Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session Q29: Geophysical Fluid Dynamics: Air-Sea Interactions |
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Chair: Luc Deike, Princeton Room: North 229 A |
Tuesday, November 23, 2021 8:00AM - 8:13AM |
Q29.00001: A viscous curvilinear model for turbulent wind-wave interaction. Part 1. Application to fast wave condition. Lian Shen, Tao Cao The problem of turbulent wind and wave interactions is challenging to model because of the complexities of the turbulence stress in the wind field. However, inspired by the data of large-eddy simulation of airflow over water waves and through developing a linear analysis framework for the wave boundary layer, we discovered that under certain conditions the interactions between the turbulent wind and waves can be explained by a linear theory without pursuing turbulence stress models. In this presentation, we report a study of one of such scenarios, wind over fast-propagating water waves. It is found that the fast wave effects on the airflow can be fully explained by a viscous curvilinear model that we recently developed. By further examining the split equations for the viscous curvilinear model, we elucidate in detail the physical mechanisms underlying the fast wave effects on the airflow and the wind-wave momentum exchange at the water surface. |
Tuesday, November 23, 2021 8:13AM - 8:26AM |
Q29.00002: A viscous curvilinear model for turbulent wind-wave interaction. Part 2. Application to opposing wave condition. Tao Cao, Lian Shen In this presentation, we report a scenario of wind-wave interactions, namely wind blowing over opposing water waves, where a viscous curvilinear model that we developed recently is applicable and the main features of the complex turbulent wind and opposing wave interaction problem can be explained through a linear analysis framework. We found that the dominant components of opposing wave effects on the overlying airflow exhibit a quasilinear behavior and can be explained by the viscous curvilinear model. Although the weak components of opposing wave effects are affected by the turbulence stress in the air, they do not change the structure of the opposing wave-induced airflow appreciably owing to their small magnitudes. The results of the viscous curvilinear model agree well with our large-eddy simulation, and the wave attenuation rates quantified using the form drag agree with the experiment results in the literature. |
Tuesday, November 23, 2021 8:26AM - 8:39AM |
Q29.00003: On the statistics of jet drops produced by breaking waves Luc Deike, Brandon Reichl, Alexis Berny, Thomas Seon, Stephane Popinet Bubbles bursting at the ocean surface are an important source of sea-spray aerosol. We describe jet drop production, from ensembles of high fidelity numerical simulations of bubble bursting, validated against experimental results. The number of jet drops, their size, and velocity are controlled by the ratio of the bubble size, and the visco-capillary length. We compute the distribution of jet drops formed by a range of bubbles present under a breaking wave which compares well against laboratory experiments. Next, we discuss the applicability of the proposed formulation, in the context of sea spray generation function, by integrating the drop production by a single breaker with the statistics of breaking waves described by the distribution of length of breaking crest. This framework is tested using a state-of-the-art spectral wave model WAVEWATCH-3 leading to a sea-state dependent sea spray generation which compares well with existing field measurements and rationalize some of the scatter in the data. |
Tuesday, November 23, 2021 8:39AM - 8:52AM |
Q29.00004: Laboratory experiments on wind-wave breaking dynamics and the associated drops, bubbles, and underwater turbulence Megan T Mazzatenta, Martin A Erinin, Baptiste Neel, Daniel Ruth, Robert D Jaquette, Fabrice Veron, Luc Deike Breaking waves influence the transfer of mass and momentum at the air-sea interface through air bubble entrainment and droplet production, which link the ocean and the atmosphere and affect the climate. To improve understanding of how wind and wave conditions affect entrainment and spray generation, an experiment was conducted at the University of Delaware Air-Sea Interaction Laboratory. Waves were produced by combining wind forcing at free-stream velocities of 9.7, 10.7, and 11.7 m/s with mechanically-generated waves with central frequencies of 1.0, 1.2, 1.4, 1.6, and 1.8 Hz and sidebands of ±0.05 Hz, in addition to wind-only forcing. Bubble entrainment in the bulk was captured with planar and stereoscopic shadowgraph techniques, and laser doppler velocimetry was used to characterize the state of underwater turbulence. On the surface, wave gauges and two white-light methods captured the wave state and surface bubbles. Measurements of droplets were made using an in-line holographic system and a shadowgraph technique with a telecentric lens. We discuss this experimental setup and the joint measurements of wind, wave, underwater turbulence, bubble entrainment, and drop generation. |
