Passive Scalar Transport in a Turbulent Cylinder Wake in the Presence of a Downstream Cylinder

This work aims to investigate how the presence of a downstream cylinder affects the passive scalar transport in a cylinder wake. The wake was generated by two tandem brass circular cylinders of the same diameter (d). The cylinder centre-to-centre spacing L/d was 1.3, 2.5 and 4.0, respectively, covering the three typical flow regimes of this flow. The upstream cylinder was slightly heated. Measurements were conducted at x/d= 10 and Re (≡ dU _∞/ν, where U _∞ is the free-stream velocity and ν is the kinematic viscosity of fluid) = 7000. A three-wire probe consisting of an X-wire and a cold wire was used to measure the velocity and temperature fluctuations, while an X-wire provided a phase reference. The phase-averaged velocity vectors and vorticity display single vortex street behind the downstream cylinder, irrespective of the flow regimes. However, the detailed flow structure exhibits strong dependence on L/d in terms of the Strouhal number, the vortex strength and its downstream evolution. This naturally affects passive scalar transport. The coherent and incoherent heat flux vectors show significant variation for different L/d.     

Interaction of a Supersonic Turbulent Wake with Acoustic Disturbances

The effect of an external acoustic field on disturbances in a supersonic turbulent wake is investigated experimentally.     

The Effect of Reynolds Number on the Passive Scalar Field in the Turbulent Wake of a Circular Cylinder

In this paper we investigate by experiments the effect of Reynolds number on a passive scalar (temperature) field in the turbulent wake of a slightly heated circular cylinder. The Reynolds number defined by Re≡U _∞ d/ν (see Nomenclature) is varied from Re= 1200 to Re= 8600. Temperature differential above ambient is chosen to be the passive scalar quantity. Present measurements are conducted using a cold wire (0.63 μm) probe. Results obtained suggest that Reynolds number in general has significant influence on the scalar mixing characteristics in the entire wake flow. Specifically, as Re increases, the mean scalar spreads out more rapidly, the scalar fluctuation intensity increases; however, its variance decays at a lower rate with downstream distance. It is also found that an increase of Re accelerates the streamwise evolution of the scalar probability density function from highly non-Gaussianity to near Gaussianity along the wake centreline. This reflects the reduction in length of the Karman-vortex street caused by an increase of Re.     

湍流涡致振动频率特性

用数值模拟方法对固定圆柱湍流涡脱落频率与弹性圆柱湍流涡致振动频率特性进行了研究,湍流计算模型采用标准κ-ε模型,压力泊松方程提法基于非交错网格系统.研究结果表明:固定圆柱湍流绕流涡脱落频率基本不随雷诺数而变,对于同一固有频率弹性圆柱,涡振频率基本不随雷诺数而变;对于某一固定雷诺数流动涡振频率在一定范围内与系统固有频率有关.     

绒毛控制圆柱涡激振动的风洞试验研究

利用绒毛对圆柱涡激振动抑制进行了风洞试验研究。通过改变附属绒毛无因次长度L/D比(L为绒毛长度,D为圆柱外径),研究L/D分别为0.6、1.2和1.8的模型在约化速度2~40的范围内对弹性支撑大质量阻尼系数圆柱涡激振动的抑制作用。试验采用激光位移传感器采集圆柱的横向(Y)和顺流向(X)位移,并用烟线测流场以揭示流动控制机理。结果表明,三种无因次长度的绒毛对大质量阻尼系数圆柱的涡激振动都有显著的抑制作用,随着L/D的增加,圆柱Y向无因次位移及功率谱密度幅值减弱,多达73.5%的无因次位移被抑制;且随着L/D的增加,圆柱附属绒毛频率比远离原始圆柱频率比。绒毛改变了圆柱的边界层分离点位置、抑制了边界层的相互作用并改变尾涡结构,从而抑制振动。     

