壁温变化非定常气动加热机理研究

研究了高超声速平板边界层因壁温时变引发的非定常气动热环境特征及机理.通过近似解析和数值模拟两种手段,得到了壁面热流随时问变化的完整过程.解析手段求解非定常可压缩边界层方程,将非定常响应表达成稳态解加上摄动级数的形式,在初始和最终稳态邻域分别求解,在适当的位置进行拼接,从而得到整个时间域上的解.在满足解析解假设的区域,数值结果与解析结果吻合较好,证明了所使用方法的可靠性.结果表明,非定常响应有两点显著特征:在壁温突然增加后短时间内,壁面热流方向改变,热边界层剖面在壁面附近出现了另一个拐点,这种新的剖面形状是典型的非定常特征.但是,高超声速情况下此种非定常响应存在的时间却很短,在考虑长时间气动加热的情况下,若只存在壁面温度时变的诱因,可以忽略流动中的非定常过程,当作准定常情况来处理.     

比热容非常值对高超声速平板边界层稳定性的影响

在高超声速条件下,边界层中气体的温度可能很高,以致气体的比热容不再是常数而与温度有关．这时边界层中的流动稳定性如何是值得研究的问题．采用线性稳定性理论,考虑比热容与温度有关时高超声速可压缩平板边界层的稳定性,并与假定比热容为常值的情况作比较,发现对第一模态和第二模态波的中性曲线、最大增长率都有影响．因此,在高超声速情况下,比热容随温度变化是研究边界层稳定性时必须考虑的一个因素．     

不同热环境下的颤振问题初探

高超声速环境下,气动加热会严重影响飞行器结构的动力与颤振特性。文章研究了不同边界条件下气动热对结构动力学特性及颤振特性的影响。以全动平尾为对象,分析了不同热条件下其振动与颤振特性的变化。此外,采用解耦求解思路,进行了高温下弹性模量变化和热应力对平尾颤振特性影响的分析。结果表明,结构加热产生的热应力是导致其颤振速度下降的主要原因;平均温度升高会引起平尾颤振速度的明显下降,应引起重视。     

轻质热防护系统波纹夹芯结构热力耦合分析

高超声速飞行器在出入大气层或持续在空间飞行时,将遭受严苛的气动加热载荷．对热防护系统进行传热分析是进行热力耦合分析的基础,而温度分布的特点直接影响到波纹夹芯结构的热应力等问题．首先对一体化热防护系统（integrated thermal protection system, ITPS）进行隔热性能分析,得到整个结构的温度场;然后采用顺序耦合的数值方法,模拟分析ITPS波纹夹芯结构单胞的热力耦合性能,给出波纹夹芯结构在静力载荷以及热力耦合载荷条件下的应力场、位移场,并对计算结果进行了讨论．结果表明波纹夹芯结构在初始尺寸及约束条件下,只满足在高温热流作用下飞行器低压区使用,而当气动压力大于等于15 000 Pa时,结构将发生破坏．     

高超声速飞行器前驻点热流数值模拟的物理准则研究

通过建立高超声速飞行器前驻点处沿驻点线的准一维流简化模型,再将头激波后流场分为无粘区和近壁粘性区,然后从N-S方程组出发,利用流场对称性和量级分析方法,最终推导出驻点邻域简化控制方程组,并证明了其流动具有边界层型特征．在此基础上,根据该控制方程组,给出了计算结果应满足的壁面驻点以及沿驻点线的相容关系,成为热流CFD计算准确与否应遵循的物理准则．最后通过算例验证了物理准则作为热流CFD计算可靠性判据的正确性．     

高超声速飞行器热防护材料与结构的研究进展

高超声速飞行器是航空航天的一个重要发展方向,在未来国防安全中起着重要作用．高超声速飞行器热防护材料与结构是高超声速飞行器设计与制造的关键技术之一,它关系到飞行器的安全．高超声速飞行器热防护材料与结构主要有金属TPS热防护系统、超高温陶瓷、C/C复合材料等．从材料制备、抗氧化、力学与物理性能表征等方面综述了热防护材料与结构的研究与应用现状,评述了其发展趋势．     

传导-辐射对沿垂直平面有热泳的瞬时自然对流的影响

研究存在热辐射时,热泳微粒的沉积,对沿垂直平面瞬态自然对流边界层流动的影响,垂直平面浸没在光密灰色流体中.分析中采用Rosseland扩散近似表示辐射热通量项.将控制方程简化为抛物线型的偏微分方程组,然后在整个时间段0≤τ<∞,利用有限差分法数值求解.还得到了小数值时间和大数值时间的渐近解,发现渐近解和数值解吻合很得好.而且,流体,20℃和1个标准大气压下的空气,即Prandtl数Pr为0.7时,用图形给出了不同物理参数,即热辐射参数Rd、表面温度参数θw和热泳参数λ,对瞬时的表面剪切应力τw、表面热传输率qw和组分浓度扩散率(传质率)mw的影响,以及对瞬时的速度、温度和浓度分布曲线的影响.     

