3 Introduction Open channel hydraulics, a subject of great importance to civil engineers, deals with flows having a free surface in channels constructed for water. Open channel flow of water has been used in aquaculture produc- tion for many years. Distribution canals, raceways and drainage ditches are some examples. Open channel flow refers to any flow that occupies a defined channel and has a free the fluid weight causes the water to flow down a slope.
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The collision is called a hydraulic jump. A hydraulic jump is characterized by a depth of flow that increases in the downstream direction. A hydraulic jump is analogous to a shock wave in aerodynamics. It is a turbulent, non-energy-conserving process that passes from rapid flow to tranquil flow.
Unlike a shock wave, its front is not vertical, but the depth increases to its final value in a distance about water flow in open channels to six times the final depth. However, momentum is conserved, and this permits us to find the final state if we know the initial state.
- Open-channel flow - Wikipedia
- Flow in open channels
- CHAPTER 6: Water Flow in Open Channels - ppt video online download
- CHAPTER 6: Water Flow in Open Channels
- Presentation on theme: "CHAPTER 6: Water Flow in Open Channels"— Presentation transcript:
We will assume the channel is horizontal, since the hydraulic jump takes place in a limited length of channel, and gravitational energies will not be important. Let the water enter the jump with velocity V1 and height y1, and leave with velocity V2 and height y2.
This is one place where the pressure plays an explicit role. The hydraulic jump curve for a wide rectangular channel is shown at the right.
Rectangular Open Channel Flow and Hydraulic Design
The dashed curve is the specific energy curve for the same discharge q. An initial state a is taken into state b by the hydraulic jump. We can combine the hydraulic jump curve with the specific energy curve to find the energy loss in the jump.
Horizontal lines through a and b will intersect the specific energy curve at the corresponding energies. Hydraulic water flow in open channels curves for other channel shapes will be similar. The length of the jump is about 6 times the greater height y.
If the channel is not horizontal, in the momentum balance we must add the weight of the water between y1 and y2 times the sine of the slope, which is S, approximately, since the angle is small.
When the slope changes from steep to mild, a hydraulic jump occurs at a point such that y2 is the normal depth on the downstream end. Depending water flow in open channels the corresponding value of y1, the jump may occur either after or before the break in slope, wherever the required value of y1 can be found.
If y1 is greater than the depth on the steep water flow in open channels, then the jump will occur on the mild slope when the height of the rapid flow reaches y1. If it is not, then the jump will occur on the steep slope to a depth that becomes the normal depth on the mild slope.
This may be illustrated by the jump shown in the figure at the left. The water comes through a gate with high velocity and a depth less than the critical depth, so the flow is rapid.
Flow in open channels
The depth conjugate to y1' is y2'. Since this is greater than the normal depth on the water flow in open channels slope, the jump will not occur immediately, because there is insufficient energy. The rapid flow decelerates on the mild slope, and the water depth increases, bringing down the conjugate depth.
Both the energy line and the water surface are slightly concave upwards in this region, though drawn as straight.