Investigation of sound-flow interaction at a bias flow liner using 3D/3C velocity measurement and Helmholtz-Hodge decomposition

Abstract

In order to reduce the sound emission of modern gas turbines or jet engines, bias flow liners are used, where the damping of the sound is based on a complex interaction of the sound wave and the bias flow injected through a perforated facing sheet. The optimization of these liners regarding a high damping efficiency necessitates a deeper understanding of complex three dimensional (3D) aeroacoustic phenomena. To this aim, a contactless threecomponent vector (3C) measurement of both the flow velocity (in the order of several m/s) and the acoustic particle velocity (down to few mm/s) is needed. However, these two velocities overlap at the measured velocity, which necessitates the decomposition of the measured velocity field in order to analyze the interaction of sound and flow. For this purpose, the natural Helmholtz-Hodge decomposition is applied for the first time at a bias flow liner. Therefore, velocity field data is obtained with a high spatial resolution of 660 μm at more than 4000 measurement points using the frequency modulated Doppler global velocimeter. The measurement system provides a high dynamic range > 10³ and low uncertainty as well as a high measurement rate up to 100 kHz and, thus allows the measurement of both the flow field and the sound field. Furthermore, the analysis of the velocity spectrum is enabled. As a result, an oscillation of the fluid velocity is measured. This velocity oscillation is dominated by sound induced vortices in the flow. Additionally, the hypothesis of an aeroacoustic source near the facing sheet of the liner is suggested. As a perspective, the work provides new insights into aeroacoustic damping phenomena.