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1.
The dispersion of a uniform two-dimensional flow of carbon dioxide gas in air over a square two-dimensional obstacle was studied experimentally in an atmospheric boundary-layer wind tunnel. The obstacle Reynolds number was about 6000 based upon the undisturbed velocity at the height of the obstacle, and the Froude number was about 1.75. Carbon dioxide was injected vertically upward from the surface at a rate of 10% of the freestream velocity. The injection surface area began one obstacle length upstream and extended upstream one and one-half obstacle lengths. A gas concentration measurement system for carbon dioxide and air mixtures was developed for use in an atmospheric wind tunnel. Centerline steady-state concentration profiles were measured for various locations downstream of the leading edge of the obstacle. The maximum concentrations of carbon dioxide occurred atop the obstacle in the thin recirculation zone above the obstacle. Immediately behind the obstacle within the larger downstream recirculation zone, concentration levels significantly decreased. Further downstream, the concentration levels continued to diminish.Experimental results were compared with the numerical solutions of Sutton et al. (1986). The experimental measurements of concentration were lower than the predicted results of the numerical solutions, which may be attributed to the higher value of the Froude number present in the experimental measurements.Associate Professor.  相似文献   

2.
Flow over a two-dimensional obstacle and dispersion of a heavier-than-air gas near the obstacle were studied. Two species, one representing air and the other representing the heavier-than-air gas were treated. Equations for mass and momentum were cast in mass-averaged form, with turbulent Reynolds stresses and mass fluxes modeled using eddy-viscosity and diffusivity hypotheses. A two-equation k- turbulence model was used to determine the effective turbulent viscosity. Streamline curvature and buoyancy corrections were added to the basic turbulence formulation. The model equations were solved using finite difference techniques. An alternating-direction-implicit (ADI) technique was used to solve the parabolic transport equations and a direct matrix solver was used to solve the elliptic pressure equation.Mesh sensitivities were investigated to determine the optimum mesh requirements for the final calculations. It was concluded that at least 10 grid spaces were required across the obstacle width and 15 across the obstacle height to obtain valid solutions. A non-uniform mesh was used to concentrate the grid points at the top of the obstacle.Experimental measurements were made with air flow over a 7.6 by 7.6 cm obstacle in a boundary-layer wind tunnel. Smoke visualization revealed a low-frequency oscillation of the bubble downstream of the obstacle. Hot-wire anemometer data are presented for the mean velocity and turbulent kinetic energy at the mid-plane of the obstacle and the mid-plane of the downstream recirculation bubble. A single hot-wire probe was found to be suitable for determining mean streamwise velocities with an accuracy of 11 %. The downstream recirculation bubble was unsteady and had a length range from 3 to 8 obstacle lengths.The experimental results for flow over the obstacle were compared with numerical calculations to validate the numerical solution procedure. A sensitivity study on the effect of curvature correction and variation of turbulence model constants on the numerical solution was conducted. Calculations that included the curvature correction model gave a downstream recirculation bubble length of 5.9 obstacle lengths while excluding the correction reduced this length to 4.4.In the second part of the study, numerical calculations were performed for the dispersion of a heavier-than-air gas in the vicinity of the two-dimensional obstacle. Characteristics of an adiabatic boundary layer were used in these calculations. The densities of the contaminant gases were 0, 25 and 50% greater than the air density. Calculations were performed with the contaminant injection source upstream and downstream of the obstacle.Use of the pressure gradient model reduced the size of the dense gas cloud by as much as 12%. The curvature correction model also affected the cloud expanse by reducing the effective turbulent viscosity in the downstream recirculation bubble. The location of the injection source had the largest impact on the cloud size. The area of the cloud within the 5 % contour was three times larger for downstream injection than for upstream injection.  相似文献   

