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1.
Subgrid-scale (SGS) modeling is a long-standing problem and a critical component in the large-eddy simulation (LES) of atmospheric boundary layer. A variety of SGS models with different levels of sophistication have been proposed for different needs, such as Smagorinsky's (1963) eddy viscosity model, Mason and Thomson's (1992) stochastic backscattering model, and Sullivan et al.'s (1994) near surface model. A modified Smagorinsky SGS model has been used in the LES version of Terminal Area Simulation System (TASS-LES). It has successfully simulated the buoyancy-dominated, convective atmospheric boundary layer flows, while simulations of the shear-dominated, slightly unstable, neutral, and stably stratified boundary layer flows are not so good. For the later, we used a simpler version of Sullivan et al.'s subgrid-scale model in which turbulent kinetic energy equation is not included and the model is still the first-order closure. A momentum profile matching approach is adopted in the proposed model. A series of simulations for shear-dominated, slightly unstable and neutral boundary layers are performed using different subgrid-scale models and different grid resolutions. The results are compared with those of Sullivan et al. (1994) and with empirical similarity relations for the surface layer. The simulations with the new SGS model appear to be far more satisfactory than those with the modified Smagorinsky model. 相似文献
2.
Simulation of Ekman Boundary Layers by Large Eddy Model with Dynamic Mixed Subfilter Closure 总被引:2,自引:0,他引:2
I. Esau 《Environmental Fluid Mechanics》2004,4(3):273-303
Theoretical analysis of boundary layer turbulence has suggested a feasibility of sufficiently accurate turbulence resolving
simulations at relatively coarse meshes. However, large eddy simulation (LES) codes, which employ traditional eddy-viscosity
turbulence closures, fail to provide adequate turbulence statistics at coarse meshes especially within a surface layer. Manual
tuning of parameters in these turbulence closures may correct low order turbulence statistics but severely harms spectra of
turbulence kinetic energy (TKE). For more than decade, engineering LES codes successfully employ dynamic turbulence closures.
A dynamic Smagorinsky turbulence closure (DSM) has been already tried in environmental LES. The DSM is able to provide adequate
turbulence statistics at coarse meshes but it is not completely consistent with the LES equations. This paper investigates
applicability of an advanced dynamic mixed turbulence closure (DMM) to simulations of Ekman boundary layers of high Reynolds
number flows. The DMM differs from the DSM by explicit calculation of the Leonard term in the turbulence stress tensor. The
Horizontal Array Turbulence Study (HATS) field program has revealed that the Leonard term is indeed an important component
of the real turbulence stress tensor.
This paper presents validation of a new LES code LESNIC. The study shows that the LES code with the DMM provides rather accurate
low order turbulence statistics and the TKE spectra at very coarse meshes. These coarse LES maintain more energetic small
scale fluctuations of velocity especially within the surface layer. This is critically important for success of simulations.
Accurate representation of higher order turbulence statistics, however, requires essentially better LES resolution. The study
also shows that LES of the Ekman boundary layer cannot be directly compared with conventionally neutral atmospheric boundary
layers. The depth of the boundary layer is an important scaling parameter for turbulence statistics. 相似文献
3.
In the present study, the prediction accuracy of a dynamic one-equation sub-grid scale model for the large eddy simulation of dispersion around an isolated cubic building is investigated. For this purpose, the localized dynamic $k_\mathrm{SGS} $ -equation model (LDKM) is employed and the results are compared with the available experimental data and two other classic sub-grid scale models, namely, standard Smagorinsky–Lilly model (SSLM) and dynamic Smagorinsky–Lilly model (DSLM). It is shown that the three SGS models give results in good agreement with experiment. However, near the ground level of the leeward wall, dimensionless time-averaged concentration, $\langle K\rangle $ , profile is not quite similar to the experimental data. It is also demonstrated that the LDKM predicts the values of $\langle K\rangle $ on the roof, leeward and side walls more acceptably than the SSLM and DSLM. Whereas, the streamwise elongation of time-averaged structures of the plume shape is more over-estimated with the LDKM than with the other two SGS models. In terms of numerical difficulty, the LDKM is found to be stable and computationally reasonable. In addition, it does not suffer from a flow dependent constant such as the Smagorinsky coefficient employed in the SSLM model. 相似文献
4.
In large-eddy simulations of atmospheric boundary layer turbulence, the lumped coefficient in the eddy-diffusion subgrid-scale
(SGS) model is known to depend on scale for the case of inert scalars. This scale dependence is predominant near the surface.
