[CFD] Calculating Initial Conditions for Turbulence

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hollus
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Re: [CFD] Calculating Initial Conditions for Turbulence

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My signature includes There Ain't No Such Thing As A Free Lunch, so I have to pitch in here.
Assuming that team X’s aerodinamicists are capable of increasing turbulence on purpose without costing performance: shouldn’t the same team have used the same resources in increasing performance instead, whatever the turbulence?
Resources are limited, also in terms of design neurons, design calculations, etc. So they should never be spent on not improving performance.
I mean, is there even the theoretical possibility that designing for turbulence instead of performance would make sense?
Are there conditions or areas under which it could be expected that increases in turbulence shlould also lead to an increase in performance?
Rivals, not enemies.

wpsiatwin
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Re: [CFD] Calculating Initial Conditions for Turbulence

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hollus wrote:
09 Aug 2018, 00:47
My signature includes There Is No Such Thing As A Free Lunch, so I have to pitch in here.
Assuming that team X’s aerodinamicists are capable of increasing turbulence on purpose without costing performance: shouldn’t the same team have used the same resources in increasing performance instead, whatever the turbulence?
Resources are limited, also in terms of design neurons, design calculations, etc. So they should never be spent on not improving performance.
I mean, is there even the theoretical possibility that designing for turbulence instead of performance would make sense?
Are there conditions or areas under which it could be expected that increases in turbulence shlould also lead to an increase in performance?
Its not that there is a situation where an increase in turbulence increases performance, although its possible that changes you make to improve performance could lead to an increase in turbulence, but it would make you harder to follow and therefor harder to overtake during races. Also more likely that a following car overheats their tires and compromises their strategy.

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strad
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Re: [CFD] Calculating Initial Conditions for Turbulence

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Its not that there is a situation where an increase in turbulence increases performance, although its possible that changes you make to improve performance could lead to an increase in turbulence, but it would make you harder to follow and therefor harder to overtake during races. Also more likely that a following car overheats their tires and compromises their strategy.
.
Exactly. IF they make a change and find they can cause turbulence without harming their performance of course they would jump at the chance. And it is my belief that they do and have.
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss

e36jon
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Re: [CFD] Calculating Initial Conditions for Turbulence

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Maybe on-topic? When they show the ultra-slow motion video of the front wings bouncing all over the place I have often wondered what that does to the wings performance. It would seem like there would be a significant tendency for detached flow when the wing is moving upward and therefore reducing the pressure on the bottom / back of all of the wing elements. And likewise an advantageous situation when the wing moves down causing the opposite effect...

Seeing it in writing I guess this same potential effect would apply to all of the downforce generating aero surfaces. And the upsetting force could be any vertical movement.

No one is talking about it, so it's probably not an issue, but to my mind the mechanics seem sound. Any thoughts?

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strad
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Re: [CFD] Calculating Initial Conditions for Turbulence

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I have often wondered the same thing.
In NASCAR they thickened the windshield and rear window plexiglass because the flexing upset the flow.
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss

e36jon
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Re: [CFD] Calculating Initial Conditions for Turbulence

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Thanks Strad. If I'm crazy at least I'm not alone...

This thread had me thinking about Top Fuel dragsters and funny cars. They regularly hit 330+ mph and yet they have what seems like only a casual regard for aero, and that's when they are parked. Once they get moving every single surface is shaking / bending / distorting like crazy (They have awesome super-slow-mo as well.).


The punch line for this topic is whether it's enough to consider static geometry (Top Fuel madness) or constant pressure gradients (F1 floppage…) when considering the onset of turbulence. I would think in the F1 case especially as it seems like they design closer to the edge / with less margin than almost anyone else in motorsport.

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strad
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Re: [CFD] Calculating Initial Conditions for Turbulence

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They do give a nod to streamlining just not so much that it's obvious.
One thing that has always bugged me is that when Al Swindahl was building my dragster I mentioned incorporating a belly pan and they thought I was crazy. Now they all do. They laughed when I wanted to strap a video camera on, before GoPro came out. I especially wanted to see frame flex and tire distortion now it's common practice.
Oh well I have often been ahead of my time. :lol:
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss

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Vyssion
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Re: [CFD] Calculating Initial Conditions for Turbulence

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e36jon wrote:
09 Aug 2018, 21:45
Maybe on-topic? When they show the ultra-slow motion video of the front wings bouncing all over the place I have often wondered what that does to the wings performance. It would seem like there would be a significant tendency for detached flow when the wing is moving upward and therefore reducing the pressure on the bottom / back of all of the wing elements. And likewise an advantageous situation when the wing moves down causing the opposite effect...