3.
Field and laboratory studies have been conducted to investigate the effect of surrounding buildings on the plume rise from low-level buoyant sources, such as distributed power generators. The field experiments were conducted in Palm Springs, California, USA in November 2010 and plume rise from a 9.3 m stack was measured. In addition to the field study, a laboratory study was conducted in a water channel to investigate the effects of surrounding buildings on plume rise under relatively high wind-speed conditions. Different building geometries and source conditions were tested. The experiments revealed that plume rise from low-level buoyant sources is highly affected by the complex flows induced by buildings stationed upstream and downstream of the source. The laboratory results were compared with predictions from a newly developed numerical plume-rise model. Using the flow measurements associated with each building configuration, the numerical model accurately predicted plume rise from low-level buoyant sources that are influenced by buildings. This numerical plume rise model can be used as a part of a computational fluid dynamics model.  相似文献   

4.
Results are presented from two sets of laboratory model experiments on the effects of an isolated seamount upon the flow of an intermediate-water slope current along a continental shelf. The experimental results for initial ambient conditions of respectively two-layer and linearly stratified fluids show that the structure of such a boundary current depends primarily on the values of the appropriate set of dimensionless dynamical parameters (namely the Burger (Bu), Ekman (Ek) and Rossby (Ro) numbers), as well as the dimensionless lateral separation of the seamount and shelf and the proportional height of the seamount relative to the distance from the bottom at which the intermediate-water flows. Comparisons of the present results with those from a previous two-layer fluid study with no obstacle present reveals that the presence of the obstacle does not alter significantly the stability of the current even when situated close to the shelf. However, for such configurations, the density, velocity and vorticity fields in the local zone of interaction between the current and the obstacle are distorted significantly by the presence of the obstacle, provided that the summit of the obstacle penetrates the level of current flow. Measurements of density, velocity and vorticity fields show no significant dependence of the flow interaction upon the detailed bathymetry of the shelf-slope. For stable intermediate-water slope currents, the nature of the interaction with the obstacle is determined primarily by (i) the lateral separation of the obstacle and the shelf edge and (ii) the Ro of the flow. For sufficiently low values of the former and high values of the latter, the interaction results in a splitting of the incident flow around the obstacle, with cyclonic and anticyclonic eddy pairs being generated in the lee. Geostrophic equilibrium is seen to be maintained in the current, even in the near wake of the obstacle. For cases in which the summit of the seamount is below the initially-undisturbed intermediate water level, no Taylor column-like division of the slope current occurs and no significant distortion of the current structure (velocity and density) occurs for the parameter ranges investigated. For linearly stratified cases, measurements show that no significant local elevation or depression of the density interfaces is observed in the interaction zone. The distributions of the local buoyancy frequencies calculated from the density profiles reveal that the minimum value of the frequency upstream of the obstacle is smaller than that downstream, indicating that the flow interactions generate local mixing downstream, with consequent erosion of the density interfaces.  相似文献   

5.
Results of laboratory and numerical experiments on both homogeneous and density-stratified flow over single, bluff obstacles of various shapes are presented. The obstacle height is in most cases of the same order as the base diameter and the major controlling (flow) parameter is the Froude number, defined here as Fh=U/Nh, where U is the (uniform) upstream velocity, h the obstacle height and N is the buoyancy frequency. Attention is concentrated, firstly, on the case of homogeneous flows over rather weakly tapered obstacles and, secondly, for bodies whose height is similar to their base width, on the case Fh=0.1, representing stratification sufficiently strong that lee-wave motions do not play a significant role in the flow dynamics. For right-circular cones it is shown that the sectional contributions to the total fluctuating side force (lift) show significant phase variations up the height of the obstacle, which are not always reflected in the developed vortex street further downstream. For some obstacle shapes, the vortex lines linking the von Karman eddies at different heights can be significantly tilted, particularly in the upper part of the wake. Vortex convection speeds do not appear generally to vary greatly with height and, as found in previous work, the shedding frequency remains constant with height, despite the strong variation of cross-stream obstacle width. By comparison with the homogeneous results, it is suggested that the stratification enhances the shedding instability, which would otherwise be very weak for squat obstacles, but does not annihilate the ability of the flow at one level to influence that at another.  相似文献   

