EFFECTIVENESS OF AN ACOUSTIC LINER FOR REDUCING FORCING AND ACOUSTIC RESPONSE DUE TO DISTORTION

Abstract Transonic fans subjected to inlet distortion will respond with unsteady pressures as the blades move through the flow field. Possible consequences of this include harmonic forces that cause blades to vibrate, and increased noise emissions. The objective of this study is to investigate how acoustic liners can be used to reduce the response due to total pressure distortion. This is done by using CFD to analyze how harmonic forces and sound power levels are affected by modifying the impedance on the end wall and comparing to reference results with a hard wall. A transmission line method is used to find dimensions of a liner that is feasible to use on the fan, and to derive realistic impedance values for different corrected speeds. Analysis results include low subsonic and transonic operating speeds. Distortion is imposed in a range of wave numbers around the cut-on limits where the response is found to be dominating. The forcing and the effectiveness of the liner exhibits a significant dependency on the acoustic cut-on/cut-off condition and the liner design. Using the proposed methodology different goals can be pursued by focusing on noise or forcing. The primary objective pursued in this study is to reduce the forcing at part speed close to the cut-on limit. In addition, substantial reduction of the emitted sound power level is observed in several relevant cases. It is also shown that the reduction of harmonic forcing and noise levels can be achieved using a realistic liner design.