Unsteady Slip Flow and Heat Transfer Analysis of Oldroyd-B Fluid Over the Stretching Wedge北大核心CSCD

研究了在速度滑移现象存在下,上随体Oldroyd-B流体绕加热的楔形体的非稳态流动。采用松弛-延迟热通量模型,模拟了传热过程和热延迟时间对传热的影响,通过考虑浮升力、热辐射和对流换热边界条件,进一步研究了流动及传热特性。利用同伦分析方法获得常微分方程组的近似解析解,发现滑移参数的增大可以促进流体的流动,以及流体的温度随热辐射参数增大而升高。此外还发现,温度场在热松弛时间和热延迟时间中出现相反的变化趋势。     

一种全速域的计算方法及其应用

针对原可压缩流动求解器不能用于低速不可压缩流动预测的缺点,采用预处理技术对控制方程特征系统、隐式求解方法进行修正,并采用预处理修正的AUSM+-up格式离散对流项.采用修正后的求解器对无粘鼓包流动、顶盖驱动粘性方腔流动以及Laval(拉瓦尔)喷管流动等算例进行数值仿真,并将数值仿真结果与基准解进行对比.结果表明将预处理技术应用于全速域流动问题的求解是可行的,经预处理修正后的求解器能够用于低速、亚音速、跨音速以及超音速流动问题的求解.     

非牛顿流体内流传热研究

在本文中,研究了注入轴对称模腔非牛顿流体非定常流动.本文的第二部份研究了上随体Maxwell流体管内热流动.对于注入模腔流动.其本构方程采用幂律流体模型方程.为了避免在表现粘度中温度关系引起的非线性.引进了一特征粘度的概念.描述本力学过程的基本方程是,本构方程、定常状态的运动方程、非定常能量方程及连续方程.该方程组在空间是二维问题,在数学上是三维问题.采用分裂差分格式求得本方程组的数值解答.分裂法曾成功应用于求解牛顿流体问题.在本文中,首次将分裂法成功地应用解决非牛顿流体流动问题.对于圆管内热流,给出了差分格式,使基本方程组化为一个三对角方程组.其结果,给出了不同时刻的模腔内二维温度分布.     

Stability analysis of supersonic entropy layers including shock effects

The prediction of the laminar/turbulent transition location in supersonic boundary layers plays an important role to accurately compute aerodynamic forces and heating rates for the aerodynamic design and control of hypersonic vehicles. The stability characteristics of supersonic boundary layers depend e.g. on nose bluntness, transverse curvature, wall temperature, shock waves, etc. Most parameters can be theoretically investigated by performing conventional stability calculations with vanishing or asymptotic perturbation conditions at the far field. In this approach the formation of a shock in front of the leading edge of a blunt body is ignored. However, to improve the understanding of the interaction between instability waves originating inside supersonic boundary layer with those coming from the inviscid entropy layer, the presence of the shock has to be taken into account. This paper presents a method, how shock effects can be physically consistently included in stability calculations. The outer free‐stream boundary conditions are replaced by appropriate shock conditions. The required perturbation equations can be derived from the linearized unsteady Rankine‐Hugoniot equations, accounting for the effect of shock oscillations due to perturbated waves which originate from the flow field windward of the shock.     

Effect of variable viscosity and viscous dissipation on the Hagen‐Poiseuille flow and entropy generation

In this article, the steady‐state flow of a Hagen‐Poiseuille modelin a circular pipe is considered and entropy generation due tofluid friction and heat transfer is examined. Because of variationin fluid viscosity, the entropy generation in the flow varies. Inhis model, Arrhenius law is applied for temperature equation‐dependent viscosity, and the influence of viscosity parameters on the entropy generation number and distribution of temperature and velocity is investigated. The governing momentum and energy equations, which are coupled due to the dissipative term in the energy equation, were solved by analytical techniques. The solutions of equations via perturbation method and homotopy perturbation method are obtained and then compared with those of numerical solutions. It is found that the fluid viscosity influences considerably the temperature distribution in the fluid close to the pipe wall, and increasing pipe wall temperature enhances the rate of entropy generation. © 2009 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 27: 529–540, 2011     

Heat transfer in a Walter’s liquid B fluid over an impermeable stretching sheet with non-uniform heat source/sink and elastic deformation

