Diffuser Profile - Convex or Concave?

[shelly]

@scarbs: yes in the past tricks like that have been used, I think till 2007.
I remember Piola discovering that on the ferrari in 2001. I think they wer related to a rule of minimum radius of the transition, which now maybe is enforced differently.
In the photos of ferrari and rbr from this year I can not see the detail - I do not know if it is still there.

I agrre with Just a fan

[shelly]

How does the exhaust come into play when the outlet is very close to the inlet of the diffuser and aimed outward toward the tire side wall? Without doubt it has no effect on the center of the diffuser. So if you believe the exhaust plays some role why would the roof in the center be the same as the roof on the out edges? Would you not expect the exhaust interaction to require a different roof shape if in fact there was such an interaction?

Brian

@brian: I think the function of 2011 blown diffuser is about vortex stretching. By stretching the footplate vortex to the diffuser, they lower the presuure of the inner core and strengthen the vortices, thus obtaining a "seal" effect bt especially a lower pressure on the flootplate and o the adjacent zone.
The roof of the diffuser is at maximum height throughout the width; the difference in pressure is managed with the fins.

I know this is difficult to understand just with words - take a look at my usual picture and I'll post some other to explain better vortex stretching.

The key of my line of thought is that the footplate vortex is already there, and the exhaust change its longitudinal speed, stretch the helix, narrow the core and lower the inside pressure level.
So exhausts are vortex enhancers, not just blowers, becuase thy stretch exsting helixes making them more effective. Google "vortex stretching" and Helmholtz theorem ffor a more educated description.

[DRCorsa]

@shelly

Interesting. So, in effect, they are trying to "aerodynamically" extend the diffuser's endplates which by regs are very short (50mm i think).
But what about this theory with the Reanult's case where we have a quite different blown exhaust concept?

[shelly]

in renault case they tangentially blow the vortex from the bargeboard on the floor leading edge, making it more powerful.

There has beeen a lot of discussion on these topics, with at least 3 threads
-exhaust blown diffuser
-diffuser function
-blown diffuser - why it is there
plus the other strange concave diffuser thread

[hardingfv32]

Shelly

I have a complete appreciation of what your propose to be the benefits of the vortex activity at the rear diffuser.

What I am challenging is if the exhaust's (vortices?) influence on the design of the diffuser roof. I do not see any indication that they effect the roof shape.

Brian

[hardingfv32]

Shelly

Why is it in the engine intake diffuser referenced above generates a pressure gain and the rear diffuser being discussed generates a negative pressure?

Is it a different type of system? At some speeds, all of the engine intake flowing through the scoop inlet is going into the engine. The engine does represent a negative pressure at the outlet.

What small detail am I missing?

Brian

[shelly]

Brian

I have tried to answer above to your question about the roof in my previous post but Ihave not explained myself well. What I meant is that we do not see roof shape variation because diffuser height is strictly limited so teams exploit maximum height allowed< then the fins placed at different angles create channels of different widthat the sides and at the center, with different expansions ratio,as they are divergent when seen from below.

As for the question about engine diffuser, there is no contradiction. Along the diffuser, except for fins effects and voritces, we see a pressure increase due to the increasing cross section. In a f1 diffuser downforce is produced by air running along a curved path across the kinkline and running fast along the flat floor

[hardingfv32]

Brian

I have tried to answer above to your question about the roof in my previous post but Ihave not explained myself well. What I meant is that we do not see roof shape variation because diffuser height is strictly limited so teams exploit maximum height allowed< then the fins placed at different angles create channels of different widthat the sides and at the center, with different expansions ratio,as they are divergent when seen from below.

I see what you mean. The roof is constant, but the fins create channel volume/shapes that are different. I had the fins only creating vortices. This adds a new complication that I must research.

Brian

[hardingfv32]

Brian

As for the question about engine diffuser, there is no contradiction. Along the diffuser, except for fins effects and voritces, we see a pressure increase due to the increasing cross section. In a f1 diffuser downforce is produced by air running along a curved path across the kinkline and running fast along the flat floor

I have less comfort with this answer. I have to rethink in the context you have provided. Maybe more question coming.

Brian

[Reca]

With a concave profile the aim is to obtain a more abrupt deceleration in the very first part, near the throat/leading edge, getting closer to the maximum ideal pressure recovery, coherently with the energy of the boundary layer, which is in fact achieved when the b.l. itself is constantly kept at limit of separation.
The same theory is valid not only for diffusers, but for the deceleration area of airfoils too (obviously, the aim is the same...), google for Liebeck airfoils to see a practical application of the concept.

