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Behaviour of a bubble in a horizontal high-speed solid-body rotating flow

Abstract : This research study focuses on bubbles released inside a horizontal high-speed solid-body rotating flow. This flow situation is interesting because it can help us to understand how bubbles behave when they meet flow regions with locally high vorticity. These high vorticity regions can be found in a variety of flow situations. In order to extract information about bubble dynamics in solid-body rotating flow, we have used an experimental apparatus containing a cylindrical Plexiglas tank of diameter 11~cm and length 10~cm which is rotated around its horizontal axis z. The bubble can be injected into the cell at the rest. For this experiment, the range of rotational velocity investigated is [600-900] rpm, i.e. from 63 rad s-1 to 94 rad s-1. Two high-speed cameras were used to determine the physical features of the bubble motion inside the tank. When the rotational velocity of the tank increases, the bubble moves close to the axis of the cell, and stretches along the horizontal axis. We first study this stretching of the bubble as a function of bubble size and of the rotation rate of the cell. We show that the bubble aspect ratio can be predicted as a function of the bubble Weber number by the model of Rosenthal (J. Fluid Mech., vol. 12, 1962, 358–366) provided an appropriate correction due to the impact of buoyancy is included. This correction accounts for the larger pressure difference between periphery and axis of the bubble, when the bubble is displaced away from the axis because of buoyancy. Then we discovered that the bubble can experience the large aspect ratio up to 2 and breaks up at certain rotational speeds for large bubble sizes. We show this break-up occurs through a resonance mechanism when the rotational velocity of the tank becomes of the order of the eigenfrequency of the bubble. We next deduce the drag and lift coefficients from the mean bubble position. For large bubbles straddling the axis of rotation we show that the drag coefficient CD is solely dependent on the Rossby number Ro, with CD ~ 1.5/Ro. In the same limit of large bubbles, we have proposed an estimate of the lift coefficient for the low Rossby number Ro. Indeed, we show that the lift coefficient CL is controlled by the shear Reynolds number Reshear = Re/Ro at the scale of the bubble. Eventually, in order to change surface tension of the bulk liquid and explore the effects of surfactants on the bubble we have used a chemical compounds called TetradecylTrimethylAmmoniumBromide (TTAB) in the liquid. We have studied two alternative surfactant solutions: one that is lower than the CMC (0.33 CMC) and one that is higher than the CMC (2 CMC), in which the bubble interface is expected to be entirely saturated by surfactants. We have carried out the same deformation and force analysis that were carried out with demineralised water with the surfactant solutions. The results reveal that as expected the deformation of the bubble is larger in both surfactant solutions than in water, and is still modelled by the model of Rosenthal (J. Fluid Mech., vol. 12, 1962, 358–366) in the case of the 2CMC solution. In the case of the 0.33CMC solution, the bubble behaves as if it was seeing an effective surface tension equal to that in the CMC solution. Regarding break-up, we observe that as for water, break-up occurs when the tank frequency is of the order of the bubble eigenfrequency. Furthermore, as compared to demineralised water as well as a fully saturated interface (2 CMC), the lift and drag coefficients for the 0.33 CMC solution are significantly lower than the fully saturated interface. We attribute this effect to a marked change in the shape of the bubble when the concentration is lower than the CMC.
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Submitted on : Monday, September 5, 2022 - 9:52:15 AM
Last modification on : Friday, September 30, 2022 - 10:56:10 AM
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  • HAL Id : tel-03768940, version 1

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Majid Rodgar. Behaviour of a bubble in a horizontal high-speed solid-body rotating flow. Other. Université de Lyon, 2022. English. ⟨NNT : 2022LYSEC025⟩. ⟨tel-03768940⟩

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