高焓湍流边界层壁面摩阻产生机制分析

高超飞行器在中低空以极高马赫数飞行时,飞行器表面会遇到湍流与高温非平衡效应耦合作用的新问题.这种高焓湍流边界层壁面摩阻产生机制是新型高超声速飞行器所关注的基础科学问题,厘清此产生机制可以为减阻方法的设计提供指导,具有重要的工程实用价值.本文选取高超声速飞行时楔形体头部斜激波后的高焓流动状态,开展了考虑高温非平衡效应的湍流边界层直接数值模拟研究,并设置同等边界层参数下的低焓完全气体湍流边界层流动作为对比,采用RD (Renard&Deck)分解技术研究了高焓湍流边界层摩阻的主要产生机制,对摩阻产生的主要贡献项积分函数分布进行了详细分析,研究了高温非平衡效应对摩阻产生的影响规律;采用象限分析技术,研究了摩阻分解湍动能生成项的主导流动事件.计算结果表明,高温非平衡效应会使得壁面摩阻脉动条带的流向和展向尺寸均减小.分子黏性耗散项和湍动能生成项是高焓湍流边界层摩阻生成的主要流动过程.分子黏性耗散项主要作用在近壁区,高焓流动的分布与低焓流动存在差异.象限分析表明,上抛和下扫运动是影响摩阻分解中湍动能生成项的主导事件.     

Explicit algebraic Reynolds stress model for shock-dominated flows

Shock waves drastically alter the nature of Reynolds stresses in a turbulent flow, and conventional turbulence models cannot reproduce this effect. In the present study, we employ explicit algebraic Reynolds stress model (EARSM) to predict the Reynolds stress anisotropy generated by a shockwave. The model by Wallin and Johansson (2000) is used as the baseline model. It is found to over-predict the post-shock Reynolds stresses in canonical shock turbulence interaction. The budget of the transport equation of Reynolds stresses computed using linear interaction analysis shows that the unsteady shock distortion mechanism and the pressure–velocity correlations are important. We propose improvement to the baseline model using linear interaction analysis results and redistribute the turbulent kinetic energy between the principle Reynolds stresses. The new model matches DNS data for the amplification of Reynolds stresses across the shock and their post-shock evolution, for a range of Mach numbers. It is applied to oblique shock/boundary-layer interaction at Mach 5. Significant improvements are observed in predicting surface pressure and skin friction coefficient, with respect to experimental measurements.     

Moving shocks through metallic grids: their interaction and potential for blast wave mitigation

Numerical simulations and laboratory measurements have been used to illuminate the interaction of a moving shock wave impacting on metallic grids at various shock strengths and grid solidities. The experimental work was carried out in a large scale shock tube facility while computational work simulated the flow field with a time-dependent inviscid and a time-dependent viscous model. The pressure drop measured across the grids is a result of two phenomena which are associated with the impact of the shock on the metallic grids. First are the reflection and refraction of the incoming shock on the grid itself. This appears to be the main inviscid mechanism associated with the reduction of the strength of the transmitted shock. Second, viscous phenomena are present during the reflection and refraction of the wave as well as during the passage of the induced flow of the air through the grid. The experimental data of pressure drop across the grid obtained in the present investigation are compared with those obtained from computations. The numerical results slightly overpredict the experimental data of relative pressure drop which increases substantially with grid solidity at fixed flow Mach numbers. The processes of shock reflection and refraction are continuous and they can be extended in duration by using thicker grids that will result in lower compression rates of the structural loading and increase the viscous losses associated with these phenomena which will further attenuate the impacting shock. Preliminary theoretical analysis suggests that the use of a graded porosity/solidity material will result in higher pressure drop than a constant porosity/solidity material and thus provide effective blast mitigation.      

Evaluation of three techniques for wall-shear measurements in three-dimensional flows

Recent improvements in three techniques for measuring skin friction in two- and three-dimensional turbulent wall-bounded shear flows are presented. The techniques are: oil-film interferometry, hot wires mounted near the wall, and surface hot-film sensors based on MEMS technology. First, we demonstrate that the oil-film interferometry technique can be used to measure the skin-friction magnitude and its direction in two- and three-dimensional wall-bounded shear flows. Second, a simple method is outlined to measure the skin friction with a wall wire located outside of the viscous sublayer. Finally, a systematic study of the parameters influencing wall-friction measurements with MEMS sensors is presented. The results demonstrate that accurate measurements of the mean skin friction with MEMS sensors are possible in two- and three-dimensional wall flows. Measurements by the three techniques are compared to each other and to past measurements in the same facility.     