The Unsteady Wake of a Circular Cylinder near a Free Surface

The behaviour of the wake Strouhal number for flow past a cylinder close to a free surface at both low and moderate Froude numbers is investigated numerically. For the low Froude number case (i.e., gravity-dominated), the results obtained are similar to those for flow past a cylinder close to an adjacent no-slip boundary. As the distance between the wall and the cylinder is reduced, the Strouhal number, as measured from the time varying lift, increases to a maximum at a gap ratio of 0.70. Further gap reduction leads to a rapid decrease in the Strouhal number, with shedding finally ceasing altogether at gap ratios below 0.16. The agreement between the results for a free surface and a no-slip boundary suggests that the mechanism behind the suppression of vortex shedding is common. For flow at a fixed gap ratio and a moderate Froude number, two distinctly different wake states are observed with the flow passing over the cylinder tending to switch from a state of attachment to the free surface, to one of separation from it, and then back again in a pseudo-periodic fashion. Even though there is a significant difference in Reynolds number, the predicted numerical two-dimensional behaviour is found to compare favourably with the experimental observations at higher Reynolds number.     

On the Interaction of Turbulent Shear Layers with Harmonic Perturbations

The problem of coherent perturbations in a turbulent shear layer is considered for the purpose of developing a mathematical model based on a triple decomposition that extracts the coherent components of random fluctuations. The governing equations for the mean and the coherent parts of flow are derived, assuming the eddy-viscosity equivalence for the random part of flow, and solved by iterations to provide a coupled solution of the problem as a whole. Calculations agree well with experimental data in the upstream part of the layer where the mean–coherent flow interaction is the most important. In this region, the interaction changes the mean flow velocity distribution in such a manner that the neutral stability curve is shifted upstream relative to its position in the undisturbed layer and the perturbation intensity decreases further downstream. Experiments show that the coherent waves suppress the turbulent Reynolds stress production downstream of this region, but the model fails to predict the layer spreading correctly probably due to an inadequate turbulence closure of the mean flow. For the case of a turbulent mixing layer, we suggest a new closure relation that takes into account this coherent-random interaction.     

On the Flow Induced by Centrifugal Buoyancy in a Differentially-Heated Rotating Cylinder

We consider the nature of thermally stratified flow in a closed cylinder rotating about the direction of gravity under conditions appropriate for terrestrial laboratory experiments. Motion is driven by centrifugal buoyancy, with outflow near the cold disk and inflow near the hot disk. Although similarity solutions for the infinite disk open-geometry problem exist and are easily found, even analytically in certain limits, there remain questions about the applicability of these spatially simplified models in a closed geometry with a vertical sidewall. This paper compares theoretical self-similar core solutions with computational simulations constructed to satisfy a wide range of sidewall thermal boundary conditions; insulating, conducting (with a linear temperature profile up the wall), hot (isothermal), or cold. The width of penetration of sidewall influence in toward the axis of rotation depends on the sidewall thermal boundary condition. However, as the cylinder radius is increased for a fixed height, a substantial region of the container about the axis is accurately described by the thermocline solutions of the theory. The non-self-similar region at large radius can include separation of the lower outflow boundary layer, a feature not evident in previous studies of this problem. Received 8 June 1999 and accepted 12 December 1999-->     

Experimental and Theoretical Sensitivity Analysis of Turbulent Flow Past a Square Cylinder

The Siemens SGT-800 3^rd generation DLE burner fitted to an atmospheric combustion rig has been numerically investigated. Pure methane and methane enriched by 80 vol% hydrogen flames have been considered. A URANS (Unsteady Reynolds Averaged Navier-Stokes) approach was used in this study along with the k ? ω SST and the k ? ω SST-SAS models for the turbulence transport. The chemistry is coupled to the turbulent flow simulations by the use of a laminar flamelet library combined with a presumed PDF. The effect of the mesh density in the mixing and the flame region and the effect of the turbulence model and reaction rate model constant are first investigated for the methane/air flame case. The results from the k ? ω SST-SAS along with flamelet libraries are shown to be in excellent agreement with experimental data, whereas the k ? ω SST model is too dissipative and cannot capture the unsteady motion of the flame. The k ? ω SST-SAS model is used for simulation of the 80 vol% hydrogen enriched flame case without further adjusting the model constants. The global features of the hydrogen enrichment are very well captured in the simulations using the SST-SAS model. With the hydrogen enrichment the time averaged flame front location moves upstream towards the burner exit nozzle. The results are consistent with the experimental observations. The model captures the three dominant low frequency unsteady motion observed in the experiments, indicating that the URANS/LES hybrid model indeed is capable of capturing complex, time dependent, features such as an interaction between a PVC and the flame front.     

