Abstract
This paper reports upon a laser-induced fluorescence visualization and time-resolved particle image velocimetry study to resolve the detailed dynamics associated with Re = 2000 and 4000 circular vortex rings colliding with 30°–75° inclined surfaces. Two-dimensional visualization results show that larger inclination angles lead to increasingly rapid size reduction in the primary vortex-ring core closer to the surface, faster formation of the secondary vortex-ring core, and subsequent ingestion by the former. In contrast, primary vortex-ring core further away from the surface becomes physically larger and incoherent more rapidly, with slower formation and entrainment of the secondary vortex-ring core. Interestingly, a vortex dipole and small vortex-ring-like structure are produced for the largest inclination angle of 75°, possibly due to vortex disconnection and reconnection processes. Results taken along the non-inclined plane show significant bulging of the primary vortex-ring cores when the inclination angle increases from 30° onwards. More importantly, additional vortex cores are observed to entwine with the primary vortex-ring core and provide strong direct evidence for the bi-helical vortex line flow mechanism put forward by Lim (Exp Fluids 7:453–463, 1989). Lastly, the behaviour of the primary and secondary vortex-ring cores further away from the surface is highly sensitive towards the state of the bi-helical lines compressed at that region. Strong compression driven by circumferential flows due to large inclination angles may explain the unique flow structures and behaviour observed for 75° inclination angle here.
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Acknowledgments
The authors gratefully acknowledge the support for this study by the Nanyang Technological University Start-Up Grant, Tan Chin Tuan Exchange Fellowship in Engineering, as well as China NSFC Grant (51106096). Assistance in the flow visualization experiments from Wang L. and Kong M. at Shanghai Jiao Tong University, and programming of the stepper motor by Kwan Z. R. are also greatly appreciated.
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New, T.H., Shi, S. & Zang, B. Some observations on vortex-ring collisions upon inclined surfaces. Exp Fluids 57, 109 (2016). https://doi.org/10.1007/s00348-016-2196-7
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DOI: https://doi.org/10.1007/s00348-016-2196-7