Interaction of a normal shock wave with a compressible turbulent flow

A speckle photographic method, which is sensitive to changes of gradients in fluid density, is applied for analyzing a compressible turbulent air flow with density fluctuations. Spatial correlation coefficients, turbulent length scales, and energy spectra are determined under the assumption of homogeneous isotropic turbulence. The experiments are performed in a shock tube where the flow is passed through a turbulence grid. Measurements are taken before and after the turbulent regime interacts with the normal shock wave reflected from the tube's end wall. Amplification of the turbulence intensity by the shock interaction process is verified quantitatively and is shown to be restricted to the lower wave numbers in the spectrum.A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988.To Professor Dr.-Ing. Klaus Gersten on the occasion of his 60th birthday     

Effect of wall distance on the prediction of variable property flow with two-equation turbulence models

This paper investigates the effect of wall distance coordinate on predicting variable property flows with two-equation turbulence models. Three of five different definitions of wall distance coordinate are employed: the wall-property definition, the integral-property definition and the local-property definition. Three different two-equation turbulence models that involve the wall distance coordinate are tested against the varying property flow: Superheated gas flow. The definition of wall distance coordinate affects the size of the viscous region. The wall-property based unit makes the wall distance to be the smallest and contributes to widen the viscous damping region, so that the skin friction factor and the Nusselt number is lowered. All the predictions with three different wall distance coordinates lie within less than 20% in the calculated Nusselt number.     

Shock waves at microscales

We present a model for the effects of scale, via molecular diffusion phenomena, on the generation and propagation of shock waves. A simple parametrization of the shear stresses and heat flux at the wall leads to the determination of new jump conditions, which show that, for a given wave Mach number at small scales, the resulting particle velocities are lower but the pressures are higher. Also, the model predicts that the flow at small scale is isothermal and that the minimum wave velocity can be subsonic. Experiments with a miniature shock tube using low pressures to simulate the effects of small scale have shown qualitative agreement with the proposed model. In fact, the effects of scale appear even more important than what has been incorporated in the model.PACS: 47.40.-xReceived: 14 November 2002, Accepted: 2 April 2003, Published online: 18 June 2003     

Viscous dissipation effects on heat transfer in flow over an inclined plate

The effects of viscous dissipation are considered for natural convection flow past a semi-infinite inclined plate with variable surface temperature. Velocity and temperature profiles, skin friction, and rate of heat transfer are obtained. The effects of Grashof and Prandtl numbers, inclination angle, exponent in the wall temperature variation law, and viscous dissipation parameter on the flow are discussed. It is shown that the time required to reach steady states increases with increasing Prandtl number of the fluid. In addition, an increase in the plate temperature due to viscous dissipation was found to lead to a rise in the average skin friction and a decrease in the average Nusselt number.     

Three-dimensional centrifugal instabilities development inside a parallelepipedic open cavity of various shape

This experimental study reports flow developments inside a parallelepipedic cavity of variable shape and dimensions. That flow is generated by the interaction between a laminar boundary layer and the cavity, which creates shear-layer oscillations. The aim is to understand the three-dimensional flow morphology varying the Reynolds number and the cavity shape. Flow visualizations are obtained in a plane situated inside the cavity in order to get the dynamical structures. Dimensional analysis of the cavity flow teaches that three dimensionless numbers are necessary for the flow reduction. This is confirmed by experimental results pointing thresholds of appearance of instabilities identified for some combinations of Reynolds number and geometric parameters. The key mechanisms for their existence are centrifugal effects induced by a vortex of spanwise axis with sufficient intensity, and viscous effects due to the wall confinement of the cavity. Their destruction is linked to flow transition to turbulence above a limiting convective velocity generated by the vortex of spanwise axis. These instabilities are generally present in a spanwise row of counter-rotating pairs of vortices, but for some cases, isolated pairs are also identified. Secondary modulations of primary instabilities are also present for particular parameters. Results permit to discriminate the relevant scales associated with the shear layer and the inner cavity flow.     

