共查询到20条相似文献，搜索用时 15 毫秒
1.
Richard M. Eckman 《Boundary-Layer Meteorology》2008,127(2):345-351
A 2006 article in Boundary-Layer Meteorology by G. Treviño and E.L Andreas presents a derivation that questions the use of time averaging for computing turbulence statistics. Their derivation shows that time averaging over a finite interval always leads to a zero integral time scale. As a result, Treviño and Andreas argue that any turbulence quantities derived from time averaging are tainted and incompatible with the Navier–Stokes equations. While Treviño and Andreas are correct that time averaging does produce integral scales that are quite different from what researchers commonly expect, this comment demonstrates that the theoretical implications are not as dire as they claim. 相似文献
2.
The turbulence closure in atmospheric boundary-layer modelling utilizing Reynolds Averaged Navier–Stokes (RANS) equations
at mesoscale as well as at local scale is lacking today a common approach. The standard kɛ model, although it has been successful for local scale problems especially in neutral conditions, is deficient for mesoscale
flows without modifications. The k–ɛ model is re-examined and a new general approach in developing two-equation turbulence models is proposed with the aim of
improving their reliability and consequently their range of applicability. This exercise has led to the replacement of the
ɛ-transport equation by the transport equation for the turbulence inverse length scale (wavenumber). The present version of
the model is restricted to neutrally stratified flows but applicable to both local scale and mesoscale flows. The model capabilities
are demonstrated by application to a series of one-dimensional planetary boundary-layer problems and a two-dimensional flow
over a square obstacle. For those applications, the present model gave considerably better results than the standard k–ɛ model. 相似文献
3.
There are two frameworks within which we can discuss turbulence energy in convective boundary layers. The first is the one provided by the Reynolds-averaged Navier–Stokes (RANS) energy equations, as interpreted by Osborne Reynolds in the late nineteenth century. The other, much newer framework is that provided by complex dynamical systems. The former gives prominence to the interpretation of local budgets of turbulence kinetic energy while the latter emphasizes the energy flows necessary to maintain turbulence in a statistically-steady state. It is argued that these frameworks constitute two incompatible paradigms, since the first localizes physical interpretation of the RANS kinetic energy budget while the second denies such a simple view. The local interpretation traces back to the way Reynolds himself interpreted his RANS energy equations, which interpretation is examined here and found to be faulty. We present a schematic model for energy flow in convective boundary layers from a complex dynamical systems’ perspective, and use it to re-interpret the RANS energy equation. 相似文献
4.
Antônio G. O. Goulart Gervásio A. Degrazia Otávio C. Acevedo Domenico Anfossi 《Boundary-Layer Meteorology》2007,125(2):279-287
The influence of turbulence on the meandering phenomenon is investigated. The study, based on the three-dimensional Navier–Stokes
equations, shows that when the turbulent fluxes can be neglected an asymptotic solution results. This solution reproduces
a horizontal wind oscillation with an infinite relaxation time. When there is turbulent forcing, on the other hand, a transition
occurs to a new order, characterized by a spatial reorganization, leading to a wind field with a well-defined direction. 相似文献
5.
We evaluate the Reynolds-averaged Navier–Stokes equations available as commercial computational fluid dynamics code for the
simulation of a neutral atmospheric boundary layer and attempt to define a proper numerical simulation procedure. Four turbulence
models, including two-equation and Reynolds stress models, were evaluated together with two near-wall models. Mesh and map
digitization sensitivity tests were also performed. The simulations were compared to experimental field data from the Askervein
Hill in Scotland. The results show that the simulations performed with ANSYS CFX 12.1 on a proper mesh and topological map
with a Reynolds stress turbulence model provided the best wind-speed predictions when compared to the experimental results. 相似文献
6.
