Fluent 12.1 - transition viscous models

kdrbrk · May 11, 2010, 09:53

Hi,

I am trying to analyze a NACA 6 series symmetric airfoil for drag coeff. with 5 metres chord and 25 m/s. It is about 8.000.000 Re. I know that these airfoils are laminar flow airfoils.

So if I analyze them with turbulence models, do I get incorrect results even with a fine mesh? I tried k-e and k-w but the results seems more than it should be.

I think the flow is laminar on most part of the NACA 6 series airfoils about this Reynolds numbers. So what viscous model should I use?

Is Transition models in Fluent 12 will give me the best result among other models?

Thanks in advance.

DoHander · May 11, 2010, 10:50

A turbulence model that also models the transition should in principle give you a more accurate Cd. However even with a simple turbulence model (like k-epsilon or S-A) you should catch the lift coefficient for the linear part (say from -5 to 8 degrees). If your Cl is also wrong then you should check your mesh for y+ variation.

A transition model is the way to go, especially if you have access to Fluent 12.

Do

kdrbrk · May 11, 2010, 11:18

Thanks Do,

By the way, it is a 3D analysis with 1.8 meters width.
My problem is about drag force, lift is not so important since it is a symmetrical airfoil. I will try to make a custom airfoil which should have better drag characteristics.

So which transition model do you recommend?

kdrbrk · June 7, 2010, 18:10

I have been trying Transition SST and k-w SST and there is very litte difference between them !
Does Transition models behave like all the zone is a transition zone? (like turbulence models behave like all the zone is turbulence zone?)

Jade M · July 29, 2010, 12:34

For CFX, ANSYS recommends the Gamma Theta Model developed by Langtry and Menter. The documentation says that this model is widely validated. I assume that this would be the same for FLUENT.

The Gamma Theta Model is based on two transport equations, one for intermittency and one for the transition onset criteria in terms of momentum thickness Reynolds number.

May 11, 2010, 09:53	Fluent 12.1 - transition viscous models	#1
kdrbrk Member Burak Join Date: May 2009 Posts: 90 Rep Power: 17	Hi, I am trying to analyze a NACA 6 series symmetric airfoil for drag coeff. with 5 metres chord and 25 m/s. It is about 8.000.000 Re. I know that these airfoils are laminar flow airfoils. So if I analyze them with turbulence models, do I get incorrect results even with a fine mesh? I tried k-e and k-w but the results seems more than it should be. I think the flow is laminar on most part of the NACA 6 series airfoils about this Reynolds numbers. So what viscous model should I use? Is Transition models in Fluent 12 will give me the best result among other models? Thanks in advance.

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May 11, 2010, 10:50		#2
DoHander Senior Member Join Date: Nov 2009 Posts: 411 Rep Power: 19	A turbulence model that also models the transition should in principle give you a more accurate Cd. However even with a simple turbulence model (like k-epsilon or S-A) you should catch the lift coefficient for the linear part (say from -5 to 8 degrees). If your Cl is also wrong then you should check your mesh for y+ variation. A transition model is the way to go, especially if you have access to Fluent 12. Do

May 11, 2010, 11:18		#3
kdrbrk Member Burak Join Date: May 2009 Posts: 90 Rep Power: 17	Thanks Do, By the way, it is a 3D analysis with 1.8 meters width. My problem is about drag force, lift is not so important since it is a symmetrical airfoil. I will try to make a custom airfoil which should have better drag characteristics. So which transition model do you recommend?

June 7, 2010, 18:10		#4
kdrbrk Member Burak Join Date: May 2009 Posts: 90 Rep Power: 17	I have been trying Transition SST and k-w SST and there is very litte difference between them ! Does Transition models behave like all the zone is a transition zone? (like turbulence models behave like all the zone is turbulence zone?)

July 29, 2010, 12:34		#5
Jade M Senior Member Join Date: Feb 2010 Posts: 148 Rep Power: 17	For CFX, ANSYS recommends the Gamma Theta Model developed by Langtry and Menter. The documentation says that this model is widely validated. I assume that this would be the same for FLUENT. The Gamma Theta Model is based on two transport equations, one for intermittency and one for the transition onset criteria in terms of momentum thickness Reynolds number.