Laboratory investigation of the effect of width and height of continuous triangular roughness on hydraulic jump characteristics in horizontal relaxation ponds

Abstract

Hydraulic jump is one of the amazing hydraulic phenomena that consumes the kinetic energy of water. Hydraulic structures such as relaxation ponds mainly use this important property of hydraulic jump to consume energy downstream of overflows, rapids and valves. The dimensions of these structures are directly dependent on the specifications of the hydraulic jump, therefore, in order to make these structures economic, extensive studies have been conducted for a long time to study the characteristics of the hydraulic jump and how to control it or minimize its dimensions. Among other things, in recent years, it has been determined that the continuous roughness of relaxation ponds can be effective in reducing the dimensions of the jump, with such hypotheses, the present research has been carried out in order to determine the extent and manner of its effect by conducting laboratory tests. With this purpose, several experiments (90 experiments) were conducted for the input landing numbers in the range of 5.19 to 12.5 in the flume with a width of 35 cm. In these experiments, triangular roughnesses with three different heights (2, 4, and 6) and three different widths (4, 8, 12) cm and their combination were installed and tested on the flume floor so that the upper surface of the roughness was on the same level as the surface of the overflow claw, and parameters such as flow rate, primary depth, secondary depth, roll length, jump length, and water surface profile were carefully measured. The data analysis showed that The roughness reduces the length of the jump by an average of 50% and the secondary depth of the jump by about 20% compared to the classic mode. The reason for this reduction can be seen as the increase in flow turbulence between the roughnesses, which causes an increase in shear stress by about 13 times. Chapter One Generalities In the design of hydraulic systems, the flow velocity should not be more than acceptable. The high velocity of water in an earthen channel or in a natural river causes wear of the channel bed and wall, which may cause irreparable damage to the facilities adjacent to the river. However, in many cases, due to various reasons, such as the steep slope of the earthen channel floor, or the large difference in energy between two sections, or the free fall of water, we encounter excessive kinetic energy of water, which inevitably requires us to design structures to consume excess kinetic energy and minimize the flow speed.

Downstream of the headwaters of the dams, due to the high height difference, the flow speed and finally the kinetic energy increases greatly, and failure to reduce this energy will cause erosion of the bottom and create a pit downstream of the dam, which in the long run will cause the dam to overturn, so we must somehow eliminate this high speed (high kinetic energy) in hydraulic systems.

Structures that reduce the flow energy and Reducing the speed to a minimum is acceptable, it is called Energy Dissipators structure.     

In summary, these structures are as follows

A- Energy consuming structures in the horizontal direction In these structures, which are mostly seen downstream of the small head of the dams (peak head) and in the canals and at the end of the breakwaters, the water energy is lost in the form of hydraulic jumps, which are types of relaxation ponds of this type.

B- Consuming structures Energy-consuming in vertical direction: these structures are mostly used in irrigation canals, among which we can mention a manifold basin (mani fold), relaxation well, and all kinds of flip buckets.

C- Energy-consuming structures in both vertical and horizontal directions: these structures include vertical and inclined slope breakers, which are mostly used in irrigation canals.

According to the above, one of the usual ones The most methods of energy consumption in the horizontal direction is to create hydraulic jump. Hydraulic jump or water jump is an interesting phenomenon that due to its great application in hydraulic science, many studies have been conducted on it. Hydraulic jump is a phenomenon that occurs due to the change of flow from supercritical to subcritical. At the same time, many disturbances are created in the form of strong eddies and reverse eddy currents, which cause a significant amount of aerated water to enter and the surface of the water has a white, foamy and disturbed appearance..

In a water jump phenomenon, y1 and y2 depths (corresponding depths), jump length Lj, energy loss, pressure distribution, and velocity profile should be measured. One of the important features of the water jump is the length of the jump, which is very important to obtain because it has a direct effect on the length of the relaxation pool. The use cases of hydraulic jump are as follows.

Reducing the energy of water flowing over dams, overflows, and other hydraulic structures and finally protecting the downstream parts.

2- Increasing the water level in the channels in order to spread the water.

3- Increasing the output flow rate from under the valves by keeping the surface or finder away.

4- Reducing the lifting pressure below. Structures by increasing the depth of water in the slope of the structure. 5- Mixing chemicals for water or sewage treatment and also for agricultural purposes. 6- Separating the trapped air from the currents in the open circular channels. Therefore, in order to create a jump right downstream of the structure and prevent it from moving downstream, you should use relaxation ponds. These basins have rectangular sections and their walls are made of concrete.

So, a horizontal concrete floor should be made at the bottom of the structure and the required depth of footing (the depth below the jump in the channel and not the secondary depth) should be created in it until the secondary depth of the jump is equal to that depth. Lower the pool or create a short vertical wall (stairs) at the end of the horizontal floor, and then by doing this, we make sure that the jump is definitely formed, but it is important that the length of the jump is sometimes 1/6 times the secondary depth, and this covers a large length, which increases the cost because the structures are concrete. Therefore, in the design of relaxation ponds, initial, end or middle blocks are also used, which helps to reduce the length of the pond. So, the main goals of building relaxation ponds is to stabilize the hydraulic jump, and the other is to help create the hydraulic jump. Shortening the length of the jump will only help to reduce costs. For this reason, different relaxation ponds were tested, each of which is useful for a described type of jump. These ponds have become types after many tests and their confirmation, and now for the design of each of them, you can refer to their main type. In general, there are two general types of relaxation ponds, one is USBR standard relaxation ponds, which itself is divided into several types, and one is SAF standard relaxation ponds, which are described below.

In the past seventy years, due to the great importance and application of hydraulic jump, extensive studies have been conducted on this matter. It is called classical, which has a two-dimensional flow pattern. This type of jump has already been studied by great researchers. The main goals of his studies are to create relationships to predict jump characteristics such as jump length, required footing depth. The amount of energy consumption, flow velocity distribution, pressure fluctuations, water level profile during the jump has been. The results of the research show that the roughness of the bed is effective in reducing the length of the hydraulic jump and the depth of the footing. Considering the importance of the two parameters of the length of the hydraulic jump and the depth of the footing in reducing the costs of energy-consuming design and construction, there is still a need for studies on this matter. During the latest studies conducted by Idu Rajaratnam (2002), which used wavy bed, the laboratory results showed that if the bed is wavy, the amount of jump length and footing depth will decrease significantly. We can also refer to the research of Shafai and Izadejo (2003). In their research, by placing wave-shaped roughnesses in the bed of the calm pond, they showed that the length of the hydraulic jump has decreased by 50%, and the proportion of conjugate depths has decreased by 30%. The studies of Carrollo et al. There are hydraulic structures in jump-type energy absorbers, it is usually created with the help of a baffle and inside the relaxation pond. So far, extensive studies have been conducted in the field of relaxation ponds. The aim of all these studies was to create economic ponds.