Diffuser Profile - Convex or Concave?

[shelly]

I think that the fisrt aim of the thread has been achieved<. demonstrate that the concave shape of the diffuser is used, show some of the reasons behind that choice, get some insight on diffuser functiong and above all moving ahead from the too much abused venturi word

[ringo]

I think the point of the thread is to find the best diffuser profile, see if it's convex and concave, neither, or a mix of both, and give an answer to why. We seem to have found reasons to support different shapes, and have had some nice evidence given, with some very informative posts.

All we lack is clear cut evidence using a whole car as a model now!

If we could get some CFD run (I know we can't take CFD results literally), to support our findings, then the thread may develop into new areas, including how things such as the exhaust, and tyre wake, affect diffuser performance, and in what way.

Would be cool if that happened!

You don't need a whole car to compare two diffusers. As long as they are in the same settings a comparison can be made. One is fundamentally better than the other so common environment is enough for a comparison.

What i mentioned earlier was that the top of the diffusers need to have the same shape when testing. Otherwise the drag and downforce results will be misleading.


the top surface is not the same here, so the results are invalid. There is no control, and results are affected by both the diffuser throat and top sufrace.

The only difference between the two should be the throat.
Finally the diffuser at least needs to be connect to a floor, maybe about 4 meters long to have steady flow at low pressure entering it.

DRcorsa can run the simulation again but with the same shape at the top surface; maybe make a rectangular block with the diffuser throats carved in.

[hardingfv32]

BreezeRacer

I have a good question about strakes in the diffuser, but first must confirm your understanding of their purpose.

Brian

[hardingfv32]

I think that the fisrt aim of the thread has been achieved<. demonstrate that the concave shape of the diffuser is used, show some of the reasons behind that choice, get some insight on diffuser functiong and above all moving ahead from the too much abused venturi word

I would say we know that concave shape is in vogue from current floor photos. Yes, we have learned that the diffuser in sensitive to ground clearance, but there is no connection between that sensitivity and the use of a concave diffuser roof (floor?). We can't assume that ground clearance sensitivity is the reason a concave shape is used.

It is not clear to me that the question has been answered, but if everyone wants to move on, I will follow as I am not going to get this question answer without help.

Brian

[shelly]

I think that one thing that has emerged from discussion is that he concave shape allows for a increased suction peak and a more aggressive pressure recovery, so it is used whenever possible. Do you agree?

[hardingfv32]

Of coarse I have been misspeaking concave it is.

I know everything we see and have discussed implies that, but what is the flow mechanism makes this so? I am just uncomfortable not know how that abrupt corner and concave shape function to provide the pressure drop in an efficient manner. Why do I not see examples of such a shape in any other field of science or industry? Just unsatisfied... hoping for a little more, because as you said, the conversation has been very cooperative.

Brian

[BreezyRacer]

I think that one thing that has emerged from discussion is that he concave shape allows for a increased suction peak and a more aggressive pressure recovery, so it is used whenever possible. Do you agree?

No I do not agree with your statement. I believe that given the packaging restrictions of F1 for example, that concave works better than the other options. However if there were no limitations such as diffusers only starting at the rear axle line, etc, a more curved profile that doesn't undergo flow separation with the diffuser surface would be preferable. That's the problem with these things .. you have to work around packaging, the rules, car speed range, and the car general design to come upon what works for that condition.

[BreezyRacer]

BreezeRacer

I have a good question about strakes in the diffuser, but first must confirm your understanding of their purpose.

Brian

I don't know that I'm the right guy to answer your strake questions, though I'll tell you what I've concluded from what I've read. There appears to be 2 general strake designs .. the full diffuser box strake and the short strake, like you see used in F1 today. In general the strakes isolate differing pressure areas within the diffuser box and typically that's good for minimizing the effects of rear tire turbulence. They are also used to control and boost the vortexes that you hopefully build within the center section of the diffuser box. The short strakes do this.

The study I referred to showed that lots of strakes add drag and also that they enable larger degrees of diffuser box rake to work. Thus if you were to build a diffuser box that was too aggressive, resulting in flow separation, you could apply strakes to get the diffuser working better. The larger angles however, don't seem to be able to increase the pressure differential without adding extra drag.

I would love to know more from someone knowledgeable about strake designs. The modern F1 diffuser is a work of art and the strakes are a large part of that.

[Pierce89]

Of coarse I have been misspeaking concave it is.

I know everything we see and have discussed implies that, but what is the flow mechanism makes this so? I am just uncomfortable not know how that abrupt corner and concave shape function to provide the pressure drop in an efficient manner. Why do I not see examples of such a shape in any other field of science or industry? Just unsatisfied... hoping for a little more, because as you said, the conversation has been very cooperative.

Brian

You do see that shape in the wings of some transonic aeroplanes it's called a Liebeck airfoil. From what I've seen, in F1, the concave profile just allows for a more aggressive diffuser in the the geometric limits set by the regs, because that shape allows for quicker pressure recovery(greater expansion in F1T parlance. Or just think more volume in the box F1 rules allow for the diffuser). As long as the flow has enough energy you don't have to have a gentle slow curve for maximum diffuser performance as is sometimes portrayed.