6.
An experimental study has been made of stagnation points and flow splitting on the upstream side of obstacles in uniformly stratified flow. A range from small to large values of Nh/U (where N is the buoyancy frequency, hm is the maximum obstacle height and U is the undisturbed fluid velocity) has been covered, for three obstacle shapes which are, respectively, axisymmetric, and elongated in the across-stream and in the downstream directions. Upstream stagnation for the first two of these models does not occur until Nhm/U > 1.05, where it occurs at z ≈ hm/2. On the central line below this point the flow descends and diverges, and we term this ‘flow splitting’. For the third model (elongated in the downstream direction), stagnation upstream first occurs at Nhm/U ≈ 1.43, at z ≈ 0. Results for this obstacle are not consistent with the ‘Sheppard criterion’, and this upstream flow stagnation is not apparently related to lee wave overturning, in contrast to flow over two-dimensional obstacles.  相似文献   

7.
Modification of a turbulent flow upstream of a change in surface roughness has been studied by means of a stream function-vorticity model.A flow reduction is found upstream of a step change in surface roughness when a fluid flows from a smooth onto a rough surface. Above that layer and above the region of flow reduction downstream of a smooth-rough transition, a flow acceleration is observed. Similar flow modification can be seen at a rough-smooth transition with the exception that flow reduction and flow acceleration are reversed. Within a fetch of –500 < x/z 0< + 500 (z 0 is the maximum roughness length, the roughness transition is located at x/z 0 = 0), flow reduction (flow acceleration) upstream of a roughness transition is one order of magnitude smaller than the flow reduction (flow acceleration) downstream of a smooth-rough (rough-smooth) transition. The flow acceleration (flow reduction) above that layer is two orders of magnitude.The internal boundary layer (IBL) for horizontal mean velocity extends to roughly 300z 0 upstream of a roughness transition, whereas the IBL for turbulent shear stress as well as the distortion of flow equilibrium extend almost twice as far. For the friction velocity, an undershooting (overshooting) with respect to upstream equilibrium is predicted which precedes overshooting (undershooting) over new equilibrium just behind a roughness transition.The flow modification over a finite fetch of modified roughness is weaker than over a corresponding fetch downstream of a single step change in roughness and the flow stays closer to upstream equilibrium. Even in front of the first roughness change of a finite fetch of modified roughness, a distortion of flow equilibrium due to the second, downwind roughness change can be observed.  相似文献   

8.
Coherent Turbulent Structures Across a Vegetation Discontinuity   总被引：3，自引：2，他引：1  
The study of turbulent flow across a vegetation discontinuity is of significant interest as such landscape features are common, and as there is no available theory to describe this regime adequately. We have simulated the three-dimensional dynamics of the airflow across a discontinuity between a forest (with a leaf area index of 4) and a clearing surface using large-eddy simulation. The properties of the bulk flow, as well as the large-scale coherent turbulent structures across the forest-to-clearing transition and the clearing-to-forest transition, are systematically explored. The vertical transport of the bulk flow upstream of the leading edge gives rise to the enhanced gust zone around the canopy top, while the transport downstream of the trailing edge leads to the formation of a recirculation zone above the clearing surface. The large-scale coherent structures across the two transitions exhibit both similarities with and differences from those upstream of the corresponding transition. For example, the ejection motion is dominant over the sweep motion in most of the region 1?<?z/h < 2 (h is the canopy height) immediately downstream of the trailing edge, much as in the forested area upstream. Also, the streamwise vortex pair, which has previously been observed within the canopy sublayer and the atmospheric boundary layer, is consistently found across both transitions. However, the inflection observed both in the mean streamwise velocity, as well as in the vertical profiles of the coherent structures in the forested area, disappears gradually across the forest-to-clearing transition. The coherence of the turbulence, quantified by the percentage of the total turbulence kinetic energy that the coherent structures capture from the flow, decreases sharply immediately downstream of the trailing edge of the forest and increases downstream of the leading edge of the forest. The effects of the ratio of the forest/clearing lengths under a given streamwise periodicity on flow statistics and coherent turbulent structures are presented as well.  相似文献   

