The Interaction Of Helmholtz Resonators In A Row When Excited By A Turbulent Boundary-Layer

The interaction between individual Helmholtz resonators when arranged in a row and excited by a turbulent boundary layer was explored experimentally. A wind tunnel wall, which had a turbulent boundary layer, provided the test region for the resonators that were placed across the span. Ten resonator orifices were flush mounted on the wall and the cylindrical resonator cavities were behind the orifice plate. In a series of tests, the effect of resonator spacing upon the response level, frequency, and the coherence and phase between resonators was determined. When two resonators were paired the oscillating cavity pressure level increased by 1 dB (at orifice spacings of one boundary layer thickness). This effect decreased with increased spacing. The same trend occurred for the correlation between resonator signals. When a row of several resonators was tested, the coherence and response level enhancement were less than that for two paired resonators. In both row and paired configurations, the oscillation between adjacent cavities was out of phase: the angle varied from 150 to 180 deg. This antiphase behavior was attributed to a fluid mechanic coupling as contrasted to acoustic coupling. The frequency of a single resonator’s response was seen to be an almost linear function of air speed. This behavior was also observed when interacting with other resonators, although the frequencies were increased by about 1%. Estimates were made of the appropriate outside end-correction factor that would be associated with these frequency changes.