the skin friction coefficient in Fluent

GJ_hit · April 14, 2013, 00:27

I simulated the low-reynolds-number flow over airfoils, there is a seperation bubble on the upper surface of the airfoil.
The experiment data shows that the skin friction near the seperation bubble is negative. But the result I calcuate by the Fluent is positive.
I don't konw why this happened for there is already have the backflow in the seperation bubble near the surface.
Is it also a reason for the Fluent can't give a good prediticion of drag?

This question puzzle me for a long time. I need your help!

shuai_manlou · April 14, 2013, 00:47

The numerical result depends on several aspects, such as mesh topology, turbulent model, Yplus, you can check this parameters and try the process again. And you can find some tutorials on the airfoil on the Internet, check how the others settle it.

stuart23 · April 14, 2013, 21:23

Skin Friction is a vector quantity. Are you plotting the directional component of skin friction? If you are just plotting "Skin Friction Coefficient", you are plotting the vector magnitude which of course can never be negative.

Stu

ck3 · April 15, 2013, 09:08

Our friend "stuart23" says true.

I had the same problem for a turbulent separated flow. To get round this problem you must use the equation of Skin Friction Coefficient given in the help:

file:///C:/Fluent.Inc/fluent6.3.26/help/html/ug/node1219.htm#217810

Good luck.

April 14, 2013, 00:27	the skin friction coefficient in Fluent	#1
GJ_hit New Member Join Date: Mar 2013 Posts: 10 Rep Power: 13	I simulated the low-reynolds-number flow over airfoils, there is a seperation bubble on the upper surface of the airfoil. The experiment data shows that the skin friction near the seperation bubble is negative. But the result I calcuate by the Fluent is positive. I don't konw why this happened for there is already have the backflow in the seperation bubble near the surface. Is it also a reason for the Fluent can't give a good prediticion of drag? This question puzzle me for a long time. I need your help!

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April 14, 2013, 00:47		#2
shuai_manlou Member CAO Liushuai Join Date: Apr 2012 Posts: 30 Rep Power: 14	The numerical result depends on several aspects, such as mesh topology, turbulent model, Yplus, you can check this parameters and try the process again. And you can find some tutorials on the airfoil on the Internet, check how the others settle it.

April 14, 2013, 21:23		#3
stuart23 Senior Member Stuart Buckingham Join Date: May 2010 Location: United Kingdom Posts: 267 Rep Power: 26	Skin Friction is a vector quantity. Are you plotting the directional component of skin friction? If you are just plotting "Skin Friction Coefficient", you are plotting the vector magnitude which of course can never be negative. Stu

April 15, 2013, 09:08		#4
ck3 Member Khaled Join Date: Mar 2009 Posts: 53 Rep Power: 17	Our friend "stuart23" says true. I had the same problem for a turbulent separated flow. To get round this problem you must use the equation of Skin Friction Coefficient given in the help: file:///C:/Fluent.Inc/fluent6.3.26/help/html/ug/node1219.htm#217810 Good luck.