Effects of Reynolds number on flow and mixing characteristics of a self-sustained swinging jet

Abstract

The effects of Reynolds number on flow and mixing characteristics of a merged single swinging jet induced by a V-shaped fluidic oscillator were studied experimentally. The jet Reynolds number was varied from 80 to 3000. A high-speed digital camera was used to capture the instantaneous flow evolution processes using the laser light-sheet-assisted flow visualisation method. Long-exposure images together with the binary edge detection technique were used to identify the jet spread width. The velocities, turbulence intensities, as well as turbulence macro time and length scales in the central axis were measured using a one-component hot-wire anemometer. Measurement results using the tracer-gas concentration detection method provided information on the dispersion characteristics of the merged swinging and non-swinging jets. Four characteristic flow modes (non-swinging, subcritical swinging, critical swinging, and supercritical swinging jets) were identified within different ranges of jet Reynolds number. At Reynolds numbers <150 ± 10, no jet swinging happened. At Reynolds numbers between 150 ± 10 and 1200 ± 23, the instantaneous velocities of the merged jet presented periodic oscillations of small amplitudes and was denoted as the subcritical swinging jet. At Reynolds numbers between 1200 ± 23 and 2350 ± 50, the merged jet proceeded irregular swinging motion and was called the critical jet. At Reynolds numbers higher than 2350 ± 50, the time-evolving velocities of the merged jet presented periodic oscillations of large amplitudes imposed by large turbulent fluctuations and was termed the supercritical swinging jet. The measured results of the axial velocities and turbulence intensities in the central axis revealed that in the regime of the supercritical swinging jet, the momentum conversion from the axial to transverse direction was rapidly performed in the near field at the axial distance <2.4 times of the jet exit width due to the induced transverse jet oscillation. The transverse oscillations of the swinging jets induced large turbulence intensities and small turbulence macro time and length scales, and led to large jet mass dispersion. Therefore, the mixing characteristics of the flow was significantly improved due to large jet mass dispersion.

KEYWORDS:

• Swinging jet
• turbulent eddies
• jet dispersion
• mixing
• coherent structure

Acknowledgements

This article was supported by Thermo-Fluid Laboratory of National Taiwan University of Science and Technology and has a funding number of 106B0224-2.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This article was supported by Thermo-Fluid Laboratory of National Taiwan University of Science and Technology and has a funding number of 106B0224-2.