Physical Analysis And Numerical Modeling Of Hypersonic Flow With Local Rarefaction Effects

In recent years,the development of near-space hypersonic vehicles and related flight technologies have attracted more and more attention from the world.As a vehicle flies through the atmosphere,it might go through continuum,slip and transition regimes due to dramatic decrease of atmosphere density with the increase of flight altitude.In addition,the surrounding flow field is usually composed of continuous flow and locally rarefied flow.Under such circumstances,the computational fluid dynamics(CFD)based on the Navier-Stokes-Fourier(NSF)equations cannot predict accurate drag and lift-drag ratio.However,the commonly used direct simulation Monte Carlo(DSMC)method for rarefied flows is computationally consuming in the near-continuum regime,which is unbearable in practical applications.Hence it is critical to develop a reliable and efficient model.Except this,the boundary-layer stability characteristics vary largely during the trajectory of a near-space vehicle through the atmosphere and might be significantly affected by the local rarefaction effects,which has seldom been studied yet.Aiming at these problems,the present thesis mainly focuses on the high-speed rarefied boundary-layer(shear layer)flows,with the aim to analyze the nonlinear transport properties of shear nonequilibrium flow,develope an improved NSF model for hypersonic near-continuum flow,and investigate the effects of gas rarefaction on the supersonic/hypersonic boundary-layer stability.The specific work and conclusions are as follows:1.The nonlinear transport properties and the inherent physical mechanism of high-speed rarefied shear flows,such as planar Couette flow and two-dimensional boundary-layer flow,have been analyzed in detail using the DSMC method.For the Couette flow,in the near-wall Knudsen layer,the local nonequilibrium effect is mainly induced by the boundary effect,and the distributions of normalized effective transport coefficients at different Mach numbers can be described by a unified scaling law.Outside the Knudsen layer,the local non-equilibrium effect is dominated by the strong shearing effect,and there exists a normal solution for the Boltzmann equation.For high Mach number flows,the normalized effective transport coefficients in the whole system(whether in the Knudsen layer or in the shear nonequilibrium region)can be well characterized by a shear nonequilibrium parameter Zh,and the quantitative relations between them have been obtained in this thesis by fitting a mass of DSMC data.It is further showed that such relation is universal irrespective of flow conditions and molecular collision model,and it can also be used to describe the transport properties of rarefied boundary-layer flows.2.Based on the above analysis,a DSMC-data improved Navier-Stokes model(Di NS)has been developed within the conventional CFD framework.In the new model,the Zh parameter is computed to identify the local shear nonequilibrium degree in the flow field and used to modify the viscosity and thermal conductivity in the NSF constitutive relations.At the same time,a consistent slip boundary condition is employed to capture the gas slip behavior near the wall.The new model has been carefully verified through a large of simulations for hypersonic near-continuum flows over different geometries,such as flat plate,sharp wedge,cylinder,blunt cone and double cone.The results show that the Di NS model reports significant improvements compared with the NSF equations in predicting boundary-layer profiles and surface aerothermodynamics,and gives good agreements with the DSMC results.At the same time,the computational efficiency of the Di NS model is nearly same with the CFD method,which is one order of magnitude faster than DSMC in near-continuum regime.3.The NSF,Di NS and DSMC method have been used to simulate the flows over blunt plate,wedge and cone to investigate the effects of bluntness,angle of attack,flight altitude,and geometric shape on the locally rarefied characteristics of the flow field and continuum breakdown on the surface aerothermodynamics,and simultaneously verify the accuracy of the Di NS model.The results show that the local flow characteristics can be significantly affected by the freestream conditions and geometric shape.For the simulated blunt plate and blunt cone cases at altitudes below85 km,the surface shear stress and heat flux predicted by the NSF equations starts to fail at an altitude of about 60 km,while the surface pressure is not sensitive to rarefaction effects.Furthermore,it is found that the inaccuracy of the NSF equations for predicting aerodynamics mainly results in the error of the axial force.For all simulated cases,the Di NS model exhibits remarkable improvements for predicting surface aerodynamic properties compared with the NSF equations,especially the axial force and the drag.4.A linear stability analysis routine for compressible rarefied boundary-layer flows has been developed based on existing linear stability theory,and the effects of rarefied effects(i.e.slip effects and failure of linear constitutive relations)on the base flow and stability characteristics of supersonic/hypersonic boundary layer have been discussed in detail.As for the base flow,the results show that the slip effect causes the boundary layer becoming thinner and the gradients of velocity and temperature becoming smaller.With considering local rarefied effects(i.e.employing the Di NS model),the computed boundary layer is thinner than a conventional one,and the gradients of velocity and temperature become larger.As for the stability,it is found that velocity slip significantly stabilizes the second-mode disturbances while largely destabilizes the first-mode perturbations.On the contrary,the temperature jump apparently enhances the second-mode instability while has little influence on the first mode.When velocity slip and temperature jump are both present,the first mode is more destabilized while a competitive effect acts on the second mode.Additional results show that the neutral stability curves for the second and third modes as well as the synchronization between fast and slow modes are delayed further downstream due to velocity slip.With considering local rarefied effects,the first and the second modes both become more destabilized than the conventional NSF results,with larger unstable region and higher growth rate.