下击暴流移动增大效应及带协同流壁面射流模拟方法

目前对移动型下击暴流的研究主要基于冲击射流模型,但研究中通常考虑的平移速度较低且缩尺比较小.为此,通过风洞试验与数值模拟方法,研究移动冲击射流风场中平移速度对出流段平均风场的影响,得到了平移速度对下击暴流出流段最大风速增大效应的经验表达式.同时考虑到采用壁面射流方法能够得到较大尺度的下击暴流出流段风场,但并未考虑下击暴...     

Numerical simulation of downburst winds

The wind field of an intense idealised downburst wind storm has been studied using an axisymmetric, dry, non-hydrostatic numerical sub-cloud model. The downburst driving processes of evaporation and melting have been paramaterized by an imposed cooling source that triggers and sustains a downdraft. The simulated downburst exhibits many characteristics of observed full-scale downburst events, in particular the presence of a primary and counter rotating secondary ring vortex at the leading edge of the diverging front. The counter-rotating vortex is shown to significantly influence the development and structure of the outflow. Numerical forcing and environmental characteristics have been systematically varied to determine the influence on the outflow wind field. Normalised wind structure at the time of peak outflow intensity was generally shown to remain constant for all simulations. Enveloped velocity profiles considering the velocity structure throughout the entire storm event show much more scatter. Assessing the available kinetic energy within each simulated storm event, it is shown that the simulated downburst wind events had significantly less energy available for loading isolated structures when compared with atmospheric boundary layer winds. The discrepancy is shown to be particularly prevalent when wind speeds were integrated over heights representative of tall buildings. A similar analysis for available full scale measurements led to similar findings.     

Numerical investigation of the influence of topography on simulated downburst wind fields

A, dry, non-hydrostatic sub-cloud model is used to simulate an isolated stationary downburst wind event to study the influence topographic features have on the near-ground wind structure of these storms. It was generally found that storm maximum wind speeds could be increased by up to 30% because of the presence of a topographic feature at the location of maximum wind speeds. Comparing predicted velocity profile amplification with that of a steady flow impinging jet, similar results were found despite the simplifications made in the impinging jet model. Comparison of these amplification profiles with those found in the simulated boundary layer winds reveal reductions of up to 30% in the downburst cases. Downburst and boundary layer amplification profiles were shown to become more similar as the topographic feature height was reduced with respect to the outflow depth.     

Numerical simulations of impinging jets with application to downbursts

Numerical simulations are employed to investigate the macro-flow dynamics and scale (Reynolds number) dependency of impinging jets with applications to downburst related high intensity winds. The flow is quasi-periodic with vortex rings, generated by the initial jet instability, impinging on the surface where they determine an unsteady separation-reattachment of the boundary layer. The maximum velocities are encountered at less than 0.05 jet diameters from the surface immediately after the touch down of the main ring-vortex. Due to the unstable separation-reattachment of the boundary layer the flow is Reynolds number (scale) dependent. The maximum in the mean velocity profiles increases and tends towards the wall as the Reynolds number increases. The unsteady Reynolds Averaged Navier-Stokes simulations provide time series which when fitted to the RFD full-scale data, (Gast, K.D., Schroeder, J.L., 2005. Extreme wind events observed in the 2002 thunderstorm outflow experiment. In: Proceedings of the 10th Americas Conference on Wind Engineering (10 ACWE), Baton Rouge, LA, USA), are used to estimate both the length and the velocity scales of the full-scale event. Moreover, these simulations provide a first spatio-temporal flow model for wind loading on structures due to downbursts.     

Pulsed wall jet simulation of a stationary thunderstorm downburst, Part A: Physical structure and flow field characterization

A pulsed wall jet has been used to simulate the gust front of a thunderstorm downburst. Flow visualization, wind speed and surface pressure measurements were obtained. The characteristics of the hypothesized ring vortex of a full-scale downburst were reproduced at a scale estimated to be 1:3000.     

A parametric study of downburst line near-surface outflows

Large Eddy Simulations (LES) of two identical downburst events, representing part of a downburst line, with a range of spatial (1, 2, and 3 km) and temporal (30, 60, 90, and 120 s) separations, are used to compare near-surface outflow features with those of an isolated event. Peak maximum wind speed and total damaging surface footprints are used to determine the expected critical wind loading conditions and likelihood of a surface structure being exposed to such an event. The results show that significant wind speed amplification factors of up to 1.55 and increases in damaging surface footprint of at least 70% can be expected from a two event downburst line. In addition, the peak maximum horizontal velocity in a downburst line is associated with smaller scale structures in the flow caused by downdraft interactions aloft, as opposed to the primary ring vortex expected for an isolated event. Subsequently, conventional downburst velocity profiles, used for an isolated event, cannot be applied in the collision region of a downburst line.     

