Boundary layer pressure fluctuation measurements in transitional and turbulent flow.

Abstract

Pressure fluctuations were measured for external flows over an axisymmetric surface. Measurements were made in laminar, transitional, and turbulent flow using piezoelectric pressure transducers. The entire surface (including the pressure transducers) was coated with a 1/16‐in. thick elastomer, in order to ensure a smooth surface. All measurements of boundary layer transition were made in an adverse pressure gradient. Results showed that root‐mean‐square pressure fluctuation levels during transition were as much as 15 dB higher than for fully developed turbulent flow. The transitional spectra were dominated by Tollmien‐Schlichting waves, and associated nonlinearities. The spectra of Tollmien‐Schlichting waves during amplification and subsequent transformation to turbulence are shown nondimensionally in comparison with Bakewell’s turbulence spectral density.