Licence information
This is a pre-copyedited, author-produced PDF of an article accepted for publication in The Journal of Fluid Mechanics following peer review
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright statements
© Cambridge University Press, 2016