Abstract: The critical penetration condition is an essential component of studies on the mechanism of sink vortex formation. However, the condition and its transition process are unknown. To address this issue, we constructed a Rankine-vortex-based fluid mechanic model, and proposed a Helmholtz-equation-based solution method to acquire the critical penetration condition. The two-phase mass suction-extraction mechanism of the Ekman boundary layer was discussed. Numerical results show that the critical penetration condition is dependent on the initial velocity components; if the initial disturbances are enhanced, the suction-extraction height and Ekman layer thickness increase. A particle image velocimetry (PIV)-based observation experimental platform was developed, and the effectiveness of the proposed method was verified. The vortex core boundary was observed first, so the radius of the vortex core could be acquired precisely.

Key words: Sink vortex; Critical penetration condition; Ekman boundary layer; Suction-extraction mechanism

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