It’s of the current trend to fit the rear of your race car with an aerodynamic device called a diffuser. These are by no means new, by a very efficient way of producing downforce. These now common devices can produce up to 30% of effective downforce. But the main question is, how do they work? For this article, I will look at the way Formula 1 has shaped the design for Formula E.
The diffuser is this angle piece of carbon fibre accelerates the air flow of air under the car, creating an area of low pressure, this low pressure is what gives you the downforce. To see how they work, we have to go back to the age old theory of the Bernoulli equation, which is all about the lowering of fluid pressure in regions where the flow velocity is increased. This applies the diffuser which is made up of several sections upon the underside of the car.
For the diffuser, we have to look at what’s called the venturi effect, whereby a reduction in fluid pressure results in a fluid flows through a constricted section to become ‘choked’. For the diffuser, we have to take three sections into account: the throat and the under tray, which both have profound effects on the pressure difference.
So let’s see how they work. The effect of such a tunnel on the air is similar to a diffuser. The air enters the diffuser in a low-pressure format, high-velocity state after accelerating under the car. By gradually increasing the cross-sectional area of the diffuser, the air gradually slows down and returns to its original free-stream speed and pressure. This is called ambient pressure. The diffuser’s aim is to decelerate the air without it separating from the tunnel walls and roof, which would cause a rapid moving flow stall, reducing the downforce and inducing a large drag force.
By installing an inverted aerofoil wing or gurney flap close to the diffuser mouth it is possible to create a low-pressure area, which essentially sucks the air from the diffuser. The diffuser and wing combination permits a higher air-mass-flow rate through the diffuser, thus resulting in higher downforce. Sharp edges on the vertical tunnel walls generate vortices from entrained air and help confine the air through the diffuser and reduce the chance it will separate.
With the aerofoil, air passing under the wing has further to travel than the air passing over the top surface. This causes the air under the wing to accelerate, resulting in a drop in air pressure, this creates a difference in pressure between the upper and lower surfaces. This difference essentially means the wing is pushed down by the higher pressure above, generating what is known as downforce, which we all know.
It’s of important nature to use strakes too, which are the vertical pieces of carbon fibre you see. While in Formula 1 you see many used, up to eight in some cases, Formula E cars are only allowed to use two by regulation. This, albeit play a very important role in flow management. As the exit of the diffuser is wide and open, the low-pressure air is prone to deteriorating, thus, give a big drag force. By implementing the strakes, the flow doesn’t have enough space to detach and break up, which mean the total drag force is much smaller, as the flow is breaking up as much.
With the speeds of the flow being very rapid, it’s a great way to keep the boundary layer happy. For the Formula E car, there not traveling as fast to an LMP1 or Formula 1 car, which is why the diffuser is smaller and need fewer strakes. Teams on race weekends will always play with diffuser height, as the lower you run the car, the better the overall effect. But as Formula E cars are battery powered, the more downforce you run, the more battery life you will use as you’re effectively pushing the car to the ground, whereby you need more battery power to push the car. This throws up a unique challenge which only Formula E engineers deal with.
It’s also very important to note the air speeds. As the initial air comes to the diffuser, it’s fairly untouched by the car, and it isn’t until the throat where things really pick up. The venturi style throat squeezes the air which picks up pace, and until it comes to the exit cross-section the flow speed decreases as the cross section is much wider, thus give the air time to expand, which slows it down. The low-pressure gradient underneath the car is vital to produce even downforce under the middle and rear.
Decreasing the flow’s velocity from the inlet of the diffuser to the outlet (so that at the outlet the flow velocity is similar to the free stream velocity), in turn, produce a pressure potential, which will accelerate the flow underneath the car resulting in reduced pressure and as such, a desired increased downforce generation. This pressure difference is a function of the ratio of the areas at the inlet and the outlet of the diffuser, where this area ratio is set by the diffuser angle and the vehicle ride height too.
Many use the term ‘pressure pumping’ for diffuser which refers to the increased cross-section area over the diffuser length, which can be used to increase the flow rate through diffuser because of pressure potential on tap. As the ratio of the inlet to outlet area becomes increasingly greater, this generates greater pressure recovery that, due to the base pressure remaining constant will increasingly depress the base pressure at the inlet.
The diffuser acts to reduce the underbody pressure due to the expansion resulting in increased flow rate under the body. This increase results in further decrease in underbody pressure, which produces the “pumping down‟. This scavenging both produces a lower pressure area under the car and also acts to reduce the boundary layer.
I hope this has shed some light on the engineering behind diffusers. If the chassis side of things is opened up in Formula E, we can definitely see more advanced designs coming, but for now, teams can get everything they need out of them.
Graphics by Thierry Courtois