*3.3. Sensitivity Analysis*

### 3.3.1. Effect of Air Pocket Sizes

It is important to identify the grea<sup>t</sup> influence of the size of the entrapped air pocket on the minimum of the subatmospheric pressure. Figure 9 shows the results taking different air pocket lengths (0.001, 0.540, 0.920, 1.320 and 2.120 m). The smaller the air pocket size, the lower subatmospheric pressure is obtained. Equation (15) shows this situation with the comparison between the gradient of the absolute pressure (*dp*<sup>∗</sup>*i* /*dt*) and the air pocket volume (*A*2(*<sup>L</sup>*2 − *Le*,<sup>2</sup>) + *<sup>A</sup>*1(*<sup>L</sup>*1 − *Le*,<sup>1</sup>)). For the air valve *S*050, subatmospheric pressures are found in the range of 9.61 *mH*20 and 10.11 *mH*20, while, for the air valve *D*040, small variations are found between 10.16 *mH*20 and 10.32 *mH*20, showing the adequacy of this air valve for the emptying process. It also shows the importance of the air valve size since it can induce critical conditions associated with the subatmospheric pressure occurrence.

**Figure 9.** Effect of the air pocket sizes on the minimum pressure attained.

### 3.3.2. Maximum Water Velocity

Figure 10 shows a comparison between computed and measured maximum water velocities. The proposed model predicts the maximum values of the water velocity for all tests. For the air valve *S*050, maximum values of water velocity are found in the range of 0.049 to 0.079 m/s, while, for the air valve *D*040, water velocities are found in the range of 0.193 to 0.397 m/s.

**Figure 10.** Comparison between computed and measured maximum water velocities.
