I have read a bit about the diffusor, but the applications specific to open wheeled cars seems to be left out of everything that I have found.
My question is this:
In an unrestricted series, what would be the optimal diffusor arrangement? Would it start in front of the side pods and be the entire underside of the car?
I am working with a friend to turn a regular symetrical Kart frame into a MicroGP car, and we were thinking about making the entire underside diffusored, but I cannot tell if that would be worthwhile.
The main mechanism is simple, it is all about accelerating the airflow underneath the car as much as possible and over as large a surface as possible. It is when trying to figure out how to do it well that the problems start, as always.
The flow underneath the car is driven mostly by what happens at the rear of the car. The air exits the diffuser at a certain velocity, and the ratio between the opening cross section at the back vs. the cross section underneath the car will determine the velocity ratio (as the air is almost incompressible). The difference of the square of the velocity underneath the car to the square of the velocity in the freestream (not diffusor exit!) will roughly give you the pressure difference underneath the car to the freestram pressure.
It is therefore essential to have a relatively clean exit for the air at the back of the diffusor so that the velocity underneath the car is as large as possible. However, even if the airflow from the diffusor is choked, there will still be a pressure decrease underneath the car since the low pressure zone behind the car that is responsible for the pressure drag will extend underneath the car via the diffusor. This is also why it is said that the rear wing on an F1 car works in conjuction with the diffusor as the wing creates an additional negative pressure underneath itself, which is then propagated forward and underneath the car.
Ideally it would be best if you can run skirts on the side of the car, such that the airflow can not spill underneath the car from the sides, and then have a flat bottom underneath as much of the car surface as possible. Running the car low is beneficial, but only up to the point where the turbulence effects will start choking the airflow between the floor and the flat underbody.
Towards the rear the diffusor needs to open up. The angle of the opening should not be too step in order to avoid airflow separation. The angle of the opening can be increased if the diffusor opening is split into compartments and separated by fences, as this would tend to produce vortices which help attaching the flow, and the fences also make the airflow exit and the pressure distribution more uniform.
You actually do not want a very long diffusor as it would tend to reduce the pressure difference by putting a large part of the underbody further away from the ground, decreasing the velocity at that point. The diffusor produces downforce on its own, but even more than that it induces the low pressure zone underneath the car forward of its position. This is usually more beneficial, assuming of course that the floor has been made smooth and possibly fenced. The ideal design is of course a diffusor that smoothly curves into the underbody.
The idea is to use teh kart frame as the starting point, and then attach a floating "swing-arm" to the rear, and a twin reverse trailing link suspension in the front to create the nose shape. At that point, the kart frame will literally be floating between the swing arm and the reverse trailing links. I wanted to rake the frame at about 7 degrees front to rear, and use teh dampers to hold everything in place.
Now, what that does is give me a flat bottom on the kart frame that can be used for some sort of diffusor effect, but it would have to vent BEFORE reaching the rear swing arm.
I am just going to have to get it together first, then look at where the air is going to need to vent, and see if I can couple the flat bottom to a diffusor mounted to the swing arm.
Who knows, but I do thank you for your info.
Does anyone have some pics of a diffusor so I can get a good visualization of what is going on?
I'm trying to install a ground effect kit on my 125cc kart as well. The circuits that I will be running on will have top corning speed around 100km/h. I hope the aero bits will help in reducing my laptime and increase the in corner stability.
Should be working on the parts by the end of this month. I'll keep you guys posted on the progress.
