Numerical analysis of acoustic resistance affected by the corner shape of a resonator orifice

Abstract

At the orifice of a Helmholtz resonator attached on a duct, acoustic resistance increases with the grazing flow speed. When the orifice corners are rounded, the increase in acoustic resistance due to the grazing flow is suppressed compared to when the orifices are right-angled. In this study, CFD analyses on the flow field including pressure fluctuation were performed for a resonator with right-angled orifice corners (NN model) and for a resonator with rounded orifice corners (RR model). The increase in acoustic resistance obtained from the CFD analysis qualitatively agrees with the previous experimental results; Acoustic resistances of both models are almost same when the grazing flow is slow. In contrast, when the grazing flow is fast, the acoustic resistance of the RR model is about half compared to that of the NN model. Moreover, Acoustic powers at cross sections normal to longitudinal direction of the duct were calculated by integrating acoustic intensity considering the effect of time averaged flow. The acoustic powers upstream and downstream of the resonator are both almost uniform, respectively, except the region in which the acoustic intensity is highly disordered. The difference in the acoustic powers at two sections upstream and downstream of the resonator coincides with the acoustic energy consumption rate calculated by using the acoustic resistance and particle velocity through the orifice. This shows that the acoustic resistance value estimated from the transfer function of sound pressures measured at two microphone positions is plausible.