Wall Pressure Fluctuations During Transition on a Flat Plate

Detailed measurements of wall pressure fluctuations have been made in the intermittent (laminar-turbulent) region of a flat plate boundary layer. Digital sampling and processing techniques were used. The properties of these pressure fluctuations were found to be similar to the previous measurements made in the fully turbulent region. The measurements were repeated with a single two-dimensional surface roughness on the plate. The only changes in the results were a decrease in the transition Reynolds number from 2 × 10⁶ to 1.6 × 10⁶, and an increase in the decay rate of the longitudinal cross-spectral density magnitude by a factor of about 1.5. Emmons’ (1951) analytical model was applied for two cases: (1) a constant source density downstream of transition and, (2) a line source distribution at transition. Both predicted burst rates as functions of intermittency appreciably higher than measured values. Wall pressure spectra scaled on dynamic head showed a strong dependency on intermittency. This dependency was largely resolved, at least for intermittencies greater than 64 percent, by scaling on turbulent mean shear stress at the wall.