Generation and growth of wind waves over a viscous liquid

Abstract : Despite numerous studies on the subject, the development of waves under the action of wind still retains a certain number of open questions. In my PhD, I approach this problem through a fairly uncommon angle: the experimental study of the deformation by wind of the surface of a highly viscous liquid. Indeed, contrary to the major part of the literature on the matter, the liquid I used is not water but a significantly more viscous liquid. Regardless of the fundamental underlying questions, this has the practical advantage of simplifying the problem. Indeed, due to the high viscosity of the liquid, the flow in the liquid stays laminar and the unamplified perturbations of the interface can only propagate over a limited distance. To observe these deformations at the liquid-air interface, I have developed a new experimental set-up upon which the Free Surface Synthetic Schlieren method of visualization was implemented. This non-intrusive technique allowed to measure with a micrometric accuracy the amplitude of the surface deformation and to access the first deformations at low wind velocity. First, experiments were conducted over a liquid thirty times more viscous than water. The experimental data obtained by FS-SS show two regimes of deformation of the liquid-air interface. At low wind velocity, the interface is populated with ``wrinkles'', small-amplitude streamwise spatially disorganized perturbations. These wrinkles can be interpreted as the effect on the interface of the pressure fluctuations in the turbulent wind. At higher windspeed, above a critical velocity, transverse waves appear with quasi-parallel crests perpendicular to the wind direction. The distinctions between the two regimes have been detailed and the nonlinearities emerging above the threshold have also been studied. Then, the viscosity of the liquid has been changed over a large range. It results from the experiments that the two regimes of surface deformation by wind can be identified for all the viscosities explored. Following these results, a model was developed to account for the evolution of the wrinkles' amplitude both with wind velocity and with viscosity.
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Submitted on : Monday, September 19, 2016 - 2:30:09 PM
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  • HAL Id : tel-01354993, version 2

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Anna Paquier. Generation and growth of wind waves over a viscous liquid. Mechanics of the fluids [physics.class-ph]. Université Paris-Saclay, 2016. English. ⟨NNT : 2016SACLS183⟩. ⟨tel-01354993v2⟩

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