Effect of the wave breaking mechanism on the momentum transfer (Abstract/Comunicazione in atti di convegno)

Type
Label
  • Effect of the wave breaking mechanism on the momentum transfer (Abstract/Comunicazione in atti di convegno) (literal)
Anno
  • 2006-01-01T00:00:00+01:00 (literal)
Alternative label
  • A. Iafrati (2006)
    Effect of the wave breaking mechanism on the momentum transfer
    in 21st International Workshop on Water Waves and Floating Bodies, Loughborough (UK), 2-5 Aprile 2006
    (literal)
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  • A. Iafrati (literal)
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  • http://iwwwfb.org/Abstracts/iwwwfb21/iwwwfb21_21.pdf (literal)
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  • Proceedings of the 21st International Workshop on Water Waves and Floating Bodies (literal)
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  • 4 (literal)
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  • PuM (literal)
  • Comunicazione (literal)
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  • INSEAN (literal)
Titolo
  • Effect of the wave breaking mechanism on the momentum transfer (literal)
Abstract
  • The flow generated by the breaking of free surface waves is numerically studied through a two-fluids Navier-Stokes solver. The aim is to investigate the differences in the vertical transfer of horizontal momentum induced by different breaking mechanisms. The study has been stimulated by the concluding remarks in reported in Melville et al. (2002). Following an earlier work (Rapp and Melville, 1990), they used the dispersive focusing technique to produce plunging breaking waves and measured the velocity field. Through ensemble average, they distinguished the mean and the turbulent components and investigated the kinetic energy decay and the momentum flux. Consistently with the previous measurements of Rapp and Melville (1990), the kinetic energy, vorticity and the Reynolds stresses were found to decay like t-1 . However, estimates of the vertical transfer of horizontal momentum, were an order of magnitude less than those implied in the quasi-steady breaking past a hydrofoil (Duncan, 1981; Duncan and Dimas, 1996) and an order of magnitude larger than those estimated by Phillips et al. (2001) in the field of wind-generated waves. The recent development of numerical approaches able to deal with complex free surface flows and topology changes of the interface, has widen the use of computational tools for such applications, see Scardovelli and Zaleski (1999) for a survey on the subject. What makes these tools very attractive is the possibility of achieving a very refined investigation of the flow field in a non-intrusive manner. This is particularly true in wave breaking flows with air entrainment as experimental measurements inside the bubbly flow is still a very challenging issue. Despite the big development of the computational tools, important limits still remain as they are usually confined to low Reynolds numbers and two-dimensional flows. Nevertheless, results published so far have shown that a reasonably good agreement with the experimental observation can be achieved in terms of more global quantities. As an example, the breaking of steep free surface waves has been investigated in Chen et al. (1999). There it is shown that, soon after the plunging event, the energy takes a t-1 decay trend, which is in good agreement with the experimental findings by Melville et al. (2002). Moreover, it was found that about 80% of the initial wave energy is dissipated within three pediods after the breaking. In a different context, Iafrati and Campana (2005) simulated the quasi steady breaking past a hydrofoil. Numerical simulations at different length scales have been performed, giving rise to different breaking regimes, ranging from plunging breaking up to microscale breaking waves in which the strong surface tension effects prevent the entrainment of air. In the latter case, it was found that a shear layer develops at the toe of the breaker and gives rise to coherent vorticity structures propagating downstream. An accurate analysis of the downstream propagating fluctuations has been done. The spectra of the fluctuations have been found in rather good qualitative and quantitative agreement with the experimental data by Walker et al (1996) and Duncan and Dimas (1996), although important differences in terms of decay properties of the fluctuations, presumably due to the two-dimensional assumption and to the much lower Reynolds number, have been noticed. In the present paper a careful investigation of the wave breaking flow is carried out. The breaking of a rather steep wave in a periodic domain and the quasi-steady wave breaking flow past a submerged hydrofoil are carefully analyzed. The analysis is mainly focused at evaluating how deep the effects of the breaking, either in terms of air entrainment and horizontal momentum, do propagate into the water and how this changes with the breaking wave mechanism. (literal)
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