Flow around a cube in a turbulent boundary layer: LES and experiment

We present a numerical simulation of flow around a surface mounted cube placed in a turbulent boundary layer which, although representing a typical wind environment, has been specifically tailored to match a series of wind tunnel observations. The simulations were carried out at a Reynolds number, based on the velocity U at the cube height h, of 20,000—large enough that many aspects of the flow are effectively Reynolds number independent. The turbulence intensity was about 18% at the cube height, and the integral length scale was about 0.8 times the cube height h. The Jenson number Je=h/z₀, based on the approach flow roughness length z₀, was 600, to match the wind tunnel situation. The computational mesh was uniform with a spacing of h/32, aiding rapid convergence of the multigrid solver, and the governing equations were discretised using second-order finite differences within a parallel multiblock environment. The results presented include detailed comparison between measurements and LES computations of both the inflow boundary layer and the flow field around the cube including mean and fluctuating surface pressures. It is concluded that provided properly formulated inflow and surface boundary conditions are used, LES is now a viable tool for use in wind engineering problems concerning flow over isolated bodies. In particular, both mean and fluctuating surface pressures can be obtained with a similar degree of uncertainty as usually associated with wind tunnel modelling.     

Effect of drying temperature on surface roughness of oak (Quercus petraea ssp. iberica (Steven ex Bieb) Krassiln) Veneer

This paper reports the effect of various drying temperatures on the surface roughness (SR) characteristics of veneer samples. Three SR parameters [average roughness (R_a), average maximum height of the profile (R_z), and root mean square roughness (R_q)] were measured on sliced veneer obtained from Oak logs (Quercus petraea ssp. iberica (Steven ex Bieb) Krassiln). The sliced veneers were dried at 100, 115 and 130 °C drying temperatures for 2 min. Roughness measurements were taken from the surface of the samples in across the grain orientation of the veneer. The results showed that the effect of drying temperatures used in practice is statistically significant on SR of the sliced veneers.     

Dynamic response and reliability analysis of tall buildings subject to wind loading

This paper explores the wind stochastic field from a new viewpoint of stochastic Fourier spectrum (SFS). The basic random parameters of the wind stochastic field, the roughness length z₀ and the mean wind velocity at 10 m height U₁₀, as well as their probability density functions (PDF), are obtained. It provides opportunities to use probability density evolution method (PDEM), which had been proved to be of high accuracy and efficiency, in computing the dynamic response and reliability of tall buildings subject to the wind loading. Principals and corresponding numerical solving algorithm of the PDEM are first presented. Then, the adopted model of the wind stochastic field is described briefly. The simulation method of the fluctuating wind velocity based on the SFS is introduced. Finally, as an example of the application of the PDEM, a 20-storey frame subject to wind loading is investigated in detail. The responses, including the mean value and the standard deviation, and the reliabilities of the frame are evaluated by the PDEM. The results demonstrate that the PDEM is applicable and efficient in the dynamic response and reliability analysis of wind-excited tall building.     

A parameterization of dust concentration (PM₁₀) of dust events observed at Erdene in Mongolia using the monitored tower data

Hourly mean time series of dust concentration (PM₁₀) measured at 3 m high and a sonic-anemometer measured momentum and kinematic heat fluxes at 8 m high above the surface have been obtained from a 20-m monitoring tower located at Erdene in the Asian dust source region of Mongolia for years of 2009 and 2010. These time series were used to identify dust events and to develop optimal regression equations for the dust concentration of dust events with the friction velocity (u_*) and the convective velocity scale (w_*). In total, 68 dust events were identified in 2009 (except for November) and 43 dust events for the period from March to August in 2010. The duration of each dust event ranged from 3-29 h in 2009 and 5-35 h in 2010. The maximum hourly mean dust concentration of the dust event was found to be 4,107 μg m^− 3 in May in 2009 and 4,708 μg m^− 3 in March in 2010 while a minimum of 251 μg m^− 3 in August in 2009 and 662 μg m^− 3 in June in 2010. The optimal regression equation for the dust concentration (C) of dust events was found to have the form of log C = a + b(u_* + cw_*)ⁿ, where a, b, c and n are constants that vary month to month. The convective velocity scale (w_*) that has not been taken into account in most dust modelings was found to enhance the dust concentration of dust events during the cold period from December to March when the soil temperature was below the freezing level for both the stable (w_* < 0) and unstable (w_* > 0) stratifications, whereas the convective velocity caused a reduction in the dust concentrations during the warm period from April to October, suggesting the importance of the convective velocity to estimate dust concentration of dust events.     