In this paper, a scale-dependent dynamic SGS model for the turbulent transport of reacting scalars is implemented in large-eddy
simulations of a neutral boundary layer. Since the model coefficient is computed dynamically from the dynamics of the resolved
scales, the simulations are free from any parameter tuning. A set of chemical cases representative of various turbulent reacting
flow regimes is examined. The reactants are involved in a first-order reaction and are injected in the atmospheric boundary
layer with a constant and uniform surface flux. Emphasis is placed on studying the combined effects of resolution and chemical
regime on the performance of the SGS model. Simulations with the scale-dependent dynamic model yield the expected trends of
the coefficients as function of resolution, position in the flow and chemical regime, leading to resolution-independent turbulent
reactant fluxes. 相似文献
5.
A method to determine flow specific first-order closure for the turbulent flux of momentum in the atmospheric boundary layer
(ABL) is presented. This is based on the premise that eddy viscosity is a flow rather than a fluid property, and the physically
more realistic assumption that the transfer of momentum and other scalar quantities in a turbulent flow takes place by a large,
but finite number of length scales, than the often used single length scale, the ‘mixing length’. The resulting eddy viscosity
is flow specific and when applied to the study of the ABL, yields the vertical profiles of shear stress and mean wind velocity
in good agreement with observations. The method may be extended to other types of turbulent flows, however it should be recognized
that each type of flow may yield a different eddy viscosity profile. Using the derived eddy viscosity the paper presents simple
analytical solutions of the ABL equations to determine observationally consistent wind speed and shear stress profiles in
the ABL for a variety of practical applications including air pollution modelling. 相似文献
6.
A series of simulations under weakly to moderately stable boundary layers (SBLs) have been performed using the proposed subgrid-scale (SGS) model implemented into the Terminal Area Simulation System (TASS). The proposed SGS model incorporates some aspects of the two-part eddy viscosity SGS model of Sullivan et al. (1994) and further refinements which include the dependence of SGS mixing length on stratification, two-part separation of the SGS eddy diffusivity of heat, and more realistic empirical forms of Monin–Obukhov similarity functions. The potential temperature profiles from simulations clearly show a three-layer structure: a stable surface layer of strong gradients, a middle layer of small gradients, and an inversion layer on the top. The wind speed profiles show the formation of low level jet (LLJ). However, the sub-layer structures under moderately SBLs differ from those under weakly SBLs. Both the momentum and heat fluxes decrease almost linearly in the lower part of the SBL. The near surface values of the normalized turbulent kinetic energy (TKE/u
*
2) in all simulations are about 4 which is much less than the typical value of 5.5 under the neutral condition. The decay of turbulence first occurs in the area with large values of Richardson number (R
i<0.2). Generally, instantaneous values of the TKE and R
i at the various grid points are negatively correlated, but there is not a unique relationship between the two parameters. 相似文献
7.
Large-eddy simulation of turbulent flow and dispersion over a complex urban street canyon 总被引:1,自引:0,他引:1
Kiyoung Moon Jeong-Min Hwang Byung-Gu Kim Changhoon Lee Jung-il Choi 《Environmental Fluid Mechanics》2014,14(6):1381-1403
Turbulent flow and dispersion characteristics over a complex urban street canyon are investigated by large-eddy simulation using a modified version of the Fire Dynamics Simulator. Two kinds of subgrid scale (SGS) models, the constant coefficient Smagorinsky model and the Vreman model, are assessed. Turbulent statistics, particularly turbulent stresses and wake patterns, are compared between the two SGS models for three different wind directions. We found that while the role of the SGS model is small on average, the local or instantaneous contribution to total stress near the surface or edge of the buildings is not negligible. By yielding a smaller eddy viscosity near solid surfaces, the Vreman model appears to be more appropriate for the simulation of a flow in a complex urban street canyon. Depending on wind direction, wind fields, turbulence statistics, and dispersion patterns show very different characteristics. Particularly, tall buildings near the street canyon predominantly generate turbulence, leading to homogenization of the mean flow inside the street canyon. Furthermore, the release position of pollutants sensitively determines subsequent dispersion characteristics. 相似文献
8.
