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 G28: Waves I |
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Chair: Kyle Brucker, SAIC Room: 32A |
Monday, November 19, 2012 8:00AM - 8:13AM |
G28.00001: The Influence of surface waves on marine current turbine performance Ethan Lust, Karen Flack, Luksa Luznik Performance characteristics are presented for a 1/25$^{th}$ scale marine current turbine operating in calm conditions and in the presence of intermediate and deep water waves. The two-bladed turbine has radius of 0.4 m and a maximum blade pitch of 17\r{ }. The hydrofoil is a NACA63-618 which was selected to be Reynolds number independent in the operational range (Re$_{C}$ = 2 - 4 x 10$^{5})$. The experiments were performed in the 116 m tow-tank at the United States Naval Academy at a depth of 0.8D measured from the blade tip to the mean free surface. The performance characteristics without waves match expected results from blade-element-momentum theory. Results show that the average power coefficient is unaffected by the presence of waves, however, the phase averaged results indicate significant variation with wave phase. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G28.00002: Laboratory study of the structure of the airflow and separation above surface waves Fabrice Veron, Marc Buckley The effects of the surface waves on the airflow dynamics greatly influence the flux of momentum between the ocean and the atmosphere. While we know that most the surface stress is supported by the wave-coherent stress carried in large part by the small gravity-capillary wind waves, the role of the airflow separation above these small waves is not well understood. We present experimental results on the details of the airflow above surface gravity waves for a several wind speeds, wave ages and slopes. The bulk of the results presented were obtained from a series of laboratory experiments that took place at the University of Delaware's Air-sea interaction facility. Airflow velocities were obtained using high resolution concatenated PIV, and wave profiles and spectra were measured by dual-beam laser-induced fluorescence. We observe direct evidence of intermittent airflow separation past the crest of the wind waves. The separation leads to dramatic along-wave variability in the surface viscous tangential stress which in turn may affect wave growth and the air-water momentum balance. Despite the intermittent aspect of this phenomenon, proper orthogonal decomposition (POD) of the wave phase-locked velocity products suggests that airflow separation generates intense mixing and transport of surface generated vorticity within the airflow. These results hold for wind speeds that would normally be considered low to moderate. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G28.00003: Impact of Plunging Breaking Wave on a Partially Submerged Cube A. Wang, C.M. Ikeda, J.H. Duncan The impact of a plunging breaking wave on a partially submerged rigid cube ($L = 30.5$~cm) is studied experimentally in a wave tank that is 14.8~m long, 1.15~m wide and 2.2~m high with a water depth of 0.91~m. A single repeatable plunging breaker is generated from a dispersively focused wave packet (average frequency of 1.14~Hz) that is created with a programmable wave maker. The water surface profiles at the vertical center plane of the cube are measured with a cinematic LIF technique. The cube is centered in the width of the tank and mounted from above with the front face oriented with its normal in the vertical long center plane of the tank and tilted at angles of 0 and 20 degrees downward relative to horizontal. For the range of horizontal cube positions used here, during the wave impact, the water free surface forms a circular arc between the water contact point on the front face of the cube and the wave crest. As the wave impact continues, this arc converges to a point and a fast-moving vertical jet is formed. The effect of the submergence and tilt angle of the cube on the jet formation are explored. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G28.00004: Crosswaves induced by the vertical oscillation of a fully immersed vertical plate Frederic Moisy, Guy-Jean Michon, Marc Rabaud, Eric Sultan Capillary waves excited by the vertical oscillation of a thin elongated plate below an air-water interface are analyzed using time-resolved measurements of the surface topography. A parametric instability is observed above a well defined acceleration threshold, resulting in a so-called cross-wave, a staggered wave pattern localized near the wavemaker and oscillating at half the forcing frequency. This cross-wave, which is stationary along the wavemaker but propagative away from it, is described as the superposition of two almost anti-parallel propagating parametric waves making a small angle of the order of 20 degres with the wavemaker edge. This contrasts with the classical Faraday parametric waves, which are exactly stationary because of the homogeneity of the forcing. Our observations suggest that the selection of the cross-wave angle results from a resonant mechanism between the two parametric waves and a characteristic length of the surface deformation above the wavemaker. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G28.00005: Numerical Simulation of a Seaway with Breaking Douglas Dommermuth, Thomas O'Shea, Kyle Brucker, Donald Wyatt The focus of this presentation is to describe the recent efforts to simulate a fully non-linear seaway with breaking by using a high-order spectral (HOS) solution of the free-surface boundary value problem to drive a three-dimensional Volume of Fluid (VOF) solution. Historically, the two main types of simulations to simulate free-surface flows are the boundary integral equations method (BIEM) and high-order spectral (HOS) methods. BIEM calculations fail at the point at which the surface impacts upon itself, if not sooner, and HOS methods can only simulate a single valued free-surface. Both also employ a single-phase approximation in which the effects of the air on the water are neglected. Due to these limitations they are unable to simulate breaking waves and air entrainment. The Volume of Fluid (VOF) method on the other hand is suitable for modeling breaking waves and air entrainment. However it is computationally intractable to generate a realistic non-linear sea-state. Here, we use the HOS solution to quickly drive, or nudge, the VOF solution into a non-linear state. The computational strategies, mathematical formulation, and numerical implementation will be discussed. The results of the VOF simulation of a seaway with breaking will also be presented, and compared to the single phase, single valued HOS results. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G28.00006: PIV measurements of the interaction between a surface wave and a moving sphere Andrew Rydalch, Eric Maxeiner, Ivan Savelyev The objective of this experiment was to study the interaction between the wake of a moving sphere and a surface wave moving in the same direction. The tests were conducted in an 8.5 x 2.8 m freshwater tank in 30cm of water. An underwater tow track, consisting of a belt-driven carriage riding on aluminum rails mounted on the bottom of the tank, was constructed in order to leave the free surface of the water unperturbed to allow for precise measurements at the air-water interface. In addition to the towing track, the tank also contains a single paddle wave-maker. PIV measurements were taken of the wake as the sphere moved in conjunction with the wave packet. The sphere velocity was constant and the motion was synchronized with the wave maker such that the sphere traveled inside the crest of a wave as it passed through the test area. In order to identify disturbances to the mean flow caused by the wave, the results were compared with measurements of the wake of the same sphere moving through quiescent water, and also of the wave field without the sphere present. In both comparisons, the measurements can be subtracted from the wave/sphere case to characterize the interaction. The results are applicable to motion control systems for vehicles operating in the presence of surface waves. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G28.00007: On the unsteady free surface wave pattern found behind a localized pressure distribution moving at speeds just below the minimum phase speed of linear gravity capillary waves N. Masnadi, J.H. Duncan The non-linear response of a water free surface to a localized pressure distribution moving at constant speed just below the minimum phase speed ($C_{min} \approx 23$~cm/s) of gravity-capillary waves is studied experimentally in a long tank. The pressure distribution is generated by blowing air onto the water surface via a vertically oriented 2-mm-ID tube that is mounted on an instrument carriage that is in turn set to move along the tank at constant speeds between 20 and 23~cm/s. A cinematic light refraction method is used to obtain quantitative measurements of the surface deformation pattern behind the air jet. At towing speeds just below $C_{min}$, an unsteady V-shaped wave pattern appears behind the pressure source. From observations of the wave pattern evolution, it is found that localized depressions are generated near the pressure source and propagate in pairs along the two arms of the V-shaped pattern. These are eventually shed from the tips of the pattern and rapidly decay. Measurements of the evolution of the speed of these localized depression patterns are compared to existing measurements of the speeds of steady three-dimensional solitary gravity-capillary waves (lumps) that appear behind the pressure source at even lower towing speeds. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G28.00008: Spatial focusing and breaking of surface waves Gerardo Ruiz-Chavarria, Patrice Le Gal, Michael Le Bars In this work, we present experimental results about the evolution of a monochromatic wave generated with a parabolic wavemaker. Experiments were carried out in the deep water limit, with frequencies ranging between 3 and 10 Hz. Measurement of surface deformation during the wave breaking was made with a laser sheet that illuminates the interface on the symmetry axis. On the other hand for small amplitude deformation we use the Schlieren method which allows a reconstruction of the fluid surface in a two dimensional domain. The spatial focusing produces a growth of the wave amplitude and the maximal value (if no breaking occurs) is attained after the wave crosses the caustic. Otherwise, wave breaking appears initially at the axis of symmetry, before the origin of the cusp. Some energy is dissipated along the wave propagation and focusing. The wave breaking is followed by a decrease in the amplitude, but in some cases the focusing stops this process. Then the amplitude grows again and another wave breaking can be observed. Our results are compared with the prediction of linear wave theory and with numerical simulation of surface non linear waves. Finally we present a discussion of the evolution of the wave before and after the breaking. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G28.00009: Mechanisms for wave generation in a turbulent air-water flow Francesco Zonta, Miguel Onorato, Alfredo Soldati Momentum and scalar transport phenomena across an air-water interface are important in many geophysical processes (absorption of CO$_{2}$ by the ocean) and industrial applications (dynamics of steam-air mixtures flowing into nuclear reactors). We used Direct Numerical Simulations (DNS) to explore the dynamics of countercurrent air/water flow. The motion of the air/water interface was computed by solving an advection equation for the interface vertical elevation (boundary fitted method). At each time step, the physical domain was mapped into a rectangular domain using a nonorthogonal transformation. Continuity and Navier-Stokes equations were first solved separately in each domain, then coupled (velocity/stress) at the interface. We performed DNS in the Weber, Froude and Reynolds number (We,Fr,Re) parameter space. Depending on We, Fr and Re, we obtained different transients for wave evolutions, from small capillary waves to longer gravity waves. For steady-state conditions, we observed that interface deformation echanced turbulence activity/transfer mechanisms across the interface. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G28.00010: Observation of star-shaped surface gravity waves Jean Rajchenbach, Didier Clamond, Alphonse Leroux We report a new type of standing gravity waves of large amplitude, having alternatively the shape of a star and of a polygon. This wave is observed by means of a laboratory experiment by vibrating vertically a tank. The symmetry of the star (i.e. the number of branches) is independent of the container form and size, and can be changed according to the amplitude and frequency of the vibration. We show that this wave geometry results from nonlinear resonant couplings between three waves, although this possibility was denied for pure gravity waves up to now. [Preview Abstract] |
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