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The dynamics of air bubbles in turbulent Rayleigh–Bénard (RB) convection is described for the first time using laboratory experiments and complementary numerical simulations. We performed experiments at $$Ra=5.5\times 10^{9}$$ and $$1.1\times 10^{10}$$ , where streams of 1 mm bubbles were released at various locations from the bottom of the tank along the path of the roll structure. Using three-dimensional particle tracking velocimetry, we simultaneously tracked a large number of bubbles to inspect the pair dispersion, $$R^{2}(t)$$ , for a range of initial separations, $$r$$ , spanning one order of magnitude, namely $$25\unicode[STIX]{x1D702}\leqslant r\leqslant 225\unicode[STIX]{x1D702}$$ ; here $$\unicode[STIX]{x1D702}$$ is the local Kolmogorov length scale. Pair dispersion, $$R^{2}(t)$$ , of the bubbles within a quiescent medium was also determined to assess the effect of inhomogeneity and anisotropy induced by the RB convection. Results show that $$R^{2}(t)$$ underwent a transition phase similar to the ballistic-to-diffusive ( $$t^{2}$$ -to- $$t^{1}$$ ) regime in the vicinity of the cell centre; it approached a bulk behavior $$t^{3/2}$$ in the diffusive regime as the distance away from the cell centre increased. At small $$r$$ , $$R^{2}(t)\propto t^{1}$$ is shown in the diffusive regime with a lower magnitude compared to the quiescent case, indicating that the convective turbulence reduced the amplitude of the bubble’s fluctuations. This phenomenon associated to the bubble path instability was further explored by the autocorrelation of the bubble’s horizontal velocity. At large initial separations, $$R^{2}(t)\propto t^{2}$$ was observed, showing the effect of the roll structure.
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Single sidewall cooling modulation on Rayleigh–Bénard convection
We experimentally explored the effect of single-sidewall cooling on Rayleigh–Bénard (RB) convection. Canonical RB was also studied to aid insight. The scenarios shared tank dimensions and bottom and top wall temperatures; the single sidewall cooling had the top wall temperature. Turbulence was explored at two canonical Rayleigh numbers, $$Ra=1.6\times 10^{10}$$ and $$Ra=2\times 10^9$$ under Prandtl number $Pr=5.4$ . Particle image velocimetry described vertical planes parallel and perpendicular to the sidewall cooling. The two $Ra$ scenarios reveal pronounced changes in the flow structure and large-scale circulation (LSC) due to the sidewall cooling. The density gradient induced by the sidewall cooling led to asymmetric descending and ascending flows and irregular LSC. Flow statistics departed from the canonical case, exhibiting lower buoyancy effects, represented by an effective Rayleigh number with effective height dependent on the distance from the lateral cooling. Velocity spectra show two scalings, $$\varPhi \propto f^{-5/3}$$ Kolmogorov (KO41) and $$\varPhi \propto f^{-11/5}$$ Bolgiano (BO59) in the larger $Ra$ ; the latter was not present in the smaller set-up. The BO59 scaling with sidewall cooling appears at higher frequencies than its canonical counterpart, suggesting weaker buoyancy effects. The LSC core motions allowed us to identify a characteristic time scale of the order of vortex turnover time associated with distinct vortex modes. The velocity spectra of the vortex core oscillation along its principal axis showed a scaling of $$\varPhi _c \propto f^{-5/3}$$ for the single sidewall cooling, which was dominant closer there. It did not occur in the canonical case, evidencing the modulation of LSC oscillation on the flow.
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- Award ID(s):
- 1912824
- PAR ID:
- 10421242
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 957
- ISSN:
- 0022-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jfm.2022.1026