Active control for a benchmark building under wind excitations

The tall building proposed in the benchmark problem is a concrete office tower, which is wind sensitive due to its slenderness. The wind loads acting on the building are defined by the results of wind tunnel tests conducted at the Sydney University and an active mass driver can be designed for installation on the top floor in order to reduce the structural response under strong wind gusts. In this paper, the control strategy presented for the third generation benchmark control problem for wind-excited buildings (J. Eng. Mech. (1999), submitted for publication) is the simultaneous treatment of both H₂ performance criteria and H_∞ attenuation constraints that demonstrates design tradeoffs and can be applied to the AMD system based on acceleration feedbacks. This control problem can be formulated by linear matrix inequalities in terms of a common Lyapunov function. Solving linear matrix inequalities is a convex optimization problem and efficient interior-point algorithms are now available to solve this problem. Simulation and design results demonstrates that, decreasing H_∞ attenuation constraint can be used to reduce the structural response under wind excitations at the expense of increasing H₂ performance index.     

Investigation of the vertically-layered structure and surface roughness parameters of the urban boundary layer

Atmospheric flow is the main factor affecting the wind load of urban buildings. The measured data observed continuously from the 325-m-tall Beijing Meteorological Tower (BMT) during 2013–2017 is employed to investigate the vertically-layered structure and surface roughness parameters of the urban boundary layer. Based on the local similarity theory and analysis results of the atmospheric stability and local friction velocity, it can be determined that the height of 80 m is near the bottom of the inertial sub-layer, the range below this height belongs to the roughness sub-layer, 140 m belongs to the inertial sub-layer, and 200 m and 280 m are in the mixing layer. The local friction velocity at 80 m can be considered a relatively reliable value as the friction velocity. Moreover, seasonal effect on local friction velocity is minimal. According to the fitting result of near-neutral strong wind samples by the log-law, it is concluded that to obtain a more accurate wind speed profile, all layers should be included when picking fitting heights. In addition, surface roughness parameters are affected by the wind direction and speed. The variation according to the wind direction corresponds to the topographical distribution surrounding the BMT, and the higher range of wind speed may be more applicable for estimating surface roughness parameters.     

Micro wind turbines in the UK domestic sector

The micro-scale wind turbine industry is expanding in the UK with institutional support and UK legislation encouraging the development of numerous companies with a profusion of design options. The application of micro wind turbines in urban environment is encouraged in the UK via a grant scheme which provides a proportion of the initial capital costs. This development is predicated on the assumption that micro wind turbines have the potential to reduce built environment CO₂ emissions. Current methods of estimating the wind speed are reported to over predict by approximately 2.0 m/s. The energy yields of a range of typical micro wind turbines (in the 0.4–2.5 kW size range) were estimated here using two wind speed datasets sited within 1 km of each other recorded with a temporal precision of 10 min. The annual energy yield of a 1.5 kW turbine was found to be 277 kWh and 2541 kWh for the two sites analysed indicating the problem with the current method of yield estimation. Between 33 and 55% of the electricity generated would be exported dependant on the dwelling's electrical demand. For the high yield site, the simple economic payback of this turbine was found to be 26.8 years i.e. beyond the likely life time of the turbine with CO₂ savings of 1093 kg CO₂. The research suggests that this technology does represent a possible route for reducing CO₂ emissions but this is unlikely to be realised unless an adequate method is found for more accurately predicting energy yield at a specific site.     

