Numerical Study of Hypersonic Separated Flow over an Expansion-Compression Surface

The flow characteristics and the significant factors instigating flow separation in high enthalpy hypersonic rarefied flow over a two-dimensional expansion-compression surface configuration are investigated through the Direct Simulation Monte Carlo approach, employing the DS2V code of Bird. The reasoning behind the chosen geometric configuration is to avoid the influence of any pre-existing boundary layer on the separation characteristics. The free-stream flow conditions correspond to experiments conducted at a total specific enthalpy of 13.4 MJ/kg in the T-ADFA free-piston shock tunnel at UNSW Canberra, thereby facilitating qualitative validation against experimental flow visualization images. The computational predictions are evaluated in terms of measurable surface quantities such as heat flux, shear stress, velocity slip and pressure. The sensitivity of these quantities to the number of simulated particles and the time to achieve a steady-state flow are presented through a convergence study. An infinitesimally sharp leading-edge case is discussed with respect to the flow structure and variations in surface quantities as influenced by the amount of rarefaction as well as geometric features. The inadequacy in quantifying the extent of separation, based on the shear stress, in a molecular description of the flow is commented upon. Owing to the manufacturing limitations in obtaining an infinitesimally sharp leading-edge, a leading-edge bluntness of 100 microns, commensurate with a model used for experiments, is also computed for comparison and an evaluation of the effect of bluntness on separation characteristics is presented. The influence of geometry on the flow structure is then discussed on the basis of changing the expansion surface angle. These results are then examined for correlation with the criterion for incipient separation.