气固两相平面混合层的直接数值模拟

本文采用直接数值模拟(DNS)方法研究了气固两相混合层的涡量场和颗粒扩散,分析了湍流混合层中涡结构的卷起和配对过程,并讨论了大尺度涡结构的配对过程对平均速度、雷诺应力的影响。同时分析了混合层中不同Stokes数的颗粒在涡结构的作用下的混合和扩散。结果显示Stokes数为1的颗粒主要分布在流场大尺度涡结构的外边界,而Stokes数为0.01在涡结构的作用下,在流场中充分混合。     

飞行器大攻角复杂流动的POD和DMD对比分析

基于非结构/混合网格、耗散自适应2阶混合格式以及脱体涡模拟（detached eddy simulation,DES）方法开展了现代战斗机模型复杂分离流动的数值模拟,并与有限的平均气动力试验数据进行了对比,结果表明计算具有合理性,在此基础上进一步应用本征正交分解（proper orthogonal decomposition,POD）和动力学模态分解（dynamic mode decomposition,DMD）方法对数值模拟流场的非定常特性进行了对比分析.研究表明飞行器背风区流场由一对边条涡的螺旋运动主导,旋涡破裂前在横向空间截面上流场是中性稳定的,同时主涡核的运动是多频耦合的.POD和DMD的对比分析则表明:两者模态配对的方式不同,但主要模态之间具有一定相关性;POD模态中包含多种频率的运动,而且能量较集中于主模态,流场重构效率更高;DMD则将流场的主要特征运动提取为一些单频模态的组合,同时能够给出模态的稳定性.      

气固两相三维圆柱绕流的直接数值模拟

本文采用直接数值模拟(DNS)方法研究了气周两相三维圆柱绕流的涡量场和颗粒扩散,并着重讨论了圆柱绕流中卡门涡街的形成和涡结构的转捩过程。同时分析了圆柱绕流中不同Stokes数的颗粒在涡结构作用下的横向扩散。结果显示Stokes数为1的颗粒主要分布在流场大尺度涡结构的外边界,而Stokes数为0．01的颗粒在涡结构的作用下,在流场中充分混合。     

高雷诺数双螺旋涡尾迹演化特性分析

螺旋状尾迹涡是直升机悬停旋翼流场的主导特征之一,其时空演化特性对旋翼气动性能具有重要影响.为了揭示悬停状态下旋翼尾迹涡的演化特征,对两桨叶刚性旋翼在高雷诺数悬停状态下的双螺旋状尾迹涡开展数值研究,采用基于流场特征的网格自适应技术,结合低耗散迎风/中心混合格式以及延迟脱体涡模拟方法对Caradonna-Tung旋翼在桨尖马赫数为0.439、桨尖雷诺数为1.92×10~6的悬停流场进行了高分辨率计算.基于欧拉和拉格朗日两种描述方法对计算结果进行了分析,揭示了双螺旋尾涡系统的演化特性:后缘尾涡面在桨尖附近的反向卷起及其与下游桨尖涡的相互作用是影响涡系稳定性以及涡-涡相互作用的重要因素;涡龄小于720°时,在固连于桨叶上的旋转坐标系中观察,涡系具有时空稳定性,涡管中心处轴向涡量随涡龄按照幂函数规律衰减.在固连于漩涡中心的局部极坐标系中,周向速度分布以及涡核半径随涡龄的变化与理论涡模型相符合,环量随涡龄的变化显示了漩涡的生长、平衡及耗散等演化阶段;模态分析结果表明,除点涡模态外,来流与点涡的复合模态在漩涡演化过程中对流动特征的转变有重要影响;涡系轴截面速度场的拉格朗日拟序结构直观地显示了漩涡场的时空演化过程,揭示了漩涡配对和共旋穿越等流动特征,同时也展示了后缘尾涡面卷起现象在漩涡演化过程中的作用.     

旋转回流的一种多重交错网格计算方法

针对圆柱坐标系下原始变量Navier Stokes方程,在有限控制容积法和压力修正的基础上,引入多重交错网格算法及非线性方程的FAS全近似格式,并对封闭圆柱空腔内的旋转流动进行数值模拟.     

Comparison between level set and phase field method for simulating bubble movement behavior under electric field

In this study, based on different numberical simulation methods, the gas-liquid two-phase flow is taken as the research object. By coupling the continuity equation of incompressible fluid, Navier-Stokes equation, electric field equation and other control equations, a multi-field coupling model for rising bubbles in viscous fluids is established, and numerical simulations are carried out. The two-phase popularity of coupled electric field is studied, and the effect of electric field on bubble motion is analyzed.The Level-set and phase field method are used to track the changes of deformation and rupture during the rising of the bubble. The accuracy and validity of the two methods are verified by mass conservation. At the same time, the calculation area is determined for the accuracy of calculation, and the optimal mesh size is calculated by using mesh independence test. Compared with the level set method, the phase field method has a certain improvement in the calculation efficiency and accuracy. Among them, the calculation efficiency of the phase field calculation method in the same grid is increased by 5 times, and by 3 times in the vertical electric field environment. Moreover, using the phase field method is easier to capture the bubbles slight changes while they are rising, and the quality of the simulation results is better.The simulation analysis of bubble rising process under coupled electric field by two methods shows that under the interaction of electrostatic force, buoyancy and surface tension, the bubble is stretched into an ellipsoid along the direction of the electric field line, and the ratio of the length to the short axis is proportional to the applied electric field strength. In addition, the bubble rising velocity is affected by the electric field, and the vertical electric field accelerates the rising of the bubble.     

