共查询到20条相似文献，搜索用时 93 毫秒
1.
Jae-Jin Kim Eric Pardyjak Do-Yong Kim Kyoung-Soo Han Byung-Hyuk Kwon 《Asia-Pacific Journal of Atmospheric Sciences》2014,50(3):365-375
The effects of building-roof cooling on flow and air temperature in 3D urban street canyons are numerically investigated using a computational fluid dynamics (CFD) model. The aspect ratios of the building and street canyon considered are unity. For investigating the building-roof cooling effects, the building-roof temperatures are systematically changed. The traditional flow pattern including a portal vortex appears in the spanwise canyon. Compared with the case of the control run, there are minimal differences in flow pattern in the cases in which maximum building-roof cooling is considered. However, as the building roof becomes cooler, the mean kinetic energy increases and the air temperature decreases in the spanwise canyon. Building-roof cooling suppresses the upward and inward motions above the building roof, resultantly increasing the horizontal velocity near the roof level. The increase in wind velocity above the roof level intensifies the secondarily driven vortex circulation as well as the inward (outward) motion into (out of) the spanwise canyon. Finally, building-roof cooling reduces the air temperature in the spanwise canyon, supplying much relatively cool air from the streamwise canyon into the spanwise canyon. 相似文献
2.
城市湍流边界层内汽车尾气扩散规律数值模拟研究 总被引：2，自引：1，他引：1
以纳维斯托克斯方程组、大气平流扩散方程、湍流动能及湍流动能耗散率方程组为基础．采用伪不定常方法,建立了一个数值模式．利用该模式列城市湍流边界层内流场结构及汽车排放污染物扩散规律进行了研究。结果表明：街谷内会形成一个涡旋型流场．汽车排放污染物浓度在地面及建筑物背风面产生堆积,且其沿高度方向的梯度变化在背风面大．迎风而小。随着街谷两侧建筑物屋顶风速的增大,峡谷内形成的涡旋流场的强度增大,污染物扩散速率增大：当屋顶来流与街道之间的夹角逐渐增大时．涡旋中心位置由街道中心偏向于背风面及更高层且污染物扩散速度加快。 相似文献
3.
The effects of street bottom heating and inflow turbulence on urban street-canyon flow are experimentally investigated using a circulating water channel. Three experiments are carried out for a street canyon with a street aspect ratio of 1. Results from each experiment with bottom heating or inflow turbulence are compared with those without bottom heating and appreciable inflow turbulence. It is demonstrated that street bottom heating or inflow turbulence increases the intensity of the canyon vortex. A possible explanation on how street bottom heating or inflow turbulence intensifies the canyon vortex is given from a fluid dynamical viewpoint. 相似文献
4.
Structure of air flow separation over wind wave crests 总被引：1，自引：0，他引：1
Sanshiro Kawai 《Boundary-Layer Meteorology》1982,23(4):503-521
Air flow over wind waves generated in a wind-wave tunnel was visualized by numerous tiny suspended particles (zinc stearate), and instantaneous air flow fields over about one wavelength of wind waves were obtained. Air flow separation was detected over the wave crest in about a half of the samples. In such cases, the separation started near the crest about half of the time, with a vortex trapped over the convergence point of the surface flow which appeared at the leeward face of the crest. This structure was much different from a previously imagined picture in which the separation started at the convergence point. The high frequency of its occurrence suggested the stability of this structure. However, even when this structure was clearly seen, the structure behind the vortex to the next wave crest had various patterns. This variety seems to be related to an instability of the high-shear layer accompanied by separation. Other varieties were also seen, such as the occurrence of separation without the above mentioned structure, as well as the existence of non-separated air flow structures. These varieties seem to be related to the variability of individual wind wave crests. An analysis of correlation between the wave form and the air flow structure over it shows that there is a critical value of local gradient of wave form, above which the air flow always separates. This fact suggests a strong coupling between the air and the water, i.e., the local stress exerted on the water surface changes the nature of a wave crest, especially its form, and as a result, the air flow structure over it changes drastically.Decreased 21 November, 1981. Final draft of the paper prepared by Professor Yoshiaki Toba, Geophysical Institute, Tohoku University. 相似文献
5.
Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Image Visualization) technique. Fluid experiments show that with calm ambient wind,the flows in the street canyon are completely driven by thermal force, and the convection can reach the upper atmosphere of the street canyon. Horizontal and vertical motions also appear above the roofs of the buildings. These are the conditions which favor the exchange of momentum and air mass between the street canyon and its environment. More than two vortices are induced by the convection, and the complex circulation pattern will vary with time in a wider street canyon. However， in a narrow street canyon， just one vortex appears. With a light ambient wind, the bottom heating and the associated convection result in just one main vortex. As the ambient wind speed increases, the vortex becomes more organized and its center shifts closer to the leeward building. 相似文献
6.
Hrvoje Kozmar 《Theoretical and Applied Climatology》2010,100(1-2):153-162
Precise urban atmospheric boundary layer (ABL) wind tunnel simulations are essential for a wide variety of atmospheric studies in built-up environments including wind loading of structures and air pollutant dispersion. One of key issues in addressing these problems is a proper choice of simulation length scale. In this study, an urban ABL was reproduced in a boundary layer wind tunnel at different scales to study possible scale effects. Two full-depth simulations and one part-depth simulation were carried out using castellated barrier wall, vortex generators, and a fetch of roughness elements. Redesigned “Counihan” vortex generators were employed in the part-depth ABL simulation. A hot-wire anemometry system was used to measure mean velocity and velocity fluctuations. Experimental results are presented as mean velocity, turbulence intensity, Reynolds stress, integral length scale of turbulence, and power spectral density of velocity fluctuations. Results suggest that variations in length-scale factor do not influence the generated ABL models when using similarity criteria applied in this study. Part-depth ABL simulation compares well with two full-depth ABL simulations indicating the truncated vortex generators developed for this study can be successfully employed in urban ABL part-depth simulations. 相似文献
7.
二维街谷地面加热引起的流场特征的水槽实验研究 总被引：5，自引：0，他引：5
利用拖曳式水槽,采用激光粒子成像速度场测量系统(PIV),模拟了街谷存在地面加热时流场特征;讨论了环境风场对其的影响。我们发现在静风条件下,街谷中环流完全由热力驱动,对流活动可伸展至街谷上方;在建筑物层顶以上,也可发现水平和垂直方向的运动。这些对流活动有助于基本风场为零时,街谷内外动量和物质的交换。当街谷较宽时,对流形成的涡旋可能为两个以上,形态较为复杂并随时间变化,当街谷变窄时,涡旋蜕化成只有一个。当有弱环境风场存在时,街谷中的对流呈现为一个主涡旋,随着风速增加,涡旋形状更加规则,其中心并向下风向移动。 相似文献
8.
The effects of a building's density on urban flows are investigated using a CFD model with the
RNG k-ε turbulence closure scheme. Twenty-seven cases with different building's density parameters
(e.g., building and street-canyon aspect ratios) are numerically simulated. As the building's density parameters
vary, different flow regimes appear. When the street canyon is relatively narrow and high, two counter-rotating
vortices in the vertical direction are generated. The wind speed along streets is mainly affected by the
building's length. However, it is very difficult to find or generalize the characteristics of the street-canyon
flows in terms of a single building's density parameter. This is because the complicated flow patterns appear
due to the variation of the vortex structure and vortex number. Volume-averaged vorticity magnitude is a very
good indicator to reflect the flow characteristics despite the strong dependency of flows on the variation of
the building's density parameters. Multi-linear regression shows that the volume-averaged vorticity magnitude
is a strong function of the building's length and the street-canyon width. The increase in the building's
length decreases the vorticity of the street-canyon flow, while, the increase in the street-canyon width
increases the vorticity. 相似文献
9.
Hrvoje Kozmar 《Theoretical and Applied Climatology》2011,106(1-2):95-104
The boundary layer wind tunnel at the Technische Universit?t München was tested for atmospheric boundary layer (ABL) simulations. The ABLs developing above rural, suburban, and urban terrains were reproduced using the Counihan method, i.e., castellated barrier wall, vortex generators, and a fetch of surface roughness elements. A series of flow-characteristic evaluations was performed to investigate the flow development and uniformity. Experimental results presented as mean velocity, turbulence intensity, integral length scale of turbulence, Reynolds stress, and power spectral density of velocity fluctuations were compared with the ESDU data and/or theoretical models. Generated ABL wind-tunnel simulations compare well with the rural, suburban, and urban ABLs. In the test section area used for experiments on structural models, the ABL simulation is developed and uniform. Results of this study indicate the boundary layer wind tunnel at the Technische Universit?t München can be successfully employed in a broad spectrum of engineering, environmental, and micrometeorological studies, where it is required to accurately reproduce ABL characteristics. 相似文献
10.
