Boundary Layer Seperation

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  • Last Post 03 June 2020
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Mac123 posted this 26 May 2020

Hello ,

I have studied the boundary layer theory in my courses 

But haven't learnt about the boundary layer separation 

What is it exactly and how does it work ?

I have scrolled through the net but vague answers were all i could find  

Thank You 

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Kremella posted this 26 May 2020

Hello,

Boundary-layer separation occurs when the pressure-gradient in the streamwise direction becomes adverse and changes sign. For fluid to flow, a favorable pressure-gradient is generally required. However, this pressure-gradient sometimes changes sign near the wall and this causes local flow reversal. It is very well explained in the following textbook:

Viscous Fluid Flow by Frank White.

I hope this helps.

Best,

Karthik

Mac123 posted this 26 May 2020

Thank You Mr Karthik

 

So as the fluid is essentially trying to overcome the viscous forces within the boundary layer

it moves towards a high pressure , which is the adverse pressure gradient ,

it stops the fluid flow(v=0) or causes it to reverse .

Right ?

Please correct me if i am wrong

Regards

 

Kremella posted this 27 May 2020

When a fluid is flowing from A to B, it moves because of dP/dx > 0. It means the pressure at point A is greater than B and is referred to as a favorable pressure gradient. In the boundary layer, because of several reasons (such as changing curvature at the wall), this pressure gradient changes sign locally. Instead of dp/dx > 0, the local gradient is dP/dx < 0. This means that the downstream pressure is greater than upstream. This is generally an adverse pressure gradient and caused local flow reversal. When the velocity gradient becomes 0 at the wall, the flow lifts off and separates. I hope this helps understand.

Please mark the most appropriate answer as 'Is Solution' to help others.

Thanks.

Best,

Karthik

Mac123 posted this 02 June 2020

Hello,

I did not quite get you there .

So basically , when dP/dx<0 , velocity becomes zero and flow reversal happens ?

Kremella posted this 02 June 2020

Yes. That is correct. When this happens near the wall, there is a local flow reversal.

Mac123 posted this 03 June 2020

So basically dP/dx<0 causes an increase in adverse pressure gradient.

Adverse pressure gradient relates to the downstream pressure .

So the point at which the velocity becomes zero , at the wall .

The flow reversal takes place ,

Right ?

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