基于非结构变形网格的间断装配法原理

在激波捕捉法计算得到的流场基础上采用辨识算法得到初始间断位置, 从ALE方程出发, 考虑离散几何守恒律, 采用变形网格和网格重构技术解决计算过程中间断运动和变形, 新旧网格之间流场采用高精度信息传递方法保持时间精度, 建立了基于非结构动网格技术的间断装配方法.通过激波管问题的二维模拟, 模拟了初始间断分解为激波和接触间断激波遇到固壁反射后与接触间断相交的非定常流动过程, 对这种新方法的基本原理进行了介绍.      

面向高超声速飞行器的激波智能预测方法

高超声速飞行器激波位置的准确预测能够有效提升数值模拟的精度和效率。一方面,对高超声速飞行器激波附近网格进行正交和加密处理,可有效提升数值计算精度;另一方面,使用高超声速飞行器激波位置对计算网格进行修正,能够加速CFD计算收敛过程。提出了一种基于机器学习的高超声速飞行器激波智能预测方法,对典型高超声速飞行器外形进行激波位置的高效准确预测。首先,针对典型高超声速飞行器外形和典型飞行状态,使用数值模拟方法获得收敛的流场,并采用基于Mach数等值线的激波提取方法,从流场中判别激波面并提取构成激波面的关键点位置,形成训练数据;然后采用有监督学习算法,学习关键点位置,并利用二次曲线沿流向拟合关键点形成初步的激波线族;最后,基于剖面压力云图,构造基于投影压力图像的智能预测神经网络,对初步形成的激波线族进行修正,并获得三维激波面。大量的实验结果表明,激波预测模型能够对高超声速飞行器激波位置做出准确预测,预测的激波面与CFD数值计算结果中提取的激波面误差在10^-4量级。     

高精度非定常激波装配法

为正确模拟高超声速绕流中,来流小扰动与弓形激波之间的干扰对流动特征的影响,将弓形激波作为动边界,利用非定常特征关系处理激波处的边界条件.应用五阶精度迎风紧致格式和六阶精度的对称格式与三阶精度的R-K方法相结合,建立高精度非定常激波装配方法.采用该方法数值模拟钝锥高超声速定常流场和二维抛物外形高超声速边界层流动的感受性问题,数值模拟来流小扰动与弓形激波干扰激波后非定常扰动流场,研究扰动波进入边界层产生边界层不稳定波的特征.     

对运动斜激波绕射波纹壁面的研究

使用计算流体动力学的方法,对经典的运动激波绕射现象做数值模拟,研究了一类复杂激波反射问题一入射的运动斜激波绕射现象.给出一组运动斜激波绕射波纹壁面的非定常过程的模拟结果。计算结果显示出由运动斜激波绕射诱导的多波干扰产生的复杂流场结构。     

双曝光全息干涉法用于风洞中激波流场的研究

本文论述了双曝光全息干涉法在激波流场研究中的应用．对于形状不规则模型在超音速风洞中产生的激波流场,由于对这种流场进行数值计算几乎不可能,从实验上对其进行定量测量就显得尤为重要．本文很好地解决了某种不规则模型超音速激波流场的定量测量问题,获得了激波流场的有限条纹全息干涉图,并利用图象处理技术对干涉图进行了细化处理,再用人机交互的方式采集数据计算出条纹的位移量,最后用拉普拉斯变换法计算出流场的密度分布值．     

应用多介质PPM方法计算斜激波与物质交界面的相互作用

利用多介质PPM方法研究斜激波与物质交界面的相互作用.采用与体积分数耦合的Euler方程组作为计算模型,用双波近似来求解一般刚性气体状态方程Riemann问题.通过体积分数的计算来获得界面的位置,在整个流场采用统一的高阶PPM格式进行计算.文中对斜激波与不同物质界面相互作用进行了数值模拟,并给出了交界面上由于斜压效应产生的涡列的演化过程,特别是强斜激波与不同物质界面的相互作用的情况.     

二维平面激波折射的理论计算方法

围绕二维平面激波与物质界面作用,建立了流场波系结构的理论计算方法.首先分析了界面两侧激波独自沿界面传播过程,识别了两类正规折射和三类非正规折射;然后,根据两侧扰动沿界面传播的相对快慢,得到了三种不同的作用情形.与已有的Catherasoo方法相比,此方法:1)考虑了激波波后紧邻波阵面流场中扰动对界面附近折射的影响,区分了扰动的强弱及其是否可以追赶上激波阵面; 2)除了少数情形外,大都基于Euler方程精确解描述流场中波系相互作用.利用该方法对马赫数1.17的激波与空气/SF6界面作用进行计算,得到的波系结构与数值结果、已有实验数据基本一致,透射波阵面与水平方向夹角吻合程度优于Catherasoo结果;给出了反射波参数、界面偏折角度等,而Catherasoo方法不能计算这些参数.对于马赫数2.00的激波,通过比较不同介质密度比和界面倾角下的理论与数值结果,发现此方法对波系类型的识别比Catherasoo方法更为准确,且识别了波后强扰动追赶上激波形成三波结构的折射类型,后者无法识别此类型.上述结果表明,新方法具有良好的适用性,对波系类型的识别较已有方法的准确度更高,可获取更为丰富的波系结构...     

