:The air passages of a plenum are investigated with Computational Fluid Dynamics (CFD) simulations. The objectives of the simulations are to examine the pressure drop between inlet (windshield base) and outlet (blower inlet), the water intrusion quantity into the HVAC module, and the velocity profile and flow rate at the outlet.An initial analysis relies on a two dimensional mesh around the chimney area. The velocity distribution at the outlet and the pressure drop (between inlet and outlet) are compared between a baseline design and a design with guide vanes.A more detailed analysis is conducted with a three dimensional mesh, to examine designs with different baffle/vane locations and inlet openings. Designs with baffles were found to reduce the water quantity entering the HVAC module. However, the pressure drop increased because the flow paths were choked. Designs with guide vanes in the chimney would reduce water intrusions and improve the outlet velocity uniformity at the cost of only a slight increase in the pressure drop.We conclude that the present CFD approach can provide useful design information in a very short time frame (one to two days for a two dimensional analysis), with greater detail available from a subsequent three dimensional analysis.