角部处理的二维方柱风压分布特性的试验研究

在低紊流度的均匀流场中,对10种不同角部处理的二维方柱模型进行了刚性模型测压试 验,试验雷诺数的变化范围为Re≈1.0×105~4.8×105。根据各模型的阻力系数随雷诺数的变化 规律将10个模型进行了分类,然后分类讨论了各模型的风压分布规律,并尝试从气流扰流的角 度揭示模型的角部处理对方柱模型气动特性的影响机理。研究表明,在试验雷诺数范围内,切 角率B/D≤15%的模型和圆角率R/D≤15%的模型的气动特性基本不随雷诺数而改变,而圆角率R/D ≥20%的模型的雷诺数效应明显。对于圆角率R/D=20%、30%和40%的模型,随着圆角率的增大, 风压分布发生改变所对应的雷诺数随之减小。     

Characteristic of wind loads on a hemispherical dome in smooth flow and turbulent boundary layer flow

A series of wind tunnel tests was performed to investigate the effects of Reynolds number on the aerodynamic characteristics of hemispherical dome in smooth and turbulent boundary layer flows. Reynolds number of this study varies from 5.3×10⁴ to 2.0×10⁶. Instantaneous pressures were measured through high frequency electronic scanner system. Mean and RMS pressure coefficients on the center meridian and the overall pressure patterns of domes were calculated for comparative study. The results indicate that, in smooth flow, the transition of separation flow occurs in-between Re=1.8×10⁵–3.0×10⁵ and pressure distributions become relatively stable after Re>3.0×10⁵. In turbulent flow, the transition of separation flow occurs at lower Reynolds number, Re<1.1×10⁵, while the pressure distributions become Reynolds number independent at Re=1.0–2.0×10⁵.     

On the Reynolds number sensitivity of the aerodynamics of bluff bodies with sharp edges

A review of recent comparisons between the aerodynamic characteristics of bluff bodies with sharp edges at low and high Reynolds number is presented. It is concluded that the relaxation of the Reynolds number similitude requirement in the study of the aerodynamics of bluff body such as long span bridge decks can lead to systematic errors. It is observed also that some bluff geometries appear to be more sensitive than others to Reynolds number effects. Parameters that could link the geometry of a bluff body with the sensitivity of its aerodynamics to Reynolds number have to be clearly defined. An ongoing study of these issues based on experiments in a pressurized wind tunnel at the National Research Council Canada is introduced and preliminary results are discussed. For rectangular prisms, the fineness ratio or width-to-depth ratio is identified as one of the parameters.     

The effect of nonharmonic forcing on bluff-body aerodynamics at a low Reynolds number

The aerodynamic response of a circular cylinder to nonharmonic forcing of the inflow velocity is studied by numerically solving the equations of two-dimensional fluid motion on an orthogonal curvilinear mesh. The effect of varying the inflow velocity waveform while maintaining other forcing parameters constant at a Reynolds number of 180 is considered in this study. The forcing frequency is 84% of the natural vortex shedding frequency in the unforced wake while the peak-to-peak amplitude of velocity oscillation is 65% of the reference velocity. Results are reported for the drag and lift coefficients and the flow field in terms of streamline patterns and vorticity distributions. It is shown that the wake is locked-on to the forcing frequency for all cases tested but the aerodynamic response is systematically modified by the imposed changes in the velocity waveform. The magnitude and the phase of the fluctuating drag and lift forces and the mean drag force are affected. These effects are associated with changes in the mechanism of vortex formation and shedding in the wake of the cylinder; it is found that the rolling up of the individual shear layers on both sides can be manipulated to promote shedding of single vortices or vortex pairs.     

Two-degree-of-freedom inclined cable galloping—Part 1: General formulation and solution for perfectly tuned system

Galloping of inclined cables and other slender structures can occur in the critical Reynolds number range and/or in skew winds due to associated changes in the static force coefficients, even for cross-sections that are otherwise stable. A complete model of the quasi-steady aerodynamic forces leading to galloping has therefore been developed, for vibrations of any cylinder in two translatory degrees of freedom. It allows for arbitrary orientations of the flow velocity and the undamped vibration plane axes relative to the cylinder, and for variation of the force coefficients with Reynolds number and the relative angles. Analytical treatment of the eigenvalue problem has then led to an explicit expression for the minimum structural damping ratio required to prevent galloping for a perfectly tuned two-degree-of-freedom system, which it has been shown can differ significantly from the damping requirement for single-degree-of-freedom motion.     

