Separation control by periodic excitation in a turbulent axisymmetric diffuser flow

This paper describes experimental investigations on separation control in turbulent flow through an 18degrees axisymmetric diffuser. The research was aimed at influencing the separated flow in the diffuser in order to reduce the separation length by means of periodic excitation at the diffuser edge and thus to minimize the related pressure losses. The measurements were conducted in a closed pipe-flow channel with an optically fully accessible test section. A laser Doppler velocimeter was used for the time-dependent velocity measurements and frequency spectra, while particle image velocimetry was applied for instantaneous and phase-averaged velocity measurements. The overall separation length could be significantly reduced to about 40% of the base flow, while the separation point was distinctly shifted downstream from the diffuser entrance edge. In certain phases of excitation the separation bubble almost disappeared. Also an enlarged intensity of the turbulent impulse transport inside the shear layer could be observed during local forcing.