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 A02: Waves: Surface Waves I |
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Chair: Dan Liberzon, Technion Room: 2B |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A02.00001: Towards a theory for the Pierson and Moskowitz spectrum Anthony Bonfils, Woosok Moon, Dhrubaditya Mitra, John Wettlaufer The spectrum of surface gravity waves was measured in the sixties by Pierson and Moskowitz for a fully developed sea, however a theory explaining their observations has been lacking. In 1957, Miles proposed an instability mechanism for single mode growth involving the solution of the hydrodynamic Rayleigh equation at the critical level, which is the height at which the phase speed of the wave is equal to the wind speed. The turbulent wind is modeled by a logarithmic profile. We show that the introduction of a viscous sublayer does not affect the growth rate. Because there is a continuous spectrum of waves, we study the interaction between the different critical layers (the counterpart of wave-wave interactions) in the framework of the quasi-linear model of Janssen (1982). The natural small parameter in this system is the air/water density ratio, which introduces a scale separation in wind-wave generation. Whereas the evolution of gravity waves occurs on a short (fast) time scale, the energy transfer from the wind occurs on a long (slow) time scale. The feedback of growing surface waves on the wind profile occurs on an intermediate time scale. A decrease of its curvature leads to the saturation of the waves, which is the first step in the transition towards a fully developed sea. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A02.00002: Wave- and shear-induced viscous stress over wind waves Kianoosh Yousefi, Fabrice Veron, Marc Buckley Detailed knowledge of the airflow over the surface of the ocean is essential for evaluating air-sea fluxes. Despite recent numerical/theoretical advances, experimental data near the water interface have been difficult to obtain, especially for scales at which viscosity plays a role. Here, we present direct measurements of the velocity field in the turbulent airflow above wind waves for a range of 10-m wind speed varying from 2 to 17 m/s. Improvements in measuring techniques have allowed us to detect the viscous sublayer in the airflow near the interface and make direct measurements of the airside viscous stresses. In a phase-averaged sense, the viscous stress is highest on the upwind face of wave crest with its peak value close to the wave crest and its minimum about the middle of the leeward side of waves. At wind speeds of 2 m/s, corresponding to winds in which wind waves are first generated, the mean tangential stress represents more than 90\% of the wind stress. The contribution of the viscous stress to the total momentum flux decreases significantly with increasing wind speed. In low winds, we also observe the viscous stress generated by the wave motion in the airflow. To the best of our knowledge, these are the first measurements of airside, wave-induced viscous stresses. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A02.00003: Observations of water waves and wind-wave interactions in the Gulf of Aqaba (Eilat) Almog Shani-Zerbib, Roni Hilel Goldshmid, Dan Liberzon The Gulf of Aqaba (Eilat) at the northern tip of the Red Sea has a unique elongated rectangular shape, a stable diurnal cycle of wind regime and a steady climate. The resulted wind induced wave field is heavily influenced by the topography and bathymetry of the gulf area. These conditions make the gulf area an alluring location for wind-wave interactions research. Here, we report on a first ever high-resolution observations of the water wave regimes in the Gulf of Aqaba, conducted during two short campaigns in 2017 and 2019. These include wave field measurements using a directional wave gauge array accompanied by wind velocity field measurements at several heights. High resolution measurements of the wind flow turbulent characteristics were also conducted using the newly developed combo probe, a collocated ultrasonic and hot-film anemometer, utilizing an in-situ Neural Network calibration procedure. We will report on the characteristics of the wind induced water wave fields and detail the wind-wave interaction dependence on the variations of the wind flow turbulence characteristics in terms of mean and fluctuating values. Empirical fits of the turbulent flow length scales vs. the wave field characteristics will also be presented. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A02.00004: Influence of the free surface on the distribution of buoyant particles in wavy flow Michelle DiBenedetto, Jeffrey Koseff, Nicholas Ouellette Using numerical and analytical techniques, we examine the effects of a varying free surface on the distribution of buoyant particles. We simulate particle dispersal scenarios under a progressive deep water wave train as well as under an idealized wave spectrum. This work implies that waves on the surface of the ocean can affect the instantaneous distribution of particles. These distribution effects can provide insight into how to interpret surface observations of particles, such as microplastics, using net trawls. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A02.00005: Detection of breaking waves in single wave gauge records of surface elevation fluctuations Dan Liberzon, Alexandru Vreme, Sagi Knobler We report the development of a new method for accurate detection of breaking water waves, which addresses the need for an accurate and cost-effective method that is independent of human decisions. The new detection method, which enables the detection of breakers using only surface elevation fluctuation measurements from a single wave gauge, supports the development of a new methodology for research relating to water waves and wind-wave interactions. According to the proposed method, detection is based on the use of the Phase-Time Method (PTM) to identify breaking-associated patterns in the instantaneous frequency variations of surface elevation fluctuations. A wavelet-based pattern recognition algorithm is devised to detect such patterns and provide accurate detection of breakers in the examined records. Validation and performance tests, conducted using both laboratory and open sea data, including mechanically generated and wind-forced waves, are reported as well. These tests allowed derivation of a set of parameters assuring high detection accuracy rates. The method is shown to be capable to achieve a positive detection rate exceeding 90 percent. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A02.00006: Predicting the breaking onset and strength of gravity water waves in arbitrary depth Morteza Derakhti, Jim Kirby, Jim Thomson, Stephan Grilli, Mike Banner We introduce a robust and local parameterization to predict the breaking onset and breaking strength of 2-D and 3-D gravity water waves in arbitrary depth. We use a LES/VOF model to simulate nonlinear wave evolution, breaking onset and post-breaking behavior for representative cases of 2-D and 3-D focused wave packets, modulated wave trains, regular and irregular waves propagating over various bed topographies featuring deep water, intermediate depths, and the shallow surf zone. We also use a 2D potential flow solver using BEM to simulate nonlinear wave evolution, focusing on breaking onset behavior. The new parameterization relates the breaking strength to a breaking strength predictor $\Gamma$ defined as the normalized rate of change of $B = U/C$ following the wave crest (with $U$ the water velocity at the crest and $C$ the crest celerity). We show that the breaking onset criterion proposed by Barthelemy et al. (2018) in deep water is also effective in shallow water (i.e., when $B$ exceeds $B_{th} \sim 0.85$ then breaking is imminent). The new parameterization is local and can handle multiple breaking events in space and time. The implementation of the new parameterization is convenient and efficient in phase-resolving models such as Boussinesq and HOS models. [Preview Abstract] |
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