Tuesday, November 23, 2021 8:52AM - 9:05AM |
Q29.00005: Spray Generation by Naturally and Mechanically Forced Wind-Waves Martin A Erinin, Baptiste Neel, Daniel Ruth, Megan T Mazzatenta, Robert D Jaquette, Fabrice Veron, Luc Deike The production of spray droplets by wind-waves is studied in laboratory experiments at the University of Delaware's Air-Sea Interaction Laboratory. Wind-waves are produced at free-stream wind speeds of 9.7, 10.7, and 11.7 m/s with the addition of mechanically forced waves with central frequencies of 1.0, 1.2, 1.4, 1.6 and 1.8 Hz and sidebands at +/- 0.05 Hz as well as a case with wind only forcing. Drop positions and diameters (d > 60 μm) are measured for each experimental condition and over multiple wave periods using an in-line holography system positioned 6.4 cm above the mean water level and at a fetch of approximately 23 meters. Droplet statistics including total number, mean diameter, and size distributions are reported at the different experimental conditions. The relative importance of spray production mechanisms is discussed and correlated to wave-field characteristics including the frequency of breaking events and the wind-wave properties. |
Tuesday, November 23, 2021 9:05AM - 9:18AM |
Q29.00006: Production of Large Marine Aerosols from Wind Forced Mechanical Breaking Waves Robert D Jaquette, Fabrice Veron Due to a lack of observations associated with sea spray generation (i.e. fluxes) at the air-sea interface, inertial droplets' role in the total spray-induced air-sea fluxes remains unclear. While these inertial droplets carry a significant amount of energy, they typically have short residence times, and remain within a significant wave height of the ocean's surface. As a result, these droplets may not reach equilibrium, further contributing to the uncertainty associated with total spray-induced air-sea fluxes. Experimental work conducted at the University of Delaware's Air Sea Interaction Laboratory studied inertial droplet size distributions and ejection velocities. Breaking waves were mechanically generated in the laboratory, in the presence of wind. Several wave age conditions were studied. Laser induced florescence techniques detected surface wave profiles, while a shadowgraph setup simultaneously tracked droplet sizes and velocities. This talk will present results from this study, with a focus on increasing wave age and wave slope's effect on average droplet production and initial dynamics. |
Tuesday, November 23, 2021 9:18AM - 9:31AM |
Q29.00007: Effects of wave kinematics on coastal atmospheric boundary layer Jagmohan Singh, Xuanting Hao, Lian Shen Air-sea interaction can significantly affect the coastal atmospheric boundary layer and consequently, the local atmosphere. Despite its importance, our knowledge of coastal air-sea interaction is limited and weather prediction models frequently generate erroneous predictions for coastal conditions due to a lack of an appropriate parameterization. Land topology, significantly varying thermal conditions across land and water, and oceanic processes such as ocean waves and wave breaking can all add to the complexity of a coastal atmospheric boundary layer, making its study difficult. In this work, we incorporate the wave effects to the simulation of coastal atmospheric boundary layer. We use an in-house large-eddy-simulation code that utilizes the immersed-boundary method to capture swells and includes a varying sea-surface roughness model. The wave kinematics are extracted from a phase-resolved wave simulation that considers a realistic bathymetry, and the sea-surface roughness is determined via a dynamic model. The findings of this research will help to improve the understanding of air-sea interaction in coastal areas and will aid in the development of coastal weather forecasting models. |
Tuesday, November 23, 2021 9:31AM - 9:44AM Not Participating |
Q29.00008: Laboratory measurement of secondary droplets generated by the impact of raindrops on water surface Xinan Liu, Xiguang Zhang, James H Duncan Secondary droplets generated by the impact of raindrops on a deep-water pool are studied experimentally in an artificial rain facility. Artificial rain is produced by a rain generator that consists of a water-filled open-surface rectangular tank with an array of 360 hypodermic needles attached to its bottom. Experiments are performed by mounting the rain generator above the water pool at a vertical distance of 2.2 m. With this distance, the impact velocities of the raindrops with a diameter of about 2.6 mm can reach 72% of their terminal velocity in natural conditions. Secondary droplets are measured at various heights above the pool's water surface by using a cinematic digital in-line holographic technique. A deep learning approach is employed to extract the diameters and 3D motions of the secondary droplets and raindrops from the high-speed hologram movies. It is found that the diameters and velocities of the secondary droplets in the rain field change drastically with the height above the pool's surface. The correlation between the energy of secondary droplets and raindrop energy is explored. |
Tuesday, November 23, 2021 9:44AM - 9:57AM |
Q29.00009: Direct numerical simulations of turbulent flow over mean-flow-misaligned waves Georgios Deskos, Shreyas Ananthan, Michael A Sprague Wind-wave misalignment is a phenomenon that occurs when waves are moving in a direction different than that of the local wind. The combination of light winds and fast-propagating waves (swell) prevails in global oceans, especially in tropical oceans such as the warm-pool area in the Pacific Ocean and has been a topic of active research for more than five decades. Better understanding of offshore wind dynamics is also key to creating optimized offshore wind farms. In this study we conduct direct numerical simulations of wind-wave misalignment at a low Reynolds number (Re=180) by considering a turbulent half-channel flow driven by a combined streamwise and spanwise pressure-gradient forcing and over idealized sinusoidal waves. For our simulations we consider wave ages corresponding to slow-, intermediate-, and fast-moving waves, two wave-steepness levels (mild and steep waves) and six different misalignment angles, from perfectly aligned wind-following waves to perfectly aligned but wind-opposing waves. The simulations are undertaken using the open-source-code Nalu-Wind from the ExaWind stack and the results show that the mean-velocity, velocity-variance, and momentum-flux vertical profiles are greatly modulated by the wave-age and wave-steepness parameters. Slow-moving waves are found to increase the wall roughness and behave like stationary bumps, whereas fast-moving waves reduce the wall roughness and act more like a partial-slip wall. Finally, the applied misalignment angle appears to affect both the first- and second-order statistics. Their effect is more pronounced for faster-moving waves and become prominent when considering perfectly aligned, wind-opposing waves. |
Tuesday, November 23, 2021 9:57AM - 10:10AM |
Q29.00010: Large-eddy simulation of gusty wind turbulence over travelling wave Xuanting Hao, Lian Shen Wind gustiness in the marine atmospheric boundary layer significantly affects the dynamics of air-sea interaction and the harvest of offshore wind energy. To understand the hydrodynamic impact of the wind gust event, we perform large-eddy simulation of wind turbulence over a travelling wave to investigate the response of the turbulent flow subject to impulsive wind speed increase and decrease. Analyses of the turbulent kinetic energy budget show that the streamwise velocity variance changes instantaneously with the mean shear while other components remain constant. We show that, because of the memory effect in turbulence evolution, the ratio of the Reynolds normal stress to the Reynolds shear stress cannot be assumed to be constant. On the other hand, the wave-coherent motions have a nearly real time response to the wind gust events and their evolution patterns exhibit a strong dependency on the wave age. For intermediate and large wave ages, the unsteady wave-coherent vertical velocity is consistent with a viscous curvilinear model developed by Cao et al. (J. Fluid Mech., vol. 901, 2020, A27) and Cao & Shen (J. Fluid Mech., vol. 919, 2021, A38). We also observe the hysteresis effect in the time evolution of the form drag and the viscous drag. Our discoveries thus provide evidence for the necessity of improved nonequilibrium turbulence and wind input modelling to account for the wind gustiness effect in future studies. |
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