Seeing it in writing I guess this same potential effect would apply to all of the downforce generating aero surfaces. And the upsetting force could be any vertical movement.

No one is talking about it, so it's probably not an issue, but to my mind the mechanics seem sound. Any thoughts?
You're starting to enter a bit into the world of "aero-elastics" with that question. Incredibly complicated subject, and not my area of expertise, so I can only explain the top level stuff and you can see whether that informs your question?

So, to start there are two main types of wings that form a type of "oscillating wing" will exhibit: A heaving wing, or a flapping wing. To start, if I just simplify the case to a heaving wing in close proximity to the ground, which has a sinusoidal motion:

Image

Your ride height now becomes a function of some sinusoidal frequency:



You also have a few other major parameters to consider:

Reduced Frequency:


Non-dimensional "Plunge" Amplitude:


And your new "effective" Angle of Attack due to the motion:


So with a heaving aerofoil, the incidence is "positive" when the aerofoil is heaving "down" (i.e. the leading edge is lower than the trailing edge).

With a periodic heaving motion, the aerodynamic forces show a period response at the same frequency as the heave motion, but with a bit of a "lag" relative to the motion itself. This "lag" of how the aerodynamic coefficients are affected depends mostly on the "Reduced Frequency" term that I wrote above.

Essentially, there are three main flow regimes present when you have something like a front wing:
  • Ground Effect
    (ventui effect we are all familiar with)
  • Incidence Effect
    (change in AoA = change in coefficients)
  • Added Mass Effect
    (the displacing of a mass of air due to motion imparts a force on the aerofoil)
There is also a vortex shedding phenomenon present in the flow as well, but it comes in two types: forced and natural. Forced shedding is an inviscid phenomenon, whilst natural shedding is caused by viscous effects.

The forced shedding is linked to the relatively weak "starting" vortex during the transient initial period when the aerofoil accelerates from zero to a certain velocity (kind of like the result of a car hitting a kerb). The total amount of circulation in the flow is pretty much constant, however, there is a change in the effective angle of attack which happens at the mean position - where angle of attack is at its maximum.

The natural shedding typically comes about from bluff bodies but can appear on an aerofoil with sharp edges when separation occurs upstream. This type doesn't need a vertical motion to occur and it basically causes the stagnation point to move around the trailing edge until the flow stalls at which point, the stagnation point then reverses its direction back the other way (sort of a "move until stall and then go back" type thing).

There is something called "Theodorsen Theory" which describes an aerofoil under sinusoidal motion in the freestream (with a small amplitude) in terms of a 2 of the 3 main effects I listed above which you can go check out if you want to.
Last edited by Vyssion on 10 Aug 2018, 19:38, edited 3 times in total.
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e36jon
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Re: [CFD] Calculating Initial Conditions for Turbulence

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Thanks for the big-boy response Vyssion! (OMG, math! Run away!!!)

Would your take be that designing around these effects would not be necessary for F1 type aero surfaces? I think of the issues some teams have had with flow reattaching when they close DRS seeming to indicate that they are designing things pretty close to the edge. Your response indicated that the turbulent transition would move around but not go into stall necessarily.

I know I'm generalizing and don't have your background so I'd love to hear your 'best guess' on what you think the designers are doing...

And apologies to the original thread owner for the mini hijack, I'm hoping this is of interest to you as well.

Jon

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Vyssion
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Re: [CFD] Calculating Initial Conditions for Turbulence

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e36jon wrote:
10 Aug 2018, 17:45
Thanks for the big-boy response Vyssion! (OMG, math! Run away!!!)
Maths isnt that bad!! :D It explains the world around you!!
e36jon wrote:
10 Aug 2018, 17:45
Would your take be that designing around these effects would not be necessary for F1 type aero surfaces? I think of the issues some teams have had with flow reattaching when they close DRS seeming to indicate that they are designing things pretty close to the edge. Your response indicated that the turbulent transition would move around but not go into stall necessarily.

I know I'm generalizing and don't have your background so I'd love to hear your 'best guess' on what you think the designers are doing...
Aeroelastics is an important aspect of finely tuned aerodynamics. I'm sure that you remember the Red Bull flexible elements issue and how the FIA now mandate specific deflections etc. The turbulent transition you're referring to I think you mean the natural vortex shedding? That effect is more akin to the Von Karman vortex shedding rather than an "anti-stall" - its more like this effect (these pics are from an aero-acoustics paper, but show the effect regardless):
Image
Image
e36jon wrote:
10 Aug 2018, 17:45
And apologies to the original thread owner for the mini hijack, I'm hoping this is of interest to you as well.
I am the original thread owner, so you're fine mate :lol:
"And here you will stay, Gandalf the Grey, and rest from journeys. For I am Saruman the Wise, Saruman the Ring-maker, Saruman of Many Colours!"

