Flow Pattern near the Outlet of a Straight Long Circular Tube : 1st Report, Experimental Study of Velocity Profile

Water issues downward from a straight vertical tube into the atmosphere. Velocity profiles near the tube outlet were measured by using a Laser-Doppler velocimeter specially designed. It was revealed that the gravity force considerably affects the flow pattern. The following results were obtained: (1) Inside of the tube, although velocity profiles at a position by one tube-radius upstream from the tube end are the same as that of the Poiseuille flow, the velocity near the tube axis increases as the flow comes close to the tube outlet. It was found that the flow pattern at the tube end is no longer that of the Poiseuille flow. (2) Outside of the tube, since the axial velocity component in the free jet increases as it goes downstream and the rate of the increase of the axial velocity near the free surface is larger than that near the jet axis, the flow pattern approaches a uniform one at a position by one tube-diameter downstream from the tube end. (3) The axial velocity near the flow axis both inside and outside of the tube increases with a decrease of the Froude number, that is, with an increase of the effect of the gravity.