固体粒子通过叶尾激波的运动特性

含尘叶轮机械如烟气轮机。高炉余气透平已得到广泛的使用。其内部是含有固体粒子的气固两相流动,又是有激波的跨音流动。叶片近尾缘区内的固粒运动受到激波的作用,使用常规的理论方法来预测气固两相流的流场会产生误差。本文通过粒子的受力分析建立了固体粒子通过激波时的运动模型。通过数值模拟可以明显的见到固体粒子受到激波的作用,粒子在激波后运动有明显的折转。获得了粒子通过激波的运动特性。     

一类新型激波捕捉格式的耗散性与稳定性分析

高超声速流动是高复杂性的可压缩黏性流动, 其中存在激波、剪切层、激波/激波干扰、激波/边界层干扰、旋涡与分离流动等复杂流场结构. 对其进行准确模拟需要使用低耗散、强鲁棒性的激波捕捉方法. 本文基于一类新型的通量项分裂方法, 提出了一种耗散低且鲁棒性好的激波捕捉格式K-CUSP-X. 对该格式的耗散性和激波稳定性进行了详细的理论分析, 得到了格式激波稳定的数值条件. 推论认为, 迎风格式激波稳定的充分条件为速度扰动量具有衰减性, 数值实验验证了该推论. 研究表明, 该格式与Toro提出的通量分裂格式K-CUSP-T相比, 在保证精确捕捉接触间断的同时, 又具有更好的稳定性, 在激波处不会产生“红玉”现象.     

激波和单列水柱、气水固多界面相互作用的数值模拟

研究可压缩多介质流场的激波和多介质界面相互作用问题.在Descartes固定网格采用level-set方法追踪界面,气/气界面边界条件处理采用OGFM方法,采用修正的rGFM方法提高气/水和气/固界面处构造Riemann问题精度,将Riemann近似解得到的界面参数外推到两侧真实和虚拟流体,采用五阶WENO方法求解流场Euler方程和界面level-set方程,给出不同时刻流场数值纹影图像.结果表明：在可压缩流场嵌入固体和水、气体等目标,本文方法可较精确地分辨平面运动激波和单列水柱及包含气/气、气/水和气/固等界面作用后产生的复杂激波结构.和传统的分区与贴体变换方法不同,为Descartes网格包含多介质界面复杂流场计算提供新途径.     

On the accuracy of EPI-based phase contrast velocimetry

For over a decade, echo-planar imaging (EPI) has been used in both the medical and applied sciences to capture velocity fields of fluid flows. However, previous studies have not rigorously confirmed the accuracy of the measurements or sought to understand the limitations of the technique. In this study, a bipolar gradient was added to a flow-compensated EPI pulse sequence to obtain rapid phase contrast images of steady and unsteady flows through two step stenoses. For steady Re = 100 and 258 flows, accuracy was measured through systematic comparisons with CFD simulations, mass flow rate measurements, and spin echo phase contrast images. On average, the EPI image data exhibited velocity errors of 5 to 10 percent, while mass was conserved to within 5.6 percent at each axial position. Compared to spin-echo phase contrast images, the EPI images have 50 percent lower signal-to-noise ratio, larger local velocity errors, and similar mass conservation characteristics. An unsteady flow was then examined by starting a pump and allowing it to reach a steady Re = 100 flow. Accuracy in this case was measured by the consistency between mass flow rate measurements at different axial positions. Images taken at 0.3 s intervals captured the velocity field evolution and showed that 50 to 100 percent errors occur when the flow changes on a time scale faster than the image acquisition time.     

On high-order shock-fitting and front-tracking schemes for numerical simulation of shock–disturbance interactions

High-order methods that can resolve interactions of flow-disturbances with shock waves are critical for reliable numerical simulation of shock wave and turbulence interaction. Such problems are not well understood due to the limitations of numerical methods. Most of the popular shock-capturing methods are only first-order accurate at the shock and may incur spurious numerical oscillations near the shock. Shock-fitting algorithms have been proposed as an alternative which can achieve uniform high-order accuracy and can avoid possible spurious oscillations incurred in shock-capturing methods by treating shocks as sharp interfaces. We explore two ways for shock-fitting: conventional moving grid set-up and a new fixed grid set-up with front tracking. In the conventional shock-fitting method, a moving grid is fitted to the shock whereas in the newly developed fixed grid set-up the shock front is tracked using Lagrangian points and is free to move across the underlying fixed grid. Different implementations of shock-fitting methods have been published in the literature. However, uniform high-order accuracy of various shock-fitting methods has not been systematically established. In this paper, we carry out a rigorous grid-convergence analysis on different variations of shock-fitting methods with both moving and fixed grids. These shock-fitting methods consist of different combinations of numerical methods for computing flow away from the shock and those for computing the shock movement. Specifically, we consider fifth-order upwind finite-difference scheme and shock-capturing WENO schemes with conventional shock-fitting and show that a fifth-order convergence is indeed achieved for a canonical one-dimensional shock-entropy wave interaction problem. We also show that the method of finding shock velocity from one characteristic relation and Rankine–Hugoniot jump condition performs better than the other methods of computing shock velocities. A high-order front-tracking implementation of shock-fitting is also presented in this paper and nominal rate of convergence is shown. The front-tracking results are validated by comparing to results from the conventional shock-fitting method and a linear-interaction analysis for a two-dimensional shock disturbance interaction problem.     