Study of the Turbulent Velocity Field in the Near Wake of a Bluff Body

An experimental characterization of the turbulent flow structure formed downstream of a vertically mounted circular bluff body is performed. Three components of an instantaneous velocity field are measured using the stereo particle image velocimetry technique at the symmetry plane. The average velocity and the turbulent properties are analyzed. The results indicate a recirculation zone consisting of a toroidal vortex with similar dimensions for all Reynolds numbers. The largest turbulent fluctuations are found at the stagnation point region. The observed anisotropy of the normal Reynolds stress components is associated with the stagnation point flow, whereas the cross-correlation component extreme occurs in high strain rate regions. An analysis of the Reynolds tensor anisotropy using the Lumley triangle is performed, revealing that the largest departures from isotropy occur at high shear regions and also within the vortex.     

On the Electric Potential Induced by a Homogeneous Magnetic Field in a Turbulent Pipe Flow

In this paper we study a turbulent pipe flow of a weakly electrical conducting fluid subjected to a homogeneous magnetic field which is applied perpendicular to the flow. This configuration forms the basis of a so-called electromagnetic induction flow meter. When the Hartmann number is small so that modification of flow by the Lorenz force can be neglected, the influence of the magnetic field results only in a spatially and temporally varying electric potential. The magnitude of the potential difference across the pipe is then proportional to the flow rate and this constitutes the principle of the flow meter. In this study the flow and electric potential are computed with help of a numerical flow simulation called Large-Eddy Simulation (LES) to which we have added an equation for the electrical potential. The results of the LES have been compared with experiments in which the electric potential is measured as a function of time at several positions on the circumference of the pipe. Both the experimental and numerical results for the mean potential at the pipe wall agree very well with an exact solution that can be obtained in this particular case of a homogeneous magnetic field. Furthermore, it is found that fluctuations in the electric potential due to the turbulence, are small compared to the velocity fluctuations. Based on the results we conclude that electrical-magnetic effects in pipe flow can be accurately computed with LES.     

Dissipation of Turbulent Kinetic Energy in a Cylinder Wall Junction Flow

The subject of this study is the discussion of the dissipation of turbulent kinetic energy and its Reynolds number scaling in front of a wall-mounted cylinder. We employed highly resolved Large-Eddy Simulation and ensured that the computational grid was fine enough to resolve most of the scales. A perceptible fraction of the total dissipation is modeled. However, this fraction - about one third - is small enough so that the total dissipation suffers only marginally from some potential shortcomings of the turbulence model. Individual terms of the pseudo dissipation tensor and their Reynolds number scaling are discussed and compared. This tensor and thus the turbulent small scale structures are not isotropic at the Reynolds numbers investigated. Furthermore, the near-wall anisotropy under the horseshoe vortex is likely to persist to larger Reynolds numbers as it can be linked to a flapping of the near-wall layer. The turbulent length scale shows a strong spatial variability. In the region of the vortex system in the cylinder front, the distribution reveals a similar shape as the one of the turbulent kinetic energy and its amplitude is in the order of magnitude of the cylinder diameter. In contrast to the region dominated by the approach flow, the turbulent length scale is independent of the Reynolds number in the region dominated by the vortex system. Even though the flow investigated is in non-equilibrium, common a priori estimations and scalings of the Kolmogorov length scale based on macro scales give satisfying results.     

圆柱绕流电磁控制影响因素的实验研究

实验与数值模拟表明,利用电介质溶液中圆柱体侧表面附近分布的电磁场产生电磁力可有效改变圆柱流体边界层,控制圆柱绕流.本文对圆柱绕流电磁控制过程中的主要影响因素进行了实验研究,电磁包覆在圆柱表面后部时,其控制效果与全部包覆相当,而包覆在其它位置时,消涡效果较差;电磁极板窄的圆柱绕流控制效果较极板宽的消涡与减阻效果好.另外,电磁作用参数N愈大,消涡减阻的效果愈好.     

