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 partic le image velocimetry measurements were made both of the jet exiting the noz zle and as it impinged on a perpendicular wall. Iso-vorticity and iso-lambd a (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 rol lers were seen to depict spanwise perturbations with the wavelength of flut es at the nozzle edge and were connected, on the same side of the jet, with streamwise 'successive ribs' of the same wavelength. This wavelength was 0 .71 of the distance between rollers and, contrary to some experiments in mi xing layers, did not double when the rollers paired. Structures not reporte d previously but evident here with iso-vorticity, lambda (2) and critical p oint theory are 'cross ribs', which extend from the downstream side of each roller to its counterpart across the symmetry plane; their spanwise period ic 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 c ontinues until the cross ribs reach the wall and merge with 'wall ribs'. Wa ll ribs remain near the wall throughout the cycle and are composed of vorti city of the same sign as the cross ribs, but the intensity level of the vor ticity within them is cyclic. Details of the expansion of fluid elements, e valuated from the rate of strain tenser, revealed that both cross and succe ssive ribs align with the principal axis and that the vorticity comprising them is continuously amplified by stretching. It is further shown, by appea l to the production terms of the phase-averaged vorticity equation, that wa ll ribs are sustained by merging and stretching rather than reorientation o f vorticity. Moreover production of vorticity is a maximum when cross and w all ribs merge and is greatest near the symmetry plane of the jet. The demi se 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.