Inlets and Angles of Convergence
Good aerodynamic design is tricky. It’s a balancing act. If you design a duct with convergence angles too steep, you will adversely affects flow since it creates a pseudo blockage in the flow path. Too shallow, and the duct becomes too long and you get a large pressure drop due to friction drag (too much exposed wall material).
There is a fine line between a well-designed, optimized duct and something that may actually inhibit flow. Use angle α and length L as basic parameters for design.