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I am looking into F1 car aerodynamics for a course that I am taking. I understand that because of Bernoulli's law, when the velocity increases, the pressure decreases, which is good for F1 cars if we want to generate downforce (by having a low pressure below the car).

However, the diffuser is described with

As the diffuser opens up the airflow accelerates further (source, but I can link many other sources.).

Which I do not get because due to conservation of mass, I would expect the velocity to go down, when a duct expands. The only time where I would expect an increase in velocity when expanding is when we have supersonic flow, but in an F1 car, we are way below that. I also looked at other diffuser designs and they all seem to really open up.

So, what am I missing?

Thanks!

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  • $\begingroup$ I suggest a bunch of writers are copying each other; you can't accelerate a fluid by increasing the area. $\endgroup$
    – Tiger Guy
    Jan 27, 2022 at 13:04
  • $\begingroup$ Hint: Acceleration is a vector. It can be decomposed into parallel and perpendicular to flow velocity. If conservation of mass says it is going slower in the flow-wise direction, what else must be happening? Complex shapes create pressure gradients (and velocity gradients) across the flow as well as along the flow. engineering.stackexchange.com/a/36918/12754 $\endgroup$
    – Phil Sweet
    Jun 27, 2022 at 2:52

3 Answers 3

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I think you are just misinterpreting the message of the passage. The diffuser opens up to create a smoother transition between the high and low velocity areas. If the transition is abrupt (sharp edge), flow separation occurs in a way that drag by viscous pressure becomes larger.

Note: drag by viscous pressure is the portion of drag that comes from the difference of pressure between the front and rear of the car.

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It is an interesting article and I will make sure to read it in full when I have time, the first paragraph of the diffuser section reads.

Airflow is accelerated under the floor where it creates a lower pressure area.[source][1]

The key word here is accelerated, i.e. work is being done to the flow and continues being done to it as it goes through the diffuser.

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  • $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Oct 25, 2022 at 23:12
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That's more aerodynamics. Faster the car goes, less traction time the wheels have on road surface per unit of surface area per second and more aerodynamic lift is gained against the car body. Thats why before defusers so many cars flipped throughout the 80s, 90s, etc. enter image description here

F1 intitated Horsepower limits and downgraded v12 to V8 for this reason, the "V12 era which lasted from 1989 to 1997". Car makers solved the issue by using whole Body panels to generate more downforce. If you can increase air flow thru the body without generating lift by exhausting that flow slightly upward and front axle partition also generates downforce by scooping up the air like a snow plow. That air also doubles as additional cooling. Race/Sports cars do everything they can to avoid AIR getting under the undercarriage. Ford did it. enter image description here

Bugatti did it. enter image description here

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