Improved modelling of downburst outflows for wind engineering applications using a cooling source approach

Large eddy simulations (LES), with a range of different practical ground roughness lengths (z₀=0.001-0.1 m), are used to compare near surface outflow features of a physically realistic cooling source downburst model, previously validated by meteorological observations, with those of the more commonly used transient impinging impulsive jet. A scaling procedure is proposed, based on length, velocity, and vorticity scales from within the outflow, allowing for direct comparison between outflows from the two models. Five scaling parameters are presented, capturing the horizontal and vertical position of maximum velocity, the ring vortex aspect ratio, the height of the ring vortex above the surface, and a non-dimensional vorticity term representative of the relative contribution of the ring vortex to the near surface wind field. It is shown that the impinging jet model is not capable of capturing the outflow features predicted by the cooling source model, due to its unrealistic forcing parameters, and is, therefore, unable to capture the physics of an actual downburst event. This difference dominates the non-dimensional vorticity term, showing that impinging jet results deviate by at least 56% from the cooling source results, at times when all other scaling parameter differences are minimized.     

Strong wind observations in the atmospheric surface layer

The content of this article is a contribution to the limited amount of available strong-wind multi-level tower observations in the atmospheric surface layer, and is primarily intended for those engineers and scientists engaged in the field of wind engineering. The observations were used to evaluate the correctness of the predictions obtained from theoretical and empirical models, the latter used frequently by the wind engineering community. The comparisons included profiles of mean wind, turbulence intensity, and gust velocities. To test the mean-velocity models for the prediction of wind speeds at locations where no recording stations were present, observations at a reference location were used to predict and to compare with the simultaneous observations at a number of locations where wind speed observations were available.The analysis of the data revealed that under strong wind conditions thermal stability effects should not be ignored. For obstacle-free open terrain significant variations of the aerodynamic roughness length are observed. The height of the surface layer that increases with roughness and wind speed is at least 150 m. Davenport's “gradient” height, not a function of wind speed, is approximately twice the height of the surface layer that applies to the strong wind data analyzed. Estimation of wind speed at locations where normally no observations are available may exceed the actual speed by as much as 50%.     

A phenomenological model for the dynamic response of wind turbines to turbulent wind

To predict the average power output of a wind turbine, a response model is proposed which takes into account: (i) the delayed response to the longitudinal wind speed fluctuations; (ii) a response function of the turbine with arbitrary frequency dependence; (iii) wind fields of arbitrary turbulence intensity. In the limit of low turbulence intensity, the dynamical ansatz as proposed in 1992 by Rosen and Sheinman is reproduced. It is shown, how the response function of the turbine can be obtained from simulation experiments of a specific wind turbine. For two idealized situations the dynamic effect of fluctuating wind is estimated at turbulence intensities 0?I_u?0.5. At the special mean wind speed , the turbine response function is determined from simulation data published by Sheinman and Rosen in 1992 and 1994.     

Air pollution and meteorological processes in the growing dryland city of Urumqi (Xinjiang, China)

Seven years (2000-2006) of monthly PM₁₀ (particulate matter, d ≤ 10 μm), SO₂, and NO₂ concentrations are reported for Urumqi, the capital of Xinjiang in NW China. Considerably high mean annual concentrations have been observed, which ranged between 150 and 240 μg m^− 3 (PM₁₀), 31 and 50 μg m^− 3 (NO₂), and 49 and 160 μg m^− 3 (SO₂). The shapes of seasonal variation of all pollutants were remarkably similar; however, winter/summer ratios of concentrations were quite different for PM₁₀ (2-3) and NO₂ (≈ 4) compared to SO₂ (up to 30). Very high consumption rates of fossil fuels for energy generation and domestic heating are mainly responsible for high annual pollution levels, as well as the (very) high winter/summer ratios. Detailed analysis of the 2000-2006 records of Urumqi's meteorological data resulted in inter-annual and seasonal frequency distributions of (a) (surface) inversion events, (b) heights of surface inversions, (c) stability classes of Urumqi's boundary layer, and (d) the “Air Stagnation Index (ASI)”. Urumqi's boundary layer is shown to be characterized by high mean annual and seasonal frequencies of (surface) inversions and by the dominance of stable dispersion classes. A further outcome of the meteorological analysis is the proof of Urumqi's strong diurnal wind system, which might have particularly contributed to the stabilization of the nocturnal boundary layer. Annual and seasonal variations of pollutant's concentrations are discussed in the context of occurrences of inversions, boundary layer, stability classes, and ASI. The trend of Urumqi's air pollution indicates a strong increase of mean annual concentrations 2000-2003, followed by a slight increase during 2003-2006. These are in strong contrast to (a) the growth of Urumqi's fleet of motor vehicles and (b) to the growing number of stable regimes of Urumqi's boundary layer climate during same period. It is concluded that the (regional and) local administrative technical countermeasures have efficiently lowered Urumqi's air pollution levels.     

