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CJO Now Includes:

Every month one particularly interesting article published in Journal of Fluid Mechanics is selected to be the subject of an extended review and discussion by an acknowledged and invited expert in the field.  This Focus on Fluids article will explain the context, importance and implications of the underlying paper to a wider scientific audience, highlighting both the key findings and breakthroughs of the paper and the implications of the research for future activity.  Authors are welcome to propose that their papers are featured in this section by contacting the associate editor responsible, Dr. C.P. Caulfield.

Could waves mix the ocean?

G. Falkovich
Physics of Complex Systems, Weizmann Institute of Science, Israel

Abstract
A finite-amplitude propagating wave induces a drift in fluids. Understanding how drifts produced by many waves disperse pollutants has broad implications for geophysics and engineering. Previously, the effective diffusivity was calculated for a random set of small-amplitude surface and internal waves. Now, this is extended by Bühler & Holmes-Cerfon (J. Fluid Mech., 2009, this issue, vol. 638, pp. 5–26) to waves in a rotating shallow-water system in which the Coriolis force is accounted for, a necessary step towards oceanographic applications. It is shown that interactions of finite-amplitude waves affect particle velocity in subtle ways. An expression describing the particle diffusivity as a function of scale is derived, showing that the diffusivity can be substantially reduced by rotation.

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Hesitant Nature

E. VILLERMAUX
Aix-Marseille Université, IRPHE, 13384 Marseille Cedex 13, France

Abstract
The addition of even minute amounts of a polymer to a liquid can modify some of its properties drastically, like the ways in which jets of the liquid break into droplets or the dynamics of dripping droplets at an orifice. The elasticity conferred by the polymers can delay the separation of the drops considerably, which hesitate between falling or running back to the orifice, ‘gobbling’ along their way the thread to which they are attached. Other free-surface flows are also of a hesitant nature.
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Understanding and predicting vortex-induced vibrations

P.W. BEARMAN
Department of Aeronautics, Imperial College, London SW7 2AZ, UK

Abstract
Vortex-induced vibration of solid structures has been known since ancient times, and remains a very important problem of fluid mechanics. Experimental studies of the vortex-induced vibration of circular cylinders have provided many fundamental insights. Recent extremely carefully controlled forced vibration experiments by Morse & Williamson (J. Fluid Mech., 2009, this issue, vol. 634, pp. 5-39), generating finely resolved data sets, are providing a fresh understanding of the mechanisms involved, especially for ocean structures with very low mass and damping.

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Cavitation in linear bubbles

MICHAEL P. BRENNER 
School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA

Abstract
Recent work has develped a beautiful model system for studying the energy focusing and heating power of collapsing bubbles. The bubble is effectively one-dimensional and the collapse and heating can be quantitatively measured. Thermal effects are shown to play an essential roles in the time-dependent dynamics.

 
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Unravelling turbulence near walls

IVAN MARUSIC
Department of Mechanical Engineering, University of Melbourne, Victoria, 3010, Australia

Abstract
Turbulent flows near walls have been the focus of intense study since their first description by Ludwig Prandtl over 100 years ago. They are critical in determining the drag and lift of an aircraft wing for example. Key challenges are to understand the physical mechanisms causing the transition from smooth, laminar flow to turbulent flow and how the turbulence is then maintained. Recent direct numerical simulations have contributed significantly towards this understanding.

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