## Ask Dr. Mesh: How can I model a rotating wheel in the external aerodynamics analysis of a vehicle?

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.

ROTATING WALL APPROACH

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.

MULTIPLE REFERENCE FRAMES APPROACH

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.

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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