A significant non-alignment between the mean horizontal wind vector and the stress vector was observed for turbulence measurements
both above the water surface of a large lake, and over a land surface (soybean crop). Possible causes for this discrepancy
such as flow distortion, averaging times and the procedure used for extracting the turbulent fluctuations (low-pass filtering
and filter widths etc.), were dismissed after a detailed analysis. Minimum averaging times always less than 30 min were established
by calculating ogives, and error bounds for the turbulent stresses were derived with three different approaches, based on
integral time scales (first-crossing and lag-window estimates) and on a bootstrap technique. It was found that the mean absolute
value of the angle between the mean wind and stress vectors is highly related to atmospheric stability, with the non-alignment
increasing distinctively with increasing instability. Given a coordinate rotation that aligns the mean wind with the x direction, this behaviour can be explained by the growth of the relative error of the u–w component with instability. As a result, under more unstable conditions the u–w and the v–w components become of the same order of magnitude, and the local stress vector gives the impression of being non-aligned with
the mean wind vector. The relative error of the v–w component is large enough to make it undistinguishable from zero throughout the range of stabilities. Therefore, the standard
assumptions of Monin–Obukhov similarity theory hold: it is fair to assume that the v–w stress component is actually zero, and that the non-alignment is a purely statistical effect. An analysis of the dimensionless
budgets of the u–w and the v–w components confirms this interpretation, with both shear and buoyant production of u–w decreasing with increasing instability. In the v–w budget, shear production is zero by definition, while buoyancy displays very low-intensity fluctuations around zero. As local
free convection is approached, the turbulence becomes effectively axisymetrical, and a practical limit seems to exist beyond
which it is not possible to measure the u-w component accurately. 相似文献
7.
8.
Large-Eddy Simulation of Flow and Pollutant Transport in Urban Street Canyons with Ground Heating 总被引：5，自引：4，他引：1
Xian-Xiang Li Rex E. Britter Tieh Yong Koh Leslie K. Norford Chun-Ho Liu Dara Entekhabi Dennis Y. C. Leung 《Boundary-Layer Meteorology》2010,137(2):187-204
Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and
pollutant dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating
the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both
the Navier–Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated
against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed turbulence, temperature,
and pollutant dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers
(Ri) were investigated. The ground heating significantly enhanced mean flow, turbulence, and pollutant flux inside the street
canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated from the free stream
and fresh air could be entrained into the street canyon at the roof-level leeward corner. Weighed against higher temperature,
the ground heating facilitated pollutant removal from the street canyon. 相似文献
9.
A study of the neutrally-stratified flow within and over an array of three-dimensional buildings (cubes) was undertaken using
simple Reynolds-averaged Navier—Stokes (RANS) flow models. These models consist of a general solution of the ensemble-averaged,
steady-state, three-dimensional Navier—Stokes equations, where the k-ε turbulence model (k is turbulence kinetic energy and ε is viscous dissipation rate) has been used to close the system of equations. Two turbulence
closure models were tested, namely, the standard and Kato—Launder k-ε models. The latter model is a modified k-ε model designed specifically to overcome the stagnation point anomaly in flows past a bluff body where the standard k-ε model overpredicts the production of turbulence kinetic energy near the stagnation point. Results of a detailed comparison
between a wind-tunnel experiment and the RANS flow model predictions are presented. More specifically, vertical profiles of
the predicted mean streamwise velocity, mean vertical velocity, and turbulence kinetic energy at a number of streamwise locations
that extend from the impingement zone upstream of the array, through the array interior, to the exit region downstream of
the array are presented and compared to those measured in the wind-tunnel experiment. Generally, the numerical predictions
show good agreement for the mean flow velocities. The turbulence kinetic energy was underestimated by the two different closure
models. After validation, the results of the high-resolution RANS flow model predictions were used to diagnose the dispersive
stress, within and above the building array. The importance of dispersive stresses, which arise from point-to-point variations
in the mean flow field, relative to the spatially-averaged Reynolds stresses are assessed for the building array. 相似文献
10.
Thomas Foken 《Boundary-Layer Meteorology》2006,119(3):431-447
In weak wind stable conditions, eddy-correlation fluxes calculated using conventional averaging times of 5 min or longer to define the perturbations are severely contaminated by poorly sampled mesoscale motions. A method is developed to identify the averaging time for each individual data record that captures the turbulence while excluding most of the mesoscale motions. The method is based on multiresolution decomposition of the heat flux, and provides an objective procedure for selecting the averaging time for calculating eddy-correlation fluxes. Eddy-correlation data collected in weak turbulence conditions over grass, snow, a pine forest and the ocean are used to demonstrate the approach.When the small-scale turbulence and mesoscale motions are clearly separated by a gap region in the heat flux cospectra, the variable window width reduces the influence of nonstationarity by more effectively filtering out mesoscale motions compared to traditional methods using constant averaging time. For records where turbulence and mesoscale motions overlap in scale, the method is not well posed, although such records occur infrequently for our datasets. These ambiguous cases correspond to significant nonstationarity at scales that overlap with turbulence scales. The improved turbulence fluxes calculated with the proposed method are the appropriate fluxes for evaluating flux-gradient relationships and Monin–Obukov similarity theory for developing improved model parameterizations of turbulence for weakly turbulent flows 相似文献
11.
