Concept for regulations to improve overtaking

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Dynamicflow
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Re: Concept for regulations to improve overtaking

Post by Dynamicflow » Mon May 15, 2017 4:18 pm

jjn9128 wrote:
Mon May 15, 2017 3:44 pm
Dynamicflow wrote:
Mon May 15, 2017 3:00 pm
I was wondering if you have run your concept car in various drafting scenarios and checked if it is any better than the current situation?
Not yet. I am no longer at my institution so don't have access to commercial CFD or a nice big/fast HPC. I just downloaded a port for running openfoam on windows so I'm going to see if my laptop can handle a simple 1/2 car simulation.

I did run a simple case with a low aspect ratio rear wing added to the test car from my PhD. Unfortunately those tests are in a paper which is currently in review (journals own the copyright of images once submitted) so I can't share any plots from it. But it does indicate what I have said about increasing the downforce contribution of the rear wing using a low aspect ratio wing.
If you like I can run it for you at various car lengths behind the lead car i.e. half, one, two, four, and eight cars lengths. It would be interesting to see how it compares to various other studies that has been done on this subject.

jjn9128
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Re: Concept for regulations to improve overtaking

Post by jjn9128 » Mon May 15, 2017 4:40 pm

Dynamicflow wrote:
Mon May 15, 2017 4:18 pm
If you like I can run it for you at various car lengths behind the lead car i.e. half, one, two, four, and eight cars lengths. It would be interesting to see how it compares to various other studies that has been done on this subject.
If that's something you want to do I'm not going to argue. CAD files are on onshape.com same as Perrinn used, search ''futureF1''. Onshape is a very simple tool so I didn't bother modeling engine, cooling or brakes, it's just an external surface.

What CFD do you use? How many cells can you generate and run? I would probably use outlets on the rad and engine intakes.
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Dynamicflow
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Re: Concept for regulations to improve overtaking

Post by Dynamicflow » Mon May 15, 2017 5:26 pm

jjn9128 wrote:
Mon May 15, 2017 4:40 pm
Dynamicflow wrote:
Mon May 15, 2017 4:18 pm
If you like I can run it for you at various car lengths behind the lead car i.e. half, one, two, four, and eight cars lengths. It would be interesting to see how it compares to various other studies that has been done on this subject.
If that's something you want to do I'm not going to argue. CAD files are on onshape.com same as Perrinn used, search ''futureF1''. Onshape is a very simple tool so I didn't bother modeling engine, cooling or brakes, it's just an external surface.

What CFD do you use? How many cells can you generate and run? I would probably use outlets on the rad and engine intakes.
Your model looks pretty simple and straight forward so it should not require that many cells. I would aim for 30 million cells for half car and between 80 to 90 million cells for the two car cases.

Any chance you could just email me the IGES files instead? I will add a simple radiator and model a pressure drop across the medium and come-up with a simplistic engine bay as well.

Ogami musashi
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Re: Concept for regulations to improve overtaking

