Experimental study of submerged hydraulic jumps generated downstream of rectangular plunging jets

•There is a small reduction in the free surface undulation along the stilling basin.•The phase change frequency behavior is similar to those reported by several authors in free hydraulic jumps.•The higher phase change detection frequencies are obtained near the bottom for local void fractions < 0.15-0.20.•The maximum change detection frequency seems to be linked with the turbulent shear stresses near the bottom.•The non-dimensional velocity near the bottom may be described by previous formulae obtained in submerged hydraulic jumps. This experimental study analyzes submerged hydraulic jumps generated in a 1.05-m wide stilling basin that receives overflow rectangular impinging jets, with the vertical distance between the sharp-crested weir and the bottom of the plunge pool being 2.20 m. Five submerged hydraulic jumps downstream of non-developed nappe jets were analyzed, with vertical falling distances to break-up length ratios between 0.76 and 0.78. The impingement velocity range was from 6.12 to 6.33 m/s, while the water depth / impingement jet thickness ratios were between 8.45 and 15.00. The velocity field and air entrapped were measured in several cross sections, with the farthest sections being located 1.0 m from the stagnation point. The entrapped air (void fraction and phase change detection) behavior measured with a phase-detection probe was classified as a function of the distance to the stagnation point / impingement jet thickness ratio and/or the mean void fraction of the vertical profiles. The maximum void fractions were observed near the free surface. However, the higher phase change detection frequencies (up to 130-140 Hz) were obtained near the bottom for local void fractions smaller than 0.15-0.20. The non-dimensional velocity distribution seems to be similar to previous published studies, extending their validity range. Furthermore, the free hydraulic jumps formula has been adapted to estimate the air rate in the shear stress layer and the growth of the characteristic length in the plunge pool.