Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q01: FreeSurface Flows: Waves 
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Chair: Andrew Jessup, University of Washington Room: 2A 
Tuesday, November 26, 2019 7:45AM  7:58AM 
Q01.00001: Direct Numerical Simulation of Wind Wave Growth Jiarong Wu, Luc Deike The growth of wind wave is still an open question as current models often diverge in both the mechanism postulated and the growth rate predicted. There have been an increasing number of numerical studies on this subject, with the development of various numerical methods and increase in computational power. The difficulty, however, arises from three aspects: the modeling of the turbulent air boundary layer; the response of water to the wind forcing; and the coupling between the two. Separate numerical studies have been done on the first two but are insufficient in revealing the whole picture of wind wave interaction. Using the open source solver Basilisk, which solves the full twophase air water incompressible NavierStokes equations with adaptive grids, we perform fully coupled wind wave simulation to study a range of parameters that factor into the growth model, including the friction velocity of air, initial wave amplitude, surface tension and viscosity, and to improve the current models of wind wave growth. [Preview Abstract] 
Tuesday, November 26, 2019 7:58AM  8:11AM 
Q01.00002: Highresolution DNS of Breaking Waves Wouter Mostert, St\'ephane Popinet, Luc Deike We present bubble and droplet size distributions resulting from breaking ocean waves in deep water, using highresolution threedimensional direct numerical simulation. We use the opensource Basilisk code to simulate the viscous NavierStokes equations in two phases with surface tension at effective resolutions of up to $4096^3$. The interface is represented and advected with a momentumconservative volumeoffluid scheme. The high effective resolutions are made possible with an octree adaptive mesh refinement scheme which is robustly implemented in Basilisk. The wave is initialized in one wavelength with an unstable thirdorder Stokes formulation, which produces local conditions leading to a plunging breaker which entrains air and ejects spray, which are directly resolved by the mesh. Varying the Bond and Reynolds numbers, which control surface tension and viscosity relative to the gravitational and inertial effects respectively, we discuss issues such as bubble breakup in turbulent flow; dimensionality in the transition to turbulence; droplet production and breakup; and numerical grid convergence. [Preview Abstract] 
Tuesday, November 26, 2019 8:11AM  8:24AM 
Q01.00003: Effect of chemical herders on wave breaking Lakshmana Chandrala, Franz O'Meally, Joseph Katz Chemical surfaceactive agents (oil herders) could be used to concentrate oil slicks to facilitate \textit{insitu} burning after an oil spill. The waterinsoluble but oilsoluble surfactants in commercial oil herders accumulate on the airwater interface and might alter the wave breaking process. In this study, the characteristics of mechanically generated breaking waves of varying energies are visualized in clean seawater and water treated with a herder containing 65{\%} Span20 and 35{\%} 2ethyl butanol at a concentration of 5.5 ml/m$^{\mathrm{2}}$. The experiments are performed in a 6x0.3x0.6 m transparent tank and the waves are generated by translating a paddle. Multiple highspeed cameras follow the evolution of both waves. For a plunging breaker in clean water, prior to impact, the wave front contains multiple ripples and small fingers. In contrast, in treated water the wavefront is smooth, resulting in entrainment of a larger volume of air, and deeper subsequent penetration of the bubble cloud. Conversely, for relatively weak spilling breakers, adding the surfactant delays the wave breaking, and dampens the formation of capillary waves on the wave crest. Once breaking occurs, visually, there is no significant difference in the appearance and penetration of the waves. [Preview Abstract] 
Tuesday, November 26, 2019 8:24AM  8:37AM 
Q01.00004: Study of ocean wave field reconstruction and effect of control parameter selection Jie Wu, Xuanting Hao, Lian Shen In this research, we study the reconstruction of ocean wave field by optimizing the initial wave field for wavephaseresolved simulation to best fit measurement data with noise. In reality, the measured data often contains only part of the information needed to reconstruct the entire wave field or is contaminated by noises. We investigate the effect of different control parameter selections on the wave field reconstruction performance using synthetic wave data. The initial condition of the wave field is constructed from the JONSWAP spectrum and its evolution is simulated using the highorder spectral method. Random noises are added to the wave field at each observation time instant to serve as the measurement error. The results show that by incorporating relationship between the free surface elevation and the free surface velocity potential, the error between the reconstructed wave field and the true wave field can be reduced appreciably. This study provides guidance on choosing appropriate control parameters to improve the reconstruction and prediction performance in ocean wave applications. [Preview Abstract] 

Q01.00005: ABSTRACT WITHDRAWN 
Tuesday, November 26, 2019 8:50AM  9:03AM 
