Ride Height/Diffuser Angle & Downforce

[Crucial_Xtreme]

I saw Somers(@SomersF1) link to the first article and then I followed a link to another.


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Introduction

The majority of you landing on this page most likely already have an idea of the effect that Ride Height has on Downforce for an F1 or DSR / LeMans Prototype Race Car. In general, the lower you can get the Underbody of the Race Car to the ground, the stronger the Ground Effect will be, and this will increase Downforce. It does however get a little more complicated than that. As you get closer to the ground, Ride Height Sensitivity increases. In addition, once the Race Car Underfloor gets too low the flow underneath can be blocked and not allow enough air through the diffuser. Once this happens, Downforce plummets.

The following article will talk about these implications in Formula One and other racing series such as DSR and LeMans or other Formula Cars or Prototypes.
Hypothesis

It is well known that the lower a Race Car rides to the ground, the more Downforce it generates. However what happens as the Underfloor is moved in a larger range, from half the normal Ride Height to several times higher? Approximately how close can the Underfloor of a Race Car get to the ground before Downforce begins to diminish? What does the Diffuser Angle do to these trends?

Lets first look at some existing results and see what we can decipher from them.

The first results we will look at are from George [1] ( Cooper referenced in the images was Wind Tunnel Data). It is clear that the Downforce increases pretty much linearly from the maximum Ride Height until brought down to a certain Underbody Ride Height. This point represents a point at or near the optimal Ride Height for generating maximum Downforce. Lowering the car beyond this optimal Ride Height results in a loss of Downforce for the Race Car. Also take note of the 2nd image showing a higher Diffuser Angle. The Wind Tunnel Data indicates worse performance, possibly due to flow separation caused by the high Diffuser Angle. However the CFD Solvers show improved performance. This is another indicator of something I've mentioned before, which is that any results I get with my own analysis at or near the regions producing stall shouldn't be taken at face value.
- See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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For the experiment covered in Milliken [2] a Symmetric Airfoil was used. This experiment used multiple Ride Heights and for each Ride Height the Angle of Attack of the body was changed. It can be seen that for a given Angle of Attack, as the Ride Height is lowered the Downforce is increased, up to a point. - See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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The basic trends from my previous article should still apply, so at reasonable Ride Heights, such as around 37.5-50mm, the data should correlate with my results from my CFD Bluff Body Diffuser Angle Article which was run at 37.5mm Ride Height. - See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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Based on the information above, I made my best guess as to what the CFD Data should resemble once all the different Diffuser Angles and Ride Heights were run. My hypothesis was that steeper Diffuser Angles would result in lost performance at higher Ride Heights, and shallow Diffuser Angles would lose performance closer to the ground. The basic sketch of this can be seen below. - See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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Simulation

My initial intention was to just make the runs at a single Diffuser Angle. However at a Diffuser Angle of 10 degrees for this Bluff Body the CFD Results did not match with the known trends. The simulations would fail to converge after a Ride Height of 35.0mm was reached. Though this could be taken as an indication the trend is being reinforced (since my solver seems to fail to converge around stall points) it was still not very confidence inspiring. I changed my meshing method to more accurately mesh the Underbody of the Race Car Bluff Body, and the trend showed up. It was at this point that I decided to make the runs over many Diffuser Angles in order to see if the meshing problem persisted for blocked flow at higher Diffuser Angles. If the meshing went smoothly without any trouble and the trends continued to show then I'd have a pretty good indication my initial hypothesis was correct.

It took about a week and a half of almost non stop CFD Simulations to get these Data. The case of the 10 degree Diffuser Angle was run over all Ride Heights in its range at 2.5mm intervals. In the interest of saving time, all other Bluff Body Diffuser Angles were only run at points of interest.

For each Diffuser Angle, a few key Ride Heights were run in order to observe the trend in the data. Once this was done, I made judgment calls as to what other Ride Heights to try for that given Diffuser Angle. The Freestream Velocity chosen was 40m/s, and standard atmosphere was used.
Analysis

First lets look at the post processing and visuals before looking at the data.
- See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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Here is an image showing a Ride Height range from 42.5mm down to 25.0mm for the 5 degree Diffuser Angle. As the Underfloor of the F1 Bluff Body gets closer to the ground the vortices seem to increase in strength. - See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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Note that for the last 2 runs for the 5 degree Diffuser Angle the mesh is different. This was due to RAM limitations and the meshing method used. - See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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It was observed that for high Diffuser Angles the blocked flow was achieved, however for low Diffuser Angles it was not as easy to show the phenomenon. It was possible that due to RAM limitations the mesh sizes needed to properly model such low ride heights just weren't achievable with the computing platform used. It took a lot of tinkering with the mesh to get the 5 degree and 7.5 degree Diffuser Angled Bluff Bodies to Stall. Since the Meshing method used to generate these data were different from the other runs the exact values for the stalled runs should not be counted on.