Hrvoje Kozmar 《Environmental Fluid Mechanics》2012,12(3):209-225
The rural atmospheric boundary layer (ABL) flow was reproduced in a wind tunnel at three different simulation length scales to investigate possible effects of the simulation length scale on flow characteristics. Performance of truncated vortex generators developed for part-depth ABL wind-tunnel simulations was tested in rural terrain exposure against the full-size Counihan vortex generators. A procedure to design the ABL developing above rural type terrain has been described. The 1:395 and 1:236 simulations were created as full-depth simulations, i.e., wind characteristics throughout an entire ABL were reproduced in the wind tunnel. The 1:208 simulation was a part-depth simulation, i.e., only a lower 70% of the ABL was experimentally modelled. The projected scaled-up ABL thicknesses are 395, 354, and 416 m full-scale in the 1:395, 1:236, and 1:208 simulations, respectively. Experimental results show similar trends in all three configurations not depending on the simulation length scale factor. This clearly indicates a possibility to physically, in the wind tunnel, reproduce the same rural atmospheric airflows at different simulation length scales. 相似文献
9.
Aggression and resource sharing among foundresses in the social wasp Polistes dominulus: testing transactional theories of conflict 总被引:3,自引:0,他引:3
Tug-of-war models of within-group conflict predict that the frequency of conflict will be positively related to the degree
of reproductive sharing within the group; in contrast, a negative relationship supports transactional models, in which reproductive
payments among group members limit the degree of within-group selfishness. We tested predictions of the tug-of-war and transactional
models by examining cofoundress interactions during the founding (preworker) phase of colony development in 30 naturally nesting
colonies of the paper wasp Polistes dominulus. We found that the mean rate of foundress aggression and the mean probability of food sharing were significantly negatively
associated, which supports the prediction of the transactional, not the tug-of-war model. Further, cofoundress aggression
significantly increased over the founding phase (independently of temperature), while the fraction of aggression initiated
by the dominant (alpha) foundress significantly decreased over this period. We show that both of these results are predicted
by the transactional model of within-group conflict. Interestingly, the alpha’s rate of aggression was significantly positively
temperature dependent, while the beta’s was not. This indicates that the alpha’s aggression level may often be near her physiological
maximum, while the beta’s aggression is limi- ted by other factors, contradicting the prediction of the tug-of-war model.
Moreover, the alpha’s aggression was significantly positively temperature dependent only in the second half of the founding
period, as predicted by the transactional model since this is when there is least reproductive sharing. Finally, our results
indicate that the alpha’s level of aggression depends on the resources controlled by the beta.
Received: 18 January 2000 / Revised: 19 June 2000 / Accepted: 24 June 2000 相似文献
10.
Estimation of critical submergence for an intake in a stratified fluid media by neuro-fuzzy approach
Most of the large surface reservoirs are stably stratified throughout most or all of the year. One means of assisting in the management is to allow for the selective withdrawal from the reservoir. The value of the intake’s submergence when the upper layer fluids begin to be drawn into the intake is known as “critical submergence”. In this study, the critical submergence for a circular intake pipe in a stratified body is investigated. Experiments were conducted on a vertically flowing downward intake pipe in a still-water reservoir. Experimental results are compared with that of the neuro-fuzzy models. 相似文献
11.
Ecological theory predicts that low productivity systems should have low biodiversity. However, despite the oligotrophic status
of the Gulf of Aqaba (Northern Red Sea) ciliate species richness was unexpectedly high. In addition, phytoplankton, as main
ciliate prey, was made up by only few genera, indicating a significant niche overlap among the grazers. Up to 97% of the ciliates
were from the same taxonomic group and of the same size range, implying very similar food niches. Ciliate diversity was highest
at times of lowest chlorophyll concentrations, during the period of stable abiotic conditions, but relatively high genetic
diversity within the ciliate prey, notably among the cyanobacteria Synechococcus and Prochlorococcus. In the absence of disturbance and with little predation pressure, the alternate explanations for the observed ciliate diversity
are either very fine niche partitioning by the ciliates, or their competitive equivalence resulting in a random assortment
of species immigrating from a larger metacommunity, in accordance with Hubbell’s, (The unified neutral theory of biodiversity
and biogeography. Princeton University Press, Princeton, 2001) neutral model. While the use of species abundance distributions (SAD’s) is far from definitive, the theoretical SAD’s that
best fit the Gulf of Aqaba ciliate data was most often not that expected by neutral theory. 相似文献
12.
Turbulence closures are fundamental for modelling the atmospheric diffusion in numerical codes and the resulting eddy diffusivities are key parameters in describing the transport and dispersion in the boundary layer. In this work, four turbulence closure schemes have been applied for reproducing a neutral flow over schematic complex terrain using the meteorological model RAMS. Two of the closures, a one-equation (E-l) and a two-equations (E-) model, have been implemented in RAMS in alternative to the ones originally available. In these cases, an analytical method based on the similarity theory for the atmospheric surface layer and boundary layer is adopted to calculate the empirical constants of the turbulence closures. Some examples of numerical studies performed to simulate the flow and turbulence over a 3-D hill in wind-tunnel experiment in neutral stratification are presented and discussed. An intercomparison of simulations related to different closures is considered by analysing the main features of the flow over the hill and by comparing calculated vertical profiles of turbulent kinetic energy with measured ones. 相似文献
13.
