Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R31: Waves: Surface Waves |
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Chair: Mohammad-Reza Alam, University of California - Berkeley Room: 312 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R31.00001: The Wave Carpet: An Omnidirectional and Broadband Wave Energy Converter M.-Reza Alam Inspired by the strong attenuation of ocean surface waves by muddy seafloors, we have designed, theoretically investigated the performance, and experimentally tested the ``Wave Carpet:'' a mud-resembling synthetic seabed-mounted mat composed of vertically-acting linear springs and generators that can be used as an efficient wave energy absorption device. The Wave Carpet is completely under the water surface hence imposes minimal danger to boats and the sea life (i.e. no mammal entanglement). It is survivable against the high momentum of storm surges and in fact can perform even better under very energetic (e.g. stormy) sea conditions when most existing wave energy devices are needed to shelter themselves by going into an idle mode. In this talk I will present an overview of analytical results for the linear problem, direct simulation of highly nonlinear wave fields, and results of the experimental wave tank investigation. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R31.00002: Cloaking water waves via an elastic buoyant carpet Ahmad Zareei, Mohammd-Reza Alam We propose a cylindrical cloak for gravity waves passing through an elastic floating carpet. This is achieved by a spatially variable flexural rigidity while mass density and water depth are kept constant. The cloak is deduced from transformation media scheme and coordinate transformation of the coupled governing equation for the buoyant carpet and the fluid underneath. The major challenge is that the governing equation is not form-invariant; while transformation media scheme requires a form-invariant governing equation. We approximate the governing equation with a form-invariant equation which is exact for a homogeneous and isotropic floating carpet. We compare the results with the solution of the exact governing equation and show the scattering waves of the cylinder are significantly suppressed, hence cloaking is achieved. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R31.00003: DNS of scalar transfer across an air-water interface during inception and growth of Langmuir circulation Amine Hafsi, Andres Tejada-Martinez, Fabrice Veron, Yi Ma Upon a blowing of a wind over an initially quiescent air-sea interface, first short capillary waves are generated which in time coexist with longer waves as part of a broad spectrum of waves. The interaction between the Stokes drift velocity induced by surface gravity waves and the mean current induced by surface wind stress leads to Langmuir turbulence (LT) characterized by Langmuir circulation (LC) consisting of parallel downwind-elongated, counter rotating vortices roughly aligned in the direction of the wind. The typical length scale of LC ranges from several centimeters when short capillary waves first appear up to tens of meters when the spectrum of waves broadens. Results are presented from direct numerical simulation (DNS) of an initially quiescent coupled air-water interface driven by an air flow with free stream speed of 5 m/s. Cases with a freely deforming interface (characterized by gravity-capillary waves giving rise to small-scale LC) and with the interface intentionally held fixed (i.e. without LC) will be compared to understand the mechanisms by which the LT enhances scalar transfer from the airside to the waterside and bulk concentration throughout the water column. Time-permitting, we will compare our results with available laboratory physical experiments. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R31.00004: Laboratory measurements of the inception and evolution of Langmuir Turbulence Yi Ma, Fabrice Veron, Andres Tejada-Martinez, Amine Hafsi When wind starts to blow over a quiescent air-sea interface, both currents and surface waves are initially generated. The interaction between the wind-driven waves and currents leads to the generation of Langmuir circulation (LC) consisting of counter rotating vortices aligned with the wind. Shortly thereafter, Langmuir turbulence (LT), that is multiple scales of LC, appear. In LT, length scales range from several centimeters when short capillary waves first appear up to tens of meters when the spectrum of waves broadens. We present results from a laboratory experiments where the evolution of the air-water interface starting from rest and the accompanying development of centimeter-scale Langmuir turbulence is investigated. We present surface infrared imagery and subsurface Particle Image Velocitmetry. We show that evolution of from organized small scale LC to LT is very rapid leading to intense surface mixing whereby momentum initially transferred to the surface through viscosity efficiently mixes the near surface layers. Subsurface turbulence measurements are presented in the context of scalar (gas) flux through the air-water interface. