second nest with higher resolution allows the solution to reach effective grid spacing well within the Kolmogorov inertial subrange of turbulence and develop an appropriate energy cascade that eliminates most of the pile-up of energy at low wavelengths. Consequently, the overestimation of velocity variances is substantially reduced and a considerably better agreement with respect to the stand-alone domain results is achieved. 相似文献
10.
Large-eddy Simulation of the Dispersion of Solid Particles in a Turbulent Boundary Layer 总被引：2，自引：2，他引：2
Ivana Vinkovic Cesar Aguirre Michel Ayrault Serge Simoëns 《Boundary-Layer Meteorology》2006,121(2):283-311
A large-eddy simulation (LES) with the dynamic Smagorinsky-Germano subgrid-scale (SGS) model is used to study the dispersion of solid particles in a turbulent boundary layer. Solid particles are tracked in a Lagrangian way. The instantaneous velocity of the surrounding fluid is considered to have a large-scale part (directly computed by the LES) and a small-scale part. The SGS velocity of the surrounding fluid is given by a three-dimensional Langevin model written in terms of SGS statistics at a mesh level. An appropriate Lagrangian correlation time scale is considered in order to include the influences of gravity and inertia of the solid particle. Inter-particle collisions and the influence of particles on the mean flow are also taken into account. The results of the LES are compared with the wind-tunnel experiments of Nalpanis et al. (1993 J Fluid Mech 251: 661–685) and of Tanière et al. (1997 Exp in Fluids 23:463–471) on sand particles in saltation and in modified saltation, respectively. 相似文献
11.
A dynamic procedure is developed to compute the model coefficients in the recently introduced modulated gradient models for both momentum and scalar fluxes. The magnitudes of the subgrid-scale (SGS) stress and the SGS flux are estimated using the local equilibrium hypothesis, and their structures (relative magnitude of each of the components) are given by the normalized gradient terms, which are derived from the Taylor expansion of the exact SGS stress/flux. Previously, the two model coefficients have been specified on the basis of theoretical arguments. Here, we develop a dynamic SGS procedure, wherein the model coefficients are computed dynamically according to the statistics of the resolved turbulence, rather than provided a priori or ad hoc. Results show that the two dynamically calculated coefficients have median values that are approximately constant throughout the turbulent atmospheric boundary layer (ABL), and their fluctuations follow a near log-normal distribution. These findings are consistent with the fact that, unlike eddy-viscosity/diffusivity models, modulated gradient models have been found to yield satisfactory results even with constant model coefficients. Results from large-eddy simulations of a neutral ABL and a stable ABL using the new closure show good agreement with reference results, including well-established theoretical predictions. For instance, the closure delivers the expected surface-layer similarity profiles and power-law scaling of the power spectra of velocity and scalar fluctuations. Further, the Lagrangian version of the model is tested in the neutral ABL case, and gives satisfactory results. 相似文献
12.
Stable Nocturnal Boundary Layers: A Comparison of One-Dimensional and Large-Eddy Simulation Models 总被引：1，自引：1，他引：0
S. Galmarini 《Boundary-Layer Meteorology》1998,88(2):181-210
13.
A long-standing problem in large-eddy simulations (LES) of the planetary boundary layer (PBL) is that the mean wind and temperature profiles differ from the Monin-Obukhov similarity forms in the surface layer. This shortcoming of LES has been attributed to poor grid resolution and inadequate sub-grid-scale (SGS) modeling. We study this deficiency in PBL LES solutions calculated over a range of shear and buoyancy forcing conditions. The discrepancy from similarity forms becomes larger with increasing shear and smaller buoyancy forcing, and persists even with substantial horizontal grid refinement. With strong buoyancy forcing, however, the error is negligible.In order to achieve better agreement between LES and similarity forms in the surface layer, a two-part SGS eddy-viscosity model is proposed. The model preserves the usual SGS turbulent kinetic energy formulation for the SGS eddy viscosity, but it explicitly includes a contribution from the mean flow and a reduction of the contributions from the turbulent fluctuations near the surface. Solutions with the new model yield increased fluctuation amplitudes near the surface and better correspondence with similarity forms out to a distance of 0.1–0.2 times the PBL depth, i.e., a typical surface-layer depth. These results are also found to be independent of grid anisotropy. The new model is simple to implement and computationally inexpensive.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
14.