应用雷诺应力湍流模型,模拟了不同高度比的城市街道峡谷的气流场。结果表明:峡谷的对称性对其内部气流场有显著影响。前高后低型峡谷下部为逆时针旋涡,上部为顺时针旋涡,峡谷越深,流场发展的越充分;峡谷内部墙面存在明显的驻点。前低后高型峡谷只存在一个大的顺时针旋涡,随着峡谷的加深,内部气流速率有减小的趋势;峡谷达到一定深度后出现驻点。对称型峡谷内部形成了顺时针旋涡,强度不大;随着峡谷的加深,内部流场转为一顺一反2个旋涡的二元结构;仅当峡谷很深时才出现明显驻点。前低后高型峡谷的气流场形式更有利于污染物的迁移、扩散,在城市规划中应尽量结合主导风向设计这类建筑布局。 相似文献
11.
影响台风移动因子的数值研究 总被引：1，自引：1，他引：0
本文采用有辐散的正压原始方程模式对影响台风涡旋移动的因子进行了数值试验,试验结果表明:(1)在无环境风条件下,台风涡旋向西北方向移动,移速与台风切向风速、台风半径和台风区f场的特征有关,台风外区(指最大风速半径以外台风区域)切向风速越大、台风半径越大,台风涡旋移速向北分量越大。(2)均一环境风场中,台风移动受基本气流的平流、β效应和指向引导气流左侧的横向加速度的作用,其中基本气流的引导作用是主要的。台风移速与引导气流速度比在东风气流中要大于西风气流中,而台风移向与基本气流之间的偏角在西风气流中要大于在东风气流中。(3)台风涡旋有沿基本气流绝对涡度梯度方向的次级运动分量。(4)台风涡旋在有切变的西风气流中比在均一西风气流中易发展加强,而它的移动更偏向引导气流的左侧。 相似文献
12.
东移低涡动力学的初步研究 总被引：6，自引：0，他引：6
用一个正压原始方程模式实施了六组试验，研究了东移低涡的动力学。结果表明：无论是切变基流与低涡的相互作用，还是涡块与低涡的相互作用，都可引起低涡强度在短暂时段内增强，但整个积分时段内低涡强度的演变仍呈下降趋势。切变基流、低涡和多个涡块的相互作用，可以改变下降的趋势。正相对涡度切变基流中低涡和涡块的合并，是东移低涡强度得以维持和发展的一个直接的原因。 相似文献
13.
On the bogusing of tropical cyclones in numerical models: The influence of vertical structure 总被引：4，自引：0，他引：4
Yuqing Wang 《Meteorology and Atmospheric Physics》1998,65(3-4):153-170
Summary In this study, idealised conditions are used to study the influence of vertical structure of the bogus vortex on its motion in numerical models by comparing the resultant forecast tracks. Two vortices were used: one has a cyclonic circulation throughout the troposphere and the other has an upper tropospheric anticyclone. Both vortices have the same structure in the middle and lower troposphere. The two vortices were inserted into four different environmental flows on a beta-plane: (a) a resting atmosphere; (b) a uniform flow; (c) a horozontal shear flow and (d) a vertical shear flow. The results show that the forecast tracks are very sensitive to the vertical structure of the bogus vortex, especially when the environmental flow is very weak, or is westerly and has a cyclonic horizontal shear. However, this sensitivity is reduced in moderate vertical shear. This motion sensitivity is found to arise from the vertical coupling mechanism by which the upper-and lower-level circulations interact with each other when a horizontal displacement occurs between them.The vertical structure of the bogus vortex can also affect the intensity of the model cyclone, depending on the configuration of the environmental flow. In general, the bogus vortex without an upper-level anticyclone will intensify quicker and will develop more intense than the one with an upper-level anticyclone. The vertical coupling mechanism can result in different asymmetric rainfall pattern in cyclone core region depending on the vertical structure of the bogus vortex. The asymmetric divergent flow associated with these convective asymmetries may in turn further influence the vortex motion. It is suggested that care needs to be taken in determining the vertical structure of the bogus vortex in numerical models.With 14 Figures 相似文献
14.