Post by Ogami musashi » Mon May 15, 2017 8:43 pm

jjn9128 wrote:
Sun May 14, 2017 11:17 pm
I should hope so. One of the papers you cite is mine! :lol:
Hopefully, i quoted several of them, so your results are in agreement with other studies and can bring additional knowledge (which is the aim of research :) )
By the way, is your article the SAE paper from may 2015?
With regard to 'turbulence' there are two distinct uses of the term. One in relation to boundary layer state and the other in relation to wakes/freestream which is then split into 'intensity' and 'length scale'. Where intensity (TI) describes fluctuations from the mean velocity and is normally presented as a percentage and length scale is the length/frequency of those turbulent eddies. The surfaces on F1 cars work the air quite hard, i.e. large pressure gradients, so boundary layers are highly turbulent and you get a thick wake, especially from the floor and rear wing. For reference freestream turbulence intensity in a F1 wind tunnel will be less than 0.15% while TI in the wake of a car can exceed 40% in places - this is from the work of Mike Wilson who has been at Force India since the late 2000s. So where I say TI in the wake has little effect is because what it serves to do (depending on length scale, for the most part the TI in a F1 wake is relatively high frequency) is force boundary layer transition to turbulence. As the operating Reynolds number of a F1 front wing is at the very least transitional, wake turbulence is going to have a very small effect on flow state, again there are a number of papers investigating the effect of turbulence on aerofoils including Wilson.
Thanks for the clarification, i agree with you then.
These front wing pressure distributions are from my work, the model I used is 25% scale so flow state at the trailing edge will be laminar even considering the pressure gradient. It is safe to say that the effect of wake turbulence on this wing will be greater than at higher Reynolds numbers. What the plots show is the baseline (clean air) compared to the effect of the wake at a 1 and 2 car separation. The dashed line is then the baseline scaled by the dynamic pressure deficit in the wake - which clearly forms a high percentage of the wake effect. And this is true for all the surfaces on the car.
https://ad5boq.by3301.livefilestore.com ... pmode=none https://qt05oq.by3301.livefilestore.com ... pmode=none
Did you run this in 2D? what are the variations of the wake effect along the span? What has been highlighted is that for the front wing, the position of the car in relation to the wake is very important i.e the geometry setting of the car-wake couple has a significant effect on front wing downforce and drag.
what is your take on it?

jjn9128
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Re: Concept for regulations to improve overtaking

Post by jjn9128 » Tue May 16, 2017 10:19 am

Yes the 2015 paper is mine. There was another one in SAE this year which I think is better.
Ogami musashi wrote:
Mon May 15, 2017 8:43 pm
Did you run this in 2D? what are the variations of the wake effect along the span? What has been highlighted is that for the front wing, the position of the car in relation to the wake is very important i.e the geometry setting of the car-wake couple has a significant effect on front wing downforce and drag.
what is your take on it?
I did not run in 2-d what you see is a slice take through the center of the wing in post-processing. The wake is certainly concentrated on the centerline of the lead car and I saw that an offset of 1/2 a car width between cars (centerline aligned to outside edge of wheels) meant a huge chunk of the downforce lost to the wake was recovered. Tracks don't really allow for drivers to take drastically different lines though, certainly in higher speed corners where the effect of downforce loss is greater. Mainly this is down to tyres, grip is enhanced by rubber being laid on the racing line so being off-line even slightly reduces adhesion, then there's the issue of the marbles. Maybe harder tyres are the way to go, Frank Dernie says this better than me... https://www.youtube.com/watch?v=MtBJKb5vwDM#t=58m42s
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Ogami musashi
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Re: Concept for regulations to improve overtaking

Post by Ogami musashi » Wed May 17, 2017 2:08 pm

I was thinking about the setup of general wind tunnel and CFD experiments. If i'm not mistaken, it is always as if the two cars follow in straight line.
In reality, at racing separation distances, there's always a lateral offset between the cars.
I wonder first about the behaviour of the wake in those conditions:
-Is the wake modified by the turning motion of the leading car? (turn is a combination of rotation and translation)
-Even if not, then we can consider that lateral separation effects are important and thus is the dynamic pressure deficit still the major factor or does the geometry of the vortex of greater importance?

P.S: i saw you sae paper from 2017, unfortunately doesn't have access to this one. But i saw the preview :)

jjn9128
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Re: Concept for regulations to improve overtaking

Post by jjn9128 » Thu May 18, 2017 1:35 pm

Ogami musashi wrote:
Wed May 17, 2017 2:08 pm
I was thinking about the setup of general wind tunnel and CFD experiments. If i'm not mistaken, it is always as if the two cars follow in straight line.
In reality, at racing separation distances, there's always a lateral offset between the cars.
I did lateral offsets in the wind tunnel and in CFD. I think I saw a picture from the OWG study with a lateral offset too, I'm really annoyed I didn't take a screen grab it at the time. Even small lateral offsets do allow the forces to recover towards the 'clean air' condition, downforce faster than drag. I think this is because the wheels of the following car (large % of total drag) are more aligned to the centerline of the lead car, so low drag, while the downforce generating surfaces operate in a higher dynamic pressure flow, with maybe small effect from down-wash helping to increase effective incidence of wings. I would argue that in corners the lateral offset is negligible as there is really only one racing line, and drivers will drive this consistently to within a few centimeters.