In this present article an analysis is carried out to study the boundary layer flow behavior and heat transfer characteristics in Walter’s liquid B fluid flow. The stretching sheet is assumed to be impermeable, the effects of viscous dissipation, non-uniform heat source/sink in the presence and in the absence of elastic deformation (which was escaped from attention of researchers while formulating the viscoelastic boundary layer flow problems)on heat transfer are addressed. The basic boundary layer equations for momentum and heat transfer, which are non-linear partial differential equations, are converted into non-linear ordinary differential equations by means of similarity transformation. Analytical solutions are obtained for the resulting boundary value problems. The effects of viscous dissipation, Prandtl number, Eckert number and non-uniform heat source/sink on heat transfer (in the presence and in the absence of elastic deformation) are shown in several plots and discussed. Analytical expressions for the wall frictional drag coefficient, non-dimensional wall temperature gradient and non-dimensional wall temperature are obtained and are tabulated for various values of the governing parameters. The present study reveals that, the presence of work done by deformation in the energy equation yields an augment in the fluid’s temperature.     

A common error made in investigation of boundary layer flows

The concept of boundary layer flow, introduced in 1904 by Prandtl, is a popular field in Fluid Mechanics for engineers, physicists and mathematicians. The present work is a critique to many papers published in the last 15 years in the field of boundary layer flow. The critique concerns the shape of velocity, temperature and concentration profiles which are truncated due to small calculation domain used during the numerical solution procedure. These truncated profiles are not compatible with the boundary layer theory and introduce errors in wall shear stress and wall heat transfer values.     

Effects of thermal radiation and magnetic field on unsteady mixed convection flow and heat transfer over an exponentially stretching surface with suction in the presence of internal heat generation/absorption

In this paper, the problem of unsteady laminar two-dimensional boundary layer flow and heat transfer of an incompressible viscous fluid in the presence of thermal radiation, internal heat generation or absorption, and magnetic field over an exponentially stretching surface subjected to suction with an exponential temperature distribution is discussed numerically. The governing boundary layer equations are reduced to a system of ordinary differential equations. New numerical method using Mathematica has been used to solve such system after obtaining the missed initial conditions. Comparison of obtained numerical results is made with previously published results in some special cases, and found to be in a good agreement.     

First stages of drop impact on a dry surface: asymptotic model

After impact of a viscous liquid drop on a dry wall surrounded by a gas, the drop surface is highly deformed, leading to the formation of an axisymmetrical lateral lamella along the wall. A local asymptotic model for the potential flow and unsteady boundary layer flow is developed to describe the lamella dynamics at early stages after impact. The second-order potential flow displaced by the unsteady boundary layer is taken into account. The lamella shape, its velocity and pressure are calculated with this model in parametrical forms. The three model parameters are evaluated here by fitting with recent experimental findings.     

Unsteady MHD stagnation-point flow with heat and mass transfer in a micropolar fluid in the presence of thermophoresis and suction/injection

The effect of suction or injection on unsteady MHD flow with heat and mass transfer in a micropolar fluid near the forward stagnation point flow with thermophoresis has been investigated. The problem is reduced to a system of non-dimensional partial differential equations, which are solved numerically using the implicit finite-difference scheme. Profiles for velocity, microrotation, temperature and concentration as well as the skin friction, the rate of heat and mass transfer are determined and presented graphically for physical parameters. The results show that the suction increases the skin friction, the rate of heat and mass transfer while opposite trend is observed for the case of injection. It is also found that the effect of thermophoresis is decrease the concentration boundary layer thickness.     

Discrete Adjoint based Sensitivity Analysis for Optimal Active Flow Control of High-Lift Configurations

Blowing and suction type of active flow control techniques can be used to delay the flow separation on the flap and to enhance the aerodynamic performance of high-lift configurations. Effective separation control and maximum enhancement in the mean lift coefficient are achieved by finding the optimal actuation parameters. The optimal set of actuation parameters can be obtained by combining the gradient based algorithms with discrete adjoints. In the present work, an unsteady discrete adjoint incompressible RANS solver is developed for the optimal active separation control. The adjoint solver is applied to the test case of active flow control on the flap of a 2D high-lift configuration. Sensitivity gradients are presented to demonstrate the accuracy of unsteady adjoint RANS solver. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Response of heat transfer from a moving flat plate in a parabolic flow

This paper deals with the solutions of steady as well as unsteady three-dimensional incompressible thermal boundary layer equations and the study of the response of heat transfer when there is a parabolic flow over a moving flat plate. The components of velocity in boundary layer are discussed by Sarma and Gupta and those results are used to analyse thermal boundary layer equations. A general analysis is made from which we deduce (i) Solutions of two-dimensional thermal boundary layer on a moving flat plate, (ii) Solutions of thermal boundary layer on a yawed flat plate, (iii) Solutions of thermal boundary layer when there is a parabolic flow over a moving flat plate by giving different values to β and Cx. Solutions are developed for large and small times and curves are drawn representing the variations of heat transfer from the plate with time for all the cases. The limiting time is also calculated.