Analysis of wind tunnel dispersion data over two-dimensional obstacles

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 TAF_N, which are shown to be in most cases closer to one than the corresponding TAF obtained without normalization.     

A wind tunnel study of turbulent flow over model hills

Detailed wind tunnel measurements have been made of mean flow and turbulence over a two-dimensional ridge and a circular hill, both having cosine-squared cross-section and maximum slope about 15 °. The measurements were made in an artificially thickened neutrally stratified boundary layer, and have been compared with results from linear models and rapid distortion theory as appropriate.Our study shows that linear theory gives generally good predictions of the mean flow on the upwind side of the hills, and especially of the flow speedup at the hill top, but that the turbulence is less well predicted. In particular, the measurements show a major increase in the vertical component of turbulence and in the shear stress on the upwind slope of both the two- and three-dimensional hills which is not predicted by either equilibrium or isotropic rapid-distortion theories, although this may be partly due to the effect of streamline curvature. Rapid-distortion theory is successful only in describing the streamwise component of turbulence in the outer region of the flow, while in the upper part of the inner region of the flow, the turbulence measurements show disagreement with both the equilibrium and the rapid-distortion theories. Our experiments also confirm that the equilibrium region is a very thin layer close to the surface, while above this region and below the outer region, there is a transitional region where all terms in the stress equation are important.The measurements over the three-dimensional hill suggest that the mean flow and turbulence are broadly similar to those over the two-dimensional ridge, but with reduced perturbation amplitudes. The major differences between the two cases are found on the upwind slope and in the wake where, respectively, horizontal divergence and convergence of the three-dimensional flow are most pronounced.     

A wind tunnel study of turbulent flow over a two-dimensional ridge

We present a wind-tunnel simulation of adiabatic atmospheric flow normal to a rough, two-dimensional ridge. The data are analyzed in physical streamline coordinates, which are described in some detail. The mean velocity speed-up on the hill top is adequately predicted by existing formulae while the behaviour of the wake flow fits into a pattern that emerges from other wind-tunnel experiments. The turbulent stresses evolve in response to the extra strain rates induced by the hill, streamline curvature and acceleration: % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWG1bWaaWbaaS% qabeaaceaIYaGbaebaaaaaaa!3456!\[u^{\bar 2}\]is coupled strongly to acceleration while % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaadaqdaaqaaiaadw% hacaWG3baaaaaa!3462!\[\overline {uw}\]and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWG3bWaaWbaaS% qabeaaceaIYaGbaebaaaaaaa!3458!\[w^{\bar 2}\]follow curvature. These differing responses lead to significant phase differences between the changes in the component stresses as the hill is traversed. An analogous response is seen in the components of turbulent stress divergence, which are computed as part of streamwise momentum budgets. Only very close to the surface is turbulent stress divergence comparable to the inertial and pressure terms in the momentum budget; over most of the flow regime, the mean flow response is approximately inviscid. Finally, we compare our results with data from other wind tunnel models and from real hills.     

A mixed spectral finite-difference model for pollutant concentrations over a hill

Given incident logarithmic profiles of wind and pollutant concentration above a rough, absorbing surface, the three-dimensional distribution of pollutant concentration over a hill of gentle slope is determined from a linearized model. The model is applied in neutrally stratified flow, without chemistry, and is integrated using spectral methods in the horizontal and a finite-difference scheme in the vertical. This approach allows for flexibility in choosing a closure scheme and a variety of surface boundary conditions. This was not possible in the analytic approach of Padro (1987) who added pollutant concentration and flux to the MS3DJH/1 model of Walmsley et al. (1980). The present model requires as input the turbulent kinetic energy, E, dissipation, , and the perturbation vertical velocity, w, from the three-dimensional boundary-layer flow model of Beljaars et al. (1987), hereinafter referred to as MSFD, The latter model also supplies wind velocity perturbations at the upper boundary, as input to upper boundary conditions on the pollutant flux perturbations.The present study describes applications of the model to idealized terrain features: isolated two- and three-dimensional hills and ridges and an infinite series of ridges. (Application to real terrain, however, presents no difficulties.) Comparisons were made with different (though uniform) surface roughnesses. Tests were performed to examine the effect of upstream terrain features in the periodic domain and to illustrate the importance of the vertical resolution of the output for interpreting results from the sinusoidal terrain case.     