3D flow around a rectangular cylinder: A computational study

The aim of this paper is to provide a contribution to the analysis of the 3D, high Reynolds number, turbulent, separated and reattached flow around a fixed sharp-edged rectangular cylinder with a chord-to-depth ratio equal to 5. The work is developed in the perspective of the benchmark on the aerodynamics of a rectangular cylinder (BARC), in terms of an exploratory computational study. First, the adopted flow modelling and computational approach are shortly described. Second, the obtained main aerodynamic integral parameters are compared with other results proposed in the literature. Hence, the 3D flow features around the nominally 2D cylinder are investigated by means of both proper orthogonal decomposition and coherence function of the side-surface fluctuating pressure field. Once the main 2D nature of the flow has been pointed out, some of the 2D mechanisms that are responsible for the variation of the fluctuating aerodynamic forces are scrutinised: the computational approach post-processing facilities are employed to look for significant relationships between the flow structures, the pressure field and the aerodynamic force components.     

On interference between two circular cylinders in staggered arrangement at high subcritical Reynolds numbers

The results of a wind-tunnel investigation on the interference between two identical parallel circular cylinders arranged in staggered configurations, carried out in a uniform smooth flow at high subcritical Reynolds number, are presented. On the basis of the pressure measurements and the flow visualization, three main flow patterns are classified and discussed. Close attention is paid to the switching phenomenon of two different pressure patterns at critical angles. This switching creates discontinuity of large lift forces on cylinders. With measurements of velocity profiles and power spectral analysis in the gap between the two cylinders, the mechanism of the appearance of this extreme aerodynamic force is discussed.     

基于气动参数识别技术的钝体雷诺数效应研究

通过有限的风洞试验数据,揭示雷诺数效应的发生、发展机理,关键在于找出能够有效表征雷诺数效应的特征参数。以经典圆柱模型为研究对象,在系列雷诺数效应风洞试验基础上,从边界层分离、转捩和旋涡脱落等方面,研究了表征雷诺数效应的气动参数及其数据识别方法。针对经典圆柱模型,采用压力系数时程和压力梯度提取有关边界层分离和流动转捩的物理信息。研究表明,在分离边界层转捩区间内分离点附近的压力系数时程会出现明显的阶跃现象,并伴有骤增的压力梯度值,可作为判定雷诺数转捩区间的辅助方法。为研究不同雷诺数下模型周围主导旋涡的作用特点,采用谱正交分解法(SPT)来识别旋涡作用及其对脉动风压场的能量贡献等,旨在对类似结构的雷诺数效应研究提供参考。     

非圆截面斜拉索气动性能风洞试验研究

大跨度斜拉桥斜拉索上的风荷载对于主梁位移和内力的贡献占整个风荷载的60%~70%,斜拉索的气动力是静力和稳定性检算的基础,准确掌握斜拉索的气动力具有重要意义。通过天平测力风洞试验,在雷诺数为10×104~42×104,对直径为120mm的圆形截面斜拉索模型和3种非圆截面模型进行测力试验,得到各个工况下阻力系数随雷诺数的变化规律,分析截面变形、风向角对阻力系数的影响。结果表明：斜拉索截面变化能够增强雷诺数效应,风向角在0°~30°情况下,阻力系数随斜拉索截面变形的程度呈单调减小的趋势;风向角在40°~90°情况下,阻力系数随斜拉索截面变形的程度呈单调增大的趋势。将气动力系数和风向角度按照亚临界、临界和超临界雷诺数区域进行分区,得到各个区域内阻力系数的数值和拟合公式,通过拟合公式,可以方便地为类似截面结构的风荷载设计提供依据。     

