考虑转动能的一维/二维Boltzmann-Rykov模型方程数值算法

研究考虑转动能的Boltzmann-Rykov模型方程,基于转动自由度对气体分子速度分布函数矩积分,引入约化速度分布函数,应用离散速度坐标法与数值积分技术,将气体运动论模型方程化为在离散速度坐标点处关于三个约化速度分布函数的联立方程组.应用拓展计算流体力学有限差分方法,数值计算考虑转动自由度的双原子气体一维、二维Boltzmann模型方程,得到高、低Knudsen数一维激波管内流动和二维竖直平板绕流问题的流场,分析验证考虑转动能的Boltzmann-Rykov模型方程全流域统一算法求解一维/二维气体流动问题的可靠性.结果表明,气体稀薄程度与分子内自由度对流场具有较大影响,且Knudsen数较高的稀薄气体流动呈现严重的非平衡流动特点.     

内聚锥形激波的非均匀强化与结构演变特征

以高超声速内转式进气道流动中的激波汇聚问题为背景,考虑工程实际中的来流和壁面几何条件这两个关键因素,分别提出了以来流攻角为研究参数的非轴向来流内锥流动模型,和以长/短轴比为研究参数的椭圆入口内锥流动模型.采用激波风洞实验观测和数值模拟相结合的方法,揭示了两类流动中激波的非均匀汇聚特征.结果表明:由来流攻角引起的激波初始沿周向强度分布的不均匀性会在汇聚过程中被放大,迎风面和背风面的激波差异不断加剧;来流攻角越大,初始激波强度不均匀性越强,在汇聚过程中激波面越容易出现不连续的拐折,且出现拐折后激波的汇聚效应会被削弱.由椭圆入口形成的等强度激波在初始时周向的几何不均匀性使激波在汇聚过程中出现沿长/短轴方向的强度差异,激波沿长轴方向上的强度增加更迅速;椭圆长/短轴比越大,激波初始几何不均匀性越强,在汇聚过程中长/短轴两个方向激波强度差异凸显得越快,波面越容易出现不连续的拐折,进而削弱激波的汇聚.在偏离轴对称达到一定程度时,这两种条件下的激波汇聚都会出现汇聚中心处从Mach反射向规则反射的转变.      

一维过驱动爆轰波形成的数值研究

利用有限速率的基元反应模型对一维过驱动爆轰波的形成过程进行数值模拟.研究表明,在上游高温、高压、高速来流作用下首先会形成一道强激波,其波面方存在诱导区和放热区,然后诱导区和放热区界面会在来流扰动的作用下发生失稳,经过复杂的波系运动过程形成过驱动爆轰波.通过对不同初始条件下界面失稳过程的模拟和分析,研究了混合气体的组元、温度,来流的压力、温度、速度对过驱动爆轰波形成的影响.     

基于第二粘性的Navier-Stokes方程组求解正激波结构

为考察气体第二粘性（体积粘性）对正激波内部流动的影响机制，数值求解含第二粘性的一维Navier-Stokes方程组.结果表明：第二粘性对激波内部的密度、热流和能量分布等物理量具有抹平效应，导致热流和熵流的峰值减小、激波厚度增加，体积粘性耗散的增加使得一部分机械能转化为内能；考虑第二粘性所计算的密度分布和激波厚度大为改善，与实验数据吻合较好；当马赫数为1.2≤Ma≤10，激波内部的Knudsen数满足0.12≤Kn≤0.4，对于马赫数Ma≤4.0的激波内部流动，考虑第二粘性的连续流Navier-Stokes方程组能够准确地模拟正激波结构.     

超黏性激波捕捉技术在激波/火焰相互作用问题中的应用

本文基于时空三阶精度的两步泰勒伽辽金有限元高阶格式(TTGC)上实现了一种超黏性激波捕捉技术.该激波捕捉技术通过引入网格相关的体积黏性项和剪切黏性项以提高对激波/湍流相互作用过程的捕捉精度和可靠性.利用经典的一维SOD激波管算例和一维Shu-Osher算例对此激波捕捉技术进行了数值验证。并将该技术用于模拟了激波与火焰的相互作用问题,分析了火焰的演化过程及激波/边界层相互作用过程,揭示了分叉结构对火焰传播特性的影响.研究表明超黏性激波捕捉技术能够有效捕捉超声速反应流中的激波、湍流和边界层等复杂流动信息,同时能很好地控制激波附近的数值耗散现象.     

