共查询到20条相似文献，搜索用时 250 毫秒
1.
A numerical model is developed for two-dimensional turbulent boundary-layer flow above gentle topography — defined as not giving rise to mean flow separation. Although the model is formulated in a framework of mixing length and turbulent energy equation models for the surface layer of the atmospheric boundary layer, it could be modified to include higher-order closure hypotheses and/or extended to model gentle topography for the planetary boundary layer or on the sea bed. Results are presented for flow above a specific shape of hill and the effects of surface roughness and hill height are investigated. 相似文献
2.
On the parameterization of drag over small-scale topography in neutrally-stratified boundary-layer flow 总被引：2，自引：0，他引：2
Predictions of the surface drag in turbulent boundary-layer flow over two-dimensional sinusoidal topography from various numerical models are compared. For simple 2D terrain, the model results show that the drag increases associated with topography are essentially proportional to (slope)2 up to the steepness at which the flow separates. For the purposes of boundary-layer parameterisation within larger-scale models, we propose a representation of the effects of simple 2D topography via an effective roughness length, z
0
eff. The form of the varation of z
0
eff with terrain slope and topographic wavelength is established for small slopes from the model results and a semi-empirical formula is proposed. 相似文献
3.
Large-eddy and mixing length model simulations of convective flows over hills have been performed for a range of hill slopes and stabilities. For low hills, the fractional speed-up and normalized pressure drag are shown to decrease with increasing instability. For hills steep enough to cause separation in neutral conditions, the effect of convection is to reduce the size and strength of the separated bubble, although the normalized pressure drag is found to be almost independent of stability. Finally, the ability of effective roughness length parametrizations to represent the effects of the hills in convective conditions is assessed. 相似文献
4.
5.
Large-eddy Simulations of Flow Over Forested Ridges 总被引：4，自引：4，他引：0
Andrew N. Ross 《Boundary-Layer Meteorology》2008,128(1):59-76
Large-eddy simulations (LES) of flow over a series of small forested ridges are performed, and compared with numerical simulations
using a one-and-a-half order mixing length closure scheme. The qualitative and quantitative similarity between these results
provides some confidence in the results of recent analytical and numerical studies of flow over forested hills using first-order
mixing length schemes. Time series of model velocities at various locations within the canopy allow the application of various
experimental techniques to study the turbulence in the LES. The application of conditional analysis shows that the structure
of the turbulence over a forested hill is broadly similar to that over flat ground, with sweeps and ejections dominating.
Differences are seen across the hill, particularly associated with regions of mean flow separation and recirculation near
the summit and in the lee of the hill. Detailed comparison of derived mixing lengths from the LES with the assumed values
used in mixing-length closure schemes show that the mixing length varies with location across the hill and with height in
the canopy. This is consistent with previous wind-tunnel measurements, and demonstrates that a constant mixing-length assumption
is not strictly valid within the canopy. Despite this, the first-order mixing-length schemes do give similar results both
for the mean flow and the turbulence in such situations. 相似文献
6.
We derive a conceptual model of the flow over heterogeneous terrain consisting of patches with contrasting Bowen ratios. Upward
moving eddies are assumed to carry heterogeneous properties, whereas downward moving eddies carry homogeneous properties.
This results in a decorrelation of temperature and humidity as the contrast between the patches increases. We show that this
model is able to reproduce the relationship developed by Lamaud and Irvine (Boundary-Layer Meteorol. 120:87–109, 2006). Some
details differ from their expression but are in accordance with data obtained over African savannah. We extend the conceptual
model to a combination of any scalars, not necessarily linked through the surface energy balance (as is the case for temperature
and humidity). To this end we introduce a new parameter that describes the surface heterogeneity in surface fluxes. The results
of the current model can be used to predict the discrepancy between similarity relationships for different scalars over heterogeneous
terrain. 相似文献
7.
Eric D. Skyllingstad 《Boundary-Layer Meteorology》2003,106(2):217-243
A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain. 相似文献
8.
