速比对混合层湍流参数影响的实验研究

本文使用PIV术对低高速侧速比为0.25、0.33和0.5时竖直通道内的混合层流动进行实验研究,基于速度差和通道水力直径的雷诺数范围15840～132000.研究发现混合层内湍流参数的分布不仅和雷诺数有关,还和速比有关.混合层内同一横截面上平均雷诺应力的最大值随雷诺数的增大而增大,而在同一横截面上相同雷诺数时雷诺应力的最大值则随速比的增大而减小.在同一横截面上平均涡量随雷诺数的增大而增大,雷诺数相同时平均涡量的最大值随速比的增大而增大.无量纲平均涡量的最大值随混合层的发展按指数规律衰减,速比越大衰减速度越快.     

用Lattice Boltzmann方法模拟1/4圆腔内的定常层流运动

分析格子Boltzmann方法中二阶精度的曲线边界处理方法.应用格子Boltzmann方法及其边界处理方法模拟1/4圆腔内的定常层流运动,引入流线图和等涡线图分析流场随Re数的变化.并且发现当Re数在10~100区间内变化时,随着Re数的增大,顺时针旋转流场的涡心位置偏离x轴的角度逐渐减小,而逆时针旋转流场的涡心位置偏离x轴的角度却越来越大.     

基于转捩SST模型凸起圆柱绕流数值研究

为了研究局部凸起对边界层转捩的影响,采用转捩SST模型分别对亚临界、临界和超临界状态下带突起的圆柱绕流问题进行了数值模拟,分析了不同Reynolds数下带突起的圆柱绕流问题的近壁面流动特征以及表面时均压力与摩擦力系数的分布和凸起对圆柱表面流动分离以及转捩的影响,对比了有无凸起两侧圆柱表面时均压力、摩擦力系数的不同. 结果表明:当来流Reynolds数处于临界区时,气流在圆柱上表面凸起处形成了3个反向旋转的漩涡,之后随着θ的增大,发生了流动分离和流动转捩现象;对于不同Reynolds数下的来流,圆柱上表面的凸起可以使气流发生转捩的位置提前;圆柱上表面的凸起使流速增大、压强降低,从而导致圆柱产生升力,随着来流Reynolds数的增大,其升力逐渐变大.      

圆柱面上再附边界层流动的级数分析

按普朗特边界展理论,将圆柱面上再附边界层流动表示成级数的形式,并根据实验所测圆柱表面上的压强分布导出再附边界层外边界上的流速分布,取有限项级数解的形式解出了圆柱表面上再附边界层内的流速分布及流动再分离点。     

低雷诺数下双向受迫振动圆柱尾涡结构分析

本文使用本团队自主开发的一款通用统一的流体力学计算软件UCFD,在Re=100和Re=500时对横、纵双向受迫振动圆柱绕流进行了数值模拟研究,研究发现了一些新的涡结构:在横向振动的基础上增加纵向振动后,在Re=100时,随着纵向振幅的增大,在某些工况下,圆柱的尾涡结构会在2P和P+S模式之间变化;当振动圆柱的纵向振幅增大至0.8D后,圆柱一周期脱落涡可达到5个;Re=500时,随着纵向振幅的增大,圆柱尾涡的2S与P+S结构分布逐渐减少,2P结构分布逐渐变多;当振动圆柱的纵向振幅增大至0.6D后,在某些工况下圆柱一周期脱落5个涡。     

绕圆柱体自由表面磁流体流动和传热的研究

本文对在不同雷诺数下,绕圆柱体的磁流体自由表面流动及传热进行了模拟,分析了磁场对绕流圆柱尾迹和涡分离的影响,获得了两种雷诺数下的电磁力密度、流场和温度场分布。结果表明,磁场不仅影响了流动的形态,而且对湍流有抑制作用,降低了自由表面的更新机制,从而减少了传热能力;在相同的Hartmann数下,相比低雷诺数下的流动换热情况,高雷诺数下的湍流不能被完全抑制,自由表面与尾迹的相互作用也较强,因而自由表面换热也较强。     

