Analysis of the Progression of Rainwater Film Over the Bonnet of a Road Vehicle

A significant route for water ingress into passenger cars is through the Heating, Ventilating, and Air-Conditioning (HVAC) system. The penetration of rainwater through the HVAC unit and the subsequent rise in moisture levels within the passenger compartment directly affect the provision of thermal comfort to the cabin occupants. It is speculated that up to 80% of water ingress into the cowl or engine bay is from water film movement over the bonnet of the car, and only the remaining 20% is from direct rain impact from above. Using a full-scale Climatic Wind Tunnel (CWT) facility, which incorporates accurate rain distribution modeling, it has been possible to study the movement of rainwater film over the exterior surface of the vehicle to ascertain the flow distribution of the film moving into the engine bay, into the cowl, advancing up and over the windscreen and shed to the sides and front of the vehicle. Using a series of catchment systems, it is also possible to determine the proportions that arrive in each location due to film progression or direct rain impingement.

This research is designed to gain an insight into the progression of the rain water film over the bonnet area of the vehicle so that methods of providing adequate separation of water from the intake air can be made as far upstream as when first contact is made with the outer surface of the vehicle.