主梁断面形状对车-桥系统气动特性影响的风洞试验研究

确定车辆和桥梁各自的气动参数是车-桥耦合振动分析的基础。为研究主梁断面形状对车辆和桥梁气动特性的影响,利用自制的三分力分离装置-交叉滑槽系统,针对8种分离式双箱主梁断面进行多工况模型风洞试验。通过对不同模型及工况试验结果的对比,讨论不同主梁断面形状下车-桥系统的雷诺数效应,得出不同行车位置处车辆和桥梁各自气动参数随主梁宽高比的变化规律以及其阻力系数的取值方法,为后续抗风设计及风-车-桥耦合振动研究提供参考。     

三维内肋管紊流换热和阻力特性的试验研究

以空气为工质,利用正交实验设计研究了雷诺数在25 000~110 000范围内的9种大管径三维内肋管的紊流换热和流动特性,优选出最佳管型,并得出努谢尔特数与范宁摩擦系数关于雷诺数和三维管肋参数的实验关联式。结果表明,三维内肋管的综合特性优于相同管径的光管,换热系数最高可达光管的3.15倍,热力性能系数最高可达光管的1.45倍,表明了大管径三维内肋管在管式换热器中运用的可行性;此外,各主要参数中,肋高对流动换热特性的影响最大,肋间距次之,肋宽最小,为强化管参数提供了优化方向。     

Wind tunnel tests on heavy road vehicles: Cross wind induced loads—Part 2

The sensitivity of heavy road vehicle aerodynamic coefficients to different testing parameters (scenario, vehicle type and turbulence intensity) is experimentally evaluated in this paper through wind tunnel experiments. The first part of the paper has investigated the aerodynamic loads (both stationary and non-stationary) acting on a high-sided lorry (VAN) in a flat ground scenario. The mean aerodynamic coefficients, the flow pattern around the vehicle and the aerodynamic admittance function have been assessed and compared for different wind turbulence conditions (boundary layer simulations). The present paper instead investigates the influence of the infrastructure scenario (flat ground, embankment, single and double viaduct), of the exposition (upwind or downwind) and of a trailed unit on the aerodynamic loads acting on the vehicle. Moreover, the mean aerodynamic coefficients and the aerodynamic admittance function of the high-sided lorry considered in the first part are compared with the ones of other heavy vehicles (tank truck, tractor-semitrailer combination and tractor-trailer combination) to assess the influence of the vehicle geometry.     

Flow interference between three equispaced cylinders when subjected to a cross flow

The drag and lift coefficients occurring on one cylinder in a group of three lying with their axes perpendicular to a uniform stream flow have been measured at a flow Reynolds number of 3 × 10⁴ at various inclination angles to the free stream. The cylinders were arranged with their centres equidistant from each other (an equilateral triangle), the spacing ratio of their centres being in the range 1.25 < S/D < 5.From the force coefficients for the individual cylinders, total force coefficients for the group as a whole and the direction in which the resultant force acts were determined. It was found that at certain inclination angles the force coefficients were similar to those pertaining to two-cylinder flow, but that in general the effect of the third cylinder was significant. The results presented should aid in the establishment of standard design codes for flow interference around groups of cylinders.     

Experimental and numerical modelling of the three-dimensional incompressible flow behaviour in the near wake of circular cylinders

An experimental investigation was carried out to study the structure of the flow field around three-dimensional circular cylinders. The study of the flow field around an obstacle was performed in a wind tunnel using a particle image velocimetry (PIV) system. The flow of a fluid around an obstacle with a different velocity to the oncoming flow was examined. The results showed the dependence of the flow structure around the obstacle on its Reynolds number, and the spacing between a pair of obstacles. Detailed quantitative information of turbulence parameters in the vicinity of the obstacle was attained. Extensive wind tunnel experimental results are presented and compared with numerical simulation. A three-dimensional numerical model with Reynolds stress model (RSM) turbulence and a non-uniform grid system were used to examine the effects of a single cylinder and two cylinders in tandem on the flow. The principal objective was to analyse three-dimensional flow past a single cylinder and two circular cylinders placed in tandem by combining the application of a PIV experimental technique and an RSM turbulence model.For the case of two cylinders in tandem, the flow patterns are characterized in the gap region as a function of the distance between the cylinders. A good level of agreement was found between the experimental results of flow and numerical simulation.     

