Direct Numerical Simulation of Two Shock Wave/Turbulent Boundary Layer Interactions

Abstract

Direct numerical simulations (DNSs) of two shock wave/turbulent boundary layer interactions (STBLIs) are presented in this thesis. The first interaction is a 24° compression ramp at Mach 2.9, and the second interaction is an 8° compression ramp at Mach 7.2. The large-scale low-frequency unsteadiness in the Mach 2.9 DNS is investigated with the aim of shedding some light on its physical origin. Previous experimental and computational works have linked the unsteadiness either to fluctuations in the incoming boundary layer or to a mechanism in the downstream separated flow. Consistent with experimental observations, the shock in the DNS is found to undergo streamwise oscillations, which are broadband and occur at frequencies that are about two orders of magnitude lower than the characteristic frequency of the energy-containing turbulent scales in the incoming boundary layer. Based on a coherence and phase analysis of signals at the wall and in the flow field, it is found that the low frequency shock unsteadiness is statistically linked to pulsations of the downstream separated flow. The statistical link with fluctuations in the upstream boundary layer is also investigated. A weak link is observed: the value of the low-frequency coherence with the upstream flow is found to lie just above the limit of statistical significance, which is determined by means of a Monte Carlo study. The dynamics of the downstream separated flow are characterized further based on low-pass filtered DNS fields. The results suggest that structural changes occur in the downstream separated flow during the low-frequency motions, including the breaking-up of the separation bubble, which is observed when the shock moves downstream. The structural changes are described based on the Cf distribution through the interaction, as well as the velocity and vorticity fields. The possible link between the low-frequency dynamics observed in the DNS and results from global instability theory is explored. It is observed that the structural changes in the downstream separated flow are reminiscent of certain global linear instability modes reported in the literature, suggesting that an inherent instability of the separated flow could be the driving mechanism for the unsteadiness. The separated shear layer in the DNS is characterized: the self-similarity of the shear layer profiles, the formation of vortical structures in the shear layer, and the low-frequency behavior of the shear layer are investigated. Based on the results, possible low-frequency mechanisms involving the shear layer are discussed. The second DNS presented in this thesis is of an attached hypersonic STBLI (8° compression ramp at Mach 7.2). The flow is described based on flow visualizations, distributions of wall quantities, as well as mean and fluctuating fields. Wall heat transfer scalings and the turbulence amplification in the interaction are discussed. The DNS results are compared to experiments performed by Smits and co-workers at similar flow conditions, and excellent qualitative agreement is observed.