Boundary layer separation

 Register Blogs Members List Search Today's Posts Mark Forums Read

 April 15, 2012, 14:40 Boundary layer separation #1 New Member   Aniket Sachdeva Join Date: Mar 2012 Posts: 22 Rep Power: 5 It is said that a boundary layer separates from the surface when the inertia of the fluid is not enough to counter the viscous and pressure forces... But increasing the flow speed will increase the tendency of the boundary layer to separate... Does that not mean that higher inertia in the fluid will make the boundary layer more susceptible to separation.. whereas from definition of separation it is clear that lower inertia should be responsible for flow separation..? Far and mb.pejvak like this.

April 16, 2012, 04:02
#2
Senior Member

Join Date: Jan 2011
Posts: 236
Blog Entries: 5
Rep Power: 8
Quote:
 Originally Posted by thegodfather But increasing the flow speed will increase the tendency of the boundary layer to separate...
In general it will not. For instance, increase of reynolds number results in moving the separation point further downstream on airfoils.

 April 16, 2012, 06:43 #3 New Member   Aniket Sachdeva Join Date: Mar 2012 Posts: 22 Rep Power: 5 For example in the case of flow past a cylinder or a sphere, a low speed fluid stream will follow the curvature and will remain attached to it.. But increasing the speed will lead to separation... Isn't it a paradox??

 April 16, 2012, 11:07 #4 Senior Member     Join Date: Jan 2011 Posts: 236 Blog Entries: 5 Rep Power: 8 You are talking about very low reynolds number regime for highly curved objects where boundary layer approximation does not work: For cylnders and spheres the flow remains stationary only for reynolds number up to order 10^1-10^2, which means that the boundary layer size is of order of the object size, i.e. bl does not really exist. For higher reynolds numbers the flow past such objects becomes nonstationary, the vortex shedding occurs and bl theory does not really work too. The bl layer theory works for high reynods numbers and mild adverse pressure gradients, e.g. for flows past airfoils in which case higher speed means later separation.

 April 16, 2012, 12:03 #5 New Member   Aniket Sachdeva Join Date: Mar 2012 Posts: 22 Rep Power: 5 Thanks for replying I did not understand, what is the meaning of boundary layer size being of the order of the object size.. n how does low value of reynolds numbr imply the same..?

April 16, 2012, 12:33
#6
Senior Member

Join Date: Jan 2011
Posts: 236
Blog Entries: 5
Rep Power: 8
Quote:
 Originally Posted by thegodfather Thanks for replying I did not understand, what is the meaning of boundary layer size being of the order of the object size.. n how does low value of reynolds numbr imply the same..?
The theory of boundary layer is an approximation where the fluid speed changes significantly only close to the surface and in the laminar flow case
ratio of the boundary layer thikness to the object size is of order 1/sqrt(Re).

In the case of spere or cylinder the flow is stationary up to Re of order 10^1-10^2. This means that bl size is about 1/3 to 1/10 of the object size, which is not really the boundary layer.

In the boundary layer theory the pressure does not change across the layer in direction normal to the surface, while it gradually changes along the direction parallel to the surface on the scale which is much greater than bl thikness. All this does not make sence when your bl thikness is 1/3 of the object curvature and Prandtl argumentation about decelleration of different parts of bl prior to separation simply does not work.

 April 17, 2012, 02:56 #7 New Member   Aniket Sachdeva Join Date: Mar 2012 Posts: 22 Rep Power: 5 Then, is there any other theory to predict flow separation for such cases...? (i.e. other than Boundary layer approximation)

April 17, 2012, 04:16
#8
Senior Member

Join Date: Jan 2011
Posts: 236
Blog Entries: 5
Rep Power: 8
Quote:
 Originally Posted by thegodfather Then, is there any other theory to predict flow separation for such cases...? (i.e. other than Boundary layer approximation)
I am not a specialist in this field, but what I have seen on this topic are only numerical simulation. BTW for very low speed regime numerical simulations are quite easy and reliable, since turbulence modelling is not involved.

April 17, 2012, 07:21
#9
Senior Member

Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 606
Rep Power: 12
Quote:
 Originally Posted by thegodfather It is said that a boundary layer separates from the surface when the inertia of the fluid is not enough to counter the viscous and pressure forces...
Specifically, the viscous drag caused by a solid boundary. Fluid viscosity is a mechanism for momentum transport, hence it's alternate name momentum diffusivity.

Quote:
 Originally Posted by thegodfather For example in the case of flow past a cylinder or a sphere, a low speed fluid stream will follow the curvature and will remain attached to it.. But increasing the speed will lead to separation... Isn't it a paradox??
For low speed fluid flows, viscosity is there to help force the fluid to follow the curvature (whereas the inertia of the flow does not!). The fluid wants to move tangentially along the surface of the cylinder, it definitely does not want to curve! Even in the inviscid case, there are many interactions needed to make the flow follow a solid boundary.

Quote:
 Originally Posted by truffaldino The theory of boundary layer is an approximation where the fluid speed changes significantly only close to the surface and in the laminar flow case ratio of the boundary layer thikness to the object size is of order 1/sqrt(Re). In the case of spere or cylinder the flow is stationary up to Re of order 10^1-10^2. This means that bl size is about 1/3 to 1/10 of the object size, which is not really the boundary layer. In the boundary layer theory the pressure does not change across the layer in direction normal to the surface, while it gradually changes along the direction parallel to the surface on the scale which is much greater than bl thikness. All this does not make sence when your bl thikness is 1/3 of the object curvature and Prandtl argumentation about decelleration of different parts of bl prior to separation simply does not work.
This discussion is referring to the formal Boundary Layer Theory by Prandtl. Boundary Layers in general do not have this restriction on their behaviour, and there is no limitation on boundary layer thickness.

For simple flows, there are engineering correlations for when and where a flow will separate and there are some models for how to treat the laminar to turbulent transition zone. The canonical example is the flat plate boundary layer. In general, transitional turbulence is a mess to deal with.

Last edited by LuckyTran; April 18, 2012 at 02:32.

 April 17, 2012, 08:54 #10 New Member   Aniket Sachdeva Join Date: Mar 2012 Posts: 22 Rep Power: 5 Thanks truffaldino for clearing my doubt... N thanks LuckyTran for deepening my understanding about the topic...

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Saturn CFX 34 October 16, 2014 05:27 HMR CFX 3 March 6, 2011 21:10 Jibran Haider FLUENT 3 August 1, 2008 00:33 Mark CFX 6 November 15, 2004 16:55 Tudor Miron CFX 15 April 2, 2004 06:18

All times are GMT -4. The time now is 15:29.