# Magnus effect, a comprehensive CFD Simulation tutorial.

Any object could exhibit the weird effect when traveling at a certain speed in a spin. It is the result of a differential pressure on the spinning object creating a force at a certain angle with the velocity vector.

The differential pressure is created due to the motion of air with the spinning direction of the ball on 1 side and against it on the other side.

# Geometry

I decided to go for a 2D simulation, the geometry comprised of a 33.3 x 11.3 m rectangle. The domain includes a small perforated box, the perforation is made by a circle of 0.3m in radius.

## Mesh

To avoid negative volume cells when enabling the dynamic mesh and 6 DOF method, I applied a tetrahedral mesh all over the fluid domain. In total, the mesh has  83,654 elements. I added a face sizing for the small box with an inflation layer on the circle wall.

## Setup

The CFD simulation is transient and 2 dimensional, therefore, I haven’t added any body forces such as gravitational acceleration. Furthermore, I used the turbulent viscous K-omega SST model due its stability and superiority over the normal K-omega and K-epsilon models.

I selected incompressible air for the fluid domain. Additionally, I created 2 zones for the domain, one internal containing the box and the other external for the rectangular domain. The boundaries comprised the following list:

-The internal domain (internal and external)

-The wall of the circle

-The outlet
As part of the simulation, I interpreted a UDF file which contained information about the mass of the circle and it Mass moment of inertia.

Don’t forget to change the extension of the UDF file from “.txt” to “.C”.
For the next step, I set up the dynamic mesh using the smoothing method and the remeshing:

Once you tick the Six DOF option a new window appears:

1. Select the circle wall (football) as a rigid body and make sure that the UDF file is attached  to “Six DOF UDF/Properties”
2. Select the internal domain (inner) as a rigid body and make sure to tick “Passive” under Six DOF panel.

Initialize your simulation and start with a small time step of 0.0001 s and then run it for as long as you wish!
Magnus effect animation video