水下爆炸冲击波数值仿真研究

由于在水下爆炸冲击波的数值仿真研究中,水的状态方程、人工黏性系数和网格尺寸对数值计算结果影响很大,采用常规TNT炸药的水下爆炸为例,以冲击波的峰值压力和比冲量为衡量指标,研究了这3个主要影响因素对数值仿真结果的影响。首先,通过采用常用的5种水的状态方程进行系列仿真,给出了各种状态方程的适用范围;其次,讨论了人工黏性系数对计算结果的影响,并给出了一次与二次人工黏性系数的建议取值范围;最后,通过对不同炸药当量及不同网格尺寸开展系列运算,从而得到不同炸药当量在满足工程计算精度要求下所对应的建议网格尺寸,并得到了不同炸药当量所对应的建议网格尺寸的表达式。     

磁涡旋极性翻转的局域能量

针对坡莫合金纳米圆盘中的单个磁涡旋结构,采用微磁学模拟研究了磁涡旋极性翻转过程中的局域能量密度.磁涡旋的极性翻转通过与初始涡旋极性相反的涡旋与反涡旋对的生成,以及随后发生的反涡旋与初始涡旋的湮没来实现.模拟结果显示当纳米圆盘样品中局域能量密度的最大值达到一临界值时,磁涡旋将会实现极性翻转,其中交换能起主导作用.基于涡旋极性翻转过程中出现的三涡旋态结构,应用刚性磁涡旋模型对局域交换能量密度进行了理论分析.通过刚性磁涡旋模型得到的磁涡旋极性翻转所需的局域交换能量密度的临界值与模拟结果符合得较好.     

自耦合射流强化掺混的数值研究

针对同向两股气流掺混的某一特定物理模型,对自耦合射流作用下的气流掺混过程进行了数值模拟研究.与未施加自耦合射流激励的掺混过程相比,自耦合射流周期性的吹/吸诱导流场内产生复杂的大尺度涡系结构,使得同向两股气流形成对流型混合,有效缩减了主流核心区长度;相对于在燃烧室单侧放置激励器的方式,双侧自耦合射流激励时温度均匀分布线较...     

不同亚格子模式在后台阶湍流流动大涡模拟中的应用

本文用大涡模拟方法研究了湍流后台阶流场中的大涡演变过程,并在此基础上研究了目前大涡模拟中比较常用的六种亚格子模式。在相同的流动几何参数以及计算条件下,给出了不同亚格子模式下湍流流动瞬时压力场以及流场中瞬时粘性大小的分布,尤其给出了流场瞬时演变的大尺度涡结构。将六种亚格子模式从回流区长度、计算时间、计算结果的准确性和流场稳定性等不同角度进行了比较。为选取合适的亚格子模式深入研究湍流耗散机理奠定了基础。     

雷诺数为15 000方腔环流的基准解和周期性分析

采用边界拟合法，结合基于结构网格的四叉树加密方法生成的高质量非均匀网格，研究雷诺数为15 000方腔环流周期性演化.利用傅里叶分析和相空间轨迹等方法研究流场演化的周期性行为以及流场动能的空间分布，确定周期性流动的基频和相对应的周期.研究发现：流场周期演化过程中，在主涡低频背景下，靠近方腔壁面、左上角、左下角以及右下角的高阶次涡具有高频演化规律.给出流场处于两种特征状态时的流场基准解.     

声波作用下球形颗粒外声流分布的数值模拟

综合考虑声学边界层内的热损失和黏性损失,建立处于平面驻波声压波节位置二维球形颗粒外声流计算模型,利用分离时间尺度的数值方法对颗粒外声流流场特征进行模拟。将模拟结果与相应的解析解和实验结果对比,验证了数值模拟的可靠性。在此基础上,研究了雷诺数Re和斯特劳哈尔数Sr对球形颗粒声学边界层内二阶声流流场结构、涡流强度及范围的影响规律。结果表明,随Sr和Re增大,声学边界层内的涡流结构尺度呈指数形式减小,其涡流尺度与颗粒直径D和激励频率f成反比,与流体介质运动黏度v成正比;且满足低Sr和高Re的声振系统可形成范围较大、更强烈的声流运动。该数值方法可用于对任意物理模型外声流特性的评估。      

Aeroacoustic performance evaluation of axial flow fans based on the unsteady pressure field on the blade surface

