### 11.3.2. Convergence Tips

The use of the alternate rotation model is an important consideration when modeling fans and blowers. Where long axisymmetric inlets exist, the absolute frame velocity has less swirl than the relative frame velocity. Because the alternate rotation model solves for the absolute frame velocity, it can reduce numerical error in such inlet sections. The model may introduce errors in the exit stream if the flow is highly swirling. Hence, the length of the inlet and exit sections can be an important factor when choosing whether to implement the model. The alternate rotation model is generally recommended, especially for axial fans. In most realistic flow situations, this model reduces (or at least will not increase) numerical errors.

Air foil drag is significant and boundary layer friction is an important modeling issue for fans and blowers. A good resolution of the boundary layer, requiring a high concentration of nodes close to the blade surfaces, is therefore important. The Shear Stress Transport model can provide relatively accurate results where the boundary layer is sufficiently resolved by the mesh.

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