Mechanical Engineering - Hydraulics and Fluid Mechanics - Discussion

3. 

In a depressed nappe

[A]. the pressure below the nappe is atmospheric
[B]. the pressure below the nappe is negative
[C]. the pressure above the nappe is atmospheric
[D]. the pressure above the nappe is negative

Answer: Option B

Explanation:

No answer description available for this question.

Kanhaiya Yadav said: (Feb 26, 2013)  
What is nappe?

Minkle said: (May 1, 2013)  
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).

Deepak Singh said: (Sep 21, 2013)  
Because we are taking pressure downward direction is positive and upward direction is negative.

Manoj Shetty said: (Jan 29, 2014)  
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.

Rohit Raj said: (Mar 21, 2014)  
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.

Rohit Sjrs said: (Aug 17, 2014)  
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.

Vikas Mahant said: (Sep 4, 2014)  
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.

Ganesh Gole said: (Oct 3, 2014)  
One half of a double cone is called a nappe.

Jatin said: (Dec 26, 2014)  
Can any one tell me about the aeration of nappe?

Fakhre Alam said: (Mar 12, 2015)  
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.

Biradar Yogesh said: (Mar 17, 2015)  
Half of double cone is called nappe.

Rajendra Singh said: (May 23, 2015)  
Because pressure toward downward direction is positive and toward upward direction is negative.

Rahul Kumar said: (Jul 4, 2015)  
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.

Gurmeet Singh said: (Jan 15, 2016)  
What is the means of negative pressure?

Sam said: (Feb 8, 2016)  
Which is the correct answer?

Saravanan said: (Mar 17, 2016)  
What is nappe?

Dad said: (Jun 4, 2016)  
What is the use of nappe and weir?

Leghari said: (Sep 5, 2016)  
I confused about nappe. Please explain it.

Suchitra said: (Sep 22, 2016)  
Thanks @Rohit Sjrs.

Dinesh said: (Oct 4, 2017)  
How it's negative. Can someone explain please?

Ayush said: (Nov 7, 2017)  
Sheet of water(top part) flowing over the notch/weir is called nappe, due to trapping of air beneath the flowing water, negative pressure is created.

Sarath said: (May 13, 2018)  
What is nappe?

Balraj said: (Jul 3, 2019)  
Half part of a double cone is called as Nappe.

Bani said: (Jul 14, 2019)  
What is the difference between Atmospheric and Vapour Pressure?

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