# FLUENT: Reference values, Boundary Conditions, Drag Co-efficient and Downforce

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 November 16, 2013, 04:35 FLUENT: Reference values, Boundary Conditions, Drag Co-efficient and Downforce #1 Member   Harshal Join Date: Oct 2013 Posts: 50 Rep Power: 3 Hello all, I have recently undertaken a project in vehicle aerodynamics and I need some guidance. In my project work, I have to do two simulations. Simulation 1: Keep the car stationary and the give the wind velocity and Simulation 2: Give the car it's velocity and keep the wind stationary. Then, I have to compare the results. I have questions regarding reference values, boundary conditions and drag co-efficient. 1) Reference values: In the 'Area' option, I have given the projected surface area of the car in the X direction, since I want to know the drag. What should I give for the 'Length' option ? The car length ? Also, for the 'Velocity' option what should I input. I think I should give 0 for Simulation 1 and the car velocity for simulation 2, but I am not sure. 2) Boundary Conditions: In boundary conditions, there is an option for 'Hydraulic Diameter'. The car, that I am analysing sits in a rectangular 3D wind tunnel. Therefore, I have given the 'Hydraulic Diameter' for Inlet and Outlet as that of a rectangular vent, namely (2*a*b) /(a+b) where 'a' is the width and 'b' is the height. Is this correct. 3) Drag Co-efficient: I am not sure whether Fluent directly gives the drag co-efficient or not. For every iteration it shows the cd value, but I think it's the cd convergence values and not the absolute cd value. Am I correct ? Or should I just calculate the cd value from the drag force ? 4) Downforce: I know that I can get the downforce from the Reports>Forces option. The question is, that in my 'Reference Values', in the 'Area' option, I plan to give the projected car surface area in x direction. Will this lead to a wrong downforce value ? (Since downforce calculation need projected area in the Z direction and not X direction) Thank You for your help, Regards, Harshal

 November 18, 2013, 03:51 #2 Member   Mohsen Join Date: Jul 2012 Posts: 43 Rep Power: 5 Hi Harshal, In simulation 1, you should consider velocity inlet and an outlet condition, and stationary wall for car. In simulation 2, you should consider a moving wall and you don't need any inlet or outlet condition. I don't know anything about calculation of drag in fluent. Be successful.

November 19, 2013, 05:20
#3
Member

Harshal
Join Date: Oct 2013
Posts: 50
Rep Power: 3
Quote:
 Originally Posted by smhosseini Hi Harshal, In simulation 1, you should consider velocity inlet and an outlet condition, and stationary wall for car. In simulation 2, you should consider a moving wall and you don't need any inlet or outlet condition. I don't know anything about calculation of drag in fluent. Be successful.
Hello smhosseini,
thank you for your replies. They have very helpful !

Regards,

Harshal

 November 19, 2013, 12:01 #4 Senior Member   Andrew Kokemoor Join Date: Aug 2013 Posts: 118 Rep Power: 5 Reference values are only used to calculate non-dimensional coefficients, such as the coefficient of drag. Their settings will not affect physical values such as force. Regarding 3), check out the Fluent User's Guide, chapter 28.15.3 on force and moment monitors.

 November 21, 2013, 08:56 #5 Senior Member   Stuart Buckingham Join Date: May 2010 Location: United Kingdom Posts: 267 Rep Power: 16 Simulation 2 will not work. Except for the rare case that your car is actually a flat plate... __________________ http://bc247.wordpress.com

November 21, 2013, 10:27
#6
Member

Mohsen
Join Date: Jul 2012
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Quote:
 Originally Posted by stuart23 Simulation 2 will not work. Except for the rare case that your car is actually a flat plate...
Hi dear Stuart,
Could you tell me why the simulation 2 won't work?
I think he can consider the external wall of the car as moving wall. Isn't it?

 November 21, 2013, 14:21 #7 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 Without moving/deforming meshes, a moving wall can only have a tangential velocity component. The only shape where this makes sense is indeed a flat plate or a rotating cylinder/sphere. stuart23 likes this.

