Effects of roughness blocks on atmospheric boundary layer flow over a two-dimensional low hill with/without sudden roughness change

Wind tunnel experiments were carried out to investigate the effects of roughness blocks on the atmospheric boundary layer flow over a two-dimensional low hill with maximum slope 0.21. Roughness blocks whose heights were one eighth of the hill height were arranged in a staggered pattern with a roughness density of 4.1% to model the rough conditions. Four situations with/without roughness change in the flow direction, i.e. a smooth hill in smooth flow, a rough hill in rough flow, a smooth hill in rough flow and a rough hill in smooth flow, were considered. The effects of the roughness blocks were clarified by comparing the flow characteristics over hill models, with emphasis on speedup ratio and turbulence structure. Experimental results were compared with the predictions obtained from linear models. The results show that speedup ratio depends strongly on the surface condition in the middle layer, the inviscid but rotational part of the outer layer defined by HLR theory [Hunt, J.C.R., Leibovich, S., Richards, K.J., 1988. Turbulent shear flow over low hills. Quart. J. Roy. Meteorol. Soc. 114, 1435-1471]. Adding or removing roughness blocks on the hill surface or inflow area changes the velocity deficit and creates a completely different turbulence structure in the wake.     

LES analysis of turbulent boundary layer over 3D steep hill covered with vegetation

Large eddy simulation (LES) is carried out to investigate the turbulent boundary-layer type of flows over a hill-shaped model with a steep slope. Also, we focus on the surface condition of a hill, such as vegetation effects as well as curvature effects. In order to model the vegetation effects for LES, we employ the feedback forcing method proposed by Goldstein et al. (1993, Modeling a no-slip flow boundary with an external force field. J. Comput. Phys. 105, 354–366). In this model, the equation of motion for trees or grasses are coupled with the Navier–Stokes equation, so turbulence in the vegetation canopy can be numerically expressed. Instantaneous velocity profile of turbulent boundary layer on the corresponding surface is imposed at inflow boundary. Both the computed results over the hill with and without vegetation are in good agreement with Meng and Hibi's (1998. An experimental study of turbulent boundary layer over steep hills. In: Proceedings of 15th National Symposium on Wind Engineering, pp. 61–66.) experimental data for a rough and a smooth hill. Also, the effects of vegetation on turbulence statistics, such as high intensity due to the coherent flow structures at the top of the vegetation and reduction of turbulence inside the vegetation, are clarified.     

Flow structure around a 3-D rectangular prism in a turbulent boundary layer

Flow around a three-dimensional (3-D) rectangular prism has been investigated by using a particle image velocimetry technique. The prism was immersed in a thick turbulent boundary layer. The ratio of the boundary layer thickness and the model height was about 0.06. Measurements were made at Reynolds number of 7.9×10³ which is based on free stream velocity and model height. Detailed flow structures and characteristics including three circulation zones were obtained by averaging over a large number of instantaneous velocity maps. The 3-D structure in a wake zone is clearly seen from mean flow streamline topology. Turbulent kinetic energy distribution is also obtained approximately. Maximum turbulent kinetic energy was found at the separated layer in the upper boundary of the separation bubble near the leading edge of a roof. The magnitude of the maximum energy is about 2.5 times that in the wake region.     

Experimental study of topographic effects on gust wind speed

This paper presents a follow-up study to our earlier work on comparing approaches to determine topographic effects in four major wind load codes. These codes are further evaluated and compared with earlier studies as well as new tests undertaken in the Texas Tech University boundary layer wind tunnel. Wind tunnel experiments with a model scale of 1:1000 were carried out to evaluate the wind speed-up effects of two main types of topography: escarpments and symmetrical ridges. Of particular interest were effects of ground surface roughness and the upwind slope of the two topographic features on wind speed-up and the space limits for speed-up applications around the crest of topography. Experimental results show that the surface roughness has significant speed-up effects for ridges rather than for escarpments. The results also indicate that wind load codes tend to be unconservative in specifying the minimum and maximum upwind slope as well as the spatial extent around the crest for application of speed-up factors.     

Measurements of spatial correlations and autocorrelations in separated and reattached flow over a blunt flat plate

Spatial correlations and autocorrelations of turbulent fluctuating velocities have been measured in separated, reattached and redeveloped flow over a flat plate of finite thickness and with a blunt leading edge. Integral time and length scales are estimated. Furthermore, measurements of probability-density distributions have been conducted. It is found from the results that turbulent eddies originating in the separated and reattached regions are extremely large compared to those existing in a turbulent boundary layer over a flat plate at zero pressure gradient, and also that the effects of separation and reattachment upon the turbulence characteristics continue far downstream.     