9.
This paper describes an experimental investigation of the behaviour of the statistics of concentration fluctuations in a passive plume dispersing over a two-dimensional hill of moderate steepness. Recently developed high frequency response Flame Ionization Detector (FID) technology with a frequency response in excess of 200 Hz was utilized to obtain an extensive set of measurements of the mean and fluctuating plume concentrations. Plumes dispersing over flat terrain and over a hill with a maximum slope of 0.3 were studied. For both cases, extensive turbulent flow measurements were also carried out.The measured mean plume concentration profiles were of a generally Gaussian form and showed the expected effects of surface reflection for the flat terrain and hill. Plume intermittency and concentration fluctuation intensity were calculated at all measurement locations. Conditional and unconditional plume concentration statistics were calculated. The conditional (in-plume) concentrations and intensities were more uniform with height than for the unconditional ones.  相似文献   

10.
Summary  Heavy precipitation events to the south of the Alps are usually associated with a southerly pre-frontal low-level jet advecting moisture toward the southern slopes of the Alps. Here we use idealised numerical simulations to assess the nature of the associated flow regimes and the mechanisms leading to vertical lifting and precipitation. The idealisations comprise: a simplified arc-shaped barrier-like orographic obstacle of Alpine scale; neglection of the tropopause; a stationary two-dimensional upstream flow configuration that includes a frontal structure and a low-level jet; hydrostatic dynamics with free-slip lower boundary conditions; and a simplified set of parameterizations to address dry, moist absolutely stable, and moist conditionally unstable upstream flow configurations. Within the dry dynamics, typical settings lead to Alpine-scale flow splitting with pronounced left/right asymmetries with respect to the incident southerly flow. Strong vertical lifting occurs over the western portion of the upstream slopes, within the stream of air that tries to circum go the elongated obstacle on the western flank. Thus, despite belonging to the “flow-around” regime, these flow configurations can exhibit vertical lifting over the whole height of the obstacle. The responsible asymmetry is primarily induced by the Coriolis effect in the presence of an elongated mountain, but it can further be intensified by the impinging low-level jet and the arc-shape of the Alpine topography. With a conditionally unstable moist upstream profile, the flow is able to surmount the obstacle without pronounced horizontal deflections. Maximum precipitation rates of are obtained. When moist convection is suppressed by using a moist absolutely stable upstream profile, the flow is again substantially deflected and shows the typical characteristics of the dry flow regime discussed above, with somewhat reduced precipitation rates as compared to the convective case. Overall there is evidence that the asymmetry introduced by the Coriolis effect, a pronounced low-level jet, and a moist upstream profile, all facilitate vertical lifting and thereby provide a suitable environment for heavy condensation and precipitation. Received March 22, 1999/Revised August 18, 1999  相似文献   

11.
A study of turbulent dispersion over hills for upstream, elevated sources was conducted, based on wind tunnel tracer gas (CO2) experiments over a gentle 2-D ridge and a 3-D circular hill, both having a cosine-square cross-section. The concentration measurements were made with four different source locations for each hill case (2-D or 3-D), and the study focused on dispersion parameters under the influence of the presence of the hills in order to provide a better understanding of the mechanisms involved.The wind tunnel measurements show that, in the case of gentle hills, the topographic impact on turbulent dispersion from upstream sources is only moderate and is more pronounced for the 3-D than for the 2-D hill. The perturbation in mean flow introduced by the hills, including streamline divergence/convergence, is shown to dominate the changes in the dispersion due to the hills in this case. The plume spread, both in the lateral and the vertical, is enhanced over the upwind hill foot and reduced over the hill top in response to the mean flow slow-down and speed-up at these places, and is further enhanced or reduced due to streamline divergence/convergence in the vertical over the hills as well as in the horizontal over the 3-D hill. These results are also compared with cases of turbulent dispersion over more steep hills (Snyder and Britter, 1987).  相似文献   

