Boundary Layer induced Rotor Noise using an Analytical Modal Approach

Kurzfassung

Upcoming aircraft designs will face new acoustic challenges related to flow distortions ingested by the engines, such as the boundary layer developing on the fuselage interacting with the fan blades. This paper presents an analytical method to predict this interaction based on the description of circumferential modes in the frequency domain. The original broadband noise model already implemented in the DLR in-house tool PropNoise has been extended to account for anisotropic turbulence and large turbulence scales. The adopted modeling approach confirms that axially stretched eddies generate bumps centered on the harmonics of the blade passing frequency. On the one hand, the small turbulent scales in circumferential direction provide a rich content in aerodynamic perturbation modes. On the other hand, large axial length scales lead to a partial correlation between several consecutive blades, where Tyler-Sofrin modes are preferably excited. The combination of both effects creates acoustic interaction modes with a low circumferential order, which is a condition for efficient sound radiation even at low frequencies. Finally, the model is applied to the academic test case FC3 of a rotor immersed in a thick boundary layer, in the framework of the “Fan Broadband Noise Prediction Workshop” organized in Dallas, 2015 and Lyon, 2016.