# Increasing air velocity through F1 car diffuser

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!

• I suggest a bunch of writers are copying each other; you can't accelerate a fluid by increasing the area. Jan 27, 2022 at 13:04
• 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 Jun 27, 2022 at 2:52

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.

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.