Unsteadiness in shock-induced separated flow with subsequent reattachment of supersonic annular jet

An axisymmetric truncated optimized contoured nozzle flow operating in overexpanded conditions is simulated in restricted shock separation regime. The numerical strategy adopted aims at computing essentially the largest dynamically active scales of the flow in order to analyze the unsteady mechanisms leading to the low-frequency peak of side loads observed in this case. It relies on an hybrid approach, based on a k-. ω model, including realizability corrections and modified in the vein of delayed detached eddy simulation. The full model operability in LES mode is shown to be inhibited by a limited resolution, which admittedly limits the representativity of interactions between intermediate scales. However, the present approach is shown to lead to surprisingly good estimates of the wall-pressure signals and resulting rms of side-loads. A representative large-scale unsteadiness is reproduced and is found to be characterized by an irregular azimuthal asymmetry of the reattached flow, superimposed on large-amplitude axial oscillations. An essential ingredient driving the flow unsteadiness is related to the inhomogenous propagation of pressure disturbances through the subsonic three-dimensional trapped vortex downstream of the cap-shock system.