Water mass modification in surface-trapped, near-field river plumes is examined using a 1.5-layer reduced gravity model and a three-dimensional numerical model. Solutions to the layer model are shown to be qualitatively similar to previous observations and three-dimensional simulations of near-field plumes. Analytic analysis of the layer model demonstrates how the near-field plume is controlled by the competing processes of mixing and spreading. The two models are then used to explore the parameter space dependence of density changes within the near-field plume and their associated cross-shore length scales. Both the magnitude of density changes and their length scales are proportional to either estuarine discharge or fresh water discharge; density changes are also inversely proportional to the estuary mouth width. One surprising feature of the parameter space solutions is that the density of water exiting the near-field plume, a measure of the net dilution of the entire near-field plume, is shown to be inversely proportional to local mixing rates. This is because when local mixing is lower, the influence of plume spreading becomes greater; this spreading accelerates the plume, requiring more net mixing to bring the plume back to subcritical flow. 相似文献
9.
余锦华 《热带气象学报(英文版)》2005,11(1):31-41
1 INTRODUCTION Being one of the important factors that govern the track and intensity change of the tropical cyclone (TC) [1, 2], topographic features are closely related with topographic parameters and the speed of latitudinal flows. As shown in a statistical study by Brand et al., with its passage through the islands of the Philippines, the TC begins to decrease the mean maximum surface wind speed from 47 m/s to 45 m/s 4 hours before reaching the Philippines. They also find that when … 相似文献
10.
A three-dimensional fog model,suitable for precipitous and complex terrain,is designed containing four prognostic variables-wind speed,potential temperature,specific humidity and mixing ratio of liquid water.The model considers turbulent exchange,the long-wave radiation on the ground surface and in the air.the short-wave solar radiation,the evaporation and condensation of moisture and the surface heat budget and so on.In order to make the model run steadily on precipitous and complex terrain,we have improved the method of calculating horizontal pressure gradient force in the terrain coordinate system.The results coincide on the whole with the actual situation. 相似文献
11.
Ben Yang Yun Qian Larry K. Berg Po-Lun Ma Sonia Wharton Vera Bulaevskaya Huiping Yan Zhangshuan Hou William J. Shaw 《Boundary-Layer Meteorology》2017,162(1):117-142
We evaluate the sensitivity of simulated turbine-height wind speeds to 26 parameters within the Mellor–Yamada–Nakanishi–Niino (MYNN) planetary boundary-layer scheme and MM5 surface-layer scheme of the Weather Research and Forecasting model over an area of complex terrain. An efficient sampling algorithm and generalized linear model are used to explore the multiple-dimensional parameter space and quantify the parametric sensitivity of simulated turbine-height wind speeds. The results indicate that most of the variability in the ensemble simulations is due to parameters related to the dissipation of turbulent kinetic energy (TKE), Prandtl number, turbulent length scales, surface roughness, and the von Kármán constant. The parameter associated with the TKE dissipation rate is found to be most important, and a larger dissipation rate produces larger hub-height wind speeds. A larger Prandtl number results in smaller nighttime wind speeds. Increasing surface roughness reduces the frequencies of both extremely weak and strong airflows, implying a reduction in the variability of wind speed. All of the above parameters significantly affect the vertical profiles of wind speed and the magnitude of wind shear. The relative contributions of individual parameters are found to be dependent on both the terrain slope and atmospheric stability. 相似文献
12.
Observations of Boundary-Layer Wind-Tunnel Flow over Isolated Ridges of Varying Steepness and Roughness 总被引：2，自引：2，他引：2
Boundary-layer wind-tunnel flow is measured over isolated ridges of varyingsteepness and roughness. The steepness/roughness
parameter space is chosento produce flows that range from fully attached to strongly separated. Measurementsshow that maximum
speedup at the hill crest is significantly lower than predictedby linear theory and that recovery in the lee of the hill is
much slower for stronglyseparated flow over steep terrain. The measurements also show that behaviour ofthe mean and turbulent
components of the flow on the downwind side of the ridgeis fundamentally different between separated and non-separated flows.
