Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G28: Waves II |
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Chair: Leonardo Chamorro, University of Illinois at Urbana-Champaign Room: Spirit of Pittsburgh Ballroom B/C |
Monday, November 25, 2013 8:00AM - 8:13AM |
G28.00001: Scaling and kinematics of a floating wind turbine under ocean waves and variable thrust: an experimental study Chris Feist, Kelley Ruehl, Michele Guala Scale model wave channel experiments were performed to study the motion of an offshore floating wind turbine in operational sea states. The model tests were conducted on a 1:100 Froude scaled Sandia National Labs 13.2 MW prototype offshore wind turbine with a barge style floating platform. The platform is modeled after the MIT/NREL Shallow Drafted Barge designed for the 5MW Offshore Baseline wind turbine. The wave environment used in the model tests is representative of the deep-water sea states off the coast of Maine as well as the Pacific Northwest. The purpose of the tests is to validate a computational model of the turbine-wave interaction where the effects of airflow are not considered. To simplify the tests and validation, the platform motion is restricted to two degrees of freedom, pitch and heave, by attaching two roller support types at the center of gravity along the pitch axis. The major aerodynamic force acting on the turbine, i.e. the rotor thrust, is provided by spinning a scaled rotor at a controlled rotational speed. A subset of experiments were performed to study the effect of a mean or fluctuating rotor thrust on the platform dynamics, exploring strategies to control the thrust as a function of platform pitch angle and minimize platform oscillations. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G28.00002: Fluid-structure interaction simulation of floating wind turbines interacting with complex, large-scale ocean waves Antoni Calderer, Xin Guo, Lian Shen, Fotis Sotiropoulos We develop a numerical method for simulating coupled interactions of complex floating structures with large-scale ocean waves and atmospheric turbulence. The Fluid-Structure Interaction (FSI) solver integrates the curvilinear immersed boundary method of Borazjani et al. (JCP 2008) with the level-set method of Kang et al. (Adv. in Water Res. 2012) and is capable of simulating the coupled dynamic interaction of arbitrarily complex bodies with airflow and waves. The large-scale wave model is based on the two-fluid coupled approach of Yang et al. (JCP 2011), which employs a high-order spectral method for simulating the water motion and a viscous solver with undulatory boundaries for the air motion. The large-scale wave field solver is coupled with the near-field FSI solver by feeding into the latter large-scale waves via the pressure-forcing method of Guo et al. (JCP 2009), appropriately adapted herein for the level set method. We validate the model under both simple wave trains and three-dimensional directional waves and compare the results with experimental and theoretical solutions. Finally, we demonstrate the capabilities of the new solver by carrying out large eddy simulation of a floating offshore wind turbine platform interacting with realistic ocean waves. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G28.00003: Impact of plunging breaking waves on a partially submerged cube A. Wang, C. Ikeda, J.H. Duncan The impact of a deep-water plunging breaking wave on a partially submerged cube is studied experimentally in a tank that is 14.8 m long and 1.2 m wide with a water depth of 0.91 m. The breakers are created from dispersively focused wave packets generated by a programmable wave maker. The water surface profile in the vertical center plane of the cube is measured using a cinematic laser-induced fluorescence technique with movie frame rates ranging from 300 to 4,500 Hz. The pressure distribution on the front face of the cube is measured with 24 fast-response sensors simultaneously with the wave profile measurements. The cube is positioned vertically at three heights relative to the mean water level and horizontally at a distance from the wave maker where a strong vertical water jet is formed. The portion of the water surface between the contact point on the front face of the cube and the wave crest is fitted with a circular arc and the radius and vertical position of the fitted circle is tracked during the impact. The vertical acceleration of the contact point reaches more than 50 times the acceleration of gravity and the pressure distribution just below the free surface shows a localized high-pressure region with a very high vertical pressure gradient. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G28.00004: Slamming pressures on the bottom of a free-falling vertical wedge C.M. Ikeda, C.Q. Judge High-speed planing boats are subjected to repeat impacts due to slamming, which can cause structural damage and injury to passengers. A first step in understanding and predicting the physics of a craft re-entering the water after becoming partially airborne is an experimental vertical drop test of a prismastic wedge (deadrise angle, $\beta = 20^{\circ}$; beam, $B = 300$~mm; and length, $L = 600$~mm). The acrylic wedge was mounted to a rig allowing it to free-fall into a deep-water tank (5.2m x 5.2m x 4.2m deep) from heights $0 \le H \le 635$~mm, measured from the keel to the free surface. The wedge was instrumented to record vertical position, acceleration, and pressure on the bottom surface. A pressure mapping system, capable of measuring several points over the area of the thin (0.1~mm) film sensor at sampling rates up to 20~kHz, is used and compared to surface-mounted pressure transducers (sampled at 10~kHz). A high speed camera (1000 fps, resolution of 1920 x 1200 pixels) is mounted above the wedge model to record the wetted surface as the wedge descended below the free surface. The pressure measurements taken with both conventional surface pressure transducers and the pressure mapping system agree within 10\% of the peak pressure values (0.7 bar, typical). [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G28.00005: Experimental investigation of the inception of a spilling breaker Dan Liberzon, Lev Shemer Conditions for the inception of a spilling breaker were studied in 18 m long tank. Peregrine breather-type wave train was excited to generate breaker at a desired location. Parameters of the breaker were obtained using wave gages and two synchronized 2 Mega pixel cameras operating at 60 fps. The instantaneous surface elevation in the vicinity of the breaker's crest was measured by 5 wave gages, while the local wave shape and the inception of breaking were identified from 18 Mpixel video records of the contact line shape variation at the side wall of the tank. An additional identical camera looking at the wave field from above was used to measure the velocity field in the vicinity of the breaking location using Particle Tracking Velocimetry (PTV). Floating particles with diameter of about 3~mm were used for that purpose. Both cameras were synchronized. The instantaneous crest location and velocity were determined from surface elevation fluctuations records. Actual local instantaneous crest velocities differ from both the phase and group velocities of the dominant wave and are compared with the instantaneous horizontal water velocities at various stages of waves breaking. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G28.00006: Generation of surface waves by an underwater moving bottom: experiments and application to tsunami modeling Leonardo Gordillo, Timoth\'ee Jamin, Gerardo Ruiz-Chavarr\'ia, Michael Berhanu, Eric Falcon Most of the ocean waves that we observe in nature are generated by processes that take place near the ocean surface. This occurs mainly because fluid layers reduce significantly the transfer of motion between the source and the free surface as the depth increases. In any case, when the disturbances at a deep source are wide and fast enough, a wave can still be generated. The archetype of this kind of process is tsunami generation: during earthquakes, the seabed of the ocean experiences a sudden net vertical displacement that can yield waves capable of flooding entire coastlines. In this talk, we will focus on laboratory experiments concerning the generation of free surface waves in a three-dimensional uniform layer whose bottom uplifts suddenly. Based on simultaneous measurements of the free surface deformation and the velocity field, we analyze the wave generation dependence on the bottom kinematics. Our results display excellent agreement with a classical linear theory of gravity waves. In addition, we develop a new theoretical approach that can be applied to improve real-time numerical simulations used by the tsunami hazard mitigation programs. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G28.00007: Measurements of turbulence in the airflow above surface waves Fabrice Veron, Marc Buckley 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 properties within and above the viscous sublayer were obtained using PIV, and wave profiles and spectra were measured by laser-induced fluorescence. We observe direct evidence of intermittent separation of the viscous sublayer 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, ensemble averages of the wave phase-locked velocity products suggests the airflow separation yield significant flux of vorticity away from the surface thereby generating intense mixing and momentum transport within the airflow. These results hold for wind speeds that would normally be considered low to moderate. Implications for models of air-sea momentum flux will be discussed. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G28.00008: Impact of Sea Spray on Air-Sea Fluxes Fabrice Veron, James Mueller The contributions of sea spray drops to the total air-sea exchanges of momentum, heat, and mass remain an open question. A number of factors obscure any simple quantification of their contribution: the number of drops formed at the ocean surface and the per-drop contribution to the fluxes. To estimate these per-droplet fluxes, we present results from a large number of drop trajectories, which are simulated with a recently developed Lagrangian Stochastic model adapted for the heavy drop transport and evaporation within the marine boundary layer. Then, using commonly accepted spray generation functions we present estimates of spray fluxes which account for the mediating feedback effects from the droplets on the atmosphere. The results suggest that common simplifications in previous sea spray models, such as the residence time in the marine boundary layer, may not be appropriate. We further show that the spray fluxes may be especially sensitive to the size distribution of the drops. The total effective air-sea fluxes lead to drag and enthalpy coefficients that increase modestly with wind speed. The rate of increase for the drag coefficient is greatest at moderate wind speeds, while the rate of increase for the enthalpy coefficient is greatest at higher wind speeds. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G28.00009: Eulerian and Lagrangian effects of surface wave on turbulence underneath Xin Guo, Lian Shen Direct numerical simulation is performed to study the effects of surface wave on underlying turbulence. In the simulations, fully nonlinear kinematic and dynamic boundary conditions are applied at the free surface. The evolution of surface elevation is obtained by advancing the kinematic boundary condition with a Runge-Kutta scheme. In the vertical direction, grid is clustered towards the free surface to ensure the boundary layers of the free surface and surface wave are fully resolved. For spatial discretization, pseudo-spectral method is used in the horizontal directions, and second-order finite difference method is used in the vertical direction. The interaction of surface wave with underlying turbulence is carefully studied in both Eulerian and Lagrangian frames. In the Eulerian frame, turbulence statistics become wave-phase dependent due to the distortion of both the free surface and the periodic wave strain field. Budget of the Reynolds normal stresses is analyzed. In the Lagrangian frame, net effect of surface wave on turbulence is identified. It is found that the net wave effect is contributed by both the Stokes drift and the correlation between the wave field and the distorted turbulence field. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G28.00010: Breaking of waves in deep water Gerardo Ruiz-Chavarria The breaking of waves is a nonlinear phenomenon during which a fraction of the energy is dissipated. In the previous stage the wave undergoes a growth of its amplitude and the wave pattern is modified in the sense that the crests become more pronounced than the troughs. The breaking has been extensively studied in the case of waves approaching the shore. However, the wave breaking in deep water remains an open problem in fluid dynamics. In this work we study the wave breaking due to focusing of an initially parabolic wave front. To this end the evolution of wave is numerically investigated using a meshless code (Smoothed Particle Hydrodynamics). We present some results about the evolution of waves excited by a parabolic wave maker, among others, the growth induced by the focusing, the behavior around the Huygens' cusp and the process of wave breaking. Then, we compare the numerical results with the criteria given in the literature about the onset of breaking and we discuss how the energy dissipates, for example by the rise of short waves. In addition we compare the numerical results with data obtained in two different experiments made by our team. [Preview Abstract] |
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