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Experiments on Vortex-Ring Dynamics in a Rotating Fluid<br />
M. A. Brend a, P. J. Thomas a and P. W. Carpenter a<br />
We present results of our first experiments investigating the dynamics of vortex<br />
rings in a rotating fluid. The experiments are conducted in our new, unique large-scale<br />
rotating-tank facility (see Fig. 1). The rig (overall height 5.<br />
7 m ) was purpose-built for<br />
our study. It constitutes a water-filled tank (height 2. 5 m , diameter 1m)<br />
mounted on<br />
a computer-controlled rotating turntable. A piston-type, computer-controlled vortexring<br />
generator is located in the centre at the top of the tank. It is rigidly attached to the<br />
tank, rotates with it and injects vortex rings vertically downwards into the tank.<br />
We summarise results from dye-visualisations and PIV measurements. We found<br />
that, in the rotating system, Coriolis forces induce several hitherto unknown flow<br />
phenomena absent for corresponding vortex rings in non-rotating flow. Coriolis<br />
forces induce, for instance, a cyclonic swirling flow in the vortex-ring wake.<br />
Additionally they establish a back flow, from the ring towards the generator nozzle.<br />
They induce shedding of secondary and, sometimes, tertiary rings from the primary<br />
ring - these have opposite vorticity in comparison to the primary ring. We further<br />
observed that background rotation decreases the translational velocity of the rings<br />
slightly and that it destabilises the rings substantially. The decay length of a ring, i.e.<br />
the distance that a ring can propagate until it has completely lost its entity, scales with<br />
a Rossby number based on the mean translational ring speed and the ring diameter.<br />
Finally, Coriolis induced secondary flows can suppress the well-known wavy Widnall<br />
instability1 that is closely associated with vortex rings in non-rotating flow.<br />
a School of Engineering, University of Warwick, Coventry CV7 7AL, United Kingdom<br />
1 Widnall and Tsai, Phil. Trans. Roy. Soc. 287, 273 (1977).<br />
Figure 1: Our rotating tank facility with members of the research<br />
team in foreground to illustrate the size of the rig.<br />
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