Reducing lift on different body shape cars

[JulianEdgar]

Fastbacks, notchbacks and squarebacks - how they produce lift.. and how to reduce it!*

I am aware that lots of people here are unconcerned with aero lift on road cars, but having given a car real-world downforce at normal road speeds (eg 100 km/h - 60 mph), I can vouch for how important it is to stability and good driveability.

(And of course, in general, a car with zero lift has lower drag than one with lift - and 99 percent of unmodified road cars have lift.)

https://www.youtube.com/watch?v=REsSOCtvyaE&t=510s

*Note that I am talking about modern cars that have attached flow on their upper surfaces, some small separation bubbles excluded.

[aerohead]

1) even though qualifiers are mentioned, I believe that the viewer would have been better served had you navigated at least one fastback which does not produce the type of lift illustrated. My experience was of overgeneralization.
2) ditto for 'wake size' attributed to drag, as examples of larger wakes with lower drag exist in the public domain.
3) and as to the tuft testing, and in light of the Porsche 911, VW New Beetle, and 2010 Audi A7 Sportback, there's a clear solution liability, by virtue of misinterpretation of observational tuft alignments, should the individual confuse 'downwash' phenomena with 'attached flow.'

[JulianEdgar]

Originally Posted by aerohead

1) even though qualifiers are mentioned, I believe that the viewer would have been better served had you navigated at least one fastback which does not produce the type of lift illustrated. My experience was of overgeneralization.

Why would I try to find the exceptions? Most fastbacks generate more lift than squarebacks and notchbacks.

2) ditto for 'wake size' attributed to drag, as examples of larger wakes with lower drag exist in the public domain.

Yes, but why would I try to find these exceptions rather than using as examples those which are most common?

3) and as to the tuft testing, and in light of the Porsche 911, VW New Beetle, and 2010 Audi A7 Sportback, there's a clear solution liability, by virtue of misinterpretation of observational tuft alignments, should the individual confuse 'downwash' phenomena with 'attached flow.'

This is a difference that you have just invented. Attached flow is attached flow, whether it is caused by downwash or not. I have already quoted the head of Jaguar aero in support of that, but as always, you believe that you are right and the true experts are wrong.

[aerohead]

Originally Posted by JulianEdgar

Why would I try to find the exceptions? Most fastbacks generate more lift than squarebacks and notchbacks.



Yes, but why would I try to find these exceptions rather than using as examples those which are most common?



This is a difference that you have just invented. Attached flow is attached flow, whether it is caused by downwash or not. I have already quoted the head of Jaguar aero in support of that, but as always, you believe that you are right and the true experts are wrong.

1) Dimensional analysis of a non-separated flow fastback would provide a reference datum from which to ponder what a subtle difference in architecture can mean to aerodynamic performance.
2) ditto
3) downwash is not attached flow. It is symptomatic of flow separation. You'll get the highest drag, vortex-drag, plus a small, but very low pressure wake, and overall higher drag. To say otherwise is delusional. It may look like a duck, but it doesn't quack like a duck.
4) If your experts are saying otherwise, then they ARE wrong.

[JulianEdgar]

Originally Posted by aerohead

4) If your experts are saying otherwise, then they ARE wrong.

Ah, back to 'Aerohead is right but the true experts are wrong'.

Just amazing.

[aerohead]

Originally Posted by JulianEdgar

Ah, back to 'Aerohead is right but the true experts are wrong'.

Just amazing.

'True' isn't a word for science. Fact will do nicely.
If 'employee' = 'true expert' what do we have?
If your 'experts' were never tasked with creating a really low drag automobile during their career, would low drag even be in their intellectual portfolio?
Why would you presume that they know anything about it unless Jaguar, or Audi, or Bentley, or Porsche, or Rover, or Tesla, etc., actually produced low drag? Historically, none of those companies have.
And so far, we've yet to experience any of their expertise as a first order reality. It's all filtered through you. And if you were having Rumsfeld moments, you wouldn't even be aware of them.
It makes things difficult. A degree of separation.

[JulianEdgar]

Originally Posted by aerohead

'True' isn't a word for science. Fact will do nicely.
If 'employee' = 'true expert' what do we have?
If your 'experts' were never tasked with creating a really low drag automobile during their career, would low drag even be in their intellectual portfolio?
Why would you presume that they know anything about it unless Jaguar, or Audi, or Bentley, or Porsche, or Rover, or Tesla, etc., actually produced low drag? Historically, none of those companies have.
And so far, we've yet to experience any of their expertise as a first order reality. It's all filtered through you. And if you were having Rumsfeld moments, you wouldn't even be aware of them.
It makes things difficult. A degree of separation.

Typical Aerohead - Aerohead is right and so many real experts are wrong.

You would think from what he writes that Aerohead has, say, led aero development for a major car company, or been a professor of aerodynamics...

[Cd]

How does downwash appear different from the surrounding tufts ?
Do tufts in a downwash remain steady, or do they flutter in any way different from areas of the car that are not in downwash ?

What clues are there that certain tufts are in downwash ?

[JulianEdgar]

Originally Posted by Cd

How does downwash appear different from the surrounding tufts ?
Do tufts in a downwash remain steady, or do they flutter in any way different from areas of the car that are not in downwash ?

What clues are there that certain tufts are in downwash ?

There is only attached or separated flow. The behaviour of the tufts makes differentiating them easy.

It doesn't matter what causes the attachment - Coanda Effect, downwash, what you had for lunch.

This is how confusion spreads here! In this case, a differentiation that no-one else makes.

Edit: Just to make this crystal clear.

1. Aerohead made up a new definition of attached and separated flows, where attached flows aren't really attached if the flow attachment is caused by downwash. This definition is in no aero textbook or tech paper, and is not supported by any aerodynamicist I've ever been in contact with. (I assume that Aerohead did this so that he could justify the template ie the fact that there is clearly attached flow on some cars that, according to his template, couldn't have been.)

2. Then someone (sorry, Cd) uncritically picks up this baloney and then tries to extend it to tuft patterns.

3. A whole new series of misunderstandings potentially developed, all based on stuff Aerohead has just literally made up!

[AeroMcAeroFace]

Originally Posted by JulianEdgar

This is a difference that you have just invented. Attached flow is attached flow, whether it is caused by downwash or not. I have already quoted the head of Jaguar aero in support of that.

Surely aerohead, it would be better to argue from the point of drag, downwash induced attached flow will be almost always higher drag than if the flow were "real" attached flow. Or reattached flow is almost always higher drag than if it was attached flow.

Arguing that the flow isn't attached and the tufts are lying and every expert is wrong seems difficult to believe. Saying that you have a different definition of attached flow seems far more reasonable to me.