Abstract: The airflow in an axisymmetric inlet with an outlet throttling ratio of 50.8% has been investigated using large eddy simulation, primarily focusing on the unsteady characteristics of the external and internal coupling oscillatory flow. The simulation is based on the software of National Numerical Windtunnel (NNW) Project. Contributions of the averaged wall pressure and its fluctuating part have been validated against experimental measurements. Various fundamental phenomena, including the flow structures, unsteady shock waves, and fluctuating pressure have been studied systematically. It is found that the shock train is induced in the throat region to match the backpressure, and accordingly three typical flow regions are classified as the supersonic region, shock train region, and subsonic region. Coupling dynamics between the shock/shear-layer/separated boundary layer are formed in the shock train region as a result of the adverse pressure gradient. With the quasi-periodic unsteady fluid motions in the shock train region, the fluctuating pressure exhibits a broadband spectral feature. The temporal and spatial distributions of the fluctuating pressure are analyzed, which indicates that the fluctuating pressure propagates in the subsonic region in a form of disturbance wave. A feedback model is proposed giving a reasonable prediction of the dominant frequency in the downstream of shock train. Based on the correlation analysis, it is demonstrated that the unsteady motions of the shock train is influenced by the coupling of the upstream and downstream flows. Specifically, it contains an unsteady motion of frequency St=0.7 which is a signature of the upstream flow, and another of frequency St=0.9 that is a signature of the downstream flow.

Key words: inlet, shock train, unsteady flow, large-eddy simulation, National Numerical Windtunnel (NNW) Project