Aeroacoustic tunnel effect in noise shielding problems

Abstract

The question is addressed, under which conditions the commonly applied neglect of shear in the flow past an acoustically shielding object may not be justified, even at low Mach numbers, representative for take-off and landing of aircraft. The numerical results generated to quantify the engine noise shielding effect of a high lift airfoil in the EU project NACRE are taken as starting point. These results showed an unexpectedly significant influence of shear on the shielding when compared to a simplified approach neglecting the shear at a low Mach number of M = 0.3. The paper gives an explanation of this phenomenon based on total reflection and accompanying evanescent waves in the shear flow near the gap slot. The problem is stripped down to the essential physical ingredients and is then reproduced in a much more simplified setup. The investigation shows that evanescent waves below a reflecting shear layer may become propagational in the presence of an edge or slot, i.e. sound may "tunnel" through even a totally reflecting shearlayer and radiate into the acoustic shadow zones predicted with simple methods, which neglect shear.