This is a car, but the point conveys. A Trucks aerodynamics are, as we all know, much worse. The reatachment point is near the edge of the windshield.
More info........
Vents and ducts serve many purposes on cars. They are used to direct air for cooling and ventilation. However, they can also be used to reduce lift and create downforce by relieving unwanted pressure differentials. Pressure differentials can occur anywhere that a fluid flow is allowed to enter an area that is difficult to exit. If this pressure differential contributes to lift, or reduces downforce, it is obviously unwanted.
One area where a large pressure differential can develop is the engine bay. The pressure differential is created in two ways. Firstly, as the air flows into the engine bay it strikes the engine, accessories, and firewall, stagnating and raising the pressure in the engine bay. The air flowing over the hood is moving quickly and is at a lower pressure, creating a pressure differential. Because the pressure below the hood is higher than above it, lift is created.
The effect is amplified by the fact that the air entering the engine bay is heated by the radiator and the engine itself. Buoyant forces (the same forces that cause a hot air baloon to rise) are created because the hot air has a lower density than the cool air above the hood. This effectively turns the hood into a hot air baloon, compounding the problem. The problem is again made worse because the engine bay is sealed on the sides and top. This allows the air only one escape path - down. The air is forced to flow down and beneath the car, increasing drag and reducing downforce. Obviously these conditions are detrimental to a car's performance.
However, simply placing a vent in the hood can help reduce the pressure differential in the engine bay, reducing the effect of these phenomena. This solution can be seen on many race cars including JGTC and DTM cars and also on very high performance road cars like the Ferrari F50, Jaguar XJ220, and Lotus Elise.
It is important to note that if the vent is placed too far back on the hood, then it may cross into the stagnation zone at the base of the windshield. If the vent is open to this high pressure area, it may actually force air into the engine bay, compounding the problem that it was installed to relieve.
However, placing the duct in this high pressure area can be used as an advantage. On older muscle cars, "induction cowls" were used to tap into this high pressure area and provide more air to the engine, making more power. For this to be most effective, the cowl should be sealed so that the air is forced into the engine and none is allowed to collect under the hood.
Our trucks cold air feed is placed in a high pressure area represented by red on this NASCAR body (Low pressure in blue)......
Remember, the Dodge NASCAR Truck team worked with the PVO team on our trucks.
Here is a link from Chrysler re; aerodynamics.......(See below the VERY interesting oil article!!)
Chrysler Corporation LLC: 7/29/07 - 8/5/07
Another from Truckin' Magazine..............
Understanding Truck And SUV Aerodynamics - Truckin' Magazine
Hope this helps!!
