Numerical Prediction of the Noise From a Rotor Ingesting a Planar Turbulent Boundary Layer

The noise from a ten-bladed rotor partially immersed in a planar turbulent boundary layer is computed using incompressible large-eddy simulation and the Ffowcs Williams-Hawkings equation. The inflow boundary layer has a thickness equal to 22% of the rotor diameter, and is at momentum-thickness Reynolds number of 1.15 × 10^4 and free-stream Mach number of 0.058. Three operating conditions, with rotor advance ratios J = 1.44 (zero thrust), 0.87 (moderate thrust) and 0.58 (high trust), are simulated, and results are compared with the Virginia Tech experimental data. Reasonable agreement is demonstrated for the sound-pressure level over a wide range of frequencies and its dependence on the rotor advance ratio. In the low and moderate thrust cases, key spectral features including the “haystacking” peaks and the accompanying valleys are correctly captured and related to blade interaction with elongated coherent structures in the boundary layer. In the high thrust case, blade-induced flow separation from the wall is observed, again consistent with experimental findings, and this leads to significantly stronger haystacking peaks in greater numbers.