November 24, 2013, 11:20
#8
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Harshal
Join Date: Oct 2013
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Quote:
 Originally Posted by flotus1 Without moving/deforming meshes, a moving wall can only have a tangential velocity component. The only shape where this makes sense is indeed a flat plate or a rotating cylinder/sphere.
Hello, flotus1,
thank you for your reply. Could you please explain what you mean by moving/deforming mesh ? I'm going through the Fluent user handbook. Do you mean moving frames of reference or moving mesh ? Also, in moving mesh there are two types : sliding and dynamic. Can you please explain which option to use ?

Thanks,

Harshal

 November 24, 2013, 12:49 #9 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 None of them. For an external aerodynamics simulation of a vehicle, the only sensible approach is number 1. Dont forget to model the floor as a moving wall.

November 24, 2013, 13:06
#10
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Harshal
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Quote:
 Originally Posted by flotus1 None of them. For an external aerodynamics simulation of a vehicle, the only sensible approach is number 1. Dont forget to model the floor as a moving wall.
Hi, thanks for the reply.
Just out of curiosity, if I give inlet velocity in BOTH the simulations and in the 1st take road as moving wall and in second give velocity to the car, would there be any difference in the results ?

Thanks,

Harshal

November 27, 2013, 06:37
#11
Member

Harshal
Join Date: Oct 2013
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Rep Power: 3
Quote:
 Originally Posted by flotus1 None of them. For an external aerodynamics simulation of a vehicle, the only sensible approach is number 1. Dont forget to model the floor as a moving wall.
Hello flotus1,
I want to ask you about the moving wall condition. I have created a velocity inlet and given the wind velocity. Now I have defined the road as a moving wall and have given the car velocity to it (which is same as the velocity-inlet velocity ). The question is should I select the road (moving wall) as 'relative to adjacent zone' or as 'absolute' velocity ?

Thanks,

Harshal

 November 27, 2013, 07:26 #12 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 Since you have no moving zones, both options should yield the same result.

November 27, 2013, 07:31
#13
Member

Harshal
Join Date: Oct 2013
Posts: 50
Rep Power: 3
Quote:
 Originally Posted by flotus1 Since you have no moving zones, both options should yield the same result.
Ok ! Thanks a lot !

Harshal

 April 10, 2014, 09:06 Doing multiple simulations for different speeds #14 Member   Harshal Join Date: Oct 2013 Posts: 50 Rep Power: 3 Hello all, I need your help. I have to find out the Cd and Cl values for a car using k-epsilon and k-omega models for multiple speeds (about 15 different speeds). Can some one please tell me how to do that ? Thanks a lot, Harshal

 April 18, 2014, 15:18 Which area does Fluent take to calculate lift force on a car #15 Member   Harshal Join Date: Oct 2013 Posts: 50 Rep Power: 3 Hello all, I wanted to know which area does fluent consider to calculate lift force for a car. From what I found out, the area considered for calculating lift force varies from object to object. So, can someone please tell me how to find out the area, which Fluent uses for calculating lift force on a vehicle. Thanks, Harshal

 April 19, 2014, 06:20 #16 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 The area Fluent uses for calculating coefficients can be found (and changed) in the reference values section.

April 20, 2014, 04:08
#17
Member

Harshal
Join Date: Oct 2013
Posts: 50
Rep Power: 3
Quote:
 Originally Posted by flotus1 The area Fluent uses for calculating coefficients can be found (and changed) in the reference values section.
Hello flotus,
as far as I know, for the Cd value, the area projected in X direction (frontal area) is considered while for Cl, the area projected in Z direction (car viewed from top) is considered. As these two values are different, I wanted to know which area value to put in the Reference Values. I want to have both, the Cd and the Cl.

Harshal

 April 22, 2014, 05:59 #18 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 You could set the area to some specific value (1mē would be quite handy) and multiply the lift and drag coefficients fluent produces by any factor you want.

 May 2, 2014, 05:51 problem with flow domain #19 New Member   Mansoor Ahmed Join Date: Apr 2014 Location: Pakistan Posts: 23 Rep Power: 3 I m new to fluent.I am doing analysis in fluent.but my domain seems upside down(inverted) How can i fix it::; kindly help me..

 August 1, 2014, 06:23 #20 Member   Harshal Join Date: Oct 2013 Posts: 50 Rep Power: 3 Hello all, I recently completed my project work. This was my first experience with CFD and I thank you all for your help and guidance. Without your support, it would not have been possible for me to complete this project. Thank You all ! Harshal

 Tags boundary conditions, downforce, drag, reference values

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