It is important to have a good mental picture of the main flow patterns around the car before trying to improve the design for aerodynamic performance.
Looking for optimal L/D Ratio
The car behaves like a wing. More incidence increases downforce and drag up to the stalling point:
Rear wing stall
Front wing stall
Each point on the curve has a given gradient (L/D ratio). As we increase incidence / downforce / drag, this gradient is reducing meaning that each incremental step in downforce is coming with more and more drag. By changing the rear wing angle, we change the L/D (aerodynamic efficiency) of the car to optimise lap time. Each track has got a ‘required efficiency’ that can be computed from lap time simulation. By knowing this required efficiency, we know where we need to be on the curve and we set up the rear wing angle to get there.
Lift-to-drag ratio curve
For example at Monza, the required efficiency will be high (4:1). At Monaco it will be low (1:1).
We can put more downforce (rear wing angle) on the car for Monaco (but stop before stall point).
At Monza, we can put more rear wing only if the incremental downforce and drag ratio is better than the track required efficiency of 4:1.
Where does downforce come from?
Downforce is mainly created by 3 highly regulated areas of a racing car: the front wing, the rear wing and the diffuser. Each one needs to be carefully designed and set-up to get the best balance possible.
Front wings can be found on single seaters whereas Le Mans car have a complete front bodywork with aerodynamic flaps. Here are some general principles regarding those features:
Front Wing Flow:
Aggressive upwash then turns into lower energy flow past the front wing
Front wing profile and slots are aiming at keeping this flow clean and energised downstream
Important to keep low energy flow outside of the car to avoid dirty air affecting diffuser and rear wing
This upstream flow is key for aerodynamic balance of the car
Highest front camber tends to be therefore away from car centerline
Perrinn F1 car front wing flow
Coming from the centre of the car where the highest energy in the flow is
Small downwash managed by turning vanes
Flow needs to be as clean as possible (high total pressure)
Mass flow to be maximum possible going under forward floor edge as this will dictate the potential for generating suction at throat of the diffuser
The quality of this flow is key for overall car efficiency (diffuser and floor is a low drag system generating very good downforce)
Static pressure P424 floor
Rear Wing Flow:
Coming from the top
Upwash over cockpit
Downwash using bodywork and engine cover and top body turning vanes
Needs to be as clean as possible (high total pressure)
This flow is key for tuning the drag level of the car (optimum configuration depends on the track layout)
Rear flow also interacts with diffuser by increasing the suction effect behind the car
Streamlines around 424
From CFD to Real Life
Once the aerodynamics of the car has been optimized through CFD, race teams need to ensure the computer simulations and observations in wind tunnel match with actual aero flows when the car is on track. This correlation work is performed thanks ton the following tools: