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Old 03-12-2016, 01:44 PM   #11 (permalink)
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avoiding large negative pressures

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Originally Posted by kach22i View Post
Nice thread, sort of wish there were extreme examples such as the Aero-Template car and a Pick-Up Truck for comparison.

Is it accurate or ignorant to say/think that avoiding large negative pressures will in turn avoid producing large positive pressures?

That is to say, if you never accelerate the air to begin with, it will not have to slow down later to equalize the mass air flow (correct term?).

EDIT:
The so-called perfect shape would have quite a bit of lift, perhaps we should turn this thing upside down?

https://surjeetyadav.wordpress.com/author/surjeetyadav/



Nice little paper on down-force without wings:
https://lucky13racing.wordpress.com/
*avoiding large negative pressures wouldn't be the issue.We'd have to have zero shape (a flat car).
*3-D streamline shapes do not produce lift as in a 2-D airfoil.The nose is held down by the favorable pressure gradient in the forebody.The tail is held down by the near-fully recovered static pressure of the separation-free aft-body.
(the T-100 showed essentially zero lift at 135-mph according to DARKO).The T-100 is also almost 50-50 weight distribution on the axles.
Here you can see the negative-lift positive pressure (blue) cancelling out the positive-lift negative pressure (yellow) acting on the 3-D body

*If we invert the streamline body,we lose attachment over the top (lift) and we force the total pressure under the vehicle (more lift).
*The streamline half-body remains very low drag.If you have dynamic stability issues,it's due to a conflict between center of pressure and center of gravity.

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Last edited by aerohead; 03-12-2016 at 01:46 PM.. Reason: add data
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Old 03-12-2016, 03:32 PM   #12 (permalink)
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Quote:
Originally Posted by aerohead
*If we invert the streamline body,we lose attachment over the top (lift) and we force the total pressure under the vehicle (more lift).
This falsifies that Colani Bonneville car, or is it an edge case?

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Old 03-12-2016, 04:51 PM   #13 (permalink)
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colani

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This falsifies that Colani Bonneville car, or is it an edge case?

Colani's body is more sophisticated than a simple inverted airfoil.His inclination would keep the top in a favorable pressure gradient,basically forbidding separation,and the underside 'fast' diffuser would help compensate for shoving all the air under there.I suspect that it would be pretty stable.
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Old 05-31-2020, 10:56 AM   #14 (permalink)
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I am reading about the laminar and turbulent boundary layers and pressure again in other sources, and I came back to EM to see what we have discussed in the past. This short thread had a lot of great images and effective dialogue between different ideas. (Also, it has a reference to The Simpsons... so, you are welcome!)
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Old 05-31-2020, 12:19 PM   #15 (permalink)
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Quote:
Also, it has a reference to The Simpsons...
And Notchbacks!
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Old 05-31-2020, 01:51 PM   #16 (permalink)
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And Notchbacks!
So, two serious questions. At the surface of the car body, the flow stream of the boundary layer stalls to zero because of vicosity and friction. The shear effects of that stalled air slows the stream above it, and to a lesser degree the stream above that. The velocity of the stream keeps turbulence down, but nonetheless turbulance develops in the boundary layer as the flow moves downstream. The boundary layer expands because it is becoming more turbulent, and the larger the area of the body, especially length, the more unstable it will become until it actually fully separates due to the turbulance. I think that is mostly correct, and if there are errors I don't know how to judge where I have made them. Maybe it is not turbulance that causes expansion, but something to do with heat and friction that causes both turbulance and expansion.

First question (vocabulary): that stalled flow at the wall is sometimes described as having "separated." I have also seen it claimed that it is capable of localized reversed flow. However, that useage of term "separation" can be confusing because it may not be refering to the same effects as when we talk about "separation" of the entire boundary layer that happens when/if the flow encounters a major change in body contour and pressure, right?

Second question: obviously there is friction drag produced by the stalled flow at the body surface and its interation with the rest of the boundary layer. That is not insignificant drag, either, apparently. What, however, is the relationship between the growing turbulance in the expanding boundary layer and actual separation drag at the rear window, c-pillars, and/or tail of the road vehicle? Is there one?

I hope this makes sense. No time to edit. Daughter and I gotta work on her car.

Thanks for replies.
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Old 05-31-2020, 02:14 PM   #17 (permalink)
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Quote:
The boundary layer expands because it is becoming more turbulent, and the larger the area of the body, especially length, the more unstable it will become until it actually fully separates due to the turbulance. I think that is mostly correct, and if there are errors I don't know how to judge where I have made them.
Until someone who knows what they're talking about comes along...

It's spelled turbulence.
Quote:
The onset of turbulence can be predicted by the dimensionless Reynolds number, the ratio of kinetic energy to viscous damping in a fluid flow. However, turbulence has long resisted detailed physical analysis, and the interactions within turbulence create a very complex phenomenon. Richard Feynman has described turbulence as the most important unsolved problem in classical physics.
The way I account for things in my own thinking is to distinguish laminar, attached turbulent and detached turbulent flow.

edit:
I could take this to a print shop and have it printed on contact paper.

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Old 05-31-2020, 04:36 PM   #18 (permalink)
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Quote:
Originally Posted by freebeard View Post
Until someone who knows what they're talking about comes along...

It's spelled turbulence.
Haha. Well, I said I didn't get to edit, so...

And the session working with my daughter on her car didn't get so far either. Funny how her promise to work on it with me waned after it was paid for.... hmmmm....

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Until someone who knows what they're talking about comes along... The way I account for things in my own thinking is to distinguish laminar, attached turbulent and detached turbulent flow.
Yeah. I didn't realize you were in the Homer category with me on this.

Anyway, thanks for the Feynman cite: "turbulence as the most important unsolved problem in classical physics." That's as interesting and more topical than Homer Simpson.
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Old 05-31-2020, 04:43 PM   #19 (permalink)
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Oh Lord, it's hard to be humble...
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Old 05-31-2020, 05:18 PM   #20 (permalink)
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If humility were easy for us, perhaps we'd be fish.

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