# Experimental Investigation of Column Separation Using Rapid Closure of an Upstream Valve
## 3 Observations and Results
### Column Separation Behaviors and Synchronous Pressure Surges
In addition, almost immediately after the cavities collapsed, a resounding noise could be heard following obvious pipeline vibration and movement. 3. Phase-2 (\(t\) = 0.157-0.237 s): The duration of visible cavitation was 0.061 s. When \(t\) = 0.157 s (Frame 7), a mist cavitation structure appeared throughout the entire pipe, while the pressure at PT6# was slightly larger than the vaporization pressure (it reached \(H_{v}\) in 0.001 s). The \"mist\" quickly developed into \"bubble clouds\" composed of large amounts of tiny bubbles with clear gas-liquid interfaces, expanding downstream with a screw-type evolutional path, as shown in Frames 7-9. The maximum length of the cavitation structure, which was longer than the one in Phase-1, was reached at 0.181 s (Frame 9). Compared with Phase-1, the tiny bubble number density in Phase-2 was lower, and the vaporous region at the top of the pipe wall in the shooting area was more obvious. Then, the front of the \"bubble clouds\" turned to \"mist\", backing up towards BV2#, as shown in Frame 10. Unlike Phase-1, repeated transformations from \"mist\" to \"bubble clouds\" could be observed. The transformation process stopped at \(t\) = 0.216 s, and the cavitation in the shooting area eventually presented in the form of mist. At 0.0004 s, the pressure at PT6# reached its the maximum, albeit lower than that of Phase-1. Importantly, a small amount of mist structure could be observed even though the pressure at PT6# was larger than that in \(H_{v}\), as shown in Frame 11 (\(t\) = 0.217 s). This structure flickered with the oscillations in pressure. This may have been the result of the release of gases dissolved in water, which would also explain the decrease in wave speed in Phase-2. 4. Subsequent phases: The durations of visible cavitation for Phase-3, Phase-4, and Phase-5 were 0.036 s, 0.031 s, and 0.012 s, respectively. The dynamic behaviors of column separation in Phase-3, Phase-4, and Phase-5 were similar to those in Phase-2. The cavitation structure shapes adjacent to BV2# and reaching maximum size are displayed in Frames 12, 13, and 14, respectively. The amount of gas released in each phase increased as time went on, which reduced the wave speed further. * After Phase-5's end (\(t>0.369\) s): The formation, expansion, and contraction of tiny bubbles could be observed in the pipeline several times, even though the pressure at PT6# was larger than that in \(H_{\varphi}\), as shown in Frame 15. After the pressure oscillations becoming flat, these bubbles moved towards the top of the pipe wall and were eventually suspended at the top wall. Figure 3: Cavitation visualization and pressure histories in the case of closing BV2# (\(H_{T2}=3.53\) m, \(V_{0}=1.027\) m/s): **(a)** frames of cavitation zones; **(b)** synchronized pressure history at PT6#. Figure 4 displays the pressure variation over time, which was measured at nine locations (PT1# to PT9#) along the whole pipeline for the test, as shown in Figure 3. As can be seen, column separation occurred at least four times in the upstream side of the pipeline of the closed BV2#. For the pressure variation in the pipeline located on the downstream side of BV2# (Figure 4a), the maximum measured pressure head of 281.75 m occurred in the first phase following the collapse of cavities at the valve. Only the pressures at PT7# and PT8# in Phase-2 and the pressure at PT7# in Phase-3 dropped to the vapor pressure level. This indicates that the vaporous cavitation region expanded to the location of PT8# in Phase-2 and the location of PT7# in Phase-3, which also explains why the bubbles at the top pipe wall in Phase-2 were larger than those in Phase-1, as shown in Frames 3 and 8 in Figure 3. For the pressure variation in the pipeline located on the upstream side of BV2# (Figure 4a), the maximum pressure head of 104.88 m occurred in PT 5# after the second collapse of cavities, accounting for only 37.22% of the maximum pressure in the pipeline system on the downstream side of the closing valve.