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Using numerical simulations, we investigate the near-field temporal variability of axisymmetric helium plumes as a function of inlet-based Richardson ( $${Ri}_0$$ ) and Reynolds ( $${Re}_0$$ ) numbers. Previous studies have shown that $${Ri}_0$$ plays a leading-order role in determining the frequency at which large-scale vortices are produced (commonly called the ‘puffing’ frequency). By contrast, $${Re}_0$$ dictates the strength of localized gradients, which are important during the transition from laminar to turbulent flow. The simulations presented here span a range of $${Ri}_0$$ and $${Re}_0$$ , and use adaptive mesh refinement to achieve high spatial resolutions. We find that as $${Re}_0$$ increases for a given $${Ri}_0$$ , the puffing motion undergoes a transition at a critical $${Re}_0$$ , marking the onset of chaotic dynamics. Moreover, the critical $${Re}_0$$ decreases as $${Ri}_0$$ increases. When the puffing instability is non-chaotic, time series of different variables are well-correlated, exhibiting only modest changes in the dynamics (including period doubling and flapping). Once the flow becomes chaotic, denser ambient fluid penetrates the core of the plume, similar to penetrating ‘spikes’ formed by Rayleigh–Taylor instabilities, leading to only moderately correlated flow variables. These changes result in a non-trivial dependence of the puffing frequency on $${Re}_0$$ . Specifically, at sufficiently low $${Re}_0$$ , the puffing frequency falls below the prediction from Wimer et al. ( J. Fluid Mech. , vol. 895, 2020). As $${Re}_0$$ increases beyond the critical $${Re}_0$$ , the puffing frequency increases and then drops back down to the predicted scaling. The dependence of the puffing frequency on $${Re}_0$$ provides a possible explanation for previously observed changes in the scaling of the puffing frequency for high $${Ri}_0$$ .
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This content will become publicly available on February 1, 2026
Measurements of Wake Concentration from the Continuous Release of a Dense Fluid Upstream of a Cubic Obstacle
Results are presented from a series of small-scale laboratory experiments designed to model dense gas dispersion around an isolated cuboid building. Experiments were conducted for a broad range of flow Richardson numbers and source discharge rates, and the concentration field in the wake of the building was measured using light-induced fluorescence (LIF). Results show that, for low Richardson numbers, the concentration of dense fluid in the wake decreases slightly with distance above the ground. However, for Richardson numbers above Ri≈3, the vertical variation is qualitatively different, as a dense lower layer forms in the wake and the concentration above the layer is much lower than for the lower Ri experiments. For these higher Richardson number flows, the primary mechanism by which dense fluid is flushed from the building wake is by the wake flow skimming dense fluid from the top of the lower layer and then moving it upstream toward the building’s leeward face. It is then transported up the leeward face of the building and then downstream. The results also generally show that, as the release rate of dense fluid increases, the density and thickness of the lower layer increases. The LIF measurements and a series of visualization experiments highlight the complex interaction of a dense fluid discharge with the wake structure behind a building.
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- Award ID(s):
- 1703548
- PAR ID:
- 10609920
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Fluids
- Volume:
- 10
- Issue:
- 2
- ISSN:
- 2311-5521
- Page Range / eLocation ID:
- 46
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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