圆柱绕流问题的初期流动数值模拟

本文继Zheng-huan Teng[1]中介绍的解Navier-stokes方程的椭圆涡团法,研究了一种新的变形涡团法,用以模拟不可压粘性流体绕圆柱的不定常流动。圆柱在静止流体中突然起动并做匀速直线运动。对整个流动区域构造完全Navier-Stokes方程的解并不容易,近十年出现很多数值研究,本文对算法有所推进。把本文的方法称作变形涡团法是因为圆柱边界附近的流体中用椭圆涡团,远离边界时用圆形涡团。计算圆柱绕流比平板绕流在满足附着条件上更为困难,本文分析了怎样在圆柱边界上给出适当的附着条件的数值方法。在算例中雷诺数分别取200、550、3000,得到了不定常边界层分离,二次涡等复杂的物理现象,这些数值结果与近年实验结果[2]是一致的。     

凹陷涡发生器非稳态流动与传热分离涡模拟

本文对具有凹陷涡发生器的冷却通道内非稳态湍流流动与传热特性进行了分离涡模拟研究。通道内流动雷诺数为50500,凹陷涡发生器深度与直径比为0.2。分离涡模拟结果与实验数据进行了对比验证,并对凹陷表面非稳态流动与传热特性进行了详细分析。研究结果表明:分离涡模拟得到的凹陷表面时均局部Nμ数分布与瞬态液晶热像实验得到的Nμ数分布相似,并且总体平均Nμ数和摩擦因子与实验数据也较好地吻合;分离涡模拟获得了详细的凹陷涡发生器表面非稳态流动和传热特征,凹陷表面前缘处的低换热区形成左右两个旋涡,经历了形成、旋转、回流、破碎四个过程,这些非稳态的涡流动显著地强化了凹陷表面的对流传热性能。     

基于间断有限元方法的并列圆柱层流流动特性

基于高阶的间断有限元方法, 数值模拟低马赫数下并列圆柱的可压缩层流流动, 捕捉并列圆柱流场中的漩涡结构, 以便分析并列圆柱尾流的流动特性. 针对二维圆柱的边界形式, 采用曲边三角形单元构造二维圆柱的曲面边界, 以适应高阶离散格式的精度. 在验证方法合理性的基础上, 分析圆柱间距及雷诺数对漩涡脱落及受力特性的影响规律. 研究结果表明: 并列圆柱的间距是影响流场流动特性的一个主要因素, 它会改变圆柱漩涡脱落的形式. 随着圆柱间距的增加, 上下圆柱的平均阻力系数及平均升力系数的绝对值随之显著下降. 雷诺数对于平均阻力系数的影响相对较小. 但随着雷诺数的增加, 上下圆柱的平均升力系数会随之降低, 而漩涡的脱落频率会随之增大.     

二维顶盖驱动半圆腔内流动MRT-LBM研究

本文采用多弛豫时间格子玻尔兹曼方法(multi relaxation time lattice Boltzmann method—MRT LBM)对二维顶盖驱动半圆腔内流动进行了数值模拟,得到了雷诺数为500～50000范围内半圆腔内流场分布情况。在二维顶盖驱动半圆腔流场中,随雷诺数的增大,流场内旋涡的数目逐渐增加,且流动依次呈现出稳定流、周期流、混沌流等状态。本文计算结果表明,MRT-LBM模型可显著提高计算的稳定性,适用于大范围的雷诺数流动情况。     

