I am wondering which surface area to use and assume as an inlet, just to perform some basic calculations in the case of NACA ducts, as the surface face area where the air enters is relatively large compared to the outlet surface area (around 14.8 in my case) giving supersonic speeds which is not possible. The NACA duct is attached to the side window of the car. I am guessing my assumptions are incorrect. Any input will be highly appreciated, thank you.
A first approximation would be the surface area of the entrance projected on the direction perpendicular to the flow (A_proj = A x cos (theta) with theta being the angle between the vector normal to the entrance and the direction vector of the flow). Because that would be the effective entrance of the duct (the area facing the flow), with simple laminar flow assumptions. In reality, the existence of the entrance would induce some turbulence and change the flux with respect to this calculation.
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