Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A14: Waves: Air-water Surface Waves |
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Chair: Di Yang, University of Houston Room: C125-126 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A14.00001: Experimental study of temporal evolution of waves under transient wind conditions. Andrey Zavadsky, Lev Shemer Temporal variation of the waves excited by nearly sudden wind forcing over an initially still water surface is studied in a small wind-wave flume at Tel Aviv University for variety of fetches and wind velocities. Simultaneous measurements of the surface elevation using a conventional capacitance wave-gauge and of the surface slope in along-wind and cross-wind directions by a laser slope gauge were performed. Variation with time of two components of instantaneous surface velocity was measured by particle tracking velocimetry. The size of the experimental facility and thus relatively short characteristic time scales of the phenomena under investigation, as well as an automated experimental procedure controlling the experiments made it possible to record a large amount of independent realizations for each wind-fetch condition. Sufficient data were accumulated to compute reliable ensemble averaged temporal variation of governing wave parameters. The essentially three-dimensional structure of wind-waves at all stages of evolution is demonstrated. The results obtained at each wind-fetch condition allowed to characterize the major stages of the evolution of the wind-wave field and to suggest a plausible scenario for the initial growth of the wind-waves. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A14.00002: Experimental observation of gravity-capillary solitary waves generated by a moving air-suction Beomchan Park, Yeunwoo Cho Gravity-capillary solitary waves are generated by a moving “air-suction” forcing instead of a moving “air-blowing” forcing. The air-suction forcing moves horizontally over the surface of deep water with speeds close to the minimum linear phase speed $c_{\mathrm{min}}=$23cm/s. Three different states are observed according to forcing speed below $c_{\mathrm{min}}$. At relatively low speeds below $c_{\mathrm{min}}$, small-amplitude linear circular depressions are observed, and they move steadily ahead of and along with the moving forcing. As the forcing speed increases close to $c_{\mathrm{min}}$, however, nonlinear 3-D gravity-capillary solitary waves are observed, and they move steadily ahead of and along with the moving forcing. Finally, when the forcing speed is very close to $c_{\mathrm{min}}$, oblique shedding phenomena of 3-D gravity-capillary solitary waves are observed ahead of the moving forcing. We found that all the linear and nonlinear wave patterns generated by the air-suction forcing correspond to those generated by the air-blowing forcing. The main difference is that 3-D gravity-capillary solitary waves are observed “ahead of” the air-suction forcing, whereas the same waves are observed “behind” the air-blowing forcing. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A14.00003: Experiments on waves under impulsive wind forcing in view of the Phillips (1957) theory Lev Shemer, Andrey Zavadsky Only limited information is currently available on the initial stages of wind-waves growth from rest under sudden wind forcing; the mechanisms leading to the appearance of waves are still not well understood. In the present work, waves emerging in a small-scale laboratory facility under the action of step-like turbulent wind forcing are studied using capacitance and laser slope gauges. Measurements are performed at a number of fetches and for a range of wind velocities. Taking advantage of the fully automated experimental procedure, at least 100 independent realizations are recorded for each wind velocity at every fetch. The accumulated data sets allow calculating ensemble-averaged values of the measured parameters as a function of time elapsed from the blower activation. The accumulated results on the temporal variation of wind-wave field initially at rest allow quantitative comparison with the theory of Phillips (1957). Following Phillips, appearance of the initial detectable ripples was considered first, while the growth of short gravity waves at later times was analyzed separately. Good qualitative and partial quantitative agreement between the Phillips predictions and the measurements was obtained for both those stages of the initial wind-wave field evolution. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A14.00004: Gravity-capillary waves in countercurrent air/water turbulent flow Francesco Zonta, Miguel Onorato, Alfredo Soldati Using the Direct Numerical Simulation (DNS) of the Navier-Stokes equations, we analyze the dynamics of the interface between air and water when both phases are driven by opposite pressure gradients (countercurrent configuration). The Reynolds number ($Re$), the Weber number ($We$) and the Froude number ($Fr$) fully describe the physical problem. We examine the problem of the transient growth of interface waves for different combinations of physical parameters. Keeping $Re$ constant and varying $We$ and $Fr$, we show that, in the initial stages of the wave generation process, the amplitude of the interface elevation grows in time as $t^2/5$. Wavenumber spectra, $E(kx)$, of the surface elevation in the capillary range are in good agreement with the prediction of the Wave Turbulence Theory. Finally, the wave-induced modification of the average wind and current velocity profiles is addressed. