Mechanical Engineering - Hydraulics and Fluid Mechanics - Discussion

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.

What is meant by Free 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, as occurred in the Vajont Dam disaster in Italy in 1963.

Nappe vibration can be managed by flow splitters in the dam wall design. So above the water their is atmospheric air.

If the atmospheric pressure exists beneath the nappe, it is known as a free nappe. A free nappe is obtained by ventilating a weir.

Nappe means a layer of water, so how pressure below nappe is the atmosphere?

Term used in hydraulics; nappe, the underside of which is not in contact with the overflow structure and is at ambient atmospheric pressure. In case of depressed nappe negative pressure between wall (dam) and fluid flow. But in free nappe atmospheric pressure created.

Nice one Thanks. @Jay.

In hydraulic engineering, a nappe is a sheet or curtain of water that flows over a weir or dam. The upper and lower water surface have well-defined characteristics that are created by the crest of a dam or weir.

Thanks for explaining. @Jay.

@Jay Maharashtra.

Good information, Thank you.