三维非静力E—ε闭合模式对山体流场及浓度分布的模拟

本文采用三维非静力E-ε闭合模式,在对流层边界层方程组中,加入湍能和耗散率方程描述边界层中的湍流运动。求解孤立三维山地地形上的流场、湍流场和浓度场,并分析地形对气流的影响及对污染物浓度分布的影响。同时做了不同坡度和不同风速下的孤立三维山体的流场、湍流场和浓度场比较。数值计算结果表明,地形能改变气流运动,气流过山时将出现分支现象,山后背坡有反向回流出现,并且回流区大小随着山体坡度的增大而增大。山体对气流的阻挡作用随山体高度的增加而更明显,随风速的增加而减弱。湍能及耗散率随高度衰减较快。随着山体坡度增加,山前气流的垂直速度也增加,湍流运动更强。风速大时湍流运动越显著。污染物在山前出现辐散,迎风坡是浓度高值区,山后背风坡也是相对高值区。山坡度越大,山前迎风坡污物浓度越高。     

Turbulence and diffusion over inhomogeneous terrain

This paper provides an overview of some aspects of atmospheric boundary-layer dispersion processes over homogeneous and complex terrain. Special emphasis is placed on a discussion of the boundarylayer scaling regimes over homogeneous terrain and the characteristics of the dispersion processes associated with each of these regimes. The paper points out that vertical concentration profiles usually deviate substantially from a Gaussian distribution. The mean flow and turbulence over a low hill is dealt with, and in the inner layer the turbulence levels are increased due to the mean flow speed-up. In the outer layer the turbulence is modified by the rapid distortion effect. In a middle layer the turbulence is reduced due to the effect of a hill-induced streamline curvature. The paper concludes that the flow perturbations introduced by large-scale hills and valleys invalidate the use of simple approximations for describing atmospheric dispersion processes, and that it is necessary to utilize the full set of equations of motion.     

A comparison of wind-tunnel experiments and numerical simulations of neutral and stratified flow over a hill

A comparison is made of numerical and experimental results for flow over two-dimensional hills in both neutral and stably stratified flow. The numerical simulations are carried out using a range of one-and-a-half order and second-order closure schemes. The performance of the various turbulence schemes in predicting both the mean and turbulent quantities over the hill is assessed by comparing the results with new wind-tunnel measurements. The wind-tunnel experiments include both neutral and stably stratified flow over two different hills with different slopes, one of which is steep enough to induce flow separation. The dataset includes measurements of the mean and turbulent parts of the flow using laser Doppler anemometry. Pressure measurements are also made across the surface of the hill. These features make the dataset an excellent test of the model performance. In general second-order turbulence schemes provide the best agreement with the experimental data, however, they can be numerically unstable for steep hills. Some modifications can be made to the standard one-and-a-half order closure scheme, which results in improved performance at a fraction of the computation cost of the second-order schemes.     

Scalar Fluxes Near a Tall Building in an Aligned Array of Rectangular Buildings

Scalar dispersion from ground-level sources in arrays of buildings is investigated using wind-tunnel measurements and large-eddy simulation (LES). An array of uniform-height buildings of equal dimensions and an array with an additional single tall building (wind tunnel) or a periodically repeated tall building (LES) are considered. The buildings in the array are aligned and form long streets. The sensitivity of the dispersion pattern to small changes in wind direction is demonstrated. Vertical scalar fluxes are decomposed into the advective and turbulent parts and the influences of wind direction and of the presence of the tall building on the scalar flux components are evaluated. In the uniform-height array turbulent scalar fluxes are dominant, whereas the tall building produces an increase of the magnitude of advective scalar fluxes that yields the largest component. The presence of the tall building causes either an increase or a decrease to the total vertical scalar flux depending on the position of the source with respect to the tall building. The results of the simulations can be used to develop parametrizations for street-canyon dispersion models and enhance their capabilities in areas with tall buildings.     

Numerical studies of neutrally stratified planetary boundary-layer flow above gentle topography

A numerical model of planetary boundary-layer flow above two-dimensional gentle topography is developed as an extension of the surface-layer model described by Taylor (1977). Comparisons are made with surface-layer predictions for flow over Gaussian hills; and the flow at various angles above hills, valleys and escarpments is modelled. Some simple case studies of the influence of gentle two-dimensional topography on pollutant dispersion are made which indicate relatively minor effects on surface pollutant concentrations in comparison with results for dispersion above a plane surface.     

The influence of topography on single-tower-based carbon flux measurements under unstable conditions: a modeling perspective

Based on high-resolution numerical simulations, the influence of topography on the one-dimensional (1-D) tower-based measurements of the net ecosystem–atmosphere exchange rate (NEE) of CO₂ was analyzed under unstable conditions. Airflow and transport of a passive scalar were simulated over undulating surfaces covered by tall trees. Compared to their flat surface counterparts, the wind and scalar mixing ratio fields are more disturbed over a steeper surface and/or under a weaker background wind condition, resulting in larger errors in the 1-D NEE estimation. The magnitude of the error is generally larger on the windward side than on the lee side when topography-induced circulation (TIC) does not occur. Applying the ensemble streamline coordinate system to estimating NEE could result in larger errors than applying the local earth coordinate system or terrain coordinate system when the atmospheric flow is significantly distorted. This is especially true when TIC occurs. NEE estimated from the 1-D framework under convective conditions with calm or weak background winds may incur significant errors even over gentle topography. Preference in the selection of a flux tower location is given to the crest area.     

