Transmission of a turbulent boundary layer wall pressure field through an elastomeric coating

Abstract

The wall pressure fluctuations beneath a turbulent boundary layer introduce flow-induced noise and vibration which limits the performance of acoustic arrays. One method for mechanically filtering this energy is to separate the acoustic sensors from the fluid–solid interface with an elastomeric coating. Long wavelength acoustic energy is transmitted across the coating, and shorter wavelength energy from convected turbulence is attenuated. Experiments were conducted in an acoustically quiet water tunnel to measure the wall pressure fluctuations under 0.635mm and 1.27mm thick elastomeric coatings. Autospectra, magnitude and phase of the coherence, and convection velocities are presented for the range of Reynolds numbers 7540<Reθ<16,100. The autospectra display an exponential decay which increases with increasing frequency and coating thickness. A modified model of that given by Blake (1984) is shown to accurately predict the attenuations in the autospectra. The cross-spectral model given by Corcos (1963) is shown to be valid for measurements of the wall pressure coherence beneath the coatings.