Van der Waals状态方程可压缩多流体流动的PPM方法

基于流体体积分数的混合型多流体数值模型，将Piecewise Parabolic Method（PPM）方法应用于可压缩多流体流动的数值模拟，采用双波近似求解多流体van der Waals状态方程的Riemann问题．模拟高密度比且含有激波的可压缩多流体流动，典型的纯界面平移问题模拟结果表明，在接触间断的界面附近，压力和速度没有任何的振荡且界面数值耗散都被控制在2—3个网格之内；一维和二维算例表明，该数值方法可以有效地处理接触间断、激波和多维滑移线等物理问题，并能够比其它多流体数值方法更精细地模拟多流体交界面．     

激波/边界层干扰对等离子体合成射流的响应特性

利用高速纹影系统和数值模拟方法研究了激波/边界层干扰对逆流喷射的等离子体合成射流的响应特性,并揭示了流动控制机理.实验在来流马赫数Ma=3.1的风洞中进行,测试模型采用钝头体和压缩斜坡的组合模型,等离子体合成射流激励器安装在钝头体头部.纹影系统捕捉了放电频率为f=1 kHz和f=3 kHz的激励对附体激波形态和分离激波运动的控制效果.等离子体合成射流使压缩斜坡激波/边界层干扰区域的起始点向下游移动,分离泡尺寸减小,附体激波强度减弱,发生弯曲,再附点移向上游,与此同时分离激波向附体激波逼近.与f=3 kHz激励相比,f=1 kHz激励的射流流量更大,对激波/边界层干扰的影响范围更广、控制效果更好.通过数值模拟,揭示了射流与来流相互作用对下游流场的作用机理:射流与来流相互作用诱导出大尺度旋涡,大尺度旋涡耗散发展增强了近壁面流场的湍流度;压缩斜坡上游近壁面的流场性质发生变化,进而导致了压缩斜坡激波/边界层干扰区域流动的变化.     

来流气流角变化对冲压叶栅性能的影响

本文研究的冲压叶栅基于激波增压这一思想,在冲压叶栅内组织了多道斜激波及一道结尾正激波,来实现叶栅的增压效应。采用雷诺平均N-S方程和Menter-SST湍流模型,对冲压叶栅的流场进行数值模拟。数值模拟结果表明,来流气流角在37°与49°之间变化时,冲压叶栅波系可以稳定存在,并且随着来流气流角增大,叶片通道内分离趋于严重,且有竹节波产生,使得叶栅流动分离损失及激波损失加剧,等熵效率降低。     

垂直振动床中的能量传递与耗散

本文采用数值方法分析了一维垂直振动床内颗粒动能/温度、能量耗散以及体积分数的分布规律.离散元模拟结果表明:当床底做低频、小振幅振动时,床层内颗粒整体随床底上下运动,沿床高方向颗粒动能逐渐增加;对于高频振动,床层内的颗粒做无规则的运动,沿床高方向颗粒动能逐渐降低.在不同振动频率(高频、低频)下体积分数、能量耗散也表现出不同的分布规律.将离散元模拟结果与动力学理论计算值对比,当系统做高频振动时,两模型所得结果基本吻合;而对于低频、小振幅振动,所得结果存在较大差异.由于低频、小振幅振动时床内颗粒并非做无规则运动,动力学理论的适用性需进一步完善.     

ρ-VOF:一种可清晰模拟自由界面的两相流方法

文章通过对EFM(effective field modeling)模型进行简化, 消除了原模型的非守恒性项和非双曲性特性项, 发展了一种基于密度的气液两相流模拟方法: ρ-VOF方法.利用体积分数信息对控制单元内的自由界面进行重构, 得到了控制单元内流体的空间分布, 并采用AUSM+-up格式获得考虑气液流体接触间断信息的对流通量.新方法可统一处理激波间断和接触间断的相互作用, 保持自由界面的尖锐性, 并且其计算量与自由界面的空间复杂度无关.最后, 数值模拟了液体激波管气液激波管和气体激波跨二维液滴传播等问题, 并与文献结果进行对比, 验证了本方法在气液两相流模拟中的准确性.      

循环流化床气固曳力模型

气固曳力是稠密气固两相流动,尤其是垂直流动中的主要作用力,相应的模型也是数值模拟中准确描述气固两相运动的关键.为了解决现有经验或半经验模型的普适性问题,合理描述流动中经常发生的颗粒团聚现象及其对气固曳力的影响,从理论分析入手,运用最小能量的概念,将传统的CFD方法与宏观的系统分析方法相结合,建立了一个新的计及颗粒团聚效应的气固曳力理论模型.与现有模型相比,新模型不仅具有相同的函数变化关系,可合理地描述气固两相相互作用的物理过程,而且避免了以往经验系数不准确导致的各种误差,为稠密气固两相流动的数值描述提供了重要依据.     

