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1.
Fernando Porté-Agel 《Boundary-Layer Meteorology》2004,112(1):81-105
An important challenge in large-eddy simulationsof the atmospheric boundarylayer is the specification of the subgrid-scale(SGS) model coefficient(s)and, in particular, how to account for factorssuch as position in the flow,grid/filter scale and atmospheric stability.A dynamic SGS model (thatassumes scale invariance of the coefficients)is implemented in simulationsof a neutral boundary layer with a constantand uniform surface flux of apassive scalar. Results from our simulationsshow evidence that the lumpedcoefficient in the eddy-diffusion modelcomputed with the dynamic proceduredepends on scale. This scale dependence isstronger near the surface, and itis more important for the scalar than for thevelocity field (Smagorinskycoefficient) due to the stronger anisotropicbehaviour of scalars. Based onthese results, a new scale-dependent dynamicmodel is developed for theeddy-diffusion lumped coefficient. The newmodel, which is similar to theone proposed earlierfor the Smagorinsky coefficient,is fully dynamic, thus not requiring anyparameter specification or tuning.Simulations with the scale-dependent dynamicmodel yield the expected trendsof the coefficients as functions of positionand filter/grid scale.Furthermore, in the surface layer the newmodel gives improved predictionsof mean profiles and turbulence spectra ascompared with the traditionalscale-invariant dynamic model. 相似文献
2.
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
3.
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
4.
Large-eddy simulation (LES), coupled with a wind-turbine model, is used to investigate the characteristics of a wind-turbine
wake in a neutral turbulent boundary-layer flow. The tuning-free Lagrangian scale-dependent dynamic subgrid-scale (SGS) model
is used for the parametrisation of the SGS stresses. The turbine-induced forces (e.g., thrust, lift and drag) are parametrised
using two models: (a) the ‘standard’ actuator-disk model (ADM-NR), which calculates only the thrust force and distributes
it uniformly over the rotor area; and (b) the actuator-disk model with rotation (ADM-R), which uses the blade-element theory
to calculate the lift and drag forces (that produce both thrust and rotation), and distribute them over the rotor disk based
on the local blade and flow characteristics. Simulation results are compared to high-resolution measurements collected with
hot-wire anemometry in the wake of a miniature wind turbine at the St. Anthony Falls Laboratory atmospheric boundary-layer
wind tunnel. In general, the characteristics of the wakes simulated with the proposed LES framework are in good agreement
with the measurements in the far-wake region. The ADM-R yields improved predictions compared with the ADM-NR in the near-wake
region, where including turbine-induced flow rotation and accounting for the non-uniformity of the turbine-induced forces
appear to be important. Our results also show that the Lagrangian scale-dependent dynamic SGS model is able to account, without
any tuning, for the effects of local shear and flow anisotropy on the distribution of the SGS model coefficient. 相似文献
5.
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
6.
Application of Dynamic Subgrid-scale Models for Large-eddy Simulation of the Daytime Convective Boundary Layer over Heterogeneous Surfaces 总被引：1，自引：0，他引：1
The sensitivity of large-eddy simulation (LES) to the representation of subgrid-scale (SGS) processes is explored for the
case of the convective boundary layer (CBL) developing over surfaces with varying degrees of spatial heterogeneity. Three
representations of SGS processes are explored: the traditional constant Smagorinsky–Lilly model and two other dynamic models
with Lagrangian averaging approaches to calculate the Smagorinsky coefficient (C
S
) and SGS Prandtl number (Pr). With initial data based roughly on the observed meteorology, simulations of daytime CBL growth are performed over surfaces
with characteristics (i.e. fluxes and roughness) ranging from homogeneous, to striped heterogeneity, to a realistic representation
of heterogeneity as derived from a recent field study. In both idealized tests and the realistic case, SGS sensitivities are
mostly manifest near the surface and entrainment zone. However, unlike simulations over complex domains or under neutral or
stable conditions, these differences for the CBL simulation, where large eddies dominate, are not significant enough to distinguish
the performance of the different SGS models, irrespective of surface heterogeneity. 相似文献
7.
One-dimensional turbulence (ODT) is a single-column simulation in which vertical motions are represented by an unsteady advective process, rather than their customary representation by a diffusive process. No space or time averaging of mesh-resolved motions is invoked. Molecular-transport scales can be resolved in ODT simulations of laboratory-scale flows, but this resolution of these scales is prohibitively expensive in ODT simulations of the atmospheric boundary layer (ABL), except possibly in small subregions of a non-uniform mesh.Here, two methods for ODT simulation of the ABL on uniform meshes are described and applied to the GABLS (GEWEX Atmospheric Boundary Layer Study; GEWEX is the Global Energy and Water Cycle Experiment) stable boundary-layer intercomparison case. One method involves resolution of the roughness scale using a fixed eddy viscosity to represent subgrid motions. The other method, which is implemented at lower spatial resolution, involves a variable eddy viscosity determined by the local mesh-resolved flow, as in multi-dimensional large-eddy simulation (LES). When run at typical LES resolution, it reproduces some of the key high-resolution results, but its fidelity is lower in some important respects. It is concluded that a more elaborate empirically based representation of the subgrid physics, closely analogous to closures currently employed in LES of the ABL, might improve its performance substantially, yielding a cost-effective ABL simulation tool. Prospects for further application of ODT to the ABL, including possible use of ODT as a near-surface subgrid closure framework for general circulation modeling, are assessed. 相似文献
8.
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
9.
Most natural landscapes are characterized by multiscale (often multifractal) topography with well-known scale-invariance properties.
For example, the spectral density of landscape elevation fields is often found to have a power-law scaling behaviour (with
a −2 slope on a log–log scale) over a wide span of spatial scales, typically ranging from tens of kilometres down to a few
metres. Even though the effect of topography on the atmospheric boundary layer (ABL) has been the subject of numerous studies,
few have focussed on multiscale topography. In this study, large-eddy simulation (LES) is used to investigate boundary-layer
flow over multiscale topography, and guide the development of parametrizations needed to represent the effects of subgrid-scale
(SGS) topography in numerical models of ABL flow. Particular emphasis is placed on the formulation of an effective roughness
used to account for the increased aerodynamic roughness associated with SGS topography. The LES code uses the scale-dependent
Lagrangian dynamic SGS model for the turbulent stresses and a terrain-following coordinate transformation to explicitly resolve
the effects of the topography at scales larger than the LES resolution. The terrain used in the simulations is generated using
a restricted solid-on-solid landscape evolution model, and it is characterized by a −2 slope of the elevation power spectrum.
Results from simulations performed using elevation fields band-pass filtered at different spatial resolutions indicate a clear
linear relation between the square of the effective roughness and the variance of elevation. 相似文献
10.
The performance of the modulated-gradient subgrid-scale (SGS) model is investigated using large-eddy simulation (LES) of the neutral atmospheric boundary layer within the weather research and forecasting model. Since the model includes a finite-difference scheme for spatial derivatives, the discretization errors may affect the simulation results. We focus here on understanding the effects of finite-difference schemes on the momentum balance and the mean velocity distribution, and the requirement (or not) of the ad hoc canopy model. We find that, unlike the Smagorinsky and turbulent kinetic energy (TKE) models, the calculated mean velocity and vertical shear using the modulated-gradient model, are in good agreement with Monin–Obukhov similarity theory, without the need for an extra near-wall canopy model. The structure of the near-wall turbulent eddies is better resolved using the modulated-gradient model in comparison with the classical Smagorinsky and TKE models, which are too dissipative and yield unrealistic smoothing of the smallest resolved scales. Moreover, the SGS fluxes obtained from the modulated-gradient model are much smaller near the wall in comparison with those obtained from the regular Smagorinsky and TKE models. The apparent inability of the LES model in reproducing the mean streamwise component of the momentum balance using the total (resolved plus SGS) stress near the surface is probably due to the effect of the discretization errors, which can be calculated a posteriori using the Taylor-series expansion of the resolved velocity field. Overall, we demonstrate that the modulated-gradient model is less dissipative and yields more accurate results in comparison with the classical Smagorinsky model, with similar computational costs. 相似文献
11.
