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Formation of vortex dipoles Y. D. Afanasyeva

Summary: Formation of vortex dipoles
Y. D. Afanasyeva
Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's,
Newfoundland A1B 3X7, Canada
Received 23 August 2005; accepted 7 February 2006; published online 28 March 2006
Evolution of a two-dimensional flow induced by a jet ejected from a nozzle of finite size is studied
experimentally. Vortex dipole forms at the front of the developing flow while a trailing jet
establishes behind the dipole. The dynamics of the flow is discussed on the basis of detailed
measurements of vorticity and velocity fields which are obtained using particle image velocimetry.
It is found that dipoles do not separate pinch-off from the trailing jet for values of the stroke ratio
up to 15, which fact can be contrasted with the behavior of vortex rings reported previously by other
authors. A characteristic time scale that is defined differently from the formation time of vortex rings
can be introduced. This time scale startup time indicates the moment when the dipole starts
translating after an initial period when it mainly grows absorbing the jet from the nozzle. A simple
model that considers the competing effects of expansion and translation is developed to obtain an
estimate of the dimensionless startup time. The dynamics of a dipole after the formation is
characterized by a reduced flux of vorticity from the jet. The dipole moves forward with constant
speed such that a value of the ratio of the speed of propagation of the dipole to the mean velocity
of the jet is found to be 0.5. A universality of this ratio is explained in the framework of a model
based on conservation of mass and momentum for the moving dipole. 2006 American Institute of


Source: Afanassiev, Iakov - Department of Physics and Physical Oceanography, Memorial University of Newfoundland


Collections: Geosciences