再入尾迹电磁特性的湍流效应研究

本文旨意在于通过探讨高超声速再入尾迹中的湍流等离子体与电磁波相互作用的机理，以及建立能反映此机理的应用性理论模型，从而提供一套可进行目标特性分析的方法，以便为工程部门的突防技术服务。本题目在再入气动物理现象研究中具有重要意义。综合分析指出，地面雷达观测到的非相干散射信号主要来源于再入尾迹的亚密湍流区产生的体积散射。因此，电磁散射特性分析主要针对尾迹亚密湍流等离子体。并且，这里所有的分析都是根据在工程应用中最成熟的一阶畸变波Born近似理论模型。再入尾迹电磁特性的湍流效应研究，着眼点就在于湍流等离子体场的研究。对湍流等离子体场理论模型，本文试图通过模式理论来表达，即求解平均化的全Navier-Stokes方程及其封闭方程k-ε-g模型，从而准确获得流动平均场和脉动场信息。这种表达方式较以前有了较大改进。注意到高超声速流动具有强烈可压缩性的特点，故使用的N-S平均方程由质量加权平均过程产生，湍流模型方程也经过可压缩性修正。方程的离散求解方法，都是运用带矢通量分裂的二阶TVD格式的有限体积法。再入尾迹湍流场的初始条件由近尾迹（底部）流动经N-S方程求解给定，初始值更加准确可靠。尾迹从层流到湍流的转捩过程采用相对成熟的半经验公式确定。飞行器的高超声速再入过程必然导致它周围的空气温度升高，使得流动表现出真实气体效应。对重点考察的湍流流动而言，真实气体效应主要表现为气体处于热化学平衡状态。就工程部门面临的实际问题，把一阶畸变波Born近似的解算方法做些改进，使其能够处理的范围从轴对称尾迹扩展到三维湍流等离子体场是必要的。这为深入的理论分析提供了有力的保障。在能够准确模拟湍流流动的刻划雷达散射截面的基础上，考察亚密湍流等离子体对电磁散射的影响。通过选择的几个有代表性的因素进行讨论，初步结果表明：湍流转捩方式、湍流尺度对尾迹雷达散射截面值计算影响不大，而电子组份脉动能初始值影响较明显，且在特定条件下湍流模型的影响亦不大。但由于湍流模型涉及脉动初始值，其影响需进一步确定。同时，一些今后开展继续此项研究工作的有益建议也提了出来。

均匀来流绕旋转及旋转振荡圆柱绕流研究

该文通过数值方法求解二维不可压Navier-Stokes方程，对均匀来流中静止、旋转和旋转振荡圆柱绕流进行了系统的数值模拟.该文采用有限体积法对控制方程进行离散，选用元结构化四边形网格剖分计算区域，关于速度-压力耦合的处理使用了SIMPLEC方法.经过了大量的数值模拟，分盺166L鸬玫搅苏饧钢秩屏鞯氖的Ｄ饨峁?该文重点是用快速傅里叶变换（FFT）方法对旋转振荡圆柱绕流中的频率耦合现象进行研究，并分析在不同频率耦合作用下涡形成、发展和脱落的规律.

An Atomistic-Continuum Hybrid Simulation Of Fluid Flows Over Superhydrophobic Surfaces

Recent experiments have found that slip length could be as large as on the order of 1 mu m for fluid flows over superhydrophobic surfaces. Superhydrophobic surfaces can be achieved by patterning roughness on hydrophobic surfaces. In the present paper, an atomistic-continuum hybrid approach is developed to simulate the Couette flows over superhydrophobic surfaces, in which a molecular dynamics simulation is used in a small region near the superhydrophobic surface where the continuum assumption is not valid and the Navier-Stokes equations are used in a large region for bulk flows where the continuum assumption does hold. These two descriptions are coupled using the dynamic coupling model in the overlap region to ensure momentum continuity. The hybrid simulation predicts a superhydrophobic state with large slip lengths, which cannot be obtained by molecular dynamics simulation alone.

Scale-similarity model for Lagrangian velocity correlations in isotropic and stationary turbulence

A scale-similarity model for Lagrangian two-point, two-time velocity correlations LVCs in isotropic turbulence is developed from the Kolmogorov similarity hypothesis. It is a second approximation to the isocontours of LVCs, while the Smith-Hay model is only a first approximation. This model expresses the LVC by its space correlation and a dispersion velocity. We derive the analytical expression for the dispersion velocity from the Navier-Stokes equations using the quasinormality assumption. The dispersion velocity is dependent on enstrophy spectra and shown to be smaller than the sweeping velocity for the Eulerian velocity correlation. Therefore, the Lagrangian decorrelation process is slower than the Eulerian decorrelation process. The data from direct numerical simulation of isotropic turbulence support the scale-similarity model: the LVCs for different space separations collapse into a universal form when plotted against the separation axis defined by the model.

