Heat transfer enhancement by flapping of streamwise-consecutive foils in a channel

We have computationally considered enhancement of forced-convection heat transfer between parallel plates of infinite spanwise extent, using an identical pair of deformable foils whose motion is driven by aeroelastic instability, over a range of Reynolds numbers (Re) relevant to air-side cooling in heat exchangers.  Computations were performed using a spectral-element discretization of the flow and heat transfer, coupled to a finite-element discretization of the foil motion.  The two foils are cantilevered in the midplane of the channel, with the leading edge of one downstream of the trailing edge of the other.  In air at Re = 250 (based on channel width), the overall heat transfer enhancement (measured in terms of a Nusselt number) considerably exceeds twice the enhancement due to a single foil, and persists considerably farther downstream.  This is directly attributable to the fact that the motion of the second foil is driven by the finite-amplitude, unsteady, asymmetric flow in the wake of the upstream foil, whereas the motion of the upstream foil is due solely to linear instability of the motionless-foil configuration with respect to the nominally steady and symmetric flow upstream of its leading edge.