# Rotating Boundary, Wheels

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 May 10, 2013, 12:24 Rotating Boundary, Wheels #1 New Member   Join Date: Jan 2012 Posts: 14 Rep Power: 7 I tried to simulate a regid body with a diffusor and rotating wheels. But it seems like it dont work. Without wheels my simulation is covergent after some iterations but i dont know how to setup the wheels. I meshed it like in this picture : Then I define the wheel as a no slip wall with a rotating velocity. But i get an error that i have a significant normal component at one or more faces and that the angle between the velocity and the surface is almost 90 degrees. Whats wrong? The ground is also moving. Should i define the wheels as complete new domain and let the domain rotate? And guidance would be great! Ps: I use CFX Pre and ICEM

 May 10, 2013, 13:44 #2 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 650 Rep Power: 13 Does global X go right through the center of all four wheels? No, It can't, so my guess is you will have to define the proper axis of rotation.

 May 10, 2013, 13:49 #3 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,329 Blog Entries: 6 Rep Power: 45 Your wheels (geometry) is not modeled correctly.

May 10, 2013, 14:03
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Erik
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Quote:
 Originally Posted by Far Your wheels (geometry) is not modeled correctly.

Oh wow, I didn't see that, I figured they were circles.
I think you have a flat tire so your car is losing control during the simulation

 May 10, 2013, 14:10 #5 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,329 Blog Entries: 6 Rep Power: 45 In reality they are somehow flat at the intersection with road, but not as modeled in pics in above post. They look like a extruded part. Correct method would be to intersect wheels with road and give corresponding velocity to road (convert circular to rectilinear)

 May 10, 2013, 15:17 #6 New Member   Join Date: Jan 2012 Posts: 14 Rep Power: 7 I heard that it wont work if i mesh the tires tangential to the road. Somebody told me that it would produce a "singularity" and wont work. I could just model the tires to the road without the vertical transition and specify the "wheel wall" as wall boundary with a rotating velocity (with right axes )and the "ground" as moving wall? Would this work? I'm thankful for your help :-)

May 10, 2013, 15:20
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Sijal
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Tangential boundary will create problems for meshing or solver? If meshing, I can show you how to handle. But solver should not have any problem.

Quote:
 with right axes
I saw this type of setting (i.e. giving different rotation axis to different parts) in Fluent but don't know how to activate this options in CFX.

 May 10, 2013, 16:39 #8 New Member   Join Date: Jan 2012 Posts: 14 Rep Power: 7 I could also do it in fluent if its easier to setup. I still wonder if i should define it as wall in one fluiddomain or should i do three bodys- For each wheel a solid and the fluid domain? I think the solution with one domain would suit better, but if the wheel is tangential to the ground i create 3 domains?

 May 10, 2013, 23:38 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,535 Rep Power: 104 Do not model rotating tyres with a rotating frame of reference. This can all be modelled in the single stationary frame of reference with a tangential velocity (which happens to rotate). Also, the funny little bits at the bottom of the tyres to eliminate the cusp point will make it MUCH easier to mesh this. If you are not interested in little details like this you will find this assists immensely. ShowponyStuart likes this.

 May 11, 2013, 01:04 #10 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,329 Blog Entries: 6 Rep Power: 45 Tangential velocity to Road?

 May 11, 2013, 03:54 #11 New Member   Join Date: Jan 2012 Posts: 14 Rep Power: 7 I think i first try to manage the right axis and if this dont work i change the mesh. But how can i find out "where" the middle of the tyre is? I have no cad the next few days. Is there a good way in icem or cfx to do this? Should the transition bits also rotate?

 May 11, 2013, 04:10 #12 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,329 Blog Entries: 6 Rep Power: 45 In ICEM put two points on periphery of tyre at location 0.25 and 0.75 or 0.0 and 0.5 and create midpoint

 May 11, 2013, 06:44 #13 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,535 Rep Power: 104 Just to check - are you doing this simulation with a rotation frame of reference or with just rotating boundary conditions? Noe that if you are doing it with rotating BC then as the effective frame of reference for the simulation is translating with the vehicle, then the rotation axis for the wheel is through its centre, with the road surface having a tangential velocity backwards.

 May 11, 2013, 11:30 #14 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,329 Blog Entries: 6 Rep Power: 45 You may find it interesting : Post # 14 and image no 3 http://www.cfd-online.com/Forums/ans...rmula-one.html For boundary conditions, now it is clear that you have to specify road a tangential velocity = omega * R (radius of wheel) in back ward direction.

 May 14, 2013, 04:39 #15 New Member   Join Date: Jan 2012 Posts: 14 Rep Power: 7 It is my "backup" plan to mesh it tangential. But i will first try it with the bits. But i think i cant find proper points to define my axis of rotation Front tyre: Start: 1200 | 515.392105 | -175 Oriantation : 800 | 515.392105 | -175 Back tyre : Start : 1 100 | -656,65112 | -175 Oriantation : 800 | -656,65112 | -175 I attached my geometry, could someone who is better than me in icem(i think almost everybody) check the geometry for the proper middle of the tyres? That would help me alot. http://dfiles.eu/files/2awsgnn1y If i mesh it like this do i have to calculate the rotating speed with 175 mm or with 160 mm? Last edited by ChrisR; May 14, 2013 at 05:33.

 May 14, 2013, 08:02 #16 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,535 Rep Power: 104 If you are trying to get the rotational speed of the tyres correct based on the sag, then the correct answer is in between these two values. But I would not worry about minor details like this, it is unlikely that this small difference in the rotational velocity makes a difference anyway. If you do not believe me run them both and compare them - I bet they are the same.

 May 14, 2013, 08:52 #17 New Member   Join Date: Jan 2012 Posts: 14 Rep Power: 7 Then my axes are probably false?

 May 14, 2013, 12:31 #18 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,329 Blog Entries: 6 Rep Power: 45 175 would be the better choice. But as indicated by glenn, difference would be very minor.

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