A comparative study of the Mellor-Yamada and k-ε two-equation turbulence models in atmospheric application

The present study systematically compares the Mellor-Yamada (MY) model and the k−ε algebraic stress model in order to verify the possibility of using the k−ε algebraic stress model in atmospheric applications. The results of the parameterization process and atmospheric application of both models confirmed that the MY model neglected the pressure redistribution effect of buoyancy due to 〈u_iu_j〉 and 〈u_iθ〉 and that of shear due to 〈u_iθ〉. In addition, the MY model overestimated the turbulent energy dissipation. Based on the formulation of the k−ε algebraic stress model, we modified the constant value C_μ(=0.09) in the standard k−ε model to obtain the variables C_μM and C_μH to account for atmospheric stability. Finally, the results of the simulation obtained from the Wangara experiment verify the possibility of using the k−ε algebraic stress model in atmospheric application.     

Parametric analysis of wind siting efficiency

The effect of wind climatic characteristics on the efficiency of the WECS is examined by means of the Weibull general model and a simple model of power curve. Two efficiency parameters—the plant utilization factor (F_u) and the site effectiveness (ε)—are considered as functions of three parameters: the Weibull shape parameter (k), a dimensionless mean wind speed (α), and the design ratio between rated and cut-in speed (φ). It is shown that only for intermediate values of α and φ the plant utilization factor can be considered independent of k, being otherwise either improved or penalized in no negligible measure by it, depending on the values of α and φ. The effect of increasing k on the site effectiveness is always beneficial. The effect of increasing α is beneficial for the plant utilization factor but largely penalizing for the effectiveness. However, no maximization is possible for neither of the efficiency parameters.It is shown that the correct use of F_u and ε for comparison is significant only for a given WECS, while the WECS data have to be used when comparing different or modified wind machines. The data of 35 commercial WECS are examined to show that the common assumption of constant rated or maximum efficiency is not acceptable.     

近地台风风场特性及低矮房屋风荷载现场实测研究

通过研制的可移动平坡屋面实验房风压及台风风场现场实测系统,研究近地台风风场特性和低矮房屋表面风荷载分布规律。基于实验房获取到的10余次近地台风风速和风压实测数据,对近地台风风场湍流特征参数如湍流强度、阵风因子、湍流积分尺度及脉动风速功率谱等,按来流不同方位地貌状况进行分类研究;同时分析了斜向强风最不利工况下,屋面角部区域风压分布特征。分析结果表明: A、B、C类地貌条件下,台风顺风向湍流强度均值分别为0.13, 0.21, 0.32;阵风因子同湍流强度正相关,湍流积分尺度随湍流强度增加而减少;与季风相比,台风眼壁区域的顺风向脉动风速功率谱密度值略大于季风的实测值,而横风向脉动风速功率谱密度值显著大于季风的实测值;在低频和惯性子区范围,台风眼壁区域的顺风向脉动风速von Karman和Harris谱拟合值与实测值吻合较好;在斜向风作用下迎风屋檐角部边缘测点区域具有较高峰值负压和脉动风压,峰值负压系数达-13.5。     

