Mechanical Engineering - Hydraulics and Fluid Mechanics - Discussion

Which is the correct answer?

What is the means of negative pressure?

Sometimes a weir is not fully ventilated, but is partially ventilated as shown in fig-2 (b). If the pressure below the nappe is negative, it is called a depressed nappe. The discharge of the nappe, in this case, depends upon the amount of ventilation and the negative pressure.

Generally, the discharge of a depressed nappe is 6% to 7% more than that of a free nappe. I hope it'll help you.

Because pressure toward downward direction is positive and toward upward direction is negative.

Half of double cone is called nappe.

Depressed Nappe:

In this type, a partial vaccum is created between the nappe and the weir. Discharge for such a flow situation is 8 to 10% greater than that with a free nappe.

Can any one tell me about the aeration of nappe?

One half of a double cone is called a nappe.

The term Nappe when used in relation to hydro-engineering refers to the sheet of water over-topping a dam. It is an important issue in dam construction because unless air can be introduced into the nappe, the vibration of the water can lead to dam failure.

Nappe:- it is that part of water which is falls downward from above, over weir.

Three types of nappe:

1. Free nappe.
2. Depressed nappe.
3. Clinging nappe.

In free nappe the water falls away from the weir wall so air gap generate beneath the nappe. This air at atmospheric pressure.

In depressed nappe the air gap remains small so the pressure of air remain below the atmospheric pressure called negative pressure.

But in clinging nappe no air gap remains so no pressure remains because water falls from the weir as a clutched with wall.