12.
北京325 m气象塔塔体对测风影响的数值模拟   总被引：1，自引：0，他引：1  
安装在气象观测塔上的仪器进行风速测量时,气象塔塔体本身会对流场有一定的影响,使其附近局部流场发生变化,产生绕流和尾流,导致所测风场数据相对于真实风场值失真,利用计算流体力学软件Fluent对北京325m框架式气象塔周围的风场进行了模拟,给出流向风速在该塔伸臂上测风位置的失真情况,及风速失真大小随风向风速的变化规律,计算表明若伸臂处于迎风面,在测风点上的风速误差均小于5%,与实际观测一致,验证了北京325m气象塔风速伸臂设计的合理性。  相似文献   

13.
Results are presented from an experimental investigation of turbulent dispersion of a saline plume of large Schmidt number (Sc=830) in a turbulent boundary-layer shear flow simulated in a laboratory water channel. The dispersion measurements are obtained in a neutrally buoyant plume from an elevated point source over a range of downstream distances, where both plume meandering and fine-structure variations in the instantaneous plume are important. High-resolution measurements of the scalar fluctuations in the plume are made with a rake of conductivity probes from which probability distributions of concentration at various points throught the plume are extracted from the time series.Seven candidate probability distributions were tested, namely, the exponential, lognormal, clipped normal, gamma, Weibull, conjugate beta, andK-distributions. Using the measured values of the conditional mean concentration, , and the conditional fluctuation intensity,i p , the Weibull distribution provided the best match to the skewness and kurtosis over all downstream fetches. The skewness and kurtosis were always overpredicted by the lognormal probability density function (pdf), and underpredicted by the gamma pdf. The conjugate beta distribution for which the model parameters are determined using a method of moments based on the fluctuation intensity,i p , and skewness,S p , was capable of modeling the distribution of scalar concentration over a wide range of positions in the plume.  相似文献   

14.
A systematic investigation of the effects of various parametrizations of dissipation, e.g. quadratic and linear frictional drag, harmonic lateral viscosity, and harmonic lateral diffusion on inertial flow over a sill and possible hydraulic control is presented. Rotation effects are ignored and the geometry is assumed to vary only slowly with downstream distance so that the flow may be considered one-dimensional. Results are given both for a single-active layer and for two-active layers with a rigid lid.If the parametrization is only a function of the dependent variables and not of their spatial derivatives, then it may be possible to hydraulically control the flow. A general expression is derived for the possible control point and the two gradients there, which are functions of the slope and possibly of flow rate. Specific energy is irreversibly removed from the flow and non-controlled as well as controlled flows can exhibit significant asymmetry in fluid depth over a sill. The upstream specific energy, and hence depth of the lower layer, of the controlled flow is greater than for an ideal fluid. Frictional effects modify the behaviour of long gravity waves, such that they are dispersive and damped with time. The system will only exhibit hydraulic control if these effects are small.For a viscous single layer of fluid, the gradient in surface elevation is always uniquely defined, so classically defined hydraulic control, as such, cannot exist. However, for values of non-dimensional lateral eddy viscosity coefficient, , where q is the flow rate, there is a narrow band of specific energies centred around that for the control solution in an ideal fluid, Ecrit, for which the surface elevation, h is very asymmetric over the sill; the solutions resemble the inviscid, hydraulically controlled solutions. Outside this range, either the fluid depth tends to zero, or the surface elevation is almost uniform over the sill. A ‘control’-type solution exists which has the conjugate values of the inviscid equation up- and downstream of the sill, where the gradient in fluid depth, and hence the viscous term, is zero. For larger values of AM, the band of specific energies is much wider, and the upstream specific energy of the ‘control’-type solution is much lower than that for an inviscid fluid. Long gravity waves are dispersive and damped with time. There is a short-wave cut-off, k2 > h/(4AM2), above which waves are stationary in the flow. Longer waves, k2 h/(4AM2), are critical if , as for an ideal fluid. If these waves can propagate significant distances, then any observed asymmetry in h will be due to inertial and not to viscous effects. The behaviour of unidirectional, two-layer flow is similar. The governing equation for viscous, two-layer exchange flow is singular, and typically excludes the ‘control’-type solutions found for unidirectional flows.Establishing the existence and behaviour of steady inertial flows in the presence of lateral diffusion between layers is more difficult. It significantly alters the single-layer solutions once the non-dimensional coefficient AH is large, i.e. . The flow rate may become zero on the downslope as all the fluid diffuses into the inert, infinitely deep, overlaying layer. The fluid depth is maintained by reverse flow from downstream. In this case, the depth of the active layer tends to zero downstream for all values of specific energy. For two-layer flow, both unidirectional and exchange, the governing equation is such that the lower-layer flow rate and interfacial height return to their upstream values.Motivation for the study is provided by the increasingly fine spatial resolution achievable in large-scale numerical models of the ocean general circulation, and the question of whether they are capable of simulating some form of hydraulic control. Application to modelling oceanic flows over a sill is discussed.  相似文献   

