We investigated tornado-like vortex induced by a fan similar to Rushton turbine placed under the top surface of a cylindrical water tank. Experiment was performed under the condition of vortex Reynolds number being 3800 and Swirl ratio being 0.45. The three-component velocity fields in two adjacent cross-sections of the tornado-like vortex were measured by dual-plane stereoscopic PIV. Single vortex column was formed in the center of the tank. While the vortex core was dominated by an upward flow, abrupt velocity deficit at the center of vortex was observed in the axial component of mean velocity profile. Further more, instantaneous axial component of velocity at the vortex center has been fluctuated in time significantly. Such a fluctuation of the velocity was observed since the traveling wave of velocity or vorticity has been passing through. As an example of our data, temporal phase lag between the waves at the two planes indicated that the wave was propagated upstream with a speed of approximately 10% of the peak axial velocity at the vortex core. In order to clarify the mechanism of the fluctuation of the axial velocity component in the center of vortex, we focused on the axial pressure gradient along the vortex. Axial pressure gradient was found to be balanced with the substantive derivative of the measured axial velocity. Thus we conclude that the fluctuation of the axial velocity component at the center of the vortex was caused by local axial pressure gradient induced by the vorticity wave traveling along the vortex.

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