Full-scale observations from two urban sites in Basel, Switzerland were analysed to identify the magnitude of different processes
that create, relocate, and dissipate turbulent kinetic energy (TKE) in the urban atmosphere. Two towers equipped with a profile
of six ultrasonic anemometers each sampled the flow in the urban roughness sublayer, i.e. from street canyon base up to roughly
2.5 times the mean building height. This observational study suggests a conceptual division of the urban roughness sublayer
into three layers: (1) the layer above the highest roofs, where local buoyancy production and local shear production of TKE
are counterbalanced by local viscous dissipation rate and scaled turbulence statistics are close to to surface-layer values;
(2) the layer around mean building height with a distinct inflexional mean wind profile, a strong shear and wake production
of TKE, a more efficient turbulent exchange of momentum, and a notable export of TKE by transport processes; (3) the lower
street canyon with imported TKE by transport processes and negligible local production. Averaged integral velocity variances
vary significantly with height in the urban roughness sublayer and reflect the driving processes that create or relocate TKE
at a particular height. The observed profiles of the terms of the TKE budget and the velocity variances show many similarities
to observations within and above vegetation canopies. 相似文献
15.
A novel dynamic mixing length (DML) subgrid-scale (SGS) model is proposed to improve the large-eddy simulations of the wind
field and contaminant dispersion around a group of buildings. Wind field and contaminant dispersion in two kinds of building
array geometries are simulated using the model, with wind-tunnel experimental data used to validate the model. The relative
errors in the lateral profiles of the streamwise mean velocities behind the sixth row of the buildings of the staggered obstacle
array and the aligned obstacle array at the half height of the building are 15 and 9%, respectively. The DML velocity fluctuations
in the staggered and aligned obstacle arrays are in agreement with those of the experiment. The results indicate that the
DML model can make a more accurate prediction of the mean velocity and velocity fluctuations. The DML model is highly suitable
for the simulation of multi-scale turbulent flow in urban canyons, of high Reynolds number turbulent flow and of complex turbulent
flow. 相似文献
16.
In recent years field experiments have been undertaken in the lower atmosphere to perform a priori tests of subgrid-scale
(SGS) models for large-eddy simulations (LES). The experimental arrangements and data collected have facilitated studies of
variables such as the filtered strain rate, SGS stress and dissipation, and the eddy viscosity coefficient. However, the experimental
set-ups did not permit analysis of the divergence of the SGS stress (the SGS force vector), which is the term that enters
directly in the LES momentum balance equations. Data from a field experiment (SGS2002) in the west desert of Utah, allows
the calculation of the SGS force due to the unique 4 × 4 sonic anemometer array. The vector alignment of the SGS force is
investigated under a range of atmospheric stabilities. The eddy viscosity model is likely aligned with the measured SGS force
under near-neutral and unstable conditions, while its performance is unsatisfactory under stable conditions. 相似文献
17.
The Effects of Canopy Leaf Area Index on Airflow Across Forest Edges: Large-eddy Simulation and Analytical Results 总被引：4，自引：3，他引：1
The structure of turbulent flows along a transition between tall-forested canopies and forest clearings continues to be an
active research topic in canopy turbulence. The difficulties in describing the turbulent flow along these transitions stem
from the fact that the vertical structure of the canopy and its leaf area distribution cannot be ignored or represented by
an effective roughness length. Large-eddy simulation (LES) runs were performed to explore the effect of a homogeneous variation
in the forest leaf area index (LAI) on the turbulent flow across forest edges. A nested grid numerical method was used to
ensure the development of a deep boundary layer above the forest while maintaining a sufficiently high resolution in the region
close to the ground. It was demonstrated that the LES here predicted first-order and second-order mean velocity statistics
within the canopy that agree with reported Reynolds-Averaged Navier–Stokes (RANS) model results, field and laboratory experiments.