Particle-Image Velocimetry Measurements of Separation and Re-attachment of Airflow over Two-Dimensional Hills with Various Slope Angles and Approach-Flow Characteristics

Airflow over two-dimensional hills was investigated in a wind tunnel using particle image velocimetry. We focus on the flow separation behaviour. A trapezoidal hill shape was used in most of the experimental runs, but the critical slope angle for flow separation was approximately the same as that established for smooth hill shapes. The re-attachment point of the separated flow became farther from the hill as the slope angle $\theta $ increased, reaching a saturation of about seven times the hill height for $\theta \gtrsim 60^\circ $ . Increasing the upwind surface roughness length was found to suppress flow separation. This tendency is analogous to the previous experimental results for turbulent boundary layers on flat plates. The boundary-layer thickness varied by the presence or absence of Counihan-type spires and a castellated fence at the test-section entrance had negligible effect on the flow separation.     

Scalar Transport over Forested Hills

Numerical simulations of scalar transport in neutral flow over forested ridges are performed using both a 1.5-order mixing-length closure scheme and a large-eddy simulation. Such scalar transport (particularly of CO₂) has been a significant motivation for dynamical studies of forest canopy–atmosphere interactions. Results from the 1.5-order mixing-length simulations show that hills for which there is significant mean flow into and out of the canopy are more efficient at transporting scalars from the canopy to the boundary layer above. For the case with a source in the canopy this leads to lower mean concentrations of tracer within the canopy, although they can be very large horizontal variations over the hill. These variations are closed linked to flow separation and recirculation in the canopy and can lead to maximum concentrations near the separation point that exceed those over flat ground. Simple scaling arguments building on the analytical model of Finnigan and Belcher (Q J Roy Meteorol Soc 130:1–29, 2004) successfully predict the variations in scalar concentration near the canopy top over a range of hills. Interestingly this analysis suggests that variations in the components of the turbulent transport term, rather than advection, give rise to the leading order variations in scalar concentration. The scaling arguments provide a quantitative measure of the role of advection, and suggest that for smaller/steeper hills and deeper/sparser canopies advection will be more important. This agrees well with results from the numerical simulations. A large-eddy simulation is used to support the results from the mixing-length closure model and to allow more detailed investigation of the turbulent transport of scalars within and above the canopy. Scalar concentration profiles are very similar in both models, despite the fact that there are significant differences in the turbulent transport, highlighted by the strong variations in the turbulent Schmidt number both in the vertical and across the hill in the large-eddy simulation that are not represented in the mixing-length model.     

NUMERICAL STUDY ON THE LOCAL WIND STRUCTURES FORCED BY THE COMPLEX TERRAIN OF QINGDAO AREA

In this paper, some 2-D features of the down- and up-slope winds and sea-land-breeze generated over the com-plex terrain of the Qingdao area and the interaction between them are numerically analysed by use of a 2-Dnon-hydrostatic mesoscale model. The simulated results in the west-east vertical cross-section show that (1) when thelarge-scale wind is a southerly gentle one, the generated easterly down-slope wind is much stronger than with an oppo-site background wind, and the down-slope wind can trigger and intensify the land breeze corresponding to the easterncoast of Jiaozhou Bay; (2) a gentle westerly background wind will reduce the eastward sea breeze and up-slope windduring the daytime due to a cold advection, but shows a little effect on the mesoscale circulation formed in the nighttime.     

Comparison of Speed-Up Over Hills Derived from Wind-Tunnel Experiments,Wind-Loading Standards,and Numerical Modelling

We evaluate the accuracy of the speed-up provided in several wind-loading standards by comparison with wind-tunnel measurements and numerical predictions, which are carried out at a nominal scale of 1:500 and full-scale, respectively. Airflow over two- and three-dimensional bell-shaped hills is numerically modelled using the Reynolds-averaged Navier–Stokes method with a pressure-driven atmospheric boundary layer and three different turbulence models. Investigated in detail are the effects of grid size on the speed-up and flow separation, as well as the resulting uncertainties in the numerical simulations. Good agreement is obtained between the numerical prediction of speed-up, as well as the wake region size and location, with that according to large-eddy simulations and the wind-tunnel results. The numerical results demonstrate the ability to predict the airflow over a hill with good accuracy with considerably less computational time than for large-eddy simulation. Numerical simulations for a three-dimensional hill show that the speed-up and the wake region decrease significantly when compared with the flow over two-dimensional hills due to the secondary flow around three-dimensional hills. Different hill slopes and shapes are simulated numerically to investigate the effect of hill profile on the speed-up. In comparison with more peaked hill crests, flat-topped hills have a lower speed-up at the crest up to heights of about half the hill height, for which none of the standards gives entirely satisfactory values of speed-up. Overall, the latest versions of the National Building Code of Canada and the Australian and New Zealand Standard give the best predictions of wind speed over isolated hills.     

