On the vortical structure in a plane impinging jet

The vortical structure of a plane impinging jet is considered. The jet was locked both in phase and laterally in space, and time series digital particle image velocimetry measurements were made both of the jet exiting the nozzle and as it impinged on a perpendicular wall. Iso-vorticity and iso-lambda (2) surfaces coupled with critical point theory were used to identify and clarify structure. The flow near the nozzle was much as observed in mixing layers, where the shear layer evolves into spanwise rollers, only here the rollers occurred symmetrically about the jet midplane. Accordingly the rollers were seen to depict spanwise perturbations with the wavelength of flutes at the nozzle edge and were connected, on the same side of the jet, with streamwise \u27successive ribs\u27 of the same wavelength. This wavelength was 0.71 of the distance between rollers and, contrary to some experiments in mixing layers, did not double when the rollers paired. Structures not reported previously but evident here with iso-vorticity, lambda (2) and critical point theory are \u27cross ribs\u27, which extend from the downstream side of each roller to its counterpart across the symmetry plane; their spanwise periodic spacing exceeds that of successive ribs by a factor of three. Cross ribs stretch because of the diverging flow as the rollers approach the wall and move apart, causing the vorticity within them to intensify. This process continues until the cross ribs reach the wall and merge with \u27wall ribs\u27. Wall ribs remain near the wall throughout the cycle and are composed of vorticity of the same sign as the cross ribs, but the intensity level of the vorticity within them is cyclic. Details of the expansion of fluid elements, evaluated from the rate of strain tenser, revealed that both cross and successive ribs align with the principal axis and that the vorticity comprising them is continuously amplified by stretching. It is further shown, by appeal to the production terms of the phase-averaged vorticity equation, that wall ribs are sustained by merging and stretching rather than reorientation of vorticity. Moreover production of vorticity is a maximum when cross and wall ribs merge and is greatest near the symmetry plane of the jet. The demise of successive ribs on the other hand occurs away from the symmetry plane and would appear to be less important dynamically than cross ribs merging with wall ribs