Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A28: Free-Surface Flows I |
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Chair: Eduardo Ramos, Center for Energy Research, National Autonomous University of Mexico Room: 32A |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A28.00001: The Rotating Polygon Instability of a Swirling Free Surface Flow Tomas Bohr, Laust Toph{\O}j, Jerome Mougel, David Fabre We present a theory of the rotating polygon instability on a swirling fluid surface [G. H. Vatistas, J. Fluid Mech. {\bf 217}, 241 (1990), Jansson et al., Phys. Rev. Lett. {\bf 96}, 174502 (2006)]. Our approach is based on potential flow theory, linearised around a potential vortex flow with a free surface. Limiting our attention to the lowest order wave-modes, we obtain an analytically solvable model showing the symmetry breaking instability, and which, together with estimates of the circulation based on angular momentum balance, reproduces the main features of the experimental phase diagram. The generality of our arguments implies that the instability should not be limited to flows with rotating bottom (implying singular behaviour near the corners) and indeed we show that we can obtain the polygons transiently in a much simpler way. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A28.00002: Interfacial Instabilities in Torsional Flows Ching-Yao Lai, Y.-T. Sun, C.-C. Chang, Y.-Y. Chen, P. Arratia, J.- C. Tsai In this presentation, we report on current findings on morphology of an oil-water interface in a torsional flow produced by rotating the upper lid of a cylindrical tank. Here, the upper half of the tank is filled with silicon oil and the lower half is filled with water. The interface morphology is investigated as a function of the Reynolds number, based on the upper fluid, and of aspect ratio between the cylinder height and radius (AR=H/R). We find that, at moderate AR, raising the rotation rate (or Re) can induce a continuous change of the oil-water interface morphology from a simple hump to a flat top (plateau), and to a double mound before the interface becomes unstable [1]. At high AR, long water threads and fluid break-ups occur either along the central axis or at a non-zero radius, reflecting the location where the upward flow exhibits a maximum. The changes in the morphology of the interface can be linked to the changes in the secondary flow in the upper fluid. The system exhibits a wealth of behaviors, including symmetry breaking, that illustrates not only the relationship between the upward (secondary) flow and the gravitational force, but also the emergence of various flow and interfacial instabilities. \\[4pt] [1] Lai and Ku, 2009 INTEL Int'l Scienand Engineering Fairs [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A28.00003: Liquid metal stirring by rotating localized magnetic field in a cylindrical container Sergio Cuevas, Michel Rivero, Eduardo Ramos We study experimentally the flow in a shallow liquid metal layer (GaInSn) driven by an array of small rotating permanent magnets (12.7 cm diameter) located at the bottom of a cylindrical plexiglas container with a diameter of 203.2 cm. The fluid layer has 13 mm and the maximum analyzed rotation frequency is 7 Hz. The explored magnet arrays vary from one single magnet up to five magnets eccentrically located but equidistant at two different fixed radius. The radial velocity component is obtained using Ultrasound Doppler Velocimetry (UDV) and analyzed through the Fast Fourier Transform. The characteristic frequencies of the flow structures are determined and global flow patterns are approximately reproduced. The flow is also analyzed by image processing in those cases where a free surface oscillation appears and these results are compared with those obtained by UDV. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A28.00004: Dynamics of interface separating two fluids under AC electric fields Asghar Esmaeeli Direct Numerical Simulations are performed to study dynamics of a horizontal interface separating two fluids under the influence of an AC electric field. A front tracking/finite difference scheme is used, in conjunction with Taylor's leaky dielectric model, to solve the governing electrohydrodynamics equations in both fluids. The interface becomes unstable beyond a critical voltage, going through a transition period, and forming an oscillatory vertical pillar that points from the liquid of higher electric conductivity to the one with a lower conductivity. It is shown that the pillar shape depends on the dielectric properties of the fluids. The correlation between the frequency of the interface oscillation and that of the imposed electric field is found and a parametric study is performed based on the governing nondimensional number of the problems. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A28.00005: Numerical Flow Analysis of Planing Boats Kyle Brucker, Thomas O'Shea, Douglas Dommermuth, Thomas Fu The focus of this presentation is to describe the recent effort to validate the computer code Numerical Flow Analysis (NFA) for the prediction of hydrodynamic forces and moments associated with deep-V planing craft. This detailed validation effort was composed of two parts. The first part focuses on assessing NFA's ability to predict pressures on the surface of a 10 degree deadrise wedge during impact with an undisturbed free surface. Detailed comparisons to pressure gauges are presented for two different drop heights, 6 inches and 10 inches. Results show NFA accurately predicted pressures during the slamming event. The second part of the validation study focused on assessing how well NFA was able to accurately model the complex multiphase flow associated with high Froude number flows, specifically the formation of the spray sheet. NFA simulations of a planing hull fixed at various angles of roll (0 degrees, 10 degrees, 20 degrees, and 30 degrees) were compared to experiments from Judge (2012). Comparisons to underwater photographs illustrate NFA's ability to model the formation of the spray sheet and the free surface turbulence associated with planing boat hydrodynamics. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A28.00006: Nonlinearity, Viscosity and Air-Compressibility Effects on the Helmholtz Resonant Wave Motion Generated by an Oscillating Twin Body in a Free Surface Palaniswamy Ananthakrishnan The problem is of practical relevance in determining the motion response of multi-hull and air-cushion vehicles in high seas and in littoral waters. The linear inviscid problem without surface pressure has been well studied in the past. In the present work, the nonlinear wave-body interaction problem is solved using finite-difference methods based on boundary-fitted coordinates. The inviscid nonlinear problem is tackled using the mixed Eulerian-Lagrangian formulation and the solution of the incompressible Navier-Stokes equations governing the viscous problem using a fractional-step method. The pressure variation in the air cushion is modeled using the isentropic gas equation pV$^{\gamma }$ = Constant. Results show that viscosity and free-surface nonlinearity significantly affect the hydrodynamic force and the wave motion at the resonant Helmholtz frequency (at which the primary wave motion is the vertical oscillation of the mean surface in between the bodies). Air compressibility suppresses the Helmholtz oscillation and enhances the wave radiation. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A28.00007: Subharmonic surface waves in a horizontally vibrated container Jose Perez-Gracia, Jeff Porter, Fernando Varas, Jose Vega Horizontal vibrations of rectangular containers first produce harmonic waves and then subharmonic waves as the forcing acceleration is increased. A theoretical analysis is performed that provides both the subharmonic instability threshold and the associated patterns. A key ingredient in the theory is an oscillatory bulk flow (OBF) produced by the vibrating container that extends horizontally to a length comparable with the container depth. The OBF involves harmonic temporal oscillations in the vertical pressure gradient at the free surface. It is precisely those pressure gradient oscillations that trigger subharmonic waves, as in the vertical forced Faraday instability. The obtained results compare well with experimental measurements/visualization that will also be presented. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A28.00008: The water entry of streamlined bodies Kyle Bodily, Tadd Truscott We present the results of an experimental study on the effects of nose shape, wetting angle, and impact angle on the water entry of axisymmetric bodies. Forces, velocity and trajectory are inferred from an inertial measurement unit embedded into the tail and validated by high-speed imaging. A hydrophone is used to record the sounds of cavity collapse to extract a unique signature for each nose shape. Horizontal motion is strongest when impacting with small oblique angles normal to the free surface and weakest when the surface of the body is coated in a half-hydrophobic and half-hydrophilic coating. Additionally, the nose shape has the largest effect on altering acoustic signature for impacts normal to the free surface. [Preview Abstract] |
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