A major source of contra-rotating open rotor (CROR) tonal noise is caused by the interaction of the front-rotor (FR) wakes with the aft-rotor blades. Inspired by chevron nozzles, which increase the mixing process in jet shear layers, serrations are implemented at the FR trailing-edge in order to increase the wake mixing and thus reduce the tones. The depth and width of the serrations are optimized with a multi-objective, metamodel-assisted evolutionary algorithm. For each member, a steady-state Reynolds-averaged Navier–Stokes (RANS) simulation is performed, which is coupled with an analytical noise prediction method in order to evaluate the noise reduction due to the serrations. The results confirm that tonal interaction noise can be reduced by means of trailing-edge serrations. It is found that the major noise reduction mechanism for wake interaction is attributed to increased destructive interferences occurring in spanwise direction. The tonal noise generated through the interaction of the rear rotor (RR) potential field with the FR trailing edge is also slightly reduced because of the circumferential and axial shift of the serrated trailing edge. Furthermore, the present study demonstrates the feasibility of performing an acoustic optimization with a hybrid approach that predicts the noise analytically and extracts the aerodynamic input data from a steady-state RANS flow solution.

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