Large- and small-scale vortical motions in a shear layer perturbed by tabs

The large- and small-scale vortical motions produced by 'delta tabs' in a t wo-stream shear layer have been studied experimentally An increase in mixin g was observed when the base of the triangular shaped tab was affixed to th e trailing edge of the splitter plate and the apex was pitched at some angl e with respect to the flow axis. Such an arrangement produced a pair of cou nter-rotating streamwise vortices. Hot-wire measurements detailed the veloc ity, time-averaged vorticity (Omega(x)) and small-scale turbulence features in the three-dimensional space downstream of the tabs. The small-scale str uctures, whose scale corresponds to that of the peak in the dissipation spe ctrum, were identified and counted using the peak-valley-counting technique . The optimal pitch angle, theta, for a single tab and the optimal spanwise spacing, S, for a multiple tab array were identified. Since the goal was t o increase mixing, the optimal tab configuration was determined from two pr operties of the flow held: (i) the large-scale motions with the maximum Ome ga(x) and (ii) the largest number of small-scale motions in a given time pe riod. The peak streamwise vorticity magnitude \Omega(x)_(max)\ was found to have a unique relationship with the tab pitch angle. Furthermore, for all cases examined, the overall small-scale population was found to correlate d irectly with \Omega(x)_(max)\. Both quantities peaked at theta approximate to +/-45 degrees. It is interesting to note that the peak magnitude of the corresponding circulation in the cross-sectional plane occurred for theta a pproximate to +/-90 degrees. For an array of tabs, the two quantities also depended on the tab spacing. An array of contiguous tabs acted as a solid d eflector producing the weakest streamwise vortices and the least small-scal e population. For the measurement range covered, the optimal spacing was fo und to be S approximate to 1.5 tab widths.