The real difficulties for wind tunnel studies are dynamic similarity, or how accurately the simulation matches flow state of the on track conditions, and representative separations. Both are something my study could have improved on but was not possible to within the limitations of our wind tunnel. We got around the separation issue to some extent by using a short bodied wake generator, which compared well to CFD with a full upstream car both aligned and with an offset, but still only managed a one vehicle length separation. The OWG group study had the same issues, and might I add with a lot more money to spend on the study. For impartiality reasons they used the Fondtech/fondmetal wind tunnel with 40% models, I estimate from the belt length and average length of a circa 2008 car that, even with the following car at the very rear of the rolling road, the maximum streamwise separation was only ~0.8 of a car length. Both TMG and Sauber wind tunnels can accommodate full scale cars so maybe a more representative separation could be achieved in these tunnels - though perhaps the only reason for that wind tunnel study was to validate one case from CFD over a large range of attitudes.

With regard to cornering, that is a tricky topic as it's not something I studied. There are CFD studies of vehicles in cornering, TotalSim https://www.totalsimulation.co.uk/corne ... -openfoam/ did some work on this with an F3 car. You can see that the wake follows the path of the car, you can also see this in wet races, but there is increased asymmetry in the wake (i.e. if you straightened the domain along the arc of the car centreline) from the effect of the yaw on surface pressures. So for the most part I would say the effect of following in the wheel tracks in a corner, as the drivers have to do if there is only one racing line, will be the same or very similar to following in the wheel tracks in a straight line.

But when you start to increase the complexity of simulations by adding cornering there's always something else to consider e.g. the effect of atmospheric conditions. I mentioned earlier in the thread that wind tunnels are designed for very low freestream turbulence (FST), this is to improve repeatability. The higher the FST the longer you have to average forces to reduce the size of your error bars. F1 teams run constant motion studies in the wind tunnel to reduce their 'wind on' time per case, and low turbulence is required so that error is not introduced, i.e. when you stick on a new part the delta you see is because of the part not because of the jet. The issue is how representative this smooth airflow is to on road conditions, so you have to start considering how clean 'clean air' is. There's a big push in the automotive sector to improve the correlation of studies to on road conditions for more accurate prediction of fuel consumption. On road/track you have cross-winds and head-winds which are interrupted by track side trees, buildings, spectators etc, so 'clean air' actually includes a baseline turbulence along a spectrum of length scales. You can model this in CFD but it increases the computational cost, normal transient simulations will only simulate 1-1.5s of time, to include atmospheric effects that would increase to 16-30s. This turbulence may alter the wake, the cross-winds change the way the wake comes off the car and how is propagates downstream... etc. Ultimately the most accurate way to test the wake effect will always be on-track but this is very expensive to implement, it's just not feasible especially if you're changing the geometry during testing, so we have to accept (to some extent) that wind tunnel and CFD are what they are, which is simulations.
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Ogami musashi
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Re: Concept for regulations to improve overtaking

Post by Ogami musashi » Sun May 21, 2017 12:42 pm

Does the wake of a car has a significant axial velocity (relative to a fixed frame like the ground)? I mean does the wake of a car actually moves downstream?

I wonder if at a fixed location, when the following car passes through the wake, it encounters only a delayed wake (the wake as the leading car produced it when it passed through that location, only evolved by its internal forces) or if it encounters a delayed wake plus the migration of wake that was produced by the leading car at later locations (that is wake structures that come from upstream).

