Published online by Cambridge University Press: 06 December 2016
A constantly accelerating circular plate was investigated towards understanding the effect of non-stationarity on shear-layer entrainment and topology. Dye visualizations and time-resolved particle image velocimetry measurements were collected for normalized accelerations spanning three orders of magnitude. Increasing acceleration acts to organize shear-layer topology. Specifically, the Kelvin–Helmholtz instabilities within the shear layer better adhered to a circular path and exhibited consistent and repeatable spacing. Normalized starting-vortex circulation was observed to collapse with increasing acceleration, which one might not expect due to increased levels of mixing at higher instantaneous Reynolds numbers. The entrainment rate was shown to increase nonlinearly with increasing acceleration. This was attributed to closer spacing between instabilities, which better facilitates the roll-up of fluid between the shear layer and vortex core. The shear-layer organization observed at higher accelerations was associated with smaller spacings between instabilities. Specifically, analogous point-vortex simulations demonstrated that decreasing the spacing between instabilities acts to localize and dampen perturbations within an accelerating shear layer.
Shear-layer dye visualizations for the low- and mid-acceleration cases (top and bottom sequences, respectively) over a diameters-traveled domain of 0 ≤ s/D ≤ 4.9. Circulation-based Reynolds numbers are provided as well.
Enstrophy fields for low-, mid- and high-acceleration cases (first, second and third rows, respectively) over a diameters-traveled domain of 0.7 ≤ s/D ≤ 1.0. Enstrophy fields from single runs are presented in the first, second and third columns, while the fourth column presents the 30-run, phase-averaged enstrophy fields. Circulation-based Reynolds numbers are indicated as well.