Numerical studies of flow through a windbreak

The pattern of flow through a porous windbreak has been investigated numerically using several well-known closure schemes (turbulence models). The shelter is included as a momentum extraction term in the streamwise momentum equation, for a fence having the value $\text{k}{}_{\mathrm{<mtext>r</mtext>}}\text{u}\text{|}\text{u}\text{|δ(x,0)s(z,H)}$ where k_r is the pressure-loss coefficient of the fence, ū is the local mean horizontal (x) velocity, δ(x,0) is the delta function and s(z,H) is a unit step function which is zero for heights (z) greater than the fence height, H. Previous experiments on neutrally stratified surface-layer flow through a porous fence were numerically simulated. Very good agreement with the observed velocity deficit in the near wake (x ? 15H where H = fence height) of the fence was obtained using a Reynolds-stress closure scheme. The predictions of the $\text{“k-?”}$ closure scheme (which includes turbulent kinetic energy and energy dissipation rate equations to estimate the eddy viscosity) and the simplest scheme tested, eddy viscosity $\text{K = K}{}_0\text{= ku}{}_{10}\text{z}$ eddy viscosity at all downwind distances equal to its value far upstream $\text{ku}{}_{10}\text{z}$ where k = von Karman's constant, $\text{u}{}_{\mathrm{1o}}\text{=}\text{friction velocity}\text{, z =}\text{height}$) were only slightly less satisfactory. Satisfactory estimates of the pattern of turbulent kinetic energy behind the fence were obtained. All simulations failed to predict the sharp speedup observed over the fence, and consequently yielded a slower rate of recovery towards equilibrium than observed. Attempts to improve prediction of the speed-over and the far wake by including corrections for mean streamline curvature were unsuccessful.Design aids for isolated windbreaks have been generated from the prediction of the second-order closure model. These give the velocity reduction to be expected in the near wake of the fence and the drag on the fence for a range of values of the fence pressure-loss coefficient, k_rmr.     

Comparison between 2.5-D and 3-D realistic models for wind field adjustment

In previous works, many authors have widely used mass consistent models for wind field simulation by the finite element method. On one hand, we have developed a 3-D mass consistent model by using tetrahedral meshes which are simultaneously adapted to complex orography and to terrain roughness length. In addition, we have included a local refinement strategy around several measurement or control points, significant contours, as for example shorelines, or numerical solution singularities. On the other hand, we have developed a 2.5-D model for simulating the wind velocity in a 3-D domain in terms of the terrain elevation, the surface temperature and the meteorological wind, which is consider as an averaged wind on vertical boundaries. Using the meteorological wind as datum, the 2.5-D model provides a 3-D local wind modified by topography and thermal gradients on the surface by solving only a 2-D optimal control problem where the boundary condition is the control. In this case, the finite element discretization consists on a triangular mesh adapted to the terrain topography and roughness length. In both models, the wind field adjusts to several wind speed measurements at several points in the 3-D domain and eventually to an average wind flux on the boundary.In this paper we introduce several advances in the 2.5-D and 3-D wind models and we compare their results on a region located in the Province of Lugo (Spain) with realistic data that have been provided by the company Desarrollos Eólicos S.A. (DESA). In order to obtain the best adjustment of models results to the measurements, the main parameters governing the models are estimated by using genetic algorithms with a parallel implementation.     

Wind structure in a rural atmospheric boundary layer near the ground

This paper describes mean velocity, turbulence intensity, Reynolds stresses and co-spectra measured during field experiments. The data were obtained using orthogonal arrays of propeller anemometers mounted on a 20 m tower. Data were recorded only when the wind speed was ～ 10 m s^?1 or higher, and the boundary layer was therefore assumed to be neutrally stable.Values of ?_uw (0) calculated both by the eddy correlation technique and from the velocity profile were found to agree well, and also to agree well with values suggested in the literature review of Counihan [1].     

An alternative model for evaluating sustainable urbanization

In recent years, there has been rapid urbanization worldwide, resulting in both benefits and problems. Sustainable urbanization has become an important aspect in promoting sustainable development. Existing studies have introduced various methodologies to guide urbanization towards sustainable practices. The application of these methods has contributed to improving urban sustainability. To further support the effective applications of the principles of sustainable urbanization, a tool is needed to evaluate whether a particular process of urbanization is sustainable. In this paper, we introduce an alternative model for evaluating sustainable urbanization by investigating the relationship between urbanization and urban sustainability. The practice of sustainable urbanization is defined as a dynamic process that enables urban sustainability to improve or to maintain a certain level of practice. By employing this definition, we introduce a sustainable urbanization elasticity coefficient e_SU, which is defined by two parameters: urbanization velocity (V_μR) and urban sustainability velocity (V_μS). The sustainability of an urbanization process is measured by the value of e_SU or read from the V_μR-V_μS coordinate. A case study demonstrates the application of the measure e_SU and the V_μR-V_μS coordinate. The proposed model is an effective tool to help policy makers understand whether the urbanization processes they support are sustainable and thus whether to correct practices. The model also allows comparison of different urbanization practices and thereby encourages the sharing of successful experiences.     

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.