In terms of beneficial downforce versus parasitic drag, a diffuser is vastly superior to any freestanding wing. New Cars. Buyer's Guide.
Type keyword s to search. Today's Top Stories. View Photos. This literally sucks the car to onto the track creating much higher grip levels than would otherwise be available simply through the tyres and suspension setup.
This is known as aerodynamic grip. The theory To understand why this works one first has to have a grasp of the basic principles of lift and down force. The illustration below shows a simple downforce generating wing profile.
The 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 higer pressure above, generating what is known as downforce. An extreme application of this theory can be seen on the Chaparral 2J car above.
A pair of fans on the rear of the car sucked the air from under the floor, pulling it onto the road, rather like a reversed hovercraft. With this in mind, the role of the diffuser on a racing car is to speed the airflow up underneath the car, reducing its pressure, creating a greater difference in pressure between the upper and lower surfaces of the car. This means more downforce and aerodyamic grip, allowing the car to corner faster. Now that we understand the basics of downforce generation we can look at the more detailed operation of a diffuser, and why they have their distinctive form.
Clearly drivers will hate the feel of the car if you allow it to get to these areas of performance change. Continuing to reduce ride height further leads to reducing energy and attachment of the flow and ultimately to complete separation over the entire width of the inlet of the diffuser.
Once a critical level is reached the higher angle diffusers achieve the same downforce at different heights. Now why might that be? If a body or a large part of one has substantial ground clearance then the chances are that there is air below that body which is relatively undisturbed. I often talk about how much energy remains in the air.
The vortices rolling up under the diffuser take high energy air from the outside of the body and introduce it into the diffuser but, perhaps more importantly, they also take high energy air from below the level of the body and mix it into the body of airflow that is expanding under the diffuser.
In Formula 1 diffusers are expanded both in side view and in plan view. This is in part because the height limit imposed by regulation is well below the aerodynamic optimum.
There is still a positive impact on pressures and hence forces on the flat part of the floor. If your diffuser or floor is wide, then there may be energy in the air inboard of the outer vortices, which is not used.
Even though this is not a conventional diffuser, you can see the vortices on the outboard parts of the floor section. The colours represent total pressure energy and the lines line integrated convolution for the CFD techies show airflow directions in the plane of the shot which is a cross section of the car near rear axle centreline.
Another little appreciated detail about motorsport diffuser performance is changing performance with speed. Not zero but secondary. Forces increase with speed times speed and ride height has a bigger impact than changing airflow states caused by different Reynolds numbers. Area ratio how much you expand the air under your diffuser is, however, an important parameter.
From the studies done at Southampton University it was found that an area ratio below 6 was relatively safe. The ratio of those areas should not exceed 5. F1 diffusers achieve high ratios thanks to putting the entire floor of the car at an angle….. Hence, despite centre of gravity disadvantages, one of the main reasons F1 cars have high rake. So they just angle the floor. Depending on the formula in which you race this could be by putting a wing above and slightly behind the diffuser not legal in F1 or it could be just surrounding the it with deflectors and winglets as is the present practice in F1.
If you add an additional fence or strake a vortex generator of any design really in each side of the floor it is possible to roll up two more vortices which take advantage of that airflow energy to create downforce. If I see high energy air exiting the floor of the car at the most critical low ride heights , I look to find a way of using it.
So, hopefully, this explains what fences in the middle of a diffuser do some part of their function in any case. Evolution Measurement is the UK based manufacturer and distributor of pressure, temperature and electrical measurement instruments. It was founded by Paul Crow Ferodo Racing is part of the Federal-Mogul Corporation and specialises in the design, development and production of disc brake pads and brake fluids for motorsp Performance Friction Corporation Brakes are the top choice in motorsports - winning more championships than any other brake supplier on the market.
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Willem Toet explains….
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