Schlieren Effect Usefulness in Motorsport

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RicerDude
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Schlieren Effect Usefulness in Motorsport

Post by RicerDude » Wed Nov 08, 2017 8:16 pm

https://m.youtube.com/watch?v=4tgOyU34D44

Found this very interesting, never heard of this effect before and was wondering if this could be/is already being used in formula 1 or any Motorsport for that matter. Seems a bit more scientific and reliable than blowing smoke at a model or painting bits with green paint.

Vyssion
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Re: Schlieren Effect Usefulness in Motorsport

Post by Vyssion » Thu Nov 09, 2017 10:09 am

RicerDude wrote:
Wed Nov 08, 2017 8:16 pm
https://m.youtube.com/watch?v=4tgOyU34D44

Found this very interesting, never heard of this effect before and was wondering if this could be/is already being used in formula 1 or any Motorsport for that matter. Seems a bit more scientific and reliable than blowing smoke at a model or painting bits with green paint.
The closest I have ever come to analyzing something similar to this is Particle Image Velocimetry (PIV) which involves submerging a model within a fluid suspension with some sort of shiny particulate within it, and then shining a laser sheet across the model at a very specific nM wavelength. You record that and then do image processing which tracks particle movements and in-out of plane movements too and you can average out velocities and pressures etc.

This effect, I feel like it "could" be used, however, you would need a wind tunnel to do it which is set up in a way that would allow the point source of light to be used. I think you might run into issues of wall shear effects (unless its an open tunnel) also being picked up within the tunnel and then refraction issues through the glass walls would make focussing the light point trickier and dimmer, since we are looking at a bulk fluid flow, any sort of turbulence would probably pose a problem. If the fluid flow is uniform, the image will be steady, but any turbulence will cause scintillation (that shimmering effect you see on hot surfaces during a sunny day). You would probably have to use a really short-duration flash to pick up what is going on rather than continuously lighting and recording what is happening.
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TwanV
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Re: Schlieren Effect Usefulness in Motorsport

Post by TwanV » Thu Nov 09, 2017 10:24 am

This type of flow-visualisation is pretty common actually, but not really practical for measuring stuff on track :lol: :
Image

What you see in Schlieren imaging is the density gradient (looks cool I admit), which can also be output from CFD, so the images could be compared. I wouldn't be surprised they use it as part of the toolbox for design validation.

Other types of flow visualisation (with e.g. smoke) look rudimentary but are very practical for checking where boundary layer separation takes place, where flow is laminar and where turbulence takes over etc. This ultimately tells you a great deal about whether the design (from CFD or other methods) is aerodynamically working as intended.

strad
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Re: Schlieren Effect Usefulness in Motorsport

Post by strad » Fri Nov 10, 2017 7:26 pm

Didn't we discuss the use of Schlieren imaging quite some time ago?
I'm pretty sure they use it.
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RicerDude
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Re: Schlieren Effect Usefulness in Motorsport

Post by RicerDude » Fri Nov 10, 2017 7:39 pm

strad wrote:
Fri Nov 10, 2017 7:26 pm
Didn't we discuss the use of Schlieren imaging quite some time ago?
I'm pretty sure they use it.
I had no idea, hence the post. The fact that teams still have correlation problems between simulation software and performance on the track proves to me that blowing smoke over a car doesn't give a clear enough picture of what's going on.

Vyssion
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Re: Schlieren Effect Usefulness in Motorsport

Post by Vyssion » Fri Nov 10, 2017 8:07 pm

RicerDude wrote:
Fri Nov 10, 2017 7:39 pm
The fact that teams still have correlation problems between simulation software and performance on the track proves to me that blowing smoke over a car doesn't give a clear enough picture of what's going on.
Gotta be careful here mate; because the issues that we currently have with correlation issues between CFD vs. Real-World Testing is because at a fundamental level, we still don't know how turbulence works when applied to complex geometry and different surfaces. Solving your typical RANS equations has your Reynolds Stress Tensor that is the main issue there of which there are tens or even hundreds of nth equation models etc aimed to predict this, and even when you get up to the LES type simulations which require stupid computational power even today, you still have to use a Sub-Grid Scale (SGS) model which makes the assumption that at a particular point in a turbulent eddy's dissipation, it will dissipate in a "univerally-same" way - which we don't know whether that is true or not. Obviously, predicting the turbulent flow over aerofoil profiles and body panels is kind of well backed up nowadays, however, most of the complexity for CFD, at least in my experience, of modelling aerodynamic phenomena comes from the four rotating tyres and the six vortices they shed each...

