Laboratory Investigation of the Effects of Rectangular Zigzag Blocks Geometry on Hydraulic Jump Characteristics in Trapezoidal Channel

Hydraulic jump is a phenomenon in which the flow regime changes from supercritical to subcritical. The characteristics of hydraulic jump are rapid expansion of the flow with turbulence, high local energy loss and increase in the height of the free surface of the water. Due to the nature of the flow, the specific force remains constant before and after the hydraulic jump.
In this phenomenon, the water depth increases in a short distance and consequently the flow velocity decreases. This increase in depth and decrease in velocity, together with a noticeable decrease in energy, causes turbulence in the flow and converts the kinetic energy of water into heat energy. The turbulence created in the stream decreases as it approaches the end of the jump. Installing obstacles with different geometries on the path of flow have an important effect on controlling the jump location, reducing the length of the hydraulic jump and increasing the energy dissipation of flow during the hydraulic jump.
The research flume is 6 meters in length and 60 centimeters in height.The flume is trapezoidal with a floor width of 16 cm and the side of the trapezoid has a slope of 75 degrees and in the length has a slope of zero degrees. Water is pumped from a storage tank to the reservoir at the beginning of the flume (Figure 1). To provide the water load required to achieve higher froude numbers, a sediment tank at the beginning of the flume, which is cylindrical in shape, has been used. This tank is measured by a gauge with an accuracy of 1 mm to read the height of the water inside the tank. In order for the flow to be developed at the beginning of the channel, a part called the jet box is used. By controlling the opening of this part and the pump flow, the desired head in the tank can be reached as well as certain froude numbers (Figure 2). A total of 60 experiments were performed. The blocks have three heights and the distance between the rows of blocks in three modes, so the arrangement of the blocks is divided into nine types. The height of the stream is considered in six different cases. To draw the jump curve, water depth was measured at three points including the end of the blocks, the point equal to the middle of the jump length, and the water depth at the end of the jump.
The ratio of the secondary depth of the hydraulic jump to the primary depth is one of the most important parameters used in the design of relaxation ponds. According to Figure 9, the ratio of hydraulic jump conjugate numbers related to the experiments of this research can be defined by the mean curve of the relation 5. Comparing this formula with the experiences of previous researchers shows a good agreement. The results of previous studies by other researchers also show a decrease in the ratio of conjugate depths to smooth surface if obstacles in the waterway are used. with the diagram of figure 10, it can be said that the secondary depth of hydraulic jump is reduced if rectangular blocks are used. Also, to reduce this depth, increasing the height of the block has a greater effect than increasing the distance between the blocks. Also by shortening the length of the hydraulic jump, the length of the required pond will be shortened and it will be more economical. According to Figure 12 and 13 the length of the hydraulic jump compared to the flat bed decreased by an average of 49.5%. In a hydraulic jump, when the flow lines collide with each other, the perturbation created in the flow path, the kinetic energy related to velocity, is converted into heat energy and causes the kinetic energy of the flow to be depleted and its regime to change from supercritical to subcritical. Therefore, calculating the amount of kinetic energy consumption and jump assessment is one of the important topics in this discussion. Figure 14 shows the ratio of energy drop to initial flow energy versus initial froude number in all experiments. According to the graph, with increasing the froude number, the value of this ratio also increases, which in the maximum value is approximately equal to 85.5% energy loss. To determine the effect of rectangular blocks on changes in this energy loss ratio, parameter G is defined as Equation 19. According to Figure 15, the maximum value of this parameter is 84.8% and the minimum value is 0.8%.
A diagram related to the dimensionless profile of the water surface shows that the profiles related to all experiments can be defined by a regression curve with a good approximation. The ratio of sequent depths of hydraulic jump with rectangular blocks to smooth bed decreases. This decrease is more noticeable within higher froude numbers. The maximum value of parameter D of the hydraulic jump in this study is equal to 9.4%, which indicates the effect of increasing the height and the distance between the blocks on the secondary depth of the jump. The maximum drop of flow energy on rectangular blocks is equal to 85.5%. Increasing the height of the blocks has a more effective role in reducing the flow energy than increasing the distance between the blocks.