Investigation the characteristics of hydraulic jump in a rough bed condition using a physical model

Background and objectives: The hydraulic jump or water jump, is one of the fast variable flow types and if the water have a supercritical condition in a part of direction, and in special situation the channel wants to change to the subcritical condition, it’s necessary in a very short path the flow’s depth increase significantly and as a result will face a significant energy depreciation. So far many studies were carried out for investigation of hydraulic jump characteristics on rough beds with different forms. Including: Ghazali et al. (2010) analyses the effect of stilling basin floor’s triangle shaped roughness on hydraulic jump characteristics. The results shows that the tailwater depth and jump length on the triangular rippled beds in compare of smooth bed are smaller 25 and 54.7 percent respectively and the sheer stress factor in a rough bed is 8.5 percent more than smooth bed situation in average. One of the solutions which can replace with the bed blocks, is the creation of roughness in flow’s bed. Also Samadi Borujeni (2013) studied the hydraulic jump parameters for six rippled bed. Results showed that the rippled bed in compare with the smooth bed shows a decrease of 25 percent in conjugate depth and a decrease of 54.7 percent in jump length and the factor of sheer stress of the rippled bed was 8.5 times bigger than smooth bed. In this research the characteristics of hydraulic jump has been studied in rough bed condition with the use of physical model. The purposes of this research includes assessing the average diameters of particles at natural roughness condition of bed on the secondary to primary depth ratio, hydraulic jump relative energy dissipation, sheer force coefficient, Comparing the experimental results with the results of other similar research on a variety of rough beds and eventually representing relationships for each of the above terms.
Materials and methods: In this research the characteristics of hydraulic jump on natural rough beds with aggregated particles carried out with an average diameter of 4.75, 3.55, 2.18, 1.59, 0.74 millimeters and a Froude number of 3.5 to 4.5 in a rectangular horizontal flume with the length of 5 meters and width of 0.075 meters and the heights of 0.175 meters. For creating a hydraulic jump, ogee spillway with the height of 0.12 meters has been used in a condition which places the rough beds in downstream. For dimensional analysis, after identification of effective parameters on hydraulic jump, dimensionless numbers, has been extracted using the Buckingham π-theorem.
Results: Relative depth, the dissipation of jump relative energy, sheer force coefficient, was obtained as a function of Froude number. The results show’s that between all of studied roughness with different values of roughness relative depth, has no significant differences in terms of influence on decreasing relative depth, increase relative energy dissipation and increase sheer modulus, but the effect of this rough values can be seen in compare with smooth bed.
Conclusion: Results show that the rough bed in compare with smooth bed, will decrease the jump relative depth in an average of 56.7 percent, and the relative energy dissipation increases 69 percent and the sheer modulus increases 2.6 percent. On the other hand, comparing of this experimental results with the other similar researches on different types of rough beds show that the graph of the fitted Curve ∆E /E_1 to Froude number, is matches the fitted graph on triangular substrates which Samadi Broujeni (2013) was studied, and changes in shear force coefficient and jump relative depth in the range of above Froude number will be increased which expresses match results.