Direct numerical simulation of shock/turbulent boundary layer interaction in a supersonic compression ramp

A direct numerical simulation of the shock/turbulent boundary layer interaction flow in a supersonic 24-degree compression ramp is conducted with the free stream Mach number 2.9.The blow-and-suction disturbance in the upstream wall boundary is used to trigger the transition.Both the mean wall pressure and the velocity profiles agree with those of the experimental data,which validates the simulation.The turbulent kinetic energy budget in the separation region is analyzed.Results show that the turbulent production term increases fast in the separation region,while the turbulent dissipation term reaches its peak in the near-wall region.The turbulent transport term contributes to the balance of the turbulent conduction and turbulent dissipation.Based on the analysis of instantaneous pressure in the downstream region of the mean shock and that in the separation bubble,the authors suggest that the low frequency oscillation of the shock is not caused by the upstream turbulent disturbance,but rather the instability of separation bubble.     

确定分布的展向Lorentz力调制下的槽道湍流涡结构

采用直接数值模拟方法,对槽道湍流中确定分布的Lorentz力的流动控制与减阻问题进行研究.讨论了Lorentz力作用于槽道湍流后,流场的特性和涡结构的特性,并对此类Lorentz力对槽道湍流的控制与减阻机理进行了讨论.研究发现:1)Lorentz力诱导的层流流场壁面附近存在梯度极大的展向速度剪切层,该剪切层容易形成流向涡结构;2)在给定合适参数的确定分布的Lorentz力作用下,湍流流场仅剩周期分布的准流向涡;3)与未控制流场相比,控制后的流场中,准流向涡的抬升高度大大降低,从而减小猝发强度,使壁面阻力下降.     

带支撑装置的CRM-WBH模型流固耦合数值模拟

共同研究模型（common research model，CRM）机翼/机身/平尾组合体构型（CRM-WBH）是第4届AIAA阻力预测研讨会（Drag Prediction Workshop IV，DPW IV）选择的基准构型，DPW IV会议的统计分析结果显示计算结果与试验结果之间存在显著差异.采用CFD方法和流固耦合方法数值模拟了带支撑装置的CRM机翼/机身/平尾组合体模型（CRM-WBHS）的气动特性，主要目的是评估支撑装置和静气动弹性变形对CRM-WBH模型气动特性数值模拟结果的影响.通过与CRM-WBH模型CFD数值模拟结果和欧洲ETW风洞（European Transonic Wind Tunnel）测力、测压和模型变形测量结果的对比分析，表明模型支撑装置导致机翼上翼面激波位置前移，升力系数、阻力系数下降，俯仰力矩系数增加；静气动弹性变形主要影响机翼上表面激波位置和外翼处激波位置前负压，导致升力系数、阻力系数进一步下降，俯仰力矩系数进一步增加.CRM-WBHS模型的流固耦合数值模拟结果更加接近试验结果.      

具有多孔介质覆面层的柱体绕流流场分析

建立多孔结构覆面柱体绕流模型, 采用含Darcy-Brinkman-Forchheimer作用力项的格子Boltzmann方程对覆盖多孔介质层的方柱绕流进行数值模拟, 研究多孔介质对钝体绕流流场特性的影响。结果表明: 相比于不可渗透壁的柱体, 引入合适参数的多孔介质覆面层后可以有效降低其升力脉动幅值, 但阻力有所增加。同时, 较高雷诺数下多孔方柱的数值模拟表明: 多孔介质壁面使得尾迹区域的剪切层相距更远, 降低了尾流处湍动能, 并将雷诺应力的峰值移动到尾迹区域, 抑制了方柱两侧的动量交换, 使动量交换的位置发生在尾迹区域, 继而使得尾迹的涡街更加规则化。     

Grid and subgrid-scale interactions in viscoelastic turbulent flow and implications for modelling

Using direct numerical simulations of turbulent plane channel flow of homogeneous polymer solutions, described by the Finitely Extensible Nonlinear Elastic-Peterlin (FENE-P) rheological constitutive model, a-priori analyses of the filtered momentum and FENE-P constitutive equations are performed. The influence of the polymer additives on the subgrid-scale (SGS) energy is evaluated by comparing the Newtonian and the viscoelastic flows, and a severe suppression of SGS stresses and energy is observed in the viscoelastic flow. All the terms of the transport equation of the SGS kinetic energy for FENE-P fluids are analysed, and an approximated version of this equation for use in future large eddy simulation closures is suggested. The terms responsible for kinetic energy transfer between grid-scale (GS) and SGS energy (split into forward/backward energy transfer) are evaluated in the presence of polymers. It is observed that the probability and intensity of forward scatter events tend to decrease in the presence of polymers.     

