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 E33: Cavitation and Ventilation |
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Chair: David Dowling, University of Michigan Room: 2022 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E33.00001: Wall-induced path variation of a large deformable rising bubble Hyungmin Park, Hyeonju Jeong In the present study, we experimentally investigate the wall-induced path variation of a large deformable bubble ($Re \sim \emph{\textbf{O}}(10^{3})$) rising near a vertical wall in quiescent water. To change the wall effect, we consider different wall materials (acrylic, PTFE and sponge) and vary the initial distance between the bubble and the wall. Depending on the conditions, various motions like a periodic bouncing, sliding, migrating away, and non-periodic oscillation without collisions are captured. Analysis on the energy balance shows that, contrary to a low-$Re$ bubble, the surface deformation plays a great role in bubble's rising behaviour. Especially, across the bubble-wall collision, the excessive surface energy compensates the deficit of kinetic energy, which enables the bubble to maintain a constant bouncing kinematics, despite the wall effect. The wall effect, appearing as a energy loss, decreases as the distance to the wall increases. Compared to the no-slip surface, the hydrophobic surface enhances or reduces the wall effect with the wall distance, whereas the porous surface reduces the energy loss due to the wall. The dependence of near-wall bubble motion on a wall configuration may give us an idea about how to predict or model the near-wall gas void-fraction. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E33.00002: Slugs in a large diameter column with air and high viscosity silicone oil Abbas Hasan, Barry Azzopardi Very little information is known about the behaviour of high viscous liquids ($>100$ Pa.s) in two phase slug flows. Experiments were carried out to study the behaviour of silicone oil (300 Pa.s) in gas bubble column using electrical capacitance tomography technique. The main aim of this paper is to study the characteristics and parameters of gas-liquid slug flows through large scale experiments with realistic liquids in a large diameter pipe (240 mm). These include; mean void fraction, Taylor bubble velocity, lengths of liquid slugs and Taylor bubbles, liquid film and fraction flowing down past the Taylor bubble. It was found that the gas mainly travels as large bubbles with ellipsoidal shape which occupy a significant portion of the pipe cross section with tiny bubbles in the liquid. In addition, the top surface of the gas-liquid column experiences a periodic oscillation (rising and falling) as the large bubbles rise to the top surface and burst. The results presented in this work have been compared with previous studies to show the effects of the viscosity and the pipe diameter on the behaviour of large bubbles in gas-liquid two phase slug flows. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E33.00003: Ventilation Inception and Washout, Scaling, and Effects on Hydrodynamic Performance of a Surface Piercing Strut Casey Harwood, Yin Lu Young, Steven Ceccio High-lift devices that operate at or near a fluid free surface (such as surface-piercing or shallowly-submerged propellers and hydrofoils) are prone to a multiphase flow phenomenon called ventilation, wherein non-condensable gas is entrained in the low-pressure flow, forming a cavity around the body and dramatically altering the global hydrodynamic forces. Experiments are being conducted at the University of Michigan's towing tank using a canonical surface-piercing strut to investigate atmospheric ventilation. The goals of the work are (i) to gain an understanding of the dominant physics in fully wetted, partially ventilated, and fully ventilated flow regimes, (ii) to quantify the effects of governing dimensionless parameters on the transition between flow regimes, and (iii) to develop scaling relations for the transition between flow regimes. Using theoretical arguments and flow visualization techniques, new criteria are developed for classifying flow regimes and transition mechanisms. Unsteady transition mechanisms are described and mapped as functions of the governing non-dimensional parameters. A theoretical scaling relationship is developed for ventilation washout, which is shown to adequately capture the experimentally-observed washout boundary. [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E33.00004: Shock wave induced shedding of cavitation clouds Harish Ganesh, Simo Makiharju, Steven Ceccio Mechanisms responsible for periodic shedding of vapor clouds from partial cavities forming on a wedge are explored using time resolved X-ray densitometry. Time resolved 2-D void fraction flow field measurements of such partial cavities are obtained to identify the mechanisms of transition from closed partial cavities to open cavities exhibiting periodic shedding of vapor clouds or large gas pockets break off. From the void fraction field measurements, presence of an advancing bubbly shock front responsible for periodic shedding is identified as a primary cause of shedding. The void fraction measurements also reveal the presence of a reentrant flow at the cavity closure that produces intermittent shedding of smaller scale cavities at slightly higher cavitation numbers than periodically shedding cavities. A discussion on the observed occurrence and properties of the bubbly shock wave, and its role in causing periodic shedding is presented based on the one-dimensional model of shock propagation in bubbly mixtures. The observed cavity shape and its dependence with cavitation number is also compared with analytically predicted cavity shape using free streamline theory. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E33.00005: Effect of Non-Condensable Gas on Cavity Dynamics and Sheet to Cloud Transition Simo Makiharju, Harish Ganesh, Steven Ceccio Partial cavitation occurs in numerous industrial and naval applications. Cavities on lifting surfaces, in cryogenic rocket motors or in fuel injectors can damage equipment and in general be detrimental to the system performance, especially as partial cavities can undergo auto-oscillation causing large pressure pulsations, unsteady loading of machinery and generate significant noise. In the current experiments incipient, intermittent cloud shedding and fully shedding cavities forming in the separated flow region downstream of a wedge were investigated. The Reynolds number based on hydraulic diameter was of the order of one million. Gas was injected directly into the cavitation region downstream of the wedge's apex or into the recirculating region such that with the same amount of injected gas less ended up in the shear layer. The cavity dynamics were studied with and without gas injection. The hypothesis to be tested were that i) relatively miniscule amounts of gas introduced into the shear layer at the cavity interface can reduce vapor production and ii) gas introduced into the separated region can dampen the auto oscillations. The authors also examined whether the presence of gas can switch the shedding mechanism from one dominated by condensation shock to one dominantly by re-entrant jet. [Preview Abstract] |
Sunday, November 23, 2014 5:50PM - 6:03PM |
E33.00006: Large eddy simulation of cavitating flows Aswin Gnanaskandan, Krishnan Mahesh Large eddy simulation on unstructured grids is used to study hydrodynamic cavitation. The multiphase medium is represented using a homogeneous equilibrium model that assumes thermal equilibrium between the liquid and the vapor phase. Surface tension effects are ignored and the governing equations are the compressible Navier Stokes equations for the liquid/vapor mixture along with a transport equation for the vapor mass fraction. A characteristic-based filtering scheme is developed to handle shocks and material discontinuities in non-ideal gases and mixtures. A TVD filter is applied as a corrector step in a predictor-corrector approach with the predictor scheme being non-dissipative and symmetric. The method is validated for canonical one dimensional flows and leading edge cavitation over a hydrofoil, and applied to study sheet to cloud cavitation over a wedge. [Preview Abstract] |
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