Mahsa Fooladgar

Numerical and experimental modeling of forced hydraulic jump by a vertical sill

Abstract

Supercritical downstream flow in some hydraulic structures causes erosion and damages to these structures. Due to the high energy dissipation, the hydraulic jump is used downstream of hydraulic structures to reduce the erosion. To ensure the formation of a jump and to control its position, hydraulic jump control structures should be utilized. Some of these structures are sill, positive and negative step.

The objective of this study is to model numerically and experimentally the forced jump upstream a sill (sharp-crested and broad-crested weirs) in a horizontal channel. OpenFOAM software is used to build the numerical models. The sills have heights of 0.02 to 0.11 meters, lengths of 0.02 to 0.3 meters and distances from the entrance section from 0.56 to 2.45 meters. The upstream Froude numbers are set at 3.2, 4.8 and 6.4. To validate the numerical results, sill upstream flow features are analyzed in both jump and jet conditions. In jet condition, pressure distributions at upstream sill brink are compared with previous experimental results. The hydraulic jump characteristics are compared with empirical relations. Experimental models are built to compare with numerical model results. The experimental model consists of a hanging baffle upstream (entrance section) to create the supercritical flow and a sill located 1.6 meters from the hanging baffle with 0.42 and 0.02 meters height and length, respectively.

Numerical simulations showed the impact of parameters such as length, height and distance from the entrance of a sill in forming the hydraulic jump. By reducing the sill relative height (height/entrance depth) having a constant relative distance (distance/entrance depth) from the entrance section, it is possible to eliminate the hydraulic jump and form the jet flow. To form a jump, the minimum sill relative height is a function of upstream Froude number and it reduces by increasing the relative distance of the sill from the entrance section. At a constant relative distance from the entrance section, minimum sill relative height and length have parabolic distributions. For relative sill lengths (sill length/critical depth) of 0.2 to 2.7, the minimum relative height increases. At a constant relative distance, further increase of sill relative height leads to the formation of submerged jump. The maximum possible relative sill height to prevent the submergence of the jump increases at higher Froude number for a constant relative distance and reduces with lower relative distance.