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 S32: Geophysical Fluid Dynamics Air-Sea Interaction |
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Chair: Jim Riley, University of Washington Room: 614 |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S32.00001: Space-time characteristics of wind turbulent pressure field at wave surface Xuanting Hao, Lian Shen The wind pressure field at the wave surface, with fluctuations several orders of magnitude smaller than the mean atmospheric pressure, has a notable impact on the wave energy growth. The time-variant and irregular wave field poses great challenge to the full space-time measurement of the surface pressure. We present a simulation-based study of the space-time characteristics of the surface pressure field. The numerical tool is a combined turbulence solver and wave-phase-resolved model, where the wind turbulence is simulated with large-eddy simulation and the nonlinear wave field simulated using a high-order spectral method. The joint probability distribution of the surface pressure and the vertical surface velocity is found to resemble that of turbulent flows over a compliant wall. We also conduct spectral analysis on the surface pressure field to obtain the wavenumber-frequency spectrum. While the turbulent pressure fluctuations have a convection velocity comparable to turbulent flows over a fixed flat wall, a distinct wave effect is identified. The wavenumber/frequency peak of the wave-coherent pressure field is found to deviate from that of the wave field, suggesting nonlinearity in the turbulence-wave interactions. [Preview Abstract] |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S32.00002: Effect of the presence of long waves on the wave growth rate of short waves Tao Cao, Lian Shen We performed a DNS study of the turbulent wind over two slowly progressive water waves with different wave lengths. It is shown that the presence of the long wave results in a significant reduction of the form drag on the short wave. To explain the underlying physical mechanism, we obtained the momentum equations for the wave-coherent motions induced by the long wave and the short wave, respectively. We first showed that the asymmetric wave-induced pressure across the surface wave crest is responsible for the form drag on the wave surface and is mainly caused by the wave-induced advection associated with the vertical wave-induced velocity that is in-phase with the surface elevation. It is further found that in the presence of the long wave, the wave-induced advection corresponding to the short wave is significantly reduced, which results in a smaller asymmetric pressure on the short wave, and thereby a lower form drag. Moreover, we observed that the extent of the reduction of the form drag on the short wave depends on the wave age of the long wave. The smaller the wave age of the long wave, the larger the reduction of the form drag on the short wave. [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S32.00003: Numerical simulations of the turbulent flow over an ocean wavefield Devin Conroy, Laura Brandt, James Rottman The air-sea dynamics for wind driven ocean waves are crucial for understanding and predicting the weather and global climate, as well as for predicting forces on ships and offshore wind farms for design and operation. The wind-wave problem has been an active area of research for many years, spanning observational, experimental, theoretical, and numerical studies. Here we leverage the Large Eddy Simulation technique with a sub-grid scale sea surface roughness model for the interfacial stress and an interface capturing Volume-of Fluid method to obtain highly accurate phase resolved simulations over a sufficiently large domain size. We investigate the dynamics of the air-sea interaction, starting with a JONSWAP spectrum, to understand the generation of turbulence, wind-wave growth, non-linear surface dynamics, and wave breaking on the overall energy budget. We find that the wind stress tends to add energy at small length scales ($<$ O(1m)), due to the surface shear and turbulent pressure fluctuations in the air above the surface. Preliminary results also indicate that under strong wind forcing (or low wave age), air separation occurs near wave crests that later undergo wave breaking. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S32.00004: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2019 11:23AM - 11:36AM |
