# A question in FLUENT modeling

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 August 18, 2012, 08:48 A question in FLUENT modeling #1 Member   mhp Join Date: Dec 2011 Posts: 33 Rep Power: 14 Hello, I am tiring to model a 2D semicircle on a rigid surface(as a bump on the ground)using FLUENT for variety of Re numbers,but I couldn't find out where the separation point is? Using XY plot there are only regions that shows the skin friction coeff. came around zero, so which of them are separation point? Is it good to use skin friction coefficient plot? How far should the grid be? I use 20D for upstream and downstream, is this assumption satisfy the modeling? At last is there any articles that do this modeling experimentally so I can check the separation point with changing the Re number? Thank you very much friends.

 August 18, 2012, 21:12 #2 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 97 Rep Power: 15 A picture would be glad

August 19, 2012, 03:12
Pictures for detail
#3
Member

mhp
Join Date: Dec 2011
Posts: 33
Rep Power: 14
So is this mesh good for this geometry? and what should I do to find separation point? it seems that there is two points near the separation ?
Is there any experimental research to validate the separation point in laminar flow for this geometry?
Thank you
Attached Images
 mesh structure.gif (41.7 KB, 12 views) plot of Cf.jpg (67.6 KB, 10 views)

 August 19, 2012, 07:49 #4 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 97 Rep Power: 15 You can not use the skin friction or wall shear because they are always positive. The separation point is defined by the change of sign of the velocity near the wall or the change of sign of the x/y-wall shear or the pressure which reaches a maximum value. I suggest you to plot the velocity vectors and look the point where the vectors are in opposite direction on either side

August 19, 2012, 16:52
#5
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mhp
Join Date: Dec 2011
Posts: 33
Rep Power: 14
Yes you're right, that Cf plot is not valid,during this day I found a way to plot Cf correctly as I want to share with you:
It seems that fluent plot the Cf with that abs value of the velocity so value of Cf would be always positive, I use custom define function part in the Define tab and write the formula for Cf with the definition of wall shear stress in 2D in the nominator of Cf fraction ,
so I think the correct plot for Cf would be like the attached picture.
I also check the velocity vector but in some Re numbers like Re=1000 there is a great vortice behind the semicircle, is this true?
Do you know any researches or articles that do the same work experimentally?
Thank you.
Attached Images
 Defined Cf plot.jpg (42.3 KB, 8 views) Velocity contour.jpg (77.2 KB, 10 views)

 August 20, 2012, 02:05 #6 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 97 Rep Power: 15

 August 20, 2012, 07:33 #7 Member   mhp Join Date: Dec 2011 Posts: 33 Rep Power: 14 Those pdf files were exellent, again thank you for your kindness.

 Tags mesh and grid, separation point