Acoustical properties of a long rectangular cavity of constant cross-section immersed in a thick boundary layer

An experimental investigation was undertaken to examine the effect of cavity length on flow oscillations produced by an open cavity placed within a subsonic turbulent boundary layer. A narrow rectangular cavity with a constant width to depth ratio, W/D, of 1 was placed within a thick fully developed turbulent boundary layer with a corresponding R e θ = 10.5 × 1 0 3 . Pressure time histories were acquired for six separate cavity lengths (or L/D values) using microphone type pressure transducers. The spectral character of these signals was analyzed in terms of their magnitude and frequency content. This study indicates that large changes in the fluctuating pressure level can occur as L/D is varied from 1.47 to 8.73. A state of acoustic resonance was only observed at L / D = 1.47 , while fluid-acoustic resonance occurred at L/D between 1.47 and 8.73. Relative SPL calculations indicate that energy within the cavity was increased by approximately 60% over this L/D range. It is also suggested that this increase was a result of vortical structure shedding and growth. In addition, the location of maximum unsteadiness was estimated to coincide with the location of vortical structure saturation.