Investigation of laminar detachment by means of simultaneous microphone and surface hot wire measurements

Abstract

An experimental study on the acoustic characterization of the laminar separation bubble on an airfoil is presented. The acoustic near- and far-fields are sampled by means of microphone arrays. Additionally the wall shear stresses are measured on the airfoil suction side by means of 24 MEMS surface hot wires in the region where the separation bubble was expected. Dierent processing strategies both in the frequency and in the time domain were applied for an acoustic characterization of the bubble. A focus of the study presented here is the application of the so called causality correlation method. The wall shear stresses on the airfoil surface together with the pressure fluctuations in the near- and far-field are recorded simultaneously. This enables the calculation of the cross-correlation between the measured quantities. An analysis of the resulting coecient matrix in the space-time domain identies perturbations traveling along the airfoil, showing a linear dependency on the sound generating process. For the case of the correlation between the wall shear stresses on the airfoil and the pressure fluctuations in the far-field the validity of the results were improved signicantly by using the focused pressure signals from the microphone-array.