An Improved Three-Dimensional Simulation of the Diurnally Varying Street-Canyon Flow 总被引：3，自引：3，他引：0
The impact of diurnal variations of the heat fluxes from building and ground surfaces on the fluid flow and air temperature distribution in street canyons is numerically investigated using the PArallelized Large-eddy Simulation Model (PALM). Simulations are performed for a 3 by 5 array of buildings with canyon aspect ratio of one for two clear summer days that differ in atmospheric instability. A detailed building energy model with a three-dimensional raster-type geometry—Temperature of Urban Facets Indoor-Outdoor Building Energy Simulator (TUF-IOBES)—provides urban surface heat fluxes as thermal boundary conditions for PALM. In vertical cross-sections at the centre of the spanwise canyon the mechanical forcing and the horizontal streamwise thermal forcing at roof level outweigh the thermal forces from the heated surfaces inside the canyon in defining the general flow pattern throughout the day. This results in a dominant canyon vortex with a persistent speed, centered at a constant height. Compared to neutral simulations, non-uniform heating of the urban canyon surfaces significantly modifies the pressure field and turbulence statistics in street canyons. Strong horizontal pressure gradients were detected in streamwise and spanwise canyons throughout the day, and which motivate larger turbulent velocity fluctuations in the horizontal directions rather than in the vertical direction. Canyon-averaged turbulent kinetic energy in all non-neutral simulations exhibits a diurnal cycle following the insolation on the ground in both spanwise and streamwise canyons, and it is larger when the canopy bottom surface is paved with darker materials and the ground surface temperature is higher as a result. Compared to uniformly distributed thermal forcing on urban surfaces, the present analysis shows that realistic non-uniform thermal forcing can result in complex local airflow patterns, as evident, for example, from the location of the vortices in horizontal planes in the spanwise canyon. This study shows the importance of three-dimensional simulations with detailed thermal boundary conditions to explore the heat and mass transport in an urban area. 相似文献
15.
S. P. Meacham K. K. Pankratov A. F. Shchepetkin V. V. Zhmur 《Dynamics of Atmospheres and Oceans》1994,21(2-3)
We show that an arbitrarily oriented ellipsoid of uniform potential vorticity, embedded in a background flow described by a quadratic streamfunction, is an exact solution of the quasigeostrophic equations governing motion in a uniformly stratified, unbounded fluid. This type of flow includes plane horizontal shear and strain as well as uniform vertical shear of a unidirectional horizontal flow. We derive ordinary differential equations describing the motion of such a vortex and discuss some aspects of their solutions. We note the existence of steady states (solutions in which the vortex is in equilibrium with the background flow), of periodic solutions near these steady states, of non-periodic trajectories which nervertheless remain in the vicinity of the steady states, and of solutions which represent the shearing out of the vortex by the background flow. We try to use this information to propose partial answers to the question of when a given horizontal or vertical shear flow is likely to destroy a vortex and when a vortex might survive external shear and strain. 相似文献
16.
Summary Conceptual models of blocking structures are constructed by reducing the two-dimensional atmospheric vorticity field to a
few point vortices. The flow is assumed to be barotropic and divergence-free, and a blocking event is represented by a point
vortex dipole. The focus is here on the motion of the blocking dipole under the influence of the zonal mean flow. This is
modelled in three different ways: A dipole embedded in a latitude-dependent zonal mean flow exhibits neutrally stable oscillations;
their period is estimated analytically. A cyclonic point vortex approaching from upstream can either pass the dipole or break
it up, so that an Ω-shaped pattern of three vortices emerges. The stationarity of a blocking between two troughs is modelled
by four point vortices.
These low-order point vortex models are compared with the dynamics of real blockings in case studies. Despite their high degree
of simplification, those models reproduce the kinematics of blocking events properly. This results from the discretization
of the flow to its actual physical states, the vortices, in contrast to the common, purely mathematical discretization to
grid points. Thus, point vortex dynamics are proposed to be a powerful completion of continuous fluid dynamics in explaining
blocking events.
Received August 30, 1999 Revised December 22, 1999 相似文献
17.
