本研究係針對一個在底部具有垂直出水管之水槽，在不同的液面初始高度、出水管口尺寸、排水孔與壁面之距離以及靜置時間，於排水及暫態自由面漩渦形成的過程中，觀察並量測流率、壓力以及初始氣凹與氣柱的生成等變化。
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。只有因為出水管開口尺寸與出水管長度的不同，以及排水孔與壁面之相對位置不同，才會改變流率曲線的變化狀況，此流率變化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，所對應的液面高度會隨著初始水位高度而上升。若靜置時間愈久，初始氣凹與氣柱生成高度便愈低，經靜置幾小時後幾乎沒什麼變化。
就壁面效應而言，可分成單壁面的影響，以及直角壁面的影響；實驗結果顯示，出水孔離壁面愈近，初始氣凹與氣柱生成所對應之液面高度便愈低，也就是說，壁面會抑制初始氣凹與氣柱的生成。若同時由兩個出水孔排放水，其間所產生之交互作用，會互相抑制對方生成初始氣凹與氣柱。若兩出水孔之圓心距離在二倍出水口徑以下，一旦產生氣柱後，其氣柱下端會在兩孔間來回跳動，無法穩定。

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, distributions of pressure, generations of dimple and critical submergence have been analyzed as initial submergence depth, size of the discharge pipe, distance between discharge pipe and wall and settling time are changed.
From the experiments, we found that the change of flowrate bears relation to the size, length and outlet design of the discharge pipe and distance between discharge pipe and wall rather than initial submergence depth. Furthermore, the cross-sectional area of discharging is the primary factor influencing the 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. The water level corresponding to when the dimple and the air column are formed rises with initial submergence depth. As settling time gets longer, the water level corresponding to dimple and air column formation decreases. After several hours, it reaches a steady value.
As to wall effects, they can be divided into two types: single wall effect, and corner effect. For a shorter distance between discharge pipe and wall, the formation of dimple and air column is slower, because the wall restrains the formation of dimple and air column. For water draining out of two discharge pipes, the formation of dimple and air column is also slower due to the mutual inhibition between the two vortices. If the distance between two discharge pipes is less than two times the pipe diameter, the air column is unstable.

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