Flow field around a bathtub vortex in a cylindrical vessel was measured. A gas core of the bathtub vortex was stationarily formed at the center of the vessel. An inlet open channel and an outlet pipe were equipped at the top tangentially and at the bottom center, respectively. Flow field was visualized using ink tracer or particle tracers, resulting in the velocity distribution measured. The downward velocity in the central region was found to be proportional to the distance from the original surface without the gas core except in the neighborhood of the outlet. Using the Laser Doppler Velocimeter, radial distribution of the downward velocity in the vessel was measured. The velocity distribution in the central region was found to be very complex. In the lower flow rate condition, the highest velocity position was the center. However, with increasing the flow rate, the highest position moved to outside. The inside of the central region was almost stagnant. The flow rate through the central region was 10 ∼ 50 % of the total flow rate. Formation of the stagnant region was related to the determination of the downward velocity gradient.

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