## 2 Performing Filling and Emptying Operations
### Big Data Platform  
This platform plays a crucial role in data collection. Figure 3 depicts an installation tested at the Instituto Superior Tecnico, the University of Lisbon, Portugal. The system has several sensors that must be appropriately connected to facilitate data collection.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Type** & **Scenario (Air Valve)** & **Characteristics** \\ \hline I & None & The filling process begins with the opening of the valve BV4, allowing the water column driven by the hydro-pneumatic tank to start filling the hydraulic system by compressing the air phase \\ III & Air valve S050 & (entrapped air pocket), generating pressure surges. Valve BV3 remains closed to induce a rapid compression of the air pocket. The blocking water column serves as a boundary condition. \\ IV & Orifice—7 mm & \\ \hline V & None & The air pocket increases by 1.5 m, while all other conditions remain identical to Types I to IV. \\ VI & Air valve D040 & The right pipe branch is emptied to configure the incremental air pocket size (1.5 m) to have \\ VII & Air valve S050 & values between 1.96 and 2.86 m. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Types of filling procedures.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Type** & **Scenario (Air Valve)** & **Characteristics** \\ \hline I & None & Drain valves are partially opened. The air pocket is distributed equally in the two inclined pipe branches. \\ \hline II & None & Drain valves are opening. The air pocket is distributed equally in the two inclined pipe branches. \\ \hline III & None & Drain valves are opening. The air pocket is distributed non-equally in the two inclined pipe branches. \\ \hline IV & Air Valve S050 & The drain valves are opening. The air pocket is distributed equally in the two inclined pipe branches. The air valve S050 was installed to admit air flow. \\ \hline V & Air valve D040 & The drain valves are opening. The air pocket is distributed equally in the two inclined pipe branches. The air valve D040 was installed to admit air flow. \\ \hline \hline \end{tabular}
\end{table}
Table 2: Types of emptying procedures.  
Figure 2: Schematic representation of filling procedures.  
Pressure pulses are captured using an S-10 WIKA pressure transducer (WIKA, Klingenberg am Main, Germany), chosen for its ability to record values below 25 bar. For the emptying and filling procedures, a pressure transducer was installed at the highest point of the hydraulic installation, as illustrated in Figures 1 and 2. A PicoScope 3424 (Pico Technology, Cambridgeshire, UK) converted these signals into pressure units, which were then stored on a computer.  
A water velocity transducer (WVT) was used, and an appropriate frequency was selected based on the range of values. For this analysis, a 4 MHz frequency was employed. The transducer was installed in the right horizontal pipe branch to capture water velocity pulses. It was positioned at an inclination of 20\({}^{\circ}\). The WVT was connected to the ultrasonic Doppler velocimetry (UDV) system from MET-FLOW (Lausanne, Switzerland) for signal processing. Water velocities were measured during the emptying procedures.  
Additionally, since the hydraulic installation was transparent, allowing for the observation of water-air movement, a camera was used to track this interface. A Sony DSC-HX200V camera (Sony, Tokyo, Japan) was utilised for the emptying and filling procedures. A high-speed camera may be able to study air-water interfaces in greater detail for faster processes. A trigger was used to synchronise all the measurements.