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Small-Scale Vortical Motions Effected by Fluttering of Self-Oscillating Reeds

The formation, shedding, and advection of a hierarchy of small-scale vortical motions effected by an aeroelastically fluttering reed cantilevered across the span of a square channel are investigated experimentally at low (laminar or transitional) Reynolds numbers using high-resolution particle image velocimetry (PIV) and hot-wire anemometry.  Formation and advection of vorticity concentrations along the surface of the reed are induced by concave/convex surface undulations associated with structural vibration modes of the reed.  These modes lead to alternate time-periodic shedding of CW and CCW vortical structures having cross stream scales that are commensurate with the cross stream amplitude of the reed motion.  The evolution of these vortices in the vicinity of the reed is strongly affected by interactions with the wall boundary layers that engender vorticity filaments spanning the entire height of the channel.  These reciprocal interactions between the reed and the embossing channel flow leads to the evolution of small scale motions of decreasing scales that is characterized by enhanced dissipation and a distribution of spectral components that are reminiscent of a turbulent flow even at the low Reynolds number of the base flow.

Supported by AFOSR

Heat Transfer Enhancement in High-Power Heat Sinks using Active and Passive Reeds
Flow-effected, enhanced heat transfer in a high aspect ratio rectangular mm-scale channel that models a segment of a high-performance, air-cooled heat-sink is characterized.  Modern, high-power air heat sinks are characterized by highly-compact designs with high density, high aspect ratio fin channels.  Forced convection heat transfer in such compact high-power heat sink designs is typically limited by the inherently low channel Reynolds number, which is associated with the available air volume flow rate.  The two coupled heat transfer processes that govern the heat transport in these heat exchangers are the local heat transfer from the fin surface, limited by the temperature gradient across the boundary layer, and the heat transport limited by the mixing of the heated air with the core flow.  These limitations are commonly overcome by increased volume flow rate, or increased fin density.  Both approaches result in a significant concomitant increase in losses and in the required blower power.

The present investigation reports a novel approach to enhanced cooling without increasing the channel’s characteristically low Reynolds number.  Heat transport is significantly increased by deliberate shedding of unsteady small-scale vortices that are induced by the vibration of a miniature, cantilevered self-oscillating reed (SOR).  The present investigation focuses on the heat transfer and fluid mechanics that are associated with the small-scale motions induced by the reed.  Heat transfer experiments were conducted using MEMS-fabricated copper heaters with integrated temperature sensors that exploit the Joule heating of the copper windings to measure the wall temperature distribution with a resolution < 0.1oC.  High-magnification particle image velocimetry (PIV) is used to characterize the interaction of tThe induced vortical structures with the channel flow.  The SOR enhances the local Nusselt number of the channel up to 110%.  The performance enhancement by reed actuation is quantified in terms of increased power dissipation over a range of flow rates compared to the baseline flow in the absence of the reed.  It is demonstrated that the channel’s coefficient of performance (COP) can be increased by a factor of 3 while accounting for the changes in channel pressure drop.

Supported by DARPA