Effects of heating on noise radiation from turbulent mixing layers with initially laminar and turbulent boundary layers

The present work focusses on large-eddy simulations of spatially developing, turbulent, compressible mixing layers. The splitter plate, which separates the two streams initially, is included inside the computational domain. Dierent inflow condition cases with initially laminar boundary layers (abbreviated as LBL) and turbulent boundary layers (abbreviated as TBL) are considered. The eect of heating on the near-field hydrodynamics and far-field noise radiation is analyzed for both keeping the velocity ratio fixed. It is observed that with heating, the initial instability is accelerated but the saturation self-similar amplitude of Reynolds stress components do not vary. The saturation amplitudes of density fluctuations were found to increase proportionally to dierence in free-stream densities whereas near-field pressure fluctuations were found to decrease with heating. A simple scaling is suggested for the near-field pressure fluctuation amplitude. For LBL, sound radiation is observed in downstream direction peaked roughly at 30 degrees. The vortex pairing and breakdown to turbulence contribute significantly to the radiated sound. For TBL, the acoustic field near the shear layer is significantly weaker and noise due to passage of boundary layer eddies over the trailing edge is observed. For both the cases, a reduction in overall sound pressure levels in the far-field is observed with heating. The analysis of relative importance of Reynolds stress and enthalpy flux covariance tensor components, that is, the acoustic source terms in Goldstein’s generalized acoustic analogy, is presented to explain the eects of heating. The Reynolds stress auto-covariance amplitudes at zero spatial separation and time-delay were found to decrease due to reduction in mean density with heating. The enthalpy flux auto-covariance and enthalpy flux-momentum flux cross-covariance were also found to have significant amplitudes with heating. But they cancel each other’s eect leading to an overall reduction in far-field sound.