CFD - Why F1 Teams Change Wings For Monza - Angle of Attack
My CFD Article on Why F1 Teams Change Wings for Monza was very popular and I got a request to go in to more detail, so in this F1 CFD Article I will focus on Angle of Attack and Drag Reduction.
In case you haven't read it yet, this old Article Covers the Coefficient of Lift itself, which is integral in the understanding of Induced Drag.
At Monza, the most important thing an F1 Team can do is to reduce Drag. You've seen this in articles by Matt Somers, Craig Scarborough, F1Technical, and others. One of the things the F1 Teams will do in search of Drag Reduction is reduce the Angle of Attack of their Wings. Particularly the Rear Wings on the Formula One Car.
It is very intuitive that reducing the Angle of Attack on an F1 Rear Wing would help with Drag Reduction. You already know this from sticking your hand out the window. What I want to do is show you some of the Engineering behind this. Want to be a Motorsports Engineering some day? You have to be able to handle some equations.
First, we will look at the Drag Polar like we did in the previous, more general article on Monza F1 Wings.
The Coefficient of Drag includes the Zero Lift Drag, and the Induced Drag. The Induced Drag is dependent on the Coefficient of Lift, the Span Efficiency, and the Aspect Ratio of the F1 Rear Wing (Wingspan/Chord).
Where is Angle of Attack in that equation? It's in the Coefficient of Lift, the only item in the equation that gets squared. This means from an outside glance, the biggest Drag Reduction can likely come from a reduction in the Coefficient of Lift. Since the Coefficient of Lift depends on the Angle of Attack, chances are it's a pretty important parameter. Lets run some CFD Simulations on this F1 Wing and see how they check with the Theory.
Here is an Animation I made from some XFOIL CFD Simulations on the NACA 2412 Airfoil (the one I'll be using for the Angle of Attack for F1 Rear Wings at Monza Article). I inverted the Wing Section such that it represents an F1 Rear Wing for Downforce rather than an Aircraft Wing for Lift. The simulation below is not on a Finite Wing, but rather on a Wing Section or Airfoil.
The Angle of Attack of the F1 Race Car Rear Wing is incrementally increased in this CFD Simulation. As the Angle of Attack increases the Coefficient of Lift increases near linearly (you can see this immediately above, and in the charts below). This is the Lift Slope in action. The XFOIL CFD Simulation is in agreement with the Airfoil Theory.
For every extra degree of Angle of Attack the Lift Coefficient increases, up to a certain point. In the case of the XFOIL CFD Simulation that seems to be somewhere around say 16 degrees. In the image immeduately above it is shown at 15 degrees.
Why the angle of attack changes the Lift Coefficient this way is beyond the scope of this article. If you are interested in knowing more, look up things like Thin Flat Plate Airfoils, Thin Airfoils, Thin Cambered Airfoils, and Lifting Line Theory (for finite wings). Finite Wings by the way are wings with a Span less than infinity. So the CFD Simulation in XFOIL was on an Airfoil whereas the simulation I am showing in Paraview was a Low Aspect Ratio Finite Race Car Wing. I'll likely write an article on these topics at a later date.
So I have gone as far as I will in this CFD Article showing that an increase in Angle of Attack of an F1 Rear Wing will increase the Coefficient of Lift in a Linear relationship. Now I will talk about why the increase in the Coefficient of Lift increases Drag.
Lift, or Downforce, is created mostly due to a difference in Pressure between the top and bottom surfaces of a Race Car Wing. I say mostly since Viscous/Shear effects also apply but in general the Pressure Difference is stronger. When this Pressure Difference is created, air at the High Pressure side of the Race Car Wing wants to move to the Low Pressure side. This creates Vortices, and Induced Drag. There are other factors in Induced Drag as seen in the equation above, but the Coefficient of Lift effects it the most (due to the squared relation).
You can see the effects of this sometimes on Formula One cars on humid days. Here, you can see the effects from Animations I made using CFD and ParaView.
Basically the larger the Pressure Difference the stronger the Vortices and Induced Drag, and an increase in Angle of Attack tends to create an increase in Pressure Difference. This known phenomenon is backed up by the CFD results on this simulated Formula One Rear Wing shown above.
Finally the image below shows the Lift, Drag, L/D, and Lift Coefficients for the Formula One Race Car Rear Wing resulting from the CFD Simulations at varying Angles of Attack.
These trends make sense [Note: The Drag was multiplied by 5 and the Coefficient of Lift was multiplied by 100 so everything could fit on one graph]. The CFD Simulation on this NACA 2412 representing a Formula One Rear Wing is validating the theory. The Angle of Attack causes a Linear increase of the Lift Coefficient. The slope of this line (CL line) is the Lift Slope, which is dependent on things such as the Airfoil or Wing Section chosen, and the Aspect Ratio of the Wing (in the case of Finite Wings). The Drag fits a Second Order Polynomial. Since the Coefficient of Lift is increasing Linearly, the Coefficient of Drag should be increasing to the Second Power as indicated by the Drag Polar Equation shown above.
The Drag itself is represented by this equation:
Air Density, Velocity, and Planform Area were all kept constant, the only thing changed was the Angle of Attack which changed the Coefficient of Drag due to the Induced Drag portion of the Drag Polar.
In this CFD Article I talked about the trends in Formula One where race teams seek Drag Reduction, and I focused on reductions in Angle of Attack to accomplish this. The Theory of the Drag Polar was introduced, and CFD Simulations were run on an NACA 2412 Profile Finite Wing was used to represent an F1 Rear Wing in order to check against the Theory. The CFD Simulations were in agreement with the Drag Polar Equation.
Hopefully you understand the Engineering behind low Angle of Attack Formula One Rear Wings at Monza a bit better now. I'd love to hear your feedback in the comment sections below. If you have any questions I'll do my best to answer them.