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Wave Breaking and Air Entrainment

Pierre Lubin

Introduction

Surface wave breaking, occurring from the ocean to the coastal zone, is a complex and challenging two-phase flow phenomenon, which plays an important role in numerous processes, including air-sea transfer of gas, momentum and energy, and in a number of technical applications, such as coastal and military applications. However, most recent modeling attempts are still struggling with the lack of physical understanding of the fine details of the breaking processes, which makes the task of parameterizing breaking effects very difficult since no universal scaling laws for physical variables have been found so far. The complex 3D structure of turbulent flows under breaking waves has not been properly addressed yet in the literature and remains poorly understood, therefore, a better knowledge of the temporal and spatial evolution of the aerated region under breaking waves is crucial.

Describing this flow process is still a key scientific challenge. Wave breaking has been shown to influence coastal erosion, climate and intensification of tropical cyclones (Veron et ah, 2015), and cause ocean ambient noise (Prosperetti, 1988). The breakup and evolution of the entrained air into numerous bubbles is a source of acoustic noise, influencing acoustic underwater communications. Bubble clouds also modify the optical properties of the water column. The hydrodynamic performance of ships is influenced by the wake, which depends heavily on the air entrainment, and the sound generated by the bubble clouds make the ships visible to detection, for military applications. The wake starts at the stem of a ship's hull, where bow waves are observed to break, playing a role in the floating stability. In hydraulic engineering, large spillways are often protected from cavitation damage by controlling aeration. Tsunami waves and storm surges are also a great threat for coastlines, where civil nuclear facilities can often be found. Numerical methods for tsunami hazard assessment need an accurate modeling of the wave-breaking phenomenon, in order to provide guidance for risk assessment on the nuclear facilities and more broadly on the human occupation of the coastal zone. Offshore structures are also very often impacted by breaking waves during extreme weather conditions, and thus need to be designed properly, taking breaking waves into account. When large tankers travel in narrow rivers or channels, river banks are impacted by breaking waves due to the wakes, and can cause major corrosion. Due to climate change (Nicholls, 2004), coastal zones are more often subjected to extreme conditions where overtopping occurs, damaging coastal dunes and beaches, and affecting dikes.

Univ. Bordeaux, CNRS, Bordeaux INP, I2M, UMR 5295, F-33400, Talence, France. Email: This email address is being protected from spam bots, you need Javascript enabled to view it popular method for simulating turbulence in breaking waves is the Large Eddy Simulation (LES) method. This method is now widely used to simulate academic test cases and highly complex industrial applications, but also to investigate the physical phenomena that occur in complex geophysical flows such as breaking waves. In LES, the contribution of the large, energy-carrying structures is exactly simulated, and the effect of the non-simulated smallest scales of turbulence has to be modeled. The scope of this chapter is to review the very recent progresses obtained so far, thanks to the LES technique, to discuss the limitations and identify the key issues which have to be overcome. This chapter is organised as follows. The first section is dedicated to the physical description of the wave breaking process in detail. The governing equations and associated numerical methods are introduced in the next section. Then the numerical results are analysed, leading to the investigation of a new three-dimensional type of vortical structure observed under breaking waves. The chapter is then concluded

Physics of breaking

Waves are generated in the ocean when wind blows and modifies the water surface. This results in an oscillatory movement of water masses, transmitted gradually and controlled by the force of gravity. Most often, when the wind blows with a certain fetch, the surface of the water oscillates randomly to form short-crested wind waves; whereas far from the windy zone, the wave motion is organized into fully developed and coherent waves to form the swell. Offshore, the waves will evolve according to their interaction with the wind and other wave trains they encounter as they propagate through the oceans. Near the coasts, wave propagation is directly influenced by variations in water depth: waves are directly modified due to the local bathymetry. Finally, waves break, air is entrained in vortical structures and turbulence is generated.

 
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