# Weird Cl for a car

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 March 19, 2014, 16:44 Weird Cl for a car #1 Member   Michele Join Date: Dec 2013 Posts: 30 Rep Power: 6 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.

 March 20, 2014, 09:24 #2 Member   Join Date: Jul 2013 Posts: 49 Rep Power: 6 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.

 March 20, 2014, 10:31 #3 Senior Member   Join Date: Nov 2010 Location: USA Posts: 962 Rep Power: 15 Is your simulation converged? Did you set up the reference values correctly?

March 21, 2014, 05:28
bho
#4
Member

Michele
Join Date: Dec 2013
Posts: 30
Rep Power: 6
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
I have run the sim with some different freestream speed:
Attached Images
 car_01.JPG (39.1 KB, 28 views) car_02.JPG (39.5 KB, 16 views) car_03.JPG (47.6 KB, 15 views)

March 21, 2014, 05:34
Report
#5
Member

Michele
Join Date: Dec 2013
Posts: 30
Rep Power: 6
this is the full report:
as you can see the contribution for the weird Cl fully comes from the wheels:
Attached Images
 car_04.JPG (27.0 KB, 20 views)

 March 21, 2014, 18:28 #6 Senior Member   Join Date: Nov 2010 Location: USA Posts: 962 Rep Power: 15 Can you plot pressure on the surface of the wheels? What does it look like? Do the values make sense?

 March 22, 2014, 10:03 #7 Member     Anonymous Join Date: Dec 2011 Location: Everywhere Posts: 83 Rep Power: 8 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 likes this.

March 22, 2014, 16:13
#8
Member

Michele
Join Date: Dec 2013
Posts: 30
Rep Power: 6
Quote:
 Originally Posted by plucas 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?
Attached Images
 carrr.JPG (27.0 KB, 13 views)

March 22, 2014, 16:50
#9
Member

Anonymous
Join Date: Dec 2011
Location: Everywhere
Posts: 83
Rep Power: 8
Quote:
 Originally Posted by Michael-STM 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?
You are correct on the equation. I am just speaking of the reference pressure below:

 March 22, 2014, 19:54 #10 Senior Member   Join Date: Nov 2010 Location: USA Posts: 962 Rep Power: 15 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 likes this.

 March 23, 2014, 16:17 #11 Member   Michele Join Date: Dec 2013 Posts: 30 Rep Power: 6 Yeah, you were right. I have set up the reference pressure to 0Pa and now the Cl is back to normality! plucas likes this.

 March 23, 2014, 16:41 #12 Member     Anonymous Join Date: Dec 2011 Location: Everywhere Posts: 83 Rep Power: 8 Good to hear!

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post yossir4 FLUENT 0 November 11, 2013 02:58 guillaume FLUENT 3 June 27, 2011 11:17 guillaume Main CFD Forum 12 June 27, 2011 11:16 guillaume Siemens 0 May 11, 2008 20:00

All times are GMT -4. The time now is 09:10.

 Contact Us - CFD Online - Privacy Statement - Top