Mutual interactions between waves and turbulence: an experiment

 

The mutual interaction between waves and turbulent currents plays a key role in the energy budget, mixing and mass transfer in the upper layer of the ocean. Turbulence is ubiquitous in the uppermost layer of the ocean, where it interacts with surface waves. Theoretical, numerical, and experimental works (e.g. [1 - 3] and others) predict that motion of non-breaking waves will increase turbulent energy, in turn leading to a dissipation of waves and, potentially, increased mixing and gas transfer between ocean and atmosphere. Conversely, waves encountering a turbulent currents will be scattered and directional seas can suffer a broadening of the directional spectrum [4,5].

In this work we study how the mutual interaction of waves and turbulent flow depends on the properties of the ambient turbulence. The measurements were performed in the water channel laboratory at NTNU Trondheim [6], able to mimic the water-side flow in the ocean surface layer under a range of conditions. An active grid at the inlet allowed the turbulence intensity and length scale to be varied for the same mean flow. The flow field was measured in the spanwise-vertical plane by stereo particle image velocimetry for various background turbulence cases with waves propagating against the current. Scattering was measured with pairs of wave probes at increasing distances from the wave-maker.

A strong increase in streamwise enstrophy (mean-square streamwise vorticity) is observed after vs before the passage of a long, Gaussian wave group. Enstrophy is intensified under troughs and reduced under crests. Scattering is observed, increasing linearly with propagation distance. The scattering rate is found to depend primarily on the energy content at the largest turbulent scales larger than a wavelength, whereas the intensification of turbulence by waves occur at length scales smaller than a wavelength.

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[2] McWilliams J. C., Sullivan P. P. and Moeng C-H. 1997 “Langmuir turbulence in the ocean” J. Fluid Mechanics 334 1-30.
[3] Thais L. and Magnaudet J. 1996 “Turbulent structure beneath surface gravity waves sheared by the wind” J. Fluid Mechanics 328 313-344.
[4] Phillips O. M. 1959 "The scattering of gravity waves by turbulence"  J. Fluid Mech. 5 177-194.
[5] Fabrikant and Raevsky 1994 "The influence of drift flow turbulence on surface gravity wave propagation" J. Fluid Mech. 262 141-156.
[6] Jooss Y., Li L., Bracchi T. and Hearst R.J. 2021 “Spatial development of a turbulent boundary layer subjected to freestream turbulence” Journal of Fluid Mechanics 911 A4.