Because of the European and global regulation concerning acoustic emission, the goal of manufacturers is to substantially decrease the noise radiated by turbomachines, and in particular axial fans, without degrading their aerodynamic performances. High rotation speed and increasing power add to the overall difficulties. The theoretical study of this paper consists of two parts: (1) an aerodynamic approach based on the vortex surface method and (2) an aeroacoustic approach which mainly concerns the prediction of the tonal noise using the Ffowcs Williams and Hawkings (FW-H) equation. One of the main goals is the evaluation of the unsteady aerodynamic forces applied on the fan blades. A 2D software analysis, based on the vortex surface method (or potential flow method), was carried out. That process enabled evaluation of the potential flow around a mobile grid; first in a steady mode, and secondly in an unsteady mode by introducing an upstream disturbance in the form of an inlet velocity variation. The sources of noise corresponding to the zones with high force fluctuation amplitude are located initially on the blade surface. These fluctuating forces are used to predict the tonal noise radiated by the fan in far field by means of the FW-H equation. Two axial flow fans were used in this study. The theoretical results will be compared to the experimental ones in order to evaluate the aeroacoustic performances of the fans.     

Investigation of turbulence models for computation of swirling flows

Different turbulence models were studied in application to calculation of swirling flows. The differential models of turbulent viscosity considering streamline curvature and the method of detached eddy simulation were used. Weakly and strongly swirling flows were considered at the example of concentrated vortex in a tube, swirling flow in a diffuser, and vortex breakdown through an abrupt expansion. The RANS models with correction to flow swirling represented well the experimental data for the weakly swirling flows. In case of strongly swirling flows, it was more correct to use the method of detached eddy simulation.     

Quantized Superfluid Vortex Filaments Induced by the Axial Flow Effect

We report the quantized superfluid vortex filaments induced by the axial flow effect, which exhibit intriguing loop structures on helical vortexes. Such new vortex filaments correspond to a series of soliton excitations including the multipeak soliton, W-shaped soliton, and anti-dark soliton, which have no analogue when the axial flow effect is absent. In particular, we show that the vortex filaments induced by the multipeak soliton and W-shaped soliton arise from the dual action of bending and twisting of the vortex, while the vortex filament induced by the anti-dark soliton is caused only by the bending action, which is consistent with the case of the standard bright soliton. These results will deepen our understanding of breather-induced vortex filaments and will be helpful for controllable ring-like excitations on vortices.     

应用笛卡尔非结构切割网格进行外挂物投放的数值模拟

描述了一种新的网格生成技术,即笛卡尔非结构切割网格技术,采用叉树数据结构,完成了几种单段和多段翼型以及三维机翼的网格生成.应用中心有限体积法,对其绕流问题进行Euler方程数值模拟,并将计算结果与实验数据进行对比.在机翼绕流数值模拟的基础上,求解出机翼带外挂物的分离投放的流场计算问题.     

过渡流下近壁插入圆柱对壁面强化传热的影响

本文利用基于复合网格系统的计算方法,对Re=50~1200的近壁插入圆柱流场进行数值模拟,研究过渡流状态下在壁面附近插入圆柱对下游壁面传热强化的影响。并基于低速循环水槽流动实验台,采用粒子成像测试法(PIV)对Re=100~500的近壁插入圆柱流场进行可视化实验研究,验证了数值模拟方法的可靠性。研究结果表明:近壁插入圆柱流场在Re=100时进入过渡流状态;Re直接影响圆柱尾流中周期性涡脱和壁面涡岛的发生位置及其洗刷效应的大小,随着Re的增大,洗刷效应明显增强,因而,过渡流范围内Re越大,圆柱下游壁面传热强化越大。     

基流对热对流涡旋结构影响的实验结果初步分析

用实验室模拟方法研究了径向温度梯度对热对流涡旋的影响,结果表明,径向温度梯度所引起的基流使热对流涡旋的对称性结构被破坏,使涡旋变成非对称结构,并使涡旋向基流下游偏移,其强度随基流的加强而减弱,直至消失.热对流涡旋越强其对称性结构越稳定,基流对它的影响越小 关键词： 转盘实验 基流 热对流涡旋     

Macroscopic far-field observation of the sub-wavelength near-field dipole vortex

The energy flow lines of the radiation emitted by a rotating electric dipole moment have a vortex structure near the source. The spatial extend of this vortex is well below an optical wavelength. This near-field vortex has a macroscopic effect which could be observed in the far field.     

Vorticity dynamics and sound generation in two-dimensional fluid flow

An approximate solution to the two-dimensional incompressible fluid equations is constructed by expanding the vorticity field in a series of derivatives of a Gaussian vortex. The expansion is used to analyze the motion of a corotating Gaussian vortex pair, and the spatial rotation frequency of the vortex pair is derived directly from the fluid vorticity equation. The resulting rotation frequency includes the effects of finite vortex core size and viscosity and reduces, in the appropriate limit, to the rotation frequency of the Kirchhoff point vortex theory. The expansion is then used in the low Mach number Lighthill equation to derive the far-field acoustic pressure generated by the Gaussian vortex pair. This pressure amplitude is compared with that of a previous fully numerical simulation in which the Reynolds number is large and the vortex core size is significant compared to the vortex separation. The present analytic result for the far-field acoustic pressure is shown to be substantially more accurate than previous theoretical predictions. The given example suggests that the vorticity expansion is a useful tool for the prediction of sound generated by a general distributed vorticity field.