Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session LH: Waves |
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Chair: James Duncan, University of Maryland Room: 101B |
Monday, November 24, 2008 3:35PM - 3:48PM |
LH.00001: A Parametric Investigation of Breaking Bow Waves using a 2D+T Wave Maker E.A. Maxeiner, M. Shakeri, J.H. Duncan An experimental study of bow waves generated by a 2D+T (Two Dimensions plus Time) wave maker in a tank that is 14.8 m long, 1.2 m wide and 2.2 m deep is presented. Rather than simulating a specific ship hull, here we use a parametric set of wave maker motions with each parameter simulating a common feature of a ship hull form. Three categories of wave maker motions are used: ``slap'' (rotation of the wave board (held flat) about the keel), ``fixed'' (translation the wave board while it is upper part remains flat and at a fixed angle relative to horizontal), and ``full'' (simultaneous rotation and translation). The wave maker motions are run over a range of speeds and, in the ``fixed'' cases, over a range of angles. The temporal histories of the wave profiles were measured using a cinematic LIF technique. The relationship between various geometrical features of the waves and the wave maker motion parameters is explored. Each category of wave maker motions produces waves that develop and break in markedly different ways, thus highlighting the complex nature of bow waves. The wave crest speeds vary between 2 and 2.5 times the maximum speed of the wave maker and, for a given class of wave maker motion, vary with wave maker speed. [Preview Abstract] |
Monday, November 24, 2008 3:48PM - 4:01PM |
LH.00002: The Cross-Stream Structure of the Crests of Breaking Waves J.H. Duncan, J.D. Diorio, X. Liu Surface profiles and flow fields in the crests of breaking waves are usually measured in vertical stream-wise planes. However, measurements of the turbulent flow in boundary layers along flat rigid walls have indicated the importance of streamwise flow structures. In the present study, breaking waves are examined in a tank that is 12.8 m long and 1.2 m wide with a water depth of 0.91 m. A programmable wave maker is used to generate wave packets (central frequencies 1.15 - 1.42 Hz) that create breakers by dispersive focusing. Different amplitudes of the wave maker motion are used to generate various breaking waves ranging from weakly spilling to plunging breakers. A cinematic 2D LIF technique is used to measure the crest profile histories and the light-sheet plane is oriented to measure both the stream-wise and cross-stream crest profiles in separate experiments. It is found that the development of ripples due to turbulence-free surface interactions is highly repeatable and that even though the waves are two-dimensional before breaking, the amplitude of the cross-stream components quickly reaches 50{\%} of the stream-wise ripple amplitude. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LH.00003: Unstable internal waves Tiron Roxana, Ann Almgren, Camassa Roberto, Sallerson Amber Recent advancements in observational techniques have revealed that internal gravity waves are an ubiquitous phenomena in the ocean and in the atmosphere. In particular, internal waves propagating in a strati?ed ocean have been observed and reported to have large amplitudes. Understanding the breaking mechanisms of these waves is crucial for explaining mixing and transport phenomena within the ocean. As experimental observations show, for near two layer stratification, waves become unstable in large amplitude regimes and the wave-breaking closely resembles Kelvin Helmholtz shear instability originating in the maximum displacement of the pycnocline region. The instability is modulated by the stream-wise variation of the shear. We simulate numerically the generation and propagation of solitary waves starting from a step function initial condition and monitor the wave-induced shear instabilities. A conservative projection method for the variable density Euler equations is implemented in this scope. The code is validated against experimental data as well as theoretical results. In an effort to elucidate whether the instabilities are an intrinsic property of the wave or they are induced by the experimental generation, we study the time evolution of traveling wave solutions. [Preview Abstract] |
Monday, November 24, 2008 4:14PM - 4:27PM |
LH.00004: Weakly nonlinear, multi-modal evolution of wind-generated long internal waves in a closed basin Larry Redekopp, Takahiro Sakai A weakly nonlinear evolution model that accounts for multi-modal interaction in a continuously stratified lake of variable depth is derived. The model for the first two vertical modes in a lake that is subject to wind stress forcing is numerically simulated. Defining modal energies, energy transfer between the first and the second vertical modes is calculated for several different forms of the density stratification. Modal energy transfer mainly occurs during reflection of mode-one waves at the vertical end walls, and it is shown that the amount of energy transfer from the first to the second mode is greatly dependent on the shape of the stratification. Also, the initial modal energy partition at the end of the wind setup is shown to depend significantly on the penetration depth of the wind stress, especially if the stress distribution extends into the upper levels of the metalimnion. [Preview Abstract] |