超声速三维空腔流气动噪声被动控制

抑制超声速武器舱空腔流噪声是航空领域中一项重要课题。大量研究表明在空腔前缘采用主/被动控制技术可以在一定程度上抑制腔内噪声水平。利用大涡模拟(large eddy simulation, LES)技术计算分析了Mach 1.4开式矩形方腔及波形、弧形两种前后壁几何修形后空腔的流动及噪声, 探索超声速来流条件下几何修形被动控制技术对开式方腔流噪声的抑制能力。计算结果表明波形和弧形空腔对腔内噪声均具有一定的抑制作用, 且波形空腔噪声控制效果更优。分析认为空腔几何修形能够改变空腔上方剪切层及腔内大尺度涡结构的发展演化, 进而实现对腔内噪声的控制。此外, 还应用LES方法计算分析了增厚的来流边界层条件下超声速方腔流, 发现来流边界层增厚可显著降低腔内噪声水平。      

Effects of dimples on laminar boundary layers

A series of direct numerical simulations of the flow past a flat plate with two and eight rows of dimples in a staggered arrangement is carried out. The Reynolds number based on the boundary layer thickness and freestream velocity near the inflow plane is 1000 and the dimples are spherical with a depth to diameter ratio of 0.1. The incoming flow is laminar and the boundary layer thickness before the dimples is half the dimple depth. At this low Reynolds number the flow is expected to remain laminar over a smooth flat plate. The presence of the dimples triggers instabilities that cause significant momentum transport. It is shown that the shear layer that forms as the flow separates over the first two rows of dimple becomes unstable and sheds coherent vortex sheets. The vortex sheets become unstable and are transformed into packets of horseshoe vortices. When these vortices evolve over a flat plate or over a series of dimples the flow dynamics are very different with important changes in momentum transport across the boundary layer.     

Free-surface flows under impacting droplets

A numerical method which fulfils the free-surface boundary conditions and extrapolates the fluid velocity into empty grid cells outside the fluid region on a fixed Cartesian grid system is presented. The complex, three-dimensional, vortex structures formed via surface/vortex interaction and induction between vortices have been computed using the proposed technique implemented within a level-set method for both vertical and oblique droplet impacts in incompressible fluids. The present results have been validated through numerical tests which confirm zero tangential shear at the free-surface and comparisons with experimental observations of cavity and vortex ring formation underneath the impact location. In some cases, transitions from a concentric vortex ring to a fully three-dimensional vortex structure has been confirmed. Whilst the primary vortex ring is initiated at the highly curved contact surface between the droplet and receiving surface, azimuthal instabilities are manifested in the shear layer around the cavity crater developing after the vertical impact, resulting in axial counter-rotating vorticity between the cavity and descending vortex ring. Underlying mechanisms which induce local deformation of the free-surface, creating a so-called scar, due to the sub-surface vortices at the oblique impacts are also discussed.     

On the influence of the Reynolds number on the intensity of vortex sound flowing around a cylindrical profile

Regularities of the emission of vortex sound (eolian tone) during air flow around stationary and rotating cylindrical profiles have been investigated. The influence of the flow Reynolds number on the intensity of vortex sound emission has been estimated from results of measuring the pressure fluctuation distribution on the surface of stationary cylindrical rods flowed around by air, as well as in the wake behind them. It is shown that the emission intensity depends on the location of the point of flow detachment from the profile surface and the track width near the profile. The ranges of the flow Reynolds numbers where the emission intensity increases with different flow velocities have been determined by analyzing the dependence of the profile lift coefficient on the Reynolds number. An independent way of determining the profile lift coefficient by measuring the vortex sound intensity is proposed. The results explain contradictions between the results of some authors, who experimentally observed different dependences of emission intensity on the flow velocity. The influence of the profile diameter on the vortex sound emission intensity has been investigated. The boundary Reynolds number above which the profile diameter does not affect emission has been established for stationary and rotating cylindrical profiles. It is shown that deposition of rough coatings on the rod surface may reduce the vortex sound emission intensity by affecting the point of flow detachment from the surface.     

