Karthik R

Hello cfd_learner,

You can have reversed flow on certain boundaries when you use 'outlet BC' and that is a perfectly acceptable solution. The fan tries to move air in a specific direction and if you are solving an incompressible flow model, there has to air flow coming into your computational domain. For a pressure outlet boundary condition, this would imply reversed flow. 

There is a lot of material in open literature about turbulent exhaust fan models and on fan curves. Please refer to these material to understand the fan performance curves. Generally, these performance curves are plotted as static pressure against volumetric flow rate.

Please check out this excellent video tutorial created by Raef Kobeissi. It explains the model set-up and boundary conditions in detail. It also outlines the mesh details necessary for your simulation.

Hope this helps.

Best Regards,