Monday, November 24, 2008 4:27PM - 4:40PM |
LH.00005: Corner waves downstream a partially submerged vertical plate Pablo Martinez-Legazpi, Javier Rodriguez-Rodriguez, Juan C. Lasheras We have studied experimentally and numerically the expansion flow developing downstream the corner of a partially submerged vertical plate. In this flow configuration, a steady wave remains attached to the corner of the plate. Both the amplitude and slope of the wave front increase with the downstream distance until, the wave breaks resulting in either a spilling or a plunging breaker. Following theoretical considerations, we propose a criterion based on a critical Froude number to determine which breaker configuration prevails. This criterion is shown to be in good agreement with the experimental results. Despite the simplicity of this flow, the observed wave pattern is remarkably similar to that one found at a dry stern in high-speed surface vessels. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LH.00006: Experimental investigation of power capture from pitching vertical cylinders in irregular waves Flocard Francois, Frandsen Janette, Finnigan Timothy Point absorbers are one of the main categories of wave energy converters being developed worldwide. These devices are classified by their dominant mode of motion relative to the water surface. Most of these converters use either a hydraulic system or direct drive electric generator as a means for power take-off (P.T.O.). These wave energy devices are highly suitable for intermediate depth locations. This paper presents the results from an experimental study on the power capture of bottom-pivoted pitching cylinders in intermediate water depth subjected to regular and irregular waves. All experiments were conducted in the University of Sydney's wave flume. The geometry of the pivoted cylinder, external damping and additional inertia (to simulate the impact of water ballasting) were taken as variable parameters in order to optimize the power capture efficiency in different wave conditions. The devices were subjected to a realistic wave climate obtained from the analysis of on-site buoy measurements from the European Marine Energy Center in Scotland, a location renowned for its wave energy potential, where existing devices have been already tested. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LH.00007: Pattern selection in a horizontally vibrated container Jeff Porter, Ignacio Tinao, Ana Laveron-Simavilla We investigate the dynamics and pattern formation properties of a fluid interface whose supporting container is subjected to horizontal vibrations. Experimental results demonstrate the prevalence of so-called subharmonic cross-waves beyond the linear stability limit of directly forced synchronous surface waves, and reveal several new and interesting properties of these subharmonic waves in large aspect ratio systems, including a preferred orientation other than 90 degrees, a tendency to form domains of distinct patterns, and a variety of low-frequency modulations. [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LH.00008: Axisymmetric Weakly Compressible Transient Pipe Flow and Water Hammer Control Jie-Zhi Wu, Li-Jun Xuan, Feng Mao Despite the partial success of existing theoretical models in explaining certain transient water flow phenomena in a long pipe, they can hardly predict the evolution of strong water hammer, in particular the one downstream the valve caused by its closing (reversed water hammer). We attack this important problem by a new perturbation theory based on the unsteady axisymmetric and compressible Navier-Stokes equations. The leading-order transient solution is~in excellent agreement with the direction simulation of the original N-S equation. We establish a simple relation~between the valve motion and adjacent pressure in reversed water hammer, by which the strategy of optimal control of reversed water hammer is analyzed and illustrated. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LH.00009: Instabilities of coupled wave packet systems Hok Shun Chiu, Kwok Chow Propagation of wave packets in layered or continuously stratified fluids will typically lead to coupled, nonlinear Schrodinger equations (CNLS). The competition between dispersion and nonlinearity will be crucial. A novel instability can arise from cross phase modulation (XPM), or the effect on a wave packet due to the presence of the other one. XPM in the hydrodynamic context is now considered from several perspectives. In the long wave regime governed by the extended Korteweg de Vries system, CNLS are derived by multiple-scale expansions and modulation instabilities (MI) of plane waves are studied. This will reveal new energy transfer mechanism. Secondly, from a scientific perspective, special, integrable higher order CNLS are considered and the presence of MI is examined. This is critical as higher (or fourth) order effects must be considered when the wave slope is sufficiently large. [Preview Abstract] |
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