Aeroacoustics research in Europe: The CEAS-ASC report on 2024 highlights, Contribution: Flow effect on measured wall pressure spectra by pinhole-mounted sensors

Kurzfassung

Wall pressure fluctuations were measured on a flat plate using pinhole-mounted Kulites with different pinhole diameters and cavity volumes. The acoustic response functions of the pinhole-mounted Kulites were determined with a laser plasma generated sound source. Fig. 19 shows the measured and corrected wall pressure spectra using acoustic calibration determined transfer function and modified transfer function where the damping factor and resonance frequency were adjusted. Fig. 20 shows the ratio of the adjusted parameters relating to the acoustic determined parameters, scaled with ω r d∕U c . The convection velocity U c represents the velocity of wall pressure fluctuations passing over the pinhole, which is pinhole size and flow condition dependent, i.e. the value changes significantly with flow pressure gradients, which were produced by placing a NACA 0012 airfoil above the plate. The ratio of the damping factor in flow and without flow, D f ∕D a , between each sensor configuration cross at the frequency sqrt(2) times larger than the frequency at the minimum value of D f ∕D a . This feature may indicate that a flow-induced resonance is superposed to the Helmholtz resonance. It is assumed that the Rossiter mode is induced when the first mode frequency coincides with the Helmholtz resonance frequency. The results showed that the spectra can be well-corrected in practice using the same formula for the Helmholtz resonance excited by acoustic sources with modified damping factor and resonance frequency.