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Vortex rings are a nice example of
coherent structures in fluids. They are created by pushing air through
the circular orifice of a cylindrical cavity with an
electromagnetically driven flat piston. Hot wire anemometry provides
accurate measurements of the velocity profile at all stages of the ring
formation. Some properties of the vortex ring core (a) can be inferred
from the calculated circulation, ring diameter (D), and average axial
velocity (U).
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Flow
visualization using a laser ligth sheet shows that the initially
undisturbed vortex ring is progressively deformed in the azimuthal
direction giving rise to a wavy azimuthal and periodic pattern in the
circumference of the ring where the product k R is an integer (k,R
instability wave number and vortex radii respectively). The wavy
pattern seems to be steady, i.e., do not rotates or translate under the
self induced velocity field. However as the vortex motion progresses in
the axial direction, the displaced portions of the ring are convected
away from the initial undisturbed position and as the local velocity
increases with displacement, the wavy pattern grows.
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Side view
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Front view
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