A 2-D numerical model of boundary-layer flow over single and multiple surface condition changes

Based on the 1-D planetary boundary layer model of Weng and Taylor with E-? turbulence closure, a 2-D numerical model is developed to study the atmospheric boundary-layer flow over single or multiple changes in surface conditions. These changes can include surface roughness, thermal and moisture properties. A constant flux wall layer is used within which approximate forms for the velocity, temperature, moisture and turbulent kinetic energy profiles are obtained by analytic solution with the assumption of production equal to dissipation of turbulent kinetic energy. We also use a simple, analytic model dealing with the surface roughness change effects in neutral stratification based on the concept of an internal boundary layer. Model results for roughness changes are discussed and compared with other models and published field data.     

Measurement of the Reynolds stress structure and turbulence characteristics of the wind above a two-dimensional trapezoidal shape of hill

Wind tunnel experiments were carried out to measure the mean velocity and turbulence structure of the wind flow over a two-dimensional trapezoidal shape of hill. The quadrant analysis technique was employed to analyze the structure of the Reynolds stress. Analysis of the turbulent velocity spectrum of the wind above the hill under different wind attack angles is conducted. The fractional speed-up ratios of the present measured results are found in agreement with the wind tunnel data of Lemelin et al. (J. Wind Eng. Ind. Aerodyn. 28 (1988) 117) for the case of the wind attack angle of 30°. Measurements of the mean velocity profiles disclose that the speed-up phenomenon is mostly manifest at Z/H=0.6 for the case of wind attack angle of 10°. Turbulence intensity profiles measured at different locations show that the turbulence intensity decreases as shifting from far upstream location of the hill (X/H=−20) to the downstream location at the center of hill (X/H=0). The decrease of the turbulence intensity is obviously at the distance close to the surface of the hill. Results of the quadrant analysis indicate that the sweep and ejection events are the major contributors to the Reynolds stress. Others like inward and outward interaction events make negative contributions. The values of the stress fractions of ejection and sweep events become the lowest as the wind attack angle is 20°. Analysis of the turbulent velocity power spectrum density shows that the spectrum density is increasing in the lower-frequency region as the wind attack angle increases. The power spectrum density is found to decrease for increase in the wind attack angle at the higher-frequency region.     

An experimental investigation of the recirculation zone formed downstream of a forward facing step

An experimental investigation of the recirculation zone formed downstream of a forward facing step immersed in a turbulent boundary layer has been undertaken using particle image velocimetry. Bluff body flow is observed with the fixed separation point located at the leading edge of the step. The recirculation region dimensions are characterised over a range of Reynolds numbers (1400-19 000), with Re_h based on the step height and the free stream velocity. Turbulent perturbations are produced in the free shear layer which develops between the recirculating flow close to the step and the free stream flow. Contour maps of amplification factor, streamwise perturbation velocity and Reynolds stresses are constructed, providing insight into optimal placement of structures within such topographical features. The mechanisms affecting the reattachment distance, namely the turbulent mixing within the boundary layer and the velocity deficit in the boundary layer, are discussed.     

Experimental study on flow and noise characteristics of NACA0018 airfoil

The flow characteristics on and around an airfoil at moderate Reynolds number are studied to understand the generation mechanism of tonal noise from a symmetrical airfoil NACA0018 in a uniform flow. The separation and reattachment of the flow on the airfoil surface are evaluated from the liquid-crystal visualization and the velocity field across the boundary layers over the airfoil are measured by particle image velocimetry (PIV). These results indicate that the separation and reattachment points of laminar boundary layers over the airfoil move along the airfoil surface, depending on the angle of attack variations. When the airfoil is inclined at a small attack angle to meet with the condition of tonal noise generation, the boundary layer on the pressure surface experiences the separation along the surface and reattaches near the trailing edge of the airfoil. Then, the periodic vortex structure is generated over the pressure surface near the trailing edge, which is followed by the formation of periodic vortex shedding in the airfoil wake. With further increase in attack angle, the flow over the suction surface separates near the leading edge of the airfoil and the boundary layer on the pressure surface keeps fully attached flows on the airfoil surface, resulting in the disappearance of tonal noise.     

Numerical studies on air flow around a cube

In this paper, air approach flow moving towards a cube will be studied using computational fluid dynamics (CFD). The Reynolds Averaging of Navier-Stokes (RANS) equation types of k-ε turbulence model are used. Some RANS predicted results are compared with different upstream air speeds. Flow separation at the corner above the top of the cube, level of separation and reattachment are investigated. Reference is made to the experimental data on wind tunnels reported in the literature.A method similar to ‘recirculation bubble promoter’ is used for different approach flow speed distributions. Problems encountered in numerical simulations due to the sharp corner are discussed with a view to obtaining better prediction on recirculation flow in regions above the top of the cube. Correlations between the turbulent kinetic energy above the cube and the recirculation bubble size are derived for different distributions of approach flow speed.By limiting the longitudinal velocities in the first cell adjacent to the sharp edge of the cube or rib, and making good use of the wall functions at the intersection cells of the velocity components, positions of maximum turbulent kinetic energy and the flow separation and reattachment can be predicted by a standard k-ε model. The results agree with those obtained in the experiments.     