This suggeststhe dominance of much increased turbulence time and length scales in the lee of thehill in association with a
production mechanism that scales with the hill length ratherthan the proximity to the surface as on the windward side of the
hill crest. 相似文献
13.
14.
Numerical simulations of flow over hills that are partially covered with a forest canopy are performed. This represents a much more realistic situation than previous studies that have generally concentrated on hills that are fully-forested. The results show that the flow over the hill is sensitive to where on the hill the forest is positioned. In particular, for low slopes flow separation is predominantly located within the forest on the lee slope. This has implications for the transport of scalars in the forest canopy. For large hills the results show more variability in scalar concentrations within the canopy compared to either a fully-forested hill or a patch of forest over flat terrain. These results are likely to have implications for a range of applications including the siting and interpretation of flux measurements over forests in complex terrain, predicting wind damage to trees and wind-farm developments. Calculation of the hill-induced pressure drag and canopy-plus-surface stress shows a strong sensitivity to the position of the forest relative to the hill. Depending on the position of the forest the individual drag terms may be strongly enhanced or reduced and may even change sign. The net impact is generally to reduce the total drag compared to an equivalent fully-forested hill, but the amount of the reduction depends strongly on the position of the forest canopy on the hill. In many cases with large, wide hills there is a clear separation of scales between the adjustment of the canopy to a forest edge (of order 6 ? 8L c, where L c is the canopy adjustment length scale) and the width of the hill. This separation means that the hill-induced pressure and flow fields and the forest-edge induced pressure and flow fields can in some sense be considered as acting separately. This provides a means of explaining the combined effects of partial forestation and terrain. It also offers a simple method for modelling the changes in drag over a hill due to partial forest cover by considering the impact of the hill and the partial canopy separately. Scaling arguments based on this idea successfully collapse the modelled drag over a range of different hill widths and heights and for different canopy parameters. This offers scope for a relatively simple parametrization of the effects of partial forest cover on the drag over a hill. 相似文献
15.
Summary Hurricanes cause a variety of damage due to high winds, heavy rains, and storm surges. This study focuses on hurricanes’
high winds. The most devastating effects of sustained high winds occur in the first few hours of landfall. During the short
period, hurricanes’ rainfall often increases, while the low-level pressure gradients continue to weaken. Latent heating does
not appear to strengthen the surface winds. The indicator is that dry mechanisms such as the boundary layer processes and
terrain are responsible for the damaging winds in the coastal areas. In this study, the design of a dry hurricane boundary
layer wind model is described. The goal is to develop a forecast tool with near-real time applications in expeditious wind
damage assessment and disaster mitigation during a hurricane landfall event.
Different surface roughness lengths and topographic features ranging from flat land to the mountainous terrain of Taiwan were
used in the model simulation experiments to reveal how the coastal environment affected the hurricane surface winds. The model
performed quite well in all cases. The experiments suggested that the downward transfer of high momentum aloft played a significant
role in the maintenance of high wind speeds at the surface. The surface wind maximums were observed on the lee sides of high
terrain. The surface streamline analyses showed that the high mountains tended to block the relatively weak flow and caused
small eddies, while they forced the stronger flow to turn around the mountains.
Due to great difficulty in data collection, the hurricane boundary layer over land remains one of the least understood parts
of the system. The dry model proves to be an effective way to study many aspects of hurricane boundary layer winds over a
wide range of terrain features and landfall sites. The model runs efficiently and can be run on a medium-size personal computer.
Received March 16, 2001 Revised September 10, 2001 相似文献
16.
