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The other day, Dr. Design came to me with a new project: “We must find the drag coefficient of a futuristic vehicle concept!!”

As usual the deadline was yesterday. With the CAD in hand, I started to set up my simulation. I had to say, the meshing part was easy. I arrived rapidly to the point of setting up my boundary conditions, but here's where I ran into some doubt. How should I model the rotating tires?? Yes, the vehicle is futuristic, but not electromagnetic yet. I could see a few methods… Which one should I choose?

It was way past the working hours so unfortunately, I couldn’t call upon my local support team - they were most likely asleep! Before deciding to try CD-adapco engineers in our other offices, I ran this question past Steve. Thankfully, Steve is always here - at 4 AM and even on weekends!

I took a quick trip into the Steve Portal and found the following article. It presents the differences between a rotating wall and a rotating region approach. I was able to carry on setting up my simulation with the confidence that I was on the right track. In the end, I didn’t miss my deadline by too long or wake anyone either!

A rotating tire causes turbulence that can have a large effect on the flow field around the wheel housing. In steady-state analysis, two different approaches can be chosen to model a rotating tire: with a rotating wall or with multiple reference frames.


The simplest approach that you can use for modeling a rotating wheel is to assign a tangential velocity to the wall boundaries faces forming the wheel.

With reference to the picture below, the following steps describe how to define a rotating condition to the front wheel, that is, the blue surface. The same approach can be used for the other wheels.


  1. Define a local coordinate system around which the wheel is rotating. See the following article: How can I define local coordinate systems?
  2. Assign a tangential velocity to the wall boundary: Go to “Physics Conditions > Tangential Velocity Specification > Local Rotation Rate”

Go to “Physics Values > Axis” and choose the local coordinate system and axis direction.

Go to “Physics Values > Wall Rotation” and assign the value of the rotational rate.

Note: the limitation of this approach is that it is applicable only for geometries of wheels which are axisymmetric. If it is not the case, refer only to the approach below.


A second approach for modeling a rotating wheel in steady-state analysis is the multiple reference frame (MRF). In this approach, a separate region enclosing the entire wheel (including rims, spokes) must be defined, and a rotating reference frame assigned to that region. This method assumes that all the fluid cells located in that region are rotating.

See also:

Note: in case of transient analysis, the MRF is not suitable anymore and the rigid body motion (RBM) approach is recommended. For a better understanding of the differences between the two approaches, see:

Should I use moving reference frames or rigid body motion for my simulation?
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