Summary The horizontal and vertical structures of an upper air cold core vortex over the subtropical latitudes of South America in
the winter of 1999 reveal many features that are different from tropical upper tropospheric cold core vortices. The vortex
in the present study is observed poleward of the subtropical jetstreak. In the middle troposphere, the center of the vortex
is cooler than the periphery by 6 °C. The relative vorticity is greatest just below the tropopause and the vortex presents
a slight eastward tilt in the vertical. The CPTEC Eta regional model simulation dataset has been used to study the life cycle
of the vortex. Intensification and movement of the system are well simulated by the combined effect of divergence and vorticity
advection at 500 hPa. The Lifted Index and CAPE values at the mature stage of the system are not sufficiently high to explain
the precipitation associated with the vortex. However, Q-vector analysis of the model simulation datasets support the observed
precipitation. The precipitation pattern closely follows the 700 hPa vertical velocity. These results indicate that the rain
associated with the vortex has been forced by dynamical lifting rather than by thermodynamic processes. Upper tropospheric
stable conditions are responsible for the shallowness of convection. The vortex presented here is different from subtropical
cyclones, which develop over oceans and have higher intensities. The present vortex development somewhat resembles lee development. 相似文献
18.
A two-dimensional, time-dependent flow model coupled with a radiative transfer module has been applied to examine the characteristics of nocturnal flow in a steep canyon in the Rocky Mountains in Colorado. The effect of nighttime surface cooling on drainage flow is examined and compared with observations. In a complementary study, tracer data have been analyzed to estimate the mass flux from a tributary canyon and to examine processes of transport and diffusion. Simulations indicate that the strength and structure of the drainage wind are controlled mainly by terrain features, ambient wind conditions, and effective radiative cooling rates. The transport of tracer from a lower secondary vortex to an upper primary vortex is largely controlled by diffusional processes; removal of tracer from the canyon is controlled by the primary vortex and its interaction with the ambient wind. Differences between mass fluxes from model simulations and those calculated from experiments involve uncertainties in both the structure of the model and the analysis of data. 相似文献
19.
Wave ray theory is employed to study features of propagation pathways(rays) of vortex Rossby waves in typhoons with asymmetric basic flow, where the tangential asymmetric basic flow is constructed by superimposing the wavenumber-1 perturbation flow on the symmetric basic flow, and the radial basic flow is derived from the non-divergence equation. Results show that, in a certain distance, the influences of the asymmetry in the basic flow on group velocities and slopes of rays of vortex Rossby waves are mainly concentrated near the radius of maximum wind(RMW), whereas it decreases outside the RMW. The distributions of radial and tangential group velocities of the vortex Rossby waves in the asymmetric basic flow are closely related to the azimuth location of the maximum speed of the asymmetric basic flow, and the importance of radial and tangential basic flow on the group velocities would change with radius. In addition, the stronger asymmetry in the basic flow always corresponds to faster outward energy propagation of vortex Rossby waves. In short, the group velocities, and thereby the wave energy propagation and vortex Rossby wave ray slope in typhoons, would be changed by the asymmetry of the basic flow. 相似文献
20.
为进一步认识高原涡对盆地西南地区暴雨过程的影响,总结该区域暴雨预报经验,本文利用2001～2011年高原涡切变线年鉴、MICAPS实况天气图、盆地西南地区气象站日降雨量资料以及NCEP再分析资料,对引起盆地西南地区暴雨过程的高原涡特征进行总结分析,得到结论:1)引发盆地西南地区产生暴雨量级以上降雨的高原涡过程多发生在每年7月;高原涡东移将对盆地西南地区产生明显降雨;48小时后大部分高原涡减弱消失,少数继续东移或东南移;2)引发盆地西南地区产生暴雨的高原涡通常是暖性高原涡,高原涡东移48小时后有明显的冷平流入侵转变成斜压性低涡;这一类高原涡常常与高原切变、西南低涡、副高、低空急流以及南亚高压等影响系统相配合,共同作用产生一次暴雨过程;3)引发的盆地西南地区暴雨的高原涡过程的温湿场特征为:500hPa高原东部到盆地上空的大气高温高湿的特征明显,700hPa和850hPa盆地高温高湿,同时垂直上升运动旺盛且随高度向北倾斜。 相似文献