The reason for this was likely that higher Diffuser Angles begin stalling at higher Ride Heights than their small Diffuser Angle counterparts. Due to this, the higher Diffuser Angle Bluff Bodies do not require extra special attention underneath the car, and my typical meshing method could be used. For the low Diffuser Angles, stall did not occur until very close to the ground at which point the mesh sizes required to accurately model this flow went beyond the capability of my laptop.

Below it can be seen that the Data very closely represent my hypothesis above. A notable exception can be seen however in the data for the 15 degree Diffuser Angle case. For some reason the 15 degree Diffuser Angle has a very obvious difference in the slope of the Lift as a function of Ride Height curve. This could be due to the fact that 15 degrees is near the stall point to begin with, or it could just be an error in the CFD. Without another form of verification all I can do is speculate.
- See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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It is clear that the 5 degree diffuser generates less Downforce than the other cases, which makes sense. It is also able to ride lower before reaching Maximum Downforce. For each incrementally higher Race Car Underbody Diffuser Angle it was clear that the minimum Ride Height before Diffuser Stall increased. The maximum Downforce generated was achieved with the 10 degree Bluff Body Diffuser at 35mm. - See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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The effect of Race Car Diffuser Angle and Ride Height on Drag was not as clear as that for Lift. The general trend was that Drag increased as Ride Height decreased and as Diffuser Angle increased. The trend was expected, but such variation in the curves was not expected. - See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf

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The trends for Lift to Drag ratio were as expected, with the exception of the lower slope for the higher Diffuser Angles. Additionally it is interesting to note that the best Lift to Drag ratio was achieved at 37.5mm, rather than at the point of maximum Downforce which was at 35mm.
Conclusion

The results matched pretty well with the hypothesis. Deviations from predictions included the slope of the lift curve for the 15 degree Diffuser Angle and convergence failure at and near the stall point for low Diffuser Angles. Once the convergence failed, a different meshing method was used and the trends showed through correctly.

As the Underbody of the Race Car got closer to the ground the Downforce was increased, until a point was reached at which the flow stalled and Downforce suddenly dropped. As Diffuser Angles increased, the Ride Height at which Stall occurred increased.

Drag tended to increase as Ride Height was decreased, and also as the Diffuser Angle increased.

Lift to Drag tended to be the greatest at or near the point of highest Downforce.
References

[1] George, A Computational Study of Idealized Bluff Bodies, Wheels, and Vortex Structures in Ground Effect

[2] Milliken, Race Car Vehicle Dynamics (He got the image from Stollery and Burns)

I write these articles to help. Please let me know if this was helpful through shares likes and comments so I know you guys find this interesting and a good area for me to focus on.
- See more at: http://consultkeithyoung.com/content/cf ... vm6FS.dpuf


The article above can be found Consultant Keith Young: CFD - Bluff Body - Ride Height

[KeithYoung]

Strange that copied so strangely, hopefully I don't have any HTML issues.

If you guys have any questions about the article feel free to ask here.

[gixxer_drew]

You cant diffuse when there is no air flow.

[henra]

Very interesting!
I'm wondering if the absolute value for optimum L/D Ride height depends on the length of the floor upfront and the speed.
I assume the problem with low ride height is related to the boundary layer of the flow over the ground and also over the surface of the floor of the car in front of the throat.
This would explain why the optimum ride height doesn't seem to change proportionally with diffuser angle and thus expansion ratio which one would normally expect.

[KeithYoung]

Hopefully I can run an analysis and write an article on that in the next couple weeks Henra.

[turbof1]

Probably smaller aero bits on the front wing, and perhaps the diffuser. The issue with passive stalling is that you need to be able to predictably and consistently guide the airflow following a specific flowspeed curve. How succesful that is, depends on the scale, knowledge, and limits set by bodywork regulations.

Something like stalling a part of the diffuser at a specific speed, the middle part has a higher drag coefficient due it connects with the Y100 winglet and the rear wing flows, therefore breaking that up drops drag all round the rear centre, is more doable since teams have been studying stalling issues for decades now. Stalling a rear wing is more difficult because teams are for one limited by body regulations and for two they still can accurately predict how, when and how long air seperation happens, ending up with downforce loss when you need downforce.

Red Bull was rumored last year to have made huge strides in the area of passive stalling last year. They suddenly gained top speed in the second half of the season, without compromising on downforce. At Italy they ran a low downforce rear wing but very interestingly kept a FW with a high AoA. It could mean they are able to stall it properly.