In the simulations reported here, the LAI was varied between 2 and 8 spanning a broad range of observed LAI in terrestrial
ecosystems. By increasing the forest LAI, the mean flow properties both within the forest and in the clearing near the forest
edge were altered in two fundamental ways: near the forest edge and into the clearing, the flow statistical properties resembled
the so-called back-facing step (BFS) flow with a mean recirculation zone near the edge. Another recirculation zone sets up
downstream of the clearing as the flow enters the tall forest canopy. The genesis of this within-forest recirculation zone
can be primarily described using the interplay between the mean pressure gradients (forcing the flow) and the drag force (opposing
the flow). Using the LES results, a simplified analytical model was also proposed to explain the location of the recirculation
zone inside the canopy and its dependence on the forest LAI. Furthermore, a simplified scaling argument that decomposes the
mean velocity at the outflow edge into a superposition of ‘exit flow’ and BFS-like flow with their relative importance determined
by LAI was explored. 相似文献
18.
The turbulent flow in and above plant canopies is of fundamental importance to the understanding oftransport processes of momentum,heat and mass between plant canopies and atmosphere,and to microme-teorology.The Reynolds stress equation model(RSM)has been applied to calculate the turbulence in cano-pies in this paper.The calculated mean wind velocity profiles,Reynolds stress,turbulent kinetic energy andviscous dissipation rate in a corn canopy and a spruce forest are compared with field observed data and withWilson's and Shaw's model.The velocity profiles and Rynolds stress calculated by both models are in goodagreement,and the length scale of turbulence appears to be similar. 相似文献
19.
Wusi Yue Marc B. Parlange Charles Meneveau Weihong Zhu René van Hout Joseph Katz 《Boundary-Layer Meteorology》2007,124(2):183-203
Turbulent flow in a corn canopy is simulated using large-eddy simulation (LES) with a Lagrangian dynamic Smagorinsky model.
A new numerical representation of plant canopies is presented that resolves approximately the local structure of plants and
takes into account their spatial arrangement. As a validation, computational results are compared with experimental data from
recent field particle image velocimetry (PIV) measurements and two previous experimental campaigns. Numerical simulation using
the traditional modelling method to represent the canopy (field-scale approach) is also conducted as a comparison to the plant-scale
approach. The combination of temporal PIV data, LES and spatial PIV data allows us to couple a wide range of relevant turbulence
scales. There is good agreement between experimental data and numerical predictions using the plant-scale approach in terms
of various turbulence statistics. Within the canopy, the plant-scale approach also allows the capture of more details than
the field-scale approach, including instantaneous gusts that penetrate deep inside the canopy. 相似文献
20.
城市建筑动力学效应对对流边界层影响的敏感性试验 总被引：3，自引：1，他引：2
本文将大涡模拟应用于城市对流边界层(CBL)湍流结构和流场特征的研究,在大涡模式中,拖曳系数取与建筑物高度及建筑物高度标准差有关的表达式以考虑次网格建筑物对风速和湍流动能(TKE)的面积平均影响.模拟结果表明,由于城市建筑物对气流的拖曳作用,使建筑物冠层及整个CBL内风速大幅度减小,城市冠层内部风速减小尤为明显,在夹卷层内,风速有一明显的跃变.在边界层中部对流运动已经发展成为较强的热泡,城市建筑物的动力学效应使热泡的水平尺度增大,CBL内平均上升气流速度和下沉气流速度减小,同时使CBL中上升气流所占比例比平坦地面增大.城市建筑物使CBL低层热通量、动量通量、速度方差和位温方差明显增大,但对近地层高度以上的湍流量影响不大. 相似文献