I first noticed orange spots on the tailgate of my truck, then, the back door of my wife’s Ford Explorer. This area seemed to have the highest concentration of tiny orange spots, fallout, industrial fallout, or rail dust.
Why do orange spots appear on the back of a truck? The same reason buses and trucks have very dirty rear sides…vehicle aerodynamics! As air flows over a vehicle, an eddy is created behind the vehicle. Depending on the vehicle’s design, a vacuum of varying strength is created. This air movement could actually pick up metallic particles and pull them toward the painted surface of your vehicle.
The size of the metal particles (also known as industrial fallout, rail dust, etc.) does impact where the particles will settle. The aerodynamic vacuum strength will pull particles of a certain size and weight against the paint, thus embedding the particles into the paint.
When the automobile industry designs a car, there are many objectives to be met by the car design. Aerodynamics may be a stronger or weaker factor in the overall design. For example, the primary objective of a bus design is to fit as many people as comfortably possible. The objective of a high performance sports car is to minimize drag. Aerodynamics objectives could include:
- Fuel efficiency.
- Comfort characteristics – ventilation/cooling the passengers, reduce noise, dirt accumulation, etc.
- Improve the driving or handling characteristics (stability, traffic safety, etc.)
Vehicle aerodynamic design includes three interacting flow fields:
- Flow past the vehicle body
- Flow past vehicle components, such as wheels, radiator, brakes, windshield,
- Flow past the vehicle compartment.
Does the shape of the vehicle impact the orange spots?
It’s been my personal experience that flat back vehicles tend to have orange spots. Tailgates of pickup trucks seem to collect orange spots before other parts of the vehicle. My wife’s Ford Explorer has a “flat” hatch back door. These designs seem to be more similar to the shape that results in a dirty rear wall of a bus.
The greater the front surface, the the more edges, the greater the air resistance. Compare a sports car to a pick up. A pick up has greater front surface and more edges, thus more aerodynamic drag, which results in lower pressure behind the vehicle.
If the purpose of this article were to discuss fuel efficiency, then, we could talk more about these low pressure areas behind the vehicle. However, we are thinking about metal particles swirling around in the low pressure area behind the vehicles. The larger the vehicle (front surface) and the more edges (square, boxy pickups) could create a greater vacuum behind the vehicle. This could increase the metal particles becoming embedded in the paint.
Why would someone spend time studying vortexes behind trucks and buses?
Fuel efficiency was probably your response. It was my first reaction as well. However, there have been studies focused on keeping the back of a city bus clean. Specifically, how could the shape of a bus impact aerodynamics that lead to the collection of dust on the back of the bus.
Perhaps more important: why would anyone care about dirt build up on the back of a bus? The answer: money. Specifically, advertising money. The back of a bus is a large, moving billboard. If you were stuck in traffic, behind a bus, looking at an advertisement for potato chips, then, you might be begin to crave chips! Kaaaaccchhhiiiinnnggg! Goes the sales register at the next convenience store you pass!
This presentation shows how aerodynamics can impact the amount of dust that builds up on the back of a bus.
The following illustration is from:
Basics of vehicle aerodynamics Prof. Tamás Lajos Budapest University of Technology and Economics Department of Fluid Mechanics University of Rome „La Sapienza” 2002
In a study about the basics of vehicle aerodynamics, Prof. Tamás Lajos, of the Budapest University of Technology conducted experimental investigations of mud deposition on bus body. He evaluated:
- Reduction of mud deposition an side walls and rear wall by using wind tunnel experiments
- simulation of moving ground with moving belt
- rotating wheels, detailed underbody
- Measurement of mud deposition
The study concluded that small changes in rear wall geometry reduces mud deposition on rear wall by 73%.
The following illustration is from:
Basics of vehicle aerodynamics Prof. Tamás Lajos Budapest University of Technology and Economics Department of Fluid Mechanics University of Rome „La Sapienza” 2002
