# Tomographic PIV measurements of a regenerating hairpin vortex

Tomographic PIV measurements of a regenerating hairpin vortex The three-dimensional formation and regeneration of a hairpin vortex in a laminar boundary layer is studied in a free-surface water channel. The vortex is generated by fluid injection through a narrow slot into a laminar boundary layer $$(Re_{\delta ^*} = 485)$$ ( R e δ ∗ = 485 ) and recorded with tomographic particle image velocimetry. The swirling strength based on the $$\lambda _2$$ λ 2 criterion shows that the hairpin initially forms at the upstream edge of the elongated ring vortex produced by the injection. The elongated ring vortex decays while the hairpin vortex strengthens. Because the hairpin vortex is of sufficient strength, it forms a kink in the legs as a result of inviscid induction. A bridging structure forms between the legs initially upstream of the kink. As this structure dissipates, another bridging structure forms downstream of the kink and closes the vortex loop between the legs. This pinches off the original hairpin head such that two distinct vortices result. The formation of the secondary hairpin head does not appear to be preceded by a reduction in the spanwise gap between the legs or significant change in height above the wall as has been seen when exposed to a mean turbulent profile. Instead, the formation is preceded by the stretching of the hairpin legs downstream of the kink, exposes the ejected fluid between the legs to boundary layer flow producing conditions similar to those that formed the initial hairpin vortex. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Experiments in Fluids Springer Journals

# Tomographic PIV measurements of a regenerating hairpin vortex

, Volume 57 (1) – Dec 14, 2015
13 pages

/lp/springer_journal/tomographic-piv-measurements-of-a-regenerating-hairpin-vortex-DYFdm7qIbG
Publisher
Springer Berlin Heidelberg
Subject
Engineering; Engineering Fluid Dynamics; Fluid- and Aerodynamics; Engineering Thermodynamics, Heat and Mass Transfer
ISSN
0723-4864
eISSN
1432-1114
D.O.I.
10.1007/s00348-015-2089-1
Publisher site
See Article on Publisher Site

### Abstract

The three-dimensional formation and regeneration of a hairpin vortex in a laminar boundary layer is studied in a free-surface water channel. The vortex is generated by fluid injection through a narrow slot into a laminar boundary layer $$(Re_{\delta ^*} = 485)$$ ( R e δ ∗ = 485 ) and recorded with tomographic particle image velocimetry. The swirling strength based on the $$\lambda _2$$ λ 2 criterion shows that the hairpin initially forms at the upstream edge of the elongated ring vortex produced by the injection. The elongated ring vortex decays while the hairpin vortex strengthens. Because the hairpin vortex is of sufficient strength, it forms a kink in the legs as a result of inviscid induction. A bridging structure forms between the legs initially upstream of the kink. As this structure dissipates, another bridging structure forms downstream of the kink and closes the vortex loop between the legs. This pinches off the original hairpin head such that two distinct vortices result. The formation of the secondary hairpin head does not appear to be preceded by a reduction in the spanwise gap between the legs or significant change in height above the wall as has been seen when exposed to a mean turbulent profile. Instead, the formation is preceded by the stretching of the hairpin legs downstream of the kink, exposes the ejected fluid between the legs to boundary layer flow producing conditions similar to those that formed the initial hairpin vortex.

### Journal

Experiments in FluidsSpringer Journals

Published: Dec 14, 2015

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