Because, if that is the case, i guess that when cornering, the wake structure is thus not exactly like straight line, because structures produced at later location would not migrate following the curved line.

Just_a_fan
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Re: Concept for regulations to improve overtaking

Post by Just_a_fan » Sun May 21, 2017 1:27 pm

Ogami musashi wrote:
Sun May 21, 2017 12:42 pm
Does the wake of a car has a significant axial velocity (relative to a fixed frame like the ground)? I mean does the wake of a car actually moves downstream?
I would have thought that the wake "follows" the car to some degree. Along with the generally unhelpful flow structures, this would have the effect of reducing the airspeed over the following car to some degree. Reduced airspeed will give reduced downforce.
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jjn9128
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Re: Concept for regulations to improve overtaking

Post by jjn9128 » Sun May 21, 2017 5:38 pm

Just_a_fan wrote:
Sun May 21, 2017 1:27 pm
Ogami musashi wrote:
Sun May 21, 2017 12:42 pm
Does the wake of a car has a significant axial velocity (relative to a fixed frame like the ground)? I mean does the wake of a car actually moves downstream?
I would have thought that the wake "follows" the car to some degree. Along with the generally unhelpful flow structures, this would have the effect of reducing the airspeed over the following car to some degree. Reduced airspeed will give reduced downforce.
This may be an issue of terminology or reference points... I'm not sure if I've said that the wake moves downstream here or if I said it in one of my papers?! In papers I might have said the wake 'moves downstream' because in the wind tunnel or CFD the car is the fixed reference while the air moves around it, whereas on track the air is the reference and the car is moving through it. On track the wake follows the car and what is experienced by the following car is reduced airspeed relative to road speed.
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hollus
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Re: Concept for regulations to improve overtaking

Post by hollus » Sun May 21, 2017 7:56 pm

I think the question was more aimed at the fact that in a corner, the wake created at any instant will move "forwards" following the tangent of the car's trajectory, and thus move away from the car's trajectory. In a straight, the wake moves forwards while staying in the car's trajectory, and thus will interact with the wake created a bit before and a bit afterwards. This would change the nature of the dusturbance to the air.
I for one find the idea that the wake might be very different in straights than it is in corners quite interesting. Doubly so since, in that case, the situation in corners is extremely difficult to reproduce in wind tunnel (regarding wake).
For every complex problem there is an answer that is clear, simple, and wrong. TANSTAAFL!

Just_a_fan
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Re: Concept for regulations to improve overtaking

Post by Just_a_fan » Sun May 21, 2017 8:23 pm

That the wake is different in corners suggests that taking a different line would allow a following driver to get closer. That this happens in the wet, where different lines work more, suggests that being able to get "off line" would be beneficial in the dry too.
Turbo says "Dumpster sounds so much more classy. It's the diamond of the cesspools."

Ogami musashi
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Re: Concept for regulations to improve overtaking

Post by Ogami musashi » Sun May 21, 2017 8:23 pm

jjn9128 wrote:
Sun May 21, 2017 5:38 pm
This may be an issue of terminology or reference points... I'm not sure if I've said that the wake moves downstream here or if I said it in one of my papers?! In papers I might have said the wake 'moves downstream' because in the wind tunnel or CFD the car is the fixed reference while the air moves around it, whereas on track the air is the reference and the car is moving through it. On track the wake follows the car and what is experienced by the following car is reduced airspeed relative to road speed.
I didn't say you said it, i asked a question (since you appear to be the one in this topic with the most knowledge on the question).

I asked, like hollus said, if each given location in the wake is independent from the others. If it is, then it means the time evolution of the wake at each location is purely down to internal forces. If it's not, it means the wake will be spoiled by adjacent locations.

