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

The transitional process of a forced plane wall jet is studied both experim entally and numerically. Experimentally, particle Image Velocimetry (PIV) a nd laser-sheet/smoke flow-visualization techniques are implemented to provi de an overall understanding of the flow features. Numerically, time-accurat e computational results are obtained by solving the two-dimensional, unstea dy Navier-Stokes equations. Comparison of PIV data and two-dimensional comp uted results shows excellent agreement in the early stages of transition, d emonstrating that the numerical study can be used to complement the experim ental one. The results show that; under the influence of external excitatio n, linear-instability growth is bypassed and a discrete shear-layer vortex is formed in the immediate vicinity of the nozzle exit. This vortex interac ts with the boundary-layer vorticity, leading to the formation of another v ortex 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 impos ed wall-boundary condition. This seems to suggest that the physical mechani sm leading to the formation of the boundary-layer vortex is an inviscid rot ational one.