On the entrapment of particles by vortical structures in wall-bounded turbulence

Coherent vortical structures are an important feature of near-wall turbulence, and understanding how particles and bubbles move with respect to these structures can be useful for many applications of measurement and flow control. While many studies have characterized bubbles and particles in terms of preferential concentration within turbulent flows, few studies have examined this relationship within the context of targeting near-wall vortices for flow control applications. The current study examines the entrapment of bubbles and particles by these near-wall turbulent vortices by simulating 106 small bubbles and heavy particles in a turbulent channel flow using a one-way coupled point particle approximation with direct numerical simulations for Reynolds numbers up to Reτ=395. Their entrapment relative to tracer particles shows a clear preference for bubbles to move into vortex cores and heavy particles to move away from vortex cores, and special limiting flow cases demonstrate the effect of flow time scales on overall entrapment. Our findings suggest that the entrapment (or lack thereof) saturates on a time scale of t+≈100 across all simulated Reynolds numbers for both particles and bubbles, and we show that bubble entrapment is limited by the lifetimes of local structures in near-wall turbulence. Therefore, the current work provides a characterization of an important constraint for potential particle-based flow control strategies.