Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session M4: Bubbles: Collapse and Coalescence |
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Chair: Gretar Tryggvason, University of Notre Dama Room: 3006 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M4.00001: Investigation of bubble-bubble interaction effect during the collapse of multi-bubble system Xueming Shao, Lingxin Zhang, Wenfeng Wang Bubble collapse is not only an important subject among bubble dynamics, but also a key consequence of cavitation. It has been demonstrated that the structural damage is associated with the rapid change in flow fields during bubble collapse. How to model and simulate the behavior of the bubble collapse is now of great interest. In the present study, both theoretical analysis and a direct numerical simulation on the basis of VOF are performed to investigate the collapses of single bubble and bubble cluster. The effect of bubble-bubble interaction on the collapse of multi-bubble system is presented. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M4.00002: Temperature considerations in numerical simulations of collapsing bubbles Eric Johnsen, Shahaboddin Alahyari Beig In naval and biomedical engineering applications, the inertial collapse of cavitation bubbles is known to damage its surroundings. While significant attention has been dedicated to investigating the pressures produced by this process, less is known about heating of the surrounding medium, which may be important when collapse occurs near objects whose properties strongly depend on temperature (e.g., polymers). Euler simulations are capable of predicting the high pressures thereby generated. However, numerical errors can occur when solving the Navier-Stokes equations for compressible interface problems. Using a newly developed computational approach that prevents such errors, we investigate the dynamics of shock-induced and Rayleigh collapse of individual and collections of gas bubbles, in a free field and near rigid surfaces. We characterize the temperature rises based on the relevant non-dimensional parameters entering the problem. In particular, we show that the temperature of a neighboring object rises due to two mechanisms: the shock produced at collapse and heat diffusion from the hot bubble as it moves toward the object. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M4.00003: Simulations of bubble collapse in viscous and viscoelastic media near a second viscoelastic medium Mauro Rodriguez, Eric Johnsen Understanding the dynamics of cavitation bubbles and the shock waves emitted by their collapse in a viscoelastic medium is important for various naval and medical applications. Two examples are histotripsy, which utilizes this phenomenon for the ablation of pathogenic tissue, and erosion to elastomeric coatings on propellers. To study these problems in a general sense, a canonical problem is considered, which involves the shock-induced collapse of a gaseous bubble in a viscous or viscoelastic medium next to a second viscoelastic or elastic medium of a certain thickness. A novel Eulerian approach, which incorporates nonlinear elasticity, is used to simulate this problem. The stresses, strains and temperatures produced during this process will be presented for different initial stand-off distances, thicknesses of the second medium and shear moduli. Additionally, studies using relevant waveforms that induce the bubble collapse will be presented. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M4.00004: Implosion of Cylindrical Cavities via Short Duration Impulsive Loading Justin Huneault, Andrew Higgins An apparatus has been developed to study the collapse of a cylindrical cavity in gelatin subjected to a symmetric impact-driven impulsive loading. A gas-driven annular projectile is accelerated to approximately 50 m/s, at which point it impacts a gelatin casting confined by curved steel surfaces that allow a transition from an annular geometry to a cylindrically imploding motion. The implosion is visualized by a high-speed camera through a window which forms the top confining wall of the implosion cavity. The initial size of the cavity is such that the gelatin wall is two to five times thicker than the impacting projectile. Thus, during impact the compression wave which travels towards the cavity is closely followed by a rarefaction resulting from the free surface reflection of the compression wave in the projectile. As the compression wave in the gelatin reaches the inner surface, it will also reflect as a rarefaction wave. The interaction between the rarefaction waves from the gelatin and projectile free surfaces leads to large tensile stresses resulting in the spallation of a relatively thin shell. The study focuses on the effect of impact parameters on the thickness and uniformity of the imploding shell formed by the cavitation in the imploding gelatin cylinder. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M4.00005: A unified physical model to explain Supercavity closure Roger Arndt, Ashish Karn, Jiarong Hong An insight into underlying physics behind supercavity closure is an important issue for the operation of underwater vehicles for a number of reasons viz. associated gas flow requirement with each closure regime, effect of cavity closure on the overall cavity behavior and collapse, differences between natural and ventilated supercavity closure etc. There have been several reports on supercavity closure since the 1950s and many empirical relationships governing different closure modes have been proposed by different authors. Yet, there is no universal agreement between results obtained at different experimental facilities. In some cases, contradictory observations have been made. In this talk, systematic investigations conducted into supercavity closure across a wide range of experimental conditions at the Saint Anthony Falls Laboratory (SAFL) are presented. A variety of closure mechanisms were observed including the ones widely reported in the literature, viz. twin vortex, re-entrant jet; new stable closure modes viz. quad vortex and interacting vortex and a host of transition closure modes. A hypothesis on the physical mechanism based on the pressure gradient across the cavity that determines the closure modes is proposed. Using this hypothesis and the control volume analysis at supercavity closure, we explain the observations from SAFL experiments as well as reconcile the observations reported by different researchers. The hypothesis explains the supercavity closure across different experimental facilities, at different blockage ratios and at different flow conditions. Thus, a unified understanding into supercavity closure from the viewpoint of fundamental physics is attempted. [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M4.00006: Coalescence of Bubbles Christopher Anthony, Sumeet Thete, Krishnaraj Sambath, Osman Basaran Drop and bubble coalescence plays a central role in industry and nature. During drop coalescence, two drops touch and merge as a liquid neck connecting them grows from microscopic to macroscopic scales. The hydrodynamic singularity that arises as two drops begin coalescing in a dynamically passive outer fluid (air) has been studied thoroughly in recent years. As a preliminary to developing a similar level of understanding when two drops coalesce in an outer fluid of non-negligible density and viscosity, we use simulation to analyze the coalescence of two identical gas bubbles (idealized as two passive spherical voids) in a liquid. This problem has recently been studied experimentally by Nagel and coworkers (2014). The simulations allow probing of the dynamics for neck radii much smaller than what is possible in experiments. At times earlier than those accessible in experiments, simulations reveal a new type of scaling response than those reported by Nagel et al. However, at larger times, the dynamics is shown to transition to regimes that have been proposed by Nagel and coworkers. Unlike in the experiments, it is shown that the observed scaling regimes can be readily rationalized by judicious interrogation of computed flow fields. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M4.00007: Bubble coalescence in channels flows Jiacai Lu, Gretar Tryggvason Direct numerical simulations (DNS) of bubbly flows in vertical channels have shown that the steady state flow structure is particularly simple and can be described by relatively elementary considerations. Similarly, DNS have been used to examine the transient evolution of both laminar and turbulent channel flows, also leading to considerable increase of the understanding of such flows. However, as the void fraction increases the assumption of bubbly flows becomes unrealistic and it is necessary to account for topological changes through coalescence and breakup and flow regime transitions. Here the transition of high void fraction laminar bubbly flows to slug flow is examined by DNS, using a front tracking method where the exact coalescence criteria can be changed and its effect studied. To quantify the transition we monitor the flow rate and the wall shear, as well as the interface structure, including the different components of the area concentration tensor, which gives the projection of the bubble surface area in different directions. The effect of the precise representation of when and how coalescence takes place is also studied. Preliminary results for turbulent flows, where both coalescence and breakup take place are also shown and the use the results to aid in the developm [Preview Abstract] |
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