Thursday, July 14, 2011

Drag Reduction by Air Release Promotes Fast Ascent in Jumping Emperor Penguins paper download link

Drag reduction by air release promotes fast ascent in jumping emperor penguins—a novel hypothesis

John Davenport1,*, Roger N. Hughes2, Marc Shorten1, Poul S. Larsen3

1Department of Zoology, Ecology and Plant Science, University College Cork, Distillery Fields, North Mall, Cork, Ireland
2School of Biological Sciences, Bangor University, Deiniol Road, Bangor, Gwynedd LL57 2UW, UK
3Department of Mechanical Engineering, Fluid Mechanics Section, Technical University of Denmark, Building 403,
2800 Kgs. Lyngby, Denmark
ABSTRACT: To jump out of water onto sea ice, emperor penguins must achieve sufficient underwater speed to overcome the influence of gravity when they leave the water. The relevant combination of density and kinematic viscosity of air is much lower than for water. Injection of air into boundary layers (‘air lubrication’) has been used by engineers to speed movement of vehicles (ships, torpedoes) through sea water. Analysis of published and unpublished underwater film leads us to present a hypothesis that free-ranging emperor penguins employ air lubrication in achieving high, probably maximal, underwater speeds (mean ± SD: 5.3 ± 1.01 m s–1), prior to jumps. Here we show evidence that penguins dive to 15 to 20 m with air in their plumage and that this compressed air is released as the birds subsequently ascend whilst maintaining depressed feathers. Fine bubbles emerge continuously from the entire plumage, forming a smooth layer over the body and generating bubbly wakes behind the penguins. In several hours of film of hundreds of penguins, none were seen to swim rapidly upwards without bubbly wakes. Penguins descend and swim horizontally at about 2 m s–1; from simple physical models and calculations presented, we hypothesize that a significant proportion of the enhanced ascent speed is due to air lubrication reducing frictional and form drag, that ­buoyancy forces alone cannot explain the observed speeds, and that cavitation plays no part in ­bubble formation.

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1 comment:

Mrc109 said...

I do not understand how the air that is trapped within the "plumage" of the Emperor penguin is retained while the penguin dives to depths far in excess of three standard atmospheres? I am assuming that a penguin cannot achieve some form of internal equilibrium (isostatic balance) of pressure within its rapidly changing surrounding environment? I would reason that a penguin descending in water depth would simultaneously undergo a rapid external compression with a subsequent loss of external surface area (S.A.). The loss of S.A. being due to the necessary volume changes occurring within the birds formerly "aerated" plumage layer. This gaseous layer or envelope of air that is trapped within the plumage of these birds would reduce in volume at a rate proportional to the density of the surrounding liquid medium and with increasing depth of dive.
For the life of me I cannot fathom how a penguin can trap air within its plumage layer in such a way that it is not rapidly expelled or expressed from within the interstices of their plumage during a rapid descent where much higher static water pressure levels exist?
It is suggested that these birds can voluntarily control and hold onto this ballast gas until such time as it is needed. How on earth can they do this? Is there some form of voluntary, and rapidly reversible form of chemisorption occurring within the boundary layer of the plumage and the trapped air?