Interaction of a rigid buoyant sphere and a deforming bubble with a vortex ring: The role of deformability

Biswas, S and Govardhan, RN (2022) Interaction of a rigid buoyant sphere and a deforming bubble with a vortex ring: The role of deformability. In: Physical Review Fluids, 7 (9).

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Abstract

In bubbly turbulent flows, the deformation of the bubble is known to play an important role in the interaction between the carrier phase and the dispersed phase. In the present paper, we experimentally study an idealized version of the problem where we look at the interaction of a deforming air bubble (nonzero Weber number) and a rigid buoyant particle (zero Weber number) with a single vortex ring (in water) for a large range of ring-circulation-based Reynolds numbers (Re=Γ/ν=6000-67 300; Γ=ring circulation, ν=kinematic viscosity of water). We are able to obtain the particle to fluid density ratio (ρparticle/ρwater≈0.008) to be very low and of the same order as that of the bubble (ρbubble/ρwater≈0.001), with the main difference being the distinct difference in their deformability. The buoyant low density (rigid) particle and the deforming bubble are directly engulfed into the core of the vortex ring and enable measurements of the effects of particles or bubbles on the vortex ring. On the particle (and bubble) dynamics side, distinct differences can be seen right from the capture phase, as unlike the rigid particle, the bubble undergoes a large axial elongation and, consequently, a quicker capture than the particle. After capture by the ring, the bubble experiences an azimuthal pressure gradient, which leads to its elongation into a thin long bubble whose diameter is relatively small compared to the vortex core size. In sharp contrast, the particle does not deform and remains effectively localized along the rings azimuthal direction. These differences in the shape of the bubble and particle within the ring lead to distinct differences in the ring's convection speed, azimuthal vorticity, and enstrophy at later times. In particular, the measurements indicate that the localized large perturbation to the vorticity in the rigid particle case (an equivalent of a rigid bubble, zero Weber number) leads to a larger disruption of the vortex ring as seen in terms of higher reduction in the rings convection speed and azimuthal enstrophy than for the deforming bubble (nonzero Weber number) case. These results could have implications in bubbly turbulent flows such as in bubble drag reduction (BDR), where many studies [like Van Gils, J. Fluid Mech. 722, 317 (2013)0022-112010.1017/jfm.2013.96] indicate that deforming bubbles are better for drag reduction than nondeforming bubbles at higher flow Re, where the dominant mechanism is the lifting of structures away from the wall. The present results suggest that in cases where vortex disruption is the primary mechanism for BDR [for example, at lower flow Re, Sugiyama, J. Fluid Mech.608,21(2008)0022112010.1017/S0022112008001183], better drag reduction would occur with nondeforming bubbles. The current study thus brings us insight into the role of deformability in the interaction of bubbles with a vortical structure, which could be important in helping us model and understand bubble-turbulence interactions.

Item Type:	Journal Article
Publication:	Physical Review Fluids
Publisher:	American Physical Society
Additional Information:	The copyright for this article belongs to American Physical Society.
Keywords:	Buoyancy; Deformation; Reynolds number; Spheres; Vortex flow; Vorticity, Air bubbles; Buoyant particles; Carrier phasis; Dispersed phasis; Enstrophy; Localised; Rigid particles; Single vortices; Vortex rings; Weber numbers, Turbulent flow
Department/Centre:	Division of Mechanical Sciences > Mechanical Engineering
Date Deposited:	04 Nov 2022 09:38
Last Modified:	04 Nov 2022 09:38
URI:	https://eprints.iisc.ac.in/id/eprint/77773

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