A modified k- model is used for the simulation of the mean wind speed and turbulence for a neutrally-stratified flow through and over a building array, where the array is treated as a porous medium with the drag on the unresolved buildings in the array represented by a distributed momentum sink. More specifically, this model is based on time averaging the spatially averaged Navier–Stokes equation, in which the effects of the obstacle-atmosphere interaction are included through the introduction of a distributed mean-momentum sink (representing drag on the unresolved buildings in the array). In addition, closure of the time-averaged, spatially averaged Navier–Stokes equation requires two additional prognostic equations, namely one for the time-averaged resolved-scale kinetic energy of turbulence,, and another for its dissipation rate, . The performance of the proposed model and some simplified versions derived from it is compared with the spatially averaged, time-mean velocity and various spatially averaged Reynolds stresses diagnosed from a high-resolution computational fluid dynamics (CFD) simulation of the flow within and over an aligned array of sharp-edged cubes with a plan area density of 0.25. Four different methods for diagnosis of the drag coefficient CDfor the aligned cube array, required for the volumetric drag force representation of the cubes, are investigated here. We found that the model predictions for mean wind speed and turbulence in the building array were not sensitive to the differing treatments of the source and sink terms in the and equations (e.g., inclusion of only the `zeroth-order' approximation for the source/sink terms compared with inclusion of a higher-order approximation for the source/sink terms in the and equations), implying that the higher-order approximations of these source/sink terms did not offer any predictive advantage. A possible explanation for this is the utilization of the Boussinesq linear stress–strain constitutive relation within the k– modelling framework, whose implicit omission of any anisotropic eddy-viscosity effects renders it incapable of predicting any strong anisotropy of the turbulence field that might exist in the building array. 相似文献
12.
K. G. Mcnaughton 《Boundary-Layer Meteorology》2006,118(1):83-107
We present a new account of the kinetic energy budget within an unstable atmospheric surface layer (ASL) beneath a convective
outer layer. It is based on the structural model of turbulence introduced by McNaughton (Boundary-Layer Meteorology, 112:
199–221, 2004). In this model the turbulence is described as a self-organizing system with a highly organized structure that
resists change by instability. This system is driven from above, with both the mean motion and the large-scale convective
motions of the outer layer creating shear across the surface layer. The outer convective motions thus modulate the turbulence
processes in the surface layer, causing variable downwards fluxes of momentum and kinetic energy. The variable components
of the momentum flux sum to zero, but the associated energy divergence is cumulative, increasing both the average kinetic
energy of the turbulence in the surface layer and the rate at which that energy is dissipated. The tendency of buoyancy to
preferentially enhance the vertical motions is opposed by pressure reaction forces, so pressure production, which is the work
done against these reaction forces, exactly equals buoyant production of kinetic energy. The pressure potential energy that
is produced is then redistributed throughout the layer through many conversions, back and forth, between pressure potential
and kinetic energy with zero sums. These exchanges generally increase the kinetic energy of the turbulence, the rate at which
turbulence transfers momentum and the rate at which it dissipates energy, but does not alter its overall structure. In this
model the velocity scale for turbulent transport processes in the surface layer is (kzɛ)1/3 rather than the friction velocity, u*. Here k is the von Kármán constant, z is observation height, ɛ is the dissipation rate. The model agrees very well with published experimental results, and provides
the foundation for the new similarity model of the unstable ASL, replacing the older Monin–Obukhov similarity theory, whose
assumptions are no longer tenable. 相似文献
13.
