While discharging water from a short tube, I noticed that a vortex began to form on the top of the discharge tube causing the jet issuing from a short tube to contract (less than the cross sectional area of the short tube).
My question is why does the jet contract below the short tube?
When the vortex forms the velocity is at a maximum:
So, the area must be a minimum.
I also observed that the contracted jet maintains this state until all the water is discharged from the container. Could this be explained in terms of head loss caused by the vortex?