Frequency-dependent flow response of a high-speed rectangular synthetic jet

摘要

As the size of electronic systems shrink, thermal management techniques must remove high heat fluxes with tight volumes. Synthetic jets can provide a compact solution, using an oscillating mechanism to produce a time-averaged jet flow with ambient fluid alone. These devices are strongly dependent on operating frequency, achieving their highest speeds near the jet resonant conditions. Unfortunately, this oft en leads to unwelcome acoustic issues, limiting their range of applications. To address this, we consider the flow performance of an impinging, rectangular synthetic jet at a range of driving frequencies, both on and off of mechanical resonant peaks. Through phase-locked particle image velocimetry (PIV), we demonstrate a significant difference in the flow structures below and above the resonant condition. Below the resonant frequency, the flow response is largely independent of frequency, with only one vortex present between the actuator orifice and an impinging wall. Above resonance, there are additional eddies near the device centerline, with three vortices located within four hydraulic diameters of the impingement point. These vortices retain their nondimensional vorticity longer than at sub-resonant conditions, resulting in nearly identical heat transfer in spite of a lower Reynolds number. These flow responses can be represented with a critical wall spacing, H_(crit) = U_o/2f, where the impingement distance is tuned to the actuator frequency.