15.
The evolution of a sea-breeze front(SBF)in parallel and offshore environmental flows was investigated by using high-resolution simulations of two SBF cases from the Bohai Bay region,China.The results show that the combination of a distinct vertical wind shear caused by the sea-breeze circulation with a neutral or slightly stable atmospheric stratification associated with the thermal inner boundary layer promoted the occurrence and maintenance of a Kelvin-Helmholtz billow(KHB).In a parallel environmental flow,the SBF evolved into a few connected segments because of the inhomogeneity of the sea-breeze direction and intensity as it penetrated inland.A significant upward vertical motion occurred at the two ends of the SBF segment owing to the sea-breeze convergence and was accelerated by the KHB.The KHB made a notable contribution to the intensity at the ends of the segment,whereas the intensity at the middle segment was primarily attributed to the convergence between the sea breeze and the parallel flow.In the offshore environmental flow,the clockwise rotation of the offshore flow varying with time increased the downstream convergence of the interface between the sea breeze and the offshore flow and pushed the downstream convergence zone to an orientation consistent with the offshore flow.The air parcels ascending from the downstream part of the SBF were continuously lifted by the downstream convergence zone during their advection,leading to a significant downstream development of the SBF.The significant upward vertical motion caused by the sea-breeze convergence behind the upstream end of the SBF was shifted to the upstream end of the SBF by the KHB,which enhanced the intensity of the upstream end of the SBF.  相似文献   

16.
李唐棣  谈哲敏 《气象学报》2012,70(3):536-548
在条件不稳定大气条件下,二维小尺度双脊钟形地形上空对流触发、传播和降水分布特征主要决定于地形上游基流强度、双脊地形配置形式、地形高度及其山谷宽度。双脊地形在沿基流方向上有两种配置:高脊地形位于上游和低脊地形位于上游。对于高脊地形位于上游的双脊地形,上游高地形将起主导作用,山地上空对流及降水特征与单脊地形类似。对于低脊地形位于上游的双脊地形,上游低地形可明显地改变下游高地形的前方来流,同时,下游高地形也能够对上游低地形背风侧流动产生影响,从而导致出现地形上空复杂的对流传播、降水分布特征。对于低脊地形位于上游的双脊地形,其山谷宽度主要决定了双脊地形与单脊地形之间在对流、降水分布等的差异;当山谷宽度较小时,双脊地形可以近似为一个包络地形,此时地形上空的对流、降水特征与单脊地形类似;当山谷宽度较大时,双脊上空流动相互影响较小,此时双脊地形可以分成两个单脊地形;当山谷宽度在一定范围内,其上空的对流及其降水分布与单脊地形有明显差异。对于低脊地形位于上游、中等山谷宽度的双脊地形上空降水主要呈现4种类型:(1)山谷与低脊迎风坡降水;(2)高脊迎风坡降水;(3)低脊山峰与高脊迎风坡降水;(4)低脊背风侧、双脊山峰准静止降水。  相似文献   

