Direct numerical simulation of large bubbles in a free shear layer
Taeibi-Rahni, Mohammad
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https://hdl.handle.net/2142/19782
Description
Title
Direct numerical simulation of large bubbles in a free shear layer
Author(s)
Taeibi-Rahni, Mohammad
Issue Date
1995
Doctoral Committee Chair(s)
Loth, Eric
Department of Study
Aerospace Engineering
Discipline
Aerospace Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Aerospace
Engineering, Chemical
Engineering, Mechanical
Language
eng
Abstract
The full Navier-Stokes equations were employed with a single-fluid model and a front tracking scheme to study large cylindrical, spherical, and ellipsoidal bubbles in a free shear layer. This approach allows direct simulation of the multiphase flow by wholly incorporating the bubble flow field in conjunction with the large scale vortical structures of the liquid. The role of large bubbles in modifying finite Reynolds number shear flow structures was investigated, specifically for bubbles whose diameter approaches the scale of the largest liquid eddies. Both two- and three-dimensional results indicate that duration of eddy crossing is the main mechanism for flow modulation, which is typically characterized by decreased vortex coherency and size, modified fluctuation statistics, and significant variations in pairing/merging phenomena. The comparison of fluctuating statistics and flow field visualization also allowed qualitative discrimination between the modulation of the non-linear eddy dynamics and fluctuations due simply to the random bubble induced perturbations. Increasing bubble deformation only has a minor effect on liquid flow modulation.
In addition to the flow modulation studies, a general formulation based on Auton et al. (1988) of the hydrodynamic forces on a finite Reynolds number large bubble in an unsteady, non-uniform, and rotational flow was developed. This formulation is used to investigate the effects of non-linear spatial and temporal gradients on dispersion of large cylindrical, spherical, and ellipsoidal bubbles in a free shear layer. Both two- and three-dimensional results indicate that the bubble dispersion in the full Navier-Stokes solution was significantly different than that given by a conventional bubble dynamic equation based on linear spatial gradients and quasi-steady flow. This is due to the forces not accounted for by such a formulation, which are related to regions of high non-uniformity and unsteadiness. Comparison of the dispersion of highly deformed bubbles with negligibly deformed ones indicates significant differences. The adjunct forces in the drag/lift direction on both two- and three-dimensional bubbles (low or high deformation) were found to be correlated with rapid variations of relative bubble velocity and acceleration/high gradients of the liquid velocity.
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