Screech in Military-Style Rectangular Nozzles

In this work, the screech resonance loop in a military-style, converging–diverging nozzle is explored using a range of experimental and modeling techniques. Schlieren measurements of both major and minor axes were performed at a range of nozzle pressure ratios (NPRs), with the images decomposed via spectral-proper orthogonal decomposition. The decomposed data allow for the identification of modal staging and also provide information about the symmetry of the waves involved in the resonance loop. Further analysis was performed using particle image velocimetry (PIV) data, with the velocity fields associated with the different screech modes obtained by modal decomposition. The PIV data was also used to provide mean flows for linear stability analysis, which is used to predict the wavenumbers of the different waves involved in the resonance loop. These predictions are used in conjunction with a weakest-link model to predict the screech frequency of the flapping modes at a range of NPRs and are found to be in good agreement with the experimentally observed tones.