| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 90 === This research investigates the transient characteristics of free surface vortex in a tank which has a vertical discharge pipe built on its bottom. The variations of flowrate, generations of dimple and critical submergence, distributions of pressure, and motions of fluid particles have been analyzed under different initial states and designs of the discharge pipe.
From the experiments, we conclude that the change of flowrate bears relation to the size and outlet design of the discharge pipe rather than to initial submergence depth. Furthermore, the cross-sectional area of discharging has direct influence on the change of flowrate. As the water-draining process goes on, a dimple forms on the free surface of liquid which is originally flat after the water level drops to a specific height. The dimple develops downward continually and an air column is thus generated. When the dimple and the air column are formed, the water level corresponding to them will rise with initial submergence depth, until the initial submergence depth reaches a certain height.
Only at the limited region adjacent to the outlet will the change of pressure result. As far as the axial direction is concerned, the farther apart from the outlet the distance is, the smaller the decrease in pressure is. As to the radial direction, the pressure in every spot inside the outlet is lower than standard atmospheric pressure. A relative low-pressure region of the round-edged discharge pipe is located at its rim, and that of the sharp-edged discharge pipe is situated at the position 0.57 to 0.72 times the pipe radius.
The observation of the streaklines shows that the flow field is controlled by the axial and radial velocities when the water level remains high. With the lowering of the water level, the tangential velocity gradually affects the flow field, which results in the three dimensional motion. Therefore, when the vortex is produced, flow field can be divided into two parts; at the upper part of the flow field, liquid particles are driven by the rotation, while near the bottom of the tank, the radial velocity dominates.
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