Christophe Sanz 《Boundary-Layer Meteorology》2003,108(1):191-197
The k - turbulence model is a standard of computational software packages for engineering, yet its application to canopy turbulence has not received comparable attention. This is probably due to the additional source (and/or sink) terms, whose parameterization remained uncertain. This model must include source terms for both turbulent kinetic energy (k) and the viscous dissipation rate (), to account for vegetation wake turbulence budget. In this note, we show how Kolmogorov's relation allows for an analytical solution to be calculated within the portion of a dense and homogeneous canopy where the mixing length does not vary. By substitution within model equations, this solution allows for a set of constraints on source term model coefficients to be derived.Those constraints should meet both Reynolds averaged Navier–Stokes equationsand large-eddy simulation sub-grid scale turbulence modelling requirements.Although originating from within a limited portion of the canopy, the predictedcoefficients values must be valid elsewhere in order to make the model capable of predicting the whole canopy-layer flow with a single set of constants. 相似文献
14.
Mitsuaki Horiguchi Taiichi Hayashi Ahoro Adachi Shigeru Onogi 《Boundary-Layer Meteorology》2012,144(2):179-198
Large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL) are investigated on the basis of observations made from the 213-m tall meteorological tower at Tsukuba, Japan. Vertical profiles of wind speed and turbulent fluxes in the ABL were obtained with sonic anemometer-thermometers at six levels of the tower. From the archived data, 31 near-neutral cases are selected for the analysis of turbulence structures. For the typical case, event detection by the integral wavelet transform with a large time scale (180 s) from the streamwise velocity component (u) at the highest level (200 m) reveals a descending high-speed structure with a time scale of approximately 100 s (a spatial scale of 1 km at the 200-m height). By applying the wavelet transform to the u velocity component at each level, the intermittent appearance of large-scale high-speed structures extending also in the vertical is detected. These structures usually make a large contribution to the downward momentum transfer and induce the enhancement of turbulent kinetic energy. This behaviour is like that of “active” turbulent motions. From the analysis of the two-point space–time correlation of wavelet coefficients for the u velocity component, the vertical extent and the downward influence of large-scale structures are examined. Large fluctuations in the large-scale range (wavelet variance at the selected time scale) at the 200-m level tend to induce the large correlation between the higher and lower levels. 相似文献
15.
多尺度大气湍流的扩散及扩散率 总被引：5，自引：3，他引：5
本文首先将大气湍流多尺度化,而将不同尺度的脉动量的Monte Carlo关系转换为Lange-vin方程,在平稳及均匀条件下可得一高阶常系数Lagrange自相关函数的常微分方程。其通解恰为一组衰减实指数函数的线性组合,系数为各尺度湍能对总湍能的比。以此可通过Taylor积分对野外实测扩散参数作出尺度分解。并可方便地将扩散参数表达为湍流参数的函数使其物理意义明确。 相似文献
16.
When modelling the turbulent dispersion of a passive tracer using Reynolds-averaged Navier–Stokes (RANS) simulations, two
different approaches can be used. The first consists of solving a transport equation for a scalar, where the governing parameters
are the mean velocity field and the turbulent diffusion coefficient, given by the ratio of the turbulent viscosity and the
turbulent Schmidt number Sc
t
. The second approach uses a Lagrangian particle tracking algorithm, where the governing parameters are the mean velocity
and the fluctuating velocity field, which is determined from the turbulence kinetic energy and the Lagrangian time T
L
. A comparison between the two approaches and wind-tunnel data for the dispersion in the wake of a rectangular building immersed
in a neutral atmospheric boundary layer (ABL) is presented. Particular attention was paid to the influence of turbulence model
parameters on the flow and concentration field. In addition, an approach to estimate Sc
t
and T
L
based on the calculated flow field is proposed. The results show that applying modified turbulence model constants to enable
correct modelling of the ABL improves the prediction for the velocity and concentration fields when the modification is restricted
to the region for which it was derived. The difference between simulated and measured concentrations is smaller than 25% or
the uncertainty of the data on 76% of the points when solving the transport equation for a scalar with the proposed formulation
for Sc
t
, and on 69% of the points when using the Lagrangian particle tracking with the proposed formulation for T
L
. 相似文献
17.