Three-dimensional finite-difference lattice Boltzmann model and its application to inviscid compressible flows with shock waves

In this paper, a three-dimensional (3D) finite-difference lattice Boltzmann model for simulating compressible flows with shock waves is developed in the framework of the double-distribution-function approach. In the model, a density distribution function is adopted to model the flow field, while a total energy distribution function is adopted to model the temperature field. The discrete equilibrium density and total energy distribution functions are derived from the Hermite expansions of the continuous equilibrium distribution functions. The discrete velocity set is obtained by choosing the abscissae of a suitable Gauss–Hermite quadrature with sufficient accuracy. In order to capture the shock waves in compressible flows and improve the numerical accuracy and stability, an implicit–explicit finite-difference numerical technique based on the total variation diminishing flux limitation is introduced to solve the discrete kinetic equations. The model is tested by numerical simulations of some typical compressible flows with shock waves ranging from 1D to 3D. The numerical results are found to be in good agreement with the analytical solutions and/or other numerical results reported in the literature.     

Diffusion of colloidal particles in swimming suspensions

Previously, we have proposed to analyse the hydrodynamic interactions in a suspension of swimmers with respect to an effective hydrodynamic diffusion coefficient, which only considers the fluctuating motion caused by the stirring of the fluid. In this work, we study the diffusion of colloidal particles immersed in a bath of swimmers. To accurately resolve the many-body hydrodynamic interactions responsible for this diffusion, we use a direct numerical simulation scheme based on the smooth profile method. We consider a squirmer model for the self-propelled swimmers, as it accurately reproduces the flow field generated by real microorganisms, such as bacteria or spermatozoa. We show that the diffusion coefficients of the colloids are comparable with the effective diffusion coefficients of the swimmers, provided that the concentration of swimmers is high enough. At low concentrations, the difference in the way colloids and swimmers react to the flow leads to a reduction in the diffusion coefficient of the colloids. This is clearly seen in the appearance of a negative-correlation region for the velocity-correlation function of the colloids, which does not exist for the swimmers.     

The effects of swirling partially premixed flame on scaled kinetic energy transport in a gas turbine-like combustor

The multi-scale interaction between combustion and turbulence is of great importance in modifying the small-scale flame structure and kinetic energy, especially in swirling flames under practical conditions. In the present study, direct numerical simulation of swirling partially premixed flame is conducted within a model combustor under gas turbine conditions. The reactive flow is compared to the corresponding non-reactive one to investigate the influence of combustion on the scaled kinetic energy transport. Kinetic energy spectra demonstrate that the turbulent kinetic energy is reduced in the dissipative subrange while enhanced in the energetic one by the flame. The critical scale is located in the inertial subrange and close to the estimated turbulent flame thickness. Filtering analyses show that the resolved-scale kinetic energy is augmented by the increased large-scale pressure-gradient work in the reactive flow, while the subgrid-scale kinetic energy is attenuated by the enhanced small-scale viscous dissipation. The backscatter prevails in the heat release regions when the filter size is larger than the laminar flame thickness, and this effect decreases with the swirling flow developing downstream. The interaction between the kinetic energy flux and the local dilatation as well as the subgrid-scale pressure-gradient work is also investigated to achieve a comprehensive understanding about the effects of combustion on the backscatter.     

Drag reduction in turbulent channel flow using bidirectional wavy Lorentz force

Turbulent control and drag reduction in a channel flow via a bidirectional traveling wave induced by spanwise oscillating Lorentz force have been investigated in the paper. The results based on the direct numerical simulation (DNS) indicate that the bidirectional wavy Lorentz force with appropriate control parameters can result in a regular decline of near-wall streaks and vortex structures with respect to the flow direction, leading to the effective suppression of turbulence generation and significant reduction in skin-friction drag. In addition, experiments are carried out in a water tunnel via electro-magnetic (EM) actuators designed to produce the bidirectional traveling wave excitation as described in calculations. As a result, the actual substantial drag reduction is realized successfully in these experiments.