Pressure Measurements at the Mixing-Layer Boundary of Supersonic Twin Jets

Pressure fluctuations at the shear-layer boundary of a supersonic round twin jet are measured by means of a phased microphone array. Measurements are taken one streamwise position at a time using 8 microphones distributed around the twin jet and placed at the boundary of the external shear layer of each jet, following mean-flow streamwise-velocity contours from RANS calculations. The spectral signature of the measured fluctuations captures the broadband mixing-layer noise component which dominates for perfectly-expanded jets, as well as the broadband shock-cell noise component and the screech resonance tones which are typical of imperfectly-expanded supersonic jets. More importantly, at frequencies where mixing noise is the dominant mechanism, the streamwise evolution of the measured pressure signals exhibits a region of linear growth that is consistent with the development of wavepackets in the twin jet flow field. The decomposition of the microphone signals into the four possible combinations (families) of symmetric and antisymmetric pressure fluctuations illustrates that symmetric fluctuations with respect to the plane containing both jets are generally more energetic than their antisymmetric counterparts, especially farther downstream from the nozzle exit, whereas symmetric and antisymmetric fluctuations with respect to the symmetry plane between both jets generally show similar energy levels. For perfectly-expanded conditions, local SPOD analyses based on the symmetric family of pressure signals reveal that the most energetic symmetric oscillation mode in the studied twin-jet system is a toroidal fluctuation, confirming the findings of previous investigations based on schlieren visualizations. For the first time, quantitative comparisons between plane-marching PSE wavepacket models and experimentally-measured pressure fluctuations are performed for the twin jet at perfectly-expanded conditions, finding a reasonable agreement between the PM-PSE mode SS0 and the symmetric experimental pressure signals for the higher frequencies of the mixing-noise region analyzed.