The effect of Strouhal number on the thermo-fluid performance of synthetic jet impingement with different waveforms

Abstract

In the present study, the flow and thermal characteristics of synthetic jets (SJs) are examined with different waveforms (sine, rectangular and ramp), for wide range of excitation frequencies (f = 10–325 Hz) across varied surface spacings (z/d = 2–24). The hot wire anemometer is utilized for measuring flow velocity, while the infrared thermal imaging camera is employed to gauge surface temperature. Numerical investigations are also performed to understand the flow behaviour of synthetic jets with different waveforms at varied Strouhal numbers. At constant amplitude of the SJ actuation, the mean centerline velocity exhibits bell-shaped behavior with excitation frequencies

which provide discrete pairs of Strouhal numbers (St) i.e., low St (StL) and high St (StH) with same Reynolds number. The thermal characteristics of SJ reveals that in near surface spacing (z/d < 6), the thermal efficacy of SJ with StH is obtained to be inferior than the StL due to the adverse effect of flow recirculation, which dominate in SJ with StH. While actuating at different waveforms, rectangular waveform outperforms the sine and ramp waveforms in near- and intermediate-surface spacing (z/d ≤ 20) at low St (StL). However, when the SJ is operated at high St (StH), though rectangular waveform shows better heat transfer rate in lower surface spacing (z/d = 2–6) but their cooling tendency decline sharply in intermediate- to far-surface spacing (z/d ≥10). It may occur due to the sudden rise and drop in their instantaneous velocity field by which momentum of SJ with high Strouhal number (StH) diminishes more quickly in the intermediate- to far-surface spacing for the rectangular waveform. © 2025 Elsevier Inc.