# Experimental Investigation of Column Separation Using Rapid Closure of an Upstream Valve
###### Abstract  
In this study, the phenomenon of column separation that occurs in the downstream pipeline of a rapid closure valve is experimentally investigated. Special attention is paid to the dynamic behavior of the formation, growth, and collapse processes of cavities, which are observed using a high-speed camera. Synchronized images of cavity patterns and measured pressure histories are analyzed to elucidate the process of water column separation, the mechanism of column separation events, and the influence of parameters on the transient flow. Experimental results indicate that during the collapse process of vapor cavities, a superposition phenomenon involving a positive pressure wave and collapse wave occurs, resulting in a nearly three times rise of Joukowsky pressure. In all test cases, the maximum pressure of the pipeline exceeded 150 times the reservoir static pressure. A new classification for a water hammer combined with cavitation (four types of pressure oscillation patterns) is proposed based on whether the duration of column separation decreases sequentially and the maximum pipeline pressure follows the first collapse of cavities at the valve. As the initial flow velocity increases, there is generally an increase in maximum pressure; however, this trend may be scattered under certain operation conditions. water hammer; column separation; cavitation; dynamic behaviors; pipeline +
Footnote †: journal: Applied  
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are distinctly different, with typical examples recorded by [PERSON] [8] and [PERSON] and [PERSON] [3]. Some scholars have conducted visualization studies on column separation by incorporating a transparent section into the pipeline system [15; 16; 17]. [PERSON] and [PERSON] [16] presented photographs of local cavities formed at the top of a pipe with a horizontal lower surface. A vaporous cavitation zone extended along the pipe's length to three quarters of the pipe's distance from the valve. [PERSON] [17] presented photographs of vapor bubbles in two situations: limited cavitation and classic severe cavitation. However, few studies have focused on column separation on the downstream side of a rapid closure valve. Regarding published reports on this topic, most researchers have merely discussed the variation characteristic of pressure over time and its influencing parameters. [PERSON] and [PERSON] [18] discussed the transient pressure and velocity changes in hydraulic transients resulting from the closure of a pipeline's upstream valve, and pointed out that the existence of cavitation bubbles creates large resistance to flow, resulting in larger actual transient pressure and velocity damping. [PERSON] et al. [19] concluded that the instantaneous pressure following the collapse of cavitation bubbles in the downstream side of a valve is much larger than the conventional Joukowsky pressure rise in the upstream side of the valve, leading to severe accidents. [PERSON] et al. [20] experimentally investigated the occurrence of column separation on the downstream side of a valve and observed that the superposition of positive pressure waves was capable of causing an extreme pressure rise close to two times the Joukowsky pressure rise. [PERSON] et al. [21; 22] performed experimental tests on transient cavitating flow in viscoelastic pipes to verify their proposed numerical model with the viscoelasticity of high-density polyethylene pipes. Other studies on column separation initiated by the closure valve located on the upstream end of a pipeline have also produced photographs of the creation and collapse of a cavity. [PERSON] [23] observed a cavitation region with plenty of tiny bubbles occupying the entire pipe cross-section, extending along the pipeline over a long distance. [PERSON] [24] presented photographs of a cavity in the downstream side of a closure valve.