Flow structure and performance of axisymmetric synthetic jets

Abstract

ABSTRACT Experimental analysis is presented of the flow structure and performance of axisymmetric synthetic jets. We report direct thrust and Particle Image Velocimetry (PIV) measurements at varying amplitude and frequency for Reynolds numbers in the range 200-2200. It is shown that the jet performance is determined by the flow length, L, defined as the length of fluid ejected during the output stroke of the synthetic jet. Two flow regimes are identified. At small flow lengths L/D ~3, the thrust equals the momentum flux ejected during the output stroke, and is proportional to L 2 . The entrainment rate of the synthetic jet was measured and found comparable to that of conventional axisymmetric turbulent jets. The mean and time-resolved flow structure in the near field of the synthetic jet varies with flow length. At small flow lengths, vortex rings form close to the orifice and loose some of the vorticity during the input stroke. At large flow lengths, vortex rings with some trailing vorticity form and move away from the orifice thus avoiding re-ingestion of vorticity during the input stroke. The amount of trailing vorticity does not affect the thrust produced by the synthetic jet.