As far as correlation between Wind Tunnel Testing vs. Real Track Testing, it comes mostly down to the shear number of "outside factors" which real track testing can throw at you, i.e. altitude, gusting winds, humidity, dust particulates, track roughness, inclination in track surface, blah blah, etc etc... You just can't cover all of that in a wind tunnel especially with the strict number of hours you're allowed each week (I think they can still trade in their terraflops for more hours, but I think it peaks at like 20hrs or something - might be wrong here) Couple that with similarity parameters affecting result translation due to matching Mach Number and Reynolds Number being very difficult across all facets of a model (teams cannot run 100% scale models anymore) and its a bit of a nightmare for correlation!
If you can't explain it simply, then you don't understand it well enough.
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jjn9128
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Re: Schlieren Effect Usefulness in Motorsport

Post by jjn9128 » Fri Nov 10, 2017 10:59 pm

Vyssion wrote:
Fri Nov 10, 2017 8:07 pm
RicerDude wrote:
Fri Nov 10, 2017 7:39 pm
The fact that teams still have correlation problems between simulation software and performance on the track proves to me that blowing smoke over a car doesn't give a clear enough picture of what's going on.
Gotta be careful here mate; because the issues that we currently have with correlation issues between CFD vs. Real-World Testing is because at a fundamental level, we still don't know how turbulence works when applied to complex geometry and different surfaces. Solving your typical RANS equations has your Reynolds Stress Tensor that is the main issue there of which there are tens or even hundreds of nth equation models etc aimed to predict this, and even when you get up to the LES type simulations which require stupid computational power even today, you still have to use a Sub-Grid Scale (SGS) model which makes the assumption that at a particular point in a turbulent eddy's dissipation, it will dissipate in a "univerally-same" way - which we don't know whether that is true or not. Obviously, predicting the turbulent flow over aerofoil profiles and body panels is kind of well backed up nowadays, however, most of the complexity for CFD, at least in my experience, of modelling aerodynamic phenomena comes from the four rotating tyres and the six vortices they shed each...

As far as correlation between Wind Tunnel Testing vs. Real Track Testing, it comes mostly down to the shear number of "outside factors" which real track testing can throw at you, i.e. altitude, gusting winds, humidity, dust particulates, track roughness, inclination in track surface, blah blah, etc etc... You just can't cover all of that in a wind tunnel especially with the strict number of hours you're allowed each week (I think they can still trade in their terraflops for more hours, but I think it peaks at like 20hrs or something - might be wrong here) Couple that with similarity parameters affecting result translation due to matching Mach Number and Reynolds Number being very difficult across all facets of a model (teams cannot run 100% scale models anymore) and its a bit of a nightmare for correlation!
I'm going to come from a different perspective as Vyssion as a more wind tunnel focussed engineer vs CFD. We rarely 'blow smoke over the car'. If you see smoke in a wind tunnel it's normally for a marketing/head office visualization, it's same the reason CFD was initially dubbed 'colourful for directors'. That said it can be useful for a quick check if you have some unexpected result and don't have a PIV system in situe, but it's never in the test program.

The correlation issues, certainly for Red Bull this year, were blockage based with the wider cars, don't know what their solution was but Force India have curved walls in their tunnel (I know they now use TMG) to combat the same issue. A lot of the teams are using bigger models than their tunnels were designed/built for. Other correlation issues as Vyssion mentions, and it's the same in the road sector for fuel targets, are atmospheric turbulence, i.e. crosswinds and gusting, vibrations/ground clearance, dynamic similarity is a big one - scale and speed restrictions is equivalent to a medium-ish speed corner...etc. Even with pneumatic tyres getting the correct contact patch, side-wall, and top tread deformation is difficult, and I still don't think has been perfectly achieved. To visualise the flow field on track also requires 'intrusive' devises, i.e. the aero rakes affect the flow field around the car.

