The prediction of the laminar-turbulent transition of boundary layers is critically important to the development of hypersonic vehicles because the transition has a first-order impact on aerodynamic heating, drag, and vehicle operation. The success of transition prediction relies on a fundamental understanding of the relevant physical mechanisms. In the 20 years since the review by Kleiser & Zang (1991) on the direct numerical simulation (DNS) of the boundary-layer transition, significant progress has been made on DNS in the hypersonic flow regime and in the spatial DNS approach. Many high-order shock-capturing and shock-fitting finite-difference methods have been developed and extensively applied to numerical simulations of the hypersonic boundary-layer transition. DNS has become a powerful research tool and has led to discoveries of new transition mechanisms. This article reviews the recent progress of DNS on hypersonic boundary-layer receptivity, instability, and transition. The current status and future directions are also presented.

http://cfdhost.seas.ucla.edu/papers/2012/Zhong-X-2012-annurev-fluid-120710-101208.pdf

Direct Numerical Simulation on the Receptivity, Instability, and Transition of Hypersonic Boundary Layers

In this paper we introduce the boundary layer stability equation solver PSE-Chem. This software employs a physical model approach to studying supersonic and hypersonic boundary layers and it incorporates the relevant physics including thermal and chemical nonequilibrium eects. Reentry F, a Mach 20 5 cone with very small nose bluntness, is used as a test case to determine the N factor at transition. Our results are compared with published results from other researchers. Results are also presented which show, using the e N method, the stabilizing eect of increasing nose radius under these conditions. The sensitivity of the methods is illustrated in the ability to calculate how small nose radius changes aect the transition location at stations hundreds of nose radii downstream.

Hypersonic Boundary Layer Stability Analysis Using PSE-Chem

Developing mechanism-based methods for estimating hypersonic boundary-layer transition in flight: The role of quiet tunnels

The influence of high levels of wall cooling on the stability of hypervelocity boundary layers is investigated. Such conditions are relevant to experiments in high-enthalpy impulse facilities, where the wall temperature is much smaller than the free-stream temperature, as well as to some real flight scenarios. Some effects of wall cooling are well known, for instance, the stabilization of the first mode and destabilization of the second mode. In this paper, several new instability phenomena are investigated that arise only for high Mach numbers and high levels of wall cooling. In particular, certain unstable modes can travel supersonically with respect to the free stream, which changes the nature of the dispersion curve and leads to instability over a much wider band of frequencies. The cause of this phenomenon, the range of parameters for which it occurs and its implications for boundary layer stability are examined. Additionally, growth rates are systematically reported for a wide range of conditions relevant to high-enthalpy impulse facilities, and the stability trends in terms of Mach number and wall temperature are mapped out. Thermal non-equilibrium is included in the analysis and its influence on the stability characteristics of flows in impulse facilities is assessed.

/pdf/stability-of-highly-cooled-hypervelocity-boundary-layers-453xn5k8ub.pdf

Stability of highly cooled hypervelocity boundary layers

*Much has been learned about the physics underlying the transition process at supersonic and hypersonic speeds through years of analysis, experiment and computation. The application has been principally to simple shapes like plates, cones and spherical nosetips. But the shapes of the new entry vehicles are not simple. They will invariably be at angle of attack so three dimensional effects will be very important, as will roughness effects due to ablation. This paper will review the physics basis of our present understanding of the transition process. Further, because of the complex geometries, it will address the need for careful experimental work as per the guidelines enunciated years ago by the U.S. Transition Study Group. Following these guidelines is essential to obtaining reliable, usable data for use in refining transition estimation techniques . I. Introduction Entry vehicles descend rapidly through the atmosphere and decelerate as the drag forces increase with the increasing atmospheric density. The vehicle starts out fully laminar. Since Reynolds numbers increase rapidly through the des cent, transition tends to move very rapidly over the whole vehicle over a narrow range of altitudes. O ne tries to minimize the aerodynamic heating loads in entry so as to minimize the thermal protection needs of the vehicle. This means delaying transition to as low an altitude as possible . The history of high -performance entry vehicles begins with the development of the ICBM in the 1950’s. These v ehicles were essentially cone -cylinders with large nose bluntness to minimize stagnation point heating. The nose materials were often subliming ablators to take advantage of the further reduction in stagnation region heating due to the surface blowing from the subliming surface. Transition would move forward from the cylinder to the cone as the vehicle descende d through the atmosphere. If transition occurred asymmetrically on the cylinder or cone (leading to drag asymmetry), it was essential that the asymmetric effects be small enough to be corrected by the control system of the missile so as to minimize the cir cle of error about the target. In some cases, the transition unexpectedly moved forward onto the spherical nose , a phenomenon named the “blunt -body -paradox.” A study in that time period by Allen and Eggers 1 showed that for orbital and sub -orbital entry ( < 8 km/sec) , the convective aerodynamic heating rate was directly related to the vehicle’s ballistic coefficient, (W/C DA). The lower the ballistic coefficient, the lower the heating rates in entry. Hence the tendency to low weight and high drag coefficient. The highly blunted capsul e shapes of the Mercury, Gemini, Apollo and Soyuz vehicles with a blat ing heat shields are a consequence of this argument . At entry speeds above about 10 km/sec , the shock layers ahead of the entering blunt shapes become lumino us and radiate. These radiative heat transfer rates are highly density dependent. Thus the blunt body may not be the best entry shape for supercircular entry speeds. Studies by Allen et al 2,3 show that to minimize total heat transfer or total ablated mass , the optimum shape gets progressively slenderer as the entry speed is increased. Not enough was done with these vehicles to identify the major transition issues.

Transition Issues for Atmospheric Entry

Analysis of boundary-layer transition data from supersonic quiet tunnels, as well as flight experiments has indicated that, in the absence of surface roughness and high levels of freestream disturbances, linear stability theory can be used as a guide for estimation of the onset of transition. Transition data from two different hypersonic flight experiments are analyzed using parabolized stability equations, including chemistry effects associated with high-temperature boundary layers. The results suggest that transition in both of these cases is caused by the amplification of second mode disturbances. The analysis shows that, consistent with previous findings for supersonic flows where first mode disturbances induce laminar-turbulent transition, N factors of about 9.5 and 11.2 correlate the transition onset locations from these two high-Mach-number experiments. Therefore, the e N method can be used for smooth body transition prediction in hypersonic vehicle design. The effect of chemistry on boundary-layer stability is also studied and is shown to be destabilizing.

Hypersonic Flight Transition Data Analysis Using Parabolized Stability Equations with Chemistry Effects

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Hypersonic Flight Transition Data Analysis Using Parabolized Stability Equations with Chemistry Effects