Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session J25: Geophysical Fluid Dynamics: Air-Sea Interaction II |
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Chair: Aditya Aiyer, Lehigh University Room: 251 C |
Sunday, November 24, 2024 5:50PM - 6:03PM |
J25.00001: Momentum and energy fluxes in wind-forced breaking waves at high wind speeds Nicolo Scapin, Jiarong Wu, J. Thomas Farrar, Bertrand Chapron, Stephane Popinet, Luc Deike We perform direct numerical simulations (DNS) of wind-forced breaking waves by solving the two-phase Navier-Stokes equations. The turbulent upper airflow drives the growth of a narrowband wave field, with wave amplitude increasing until breaking occurs. After the breaking stage, the waves continue to grow under wind forcing. We examine these cycles in the high-wind speed regime, characterized by the ratio of friction velocity to wave speed $u_\ast/c$ in the range [0.3−0.9]. We analyze the momentum and energy fluxes exchanged among the airflow, wave field, and water currents. The total momentum flux is mainly governed by the pressure contribution, which increases with $u_\ast/c$ in the pre-breaking stage and suddenly reduces during breaking stage. These effects balance, leading to a saturation of the pressure force over a breaking cycle, similar to the drag force saturation at high wind speeds observed in experiments and fields observations. The water column's energy budget shows the pressure input's dominance during wave growth and dissipation during breaking. These observations align with scaling laws for wave growth [1] and breaking-induced dissipation [2]. |
Sunday, November 24, 2024 6:03PM - 6:16PM |
J25.00002: Measurements of sub-surface turbulence beneath laboratory wind-driven waves Daniel Ruth, Pim Bullee, Matteo Clementi, Raffael Meier, Claudio Mucignat, Filippo Coletti We explore the characteristics of the turbulence beneath young wind-driven surface waves. We leverage a wind-wave tunnel in which the ratio between the air-side friction velocity and wave phase speed varies between 0.8 and 1.5. Specifically, we perform simultaneous particle image velocimetry (PIV), which provides the sub-surface velocity field, and planar laser-induced fluorescence (PLIF), which provides the overhead water surface position and enables the inference of the wave-induced orbital velocity. In one experimental configuration, two-dimensional PIV and PLIF are performed in a streamwise vertical plane, resolving the wind-induced drift and the two primary components of the orbital motion; in a second configuration, three-component PIV is performed in a vertical plane normal to the flow, resolving the wave-coupled structures characteristic of the wind drift layer. |
Sunday, November 24, 2024 6:16PM - 6:29PM |
J25.00003: Wind-generated waves on a water layer of finite depth: A linear stability study Ramana Patibandla, Yashodhan Kadam, Amit Tandon, Anubhab Roy In this work, we study the linear stability of an air-water two-phase system. Motivated by recent experiments, we consider a quadratic velocity profile in the finite-depth water layer and an exponential velocity profile in the air layer. In the inviscid stability problem, it is known that the exponential velocity profile is amenable to analytical calculation. In this work, we provide an analytical solution to the Rayleigh equation with the quadratic velocity profile in terms of spheroidal wave functions. A comparison with recent experiments shows a good match with the rippling instability (the instability due to shear flow in the water layer) growth rates, indicating that rippling instability is more important than the Miles instability (the instability due to shear flow in the air layer) in parameter regimes corresponding to the experiment. Further, we provide analytical expressions for the neutral stability curve and long-wave limit growth rate asymptote. We perform a viscous stability study to support the inviscid stability analysis results. Interestingly, it reveals that the energy contribution to the viscous versions of the instability is different than the inviscid ones (rippling and Miles instabilities). |
Sunday, November 24, 2024 6:29PM - 6:42PM |
J25.00004: Evaluating the efficacy of wind-wave interaction models to calculate wave-induced stresses for "wave-modeled" Large Eddy Simulation Aditya K Aiyer The sea-state is characterized by a combination of wind-driven waves and long-wavelength swell waves, both interacting with the wind to exchange energy and momentum. Recently, Aiyer et al. (2023,2024) introduced a wall-modeling framework to calculate drag from the interaction of airflow with generalized moving surfaces, focusing on the pressure (form) drag with the effects of long-wavelength swell waves empirically parameterized using a phase-averaged approach. However, kinematic effects related to the vertical motions of the wave surface remains a challenge to resolve for wall-modeled Large-eddy Simulation (LES) due to the absence of vertical surface motions, leading to an underestimation of wave-induced stresses. This study evaluates the suitability of four wind-wave models: Miles' (1959) critical layer model, Townsend's (1980) rapid-distortion model, Belcher's (1998) non-separated sheltering model, and Kudryavtsev et al.'s (2014) wind-over-waves model for use in LES. Each model calculates wave-induced pressure fluctuations (causing form drag through pressure-slope correlations) and/or wave-induced stresses. We discuss the applicability and implementation strategies of each model within the context of wall-modeled Large-Eddy Simulation. |
Sunday, November 24, 2024 6:42PM - 6:55PM |
J25.00005: The effect of mild wind on droplet generation in a plunging breaker Chang Liu, James H Duncan, Xinan Liu An experimental study of droplet generation in a deepwater plunging breaker in the presence of initially still air (case Water) and mild wind in the wave propagating direction with two mean free stream speeds, 1.5 m/s (case W150) and 3.0 m/s (case W300), is presented. The breakers are generated by a programmable wave maker that is set with a single motion profile that produces a highly repeatable dispersively focused 2D wave packet with a central frequency of f0 = 1.15 Hz and a corresponding wavelength of λ0 = 1.18 m (by linear theory). The wave profiles from the time of (plunging) jet formation to the time of jet impact are measured using a high-speed laser-induced fluorescence (LIF) imaging technique. The droplets produced are measured with an in-line cinematic holographic system operating with measurement volumes that span the width of the tank. The positions, diameters (d ≥ 100 μm), times and velocities of droplets are measured as they move up across a prescribed horizontal measurement plane. The LIF measurements demonstrate strong effects of mild winds on the geometry and dynamical behaviour of the pre-breaking wave profiles. In the droplet measurement, it is found that mild winds significantly modify the overall number and the diameter and velocity distributions of the droplets associated with each of the three droplet production mechanisms, which were distinguished in a typical plunging breaker in the work of Erinin et al., J. Fluid Mech., vol.967, 2023, A36. |
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