Drag Forces on Baffle Blocks in Hydraulic Jumps

Large bluff-shaped objects (baffle blocks) placed in the jump improve the hydraulic jump performance characteristics. Knowledge of the drag force on the blocks would aid in determining the optimum block geometry. The highly turbulent and nonuniform flow in the jump prevent a theoretical solution to the problem. An experimental technique similar to that employed by aerodynamicists was used to directly measure the drag forces. Block location, height, width, spacing, second-row location, and shape were the geometrical variables investigated. The study was limited to nonsubmerged jumps in wide, rectangular, horizontal channels. A drag force ratio using the free jump, sequent depth pressure force proved more practical for data correlation than a classical drag coefficient. Resultant downstream water depths computed using the measured drag force compared favorably with experimentally measured values. The results enable the designer to calculate the drag force for a wide range of block geometries and jump inlet Froude numbers from 3 to 10.