In many AEC applications, flow is entrained from exterior vents by fans within ducts. Unlike many applications in which the flow rate or pressure is known, these applications should be approached in a slightly different manner. Some general solution strategies for simulating entrainment flow include:
Model Openings
Use a 1/4 to 1/8 "globe" (semi-hemisphere) around the entrainment port(s) - some care may need to be modeled using the extension length and total radius. It may be necessary to experiment in 2D first before moving to a 3D model.
Boundary Conditions
Assign an Unknown BC on all entrainment ports, even if incompressible. This allows the pressure to float, and is more physically realistic than a 0 psi Pressure condition.
Advection Scheme
ADV2 works well for these flow. (To enable ADV2, open the Solve dialog, and click the Solution control button. Click the Advection... button to open the Advection dialog.
Mesh
Refine the mesh on the entrainment ports and in the transition regions from the ports to the main flow regions.