Coherent Helical Structures Inherent to Unsteady Shock Trains in Circular Duct

Circular isolators demonstrate a promising prospect due to their superior performance compared to traditional rectangular isolators for a scramjet. In this study, we conduct high-resolution implicit large-eddy simulations of the flow in a backpressured circular duct at two typical inflow Mach numbers of 2.00 and 2.45. For this distinct shock train system, the leading shocks are shaped like hourglasses composed of nearly axisymmetric [Formula: see text] bifurcated normal shocks; the following shocks are significantly twisted, distorted, and observed to exhibit unsteady large-scale motions at the tail part of the shock train region. Through the azimuthal discrete Fourier transform and proper orthogonal decomposition, these large-scale motions are shown to be closely linked with the coherent helical structures, which are present in two typical forms: the canonical-form ones concentrated in the bulk flow and the two-layer ones occurring in both the bulk and near-wall flow. Moreover, quantitative analysis of correlation functions and power spectral density of wall pressure fluctuations show that the two-layer helical structures greatly contribute to the peaks of wall pressure fluctuation coefficients near the tails of shock trains.