**4. Discussion**

An extensive experimental work for AP behavior cognition has been carried out in Instituto Superior Técnico (IST) experimental laboratory. Tests are done with three WH events: 1WH, 5WH and 9WH. Also, two situations of the BV in the CAV, i.e., closed BV and open BV were conducted. In the closed BV tests, both peak pressure and stored pressure were raised by increasing the Re number and decreasing the VFR. In addition, stored pressure is much lower than peak pressure for small VFR, i.e., VFR < 11%. The pressure storage process is very regular and predictable for the VFR > 11%, but for smaller VFR values, the pressure storage starts with a high peak at the first WH occurrence. The stored pressure is always higher than the peak pressure (Ps > P1p) for Re ≤ 75,000. Also, the stored pressure is always higher than the peak pressure (Ps > P1p) if VFR > 11%. Despite being more frequent in 9WH tests, the stored pressure is not always higher than for other events. In fact, for the VFR < 30% and Re > 93,000, the stored pressure from 9WH is smaller than for 5WH. For reasoning, it can be stated that in one level of WH repetitions, the air reaches maximum compression for this pressure range. After that, more WH actions do not compress the air but lead to pressure loss by solving some portion of air into the water. Regular and equal pressure steps can be seen in the rising pressure of several WH events for Re < 75,000. For higher Re numbers, the pressure change route starts with a peak and dissipates fast, continuing with smaller steps. The stored pressure is in correlation with Re but in contrast to the VFR.

The pressure in the pipe system shows that increasing the VFR or AP size will diminish the pressure rising within the pipe. The final pressure in the pipe is more or less the same for the VFR > 20%. So, it seems that the VFR in the range of around 20% to 30% presents good ability to control the pressure without expanding the air vessel size.

It is very important for the optimal size of a CAV that the rising water level (Δh) stays in an acceptable range. The VFR from 20% showed good performance in controlling the pressure surge of WH events. For this range of VFR, the Δh remains below 20% of the CAV length.

In the "open BV test", the condition of using the pressure energy of the WH to supply water as a ram pump was examined. By increasing the Re number, the outflow water volume will be increased too. The water level inside the CAV keeps the same elevation due to confinement and

vacuum conditions. For the VFR > 32%, the WH events create a constant outflow from the CAV with regular compression and expansion in the AP in the 9WH tests.

This work presents an extensive experimental study of different WH events that can be used as a benchmark for future calculations using WH equations.

Finally, authors believe that the main effective parameter in pressure change inside the AP is the volume of the AP and flow velocity which have been referred by the dimensionless parameters VFR and Re respectively in the graphs. However, for different CAV sizes, namely diameter or height, the results may change. As a suggestion, researchers may examine the effect of different air vessel sizes on pressure change and other variables of an AP.

**Supplementary Materials:** The following videos from "open BV test" and "closed BV test" for 5WH and 9WH events are available showing the continuity of the flow discharge for APs of 20 and 40 cm in case of 9WH events. Videos AP = 5\_5WH, AP = 5\_9WH, AP = 10\_5WH, AP = 10\_9WH, AP = 20\_5WH, AP = 20\_9WH, AP = 40\_5WH, AP = 40\_9WH. The videos are available online at www.mdpi.com/2073-4441/9/1/63/s1.

**Acknowledgments:** Authors appreciate the financial support from Ph.D. gran<sup>t</sup> ref. PD/BD/114459/2016 issued by FCT under the IST-LNEC Joint Doctoral Initiative supported by the Civil Engineering, Research, and Innovation for Sustainability (CEris) Center, Department of Civil Engineering, Architecture and Georesources, Instituto Superior Técnico (IST), University of Lisbon (ULisbon), Portugal and providing the experimental facilities for conducting the tests.

**Author Contributions:** Mohsen Besharat has fulfilled the experimental activities. Results are analyzed and the paper is written with the contribution of Helena M. Ramos and Maria Teresa Viseu.

**Conflicts of Interest:** The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses and interpretation of data; in the writing of the manuscript; and in the decision to publish the results.
