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 3820 and swirl ratio being 2.2. Velocity fields of the tornado-like vortex were measured by stereoscopic-PIV and dual-plane stereoscopic-PIV. We observed that the tornado-like vortex has the one-celled structure in the tank. The vorticity profile has reasonable agreement with Burgers solution based on the measured axial velocity gradient and circulation. Furthermore, we observed that axial velocity fluctuation and vorticity traveling wave were propagated upstream on the vortex center. Because of vorticity fluctuation on the vortex center, the fluctuation of local axial pressure gradient was induced. Since this axial pressure gradient balanced with the substantive derivative of the measured axial velocity, we concluded that the fluctuation of the axial velocity on the vortex center was caused by the vorticity fluctuation via the axial pressure gradient.

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