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A14.00005: Wind-wave generation using interface tracking Espen Akervik The wind-wave generation process in a periodic open channel is studied by means of Large Eddy Simulation, using the Volume of Fluid method to track the interface. The coupled system is initiated by imposing a turbulent air flow at $Re_\tau=395$ on top of water at rest. Surface tension effects are excluded and the Froude number is chosen so as to fit equilibrium slow moving waves inside the computational domain. In the initial stage, the surface deformation consists of streamwise elongated narrow structures. These may be seen as footprints of the near wall streaks in the turbulent air flow. This phase is associated with linear growth in amplitude, and the behavior of the air flow is largely unaffected by the surface deformations. In the second stage, localized slow moving ($c/u_\tau<10$) wave packets appear, and the air flow becomes linked to the waves. This phase is associated with exponential growth of the waves. In the third stage, non-linear interactions occur, resulting in redistribution of energy. The growth rates are compared to previous simulations and theoretical results. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A14.00006: Numerical investigation of energy transfer in coupled wind and wave system Xuanting Hao, Lian Shen Energy transfer in the wind-wave system is one of the key physical processes in air-sea interactions. In open oceans, the wind input, nonlinear wave interaction, and wave dissipation are three key mechanisms of energy transfer that govern the wave field evolution. In this study, we conduct a series of high-fidelity numerical experiments using dynamically coupled large-eddy simulation for the turbulent wind and high-order spectral simulation for the waves. By directly analyzing the wave statistics data obtained from our wave-phase-resolved deterministic simulations, we monitor the spectral evolution of the wave field. We observe the frequency downshift phenomenon and the self-similarity of the wind-forced nonlinear wave field throughout the numerical experiment. Further analysis quantifies the wind input, nonlinear wave interaction, as well as wave dissipation. The nonlinear wave interaction is found to be dominant over the wind input and wave dissipation locally in the spectral domain despite its overall energy-conserving property. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A14.00007: Wave-interference Effects in the Presence of a Shear Current Yan Li, Simen Ellingsen, Francis Noblesse Wave-interference effects, based on a 2-point wavemaker model of monohull ships, are analysed when a shear current of uniform vorticity is present. Indeed, wave interferences in the presence of a shear current, similar to the cases in finite water depth, are considerably more complicated than in deep water without vorticity. The effects of a shear current on far-field waves that are formed by 2-point wavemaker models greatly depend on the shear Froude number VS/g, where V is the speed of the ship, S is the uniform vorticity of the shear, g is the gravitational acceleration, as well as the angle between the ship's motion direction and the shear current. Various circumstances, under which ray angles of the highest waves that are associated with constructive interferences between waves are much narrower than the wake angles of the cusps or the asymptotes of wave patterns formed by Kelvin's classical 1-point wavemaker, are shown. In particular, cusp shear Froude numbers Frscusp where ray angles of the highest waves are equal to the cusp angles are determined. As for shear Froude numbers VS/g that are larger than Frscusp, the apparent angles where highest waves are found are significantly smaller than the cusp/asymptote angles. Furthermore, the asymmetry due to the presence of a shear current results in remarkable differences between the cases where a ship moves upstream or downstream. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A14.00008: Shear turbulence, Langmuir circulation and scalar transfer at an air-water interface Amine Hafsi, Andres Tejada-Martinez, Fabrice Veron DNS of an initially quiescent coupled air-water interface driven by an air-flow with free stream speed of 5 m/s generates gravity-capillary waves and small-scale (centimeter-scale) Langmuir circulation (LC) beneath the interface. In addition to LC, the waterside turbulence is characterized by shear turbulence with structures similar to classical ``wall streaks'' in wall-bounded flow. These streaks, denoted here as ``shear streaks'', consist of downwind-elongated vortices alternating in sign in the crosswind direction. The presence of interfacial waves causes interaction between these vortices giving rise to bigger vortices, namely LC. LES with momentum equation augmented with the Craik-Leibovich (C-L) vortex force is used to understand the roles of the shear streaks (i.e. the shear turbulence) and the LC in determining scalar flux from the airside to the waterside and vertical scalar transport beneath.~ The C-L force consists of the cross product between the Stokes drift velocity (induced by the interface waves) and the flow vorticity. It is observed that Stokes drift shear intensifies the shear streaks (with respect to flow without wave effects) leading to enhanced scalar flux at the air-water interface. LC leads to increased vertical scalar transport at depths below the interface. [Preview Abstract] |
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