[hardingfv32]

You do see that shape in the wings of some transonic aeroplanes it's called a Liebeck airfoil. From what I've seen, in F1, the concave profile just allows for a more aggressive diffuser in the the geometric limits set by the regs, because that shape allows for quicker pressure recovery(greater expansion in F1T parlance. Or just think more volume in the box F1 rules allow for the diffuser). As long as the flow has enough energy you don't have to have a gentle slow curve for maximum diffuser performance as is sometimes portrayed.

1) I can't make the Liebeck airfoil work. It looks to me that the surface that resembles the concave diffuser create a high pressure zone near the leading edge on the airfoil.

2) When discussing the concave diffuser, the only thing I can thing of is some kind of Kamm effect.

Brian

[hardingfv32]

They are also used to control and boost the vortexes that you hopefully build within the center section of the diffuser box. The short strakes do this.

My question about strakes is related to the property you mentioned that generates vortices and I assume helping to keep the flow attached to the roof (floor).

In the case to the concave diffuser it would SEEM that we need all the help we can get keep the flow attached at the front of the diffuser. I don't see how the strakes can be much help at the front. Can the vortices form fast enough to help the flow stay attached close to the leading edge of the diffuser?

Brian

[BreezyRacer]

They are also used to control and boost the vortexes that you hopefully build within the center section of the diffuser box. The short strakes do this.

My question about strakes is related to the property you mentioned that generates vortices and I assume helping to keep the flow attached to the roof (floor).

In the case to the concave diffuser it would SEEM that we need all the help we can get keep the flow attached at the front of the diffuser. I don't see how the strakes can be much help at the front. Can the vortices form fast enough to help the flow stay attached close to the leading edge of the diffuser?

Brian

Well I'll tell you what I found .. the vortexes begin at the leading edge of the undertray. The leading edge of the undertray/splitter has to be shaped to create a vortex and then you nurse that vortex (1 per side) all along the floor to the diffuser. At the diffuser the vortexes expand and become quite strong , leaving a powerful low pressure area between the two vortexes. As they merge into the diffuser box they are spinning in opposite directions and they work to feed from each other and pull towards each other. With a setup like this you can create a low pressure area far ahead of the diffuser box itself, and it's strong and stable.

A weak diffuser setup will create sudden vortexes at the front corners of the diffuser box and extreme pressure variations all over the place. The low pressure will be short and centered around the diffuser entry point. Not good.

My understanding of the concave diffuser is that you build a rolling airflow in the void of the concave area.Below that rolling airflow you have the regular airstream. It seems as if the rolling airflow adjusts the diffuser box to minimize turbulence/separation in the normal flow area, which is no longer the roof of the diffuser, but rather the bottom of the spinning airflow in the cavern of the concave diffuser box. The downside is that it's more speed sensitive because at low speeds there just isn't enough spinning energy to pressurize the cavern. BUT all this is just speculation on my part. It's gleaned from reading anything I could find for many years about diffusers and comparing diffuser shapes, etc. I haven't had the chance to sit with Adrian and chat about for an afternoon. I just don't have that much free time these days

What I would love to know is whether today's short strakes reside outside the vortexes or if they can actually enhance the vortexes, amongst other things of course.

[Lightspeedrt]

I think the point of the thread is to find the best diffuser profile, see if it's convex and concave, neither, or a mix of both, and give an answer to why. We seem to have found reasons to support different shapes, and have had some nice evidence given, with some very informative posts.

All we lack is clear cut evidence using a whole car as a model now!

If we could get some CFD run (I know we can't take CFD results literally), to support our findings, then the thread may develop into new areas, including how things such as the exhaust, and tyre wake, affect diffuser performance, and in what way.

Would be cool if that happened!

You don't need a whole car to compare two diffusers. As long as they are in the same settings a comparison can be made. One is fundamentally better than the other so common environment is enough for a comparison.

Not 100% sure I agree with that. I'm not very experienced, but i wouldn't dismiss this idea so suddenly. I think it would be worth trying, one may perform better against the other if tyre wake etc. is added. I understand that you are trying to stick to the one "variable" but it's worth a crack. I may be wrong, just an opinion.

[hardingfv32]

Not 100% sure I agree with that. I'm not very experienced, but i wouldn't dismiss this idea so suddenly. I think it would be worth trying, one may perform better against the other if tyre wake etc. is added. I understand that you are trying to stick to the one "variable" but it's worth a crack. I may be wrong, just an opinion.

Of coarse what you say is true if we had a way to implement such an investigation, but...

1) There seems to be different opinions about how the tire wake is being or best managed.

2) At this point we are never going to see a valid model of diffuser and tire interaction.

3) Our only hope of evaluating diffuser and tire interaction is to first get a fundamental understanding the concave diffuser's aero properties.

Brian

[DRCorsa]

@ringo
I didn't really understand the different top surface subject.
Maybe you mean the different angle of the diffusers between those 2 images? If yes, this has nothing to do with the analysis, the voncave one is just rotated on the post processor. The flow direction is parallel to the throat on both examples.