Prediction of Diffuser Core Flow Fluctuations Caused by Oscillating Turbulent Flow

Abstract

Theoretical consideration of the time-dependent diffuser core flow, involving two limiting models (highfrequency and low-frequency) for the time-dependent effective core flow area, results in predictions for decay of the first harmonic core velocity and wall pressure amplitudes with streamwise distance. In order to predict these decays, it is necessary to specify only time mean core flow and either time-dependent diffuser inlet or exit conditions. The theoretical models are compared with experiments for diffuser inlet conditions, spanning a tenfold range of dimensionless frequencies. The high-frequency model best predicts the decay of core velocity fluctuations and wall pressure fluctuations in the streamwise direction. A backward calculation scheme, which uses diffuser exit conditions as initial conditions, enables good prediction of the fluctuating wall pressure distributions when the diffuser exit exhausts into the atmosphere. An order-of-magnitude analysis yields a simple expression for decay of the core velocity fluctuations when the dimensionless frequency is approximately unity.

Problems in Forced Unsteady Fluid Mechanics.

“…50 Their investigations had a different perspective because they recognized that diffusers often need to operate with an unsteady inlet flow and that the effect of this time dependency on the overall performance is of significant interest. In the present case, the time ;candency is forced in an attempt to improve the performance of the device.…”