Noise from a Rotor Ingesting Inhomogeneous Turbulence

Abstract

On-blade hot wire anemometry measurements as well as far field sound measurements at several receiving angles have been previously made for a rotor partially embedded in a boundary layer. The inflow distortion effect on the rotor angle of attack distribution was determined directly from the on-blade measurements, and was found to minimally affect the angle of attack at the blade tips and lower the angle attack in the rotor disk plane as the radial location moves towards the hub. A narrow, sharp increase in angle of attack as the rotor blades approached the wall was also observed, indicating blade interaction with flow reversal. The haystacking pattern, or spectral humps that appear at multiples of the blade passage frequency, was studied for a wide range of advance ratios. At high advance ratios, evidence of vortex shedding from the blade trailing edges was observed. For low advance ratios, the haystacks narrowed, became more symmetric and increased in number. A method of determining the average acoustic signature of an eddy passage through a rotor was developed from time delay aligning multiple microphone signals and eddy passages detected using the continuous wavelet transform. It was found that the eddy passage signatures were similar to a cosine wave with a Gaussian window. It was also found that normalized timescales obtained directly from the eddy passage signatures remained somewhat constant with advance ratio, but increases slightly for fixed free stream velocities with increasing rotor RPM. For advance ratios less than 0.6, the eddy passage signatures were dominated by a tonal component due to rotor ingestion of misaligned flow caused by a boundary layer separation at the wall. This indicates that flow reversal known as the Pirouette Effect is interacting with the rotor blades.