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-   -   Weird Cl for a car (http://www.cfd-online.com/Forums/star-ccm/131711-weird-cl-car.html)

Michael-STM March 19, 2014 16:44

Weird Cl for a car
 
I'm simulating a classical flow around a car...i have included obviously a moving street and rotating wheels.
The simulation is steady with a k-e model.
I have done the setup for the reports for Cd and Cl.
The Cd is normal...but the Cl of the car is crazy!...the Cl is around 27 watching the details of the report a huge contribution for the Cl is coming from the wheels...what is happening??
Thanks.

Alex C. March 20, 2014 09:24

It is normal for rotating cylinder to provide a force that is perpendicular to the flow which is a lift.
Here's a wiki on that.

Maybe your results are valid. (Maybe they are not)

You could check the magnitude of the lift force (not lift coefficient) and compare it to gravity force. Rotation of the wheels is not something that can overcome gravity... There should be several order of magnitude of difference.

EDIT : also, remove the rotation from the wheel and run your simulation once. It should be near 0.

me3840 March 20, 2014 10:31

Is your simulation converged? Did you set up the reference values correctly?

Michael-STM March 21, 2014 05:28

bho
 
3 Attachment(s)
Taking the Magnus effect into the game the Cl is still non-physical.\
The solution converged and the reference values are the same of the Cd report wich is ok.
I`m feeling like i`m missing something in the setup...i do not know :confused:
I have run the sim with some different freestream speed:

Michael-STM March 21, 2014 05:34

Report
 
1 Attachment(s)
this is the full report:
as you can see the contribution for the weird Cl fully comes from the wheels:

me3840 March 21, 2014 18:28

Can you plot pressure on the surface of the wheels? What does it look like? Do the values make sense?

plucas March 22, 2014 10:03

What do you have your reference pressure at in setting up the coefficient of lift? Changing this value will drastically change your coefficient value

Michael-STM March 22, 2014 16:13

1 Attachment(s)
Quote:

Originally Posted by plucas (Post 481471)
What do you have your reference pressure at in setting up the coefficient of lift? Changing this value will drastically change your coefficient value

I think i`m missing something...this was my idea: the software integrate the shear force and the pressure on the entire model surface it project the vector obtained to a normal vector parallel to the freestream and then it calculate the ratio (F)/(0.5 density v^2 A). Is this correct? in this case the refernce pressure does not influence the result...if this is not true i have setted to 1atm is this wrong?

Anyway...this is the pressure on the wheels...does not look weird? :p

plucas March 22, 2014 16:50

Quote:

Originally Posted by Michael-STM (Post 481513)
I think i`m missing something...this was my idea: the software integrate the shear force and the pressure on the entire model surface it project the vector obtained to a normal vector parallel to the freestream and then it calculate the ratio (F)/(0.5 density v^2 A). Is this correct? in this case the refernce pressure does not influence the result...if this is not true i have setted to 1atm is this wrong?

Anyway...this is the pressure on the wheels...does not look weird? :p

You are correct on the equation. I am just speaking of the reference pressure below:

http://i1291.photobucket.com/albums/...pscacdff05.jpg

me3840 March 22, 2014 19:54

Those pressures look fine. Are you sure you have the reference values set correctly? I don't think you do. What's the physical value of the lift force?

Michael-STM March 23, 2014 16:17

Yeah, you were right. I have set up the reference pressure to 0Pa and now the Cl is back to normality!

plucas March 23, 2014 16:41

Good to hear!


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