On the persistence of regular reflection

It is well accepted that the persistence of regular reflection of a shock wave off a wedge beyond the ideal theoretical prediction is due to viscous and thermal boundary layers induced behind the reflection point. Experiments have been done by reflecting two shock waves of equal strength off each other so that the plane of symmetry between them becomes an ideal inviscid and adiabatic reflection plane thereby experimentally mimicking the assumptions of the theory. There is one definitive experiment done at a wall angle of 40° using a bifurcated shock tube that indicates that the actual transition angle is the theoretical detachment condition. This paper extends these results to two cases near limiting conditions; one at a very low incidence shock Mach number and one at a wall angle very close to the theoretical transition limit. The first confirms the reasons for the von Neumann Paradox but cannot discriminate between sonic and detachment conditions, but is within about 0.5% of them, and the second shows transition much closer to the sonic than the detachment condition but with both within the experimental error bounds. In both cases, the results are notably different from transition conditions off a wedge and confirm the effects of transport properties being the cause of persistence of regular reflection.     

Application of hot-wire anemometry in shock-tube flows

Several techniques associated with the use of hot wire anemometry in compressible turbulence measurements are described and tested in shock tube flows. These techniques include 1. in-situ calibration of the hot-wire probe by firing several shock waves of different strengths in the shock tube; 2. on-line analog frequency compensation or off-line digital compensation of the temperature-wire; 3. simultaneous acquisition of time-dependent flow velocity and temperature of the flow without invoking Morkovin's hypothesis of strong Reynolds analogy. The techniques were tested in two different shock tube facilities, where a grid generated turbulent flow interacting with a moving shock was set up.The financial support provided by National Science Foundation and NASA is greatly acknowledged.     

Mixed convection boundary layers in the stagnation-point flow toward a stretching vertical sheet

An analysis is made for the steady mixed convection boundary layer flow near the two-dimensional stagnation-point flow of an incompressible viscous fluid over a stretching vertical sheet in its own plane. The stretching velocity and the surface temperature are assumed to vary linearly with the distance from the stagnation-point. Two equal and opposite forces are impulsively applied along the x-axis so that the wall is stretched, keeping the origin fixed in a viscous fluid of constant ambient temperature. The transformed ordinary differential equations are solved numerically for some values of the parameters involved using a very efficient numerical scheme known as the Keller-box method. The features of the flow and heat transfer characteristics are analyzed and discussed in detail. Both cases of assisting and opposing flows are considered. It is observed that, for assisting flow, both the skin friction coefficient and the local Nusselt number increase as the buoyancy parameter increases, while only the local Nusselt number increases but the skin friction coefficient decreases as the Prandtl number increases. For opposing flow, both the skin friction coefficient and the local Nusselt number decrease as the buoyancy parameter increases, but both increase as Pr increases. Comparison with known results is excellent.     

壁面展向周期振动的槽道湍流减阻机理的研究

利用直接数值模拟研究了带有壁面展向周期振动的槽道湍流.壁面在展向的周期运动使湍流受到抑制,并使壁面摩擦阻力减小.通过对雷诺应力输运方程的分析研究了壁面展向周期振动的减阻机理,进一步揭示了压力变形项在湍流抑制中的关键作用.     

Impact of vacuum degree on the aerodynamics of a high-speed train capsule running in a tube

Motivated by the growing scientific and engineering interest in evacuated tube railway transportation systems, in this paper we numerically study the influence of the vacuum degree on the flow field around a train capsule running in an evacuated tube with circular section. The vacuum degree is increased by lowering the nominal pressure inside the tube. The numerical simulations are fully verified by wind-tunnel experimental data of supersonic flows around a blunt body and in a scramjet combustion chamber, as well as by several numerical results in other related studies. The flow around the train capsule is characterized by a compression region in front of the train, a chocked flow near the train, and a complex highly unsteady region behind the train, where expansions waves and reflecting oblique shock waves exist. The total aerodynamic drag and the vacuum degree are found to be linearly related, revealing that lowering the nominal pressure can have a significantly beneficial effect on the aerodynamic performance of the train capsule. The aerodynamic heating due to compressibility effects and the increased pressure are more prominent along the centreline of the tube than on the tube wall. As the vacuum degree increases, the temperature and pressure differences between the front and the tail of the train and the intensity of the reflected shock waves become less significant, so that the extension of the expansion region in the train wake shortens.     

Computation of shock wave/turbulent boundary layer interactions using a two-equation model with compressibility corrections

In this paper computational results for two different types of shock wave / turbulent boundary layer interaction flows are presented. It is shown that upstream effects of the shock induced separation cannot be reproduced by Wilcox's (1991) k--model, whereas downstream of the interaction, predictions of pressure distribution and skin friction are acceptable. The inclusion of the compressible part of the dissipation rate and the pressure dilatation in the model has noticeable, but not dramatic effects on wall pressure and skin friction in the selected flow cases.     