Space-time correlations of fluctuating velocities in turbulent shear flows

Space-time correlations or Eulerian two-point two-time correlations of fluctuating velocities are analytically and numerically investigated in turbulent shear flows. An elliptic model for the space-time correlations in the inertial range is developed from the similarity assumptions on the isocorrelation contours: they share a uniform preference direction and a constant aspect ratio. The similarity assumptions are justified using the Kolmogorov similarity hypotheses and verified using the direct numerical simulation DNS of turbulent channel flows. The model relates the space-time correlations to the space correlations via the convection and sweeping characteristic velocities. The analytical expressions for the convection and sweeping velocities are derived from the Navier-Stokes equations for homogeneous turbulent shear flows, where the convection velocity is represented by the mean velocity and the sweeping velocity is the sum of the random sweeping velocity and the shearinduced velocity. This suggests that unlike Taylor’s model where the convection velocity is dominating and Kraichnan and Tennekes’ model where the random sweeping velocity is dominating, the decorrelation time scales of the space-time correlations in turbulent shear flows are determined by the convection velocity, the random sweeping velocity, and the shear-induced velocity. This model predicts a universal form of the spacetime correlations with the two characteristic velocities. The DNS of turbulent channel flows supports the prediction: the correlation functions exhibit a fair good collapse, when plotted against the normalized space and time separations defined by the elliptic model.

An atomistic-continuum hybrid simulation of fluid flows over superhydrophobic surfaces

Recent experiments have found that slip length could be as large as on the order of 1 mu m for fluid flows over superhydrophobic surfaces. Superhydrophobic surfaces can be achieved by patterning roughness on hydrophobic surfaces. In the present paper an atomistic-continuum hybrid approach is developed to simulate the Couette flows over superhydrophobic surfaces in which a molecular dynamics simulation is used in a small region near the superhydrophobic surface where the continuum assumption is not valid and the Navier-Stokes equations are used in a large region for bulk flows where the continuum assumption does hold. These two descriptions are coupled using the dynamic coupling model in the overlap region to ensure momentum continuity. The hybrid simulation predicts a superhydrophobic state with large slip lengths which cannot be obtained by molecular dynamics simulation alone.

Solute transport in nanochannels with roughness-like structures

Newfound attention has been given to solute transport in nanochannels. Because the electric double layer (EDL) thickness is comparable to characteristic channel dimensions, nanochannels have been used to separate ionic species with a constant charge-to-size ratio (i.e., electrophoretic mobility) that otherwise cannot be separated in electroosmotic or pressure- driven flow along microchannels. In nanochannels, the electrical fields within the EDL cause transverse ion distributions and thus yield charge-dependent mean ion speeds in the flow. Surface roughness is usually inevitable during microfabrication of microchannels or nanochannels. Surface roughness is usually inevitable during the fabrication of nanochannels. In the present study, we develop a numerical model to investigate the transport of charged solutes in nanochannels with hundreds of roughness-like structures. The model is based on continuum theory that couples Navier-Stokes equations for flows, Poisson-Boltzmann equation for electrical fields, and Nernst-Planck equation for solute transports. Different operating conditions are considered and the solute transport patterns in rough channels are compared with those in smooth channels. Results indicate that solutes move slower in rough nanochannels than in smooth ones for both pressure- driven and electroosmotic flows. Moreover, solute separation can be significantly improved by surface roughness under certain circumstances.

Numerical Investigation On The Flowfield Of "Swallowtail" Cavity For Supersonic Mixing Enhancement

A "swallowtail" cavity for the supersonic combustor was proposed to serve as an efficient flame holder for scramjets by enhancing the mass exchange between the cavity and the main flow. A numerical study on the "swallowtail" cavity was conducted by solving the three-dimensional Reynolds-averaged Navier-Stokes equations implemented with a k-epsilon turbulence model in a multi-block mesh. Turbulence model and numerical algorithms were validated first, and then test cases were calculated to investigate into the mechanism of cavity flows. Numerical results demonstrated that the certain mass in the supersonic main flow was sucked into the cavity and moved spirally toward the combustor walls. After that, the flow went out of the cavity at its lateral end, and finally was efficiently mixed with the main flow. The comparison between the "swallowtail" cavity and the conventional one showed that the mass exchanged between the cavity and the main flow was enhanced by the lateral flow that was induced due to the pressure gradient inside the cavity and was driven by the three-dimensional vortex ring generated from the "swallowtail" cavity structure.

