Experimental and computational studies on propeller noise due to inflow distortion

Large-scale non-uniformities in the approach flow to a propeller result in periodic unsteady blade forces that radiate tonal noise at the blade passing frequency and its higher harmonics. The current research focuses on a combined theoretical and experimental approach to study this propeller aeroacoustic problem. The theoretical study employs the classical acoustic analogy in conjunction with two-dimensional unsteady aerodynamic theory. The experimental side of this research is conducted in an anechoic flow facility and employs a four-bladed marine propeller. Both isolated and cyclical distortions are investigated. The radiative properties due to isolated distortions is first studied by operating the propeller behind the wake of a rod. Qualitative comparisons of the measured directivities match well with the analyses from theory. Next, the analysis is extended to cyclical distortions in the incoming flow. The theoretical analysis accounts for the effect of mean blade loading on the unsteady blade forces and provides an upper and lower bound for the resulting far field noise radiation. The experimental study employs non-uniform flow generators that provide a three-cycle and four-cycle azimuthal variation in the approach flow. An extensive comparison of the measured noise levels with the theoretical predictions shows an excellent agreement both in magnitude and directivity.