Variation of the Sectional Drag Coefficient of a Group of Buildings with Packing Density 总被引：2，自引：2，他引：0
Reynolds-averaged Navier–Stokes (RANS) simulations of turbulent flow over groups of buildings with different packing densities
are reported. The results for a selected packing density are compared with direct numerical simulations (DNS) previously validated
against wind-tunnel data. The present study is focused on average properties of the flow, especially on the drag coefficients,
and is a first attempt to provide information on these parameters (their values are not generally known) for a range of packing
densities, for a given staggered arrangement of cubes using RANS methods. However, some of the limitations of RANS have come
to light. Hence, it is recommended that such simulations are ‘calibrated’ against experimental or DNS data, as is done here. 相似文献
18.
Spectral similarity between scalars at very low frequencies in the unstable atmospheric surface layer over the Tibetan plateau 总被引：1，自引：7，他引：1
Jun Asanuma Ichiro Tamagawa Hirohiko Ishikawa Yaoming Ma Taiichi Hayashi Yongqiang Qi Jiemin Wang 《Boundary-Layer Meteorology》2007,122(1):85-103
Similarity between temperature and water vapour was investigated in the Fourier domain by using their coherency spectra, defined
as the correlation coefficient between their Fourier modes, and the relative efficiency of their vertical transport. The class-averaged
values of these indices were computed from the turbulence measurements over sparse grasslands on the Tibetan plateau during
the intensive observations of GEWEX (Global Energy and Water Experiment) Asian Monsoon Experiment (GAME). It was found that
the energy-containing eddies at scales up to 100z (z being height) are characterised by well-maintained similarity between these scalars. The scalars are highly correlated with
each other, and their transport efficiencies are almost equal within this scale range. In contrast, similarity was not always
maintained at scales larger than 1000z. Detailed analyses showed that this breakdown of similarity occurs occasionally or sporadically, suggesting that it is caused
by events whose average return period is not much smaller than the current averaging time, i.e. 30 min. We speculate that
entrainment of drier and warmer air at the top of the atmospheric boundary layer caused the scalar dissimilarity at this scale
range. 相似文献
19.
We propose a new approach to derive spatially-averaged momentum equations for an urban canopy model that resolves buildings
vertically and not horizontally. First, in order to mathematically describe the actual momentum field as a completely continuous
field, the underling concepts of the immersed boundary method are employed, where we assume that (i) the entire simulation
space, including that occupied by buildings, is filled with a fluid, and (ii) an external body force field exists that reduces
the wind speed to zero at all positions coinciding with the space occupied by the buildings. Then, in order to obtain the
required spatially-averaged momentum equations in a self-consistent manner, a spatial-averaging operation is applied to the
Navier–Stokes equations that include a term representing the external force field. The applied spatial-averaging operation
is equivalent to the conventional spatial filtering operation used in large-eddy simulations. To examine the significance
of the subgrid-scale (SGS) stresses of the spatially-averaged momentum equations, a numerical simulation is performed for
a flow around a regular array of cubical blocks with a grid resolution that is sufficient to resolve the blocks. By estimating
the individual terms in the spatially-averaged momentum equations using the simulation results, we show that the SGS stresses
contribute significantly to the spatially-averaged momentum budget, and therefore they should not be neglected in urban canopy
modelling. 相似文献
20.
Measurements of concentration fluctuation intensity, intermittency factor, and integral time scale were made in a water channel
for a plume dispersing in a well-developed, rough surface, neutrally stable, boundary layer, and in grid-generated turbulence
with no mean velocity shear. The water-channel simulations apply to full-scale atmospheric plumes with very short averaging
times, on the order of 1–4 min, because plume meandering was suppressed by the water-channel side walls. High spatial and
temporal resolution vertical and crosswind profiles of fluctuations in the plume were obtained using a linescan camera laser-induced
dye tracer fluorescence technique. A semi-empirical algebraic mean velocity shear history model was developed to predict these
concentration statistics. This shear history concentration fluctuation model requires only a minimal set of parameters to
be known: atmospheric stability, surface roughness, vertical velocity profile, and vertical and crosswind plume spreads. The
universal shear history parameter used was the mean velocity shear normalized by surface friction velocity, plume travel time,
and local mean wind speed. The reference height at which this non-dimensional shear history was calculated was important,
because both the source and the receptor positions influence the history of particles passing through the receptor position. 相似文献