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R31.00005: Influence of wave age on the structure of the airflow above surface waves. Marc Buckley, Fabrice Veron The role of the surface waves on the airflow dynamics is known to be significant but our physical understanding remains incomplete. In this talk, we present detailed airflow measurements taken in the laboratory for 17 different wind-wave conditions with wave ages $C_p/u_*$ ranging from 1.4 to 66.7. For these experiments, a combined Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) technique was developed. Two-dimensional airflow velocity fields were obtained as low as 100 $\mu$m above the air-water interface. When the wind stress is too weak to generate surface waves, the mean velocity profile follows the law of the wall. With waves present, turbulent structures are directly observed in the airflow, whereby low horizontal velocity air is ejected away from the surface, and high velocity fluid is swept downward. Airflow separation is observed above young wind waves ($C_p/u_* < 3.7$) and the resulting spanwise vorticity layers detached from the surface, produce intense wave coherent turbulence. On average, the airflow is sheltered downwind of wave crests, above the critical height (defined by $U(z_c) = C_p$). Below $z_c$, the coupling of the airflow with the waves causes a reversed, upwind sheltering effect. Finally, we also show preliminary field measurements. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R31.00006: Surface waves in a square container due to its resonant horizontal elliptic motion Mitsuaki Funakoshi, Ai Hiramitsu Surface waves in a square container due to its resonant horizontal elliptic or linear motion are investigated theoretically. The motion of the container is characterized by the ratio, expressed as $\tan{\phi}$, of the length of the minor axis to the length of the major axis of its elliptic orbit, and by the angle $\theta$ between the directions of the major axis and one of its sidewalls. Using the reductive perturbation method, nonlinear time evolution equations for the complex amplitudes of two degenerate modes excited by this motion are derived with the inclusion of linear damping. When $\tan{\phi}$ is small, for any $\theta$ these equations have two kinds of stable stationary solutions corresponding to regular co-rotating waves whose direction of rotation is the same as that of the container, and regular counter-rotating waves of the opposite direction of rotation. As $\tan{\phi}$ increases to one, the region of forcing frequency in which stable regular counter-rotating waves are observed shrinks and then disappears for any $\theta$. Solutions with chaotic or periodic slow variations in amplitude and phase of excited surface waves are also obtained for forcing frequencies where no stable stationary solutions exist. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R31.00007: PTV measurements of Lagrangian particle transport by surface gravity wave groups Ton van den Bremer, Colin Whittaker, Alison Raby, Paul Taylor We present detailed PTV (particle tracking velocimetry) measurements of the Lagrangian transport and trajectories of neutrally buoyant particles underneath two-dimensional surface gravity wave groups in a laboratory flume. By focussing our attention on wave groups of moderate steepness, we confirm the predictions of standard second-order multi-chromatic wave theory, in which the body of fluid satisfies the potential flow equations. Particles near the surface are transported forwards and their motion is dominated by Stokes drift. Particles at sufficient depth are transported backwards by the Eulerian return current that was first described by Longuet-Higgins \& Stewart (1962) and forms an inseparable counterpart of Stokes drift for surface wave groups ensuring the (irrotational) mass balance holds. Finally, we provide experimental validation of a simple scaling relationship, derived based under the assumption of separation of scales, for the transition depth: the depth above which Lagrangian particles are transported forwards by the Stokes drift and below which such particles are transported backwards by the return current. We present results for a range of effective water depths. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R31.00008: Measurements of wind-waves under transient wind conditions. Lev Shemer, Andrey Zavadsky Wind forcing in nature is always unsteady, resulting in a complicated evolution pattern that involves numerous time and space scales. In the present work, wind waves in a laboratory wind-wave flume are studied under unsteady forcing`. The variation of the surface elevation is measured by capacitance wave gauges, while the components of the instantaneous surface slope in across-wind and along-wind directions are determined by a regular or scanning laser slope gauge. The locations of the wave gauge and of the laser slope gauge are separated by few centimeters in across-wind direction. Instantaneous wind velocity was recorded simultaneously using Pitot tube. Measurements are performed at a number of fetches and for different patterns of wind velocity variation. For each case, at least 100 independent realizations were recorded for a given wind velocity variation pattern. The accumulated data sets allow calculating ensemble-averaged values of the measured parameters. Significant differences between the evolution patterns of the surface elevation and of the slope components were found. Wavelet analysis was applied to determine dominant wave frequency of the surface elevation and of the slope variation at each instant. Corresponding ensemble-averaged values acquired by different sensors were computed and compared. Analysis of the measured ensemble-averaged quantities at different fetches makes it possible to identify different stages in the wind-wave evolution and to estimate the appropriate time and length scales. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R31.00009: Luneburg modified lens for surface water waves Helene Pichard, Agnes Maurel, Phillipe Petitjeans, Paul Martin, Vincent Pagneux It is well known that when the waves pass across an elevated bathymetry, refraction often results in amplification of waves behind it. In this sense, focusing of liquid surface waves can be used to enhance the harvest efficiency of ocean power. An ocean wave focusing lens concentrates waves on a certain focal point by transforming straight crest lens of incident waves into circular ones just like an optical lens. These devices have attracted ocean engineers and are promising because they enable the effective utilization of wave energy, the remaining challenge being to increase the harvest efficiency of the lens. In this work, in order to improve well known focusing of surface liquid waves by lens, the propagation of liquid surface waves through a Luneburg modified lens is investigated. The traditional Luneburg lens is a rotationally symmetric lens with a spatially varying refractive-index profile that focuses an incident plane wave on the rim of the lens. The modified Luneburg lens allows to choose the position of the focal point, which can lie inside or outside the lens. This new degree of freedom leads to enhanced focusing and tunable focusing. The focusing of linear surface waves through this lens is investigated and is shown to be more efficient than classical profile lenses. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R31.00010: Surfboard Dynamics Eline Dehandschoewercker, David Quere, Christophe Clanet There are two main phases in surfing : catching and riding the wave. Field observations reveal that the board shape and mass distribution play a major role in both phases. To understand and optimize wave transport, we have developed an experimental setup that allows us to control the different physical parameters. A wave-maker generates either propagating periodic waves (whose wavelength and amplitude are accurately controlled) or breaking waves (whose height and velocity can be changed). Balsa boards (with different aspect ratios, mass distributions and submerged volumes) are used as floating bodies with variable friction on the water waves. We thus study the motion of boards towed onto waves. We first focus on the conditions needed for the board to be captured by the wave. We also determine the surfboard dynamics by using a force sensor responsive to the propulsive force of the wave on the surfboard. Finally, we show that the main parameters that affect this force are the slope of the wave and the shape of the board. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R31.00011: Resonant triad interactions of acoustc--gravity waves Usama Kadri, T.R. Akylas Surface--acoustic wave disturbances in water of constant depth over a rigid bottom, due to the combined action of gravity and compressibility, are studied. In the linear theory, apart from free-surface (gravity) waves, there is also a countable infinity of acoustic (compression) modes. As the sound speed in water, typically, far exceeds the maximum gravity wave phase speed, these two types of modes feature vastly different spatial and/or temporal scales, and their linear coupling is weak. It is possible, however, to realize significant energy exchange between gravity and acoustic waves via nonlinear interactions. This scenario is analyzed for resonant wave triads that comprise two counter-propagating gravity waves and a long-crested acoustic mode. Owing to this disparity in length scales, the interaction time scale as well as the form of the amplitude evolution equations differ from those of a standard resonant triad. In the case of a perfectly tuned triad of uniform monochromatic wave trains, nearly all the energy initially in the gravity waves can be transferred to the acoustic wave. This mechanism, however, is less efficient when the interacting waves are modulated wavepackets. [Preview Abstract] |
Tuesday, November 24, 2015 3:13PM - 3:26PM |
R31.00012: Quantifying wave-breaking dissipation using nonlinear phase-resolved wave-field simulations with a phenomenological-based wave breaking model Yusheng Qi, Dick Yue We use direct nonlinear phase-resolved simulations based on a High-Order Spectral (HOS) method (Dommermuth {\&} Yue 1987) to understand and quantify wave-breaking dissipation in the evolution of general irregular short-crested wave-fields. We achieve this by incorporating a robust phenomenological-based wave breaking model in HOS simulations to account for energy dissipation. This model can automatically simulate the onset of wave breaking, and the simulated wave-breaking dissipation strength differentiates corresponding to different wave breaking type (such as spilling or plunging breaking waves). The efficacy of this model is confirmed by direct comparisons against measurements for the energy loss in 2D and 3D breaking events. By comparing simulated wave-fields with and without the dissipation model in HOS, we obtain the dissipation field, which provides the times, locations and intensity of wave breaking events. From the dissipation field we further calculate the distribution of total length of breaking wave front per unit surface area per unit increment of breaking velocity (Phillips 1985), and obtain qualitative agreement with Phillips theoretical power-law. [Preview Abstract] |
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