Similarities of pressure induced by separation bubble in grid-generated turbulent flow

Some existing wind-tunnel data on mean pressure and fluctuation pressure distributions, induced by a separation bubble near the leading edge of a blunt plate, in the presence of grid-generated freestream turbulence, are systematically analyzed in this paper. The mean pressure distributions, when expressed in reduced coordinates, are self-similar and may be approximated by the pressure variation induced by an inviscid rotational vortex hovering above a flat surface in uniform smooth flow. These coordinates are expressed in terms of characteristic parameters of the mean flow field and each parameter has been correlated with the longitudinal component of the turbulence intensity and scale non-dimensioned by the plate thickness. In analyzing the pressure fluctuation, a similar approach is used. The results from the rotational vortex are consistently useful when considering the leading portion of the bubble characterized by the location of maximum fluctuation. The remaining portion may be represented by an exponential function characterized by the turbulence intensity only. The outcome of this study may be used as a prediction method whose validity has been checked with experimental data.     

Numerical calculation of separated flow past a circular cylinder using panel technique

In the present investigation, a potential flow model based on panel technique, has been developed for calculation of bubble type separated flow past smooth and rough circular cylinder. Free vortex lines are assumed to emanate from the points of separation that converge downstream of the body. The converged wake shape is iteratively obtained by integrating the velocity vectors at the collocation points on the wake panels. Effect of vorticity dispersion in the wake, which plays the dominant role in the flow separation phenomena, is incorporated in the flow model. It has been observed that separated flow past circular cylinder for different Reynolds number and surface roughness can be calculated with reasonable accuracy with appropriate values of vorticity dispersion factor (λ).     

Computation of turbulent flows past arbitrary two-dimensional surface-mounted obstructions

The modelled equations of turbulent flow over two-dimensional bluff surface obstructions are solved numerically, using body-fitted coordinates and the k-? two-equation model of turbulence. Emphasis is placed on demonstrating a method capable of treating arbitrary geometrical configurations.Computations have been made for a number of geometrical configurations, including, amongst others, a rectangular block as well as triangular and semi-circular obstructions. Upstream conditions were purposely selected to simulate a neutrally stable atmospheric boundary layer.The low-Reynolds number version of the turbulence model was used in order to accommodate near-wall viscous effects, and some effort was devoted to the choice of empirical constants based on comparison with empirical information. Making due allowance for a number of discrepancy-causing factors, reattachment lengths obtained are deemed to be under-predicted. This is attributed to theoretical weaknesses of the two-equation model as well as numerical inaccuracies associated with the difference scheme.     

Discussion on “Experimental issues in atmospheric boundary layer simulations: roughness length and integral length scale determination” by A.K.S. Iyengar, C. Farell

In their paper, Iyengar and Farell, Experimental issues in atmospheric boundary layer simulations: roughness length and integral length scale determination, J. Wind Eng. Ind. Aerodyn. 89 (2001) 1059, compare Reynolds stress measurements in a boundary layer developed over an array of cubes with the drag force on the cube array, expecting these two forms of measurement to be equivalent, but finding that they differ by more than 15%. This leads to inconsistencies in the estimates of aerodynamic roughness and zero-plane displacement from the measurements. It is suggested in this discussion that not all the drag on the elements is mobilised into the boundary layer as Reynolds stresses when the aerodynamic zero plane is a significant proportion of the roughness height. Accordingly, balance measurements exaggerate the equivalent surface shear stress and direct measurement of the Reynolds stress should be used instead.     

Wind tunnel tests of effects of atmospheric stability on turbulent flow over a three-dimensional hill

This paper presents a wind tunnel study on the turbulent structure of the airflow around a three-dimensional hill model placed in a boundary-layer flow. The effect of atmospheric stability: stable, neutral and unstable on the flow field of the boundary layer is examined. The wind velocity is measured with a three-dimensional laser doppler anemometer (LDA). Measurements analysis includes mean velocity, turbulent velocity, Reynolds stress and turbulence energy profiles around the hill. The main results are as follows: (1) The mean wind velocity does not vary with the stability at the hilltop; it reaches a maximum at the back of the hill, for the unstable case. (2) The turbulent velocity at the back of the hill reaches its peak value at the height of the hilltop. It takes maximum value for the stable boundary layer flow, and become smaller for the neutral flow and the unstable flow. Buoyancy production has little effect on the turbulence energy. (3) A clear peak of /U_H² is observed at a height near Z/H=1. The peak value becomes the largest for the stable case and the smallest for the unstable case.     

