Targeted disruption of vortical structures

Coherent structures act as engines of momentum transport and contribute to high wall shear stress and drag.   Polymer molecules disrupt such vortical structures and hence lead to drag reduction. While moderately useful for internal flows, the large quantities of polymer required for external flows impose serious cost and pollution constraints. The majority of previous studies have considered the polymer to be uniformly present in the flow, or involved bleeding the polymer into the entire near-wall turbulent boundary layer.  On the other hand, we examine the effect of the hypothetical ability to place the polymer only within or immediately adjacent to specific drag producing vortical structures. Direct numerical simulations of a hairpin vortex in a channel flow are performed and a scalar field is introduced in its neighborhood. On entering the vortical structure, the scalar field is observed to remain within the structure for the duration of the simulation.  While this is a consequence of Kelvin’s theorem, we demonstrate its applicability to the near wall viscous regime.  Our simulations also show that if the scalar field is associated with a FENE-P fluid concentration, the hairpin is disrupted suggesting the viability of this targeted control.