# Experimental Investigation of Column Separation Using Rapid Closure of an Upstream Valve
## 3 Observations and Results
### Column Separation Behaviors and Synchronous Pressure Surges  
To capture cavitation behaviors in pipelines, the high-speed camera was located downstream of the operating valve, BV1# or BV2#, as shown in Figure 2. A sequence of photographs and the associated pressure variations were synchronously illustrated to show cavity behavior during a column separation event. Figure 3 presents a series of photographs and the synchronous pressure variations at PT6# to illustrate the behaviors of column separation in the downstream side during the fast closure of BV2#. The initial steady-state conditions were as follows: \(H_{T2}\) = 3.53 m, \(V_{0}\) = 1.027 m/s, \(t_{\text{c}}\) = 0.05 s. To facilitate discussion, the pressure oscillation process during one period of the water column's separation and collapse, named a \"Phase\", is shown in Figure 3. It illustrates that there are five episodes of pressure dropping to the vapor pressure level in this test-column separation that occurs five times. According to Figure 3, the sequence of a column separation event is described as follows: \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Operation Valve**} & \multirow{2}{*}{**Flow Direction**} & \multicolumn{2}{c}{**Static Pressure Head at the Reservoir**} & \multirow{2}{*}{**Experiment Numbers**} \\ \cline{3-3} \cline{5-5}  & & \(H_{T1}\) & \(H_{T2}\) & \\ \hline \multirow{3}{*}{BV1\# or BV2\#} & \multirow{3}{*}{From Tank 1 to Tank 2} & 1.65–2.55 m, with \(\Delta H\) = 0.01 \(\pm\) 0.002 m & 1.55 m & 10 \\  & & 2.65–3.55 m, with \(\Delta H\) = 0.01 \(\pm\) 0.002 m & 2.55 m & 10 \\  & & 3.65–4.35 m, with \(\Delta H\) = 0.01 \(\pm\) 0.002 m & 3.55 m & 8 \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of experimental cases. Figure 2: Shooting position of the high-speed camera when BV2# is closing. 1. Closing the valve (\(t\) = 0.0-0.05 s): When the static pressure head in each tank reached the target values (here, \(H_{T1}\) = 4.22 m and \(H_{T2}\) = 3.53 m, then \(V_{0}\) = 1.027 m/s), the ball valve began to close (\(t\) = 0.0 s). The operation time of the ball valve was 0.05 s. Once the valve action stopped, obvious vibration and movement were observed. 2. Phase-1 (\(t\) = 0.048-0.157 s): The duration of visible cavitation was 0.09 s. Before the valve stopped action, the pressure at PT6# dropped to the vapor pressure level (at \(t\) = 0.048 s). Meanwhile, a cavity resembling \"white clouds\" had already appeared in the position of the pipeline flange, as shown in Frame 1. There was no clear gas-liquid interface for this cavitation structure. Then, the cavity dilated rapidly, and the gas-liquid interface close to PT6# became clear, eventually occupying the entire pipe cross-section, as shown in Frames 2 and 3. At 0.064 s (Frame 3), tiny bubbles appeared at the top of the pipe wall and expanded towards Tank 2#, indicating that a region of vaporous cavitation had formed. After this, the \"white clouds\" slowed their expansion rate and eventually reached their maximum size at 0.088 s (Frame 4). During this period, the mist cavity structure developed gradually into several little bubbles with clear gas-liquid interfaces. Then, the bubble number density increased quickly and eventually stopped at 0.108 s (Frame 5). Coinciding with the maximum bubble number density, the tiny bubbles in the cavity structure were eliminated gradually, showing a screw-type evolutional path. Following the complete elimination of the vaporous region at the top of the pipe wall (\(t\) = 0.128 s, see Frame 6), the number of tiny bubbles in the cavity structure rapidly diminished, while some bubbles gradually developed into a mist structure. All cavities vanished entirely by 0.139 s, and the pressure at PT6# began to rise, almost instantaneously reaching a maximum value of 281.75 m in 0.004 s, which is nearly three times that of the Joukowsky pressure rise. Subsequently, the damped pressure oscillation at PT6# showed high frequency and high amplitude characteristics.