Q01.00006: Hysteresis phenomena in gravitycapillary waves on deep water generated by a moving twodimensional/threedimensional airblowing/airsuction forcing Yeunwoo Cho, Beomchan Park Hysteresis phenomena in forced gravitycapillary waves on deep water where the minimum phase speed $c_{\mathrm{min}}=$23cm/s are experimentally investigated. Four kinds of forcings are considered; 2D/3D airblowing/airsuction forcings. For a still water initial condition, as the forcing speed increases from zero towards a certain target speed ($U)$, there exists a certain critical speed ($U_{\mathrm{crit}})$ at which the transition from linear to nonlinear states occurs. When $U$\textless $U_{\mathrm{crit}}$, steady linear localized waves are observed (state I). When $U_{\mathrm{crit}}$\textless $U$\textless $c_{\mathrm{min}}$, steady nonlinear localized waves including steep gravitycapillary solitary waves are observed (state II). When $U\approx c_{\mathrm{min}}$, periodic shedding phenomena of nonlinear localized depressions are observed (state III). When $U$\textgreater $c_{\mathrm{min}}$, steady linear nonlocal waves are observed (state IV). Next, with these stateII, III and IV waves as new initial conditions, as the forcing speed is decreased towards a certain target speed ($U_{\mathrm{final}})$, a certain critical speed ($U_{\mathrm{crit,2}})$ is identified at which the transition from nonlinear to linear states occurs. When $U_{\mathrm{crit,2}}$\textless $U_{\mathrm{final}}$\textless $U_{\mathrm{crit}}$, steeper gravitycapillary solitary waves are observed. When $U_{\mathrm{final}}$\textit{\textless U}$_{\mathrm{crit,2}}$, linear stateI waves are observed. These are hysteresis phenomena which show the dependence of a state on its history starting from different initial conditions. [Preview Abstract] 
Tuesday, November 26, 2019 9:03AM  9:16AM 
Q01.00007: Airinduced axisymmetric sloshing waves on a water surface Utkarsh Jain, Francesco Viola, Detlef Lohse, Devaraj Van Der Meer Here we experimentally study the surface wave trains generated by an oscillating disk placed above a water basin. The harmonic vertical motion of the disk yields a radial, periodic flow of air in the thin gap between the disk and the water. Although the disk is never in contact with the water, the oscillating air pressure in the gap is sufficient to excite a system of standing and travelling capillarygravity waves at the freewater surface. The dynamics of these waves are measured both inside and outside the disk's projection over the free water surface using an inhouse experimental method based on total internal reflection. This allows us to reconstruct the instantaneous free surface elevation in the whole basin. To rationalize our experimental observations, we analytically solve the air flow below the disk using the lubrication equations. The resulting oscillating pressure is then coupled to the water phase through the dynamic condition at the free water surface, which forces axisymmetric waves on the entire liquid surface, for which we solve by means of Hankel transforms. Theoretical calculations and experiments show qualitatively similar behavior, and the wavenumbers measured experimentally are well reproduced by theory over a large frequency range. [Preview Abstract] 
Tuesday, November 26, 2019 9:16AM  9:29AM 
Q01.00008: Generation of Water Waves by Underwater MultiPoint Action Leonardo Gordillo, Juan F. Mar\'in, Isis Vivanco, Bruce Cartwright Wave generation in channels is usually achieved through wavemakers (moving paddles) acting on the surface of water. Although practical for this purpose, wavemakers have issues: they perform poorly in the generation of long waves and create evanescent waves in their vicinity. In this talk, we introduce a framework for wave generation through the action of an underwater multipoint mechanism. We analyze the linear response of waves in a uniform channel in terms of the frequency and wavelength of the bottom action. The system behaves as a longpass filter in space and a highpass filter in time with a sharp resonance limited by viscosity. The framework naturally solves the problem of the performance for long waves and reduces evanescent waves to thin boundary layers at the bottom. We also show that a proper synchronization of an orbital motion on the bottom can produce waves that mimic deep water waves with great accuracy. This last feature has been proved to be useful and efficient to study fluidstructure interaction in simulations based on smoothedparticle hydrodynamics. [Preview Abstract] 
Tuesday, November 26, 2019 9:29AM  9:42AM 
Q01.00009: On the Relationship between the Thermal Signature of the Cooling Foam and the Bubble Plume Dynamics in Breaking Waves Naeem Masnadi, Chris Chickadel, Andrew Jessup This study is motivated by the observation that after a wave breaking event in the ocean, the residual surface foam left in the wake of the breaker rapidly cools down. The relationship between the cooling foam and the characteristics of the breaking wave such as bubble plume dynamics, visible surface foam, and energy dissipation is investigated experimentally. Previous studies have suggested that the decay time of the visible foam can be used to determine the dynamics of the subsurface bubble plume and to estimate the energy dissipation by the breaking process but the foam decay process can be greatly affected by the surfactants concentration in the ocean. We present a new approach that utilizes the thermal signature of the cooling foam to infer the breaking characteristics. The experiments are conducted in a wave flume that is equipped with a pistontype wavemaker and is filled with salt water. Breaking waves are generated using the focusing wavepacket technique and are designed to cover a wide range of slope and breaking intensity. The bubble plume and the surface foam are imaged using visible cameras and the surface temperature is captured using an IR camera. It is found that the onset of cooling of the foam scales with energy dissipation and the slope of the breakers. It is observed that the foam decay time is prolonged by the presence of additional surfactants but the onset of cooling is not significantly affected. [Preview Abstract] 
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