Separation control in an axisymmetric diffuser flow by periodic excitation

This paper describes first experimental investigations on separation control in turbulent flow through an 18degrees-axisyminctric 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 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.