On the turbulent flow characteristics of wall-attaching synthetic jets at varying offset height ratios

This study investigates the influence of offset height ratio on the unsteady flow characteristics and coherent structures of wall-attaching synthetic jets using improved delayed detached eddy simulations. The simulations were conducted at a Reynolds number based on the average jet-exit velocity and nozzle diameter of Re=565, with an actuation frequency of 300 Hz. Four offset ratios (G/d=1, 2, 3, and 4, where G is the distance from the nozzle centerline to the bottom wall and d is the nozzle diameter) and a reference free synthetic jet (FSJ) were examined using instantaneous, phase-averaged, and time-averaged statistics, including Q-criterion, vortex-core circulation tracking, and proper orthogonal decomposition (POD). The results show that reducing G/d promotes stronger jet deflection and earlier attachment of the synthetic jet on the wall, leading to the development of a wall synthetic jet. At G/d=1, the vortex rings attach almost immediately on the wall, inducing strong spanwise stretching, vorticity redistribution, and rapid breakdown and decay of the jet. Increasing the offset ratio to G/d=2 delays wall attachment but introduces lower shear layer asymmetry that alters the evolution of the counter-rotating vortex pair. For G/d=3 and 4, the vortex evolution and the flow field are similar to the FSJ and also exhibit self-similarity in the far field. POD analysis reveals that, as the offset height ratio decreases, the strong jet-wall interactions significantly increase the range of turbulent scales, leading to an increased modal requirement in flow field reconstruction.