Large-Eddy Simulation of Atmospheric Boundary-Layer Flow Over Fluvial-Like Landscapes Using a Dynamic Roughness Model 总被引：2，自引：2，他引：0
A recently developed dynamic surface roughness model (Anderson and Meneveau, J Fluid Mech 679:288–314, 2011) for large-eddy simulation (LES) of atmospheric boundary-layer flow over multi-scale topographies is applied to boundary-layer flow over several types of fluvial-like landscapes. The landscapes are generated numerically with simulation of a modified Kardar–Parisi–Zhang equation (Passalacqua et al., Water Resour Res 42:WOD611, 2006). These surfaces possess the fractal-like channel network and anisotropic features often found in real terrains. The dynamic model is shown to lead to accurate flow predictions when the surface-height distributions exhibit power-law scaling (scale invariance) in the prevalent mean flow direction. In those cases, the LES provide accurate predictions (invariant to resolution) of mean velocity profiles. Conversely, some resolution dependence is found for applications in which the landscape’s streamwise spectra do not exhibit pure power-law scaling near wavenumbers corresponding to the LES grid resolution. 相似文献
12.
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
13.
14.
Large-eddy simulation (LES) is a well-established numerical technique, resolving the most energetic turbulent fluctuations
in the planetary boundary layer. By averaging these fluctuations, high-quality profiles of mean quantities and turbulence
statistics can be obtained in experiments with well-defined initial and boundary conditions. Hence, LES data can be beneficial
for assessment and optimisation of turbulence closure schemes. A database of 80 LES runs (DATABASE64) for neutral and stably
stratified planetary boundary layers (PBLs) is applied in this study to optimize first-order turbulence closure (FOC). Approximations
for the mixing length scale and stability correction functions have been made to minimise a relative root-mean-square error
over the entire database. New stability functions have correct asymptotes describing regimes of strong and weak mixing found
in theoretical approaches, atmospheric observations and LES. The correct asymptotes exclude the need for a critical Richardson
number in the FOC formulation. Further, we analysed the FOC quality as functions of the integral PBL stability and the vertical
model resolution. We show that the FOC is never perfect because the turbulence in the upper half of the PBL is not generated
by the local vertical gradients. Accordingly, the parameterised and LES-based fluxes decorrelate in the upper PBL. With this
imperfection in mind, we show that there is no systematic quality deterioration of the FOC in the strongly stable PBL provided
that the vertical model resolution is better than 10 levels within the PBL. In agreement with previous studies, we found that
the quality improves slowly with the vertical resolution refinement, though it is generally wise not to overstretch the mesh
in the lowest 500 m of the atmosphere where the observed, simulated and theoretically predicted stably stratified PBL is mostly
located.
The submission to a special issue of the “Boundary-Layer Meteorology” devoted to the NATO advanced research workshop “Atmospheric Boundary Layers: Modelling and Applications for Environmental Security”. 相似文献
15.
Resolution Sensitivity and Scaling of Large-Eddy Simulations of the Stable Boundary Layer 总被引：1，自引：5，他引：1
Large-eddy simulations (LES) of the continuously turbulent quasi-equilibrium stable boundary layer (SBL) are conducted with grid lengths in the range of 12.5 m to 2 m, in order to explore resolution sensitivity, and determine at what point grid convergence occurs. The structure of the mean potential temperature, winds, and turbulent fluxes varies significantly over this resolution range. The highest resolution simulations show a significant degree of convergence. The dimensionless momentum diffusivity asymptotes to a value of 0.06, corresponding to a limiting flux Richardson number of 0.15.Using the converged simulations, some scaling hypotheses underpinning first-order and second-order closure models are revisited. The effective Richardson number stability functions of the LES are compared with the forms often used in numerical weather prediction (NWP). The mixing implied by the LES is less than that used in NWP. The commonly used similarity profiles for heat and momentum fluxes, and the scalings for dissipation and pressure covariances are compared with the LES. This information could provide guidance for the next generation of SBL parametrization schemes. 相似文献
16.
17.
Gervásio Annes Degrazia Umberto Rizza Franciano Scremin Puhales Antônio Gledson Goulart Jonas Carvalho Guilherme Sausen Welter Edson Pereira Marques Filho 《Boundary-Layer Meteorology》2009,131(2):277-292
A variable vertical mesh spacing for large-eddy simulation (LES) models in a convective boundary layer (CBL) is proposed.
