Experimental Investigation of Supercritical Flow Energy Dissipation in Sudden Contraction with Wall Roughness

Introduction
One of the most important problems in hydraulic structures is the kinetic energy of the flow. If this destructive energy is not controlled, the structures downstream will be damaged and cause significant damage. One of the functions of energy dissipator structures is to change the flow regime, reduce the flow velocity and eliminate excess flow energy. One of the methods that can deplete the flow is to create a narrowing along with the roughness in the flow path. By creating a constriction in the flow path and using roughness in the constriction wall and forming a hydraulic jump, a significant part of the destructive energy of the flow can be dissipated. Hydraulic jump is a common phenomenon downstream of hydraulic structures that increases the flow depth by rapidly converting the flow from supercritical to subcritical and plays an important role in influencing hydraulic parameters. Examining previous studies on stenosis with natural wall roughness, what is certain is that no laboratory and numerical studies have been observed so far.
Methodology
A laboratory flume with a rectangular cross-section 5 meters long, 0.3 meters wide, 0.5 meters high, and with Plexiglas floors and walls was used to perform the experiments. Two pumps each with a capacity of 450 liters per minute enter the flow into the flume and the flow inlet flow is read by two rotameters with an error of ± 2%. A point gauge with an accuracy of one millimeter was used to measure the water depth, and a construction meter with an accuracy of one millimeter was used to measure the length of the jump length. To create narrowing in the cross section of the channel from glass boxes with a fixed height of 0.5 m, widths of 2.5, 5, 7.5 cm on each side and to create supercritical current upstream of the section Narrowing A valve with a width of 0.3 m, a thickness of 3 mm with an opening of 2 cm, which is located at a distance of 1.5 m from the narrowing, has been used.
Figure 1, schematic of the canal and the equipment installed on it and Figure 2, an example of stiffening placement, hydraulic jump formed in the flow path and rough placement method with three average diameters of 2.08, 1.28, 0.8 Shows centimeters on the wall.
Results and Discussion
In order to achieve the objectives of the present study, flow path constrictions have been provided using glass boxes and sand materials have been used to roughen the constriction walls. In total, 270 experiments were performed in the range of Froude number 2.5 to 7.5 and relative contraction range of 8.9 to 12.42. By adjusting the laboratory model and opening the pump, the flow enters the channel supercritically after passing through the vertical valve and moves towards the narrowing section.
As the downstream Froude number increases, the relative energy dissipation increases and this amount is greater than the energy dissipation in the constriction of 15 cm. The reason for this is that due to the collision of supercritical flow with the constriction section, the backwater profile at the point of water collision with the sides of the constriction elements and also the formation of hydraulic jump upstream of the constriction section increase turbulence and climate interference and consequently it increases the relative energy dissipation.
Changing the diameter of the rough particles has little effect on the amount of flow energy dissipation. But it can be seen that the effect of roughness with an average diameter of 1.28 cm is slightly more than other roughnesses. The reason for this is that the roughness of 1.28 cm has more contact surface with the flow and also the empty space between the grains increases the surface tension and shear stress. Therefore, some of the energy is wasted by hydraulic jumping and some of it is consumed by the backflow of the flow.
Conclusion
Comparison of the models with each other as a control showed that with increasing the amount of narrowing of the channel width, the relative energy consumption increases. According to laboratory results, the use of roughness significantly increases the relative energy consumption relative to the upstream. Energy dissipation in 15 cm constriction is 25.48%, 20.88% and 23.83% less than 0.8, 1.28 and 2.08 cm roughnesses in control mode, respectively. Energy dissipation in 10 cm constriction is less than 44.34, 43.68, and 40.63% in roughness compared to 0.8, 1.28, and 2.08 cm roughnesses, respectively. Energy dissipation at 5 cm constriction is 50.75, 51.19, and 40% less than 0.8, 1.28, and 2.08 cm roughnesses in control mode, respectively.