Flow over the summit of an isolated hill

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.     

Mean temperature and mean concentration distributions over a physically modelled three-dimensional heat island for different stability conditions

Results of flow visualization, and the mean temperature measurements over a physically modelled three-dimensional heat island in a wind tunnel capable of simulating stratified atmospheric boundary layers are presented. Concentration measurements of Kr85 released from an upwind two-dimensional continuous line source show good correlation with flow visualization and mean temperature distributions. The results indicate the unique features of three-dimensional flow over a heat island - lateral low-level convergence, upward vertical motions, and upper-level horizontal divergence.     

A simple spectral model for the modification of turbulence in flow over gentle hills

A model is presented which calculates the changes of the velocity variances and stress uw in flow over gentle isolated hills. At intermediate frequencies spectra of the velocity components are modified according to rapid distortion theory. At low frequencies spectral densities change in proportion to the square of the mean wind. The inner and outer layer of the flow are distinguished. Streamline curvature effects are accounted for in the vertical velocity variance and the covariance.The sensitivity of the model to several parameters is investigated. Then, its results are compared with measurements of turbulent flow over various hills and an escarpment. The model is able to simulate the structure of the modified variance and covariance fields although larger differences occur at individual positions. The calculated modified spectra compare well with observed spectra.     

The Influence of Thermally-Induced Mesoscale Circulations on Turbulence Statistics Over an Idealized Urban Area Under a Zero Background Wind

The influence of mesoscale circulations induced by urban-rural differential surface sensible heat flux and roughness on convective boundary-layer (CBL) flow statistics over an isolated urban area has been examined using large-eddy simulation (LES). Results are analyzed when the circulations influence the entire urban area under a zero background wind. For comparison, the CBL flow over an infinite urban area with identical urban surface characteristics under the same background meteorological conditions is generated as a control case (without circulations). The turbulent flow over the isolated urban area exhibits a mix of streaky structure and cellular pattern, while the cellular pattern dominates in the control case. The mixed-layer height varies significantly over the isolated urban area, and can be lower near the edge of the urban area than over the rural area. The vertical profiles of turbulence statistics over the isolated urban area vary horizontally and are dramatically different from the control case. The turbulent kinetic energy (TKE) sources include wind shear, convergence, and buoyancy productions, compared to only buoyancy production in the control case. The normalized vertical velocity variance is reduced compared to the control case except in the central urban area where it is little affected. The low-level flow convergence is mainly responsible for the enhanced horizontal velocity variance in the central urban area, while wind shear is responsible for the additional local maximum of the horizontal velocity variance near the middle of the CBL outside the central area. Parameterizations in the prognostic equation for TKE used in mesoscale models are evaluated against the LES results over the isolated urban area. We also discuss conditions under which the urban-induced circulations occur and when they may affect the entire urban area. Given that urban-induced circulations can influence the entire urban area within hours for an urban area of a realistic size, it is inappropriate to directly apply empirical relations of turbulence statistics derived under horizontally-homogenous flow conditions to an urban area.     

Estimating the components of the sensible heat budget of a tall forest canopy in complex terrain

Ultrasonic wind measurements, sonic temperature and air temperature data at two heights in the advection experiment MORE II were used to establish a complete budget of sensible heat including vertical advection, horizontal advection and horizontal turbulent flux divergence. MORE II took place at the long-term Carbo-Europe IP site in Tharandt, Germany. During the growing period of 2003 three additional towers were established to measure all relevant parameters for an estimation of advective fluxes, primarily of CO₂. Additionally, in relation to other advection experiments, a calculation of the horizontal turbulent flux divergence is proposed and the relation of this flux to atmospheric stability and friction velocity is discussed. In order to obtain a complete budget, different scaling heights for horizontal advection and horizontal turbulent flux divergence are tested. It is shown that neglecting advective fluxes may lead to incorrect results. If advective fluxes are taken into account, the sensible heat budget based upon vertical turbulent flux and storage change only, is reduced by approximately 30%. Additional consideration of horizontal turbulent flux divergence would in turn add 5–10% to this sum (i.e., the sum of vertical turbulent flux plus storage change plus horizontal and vertical advection). In comparison with available energy horizontal advection is important at night whilst horizontal turbulent flux divergence is rather insignificant. Obviously, advective fluxes typically improve poor nighttime energy budget closure and might change ecosystem respiration fluxes considerably.