Calculation of Cf in Openfoam

rajeshamech · January 21, 2014, 22:46

Dear all,

I am doing simulation of flow past Selig airfoil.. I am wondering how to calculate co-efficient of friction?? if anybody knows. Pls respond.

msuaeronautics · January 22, 2014, 12:05

Use the "wallShearStress" utility and find the resultant of the three components that are output for you. This gives you $\tau _w$ . Divide that by freestream dynamic pressure and you get Cf.

rajeshamech · January 23, 2014, 05:35

Dear Eric

Thanks for your reply. I calculated resultant of the three components as
V = sqrt(u_sq+v_sq+w_sq). Is this the way to calculate resultant of 3 components? By this i get only positive values. If there is separation in the flow, i should get negative Cf.

msuaeronautics · January 23, 2014, 07:56

That is correct.

Since your flow has separation, there will be a local minimum value of Cf accompanied by an increase. I'm other words, your graph will approach zero Cf but will begin to diverge away from it. You must locate this value and manually place a negative sign next to the values in your region after separation.

rajeshamech · January 27, 2014, 04:09

Dear Eric,

Thanks a lot

January 21, 2014, 22:46	Calculation of Cf in Openfoam	#1
rajeshamech New Member rajesh Join Date: Jun 2012 Location: Bangalore Posts: 9 Rep Power: 13	Dear all, I am doing simulation of flow past Selig airfoil.. I am wondering how to calculate co-efficient of friction?? if anybody knows. Pls respond.

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January 22, 2014, 12:05		#2
msuaeronautics Member Eric Robertson Join Date: Jul 2012 Posts: 95 Rep Power: 14	Use the "wallShearStress" utility and find the resultant of the three components that are output for you. This gives you $\tau _w$ . Divide that by freestream dynamic pressure and you get Cf.

January 23, 2014, 05:35		#3
rajeshamech New Member rajesh Join Date: Jun 2012 Location: Bangalore Posts: 9 Rep Power: 13	Dear Eric Thanks for your reply. I calculated resultant of the three components as V = sqrt(u_sq+v_sq+w_sq). Is this the way to calculate resultant of 3 components? By this i get only positive values. If there is separation in the flow, i should get negative Cf.

January 23, 2014, 07:56		#4
msuaeronautics Member Eric Robertson Join Date: Jul 2012 Posts: 95 Rep Power: 14	That is correct. Since your flow has separation, there will be a local minimum value of Cf accompanied by an increase. I'm other words, your graph will approach zero Cf but will begin to diverge away from it. You must locate this value and manually place a negative sign next to the values in your region after separation.

January 27, 2014, 04:09		#5
rajeshamech New Member rajesh Join Date: Jun 2012 Location: Bangalore Posts: 9 Rep Power: 13	Dear Eric, Thanks a lot