The argument is based on the fact that in the vertical direction the turbulence near the surface in a CBL is inhomogeneous
and therefore the subfilter-scale effects depend on the relative location between the spectral peak of the vertical velocity
and the filter cut-off wavelength. From the physical point of view, this lack of homogeneity makes the vertical mesh spacing
the principal length scale and, as a consequence, the LES filter cut-off wavenumber is expressed in terms of this characteristic
length scale. Assuming that the inertial subrange initial frequency is equal to the LES filter cut-off frequency and employing
fitting expressions that describe the observed convective turbulent energy one-dimensional spectra, it is feasible to derive
a relation to calculate the variable vertical mesh spacing. The incorporation of this variable vertical grid within a LES
model shows that both the mean quantities (and their gradients) and the turbulent statistics quantities are well described
near to the ground level, where the LES predictions are known to be a challenging task. 相似文献
18.
The Influence of Thermally-Induced Mesoscale Circulations on Turbulence Statistics Over an Idealized Urban Area Under a Zero Background Wind 总被引：2，自引：2，他引：0
Weiguo Wang 《Boundary-Layer Meteorology》2009,131(3):403-423
The influence of mesoscale circulations induced by urban-rural differential surface sensible heat flux and roughness on convective
boundary-layer (CBL) flow statistics over an isolated urban area has been examined using large-eddy simulation (LES). Results
are analyzed when the circulations influence the entire urban area under a zero background wind. For comparison, the CBL flow
over an infinite urban area with identical urban surface characteristics under the same background meteorological conditions
is generated as a control case (without circulations). The turbulent flow over the isolated urban area exhibits a mix of streaky
structure and cellular pattern, while the cellular pattern dominates in the control case. The mixed-layer height varies significantly
over the isolated urban area, and can be lower near the edge of the urban area than over the rural area. The vertical profiles
of turbulence statistics over the isolated urban area vary horizontally and are dramatically different from the control case.
The turbulent kinetic energy (TKE) sources include wind shear, convergence, and buoyancy productions, compared to only buoyancy
production in the control case. The normalized vertical velocity variance is reduced compared to the control case except in
the central urban area where it is little affected. The low-level flow convergence is mainly responsible for the enhanced
horizontal velocity variance in the central urban area, while wind shear is responsible for the additional local maximum of
the horizontal velocity variance near the middle of the CBL outside the central area. Parameterizations in the prognostic
equation for TKE used in mesoscale models are evaluated against the LES results over the isolated urban area. We also discuss
conditions under which the urban-induced circulations occur and when they may affect the entire urban area. Given that urban-induced
circulations can influence the entire urban area within hours for an urban area of a realistic size, it is inappropriate to
directly apply empirical relations of turbulence statistics derived under horizontally-homogenous flow conditions to an urban
area. 相似文献
19.
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
20.
Some Basic Properties of the Surrogate Subgrid-Scale Heat Flux in the Atmospheric Boundary Layer 总被引：3，自引：3，他引：0
Fernando Porté-Agel Charles Meneveau Marc B. Parlange 《Boundary-Layer Meteorology》1998,88(3):425-444
The development of improved subgrid-scale (SGS) models for large-eddy simulation of scalar transport in the atmospheric boundary layer requires an improved understanding of basic properties of the SGS fluxes. High frequency atmospheric wind speed and temperature data sampled at a height of 1.7 m are used to measure SGS heat fluxes and dissipation of temperature variance, by means of one-dimensional filtering and invoking Taylor's hypothesis. Conditional averaging is used to isolate interesting features of the SGS signals, and to relate them to the large-scale characteristics of the flow, such as the presence of coherent structures. Both mean and conditionally averaged SGS quantities are compared with those obtained using a standard eddy-diffusivity model. Within the limitations imposed by the one-dimensional data analysis, we observe that the model appears unable to reproduce important features of the real signals, such as the negative dissipation of temperature variance associated with strong negative resolved temperature gradients due to the ejection of warm air under unstable atmospheric stability conditions. 相似文献