风暴移动对下击暴流风场特性的影响研究

气象观测表明,下击暴流的风暴中心是移动的,风暴的移动会对风场特征产生重要影响。为研究风暴移动对下击暴流风场特性的影响,基于计算流体动力学方法建立考虑移动效应的下击暴流数值模型,并通过可移动喷口的冲击射流试验对数值模拟结果进行了验证。结果表明：移动下击暴流风场在风暴中心前缘存在较大环涡,形成弓形分布的极值风速区域,最大水平风速出现在距离风暴中心r=1.0Djet（Djet为喷口直径）左右,极值风速可达1.5Vjet（Vjet为射流速度）;风暴后方水平风速被减弱,最大风速出现在r=1.5Djet附近,极值风速约为0.8Vjet。近地面竖向风速随高度的增大而增大,受风暴移动的影响,在风暴中心区域,竖向风速向风暴移动方向一侧倾斜;在风暴中心区域以外,竖向风速主要由环涡引起,较大竖向风速主要分布于风暴中心后方以及风暴中心前方1.0Djet~2.0Djet的区域。风暴的移动导致风场中测点风速和风向随时间变化显著。     

基于FLUENT的下击暴流风场的数值模拟

基于冲击射流理论,利用CFD（Computational FluidDynamics）通用商业软件FLUENT对下击暴流风场建立三维数值模型,分析了下击暴流风场径向速度的竖向剖面分布,并将此结果与三种经验风速剖面和一种实测风速剖面进行对比,认为此模拟方法所得到的下击暴流风场可信,这对进一步研究实际工程的抗风设计有一定的借鉴意义。     

Experimental measurement and numerical simulation of an impinging jet with application to thunderstorm microburst winds

Wind profiles and characteristics in a thunderstorm downburst are significantly different from that of regular boundary layer winds. This paper deals with the experimental and numerical simulation of a type of thunderstorm wind, namely the microburst, to study the outflow velocity characteristics. The microburst is simulated as a round jet, impinging onto a flat plate. A generic empirical equation for radial velocity profile is developed based on the experimental data, using hotwire, pressure rakes and particle image velocimetry (PIV). The experimental results are used to validate CFD simulations and to find the applicability of different turbulence models for this kind of flow. Favorable agreement between numerical and experimental studies indicates that CFD can be used for this kind of complex flow.     

稳态冲击射流作用下平地及坡地高层建筑的风荷载特性

现行规范的设计风荷载以普通大气边界层风场为主,其平均风剖面为指数型或对数型。下击暴流风场的典型风剖面与普通边界层风场差异很大。基于稳态冲击射流试验和数值模拟分析,考虑平地与坡地两种地形以及坡地坡度的影响,研究冲击射流风场中的高层建筑物表面风压分布特性与风荷载情况。风洞试验和数值模拟结果表明：冲击射流风场中建筑表面风压分布不同于大气边界层风场,迎风面最大风压出现在建筑下部;坡地上建筑物迎风面风压小于同等情况下平地上建筑物迎风面风压,而在侧面和背风面则呈现出相反的规律。对不同坡度坡地上的模型分析结果表明,随着坡地坡度的增大,建筑迎风面风压逐渐减小。     

雷暴冲击风作用下高层建筑风压幅值特性研究

采用冲击射流装置模拟雷暴冲击风,对4个不同深宽比的高层建筑模型进行测压试验,分析了各模型8个不同径向位置处的风压幅值特性,并与大气边界层风作用下的建筑表面风压系数进行了对比。结果表明：雷暴冲击风作用下,建筑迎风面为正压,侧面和背风面均为负压;迎风面平均和脉动风压受模型深宽比影响较小,侧面和背风面受深宽比影响较大;随着径向距离的增加,迎风面平均风压系数逐渐减小,脉动风压系数先增大后减小,侧面平均风压系数绝对值以及脉动风压系数先增大后减小,背风面平均和脉动风压系数变化较为平缓;各模型迎风面风压系数沿高度呈“鼻子”状分布,最大风压出现在0.25H(H为模型高度);与大气边界层风作用下建筑表面风压幅值相比,雷暴冲击风作用下高层建筑模型的迎风面中下部区域以及侧面前缘部位风压系数较大,考虑雷暴冲击风作用的高层建筑设计时,应对这些区域的风荷载取值进行适当放大。     

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.     

The use of impulse ventilation for smoke control in underground car parks

Impulse ventilation systems (IVS) are used to provide ventilation for covered car parks and to control the smoke in the event of fire. In this paper the interaction between the fire ceiling jet and the flow driven by jet fans is studied using CFD simulations. A sensitivity analysis considering important parameters as position and intensity of fire source, transversal distance between jet fans, restriction of exhaust flow rate and dimension of car park exhaust opening is carried out and rules for the design of 50 N thrust jet fans are deduced. An analytical model for the flow field near the ceiling is developed and compared with CFD simulations. This model is intended to support a first approach of the design of IVS.     

