Oscillating Turbulent Flow in a Conical Diffuser

Abstract

Turbulent boundary layers experiencing a time mean adverse pressure gradient and a controllable periodic oscillation are examined experimentally over a tenfold range of dimensionless frequencies, and amplitudes less than ten percent of the lime mean diffuser inlet velocity, for well defined diffuser inlet conditions. Organized velocity fluctuations of the diffuser core and boundary layer flow fields, along with organized wall pressure fluctuations, are characterized using a phase averaging process. Amplitudes of the core velocity fluctuations and wall pressure fluctuations decay in the streamwise direction. Depending on the dimensionless excitation frequency, the amplitude of the organized velocity fluctuations in the boundary layer can exceed the local core flow amplitude by as much as an order of magnitude. At relatively low dimensionless excitation frequency, the organized velocity fluctuation amplitude distribution within the boundary layer grows in an orderly fashion with streamwise distance, and the phase of these fluctuations consistently leads the local core flow fluctuation. At relatively high dimensionless excitation frequency, phase lags of the velocity fluctuations within the boundary layer, relative to the local core flow fluctuation, can extend across the entire boundary layer. Over the range of frequencies examined, the mean flow field of the diffuser remained essentially unaltered.