Then during a corner while the wake will mainly follow the racing line, inside the wake asymmetry would develop because at a given location because adjacent locations would follow a tangent trajectory hence not the racing line.
The CFD example of the F3 test shows asymmetries inside the wake even if the main wake is clearly curved along the racing line.

See this video example (https://www.youtube.com/watch?v=E1ESmvyAmOs). It is a very famous plane wake smoke test. You can see the vortex that develops doesn't see to stay stationary. I ask myself about a similar effect with cars (especially given the proximity of the ground)

jjn9128
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Re: Concept for regulations to improve overtaking

Post by jjn9128 » Mon May 22, 2017 12:53 pm

Ogami musashi wrote:
Sun May 21, 2017 8:23 pm
I asked, like hollus said, if each given location in the wake is independent from the others. If it is, then it means the time evolution of the wake at each location is purely down to internal forces. If it's not, it means the wake will be spoiled by adjacent locations.

Then during a corner while the wake will mainly follow the racing line, inside the wake asymmetry would develop because at a given location because adjacent locations would follow a tangent trajectory hence not the racing line.
The CFD example of the F3 test shows asymmetries inside the wake even if the main wake is clearly curved along the racing line.

See this video example (https://www.youtube.com/watch?v=E1ESmvyAmOs). It is a very famous plane wake smoke test. You can see the vortex that develops doesn't see to stay stationary. I ask myself about a similar effect with cars (especially given the proximity of the ground)
Okay I think I understand what you're asking now. Apologies for the misunderstanding.

As I said before the wake in curved flow is not something I have studied so is not something I can definitively answer about, but I can make some assumptions from other studies. These papers all concern an isolated front wing in yawed or curved flow.
https://www.researchgate.net/publicatio ... v=prf_high
https://www.researchgate.net/publicatio ... und_Effect
https://www.researchgate.net/publicatio ... g_Vehicles
https://www.researchgate.net/publicatio ... erted_wing
During actual cornering, a vehicle turns at a specific angular velocity. Because of this, the air flows at the front and rear of the vehicle have different inflow angles... The difference in front-rear inflow angle is particularly pronounced during low and medium speed cornering, when the angular velocity is high in relation to vehicle speed...
Ogawa from the 3rd era Honda aero papers. From this I would say that when the effect of cornering on car performance and therefore the wake is greatest, low speed corners, the effect of the wake will be smallest as the downforce is proportional to speed squared. i.e. if the effect is a 30% drop of downforce, at 200mi/hr this would be a 7.5kN loss of downforce while at 100mi/hr this would be a 1.8kN loss of downforce (rounded and assuming CzS ~5).

I do not think it is so simple as to say there is a tangential component to the wake as the effect of cornering is a difference in the way the inboard and outboard vortices form, compared to straight ahead testing. On these front wing studies the inboard vortex is more significantly affected than the outboard one. I would say that from the Ogawa quote the effect on the rear wing will be the opposite, if the inflow angle is reversed, so the formation and path of the outboard vortex would be more affected, I think this squares with another image I have seen from Sauber(?).

With regard to the comparison to planes just look at how far the contrails remain in the wake behind a plane compared to an F1 car, this is why wake turbulence is a bigger factor in aerospace as when these structures breakdown the other features of the wake are gone and the result is only from turbulence. There is a downwash component in the wake of a plane because it is not being reacted by anything. With an F1 car there is low pressure formed on the ground which reacts the wake to some extent meaning there is little up-wash (net not local).

I'm still not sure if this answered your question?! I think the effect of cornering on wakes and subsequently the wake effect would certainly be a great topic for study and maybe even would go as far as to suggest it is something Brawn and Somerville should look into when defining the new aerodynamic rules.
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Tommy Cookers
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Re: Concept for regulations to improve overtaking

Post by Tommy Cookers » Mon May 22, 2017 2:44 pm

at high Cl in a plane lift-independent drag is far less relative to lift-dependent drag than it is in a car
that's why the tip part of the wake is less important in a car than it is in a plane