Excitation of a Helmholtz resonator by a turbulent boundary layer. [with turbulent flow excitation source]

A turbulent boundary layer on the fuselage of a glider was used to excite a Helmholtz resonator. The resonator orifice was flush with the surface and smaller than the boundary‐layer thickness. 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. Four of the nine resonators tested were ’’strongly’’ excited and radiated considerable acoustic sound. This phenomenon occurs when turbulent eddies about the size of the orifice are convected past the orifice with the proper velocity to impose a frequency that matches a resonant frequency of the resonator. Turbulent eddies which are either smaller or larger than the orifice will not ...