Flow over the summit of an isolated hill 总被引：5，自引：0，他引：5
P. J. Mason 《Boundary-Layer Meteorology》1986,37(4):385-405
Observations of the mean flow and turbulence statistics over the summit of an isolated, roughly circular hill, Nyland hill, are presented, Nyland hill rises 70 m above the surrounding terrain and has a base diameter of about 500 m. The summit of the hill is very smooth and allows representative measurements to be made close to the surface. The flow speed 8 m above the summit is increased by a factor of 2 over the upstream speed 8 m above level terrain, and flow separation occurs in the lee of the hill. The mean velocity profile over the summit shows an increase in velocity with height up to about 2 m and then a near constant velocity between 2 and 16 m. The flow perturbation relative to the upstream profile is thus a maximum at about 2 m. The measurements of turbulence structure show how the influence of the hill depends on the length scale of the turbulent eddies involved. Scales greater than the scale of the hill are modified through the flow speed-up whilst scales shorter than the hill suffer complex changes. The short-scale turbulence over the summit is only in local equilibrium in the lowest fraction of a metre. Above this equilibrium region, there is a complex adjustment towards the rapid distortion dynamics which appear to dominate at heights above about 8 m. The detailed results are compared with previous studies and available theories. 相似文献
17.
Sang Jianguo 《Acta Meteorologica Sinica》1991,5(1):71-78
The solutions of downslope motions over a sloping terrain are solved analytically in terms of the atmos-pheric wave equations with a two-layer model.The physical meanings of the solutions are discussed.As thelower layer of the atmosphere is stable and deep with strong wind the solution represents strong downslopewind,while as the lower layer is strong stable with light wind the drainage flow is obtained.The dependenceof the strength of downslope motion on the atmospheric stratification,wind field structure as well as Scorerparameter is also examined. 相似文献
18.
Daniel Reinert Volkmar Wirth Joachim Eichhorn Walter-Georg Panhans 《Boundary-Layer Meteorology》2007,125(1):109-132
This paper presents the dry version of a new large-eddy simulation (LES) model, which is designed to simulate air flow and
clouds above highly complex terrain. The model is three-dimensional and nonhydrostatic, and the governing equations are sound
filtered by use of the anelastic approximation. A fractional step method is applied to solve the equations on a staggered
Cartesian grid. Arbitrarily steep and complex orography can be accounted for through the method of viscous topography. The
dynamical model core is validated by comparing the results for a spreading density current against a benchmark solution. The
model accuracy is further assessed through the simulation of turbulent flow across a quasi two-dimensional ridge. The results
are compared with wind-tunnel data. The method of viscous topography is not restricted to moderately sloped terrain. Compared
to models using curvilinear grids, it allows this model to be applied to a much wider range of flows. This is illustrated
through the simulation of an atmospheric boundary-layer flow over a surface mounted cube. The results show that the dry model
version is able to accurately represent the complex flow in the vicinity of three-dimensional obstacles. It is concluded that
the method of viscous topography was successfully implemented into a micrometeorological LES model. As will be shown in Part
II, this allows the detailed study of clouds in highly complex terrain. 相似文献
19.
山区复杂地形上区域大气质量模式的试验研究 总被引：3，自引：3，他引：3
由于复杂地形上的气流多变，所以，进行区域空气质量模拟是比较困难的。根据大气污染总量控制和环境规划的需要，发展了一种可适用于山谷城市的大气质量模式，该模式的主要特点；（１）采用目前尚少的复杂地形上的三维边界层模式作为大气扩散模式所需城三维边界层要素场；（２）用复杂地形上的三维以模式和斯扩散氏架源（包括面源）和主架源所造在的地面ＳＯ２浓度，然后将两者的模拟浓度在相应网各上叠加。验证结果表明，该混合模式 相似文献
20.
Several two-equation turbulence models using isotropic eddy viscosity and wall functions are assessed by solution of the neutral atmospheric boundary layer over a flat surface and wind flow over two- and three-dimensional models and real terrain. Calculations are presented for wind flow over the Sirhowy Valley in Wales, an embankment along the Rhine in Germany and the Askervein Hill in Scotland. Comparisons of predictions with previous work, and laboratory and field data, show that the RNG-based k– model gives the best agreement with respect to the flow profiles and length of the separated flow region. The results of this model are analyzed with a non-linear stress-strain relation to gauge the potential effect of turbulence anisotropy. 相似文献