The problem is that, exactly because the maximum pressure recovery demands pushing the b.l. to limit of separation, it's a potentially more critical design that operates optimally in a rather limited range of conditions and dramatically loses efficacy at the minimal variation, and that makes often preferable a more conservative convex profile, that induces a more limited peak deceleration the b.l. is more easily able to cope with, sacrificing pressure recovery in favor of versatility.

That presumably explains the difference we see in F1 diffusers, with the central, narrow, part typically convex, while lateral channels are concave.

In the middle the floor gets quite close to the ground, so the area at the throat is small and changes greatly with small variations of ride height, and geometry is not particularly favorable being narrow and open at the sides, things making more difficult to control the flow thus increasing risk of separation; a smoother transition with a gradual change of slope, imposing a smoother deceleration and keeping the b.l. more distant from limit of separation, is preferable in order to get a more stable behavior for all the working conditions (obviously any trick to improve the situation, like blowing thru the starter hole, energizing the flow, can allow to be more "aggressive" and move a bit towards the ideal)

For the lateral channels, more distant from ground at the throat, thus less sensitive to ride height variations, that problem is lot less evident and they can afford using a more effective profile, concave, closer to the ideal shape for maximum pressure recovery.

BTW, concave profile is nothing new, as you can see in this WT model of Ferrari's F310B (1997) floor, that was on display at Galleria Ferrari in 2005 showing pressure distribution, with distinctly in evidence the peak of pressure localized at the kink:

[shelly]

Wonderful post Reca!

[hardingfv32]

I believe aerodynamics has evolving to include other possible causes of lift in wing design beyond camber, say angle of attach of attack and vortex-induced lift. I say this from the point of view of a layman on the subject.

While there is mention of vortex activity for flow attachment on this forum, there is little about vortex-induced downforce. Could vortex-induced downforce play a greater roll than camber when discussing downforce production?

Brian

[DRCorsa]

It is possible that the blown exhaust concept follows the relatively old concept of "boundary layer control system" on aircrafts. So it maybe has nothing to do with "squeezing" the vortices but mainly to energise the boundary layer in areas where flow seperation normally occurs. The result will be that the flow stays attached and those areas contribute to downorce generation.

In my CFD analysis of the concave diffuser we saw that we get a low pressure on the "kink" line but we get seperation, stall and even lift on the curved area after the kink. Maybe the blown exhaust's job is to energise the boundary layer after the kink preventing the stall?

[Tozza Mazza]

With a concave profile the aim is to obtain a more abrupt deceleration in the very first part, near the throat/leading edge, getting closer to the maximum ideal pressure recovery, coherently with the energy of the boundary layer, which is in fact achieved when the b.l. itself is constantly kept at limit of separation.
The same theory is valid not only for diffusers, but for the deceleration area of airfoils too (obviously, the aim is the same...), google for Liebeck airfoils to see a practical application of the concept.

The problem is that, exactly because the maximum pressure recovery demands pushing the b.l. to limit of separation, it's a potentially more critical design that operates optimally in a rather limited range of conditions and dramatically loses efficacy at the minimal variation, and that makes often preferable a more conservative convex profile, that induces a more limited peak deceleration the b.l. is more easily able to cope with, sacrificing pressure recovery in favor of versatility.

That presumably explains the difference we see in F1 diffusers, with the central, narrow, part typically convex, while lateral channels are concave.

In the middle the floor gets quite close to the ground, so the area at the throat is small and changes greatly with small variations of ride height, and geometry is not particularly favorable being narrow and open at the sides, things making more difficult to control the flow thus increasing risk of separation; a smoother transition with a gradual change of slope, imposing a smoother deceleration and keeping the b.l. more distant from limit of separation, is preferable in order to get a more stable behavior for all the working conditions (obviously any trick to improve the situation, like blowing thru the starter hole, energizing the flow, can allow to be more "aggressive" and move a bit towards the ideal)

For the lateral channels, more distant from ground at the throat, thus less sensitive to ride height variations, that problem is lot less evident and they can afford using a more effective profile, concave, closer to the ideal shape for maximum pressure recovery.

Fantastic post Reca, very insightful,

Thanks.

[shelly]

@drcorsa> I think there is something more to this year ebd than just energizing the boundty layer. Entrainment plays a part, but I think that interaction with vortices is a dominant effect