Large-Eddy Simulations of a Normal Shock Train in a Constant-Area Isolator

Large-eddy simulation of a normal shock train in a constant-area isolator model (M-infinity = 1.61, Re-theta approximate to 1660) is carried out to investigate solution sensitivity with respect to a variety of physical modeling assumptions. Simulations with spanwise periodic boundary conditions are first performed, the results of which are compared with experiment and validated with a three-level grid refinement study. Due to the computational cost associated with resolving nearwall structures, the large-eddy simulation is run at a Reynolds number lower than that in the comparison experiment; thus, the confinement effect of the turbulent boundary layers is not exactly duplicated. Although this discrepancy is found to affect the location of the first normal shock, the overall structure of the shock train and its interaction with the boundary layers matches the experiment quite closely. Observations of pertinent physical phenomena in the experiment, such as a lack of reversed flow in the mean and the development of secondary shear layers, are confirmed by the simulation. Next, three-dimensional effects due to the side walls of the isolator are investigated by performing large-eddy simulation of the same shock train interaction in a three-dimensional, low-aspect-ratio rectangular duct geometry. It is observed that the same pressure ratio that results in a stable shock train with periodic boundary conditions may result in isolator unstart when side-wall effects are fully resolved, further emphasizing the profound role of geometry and boundary-layer confinement on the dynamics of the shock train system. Additionally, it is found that measures of local flow blockage, such as the ratio of boundary-layer thickness to duct height, are more important than total pressure loss in locating the initial shock within an isolator.