Flow field features of chevron impinging synthetic jets at short nozzle-to-plate distance

The flow field of a chevron synthetic jet in impinging configuration at Reynolds number Re=4500, dimensionless stroke length L0/D=28 and nozzle-to-plate distance H/D=2 is experimentally investigated by means of Stereoscopic Particle Image Velocimetry. This study is motivated by the need of explaining the heat transfer behaviour of the chevron synthetic jet highlighting its differences with respect to the circular synthetic jet. Two different experiments are carried out. The first experiment is devoted to the characterisation of the flow field evolution on the impingement plate. The second experiment is a three-dimensional reconstruction of the entire flow field at three characteristic phases. The results show that the circular synthetic jet presents a circular velocity pattern on the plate, while the chevron exit leads to the acceleration of the fluid, thus to a larger impinging velocity with a star-shaped pattern. Furthermore, in both configurations a secondary vortex ring is formed on the wall at a radial position of about 2 diameters. For the circular synthetic jet, the intense turbulence levels are located in two annular regions: the inner region is due to the turbulent wall jet generated by the impinging trailing jet and the outer region is due to the effect of the generation of the secondary vortex ring. For the chevron synthetic jet, the intense turbulence level is produced by the turbulent wall jet along preferential paths. These preferential paths are caused by the star-shaped distribution of impinging velocity and the impinging streamwise coherent vortices. These features characterizing the impinging flow field are fundamental to understand and explain the mechanisms behind the heat transfer behaviour of such devices.