Collapse of a water jet flowing out from a nozzle to the atmosphere was examined. The diameters of nozzles used in the experiments were 3, 6 and 8 mm. The flow state of the water jet was recorded with a high speed video camera. The collapse length was derived from recorded images. When the flow velocity was quite low, the surface of the water jet was smooth and small perturbations appeared at the lower position of the water jet. As the flow velocity was increased, the position where the small perturbations appeared came close to the nozzle outlet. The perturbations grew as these went downstream and lumps of water were formed at the lower position. When the flow velocity was further increased, successive waves came around on the surface of the water jet. The collapse of the water jet occurred in such a state that the lump of water was torn off from the jet. When the water jet velocity was high, the jet turned into a dispersed flow and the collapse occurred. The agreement of the measured results and the predicted results was poor. It was considered that the instability of the surface of the water jet seemed important for the jet collapse in the present experimental range. The Kelvin-Helmholtz instability wave length was compared with the measured wave length on the water jet. When the wave length reached the Kelvin-Helmholtz instability wave length, the jet collapse occurred except the case that the transition to the dispersed flow caused the jet collapse. The air rolling into the water jet was not observed in the present experimental conditions. The two-phase region was not formed inside the water jet contrary to what is explained in the literature.

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