流量配比对Coil增益分布影响的数值研究

利用三维CFD技术,通过求解层流Navier-stokes方程与组分输运方程,对简化后的化学氧碘激光RADICL模型进行数值模拟与分析,结合10种组分和21个基元反应的化学反应模型,对COIL亚声速段横向射流情况下,不同的主副流流量配比对化学氧碘激光器性能的影响进行分析与比较.结果证明,过高或过低的碘分子浓度状态均不利于合理、可观的小信号增益系数产生.存在一个最佳流量配比范围,与之对应的工作状态下,COIL的小信号增益系数会得到显著提高.

氧碘化学激光器超声速段射流工作方式性能的数值研究

氧碘化学激光器(COIL)的混合喷管内发生的是气体动力学、化学反应动力学以及光学等相互耦合的复杂过程,每个过程都对COIL性能有着至关重要的影响.利用3维计算流体动力学技术,通过求解层流Navier-Stokes方程与组分输运方程对简化的氧碘化学激光RADICL模型进行数值模拟与分析,结合10种组分和21个基元反应的化学反应模型,对COIL超声速段射流情况下喷管内的流动及混合情况,尤其是产率、分解率、泵浦率和小信号增益系数的细致3维空间分布进行研究.结果证明超声速段进行射流有利于提高COIL的性能表现,可以充分利用高增益区,光腔位置增益可以达到0.012 cm~(-1),与亚声速段射流相比总压恢复性能提高,混合有待加强.

计算气动弹性若干研究进展

郭永怀和钱学森先生早在1946年提出了上临界马赫数的概念,即对于亚声速的二维无旋流动,当来流速度达到下临界马赫数时开始出现声速.稍增加来流速度,光滑无旋的亚、超声速混合流动可以继续存在,理论上只有当来流速度达到上临界马赫数出现激波后,光滑无旋流动才被破坏.随后,航空工程界先驱们为提高阻力发散马赫数,降低马赫数1附近的飞机阻力,为突破声障,提出了超临界翼型设计技术,引进了后掠翼设计概念,提出了跨声速面积律理论,导致了20世纪军民用航空飞行器的大规模发展,随着计算机技术和计算方法的进步,不同程度地简化流体控制方程的求解方法得到大发展.基于雷诺平均Navier-Stokes方程的计算流体力学已广泛应用于飞机性能评估、复杂流动机理分析.目前,气动外形优化设计、气动/结构耦合干扰、气动噪声等多学科问题成为空气动力学的研究热点.该文介绍作者的团队近年来在计算气动弹性研究方面的若干进展,作为对郭永怀先生诞辰100周年的怀念.

A CE/SE Scheme for Flows in Porous Media and Its Applications

In this paper, a new computational scheme for solving flows in porous media was proposed. The scheme was based on an improved CE/SE method (the space-time Conservation Element and Solution Element method). We described porous flows by adopting DFB (Brinkman-Forchheimer extended Darcy) equation. The comparison between our computational results and Ghia's confirmed the high accuracy, resolution, and efficiency of our CE/SE scheme. The proposed first-order CE/SE scheme is a new reliable way for numerical simulations of flows in porous media. After investigation of effects of Darcy number on porous flow, it shows that Darcy number has dominant influence on porous flow for the Reynolds number and porosity considered.

高超声速前台阶绕流的计算分析

气动力和气动热的精确计算是高超声速流动分析的关键和难点，一直以来没有得到解决。本 文选择前台阶绕流这一经典问题，分析来流马赫数为10 的氩气流动以及台阶壁面的气动性能。通 过求解Navier-Stokes 方程和直接Monte-Carlo 粒子模拟，得到了高超声速前台阶绕流的精确流 动，并详细分析了计算网格对Navier-Stokes 方程求解结果的影响，同时解释了高超声速流动中热 流和摩阻计算困难的原因。通过壁面边界层的准一维假设，首次提出了壁面热流和剪切力的层流壁 面函数。研究发现，壁面函数很好地描述了高超声速层流边界层内的温度分布和速度分布，为高超 声速流动的气动力和气动热预估提供了有力的工具。

Simulation of gas flow over a micro cylinder

Gas flow over a micro cylinder is simulated using both a compressible Navier-Stokes solver and a hybrid continuum /particle approach. The micro cylinder flow has low Reynolds number because of the small length scale and the low speed, which also indicates that the rarefied gas effect exists in the flow. A cylinder having a diameter of 20 microns is simulated under several flow conditions where the Reynolds number ranges from 2 to 50 and the Mach number varies from 0.1 to 0.8. It is found that the low Reynolds number flow can be compressible even when the Mach number is less than 0.3, and the drag coefficient of the cylinder increases when the Reynolds number decreases. The compressible effect will increase the pressure drag coefficient although the friction coefficient remains nearly unchanged. The rarefied gas effect will reduce both the friction and pressure drag coefficients, and the vortex in the flow may be shrunk or even disappear.