Investigation of the time it takes to unstart at hypersonic conditions, with relation to the geometry of an axisymmetric nozzle

An axisymmetric nozzle under hypersonic conditions is investigated to determine how the geometric parameters alter the time taken for the internal shock waves to propagate out of the internal section of the nozzle, a process known as unstart. The study compares the results to the Kantrowitz limit, a criterion used at lower Mach numbers to determine when unstart will occur in inlet nozzles, recent alterations to this limit, and expands this area of research by examining the geometry’s effect on the time taken for the shockwave to reach the leading edge of the nozzle. This study is designed to inform the construction of an axisymmetric ‘tick’ geometry, which will be used to study flow separation in the ANU T2 free-piston shock tunnel. The study computed the time through CFD simulations, using ANSYS Fluent software. The results of these simulations created a ‘map’ of results relating the time for the shock to propagate to the inlet entrance with the inlets geometrical parameters and enables analysis of the flow phenomena causing the propagation. Results demonstrated that the Kantrowitz limit is not consistent with CFD results in hypersonic regions, and that time does not correlate with the likelihood of unstart. These results are compared with DSMC results, obtained through the DS2v code to compare the unstart behaviors in continuum and rarefied flow regimes.