Full-scale pressure measurements on a Sparkman and Stephens 24-foot sailing yacht

The aerodynamics of a Sparkman and Stephens 24-foot sailing yacht was investigated. Full-scale pressure measurements were performed on the mainsail and the genoa in upwind condition. Pressure taps were adopted to measure the pressures on three horizontal sections on the windward and leeward sides of the two sails. Several trims and apparent wind angles were tested. The present paper shows the pressure distributions on the sails and correlates the measured pressures with the flow pattern. In particular, leading-edge laminar separation bubble, turbulent reattachment and turbulent separation are discussed. Pressure measurements are also adopted to draw some trim guidelines.     

圆角处理对二维矩形柱体模型表面风压频域特性的影响研究

利用刚性模型测压试验的方法,在低湍流度的均匀流场中测量了4种圆角处理（即圆角半径与模型迎风面厚度之比（圆角率）分别为0、5%、10%和15%）的截面宽厚比（截面顺风向宽度与迎风面厚度之比）分别为2、3和4的二维矩形柱体模型的表面风压,试验雷诺数为3.4×10⁵。研究了圆角处理对3种截面宽厚比模型表面的3个典型测点的风压系数功率谱密度的影响,并分析了每种截面宽厚比模型的侧面风压分布及其风压相关性随圆角率的变化规律。研究表明：当圆角率为0和5%时,截面宽厚比为2的模型侧面不会出现气流再附现象,而当圆角率为10%和15%时,气流会再附于截面宽厚比为2的模型的侧面;圆角处理主要影响截面宽厚比为3和4的模型侧面的气流再附区的风压分布,而侧面气流分离区的负压绝对值基本不变;3种截面宽厚比的模型中,圆角处理对截面宽厚比为2的模型的侧面风压相关性影响最大。     

Scale, boundary and inlet condition effects on impinging jets

The sensitivity of the orthonormal impinging jets with respect to scale (Reynolds number), boundary conditions (geometry and surface roughness) as well as inlet conditions is investigated. Due to the unsteady separation in the near-wall region the flow field is Reynolds number dependent. The depth of the boundary layer formed on the impinging surface decreases, while the maximum radial velocity increases with Reynolds number below a critical, Re_cr, value. Above one order of Re_cr the flow becomes asymptotically independent of Reynolds number. When Reynolds number reaches a fully roughness region the depth of the surface layer increases with roughness height only. The flow is found to be only weakly dependent on the distance between the jet and the surface for distances larger than the ring-vortex formation length. Radial confinements of diameters less than approximately 10 jet diameters and axial confinements placed at less than 1 jet diameter above the surface affect the pressure distribution on the impinging plate. The inlet turbulence affects mostly the free-jet flow region.     

Effects of inflow turbulence intensity on flow and pollutant dispersion in an urban street canyon

The effects of inflow turbulence intensity on flow and pollutant dispersion in an urban street canyon with a street aspect ratio of 1 are examined using a two-dimensional numerical model. As the inflow turbulence intensity increases, turbulent kinetic energy and turbulent diffusivity in the street canyon increases. Also, the mean horizontal velocity near the roof level increases and the street-canyon vortex strengthens. The analyses of the time series and residue ratio of pollutant concentration show that the inflow turbulence intensity significantly affects pollutant concentration in the street canyon. As the inflow turbulence intensity increases, the pollutant concentration in the street canyon becomes low and hence more pollutants escape from the street canyon.     

Analysis of flow over backward facing step with transition

The present study deals with the study of the velocity distribution and the separation phenomenon of flow of air over a two dimensional backward facing step. The flow of air over a backward facing step has been investigated numerically using FLUENT. Flow simulation has been carried out in a backward facing step having an expansion ratio (ratio of the height before and after the step) of 1:1.94 and the results obtained are compared with the published experimental results. Comparison of flow characteristics between steps with three different transitions is made. The variation of reattachment length for all the three cases are analyzed for wide range of Reynolds number ranging from 100 to 7000 which covers the laminar, transition and turbulent flow of air. Simulation of the flow over steps with expansion ratios of 1:1.24, 1:1.38, 1:1.47, 1:1.53, 1:1.94, 1:2.20 are also carried out to examine the effect of different expansion ratios on the reattachment length. It is found that the primary reattachment length increases with increase in the expansion ratio. The primary reattachment length at the bottom wall downstream of the step is minimum for the step with round edged transition and maximum for the step with a vertical drop transition.