Following on from the discussions in the Fiorano test thread:
Pages 10 and 11 of this thread:
http://www.thescuderia.net/forums/sh...210#post266210
The topic of problems with the diffuser came up.
The following is an explaination based on various websites and my own knowledge, and may be amended in future if it is either incorrect or not explained adequately.
edits shown in red
The diffuser
Firstly, the diffuser is basically a device that expands the air from under the car and slows it down. This creates a lower pressure under the floor of the car, effectively 'sucking' the car down (term sucking used loosely).
This is Bernoulli's principle of a higher velocity generating a reduction in pressure. Since the pressure above the car is higher than that bellow it, the car is pushed down onto the track.
The diffuser is made up of a series of sloped 'ducts' increasing in height as they leave the floor section of the car.
Stalling
In aerodynamics, stalling is when flow separation occurs.
What this means is that the air flowing closely around the aerofoil starts to detach from the surface and become turbulent.
This occurs when the angle of attach (angle of the aerofoil to the flow direction) becomes too great, and the air cannot remain attached to the top surface of the aerofoil.
(below is an image of the flow around a stalling aerofoil - note the turbulent flow above it)
This causes the drag and lift to become eratic.
Diffuser Stall
On an F1 car diffuser stall is where the air separates from the surface of sloped exits or by 'vortex burst'
Since the angle of attack will not change significantly on the diffuser, flow separation will happen when the speed of the air drops such that it will detach.
The task is therefore to match the slope of the diffuser sections to the expected flow speeds through those areas.
Stalling will occur during the following conditions:
Under braking, the car pitches forward, and the back of the car will get further from the ground. This (again through Bernoulli's principle) will increase the pressure by allowing more air in from the sides, and thus slowing the air.
At this point, the diffuser will stall if poorly designed.
Stalling can also occur when the car is running level.
The side parts of the diffuser are fed with air from under the sidepods, which will have partly mixed with slower air from either side of the car. This will slow the overall flow, and if the slope of the diffuser is too high - lead to stalling.
In the diffuser there are various fences which control the flow.
When overloaded aerodynamically, the vorticies generated off the fences can burst releasing slow 'lossy' air right in the middle of the diffuser.
Finally, the diffuser can stall if the car ‘bottoms out’. This is when the car runs so low under high downforce and over bumps that the skid block/plank grounds out on the track, and air flow to the diffuser is significantly reduced if not interrupted for brief periods.
Having thought about it further, the stuff below is probably not totally correct - please see further discusion for latest thoughts on the Ferrari 248 F1 diffuser.
Ferrari 248 F1 Diffuser
In the case of the new Ferrari, they appear to be complaining of increased drag at high speeds.
This would suggest stalling during level running, probably due to the arrangement of the fences within the diffuser.
Alternatively, they may have gone too aggressive with the design of the diffuser and made the exits too steep – leading to stalling.
Of these the first is easier to solve with a decent bit of windtunnel time, the second could require a major re-design of the diffuser.
It is unlikely to be due to ‘bottoming out’ as this is simply resolved by increasing ride height slightly, or increasing the bump stiffness of the suspension.
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