Excitation of a Helmholtz resonator under a turbulent boundary layer

A zero-pressure-gradient boundary layer on the fuselage of a glider was used to excite a Helmholtz resonator. The resonator was constructed so the orifice was flush with the surface. Nominal resonator frequencies were chosen so they would tune with different portions of the boundary layer wall pressure spectrum. The resonators were excited at both the Helmholtz frequency and a standing wave frequency. The results show a shift in the Helmholtz frequency when the boundary layer is present. This shift indicates the degree to which the turbulence interacts with the acoustic motion in the orifice and modifies the end correction. In several cases extremely high cavity responses were observed. These cases produced measurable sound in the free stream. The existence of this phenomenon correlates with the parameter ωd/u* (ω = resonance frequency, d = orifice diameter, u* = friction velocity). [This work was supported by NASA Ames Research Center.]