Where I don't see this technique as being useful, and it's something I bang on about, is giving value. For something to be useful in an engineering application it has to add something. PIV gives velocity vectors, so you see where flow is going and how fast. Pressure probes, 5,7,13-hole, will give pressures but also velocity and directionality. Obviously force and surface pressures are measured...etc.

n_anirudh
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Re: Schlieren Effect Usefulness in Motorsport

Post by n_anirudh » Sat Nov 11, 2017 7:57 pm

How does changing to a circular wind tunnel help reduce blockage.

I am quite surprised that RBR did not pick up this issue in 2016 regarding the blockage effects and the corrections that need to be applied.

I guess teams have PIV setup behind rear wheels and I am not 100% sure if Pirelli test tires replicate the on track behaviour with with their miniature WT tires.

Schlieren imaging is only at high mach numbers- where density plays a role - not at 100m/s

Vyssion
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Re: Schlieren Effect Usefulness in Motorsport

Post by Vyssion » Sat Nov 11, 2017 9:31 pm

n_anirudh wrote:
Sat Nov 11, 2017 7:57 pm
How does changing to a circular wind tunnel help reduce blockage.

I am quite surprised that RBR did not pick up this issue in 2016 regarding the blockage effects and the corrections that need to be applied.

I guess teams have PIV setup behind rear wheels and I am not 100% sure if Pirelli test tires replicate the on track behaviour with with their miniature WT tires.

Schlieren imaging is only at high mach numbers- where density plays a role - not at 100m/s
The wind tunnel isnt a circular one, the walls are just curved. Kinda like a bulge outward within the test section instead of a straight rectangle; you can even get some wind tunnels which have hydraulic panels which they can move in and out to change the test section volumetric shape on demand.

Its basically aimed at reducing the wall effects on the test object. Whenever there are streamlines which are turning/angled/rotating/etc, there is a pressure imbalance on one side of the streamline to the other which is causing the curvature. This isnt localized to one streamline, however, and so the streamline "above" (lets say) also is subjected to a pressure differential and curves too (albeit at a lesser amount). If this pressure "bubble of influence" reaches the wall surface, then the wall will not be able to be "curved" because it is rigid. The force that this bubble exerts on the wall will be reacted back into the flow, which will cause an artificial pressure increase within the test section. This artificial pressure increase can sometimes reach back to the test object and then further influence how the object performs aerodynamically. Obviously this is more likely to occur whilst testing things that have a higher blockage ratio as there is less room for the air to be moved about before it hits the wall surfaces.

It is kind of a similar thing that you need to be mindful of within CFD; there is some variation here (so don't take this as scripture - a domain sensitivity analysis is still required!!), but typically you will start with 3-4 lengths in front, 6+ behind, and 2-3 outwards and upwards to allow for this streamline deflection without the artificial increases being an issue.

regarding Pirelli and their WT supplied pneumatic tyres etc, I'm sure they do extensive testing on them, but there are just too many variables to consider when trying to exactly match performance - each of which influences the aerodyanmics of the most draggy part of an F1 car...
If you can't explain it simply, then you don't understand it well enough.
The great thing about facts is that they are true, whether or not you believe them. - Neil deGrasse Tyson
Vyssion Scribd - Aerodynamics Papers
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n_anirudh
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Re: Schlieren Effect Usefulness in Motorsport

Post by n_anirudh » Sun Nov 12, 2017 8:26 am

Thanks a lot for the detailed explanation. :D
Image
Usually blockage ratio is less than 4% in WT, perhaps I didnt think much of higher block effects like the image above.Its "easier" in CFD with symmetry or free slip boundary conditions :).

jjn9128
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Re: Schlieren Effect Usefulness in Motorsport

Post by jjn9128 » Sun Nov 12, 2017 9:45 am

n_anirudh wrote:
Sun Nov 12, 2017 8:26 am
Thanks a lot for the detailed explanation. :D
https://www.jamesallenonf1.com/wp-conte ... 549119.png
Usually blockage ratio is less than 4% in WT, perhaps I didnt think much of higher block effects like the image above.Its "easier" in CFD with symmetry or free slip boundary conditions :).
We've drifted off topic, but that's a slotted wall wind tunnel (a sort of hybrid between closed and open section but not quite a 3/4 open) which can run slightly higher blockages. Looks like one of the fondmetal/fondtech/aerolab wind tunnels either with the Lotus or when they did validation for Force India.