Why it is necessary to ventilate the space below the nappe?
A free nappe, which is ventilated to maintain atmospheric pressure below, does not come into contact with the underside of the weir. The negative pressure leads to a 6\% to 7\% increase in discharged water compared to a free nappe. Clinging nappes have no air beneath, and the stream flows along the face of the weir.
What is ventilation of weir why it is required?
Ventilation of notch or weirs: The air below the nappe and the walls are trapped. This air gradually carried along with flowing water and the pressure between the nappe and wall is reduced below atmospheric pressure. Eventually negative pressure is developed and as a result, the nappe starts to depress.
Why it is important to study the types of nappe?
It is especially important to observe the behavior of the weir nappe to ensure the design provides hydraulic optimization and to account for pressure fluctuations, possible vibrations, resonance effect, noise and flow surging.
What does nappe and sill mean?
While the stream of water discharged by an orifice is called a jet, the sheet of water discharged by a notch or weir is called a nappe or vein. The upper surface of the notch or weir over which the water flows is called the Crest or Sill.
When there is no air left below the nappe?
The process of removal of air continues further and at a certain time, no air is left in the space. The nappe in this state adheres or clings to the downstream side of the weir. Such nappe is called clinging or adhering nappe. The discharge of this clinging nappe is 25 to 30\% more than the discharge of the free nappe.
What is weir in fluid mechanics?
A weir /wɪər/ or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. They are also used to control the flow of water for outlets of lakes, ponds, and reservoirs.
What happens when the water flows approaching a weir?
Consider a liquid flowing over a sharp crested weir as shown in Figure 8.8. Let the upstream level of the liquid be z0 above the level of the weir crest. As the liquid approaches the weir, the liquid level gradually drops and the flow velocity increases. Downstream from the weir, a jet is formed.
What is aeration of nappe?
The Need for Nappe Aeration In a suppressed rectangular weir, the nappe stretches from channel wall to channel wall. This reduction of air under the nappe creates a pressure gradient. Ultimately the nappe collapses against the downstream face of the weir unless new air is allowed to replenish the entrained air.
What nappe means?
nappe, in geology, large body or sheet of rock that has been moved a distance of about 2 km (1.2 miles) or more from its original position by faulting or folding.
When no air is left below the Nappe and water stream adheres to the down?
How does the nappe of a water pump work?
The nappe emerging out from the notch or weir touches the side walls of the channel. The air below the nappe and the walls are trapped. This air gradually carried along with flowing water and the pressure between the nappe and wall is reduced below atmospheric pressure.
Why does the nappe of a toilet depress?
The air below the nappe and the walls are trapped. This air gradually carried along with flowing water and the pressure between the nappe and wall is reduced below atmospheric pressure. Eventually negative pressure is developed and as a result, the nappe starts to depress.
How do you measure the discharge over notch?
The discharge over notch is measured by measuring the head acting over the notch. As water approaches the notch, its surface becomes curved. Therefore, the head over the notch is to be measured at the upstream of the notch where the effect of curvature is minimum.
Do weirs present a problem with discharge measurement?
Rating tables and dimensions of weirs for measuring discharges up to 1300 cfs are presented. Weirs present problems if the flow is transporting sediment or debris. Sedimentation tends to occur immediately upstream from the weir causing shifts in the stage–discharge relationship and requiring frequent clean out.