A question in FLUENT modeling
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.:) 
A picture would be glad

Pictures for detail
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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 
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/ywall 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 
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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. 

Those pdf files were exellent, again thank you for your kindness.

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