The impact of water jet angle on the characteristics of hydraulic jump

Document Type : Complete scientific research article


Associate Professor, Department of Water Engineering, College of Agriculture, Ferdowsi university of mashhad


Background and Objectives : Hydraulic jump is one of the common methods of dissipation of energy of the super-critical flows in open channels. This phenomenon is a kind of rapid, varied flow that exists when the super - critical state of of flow converts to the sub-critical state. It causes to considerable dissipation in value of the energy. In this research a new method introduced for decreasing both values of the conjugate depths and the hydraulic jump's length. This method was based on using the specifications of a rectangular free-jet for affecting to the jump features. In fact, fast impact of the jet into the jump and shifting the momentum value to it, affected the jump's specifications and situation.
Material and methods : This research was an experimental study.The experimental setup was including the flume with the walls of the glass materials in the dimensions of the 1200*30*50 cube centimeter. A contorl gate used to fixing the situation of the jump, in the downstream of the flow. The depth of the flow measured by two methods of direct and indirect named. Indirect method included installing the piezometers in the flume bed and reading the heigth of the water column in the piezometers by a camera with a high resolution.Then it measured by a plotter of the flow’s depth software.In order to the investigation of the effect of the discharge and jet angle on the jump’s specifications, the tests performed in various conditions. These conditions include of the three different jet’s discharges of 2, 2.5 and 3.2 litre per second, the jet’s angles of 60 and 90 degree versus the horizontal direction and two specific angles that their direction considered toward the maximum displacement from the beginning point of the jump direction and without any displacement and change at the beginning point of the jump direction.
Findings : The hydraulic jump at a special jet angle had not any displacement that this angle was named as the neutral angle. As the jet’s angle increased, the jump moved toward the upstream until the angel reached to the extent with no movement of the jet to the upstream, this angle called as the maximum angle of the jump displacement. Change in the angle and discharge of the jet resulted in changes in the secondary depth, jet length, relative energy loss and bed shear stress. At the maximum angle of the 60 degrees, and the discharge equal to the 3.2 liter per second in the minimum Froude number, the secondary depth ratio increased by 25.4 percent. Using a jet with the angle of the 60 degrees and the discharge of the 3.2 liter per second in the maximum Froud number of the flow caused the increasing in the secondary depth ratio by 8.7 percent. The maximum reduction in the jet length (e.g.48.3 percent) occurred at the maximum jet angle and discharge equal to the 3.2 liter per second in the minimum Froude number. The maximum increasing in the jet length (e.g.15.7 percent) happened in the angle of the 60 degrees, discharge about 3.2 liter/s and the maximum Froude number. In the maximum angle of the jet, discharge about 3.2 liter/s and minimum Froude number, the relative energy loss, increased about 13.9 percent and finally using the jet’s angel equal to the 60 and 90 degrees caused decreasing and increasing of the bed shear stresses , respectively.
Conclusion: Applying the jet to the jump with an angle greater than ineffective angle caused decreases in the secondary depths ratio and jump length while increasing in the energy loss and bed shear forces


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