Mean Flow and Linear Stability Analysis of Twin Supersonic Jets

The mean flow and stability properties of perfectly-expanded supersonic twin-jet configurations are investigated by means of RANS calculations, spectral proper orthogonal decomposition (SPOD) based on experimental measurements and plane-marching parabolized stability equations (PM-PSE). A tailored twin-jet mean flow model is considered to assess its accuracy with respect to three-dimensional RANS twin-jet calculations. Mean-flow comparisons for different jet spacings suggest that the tailored model can provide a successful prediction of the actual twin-jet mean flow characteristics in the region close to the nozzle exit plane. However, further downstream, the tailored flow field presents strong deviations from the actual twin-jet mean flow, which increase in magnitude as the jet spacing is reduced. SPOD analyses applied to high-speed Schlieren visualizations provide the dominant coherent structures for the cases under study. Tonal peaks associated with Kelvin-Helmholtz instabilities showing flapping and toroidal fluctuations of the jet structure are found to concentrate a large portion of the spectral energy for all jet spacings. Complementary PM-PSE calculations based on the actual twin-jet RANS solutions show a very good qualitative agreement with SPOD modes in terms of the amplitude functions, enabling the identification of individual modes of oscillation of the twin-jet system