叶栅内定常及非定常粘性流动数值模拟

本文给出了一个模拟叶栅内准三维定常和非定常粘性流动的数值方法。对于定常流动,采用ＴＶＤ Ｌａｘ－Ｗｅｎｄｒｏｆｆ格式和代数湍流模型求解雷诺平均Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程,使用当地时间步长和多网格技术使计算加速收敛到定常状态;对于非定常流动,使用双时间步长和全隐式离散,采用与求解定常流动相似的多网格方法求解隐式离散方程。文中给出了ＶＫＩ透平叶栅内的定常流结果和１．５级透平叶栅内的非定常数值结果。     

High-order shock-fitting methods for direct numerical simulation of hypersonic flow with chemical and thermal nonequilibrium

In recent years, much progress has been made in the direct numerical simulation of laminar-turbulent transition of hypersonic boundary layer flow. However, most of the efforts at the direct numerical simulation of transition previously have been focused on the idealized perfect gas flow or “cold” hypersonic flows. For practical problems in hypersonic flows, high-temperature effects of thermal and chemical nonequilibrium are important and cannot be modeled by a perfect gas model. Therefore, it is necessary to include the real gas models in the numerical simulation of hypersonic boundary layer transition in order to accurately predict flow field parameters. Currently most numerical methods for hypersonic flow with thermo-chemical nonequilibrium are based on shock-capturing approach at relatively low order of accuracy. Shock capturing schemes reduce to first-order accuracy near the shock and have been shown to produce spurious oscillations behind curved strong shocks. There is a need to develop new methods capable of simulating nonequilibrium hypersonic flow fields with uniformly high-order accuracy and avoid spurious oscillations near the shock. This paper presents a fifth-order shock-fitting method for numerical simulation of thermal and chemical nonequilibrium in hypersonic flows. The method is developed based on the state-of-the-art real gas models for thermo-chemical nonequilibrium and transport phenomena. Shock-fitting approach is used because it has the advantage of capturing the entire flow field with high-order accuracy and without any oscillations near the shock. The new method has been tested and validated for a number of test cases over a wide span of free stream conditions. The developed method is applied for the study of receptivity of free stream acoustic waves over a blunt cone for hypervelocity flow. Some preliminary results of the computations of the high order shock fitting method for the above mentioned study have also been presented.     

Visualization of a wall shear flow

In this paper we propose a new method which might be useful to investigate the flow fields close to vaulted walls with spatial and temporal resolution. This kind of flow visualization is important in the field of biofluid mechanics, since a close relationship is assumed between flow and biological phenomena. This new method is non-invasive, and is also applicable for unsteady flows. It has been used to investigate the steady and the unsteady laminar flow in a rectangular duct, as well as the steady, laminar flow in two different U-shaped ducts, both with a backward facing step, one having a rectangular cross-section, the other a nearly elliptical cross-section. The results concurred well with analytical or numerical solutions.     

N-S方程隐式分区并行计算

将Wu Z N和Zou H提出的一套适合三点块三对角隐式格式的定常和非定常可压无粘流的覆盖分区并行计算方法推广到N-S方程的计算.在二维定常亚音速、跨音速和非定常情况下都得到了较好的绝对并行效率.     

Generalized potentiality of inhomogeneous media flows

A method for specifying a class of potential flows of inhomogeneous continuous media is developed. The general approach is based on expanding a medium material symmetry group to a special volume-preserving group, allowing us to obtain a law for the conservation of vorticity and, when there is no vorticity, to derive the unsteady Bernoulli equation. As illustrations, plane steady stationary flows of an inhomogeneous incompressible fluid and variable-entropy gas are considered. The problem of an inhomogeneous gas flow around a wedge yielding the formation of a shock wave is solved.     

Numerical simulation of nonequilibrium flows by using the state-to-state approach in commercial software

Nonequilibrium flows of a two-component oxygen mixture O₂/O behind a shock wave are studied with due allowance for the state-to-state vibrational and chemical kinetics. The system of gas-dynamic equations is supplemented with kinetic equations including contributions of VT (TV)-exchange and dissociation processes. A method of the numerical solution of this system with the use of the ANSYS Fluent commercial software package is proposed, which is used in a combination with the authors’ code that takes into account nonequilibrium kinetics. The computed results are compared with parameters obtained by solving the problem in the shock-fitting formulation. The vibrational temperature is compared with experimental data. The numerical tool proposed in the present paper is applied to study the flow around a cylinder.     

Shock formation distance in real air

A simple derivation of the shock formation distance in spherically symmetric, unsteady, non-equilibrium flow is presented. The one-dimensional unsteady flow and the ideal gas flows emerge as special cases. The results are applied to atmospheric air and the ambiguity and limitations of the shock formation concept are discussed.