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 E16: Free-Surface Flows III: Turbulence and Wakes |
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Chair: James Duncan, University of Maryland Room: 2000 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E16.00001: Turbulent hydraulic jumps: characterization of macro and micro bubble generation Milad Mortazavi, Ali Mani Bubble generation is a ubiquitous two-phase flow phenomenon occurring constantly in nature and in a wide variety of industrial processes. This work considers a hydraulic jump as a canonical setting to investigate bubble generation by nonlinear breaking waves. We have performed direct numerical simulation of a turbulent hydraulic jump with inlet Froude number of 2.0 and Reynolds number of 11000. We show remarkable similarities in the bubble size distribution and its evolutions between this statistically stationary wave and reported results for transient breaking waves. Additionally, in the hydraulic jump large bubbles are observed to be generated in patch-like structures with a distinct frequency which matches a dominant frequency in the velocity spectrum. It is speculated that this frequency is associated with the roller frequency generated at the toe of the jump. Curvature and relative velocity at the liquid-liquid impact points are investigated as an attempt to model microbubble generation by making connections to the Mesler entrainment mechanism. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E16.00002: Accuracy of the 2D+t Approximation for a Laminar Wake in Surface Waves Laura Pauley, Chris Mathiot Wakes in the ocean can be produced by a stationary object in a current or by a moving object in stationary water. When viewed in a reference frame moving with the object, the wake can persist thousands of object diameters downstream. Due to the extensive domain, an unsteady two-dimensional (2D+t) computation is often used to sweep downstream through the wake development. The 2D+t computation approximates the development of the wake at a fixed location as an object moves past but applies cyclical boundary conditions in the streamwise direction. A Parabolized Navier-Stokes (PNS) method has the same numerical efficiency as the 2D+t method but includes additional streamwise gradient terms found in the three-dimensional governing equations. The present paper investigates the accuracy of the 2D+t approximation for a laminar wake interacting with a surface wave described by the Stokes drift velocity distribution. When the laminar wake and Stokes drift are in the same direction, a secondary recirculating flow develops in the cross-span plane. The 2D+t results are compared with results from 3D Navier-Stokes computations and results from PNS computations to identify criteria at which the 2D+t method will yield accurate results. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E16.00003: A Laboratory Study of Rain-Induced Underwater Turbulence Using Particle Image Velocimetry R. Liu, X. Liu, J.H. Duncan The characteristics of rain-induced turbulence under a free surface are studied experimentally with Paticle Image Velocimetry (PIV) techniques in a 1.22-m-by-1.22-m water pool with a water depth of 0.3~m. A rain generator consisting of an open-surface water tank with an array of 22-gauge hypodermic needles attached to the tank bottom is mounted above the water pool. The tank is connected to a 2D translation stage to provide a small-radius horizontal circular motion to the needles, thus avoiding repeated drop impacts at the same location under each needle. The drop diameter is 2.6~mm and the height of the rain generator above the water surface of the pool is varied from 1~m to 2.5~m to provide different impact velocities. Both the flow field of a single drop impact and the turbulent layer under the free surface during rain simulations were measured with PIV. It was found that the drop penetration, the thickness of the turbulent layer under the free surface and the RMS velocity fluctuation are strongly correlated to the impact velocities of raindrops. The influence of this turbulence on the height of rebounding jet stalks from drop impacts is discussed. [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E16.00004: Vorticity-based correction for modelling of free-surface wave interacting with turbulent current Wei Zhang This paper describes a new vorticity-based correction model for studying the interaction between free-surface wave and turbulent current. To track free-surface movements, the volume of fluid (VOF) method is employed. The momentum equations are rewritten to avoid the numerically generated vorticity effects along the air-water interface. Simultaneously unsteady RANS equations are used, while standard k-epsilon model is adapted with modification to the production term by introducing the vorticity to limit the production of turbulent kinematic energy at free surface. To validate the numerical model used here, standalone wave and current cases are studied to ensure the accuracy of each component of the numerical model. The model is then used to simulate the interaction between the second-order stokes wave and turbulent current for both wave following and countering in a setting of shallow water wave flume. The results are compared with experimental measurement available in the literature. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E16.00005: Water Surface Ripples Generated by the Turbulent Boundary Layer of a Surface-Piercing Moving Wall N. Washuta, N. Masnadi, J.H. Duncan Free surface ripples created by subsurface turbulence along a surface-piercing moving wall are studied experimentally. In this experiment, a meter-wide stainless steel belt travels horizontally in a loop around two rollers with vertically oriented axes, which are separated by 7.5~meters. One of the two 7.5-m-long belt sections between the rollers is in contact with the water in a large open-surface water tank and the water level is adjusted so that the top of the belt pierces the water free surface. The belt is launched from rest with a 3$g$ acceleration in order to quickly reach a steady state velocity. This belt motion creates a temporally evolving boundary layer analogous to the spatially evolving boundary layer created along the side of a ship hull moving at the belt velocity, with a length equivalent to the length of belt that has passed the measurement region. The water surface ripples generated by the subsurface turbulence are measured in a plane normal to the belt using a cinematic LIF technique. It is found that the overall RMS surface fluctuations increase linearly with belt speed and that the spatial distributions of the fluctuations show a sharp increase near the wall. [Preview Abstract] |
Sunday, November 23, 2014 5:50PM - 6:03PM |
E16.00006: Surface Ripples Generated in a Couette Flow with a Free Surface N. Masnadi, N. Washuta, J.H. Duncan Free surface ripples created by subsurface turbulence in the gap between a vertical surface-piercing moving wall and a parallel fixed wall are studied experimentally. The moving wall is created with the aide of a meter-wide stainless steel belt that travels horizontally in a loop around two rollers with vertically oriented axes, which are separated by 7.5~meters. One of the two 7.5-m-long belt sections between the rollers is in contact with the water in a large open-surface water tank and forms the moving wall. The fixed wall is an acrylic plate located 4~cm from the belt surface. The water surface ripples are measured in a plane normal to the belt using a cinematic LIF technique. Measurements are done at a location about 100 gap widths downstream of the leading edge of the fixed plate in order to have a fully developed flow condition. It is found that the overall RMS surface fluctuations increase linearly with belt speed. The frequency-domain spectra of the surface height fluctuation and its temporal derivative are computed at locations across the gap width and are used to explore the physics of the free surface motions. [Preview Abstract] |
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