双旋转附属圆柱对主圆柱尾流控制的影响

基于Fluent软件平台,采用数值模拟方法对非稳态圆柱体结构尾流流动特性进行了研究。对在Re=50～200范围内,双旋转附属圆柱的转速对主圆柱体尾流流动特性的影响进行了分析。研究结果表明:随着附属圆柱旋转速率的增加,主圆柱体表面所受阻力系数平均值与均方根值、升力系数均方根值均会减小。同时,旋转速率的变化对柱体结构表面压力分布的影响显著,压力系数在附属圆柱的位置产生了跳跃性变化。另外,当附属圆柱转速达到临界值时,尾流涡街变窄,涡脱落现象消失,并且系统的能量效率到达最佳状态。     

高阻尼比低质量比圆柱涡激振动试验研究

针对圆柱的涡激振动问题,设计开发了高性能循环水槽与超声位移传感器,研究了高阻尼比、低质量比条件下,弹性支撑的刚性圆柱的涡激振动变化规律。结果表明:(1)高阻尼比条件下,振幅主要受独立参数阻尼比、质量比的影响;低质量比条件下,振动频率随流速增大而增大,"锁定区间"较高质量比范围扩大。(2)高阻尼比、低质量比条件下,阻尼的增大会导致振幅减小且"锁定区间"变窄;但振动频率在"锁定区间"内变化趋势一致。(3)高阻尼比、低质量比有利于涡激振动的能量转化,但阻尼不可过大,否则振幅与"锁定区间"均变小,影响电能转化。     

Hilbert-Huang变换方法在Duffing方程和圆柱尾迹流分析中的应用

本文采用N．E Huang最近提出的Hilbert-Huang变换方法分析了使用三阶Runge-Kutta方法数值求解的Duffing方程的性质和圆柱绕流尾流特性,分解得到了有限阶经验模态（Empirical Modes）,给出了相应的Hilbert谱。结果表明,Hilbert-Huang变换得到的主要含能模态物理意义清晰,相应的瞬时频率存在着明显的波内频率调制,很好地刻画了信号的时空局部特性。     

振荡流中圆柱受力测量研究

本文叙述了测量振荡流中圆柱受力大小和性质的方法和步骤。给出了测量结果,并首次给出这种力的频谱特性。     

Oscillations of a Cylinder Induced by Free Surface Flow

The results of experiments in which a cylinder freely located on a fixed shaft performs small harmonic oscillations driven by a subcritical free stream in an open channel are given. Examples of the strong feedback effect of the cylinder oscillations on the hydrodynamic wake and surface gravity waves are obtained.     

Effects of Body Forces on Turbulent Kinetic Energy Transport in Premixed Flames

Flow, Turbulence and Combustion - The effects of buoyancy on turbulent premixed flames are expected to be strong due to the large changes in density between the unburned and fully burned gases. The...     

方柱尾流流态对床面冲淤形态的影响

应用平面二维悬沙数学模型对方柱尾流区的泥沙输运及床面调整进行了数值模拟，并对重要物理参数做了分析说明。数值计算采用时间分裂一全隐式有限差分格式，流场的计算基于沿水深平均的RANS方程。通过系统的数值模拟，揭示了不同来流情况下，方柱尾流区的流态变化，及与之相应的床面变化规律，并对不同流态下的泥沙运动作了分析。计算结果表明在方柱尾流区，不同的流场流态对尾流区的床面调整有明显影响。流场较弱的情况下，尾流区中湍动强度相应较弱，此时悬浮泥沙由于流速减小而普遍落淤，床面应力的减小也致使床面冲刷量减小。随着流场强度的逐渐增大，尾流区中湍动强度相应增强，床面应力增大，同时湍流的强烈交换作用增强了对泥沙的输运作用，床面变化不再是简单的淤积状态，部分区域出现了冲刷。