"Generation of energy and vorticity production by surface waves through two-dimensional turbulence effects"

We study energy condensation in quasi two-dimensional turbulence that is driven by surface waves. This physical mechanism is investigated with regard to its potential for energy production.
In two-dimensional turbulence the net energy is transferred from small scales to large scales. Energy condensation develops when large scale friction is low and energy piles up at large scales. In this way, energy condensation produces large ordered flow structures from disordered small scale forcing that drives the two-dimensional turbulence. It was shown only recently that two-dimensional turbulence can also be driven by surface waves [von Kameke et al. 2011].

However, it is unclear if two-dimensional turbulence and energy condensation can also be driven by more naturally occurring unordered forcing as for instance provided by oceanic surface waves. Further, it is not yet fully understood how non-breaking surface waves generate horizontal vorticity, and if the waves have to possess certain properties, i.e., if they need to be standing, non-linear or monochromatic [Francois et al. 2014, Filatov et al. 2016]. Additionally, the necessary boundary conditions for energy condensation are vague and need clarification. And, it needs to be addressed if the process of energy condensation is stable to the introduction of further sources of drag, i.e., when a turbine is plugged into the fluid flow in order to retrieve energy.

Here, these open points are to be investigated using a Faraday experiment [von Kameke et al. 2010, von Kameke et al. 2011, von Kameke et al. 2013]. The generation of vorticity by the surface waves and the influence of the boundary- and forcing- conditions on energy condensation will be studied as well as the velocity statistics. To this end the full unsteady three-dimensional velocity field at the water surface and below the water surface needs to be recorded which has not been investigated so far. The latest optical methods will be used, such as time-resolved high speed planar particle image velocimetry and time-resolved three-dimensional particle image velocimetry and particle tracking. The complete velocity data allows to doubtlessly verify, if the flow obtained in each case is two-dimensional and, if energy condensation takes place. Two-dimensionality is analyzed on the basis of energy and enstrophy spectra and spectral fluxes, calculated with the aid of a novel filtering method [Eyink, 1995, von Kameke et al. 2011, von Kameke et al. 2013]. Moreover, existing three- dimensional flow structures will be identified and characterized. The forcing, exerted by the surface waves on the fluid-particles, and the resulting vorticity generation will be quantified by measuring the fluid surface elevation simultaneously to the PIV measurements and the subsequent usage of Lagrangian methods [von Kameke et al. 2011, von Kameke et al. 2013, LaCasce 2008] that allow to correlate both movements. The objective of this study is to uncover a new effective mechanism to retrieve renewable energy and will broaden insight into surface wave physics and two-dimensional turbulence. 

Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 395843083