Aeroacoustic analysis using natural Helmholtz-Hodge decomposition

Abstract

We present the first application of the Helmholtz-Hodge decomposition (HHD) to measured velocity data in aeroacoustics in order to analyze damping phenomena at an aeroacoustic sound absorber. As an example, a generic bias flow liner is investigated, i.e. a perforated acoustic liner whose damping is resulting from an interaction of sound and flow but provides an increased damping efficiency due to an additional injected flow through the perforation. For the aeroacoustic investigation, three-dimensional optical measurements of the velocity vector are performed at 4096 locations with a spatial resolution of 660μm and a measurement rate of 100 kHz. Applying the natural HHD to the phase-resolved oscillation velocity regarding the acoustic excitation frequency of 1122 Hz yields an irrotational velocity term, the acoustic particle velocity, and a solenoidal velocity term, the aerodynamic flow velocity. As a result, the interaction between the sound and the flow results in a coherent flow vortices dominating the velocity oscillation. In addition, a local acoustic source near the perforation is supposed, having a local acoustic particle velocity which is three times higher compared to the excitation sound wave. The HHD result shows slight artifacts, whose cause is identified as the spatial discretization of the data and, thus, can be suppressed by using a finer data grid size in the future. As a consequence, the HHD will be a valuable tool for analyzing and eventually optimizing bias flow liners for enhanced sound absorption in jet engines and stationary gas turbines.