Numerical investigation of the tangential stress effects on a fluid flow structure in a partially open cavity

Mathematical and numerical modeling of fluid flows in the domains with free boundaries under co-current gas flow is widely investigated nowadays. A stationary problem of fluid motion in a rectangular cavity with a non-deformed free boundary is studied in a two-dimensional statement. The tangential stresses created on the free boundary by an adjoint gas flow are considered to be a driving force for a fluid motion. The influence of the cavity geometry (cavity aspect ratio) and of the free boundary (length of the open part of the boundary) on the velocity field is investigated numerically. The simulations are carried out for different values of the gas Reynolds numbers. The characteristic values for the flow parameters as well as geometrical characteristics described in this paper are motivated by the main features of the CIMEX-1 experiments prepared for the International Space Station. The paper presents examples of the fluid flow structure in the open cavities and conclusions.     

Observation of vortex packets in direct numerical simulation of fully turbulent channel flow

A number of experimental studies have inferred the existence of packets of inclined, hairpinlike vortices in wall turbulence on the basis of observations made in two-dimensional x−y planes using visualization and particle image velocimetry (PIV). However, there are very few observations of hairpins in existing three-dimensional studies made using direct numerical simulation (DNS), and no such study claims to have revealed packets. We demonstrate, for the first time, the existence of hairpin vortex packets in DNS of turbulent flow. The vortex packet structure found in the present study at low Reynolds number,Re _t=300, is consistent with and substantiates the observations and the results from twodimensional PIV measurements at higher Reynolds numbers in channel, pipe and boundary layer flows. Thus, the evidence supports the view that vortex packets are a universal feature of wall turbulence, independent of effects due to boundary layer trips or critical conditions in the aforementioned numerical studies. Visualization of the DNS velocity field and vortices also shows the close association of hairpin packets with long low-momentum streaks and the regions of high Reynolds shear stress.     

A numerical analysis on flow field and heat transfer by interaction between a pair of vortices in rectangular channel flow

This paper is a numerical study of the effect of flow field and heat transfer created by interactions between a pair of vortices generated by a vortex generator in a rectangular channel flow. In order to analyze the vortices produced by the vortex generator, the pseudo-compressibility method is introduced into the Navier–Strokes (NS) equation of a three-dimensional unsteady, incompressible viscous flow. A two-layer k–ε turbulence model is used on the flat plate three-dimensional turbulence boundary to predict the turbulence characteristics of the vortices. The computational results accurately predict the vortex characteristics, which are related to Reynolds stress, turbulent kinetic energy, and flow field. Also, in the prediction of thermal boundary layers, skin friction characteristics, and heat transfers, the present results are reasonably close to the experimental results obtained by other researchers.     

Construction of curved global vortex

The complex scalar field models which possess, in the spectra of solutions, the vortices with cylindrical symmetry are considered. The rigorous construction of the curved, constant width, vortices is presented. In the framework of the used formalism an example of the curved vortex of constant width is discussed in detail.     

基于涡流发生器的风洞试验段附面层控制数值模拟

沿试验段侧壁发展的附面层是影响飞行器半模型实验数据精准度的主要因素之一.利用数值模拟方法验证了涡流发生器减小附面层影响的可行性,重点分析了安装角度、结构尺寸、安装位置及个数等设计参数对附面层内速度分布的影响规律,对涡流发生器尾涡强度以及沿流向的发展规律进行了初步探讨.结果表明,涡流发生器产生的尾涡能够有效改善附面层内的速度分布,进而减小附面层厚度,降低附面层影响;涡流发生器的后缘应略高于当地附面层厚度,安装角度、位置、个数等参数必须合理设计以减小涡流发生器对试验段主气流的影响.基于计算结果初步设计了可用于2.4 m跨声速风洞半模试验段的涡流发生器,在亚声速范围内能够减小模型区侧壁附面层厚度66%左右,对核心流Mach数影响小于0.003,为涡流发生器的实际应用提供了依据.