## [CFD] Calculating Initial Conditions for Turbulence

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hollus
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Joined: Sun Mar 29, 2009 12:21 am
Location: Copenhagen, Denmark

### Re: [CFD] Calculating Initial Conditions for Turbulence

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?
For every complex problem there is an answer that is clear, simple, and wrong. TANSTAAFL!

A15013950
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Joined: Tue Jun 12, 2018 3:34 pm

### Re: [CFD] Calculating Initial Conditions for Turbulence

hollus wrote:
Wed Aug 08, 2018 11:47 pm
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.

249
Joined: Sat Jan 02, 2010 12:57 am

### Re: [CFD] Calculating Initial Conditions for Turbulence

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.
Motorsport without danger is like cooking without salt
Sir Stirling Moss

e36jon
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Joined: Mon Apr 25, 2016 1:22 am
Location: California, USA

### Re: [CFD] Calculating Initial Conditions for Turbulence

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?

249
Joined: Sat Jan 02, 2010 12:57 am

### Re: [CFD] Calculating Initial Conditions for Turbulence

I have often wondered the same thing.
In NASCAR they thickened the windshield and rear window plexiglass because the flexing upset the flow.
Motorsport without danger is like cooking without salt
Sir Stirling Moss

e36jon
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Joined: Mon Apr 25, 2016 1:22 am
Location: California, USA

### Re: [CFD] Calculating Initial Conditions for Turbulence

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.

249
Joined: Sat Jan 02, 2010 12:57 am

### Re: [CFD] Calculating Initial Conditions for Turbulence

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.
Motorsport without danger is like cooking without salt
Sir Stirling Moss

Vyssion
138
Joined: Sun Jun 10, 2012 1:40 pm

### Re: [CFD] Calculating Initial Conditions for Turbulence

e36jon wrote:
Thu Aug 09, 2018 8:45 pm
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 are entering into the world of "aero-elasto-dynamics" with that question. Incredibly complicated subject, so I'll 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:

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

$h\quad =\quad { h }_{ 0 }\quad -\quad \frac { a }{ 2 } \sin { \left( 2\pi ft \right) }$

You also have a few other major parameters to consider:

Reduced Frequency:
$k\quad =\quad \frac { \pi fc }{ { U }_{ \infty } }$

Non-dimensional "Plunge" Amplitude:
$\frac { a }{ c }$

And your new "effective" Angle of Attack due to the motion:
${ \alpha }_{ effective }\quad =\quad { \alpha }_{ 0 }\quad +\quad \arctan { \left( \frac { \cfrac { dh }{ dt } }{ { U }_{ \infty } } \right) }$

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 Fri Aug 10, 2018 6:38 pm, edited 3 times in total.
If you can't explain it simply, then you don't understand it well enough.
- Albert Einstein

The great thing about facts is that they are true, whether or not you believe them.
- Neil deGrasse Tyson

Vyssion Scribd - Aerodynamics Papers

e36jon
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Location: California, USA

### Re: [CFD] Calculating Initial Conditions for Turbulence

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

Vyssion
138
Joined: Sun Jun 10, 2012 1:40 pm

### Re: [CFD] Calculating Initial Conditions for Turbulence

e36jon wrote:
Fri Aug 10, 2018 4:45 pm
Thanks for the big-boy response Vyssion! (OMG, math! Run away!!!)
Maths isnt that bad!! It explains the world around you!!
e36jon wrote:
Fri Aug 10, 2018 4:45 pm
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):

e36jon wrote:
Fri Aug 10, 2018 4:45 pm
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
If you can't explain it simply, then you don't understand it well enough.
- Albert Einstein

The great thing about facts is that they are true, whether or not you believe them.
- Neil deGrasse Tyson

Vyssion Scribd - Aerodynamics Papers

e36jon
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Joined: Mon Apr 25, 2016 1:22 am
Location: California, USA

### Re: [CFD] Calculating Initial Conditions for Turbulence

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...

jjn9128
99
Joined: Tue May 02, 2017 10:53 pm

### Re: [CFD] Calculating Initial Conditions for Turbulence

e36jon wrote:
Mon Aug 13, 2018 5:04 pm
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.
Jin.

e36jon
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Joined: Mon Apr 25, 2016 1:22 am
Location: California, USA

### Re: [CFD] Calculating Initial Conditions for Turbulence

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