[tuj]

Is it really worth the effort to stall the diffuser? I was under the impression that the diffuser had one of the lowest drag penalties for any down-force producing device, no? As for the front wing elements stalling, do you think they are doing this by ducting air to the front wing assembly or are they using something like flexi-aero to stall it?

[turbof1]

Is it really worth the effort to stall the diffuser? I was under the impression that the diffuser had one of the lowest drag penalties for any down-force producing device, no? As for the front wing elements stalling, do you think they are doing this by ducting air to the front wing assembly or are they using something like flexi-aero to stall it?

Yes it has one of the lowest drag penalties. The reason you still might want to stall is to prevent the upwash connecting with the monkey seat flow (the old beam wing flow) and the rear wing flow. Breaking one flow will set a chain reaction in the other 2 flows. So what you are really doing is not so much remove drag from the diffuser, but from the monkey seat and the rear wing.

The interaction between those flows was a big talking point coming into 2014. The goal was for the diffuser's upwash to extract low pressure air from the rear wing. Since it's quite a big gap between diffuser and the underside of the rear wing, teams used the beam wing as a stepping stone. With that gone, teams nowadays use the combination of the exhaust and a slightly bigger monkey seat to work alteast the middle part of the rear wing harder. The current Red Bull is a testimony of that, employing a Y100 winglet very reminiscent of a beam wing (well the middle part of it).

Of course letting the rear wing work harder also means it produces more drag. You could remove this drag by stalling the diffuser upwash.

About the front wing: I honestly can't say. My intuition tells they count on certain vortices manifesting above a certain speed, which stalls parts of the wing. But I could be wrong. flexing is more predictable to use, but made fairly difficult through regulations.

[trinidefender]

Is it really worth the effort to stall the diffuser? I was under the impression that the diffuser had one of the lowest drag penalties for any down-force producing device, no? As for the front wing elements stalling, do you think they are doing this by ducting air to the front wing assembly or are they using something like flexi-aero to stall it?

Yes it has one of the lowest drag penalties. The reason you still might want to stall is to prevent the upwash connecting with the monkey seat flow (the old beam wing flow) and the rear wing flow. Breaking one flow will set a chain reaction in the other 2 flows. So what you are really doing is not so much remove drag from the diffuser, but from the monkey seat and the rear wing.

The interaction between those flows was a big talking point coming into 2014. The goal was for the diffuser's upwash to extract low pressure air from the rear wing. Since it's quite a big gap between diffuser and the underside of the rear wing, teams used the beam wing as a stepping stone. With that gone, teams nowadays use the combination of the exhaust and a slightly bigger monkey seat to work alteast the middle part of the rear wing harder. The current Red Bull is a testimony of that, employing a Y100 winglet very reminiscent of a beam wing (well the middle part of it).

Of course letting the rear wing work harder also means it produces more drag. You could remove this drag by stalling the diffuser upwash.

About the front wing: I honestly can't say. My intuition tells they count on certain vortices manifesting above a certain speed, which stalls parts of the wing. But I could be wrong. flexing is more predictable to use, but made fairly difficult through regulations.

turbof1 I have to address a few points you made.

1. It wouldn't be the diffuser is causing the rear wing to work harder. It would be the rear wing is causing the diffuser to work harder. This is because below the rear wing, there is a low pressure side. Air always flows from high pressure to low. Keep that in mind.

Now imagine the car without a rear wing. Above and behind the diffuser, where the airflow from the diffuser flows to, there is a certain pressure. Now add the rear wing in at its usual position. Below the rear wing there is a large low pressure region. This low pressure region is above and behind the diffuser. The same place we talked about the diffusers air flowing. So imagine now that we have added in the rear wing and that low pressure zone is created, it means the airflow from the diffuser flows there faster to try to fill in the pressure "void" (horrible terminology).

This helps speed up airflow through the diffuser and with it the diffuser has a lower pressure underneath. This lower pressure in the diffuser creates more downforce. Simple as that.

2. If you have an up flow of air from a lower (downforce producing) wing, in this case the diffuser and/or exhaust flowing onto a rear wing then it actually reduces the local angle of attack on the rear wing. Of course this can be offset by the exhausts higher speed exhaust gas flow reducing pressure below the wing. When you reduce the local angle of attack on a wing you actually reduce the downforce it creates. The only thing that having an up flow onto the rear wing does that is beneficial is it 1. Reduces the likelyhood of the wing stalling and 2. Allows the wing to be run at a slightly higher angle of attack meaning that the top (high pressure side) of the wing will work a little bit harder being at a higher AoA.

[turbof1]

Is it really worth the effort to stall the diffuser? I was under the impression that the diffuser had one of the lowest drag penalties for any down-force producing device, no? As for the front wing elements stalling, do you think they are doing this by ducting air to the front wing assembly or are they using something like flexi-aero to stall it?