A large-eddy simulation with transitional structure function(TSF) subgrid model we previously proposed was performed to investigate
the turbulent flow with thermal influence over an inhomogeneous canopy, which was represented as alternative large and small
roughness elements. The aerodynamic and thermodynamic effects of the presence of a layer of large roughness elements were
modelled by adding a drag term to the three-dimensional Navier–Stokes equations and a heat source/sink term to the scalar
equation, respectively. The layer of small roughness elements was simply treated using the method as described in paper (Moeng
1984, J. Atmos Sci. 41, 2052–2062) for homogeneous rough surface. The horizontally averaged statistics such as mean vertical profiles of wind velocity,
air temperature, et al., are in reasonable agreement with Gao et al.(1989, Boundary layer meteorol. 47, 349–377) field observation (homogeneous canopy). Not surprisingly, the calculated instantaneous velocity and temperature
fields show that the roughness elements considerably changed the turbulent structure within the canopy. The adjustment of
the mean vertical profiles of velocity and temperature was studied, which was found qualitatively comparable with Belcher
et al. (2003, J Fluid Mech. 488, 369–398)’s theoretical results. The urban heat island(UHI) was investigated imposing heat source in the region of large
roughness elements. An elevated inversion layer, a phenomenon often observed in the urban area (Sang et al., J Wind Eng. Ind. Aesodyn. 87, 243–258)’s was successfully simulated above the canopy. The cool island(CI) was also investigated imposing heat sink to
simply model the evaporation of plant canopy. An inversion layer was found very stable and robust within the canopy. 相似文献
14.
Oscar Castro-Orgaz 《Environmental Fluid Mechanics》2011,11(4):427-435
Seepage flow is an agent related to the transport and dispersion of contamination in groundwater. Steady two-dimensional seepage
flow is governed by Laplace’s equation, for which several solution techniques are available. Because computations are complex
from a practical point of view, simplified models encompass the Dupuit-Forchheimer approach assuming a horizontal flow. However
this approach is inaccurate in seepage problems involving steep drawdowns. In this research, a new theoretical model for 2D
seepage flow is proposed based on Fawer’s theory for curved flows Castro-Orgaz (Environ Fluid Mech 10(3):2971–2310, 2010),
from which a second-order equation results describing the seepage surface. From this development, a numerical solution for
the rectangular dam problem based on the second-order model is presented, whereas a simple first-order equation is found to
describe flow to drains under a uniform rainfall. The results of this new model are compared with the full 2D solution of
Laplace’s equation for typical test cases, resulting in an excellent agreement. 相似文献
15.
Among procellariiform seabirds, many burrowing petrels show good olfactory abilities especially in recognising their nest.
In particular, it has been reported that Antarctic prions (Pachyptila desolata) discriminate their own and their mate’s odours and, in Y-maze experiments, prefer the odour of a conspecific bird to their
own. While traditionally examined from the perspective of homing mechanisms, these recent results have drawn attention to
the possible use of chemical signals in birds’ social behaviours. Indeed, the life history of petrels suggests that a mate
choice mediated by olfactory mechanisms may have evolved in this group to ensure genetic compatibility. This study was undertaken
to validate and extend results obtained on petrels’ olfactory discrimination capabilities. Following the Y-maze experiment
protocol, blue petrels (Halobaena caerulea) were offered three different choices: (1) mate versus conspecific’s odour, (2) own versus mate’s odour and (3) own versus
conspecific’s odour. We discovered that birds prefer the odour of their mate not only when presented against conspecific’s
odour but also against their own. We further verified that blue petrels also avoid their own odour when presented against
conspecific’s odour. Our results confirm that olfactory discrimination in burrowing petrels goes beyond self-recognition and
that self-odour avoidance may be widespread. We use two mutually non-exclusive behavioural frameworks for the interpretation
of our results, homing and mate choice, and explain why homing mechanisms cannot account for all of our observations. This
study opens the door to further research on olfactory mechanisms that, in petrels, might mediate individual recognition and
mate choice. 相似文献
16.
A. Vassilopoulos D. R. Green T. H. Gournelos N. Evelpidou P. Gkavakou S. Koussouris 《Journal of Coastal Conservation》2008,11(4):209-213
In this paper the counteractive dynamic marine and river environments responsible for a river mouth’s and a delta’s modulation
is studied. The Acheloos River (West Greece) was chosen as the case study due to the recent changes taken place in its basin.
