Effects of disturbed nozzle-exit boundary layers on acoustic waves from ideally-expanded supersonic jet

The effects of disturbed boundary layer at the nozzle exit on acoustic waves of supersonic jets of the Mach and Reynolds numbers of 2.0 and 900,000, respectively, are investigated by large-eddy simulations. The high order compact schemes and sufficient grid points are used to solve the compressible Navier Stokes equations. The inflow is tripped by the method used in the previous study for a subsonic jet computation in which a random vortex is imposed inside the boundary layer of the nozzle. Two disturbed boundary layer cases (disturbed cases) with different disturbance strength and one laminar boundary layer case (laminar case) are investigated. The flow seems to be much disturbed by the tripping, and the slower growth of the shear layer thickness for the disturbed cases is observed than that for the laminar case. This slower growth for the disturbed case leads to its longer potential core length. The laminar case has stronger peaks inside the nozzle near the nozzle exit and it corresponds to the turbulent transition. With regard to the acoustic fields, the region where the most strong sound pressure level (SPL) is observed is the end of the potential core for the disturbed cases, while the laminar case have higher SPL around the transition region due to the strong Mach wave generation by the transition. The SPL of the laminar case is 5dB higher than disturbed cases at the far field, and the spectral of the laminar case is entirely higher than those of disturbed cases in the wide range of the frequency. Disturbance strength for disturbed case does not affect the flow and acoustic fields much in the range of disturbance strength we investigated.