Journal
JOURNAL OF THE ROYAL SOCIETY INTERFACE
Volume 8, Issue 61, Pages 1090-1103Publisher
ROYAL SOC
DOI: 10.1098/rsif.2010.0481
Keywords
copepod jump; viscous vortex ring; impulsive stresslet; impulsive Stokeslet; hydrodynamic camouflage; non-dimensional 'jump number'
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Funding
- National Science Foundation [NSF OCE-0352284, IOS-0718506]
- WHOI's Ocean Life Institute
- Danish Research Council for independent research
- Niels Bohr Foundation
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Copepods swim either continuously by vibrating their feeding appendages or erratically by repeatedly beating their swimming legs, resulting in a series of small jumps. The two swimming modes generate different hydrodynamic disturbances and therefore expose the swimmers differently to rheotactic predators. We developed an impulsive stresslet model to quantify the jump-imposed flow disturbance. The predicted flow consists of two counter-rotating viscous vortex rings of similar intensity, one in the wake and one around the body of the copepod. We showed that the entire jumping flow is spatially limited and temporally ephemeral owing to jump-impulsiveness and viscous decay. In contrast, continuous steady swimming generates two well-extended long-lasting momentum jets both in front of and behind the swimmer, as suggested by the well-known steady stresslet model. Based on the observed jump-swimming kinematics of a small copepod Oithona davisae, we further showed that jump-swimming produces a hydrodynamic disturbance with much smaller spatial extension and shorter temporal duration than that produced by a same-size copepod cruising steadily at the same average translating velocity. Hence, small copepods in jump-swimming are in general much less detectable by rheotactic predators. The present impulsive stresslet model improves a previously published impulsive Stokeslet model that applies only to the wake vortex.
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