On the structure and resolution of wall-pressure fluctuations associated with turbulent boundary-layer flow

In a wind tunnel designed for flow-acoustic measurements, the wall-pressure fluctuations beneath a turbulent boundary layer have been investigated. The measurements were carried out with variously sized pressure transducers (19 les d + les 333) and with an array of four small transducers (separation distance Δ x + = 75). It is shown that the dimensionless diameter d + = 19 of the transducers is sufficient to resolve the essential structures of the turbulent pressure fluctuations. The power spectrum Φ (ω +) measured with the smallest transducer d + = 19 partly exhibits power-law decay ^7{3}, which has been theoretically predicted for locally isotropic turbulence. By visual analysis and signal averaging in the time domain, pressure structures with high amplitudes could be detected which have the shape of short wavetrains or pulses. Their characteristic frequency and longitudinal wavelength have the mean values ω + = 0. 52 and λ + = 145 respectively, and their mean convection velocity amounts to u c / u ∞ = 0. 53. It was calculated from the measured probability density that these characteristic structures play an important role, although the probability of their occurrence is low. The sources of these wall-pressure structures can be located in the buffer layer of the boundary layer.