# Experimental Investigation of Column Separation Using Rapid Closure of an Upstream Valve
## 4 Conclusions  
In this paper, a series of experimental studies on column separation events on the downstream side of a closure valve were conducted to reveal the dynamic behaviors of column separation and the effects of initial flow velocity, reservoir static pressure head, and pipeline length on the pressure surge. The main conclusions of this study are as follows:  
1. The dynamic behaviors and transient regimes of column separation can be summarized as follows: (a) when the pressure drops to vapor pressure level, \"mist clouds\" initially appear near the valve and rapidly expand downstream until they occupy the entire pipe cross-section; (b) the cavitation structures can manifest as \"mist clouds\" with distinct gas-liquid interfaces, \"bubble clouds\" composed of numerous tiny bubbles with clear gas-liquid interfaces, or a combination thereof that can transform into  
Figure 14: Maximum pipeline pressure measurements: (**a**) closure valve BV2#; (**b**) closure valve BV1#.  
each other; (c) the rate of expansion-condensation and the process of shape changes for bubbles at different positions within a cavitation structure are not the same, with several repeated expansion-condensation processes occurring before the cavitation structure vanishes completely; (d) following the collapse of a cavitation structure, an instantaneous sharp pressure oscillation with high frequency and amplitude propagates along the pipeline; (e) after the cavities collapse, gas release is observed, which increases over time in each phase, leading to reduced wave speeds.
2. Based on variations in the duration of column separation (sequential decrease or not) and the occurrence position of maximum pressure (following the first cavity collapse in the valve or not), four types of pressure oscillation pattern were identified in these experimental tests.
3. The influence of piping system quantities on pressure oscillation patterns can be summarized as follows: (a) under the condition of a constant downstream reservoir static head, when the initial velocity increases, the type of pressure oscillation pattern transitions from a nonsequential decrease in the duration of column separation (i.e., Type 3 or 4) to a sequential decrease in duration (i.e., Type 1 or 2); (b) for similar initial flow velocities, the types of pressure oscillation pattern remain consistent, indicating that the reservoir static head has a slight impact on the type of pressure oscillation pattern in the column separation event on the downstream side of the closure valve; (c) a shorter pipeline results in shorter cavitation duration (approximately 40% for Phase-1), but this is not necessarily the same for maximum pipeline pressure.
4. In all test cases, the maximum pipeline pressure exceeded 150 times the reservoir static pressure. For constant reservoir static pressure, increasing initial flow velocity initially raises and then lowers the maximum pipeline pressure before increasing it again. This trend becomes more pronounced with decreasing reservoir static pressure.  
Conceptualization, methodology, software, and writing--original draft preparation, H.W.; investigation, validation, formal analysis, H.W. and L.Z.; resources, writing--review and editing, project administration, L.Z.; supervision, and revision, D.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant numbers 51839008 and 51679066, and the Fok Ying Tong Education Foundation grant number 161068.  
Not applicable.  
The data presented in this study are available from the corresponding author upon reasonable request. The data are not publicly available due to privacy.  
The authors are grateful to [PERSON], Department of Mathematics and Computer Science, Eindhoven University of Technology, and [PERSON], The Academic and Research Network of Slovenia, for assistance with experimental data analysis.  
The authors declare no conflict of interest.