壁湍流相干结构和减阻控制机理

剪切湍流中相干结构的发现是上世纪湍流研究的重大进展之一,这些大尺度的相干运动在湍流的动力学过程中起重要作用,也为湍流的控制指出了新的方向.壁湍流高摩擦阻力的产生与近壁区流动结构密切相关,基于近壁区湍流动力学过程的减阻控制方案可以有效降低湍流的摩擦阻力,但是随着雷诺数的升高, 这些控制方案的有效性逐渐降低.近年来研究发现, 在高雷诺数情况下外区存在大尺度的相干运动,这种大尺度运动对近壁区湍流和壁面摩擦阻力的产生有重要影响,为高雷诺数湍流减阻控制策略的设计提出了新的挑战.该文将对壁湍流相干结构的研究历史加以简单的回顾,重点介绍近壁区相干结构及其控制机理、近年来高雷诺数外区大尺度运动的研究进展,在此基础上提出高雷诺数减阻控制研究的关键科学问题.      

Shock waves from an open-ended shock tube with different shapes

A new method for decreasing the attenuation of a shock wave emerging from an open-ended shock tube exit into a large free space has been developed to improve the shock wave technique for cleaning deposits on the surfaces in industrial equipments by changing the tube exit geometry. Three tube exits (the simple tube exit, a tube exit with ring and a coaxial tube exit) were used to study the propagation processes of the shock waves. The detailed flow features were experimentally investigated by use of a two-dimensional color schlieren method and by pressure measurements. By comparing the results for different tube exits, it is shown that the expansion of the shock waves near the mouth can be restricted by using the tube exit with ring or the coaxial tube exit. Thus, the attenuation of the shock waves is reduced. The time histories of overpressure have illustrated that the best results are obtained for the coaxial tube exit. But the pressure signals for the tube exit with ring showed comparable results with the advantage of a relatively simple geometry. The flow structures of diffracting shock waves have also been simulated by using an upwind finite volume scheme based on a high order extension of Godunov's method as well as an adaptive unstructured triangular mesh refinement/unrefinement algorithm. The numberical results agree remarkably with the experimental ones.     

Superorbital expansion tube tests of a caret waverider

A computational and experimental study was conducted to assess the potential of testing waverider configurations in a high-performance, short-duration expansion tube facility. The tests were performed in the newly commissioned X3 superorbital expansion tube and provide the first experimental data of a waverider tested at a stagnation enthalpy and equivalent flight speed exceeding 40 MJ/kg and 9 km/s, respectively. Two simple caret configurations were chosen as benchmark test cases to test the use of the facility, instrumentation and numerical models to investigate these flows. The general performance of the sharp and blunt leading edge waveriders at angles of attack ranging from 0° to 5° were analyzed and compared to CFD and theoretical predictions. For the conditions tested, the presence of a strong viscous interaction caused the shock wave to be detached from the leading edge of the models resulting in a significant loss in performance. An analytical model was developed to account for the strong coupling between the shock wave and boundary layer. Results were shown to be in very good agreement with CFD estimates for both configurations at all angles of attack considered. Finite-rate chemistry CFD simulations indicated that real gas effects other than the residual levels of nonequilibrium freestream dissociation present in the expansion tube flow were negligible for the conditions tested. The study also revealed that a past flow visualization technique gave a false indication of the leading edge shock location. An improved experimental visualization technique was successfully tested with results from these tests correlating well with computational estimates. This study successfully demonstrated the use of the facility to study waverider performance at speeds representative of orbital flight.      

Effect of viscous dissipation on the mixed convection heat transfer from an exponentially stretching surface

The mixed convection flow and heat transfer from an exponentially stretching vertical surface in a quiescent fluid is analyzed using similarity solution technique. Wall temperature and stretching velocity are assumed to have specific exponential function forms. The influence of buoyancy along with viscous dissipation on the convective transport in the boundary layer region is analyzed in both aiding and opposing flow situations. The flow is governed by the mixed convection parameter Gr/Re². The velocity and temperature inside the boundary layer are observed to be influenced by the parameters like Prandtl number Pr, Gebhart number Gb. Significant changes are observed in non-dimensional skin friction and heat transfer coefficients due to viscous dissipation in the medium. The flow and temperature distributions inside the boundary layer are analyzed and the results for non-dimensional skin friction and heat transfer coefficients are discussed through computer generated plots.