The aim of this study is to analyze the present dominant environment and the impact of human activity on this balance. A GIS
database was developed and updated in order to provide access for future use and computer analysis (both spatial and chronological).
The study, includes an estimation of the delta forming processes, the impact of the anthropogenic parameters on these processes,
and, finally, their representation. 相似文献
17.
Elvan Ceyhan 《Environmental and Ecological Statistics》2010,17(3):247-282
For two or more classes (or types) of points, nearest neighbor contingency tables (NNCTs) are constructed using nearest neighbor
(NN) frequencies and are used in testing spatial segregation of the classes. Pielou’s test of independence, Dixon’s cell-specific,
class-specific, and overall tests are the tests based on NNCTs (i.e., they are NNCT-tests). These tests are designed and intended
for use under the null pattern of random labeling (RL) of completely mapped data. However, it has been shown that Pielou’s
test is not appropriate for testing segregation against the RL pattern while Dixon’s tests are. In this article, we compare
Pielou’s and Dixon’s NNCT-tests; introduce the one-sided versions of Pielou’s test; extend the use of NNCT-tests for testing
complete spatial randomness (CSR) of points from two or more classes (which is called CSR independence, henceforth). We assess the finite sample performance of the tests by an extensive Monte Carlo simulation study and demonstrate
that Dixon’s tests are also appropriate for testing CSR independence; but Pielou’s test and the corresponding one-sided versions
are liberal for testing CSR independence or RL. Furthermore, we show that Pielou’s tests are only appropriate when the NNCT
is based on a random sample of (base, NN) pairs. We also prove the consistency of the tests under their appropriate null hypotheses.
Moreover, we investigate the edge (or boundary) effects on the NNCT-tests and compare the buffer zone and toroidal edge correction
methods for these tests. We illustrate the tests on a real life and an artificial data set. 相似文献
18.
This study discusses some statistical methods used in quantitative structure–property relationships model’s validation and
comparison. The paper also introduces a series of online tools for model’s validation. The implemented diagnostic tool that
incorporated analytical approaches are exemplified on a series of models which estimate and predict the boiling points of
a sample of 73 alkanes. The used parameters and methods are presented, and some reference values are provided. 相似文献
19.
In 1990 the Dutch government decided to stop any further long-term landward retreat of the coastline. This policy choice for
a ‘dynamic preservation’ is primarily aimed at safety against flooding and at sustainable preservation of the values and interests
concerning the dunes and beaches. Five years later, a first overview of the benefits and bottlenecks of the new coastal defence
policy could be presented, which was published in the second governmental coastal report ‘Kustbalans 1995’ (coastal balance
1995). This consists of three elements: (1) evaluation of the implementation of ‘dynamic preservation’, (2) the consequences
of several natural and anthropogenic developments in the coastal zone and (3) integrated coastal zone management. The present
report describes experiences of Dutch coastline management and summarizes the main conclusions of the second governmental
report.
The overall conclusion of the evaluation study is that the 1990 choice for ‘dynamic preservation’ was right. Sand supply is
an effective method of coastline maintenance, which also serves functional uses in the beach and dune area. However, nearly
a doubling of the supply volume is necessary to compensate for sand losses in the coastal zone. A more integrated management
of the coastal zone is necessary to find an equilibrium between the interests of socio-economic development and the maintenance
of a natural, dynamic system. 相似文献
20.
In 1990 the Dutch government decided to stop any further long-term landward retreat of the coastline. This policy choice for
a ‘dynamic preservation’ is primarily aimed at safety against flooding and at sustainable preservation of the values and interests
concerning the dunes and beaches. Five years later, a first overview of the benefits and bottlenecks of the new coastal defence
policy could be presented, which was published in the second governmental coastal report ‘Kustbalans 1995’ (coastal balance
1995). This consists of three elements: (1) evaluation of the implementation of ‘dynamic preservation’, (2) the consequences
of several natural and anthropogenic developments in the coastal zone and (3) integrated coastal zone management. The present
report describes experiences of Dutch coastline management and summarizes the main conclusions of the second governmental
report.
The overall conclusion of the evaluation study is that the 1990 choice for ‘dynamic preservation’ was right. Sand supply is
an effective method of coastline maintenance, which also serves functional uses in the beach and dune area. However, nearly
a doubling of the supply volume is necessary to compensate for sand losses in the coastal zone. A more integrated management
of the coastal zone is necessary to find an equilibrium between the interests of socio-economic development and the maintenance
of a natural, dynamic system. 相似文献