17.
条件不稳定湿大气中三维理想地形上空对流的动力学特征   总被引：5，自引：1，他引：4  
董继立  谈哲敏 《气象学报》2008,66(3):293-309
条件不稳定湿大气情况下,气流经过三维地形可以形成不同性质的对流系统以及不同特征的地形流结构,其对流系统、地形流的性质主要取决于地形上空的对流触发、对流-地形流-重力波三者之间的相互作用,同样这些过程对于地形降水的性质、分布起重要的作用.根据不同湿Fr数(Froude number),湿条件不稳定大气经过三维小尺度山地上空时其对流和地形流动存在4种不同的流域(flow regirnes):(1)下游传播对流模态;(2)上游传播和下游传播共存对流模态;(3)山峰附近准静止和下游传播共存对流模态;(4)下坡稳定和下游传播对流共存模态.地形上空对流系统主要可以通过两种不同机制形成:(1)地形直接的抬升或减速作用;(2)在地形流形成后,由于地形流本身特性(如上游分离、背风涡旋和下坡重力波破碎)触发.在较大的Fr数情况下,地形上空对流生成后反过来可以破坏上、下游的地形流结构,但对背风坡的重力波破碎影响较小.不同初始对流有效位能(CAPE)不仅可以影响对流系统的传播、发展,而且可以影响整体地形流性质.较低的初始CAPE有利于地形流的形成,此时对流对地形流结构特征的影响相对较小,其流场性质与低Fr数流域性质相似.  相似文献   

18.
Summary. ?The airflow over an idealized orography with two mountain peaks and a valley between is investigated using a non-linear numerical model. The flow is assumed to be two-dimensional and nonrotational. Surface friction is neglected. This setup is a first step in studying the modifications a finely structured “real” topography introduces to the well-studied flow over one isolated obstacle. The sensitivity of the flow behavior to the valley width is examined for the case of specified mountain volume as well as constant non-dimensional mountain height. Flow patterns for linear, weakly nonlinear, wave breaking and upstream blocking cases are examined. Whereas the nondimensional mountain height is still the main measure of the nonlinearity of the flow, the differing steepness of upslope and downslope caused by the separating valley, strengthens nonlinear effects. It also modifies wave breaking and upstream blocking. For wide enough valleys wave breaking regions can form above both peaks. Received January 20, 1999/Revised June 28, 1999  相似文献   

19.
The Bolund Experiment,Part I: Flow Over a Steep,Three-Dimensional Hill   总被引：1，自引：0，他引：1  
  相似文献   

20.
An analysis of wind tunnel data of dispersion from elevated point sources over a flat floor and gently sloping, 2-D hills is performed. (The data were obtained by Khurshudyan et al., 1981.) Formulas for the mixing lengths and the Lagrangian time scales are tested, suitable for use in various dispersion models. Some expressions for the vertical first moments of the plume concentration distribution suitable for shear flow (Hunt, 1985) are also tested.Then, a normalization is suggested, based on the source mean flow and turbulence parameters, for the ground-level concentration maximum value and position along the plume centerline. Using this normalization, the maximum position is almost constant, regardless of source height variation and of whether the hill is present or not, at least for source positions upstream or over the top.The maximum values allow the determination of normalized terrain amplification factors TAFN, which are shown to be in most cases closer to one than the corresponding TAF obtained without normalization.  相似文献   

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