Yes it has one of the lowest drag penalties. The reason you still might want to stall is to prevent the upwash connecting with the monkey seat flow (the old beam wing flow) and the rear wing flow. Breaking one flow will set a chain reaction in the other 2 flows. So what you are really doing is not so much remove drag from the diffuser, but from the monkey seat and the rear wing.

The interaction between those flows was a big talking point coming into 2014. The goal was for the diffuser's upwash to extract low pressure air from the rear wing. Since it's quite a big gap between diffuser and the underside of the rear wing, teams used the beam wing as a stepping stone. With that gone, teams nowadays use the combination of the exhaust and a slightly bigger monkey seat to work alteast the middle part of the rear wing harder. The current Red Bull is a testimony of that, employing a Y100 winglet very reminiscent of a beam wing (well the middle part of it).

Of course letting the rear wing work harder also means it produces more drag. You could remove this drag by stalling the diffuser upwash.

About the front wing: I honestly can't say. My intuition tells they count on certain vortices manifesting above a certain speed, which stalls parts of the wing. But I could be wrong. flexing is more predictable to use, but made fairly difficult through regulations.

turbof1 I have to address a few points you made.

1. It wouldn't be the diffuser is causing the rear wing to work harder. It would be the rear wing is causing the diffuser to work harder. This is because below the rear wing, there is a low pressure side. Air always flows from high pressure to low. Keep that in mind.

Now imagine the car without a rear wing. Above and behind the diffuser, where the airflow from the diffuser flows to, there is a certain pressure. Now add the rear wing in at its usual position. Below the rear wing there is a large low pressure region. This low pressure region is above and behind the diffuser. The same place we talked about the diffusers air flowing. So imagine now that we have added in the rear wing and that low pressure zone is created, it means the airflow from the diffuser flows there faster to try to fill in the pressure "void" (horrible terminology).

This helps speed up airflow through the diffuser and with it the diffuser has a lower pressure underneath. This lower pressure in the diffuser creates more downforce. Simple as that.

2. If you have an up flow of air from a lower (downforce producing) wing, in this case the diffuser and/or exhaust flowing onto a rear wing then it actually reduces the local angle of attack on the rear wing. Of course this can be offset by the exhausts higher speed exhaust gas flow reducing pressure below the wing. When you reduce the local angle of attack on a wing you actually reduce the downforce it creates. The only thing that having an up flow onto the rear wing does that is beneficial is it 1. Reduces the likelyhood of the wing stalling and 2. Allows the wing to be run at a slightly higher angle of attack meaning that the top (high pressure side) of the wing will work a little bit harder being at a higher AoA.

Yes you are more or less right (I actually feel the diffuser does help the rear wing, and vice versa the rear wing the diffuser. It's a unison of both that makes the complete package work harder). I only very simply put it down, because we are still talking about a specific car and not just a specific manner of creating downforce. Do remind that not the whole 'void' between the rear wing and diffuser is completely low pressure. We have the cooling outlets throw a lot of dirty high pressure air into it, and there's a reasonable gap. That's why you need a stepping stone of some sorts to make interaction between diffuser and rear wing possible.

[Per]

Trinidefender is correct in stating that it is the rear wing that helps the diffuser because it creates a low-pressure zone behind the car (i.e. behind the diffuser).

He is also correct that the diffuser upwash results in a lower effective AoA for the rear wing, but this is actually interesting: doesn't that mean that stalling the diffuser (and reducing its upwash) results in a higher AoA for the wing, possibly causing the wing to stall as well? It would be very neat if you could get that to work but I guess it is extremely difficult to do properly and reliably. (and this is what turbof1 was saying: causing the diffuser to stall can also reduce downforce and drag on the rear wing)

[flyboy2160]

You guys keep throwing around the term "stalling the diffuser." What, exactly, do you mean by this?

[trinidefender]

You guys keep throwing around the term "stalling the diffuser." What, exactly, do you mean by this?

I haven't used the term because I don't think I know enough about it. My understanding of it is that above a certain speed the kink (the abrupt angle change evident of diffusers in modern F1 cars) in the space, where the floor goes from flat to angled for the diffuser, will cause the diffuser to stall and probably shedding drag.

From what I know though this speed is quite high and as such the diffuser rarely stalls as a result of a cars speed, maybe only at the end of the highest speed straights.

[henra]

You guys keep throwing around the term "stalling the diffuser." What, exactly, do you mean by this?

I guess what is meant is: Flow detaching at the throat of the diffuser (aka stalling similar to a wing stalling where the air detaches from the upper side). This will decrease its effective expansion ratio and thus speed at the throat.

[flyboy2160]

Bumped for incoming diffuser stall posts.