Wind load on a container crane located in atmospheric boundary layers

Wind environment nearby a container crane located in a port was simulated in a wind tunnel test section. A scale-downed model of the container crane was embedded in an atmospheric boundary layer (ABL). Wind load acting on the container crane model was measured using a seven-component balance with varying wind direction (yaw angle) in two different ABLs. Reynolds number effect on the aerodynamic coefficients was investigated with varying Reynolds number. Effects of ABL, adjacent structures and boom positions on the wind load variation were evaluated experimentally. In addition, the effect of the minute structures attached at the crane model such as stairways, wheels, handrails, etc., were also investigated in this study.     

CFD modelling of the air and contaminant distribution in rooms

The k-ϵ model is a widely used model in engineering practice in handling indoor air quality problem. However, difficulties may arise when using the high Reynolds number k-ϵ model to simulate air flow patterns close to the boundaries of air and the stagnant component as well as the low air flow fluctuation elsewhere in a room. When using the k-ϵ model for low Reynolds number cases, the correlations between turbulent coefficients and turbulent Reynolds number must also be defined. By using the so-called Kolmogorov micro scale method, a new set of turbulent coefficient functions was deduced in this paper for the k-ϵ model in a case of low Reynolds number flow. Using the standard wall function leads to large differences between the measured and calculated heat transfer coefficient. A special wall function valid for a viscous sublayer, a buffer zone and a fully turbulent log-law zone is recommended in this paper. In addition, the modelling of air terminal devices in CFD simulations is summarized by using a literature collection.     

Numerical study of aerodynamic characteristics of a square prism in a uniform flow

Aerodynamic characteristics of a square prism in a uniform flow for various angles of attack have been investigated using LES turbulence model. The results show that mean aerodynamic coefficients, surface pressures, and flow patterns for all angles of attack agree favorably with experiments. In addition, spanwise length of computational domain is found to slightly influence the mean aerodynamic coefficients whereas it shows strong impacts on fluctuating aerodynamic coefficients, which motivates a development of grid-independent estimation method for quantitative evaluations of fluctuations. The predicted power spectral densities for fluctuating aerodynamic coefficients are in good agreements with experiments in primary frequencies, which can be explained by the flow patterns. Strouhal numbers obtained from the simulations also agree well with experiments and acute change observed around 14° is successfully captured. Finally, a method for estimating grid-independent fluctuating aerodynamic coefficients is proposed by using a systematic elongation of spanwise length and is validated by the numerical tests.     

Aerodynamic behaviour investigation of the new EMUV250 train to cross wind

In this work a numerical-experimental procedure for the aerodynamic optimisation of the new train AnsaldoBreda EMUV250 in terms of behaviour to cross wind, will be presented. The first step of the research consists in evaluating the aerodynamic forces acting on the original geometry of the train by means of wind tunnel tests on scale model and of CFD numerical simulations. Thanks to the information provided by the CFD analysis (pressure distribution, velocity field, etc.), two different new versions of the train, specifically designed to achieve a better cross wind behaviour through a modification of both the train roof and the nose, have been tested in the Politecnico di Milano Wind Tunnel. A final comparison between the three versions of the train considered in this work is presented in terms of force/moment aerodynamic coefficients.     

Edge degree-of-sharpness and free-stream turbulence scale effects on the aerodynamics of a bridge deck

This paper discusses the sensitivity of the aerodynamic behavior of a trapezoidal-shaped bridge deck cross-section to its lower corner degree-of-sharpness and to the incoming flow turbulence integral length scale in conjunction with low turbulence intensity. Since these features are hard to set and measure in experimental facilities, the aerodynamic behavior of the body has been investigated through the computational simulation of the flow around it. The results are given in term of force coefficients, Strouhal number, pressure distribution along its surface, and the mean and instantaneous flow patterns. Dramatic changes in the force coefficients and Strouhal number occur following small changes in the parameter values. These changes have been found to be due to significant modifications in the topological structure of the flow. Special emphasis has been given to the analysis of the separation and reattachment points, the recirculation bubble length, the vortex shedding mechanisms and the wake structures. On the basis of the results, four aerodynamic regimes have been pointed out in analogy with the well-known individual Re number regimes. Some of these regimes have also been recognized on the deck section with the considered sharpest and smoothest lower edge treatments, i.e. for the highest and lowest degrees-of-sharpness.