Diffuser Profile - Convex or Concave?

[DRCorsa]

@ringo

Before the... bell rings, please wait to see some results of the pure convex diffuser..

[ringo]

If you actually have any idea what you are talking about, please relate it to the images that have been posted. Just pick one or two of the many variations you say are possible and explain how they apply. Demonstrate that anything you have mentioned is relevant to the discussion.

No one was listening. It's the shear stress at the surface that should be addressed. That affects the pressure recovery along the diffuser.

I've learned not to throw too much info out as it isn't even referred to, because it's not on a fancy website. Can't blame you guys still as i wouldn't trust me either.

[hardingfv32]

Those percentage figures you have Brian seem quite precise.

Where did you get them from?

Can you draw out this profile inside a box 350mm long and 125mm (75mm from step plane) high?

The percentages are based on observation of many views of the RB7 Monza diffuser.

You will not see any computer generated drawings from me. Not in my skill set. I could fabricate something faster than trying to draw on the computer. I assume old school drawings are not an acceptable substitute, as they are almost never seen on this form.

Brian

[Tozza Mazza]

A scale drawing is a scale drawing, whether it be CAD or just Pen and paper!

[hardingfv32]

If you guaranty no prejudice from the use of pen and paper, I will have a go at it.

Brian

[hardingfv32]

This is my estimation of the RB7 Monza diffuser floor. The initial surface could actually be straight with a curved transition to the next straight section.

Added the curved "Gurney" that is now in vogue. I believe it can be 20 mm above the dimension box, but I have no idea how far forward it can extend.

A chicken and egg type question: Is the convex shape at the end more important to the flow above the diffuser floor (roof) or the flow below it?

Brian

[PlatinumZealot]

My view on this is that the concave part is to create a wall of air in the "roof" of the diffuser right after the kink. This actually replaces a "solid" roof of the convex diffuser.

The active air stream will still look like it is moving through a convex diffuser. The only difference it that there is a point in time when the boundary layer energy loss is less (since it is air to air at the boundary instead of carbon fibre to air), So you can obtain a larger expansion ratio at low speeds without separation.

In layman terms I think it creates an air to air bearing in the concave part. The shear forces are very low (again less boundary layer losses).

[ringo]

This is my estimation of the RB7 Monza diffuser floor. The initial surface could actually be straight with a curved transition to the next straight section.

Added the curved "Gurney" that is now in vogue. I believe it can be 20 mm above the dimension box, but I have no idea how far forward it can extend.

A chicken and egg type question: Is the convex shape at the end more important to the flow above the diffuser floor (roof) or the flow below it?

Brian

I think your first curve is a little too extreme. The shape at the end is not as critical as the shape at the throat. The general idea is correct though, and this will change when the diffuser is no longer blown.

[Mystery Steve]

You guys biting off more than you can chew with beam wings and what not.
Just take it easy and focus on the diffuser alone.

How is it biting off more than you can chew when it is very much relevant to the functionality of the diffuser? Like I said before, beginning with the simple model is the definitely best place to start, and then adding complexity to either address an issue or as the simple components become better understood. In this case, we saw an issue with flow separation, so the beam wing was suggested as a way of addressing that issue without sacrificing the aggressiveness of the diffuser profile.

It's not like we have a deadline to meet here; we just want to systematically learn the fundamental physics of F1 cars, which includes the beam wings as they are clearly an important component of the car. If they weren't, we wouldn't see the unusual McLaren sidepod design or the "coke bottle" sidepods.

[hardingfv32]

The shape of the convex section at the exit:

Does this shape benefit the flow over the top more than the flow underneath? Or is it ideal for both flow systems?

Brian

[ringo]

It help the underneath, as the flow over the top will entrain the flow coming out of the diffuser. The drag will increase though.
This is my opinion though, i haven't looked on it in detail.

[ringo]

You guys biting off more than you can chew with beam wings and what not.
Just take it easy and focus on the diffuser alone.

How is it biting off more than you can chew when it is very much relevant to the functionality of the diffuser? Like I said before, beginning with the simple model is the definitely best place to start, and then adding complexity to either address an issue or as the simple components become better understood. In this case, we saw an issue with flow separation, so the beam wing was suggested as a way of addressing that issue without sacrificing the aggressiveness of the diffuser profile.

It's not like we have a deadline to meet here; we just want to systematically learn the fundamental physics of F1 cars, which includes the beam wings as they are clearly an important component of the car. If they weren't, we wouldn't see the unusual McLaren sidepod design or the "coke bottle" sidepods.

Why i say that is that jumping into beam wings and wheels before we even fully understand what the diffuser has to do to perform optimally can be seen as glossing over the subject without grasping it fully.
If there was no interjection about multiple curves, then we would be stuck on thinking a single concave curve is best, but needs a beam wing to function. So those early conclusions can be misleading and maybe harmful.

[marekk]

Guys, maybe we've gone a bit to far with CFD and 1% numbers, without answering the very basic question (and understanding the answer): how does the diffuser produce downforce in first place ?

My way of thinking is:

1. As the car moves around the track, on the calm day, air molecules just in front of the leading edge of the floor have no (mean) velocity in any direction at all, relative to the rest of the atmosphere and tarmac.
2. To make them generate any bernoulli downforce/lift, we have to accelerate them (still relative to other air molecules, not relative to the car).
3. As there is (contrary to how the wings work) no significant longitudal circulation around the floor, the only way to accelerate the molecules is pressure difference.
4. As the pressure just in front of the leading edge is ambient, we need low pressure behind the car to make the gradient.
5. From the energy efficiency point of view, the ideal situation would be to accelerate the flow as much as possible where it makes maximum of downforce (i.e. near the horizontal parts of the floor) and then slow down back to zero - this way it would cost nothing (no work done in the cycle). Sucking the air from under the floor AND slowing the molecules as much as possible has to be done by diffuser - there is no other magic device here.
6. Now we can add boundary layers, shear stresses, optimal pressure recovery profiles, beam and rear wings, gurneys, strakes, vorticies, hot blown gases, rear tyres wakes, dynamic rake and ride height change and F1 rules on top of that, and try to achieve the goal of point 5 - in my view it's best to start thinking from behind the car, but doesn't really matter, pressure changes propagate in fluids in every direction, and equations are elipitcal (egg or chicken ?) anyway.

Without any CFD (or real knowledge of the problem, to be honest) i can imagine this way, that concave shape gives very strong pump just behind the kink and nice geometry to gradually expand and slow down the flow, with less and less expansion ratio as the pressure gradient decreases. Convex shape will do the opposite.

[hardingfv32]

marekk

I think your way of thinking about diffusers is flawed, but I will not dare to state what the true definition is.

4) The simple CFD simulations from DRCorsa do not indicate that the low pressure from the diffuser makes it to the leading edge. Looks more like the leading edge is responsible for low pressure at the front of the floor. There is also the issue of leakage along the floor sides.

Other points you state really do not form a conclusion that the concave shape is best. Up to this point in the discussion the success of the concave shape really seems to depends on the shape of the diffuser floor after the concave section and the condition of the air flow above it.

Brian

[DRCorsa]

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: