L. Laguarda, S. Hickel (2024)
Physics of Fluids 36: 081708. doi: 10.1063/5.0227332
We revisit the origin of low-frequency unsteadiness in turbulent recirculation bubbles (TRBs), and, in particular, the hypothesis of a dynamic feedback mechanism between unconstrained separation and reattachment locations. Our results demonstrate, for the first time, effective suppression of the low-frequency characteristics of the TRB without reducing its size, strongly supporting our hypothesis.
In this paper, we provided evidence that the low-frequency dynamics of TRBs may originate from a dynamic coupling between unconstrained separation and reattachment lines. This coupling can be effectively intercepted by placing a backward-facing step underneath the TRB, a strategy not previously explored in the literature. The pronounced expansion at the step is found to progressively alter the flow topology of the TRB as the step height increases, keeping the reverse-flow region shallow and confined close to the wall upstream of the step. This alteration allows mixing and fluid entrainment to be significant only downstream of the step, effectively compartmentalizing the TRB. We show that this compartmentalization strongly correlates with the overall suppression of low-frequency dynamics, thereby supporting our hypothesis about their origin.
In agreement with previous works we identify streaks and streamwise-aligned vortices—often referred to as Görtler-like vortices—at reattachment. Although their exact role in the low-frequency dynamics of the TRB remains inconclusive, our results show that these structures persist even when low-frequency dynamics are suppressed. This suggests that streamwise-aligned vortices either do not play a pivotal role in such dynamics, or they must originate from strong concave streamline curvature at separation to be dynamically significant, a condition that is absent in the large-step-controlled TRBs.