S32.00005: A kinetic approach to estimate air-sea exchanges driven by sea spray in high winds. Fabrice Veron, Luc Mieussens Sea-spray is known to be a fundamental component of air-sea heat flux in high wind speed conditions where water drops are frequently ejected from the sea surface because of breaking waves and breaking related phenomena such as bubble entrainment and whitecaps. Once ejected from the ocean surface, these drops are transported and dispersed in the Atmospheric Boundary Layer (ABL) where they interact and exchange momentum, heat, and moisture with the ambient atmosphere. However, understanding of these spray fluxes pathways, and our ability to model them remains limited. In this work, we borrow the framework from established kinetic gas theory, and apply these mathematical tools to model the transport of spray droplets and the exchanges of heat, momentum, and moisture between the drops and the atmosphere. Within this framework, one of the most vexing component of this spray flux problem, i.e. the feedback from the drops on the atmosphere, is relatively straightforward to account for. This work is largely exploratory and in the early stages of development. We will present an overview of the approach as well as preliminary results. [Preview Abstract] |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S32.00006: An Experimental Study of Droplet Generation by Plunging Breaking Water Waves Martin A. Erinin, Sophie Wang, Ren Liu, David Towle, Xinan Liu, James H. Duncan The production of droplets by strong, moderate, and weak plunging breakers is studied experimentally in a laboratory. The water waves are generated mechanically using a dispersively focused wave packet technique with an average wave packet frequency of 1.15 Hz for all three waves. Surface profile histories of the breaking wave crests are measured using a cinematic laser-induced fluorescence technique. The temporal evolution of the phase averaged surface profile of the breaker, obtained from 10 runs for each wave, are used to characterize the breakers. Droplets are measured using a cinematic digital in-line holographic system positioned at 28 streamwise locations along a horizontal plane (herein called the measurement plane) that is 1 cm above the maximum wave crest height. The droplet radii (r $ \geq 100$ $\mu$m), positions and trajectories are determined from the holograms. Counting only the droplets that are moving up across the measurement plane, the spatio-temporal distribution of droplet generation by the breakers is obtained. Droplet statistics including total number, mean diameter and speed are presented. The relative importance of the various droplet generation mechanisms in the three waves are discussed and correlated with the mean wave profile characteristics. [Preview Abstract] |
Tuesday, November 26, 2019 11:49AM - 12:02PM |
S32.00007: Size Distributions and Dynamics of Sea Spray Droplets Produced by Breaking Waves in Various Wind Speeds Schuyler Moss, Robert Jaquette, Fabrice Veron Sea spray is thought to be an essential component of the total air-sea fluxes of momentum, heat, and moisture. While exchange rates for momentum, heat and moisture for suspended water droplets interacting with their ambient surroundings are generally well understood, spray-induced air-sea fluxes particularly from large spume droplets remain hindered by uncertainty associated with the generation (i.e. fluxes) of drops at the air-sea interface. Size and velocity distributions of sea spray droplets generated by mechanical breaking waves in various wind speeds were captured utilizing a multicolored double exposure particle tracking velocimetry experimental setup. Repeated trials at incremental distances downstream from the breaking location provided the size and velocity distributions necessary for estimating droplet surface fluxes. Results from these trials indicate a correlation between longer droplet lifetimes, and increased air velocity. Possible explanations for this relationship include increased ejection velocities or extended suspension periods induced by the airflow. However, a complete understanding of spume droplet ejection velocities remains unresolved and are thus an integral component for further insight into droplet dynamics and exchange fluxes. [Preview Abstract] |
Tuesday, November 26, 2019 12:02PM - 12:15PM |
S32.00008: Large-eddy simulation (LES) of small-scale Langmuir circulation and scalar transport Andres Tejada-Martinez, Amine Hafsi, Cigdem Akan, Fabrice Veron Previous laboratory experiments have revealed that small-scale (centimeter-scale) Langmuir circulation (LC) beneath an initially quiescent air-water interface appear shortly after the initiation of wind-driven gravity-capillary waves and provide the laminar-turbulent transition in wind speeds between 3 and 6 m/s. LES reveals Langmuir turbulence characterized by multiple scales ranging from small bursting eddies at the surface that coalesce to give rise to larger (centimeter-scale) LC over time. The growing LC scales lead to increased vertical scalar transport at depths below the interface and thus greater scalar transfer efficiency. Simulations were performed with a fixed wind stress corresponding to a 5 m/s wind speed but with different wave forcing parameters. It is observed that longer wavelengths lead to more coherent, larger centimeter-scale LC providing greater contribution to the turbulent vertical scalar flux. In all cases, the molecular diffusive scalar flux at the water surface relaxes to the same statistically steady value after transition to Langmuir turbulence occurs, despite the different wave forcing across the simulations. [Preview Abstract] |
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