Effects of a turbulent grazing flow on the impedance of a single-orifice Helmholtz resonator

The databases of several direct numerical simulations of a cavity-backed circular orifice, characteristic of a single degree-of-freedom, locally reacting acoustic liner, are used to predict the liner’s impedance and to guide development of a reduced-order model (ROM). The simulations contain incident monochromatic acoustic waves of amplitudes and frequencies ranging from 130–160 dB and 2–3 kHz, respectively, in the presence of laminar and turbulent grazing boundary layers with freestream Mach numbers of 0.5. The two microphone method attributed to Dean (Journal of Sound and Vibration, Vol. 34(1), 1974) is used to estimate the simulation-predicted impedance and compared to corresponding experimental data from NASA Langley. While encouraging agreement is found, the results suggest that selection of the impedance eduction method is critical. A time-domain ROM is developed using the simulations as a guide to suggest appropriate simplification. The model contains one semi-empirical element where the discharge coefficient of the orifice is connected to the fluid velocity at the orifice opening. The newly-developed ROM reproduces the simulation data to a large degree from sound amplitudes at and below 150 dB but begins to lose accuracy at the highest amplitude case of 160 dB. Simulations of a multiple orifice liner model preliminary confirms the credibility of the porosity-scaled resistance comparison.