On the simulation of the plume from stacks of buildings

A three-dimensional numerical model is used for a turbulent buoyant jet. The standard k–ϵ model has been modified to focus on the buoyancy-production term. The usual and modified buoyancy production coefficients are used for comparisons with experimental data reported in the literature. Imported numerical results are obtained with the modified coefficient for the stack–exit velocities and temperatures. The effects of these parameters on flow characteristics are discussed.     

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).     

碰撞射流通风供暖房间热环境特征的研究

采用数值模拟方法研究了碰撞射流通风系统的供热特性,对供热房间内热环境特征以及热舒适性的分析结果表明：在典型送风参数工况下,碰撞射流供热房间内靠近送风口附近位置处存在垂直温度分层,空气温度随房间高度增加而降低,工作区热舒适情况良好。     

Numerical study on wind pressure characteristics of Chinese yurt building under downburst wind

Inner Mongolia is a high-frequency thunderstorm region in China, and the downburst caused by the thunderstorm weather is a severe threat to buildings. In order to study the influence of downburst on the wind pressure characteristics of the yurt building, the wind field model of the yurt building under downburst is established based on the computational fluid dynamics method, and the effect of the wall treatment method and turbulence model on the numerical simulation of wind pressure of the yurt building under downburst is analyzed. The results demonstrate that the maximum positive pressure at the windward side of the yurt building occurs at 3/4 of the yurt building height under downburst, and the maximum negative pressure at the roof of the yurt building appears at the center of the roof. Compared with the experimental results, the Shear Stress Transport (SST) k-ω model is suitable for simulating both sides of the yurt building, while the Reynolds Stress equation Model (RSM) is suitable for simulating the windward side, roof, and leeward of the yurt building. The enhanced wall treatment is appropriate for simulating the remaining sides of the yurt building while the standard wall function is appropriate for simulating both sides of the building.     

Some effects of finite solidity on the aerodynamics of horizontal-axis wind turbines

This paper describes an exploratory, computational study of the effects of local solidity on the aerodynamics of a horizontal-axis wind turbine. The parameters governing the path of the trailing vorticity in the crude model employed were chosen to match the circulation measured behind a two-bladed model turbine by previous workers. Traditional blade element theory, which assumes two-dimensional aerofoil characteristics, significantly underestimated the measured circulation at high angles of incidence. The calculations suggest that the major departure from two-dimensional behaviour is a reduction in the surface vorticity near the leading edge due to the trailing vorticity of a blade. This reduction, which is a consequence of finite local solidity, appears sufficient to prevent boundary layer separation at angles at which this would occur for two-dimensional flow. The magnitude of the reduction decreases as the angle of incidence is reduced. The results suggest that the optimization of wind turbine design will require fully three-dimensional modelling.     

Survival dynamics of fecal bacteria in ponds in agricultural watersheds of the Piedmont and Coastal Plain of Georgia

Animal agriculture in watersheds produces manure bacteria that may contaminate surface waters and put public health at risk. We measured fecal indicator bacteria (commensal Escherichia coli and fecal enterococci) and manure pathogens (Salmonella and E. coli 0157:H7), and physical-chemical parameters in pond inflow, within pond, pond outflow, and pond sediments in three ponds in agricultural watersheds. Bishop Pond with perennial inflow and outflow is located in the Piedmont, and Ponds A and C with ephemeral inflow and outflow in the Coastal Plain of Georgia. Bromide and chloride tracer experiments at Bishop Pond reflected a residence time much greater than that estimated by two models, and indicated that complete mixing within Bishop Pond was never obtained. The long residence time meant that fecal bacteria were exposed to solar UV-radiation and microbial predation. At Bishop Pond outflow concentrations of fecal indicator bacteria were significantly less than inflow concentrations; such was not observed at Ponds A and C. Both Salmonella and E. coli 0157:H7 were measured when concomitant concentrations of commensal E. coli were below the criterion for surface water impairment indicating problems with the effectiveness of indicator organisms. Bishop Pond improved down stream water quality; whereas, Ponds A and C with ephemeral inflow and outflow and possibly greater nutrient concentrations within the two ponds appeared to be less effective in improving down stream water quality.     

水平喷射火撞击垂直障碍物的温度特性研究

为了研究喷射火撞击邻近建筑物后火羽流的温度场特性,采用数值模拟软件建立喷射火模型,模拟不同长宽比的矩形泄漏孔甲烷水平喷射火撞击障碍物场景,分析喷射火温度在水平方向和障碍物壁面垂直方向的分布特征。结果表明：障碍物壁面温度场受燃气喷射速率和泄漏孔形状特征影响显著;泄漏速率越大,温升区宽度更宽,泄漏孔长宽比越大,温升区域的宽度弱化越显著;泄漏速率增加,使温升区在垂直方向上和水平面上的影响范围更宽,且泄漏孔长宽比增大使温升区厚度增加,水平面的影响宽度发生变化。