Phase-resolved PIV measurements in a transitional plane wall jet: a numerical comparison

Abstract

The transitional process of a forced plane wall jet is studied both experimentally and numerically. Experimentally, Particle Image Velocimetry (PIV) and laser-sheet/smoke flow-visualization techniques are implemented to provide an overall understanding of the flow features. Numerically, time-accurate computational results are obtained by solving the two-dimensional, unsteady Navier–Stokes equations. Comparison of PIV data and two-dimensional computed results shows excellent agreement in the early stages of transition, demonstrating that the numerical study can be used to complement the experimental one. The results show that, under the influence of external excitation, linear-instability growth is bypassed and a discrete shear-layer vortex is formed in the immediate vicinity of the nozzle exit. This vortex interacts with the boundary-layer vorticity, leading to the formation of another vortex in the inner layer. These two vortices form a vortex couple that for high forcing convects downstream in a stable manner. By adoption of either a no-slip or a slip boundary condition in the numerical computation, it is determined that the flow development is relatively insensitive to the imposed wall-boundary condition. This seems to suggest that the physical mechanism leading to the formation of the boundary-layer vortex is an inviscid rotational one.