本研究針對一個在底部具有垂直出水管之水槽，觀測其隨時間改變之暫態自由面渦流流場現象。實驗其在不同的初始狀況和出水管口設計下，流率趨勢、初始氣凹與氣柱的形成、壓力場分佈以及流體運動情形所產生的變化。
實驗結果顯示，初始進水的水位高低並不會改變流率趨勢的變化，只有因為出水管本身尺寸及出水口設計的不同，才會改變流率的曲線變化狀況。出水管所造成的流率變化，往往與其排水截面積有關。在實驗過程中，一開始液體自由面外形平坦，隨著排水而使液面降至一定高度後，自由液面即產生初始氣凹，進而向下延伸而變成一氣柱。初始氣凹和氣柱生成時，所對應的水面高度雖然會隨著初始水位高度而上升，但是在高於一定水位後，兩者的生成高度即不再有大幅變化。
只有靠近出水口附近的狹窄區域，暫態流場的壓力場分佈才會產生特殊的變化。在軸向部分，愈遠離出水口，壓降愈小。而在徑向方面，出水口內任一位置的壓力都會降至一大氣壓以下；其中鐘形出水口的相對低壓區位於出水管內側壁緣，而銳緣出水口則位於0.57至0.72倍的出水管半徑處。
藉由觀察煙線的分佈情形，暫態渦流流場在水位尚高時，主要受到徑向及軸向速度影響；隨著水位降低，流場逐漸受到切線向速度的帶動而產生三維運動。因此渦流產生後，流場即可概略分為兩部分，在流場較高位置，液體分子受到旋轉運動控制，但是在水槽底部，則仍然以徑向速度為主。

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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