Small Silicon Pressure Transducers for Space-Time Correlation Measurements in a Flat Plate Boundary Layer

Silicon based pressure sensors have been used to measure turbulent wall-pressure fluctuations in a two-dimensional flat plate boundary layer, Re_θ = 5072. The side lengths of the diaphragms were 100 μm (d⁺ = 7.2) and 300 μm (d⁺ = 21.6), giving a ratio of the boundary layer thickness to the diaphragm side length of the order of 240 and a resolution of eddies with wave numbers less than ten viscous units. Power spectra were measured for the frequency range 13 Hz < f < 13 kHz. Scaled in outer and inner variables a clear overlap region between the mid and high frequency parts of the spectrum is shown. In this overlap region the slope was found to be ω⁻¹, while in the high frequency part it was ω⁻⁵. Correlation measurements in both the longitudinal and transversal directions were performed and compared to other investigations. Longitudinal space time correlations, including the high frequency range, indicated an advection velocity of the order of half the freestream velocity. A broad band filtering of the longitudinal correlation showed that the high frequency part of the spectrum is associated with the smaller eddies from the inner part of the boundary layer, resulting in a reduction of the correlation.