Diffuser Pressure Peak

[tomspotley]

I've searched everywhere for this but havn't found any satisfactory explanations.

Why is there a (low) pressure peak at the kick-up point of the diffuser (or tunnels generally)? Is it just because air from the side wants to attach to the diffuser and so the airflow converges laterally causing acceleration at the kick up point? Or is there something going on with vortices, tyre squirt or similar effects. I've also read something about changes in radius causing acceleration but not sure how this would apply.

I have a pretty decent theoretical understanding of fluid dynamics so I guess I'm looking for an answer in that context.

[bhall]

Unless I've completely misinterpreted your question, it's because the idea behind a diffuser is to expand and decelerate, or diffuse, the high speed air flow underneath the car that creates low pressure (downforce). Naturally, the low pressure peak of the diffuser is then at the front of it, where the the floor ends and the diffuser begins, because that's where the effect of the diffuser is weakest.

[tomspotley]

Unless I've completely misinterpreted your question, it's because the idea behind a diffuser is to expand and decelerate, or diffuse, the high speed air flow underneath the car that creates low pressure (downforce). Naturally, the low pressure peak of the diffuser is then at the front of it, where the the floor ends and the diffuser begins, because that's where the effect of the diffuser is weakest.

Why does the air that is flowing under the floor accelerate and drop in pressure as it initially enters the diffuser? The diffuser is designed to do the opposite. Hopefully that's a bit clearer.

There is something that causes a "suction" (a pressure gradient) at the kick up point. If you look at the pathlines in CFD, there is definitely something encouraging the flow (especially from the sides) to converge towards the diffuser.

[bhall]

The air pressure under a car is only low when compared to the air pressure around the car. Otherwise, air flow under the car is of higher pressure than that in the diffuser. Hence, the "suction" effect of the diffuser.

Think of a bulb syringe that never stops expanding.

[xpensive]

Bernoulli says that total pressure is constant: Static pressure + Dynamic pressure, the latter as density * speed^2 / 2.

What is creating downforce is static pressure, which is at its lowest where the air-speed is at its highest, see above.

The diffuser speeds up the air under the car vs over and slowing it down through itself to match the latter at the exit.

[bhall]

To expand on that a bit, a diffuser speeds up air flow under the car by giving that air somewhere to easily go. The bigger the diffuser, the more it can accept, which increases the efficiency of ground effect.

I only mention that because it took me forever and a day to figure out how a diffuser works, because it's sometimes stated, without explanation, only that a diffuser speeds up air flow under a car. As such, I took that literally to mean a diffuser physically accelerates air flow by somehow imparting energy to it, and that made no sense whatsoever. It wasn't until I realized they speed up air flow by keeping the flow's path clear, so to speak, that I understood the concept correctly.

(Plus, I can't sleep.)

[xpensive]

Some numerical values on the downforce benefits of the diffuser can be found here;

viewtopic.php?f=6&t=6545&hilit=Double+diffuser

Anyway, I don't really get the initial question, please identify kick-up point first of all?

[tomspotley]

The kick-up point is at the transition from flat floor to diffuser.

That image shows that the pressure on the flat floor is lower than everywhere in the diffuser except around the kick-up point. My question is why does the air accelerate and lose pressure as it enters the diffuser (creating a low pressure peak at the kick up point).

[xpensive]

Probably just a CFD-phenomena related to the sharp transition, try and model a curvature and I'm sure it will go away.

[tomspotley]

Probably just a CFD-phenomena related to the sharp transition, try and model a curvature and I'm sure it will go away.

I have and it doesn't, it's a fairly well documented phenomena.

[bhall]

I think this might help, especially now that I understand the question.

I REPOST THE RESULTS OF THE "BELL SHAPED" NEXT TO THE CONVEX DIFFUSER ADDING DOWNFORCE-DRAG FIGURES

"BELL" DIFFUSER
Downforce: 1381N
Drag: 220N

CONVEX DIFFUSER
Downforce: 1705N
Drag: 259N

After studying Brian's latest picture of the RB7 floor, i came up with this design which follows the F1 rules (125mm height, 350mm length) so the downforce-drag levels are not comparable with the previous analyses.
Very smooth concave at the start and convex shape at the end. The flow is almost perfectly attached, the suction peak at the kink line is unchanged, plus we have a second (lower intensity) suction peak at the transition between concave and convex.
With a bit of design fine-tuning we could come up with a stronger suction peak at the second transition and also with an improvement of the pressure at the rearmost edge of the throat.






Notice the blue areas on both the diffuser -but more strongly on the convex one- due to the vortices!






And here is a pressure plot on a plane closer to the diffuser lateral wall, where the second suction peak is more evident.




And lastly, a velocity streamline on this same plane:

[stez90]

Around that transition zone streamlines are closer each other than in flat zones, meaning higher speed and lower pressure..

[bhall]

Also...

Yes exactly,they are the tip vortices of the diffuser. You can see in this picture of yours


The strong blue low pressure zone they produce on the downfacing flat surface of the footplate. EBD is about accelerating d enhancing these vortices in my opinion.

These have all been taken from this thread: viewtopic.php?f=6&t=10943

[xpensive]

The reason could be that the air accelerates when approaching the transition point, like at the nozzle of a vacuum-cleaner.

Perhaps if you had finer resolution, ie more blue shades, this acceleration could be seen from a farther distance?

[tomspotley]

The reason could be that the air accelerates when approaching the transition point, like at the nozzle of a vacuum-cleaner.

Perhaps if you had finer resolution, ie more blue shades, this acceleration could be seen from a farther distance?

Could you explain the vacuum-cleaner analogy? Isn't a diffuser the opposite of a nozzle?



You can see in this image that the flow definitely converges laterally (and therefore speeds up if incompresible) as it approaches the "kick up point". The flow from the sides seems to be sucked inwards.

I feel like the air is "competing" to get into the diffuser, like people scrambling for the exit at a concert (except because of incompressibility it has to speed up). I just can't work out what exactly is causing the air to rush into the diffuser rather than just go around it. I could have it the wrong way round though and the only reason the air is sucked in is because there is a low pressure there in the first place (caused by some other mechanism).