Going fast on my motorcycle or in my car has been a hobby of mine for a few years now. When it comes to watching racing, I’ve been more of a MotoGP (motorcycle) person and my roommate has been more of an F1 (automotive) person. He recently told me an interesting fact about F1 racing: the manhole covers on the street courses have to be welded and bolted down, because otherwise they would fly up when the cars drive over them!
I found many articles on the internet stating this fact, but it wasn’t too clear on exactly why this happened. I decided to run this in SolidWorks Flow Simulation to understand this phenomenon better. I did a bit of research online and got some of the general dimensions of an F1 car, as well as the size and weight of a typical manhole cover. I also found out that between the axles of the car, the ground height has to stay constant but behind the rear axle, there is a rear diffuser:
The diffuser acts like an airfoil with a negative angle of attack (AOA), creating a region of low pressure and helps the car stick to the road surface by a phenomenon called “ground effect downforce”. Just as a wing on a plane with a positive AOA creates lift due to a higher pressure underneath and a lower pressure above, a rear diffuser creates downforce. An interesting read on this can be found at http://warp.povusers.org/grrr/airfoilmyth.html.
Learning this, I had a theory that when the F1 car drives over a manhole cover at high speed, it creates a low pressure pocket above it and the pressure beneath it forces it upward. I ran this in SolidWorks Flow Simulation using simple geometry, which showed the following:
As you can see, there is a significant low pressure region where the rear diffuser is located. We can back-calculate the force acting on the manhole cover by taking the pressure difference above and below it and multiplying it by the surface area. If the force pushing the manhole cover up is greater than or equal to the weight of it, you have the potential for trouble! Don’t worry though, even at freeway speeds, normal production cars cannot come close to putting out that much downforce.
The next time you need to figure out the aerodynamics of your next project car (or just trying to put an aftermarket rear diffuser on the back of the family minivan), remember SolidWorks Flow Simulation can show you how to stay firmly planted to the ground!