## 2 Experimental Studies  
### Experimental Setup  
An experimental facility measuring 4.00 m in length and 2.35 m in width was constructed in the Hydraulics Department of LNEC and digitally reproduced for flow numerical analysis, as presented in the next section. This setup was specifically designed to generate vertical free-water jets with a maximum velocity of 18.0 m/s, as depicted in Figure 1a, from a 72 mm diameter nozzle, \(D_{i}\), 1.00 m above the facility base (Figure 1b). The setup includes two flap gates, each 0.65 m wide, located 2.00 m from the jet axis along the longitudinal direction. These gates control water levels within the facility, ranging from 0.30 to 1.10 m, thus enabling the creation of both plunging and submerged jets, given that the maximum pool depth can submerge the nozzle exit [14].  
The nozzle directs the jet flow vertically onto a 1.00 m \(\times\) 1.00 m metallic plate equipped with pressure transducers. Figure 2 illustrates the placement of these sensors, with the projection of the nozzle cross-section marked with the green circumference. The pressure transducers are distributed within a single quadrant of the metallic plate based on the confirmed symmetry of pressure readings across all quadrants. This symmetry was verified through preliminary tests in which pressures were measured in the other three quadrants of the bottom plate. A total of nineteen transducers were deployed throughout the study at various locations. The dimensionless distance from the projection of the jet's central point on the plate to the center of each sensor, \(r/D_{i}\), varies from 0.25 to 4.51. Four transducers were intentionally placed near the jet stagnation point \(r/D_{i}<0.50\), and two pairs with identical \(r/D_{i}\) ratios (0.35 and 2.04) were positioned to further double-check for symmetry conditions during the testing procedure. The number of sensors positioned in the jet stagnation zone is relevant since this approach intends to address existing knowledge gaps regarding pressure levels in this area, where previous research has typically considered only one or two sensors in this zone [3; 9; 10; 11].  
Figure 1: Experimental facility: (**a**) general view, (**b**) front view with the nozzle and flat gate.  
Figure 2: Pressure transducers’ non-dimensional locations, as a function of \(r/D_{i}\).  
Five Kulite XTM-190 (M)-17 BAR-A transducers were arranged near the jet central axis (3.9 mm diameter, 17 bar range, and 1% accuracy), and fourteen ADZ Nagano SMF in the remaining measurement locations (25.4 mm diameter, 20 bar range, and 1% accuracy), as shown in Figure 2. The Kulite transducers were predominantly positioned near the jet axis to optimize measurement accuracy, as their diaphragm area is very small. The data acquisition system comprises three HBM Spider 8 data loggers controlled by a laptop. Measurements were acquired at acquisition frequencies between 600 Hz and 2400 Hz. The data series generated include 131,072 (\(2^{17}\)) values for the 600 Hz acquisition frequency and 524,288 (\(2^{19}\)) values for the 2400 Hz, spanning approximately 218 s of measurements. These dual frequencies were selected based on preliminary tests, which indicated that the 600 Hz frequency was insufficient for fully capturing the pressure spectrum at high velocities and lower pool depths [25].