The large- and small-scale vortical motions produced by
‘delta tabs’ in a two-stream
shear layer have been studied experimentally. An increase in mixing was
observed
when the base of the triangular shaped tab was affixed to the trailing
edge of the
splitter plate and the apex was pitched at some angle with respect to the
flow
axis. Such an arrangement produced a pair of counter-rotating streamwise
vortices.
Hot-wire measurements detailed the velocity, time-averaged vorticity
(Ωx) and small-scale turbulence features in the three-dimensional space downstream of
the tabs. The
small-scale structures, whose scale corresponds to that of the peak in
the dissipation
spectrum, were identified and counted using the peak-valley-counting technique.
The
optimal pitch angle, θ, for a single tab and the
optimal spanwise spacing, S, for a
multiple tab array were identified. Since the goal was to increase mixing,
the optimal
tab configuration was determined from two properties of the flow field:
(i) the large-scale motions with the maximum Ωx,
and (ii) the largest number of small-scale
motions in a given time period. The peak streamwise vorticity magnitude
[mid ]Ωx−max[mid ] 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
directly with
[mid ]Ωx−max[mid ]. Both quantities peaked
at
θ≈±45°. It is interesting to note that the peak
magnitude of the corresponding circulation in the cross-sectional plane
occurred
for θ≈±90°. For an array of tabs, the two
quantities also depended on the tab
spacing. An array of contiguous tabs acted as a solid deflector producing
the weakest
streamwise vortices and the least small-scale population. For the measurement
range
covered, the optimal spacing was found to be S≈1.5 tab widths.
Evidence of strong small‐scale population structure in the Antarctic freshwater copepod
Boeckella poppei
in lakes on Signy Island, South Orkney Islands