Howling in a Model-Scale Nozzle Related to Shock-Induced Boundary-Layer Separation at the Nozzle Exit

Abstract

The howling produced by a model-scale nozzle operated at high-subcritical pressure ratios is shown be related to shock-induced boundary-layer separation just upstream of the nozzle’s exit. A combination of proper orthogonal decomposition (POD) approaches are used to analyze high-speed schlieren (spectral POD) and particle-image velocimetry (snapshot POD) of the jet flow, showing that the boundary layer periodically separates from the nozzle wall at the frequency of the fundamental acoustic tone produced. Reynolds-averaged Navier-Stokes (RANS) simulations suggest that, when howling occurs in the experiments, a region of supersonic flow exists just upstream of the nozzle exit that is terminated by a shock which induces boundary-layer separation. The supersonic flow is caused by the flow passing around a small-radius, convex bend in the nozzle wall. Experimentally, it is shown that tripping the boundary layer just upstream of this bend suppresses the howling entirely. Hybrid RANS/large-eddy simulations reveal that the shock-induced separation occurs periodically as part of an unsteady shock-wave/boundary-layer interaction (SWBLI). We hypothesize that the SWBLI oscillations lock on to the frequency of an acoustic mode inside the nozzle.