#aerosaruman

"No Bubble, no BoP, no Avenging Crusader.... HERE COMES THE INCARNATION"!!"

e36jon
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Re: [CFD] Calculating Initial Conditions for Turbulence

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Thanks for another detailed reply!

I'm still feeling like we may not be on the same page, so here's my last attempt to ask this:

Looking at the top picture, if that airfoil was suddenly moved up vertically, my brain says that there would be an increase in negative pressure on the underside. My question is, does that increase in negative pressure cause large scale / complete flow separation, similar to a stall condition? If it does, then it would seem like a primary concern for the designers, given how often the car is hitting curbs. Given our conversation thus far, it seems like this is more of a 'corner condition' and is something they would check after the fact...

Edit: I looked at the top photo again a realized that the flow isn't attached already! There's a recirculation pocket that I missed... So, the question still stands as 'detached turbulent flow, as in a stall condition'...

Thanks again for the fun conversation. I really appreciate your level of engagement.

Jon
B.S. Mechanical Engineering, but math still scares me...

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jjn9128
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Re: [CFD] Calculating Initial Conditions for Turbulence

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e36jon wrote:
13 Aug 2018, 18:04
Thanks for another detailed reply!

I'm still feeling like we may not be on the same page, so here's my last attempt to ask this:

Looking at the top picture, if that airfoil was suddenly moved up vertically, my brain says that there would be an increase in negative pressure on the underside. My question is, does that increase in negative pressure cause large scale / complete flow separation, similar to a stall condition? If it does, then it would seem like a primary concern for the designers, given how often the car is hitting curbs. Given our conversation thus far, it seems like this is more of a 'corner condition' and is something they would check after the fact...

Edit: I looked at the top photo again a realized that the flow isn't attached already! There's a recirculation pocket that I missed... So, the question still stands as 'detached turbulent flow, as in a stall condition'...

Thanks again for the fun conversation. I really appreciate your level of engagement.

Jon
B.S. Mechanical Engineering, but math still scares me...
I'm going to try... aerodynamics are not steady state, nice smooth contour plots in CFD and wind tunnel results are the result of long time averaging intervals. In reality the trailing edge flow will be as Vyssion showed 2 posts back - more akin to a Karman vortex street. Wing's are also not infinitely stiff so this may result in some oscillations which will affect pressure distribution - which will cause the wing to pitch and heave - but the oscillatory forces are rather small so it's unlikely to create very big deflections. However if you watched the Haas sharkfin and T-wing last year you'll see those unsteady effects can get quite big - even to the extent that parts fail because of it.

On your kerb example the deflections can get rather big, even causing the endplates to hit the the ground - as those are set 85mm high when the car is flat to the ground those are some big oscillations. So yes as the wing is slapping up and down it will change the pressure distribution - if the car was static you may read some quite big forces because the front wing is a big surface area. However, the cars are generally travelling above 100mi/hr where the air under the wing is travelling at 4 or 5x that - so these oscillatory effects will be quite small as a percentage of total downforce. So it's unlikely to change the state of flow significantly.
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e36jon
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Re: [CFD] Calculating Initial Conditions for Turbulence

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Thanks Jin!

The concept of "a small percentage of overall downforce" is something I can get my head around. I hadn't been considering the effect I was curious about in-context, so that really helped.

Cheers,

Jon

Snail With a Shotgun
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Re: [CFD] Calculating Initial Conditions for Turbulence

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Hi, sorry for raising this one from the dead, but would it be possible to update the pictures? I've been trying to get these sorted for far longer than I'm willing to admit, but either (seemingly) conflicting or unclear info I found online means I've been stuck at this for a while now.

Cheers!

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Vyssion
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Re: [CFD] Calculating Initial Conditions for Turbulence

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Snail With a Shotgun wrote:
09 Jul 2022, 05:55
Hi, sorry for raising this one from the dead, but would it be possible to update the pictures? I've been trying to get these sorted for far longer than I'm willing to admit, but either (seemingly) conflicting or unclear info I found online means I've been stuck at this for a while now.

Cheers!
Should be all fixed for you mate (ty steven). Let me know if you have any questions on what I wrote out.
"And here you will stay, Gandalf the Grey, and rest from journeys. For I am Saruman the Wise, Saruman the Ring-maker, Saruman of Many Colours!"

#aerosaruman

"No Bubble, no BoP, no Avenging Crusader.... HERE COMES THE INCARNATION"!!"