Mechanical Engineering - Hydraulics and Fluid Mechanics - Discussion

Here we are considering the down side of the nappe is negative and the above portion will be exposed to atmosphere so the option B and C are correct.

This air is carried away by the flowing water, which results in creating a negative pressure beneath the nappe. The negative pressure drags the lower side of the nappe towards the surface of the weir wall. This results in more discharge than the normal discharge.

In order the keep the atmospheric pressure in the space below the nappe holes are made through the channel walls which are connected through the pipes to the atmosphere as shown in figure. Such holes are called 'Ventilation' of a weir.

Because we are taking pressure downward direction is positive and upward direction is negative.

Here is the answer to your question.

One half of a double cone is called a nappe.

Here, each part is called nappe.

Or

One of the two pieces of a double cone (i.e., two cones placed apex to apex).

What is nappe?