EcoModder Forum comment on 1st-gen Porsche 911/912 lift

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 Originally Posted by kach22i Pressure drag? Why would we limit ourselves to that? A vortex may be a simple planar pressure difference at the surface, and it's true expression as a force of power forms after. As I understand it, drag causing vortexes can be delayed by rake fences and fins (vertical stabilizer wings), but they are similar to tentacles that hold the vehicle back like a parachute. If we look only at tufts or numbers at the surface we miss the bigger picture.
Because pressure drag makes up 80-90 per cent of total drag on current road cars.

And the 'tentacles' of vortices can only hold the vehicle back through pressures acting on the surface.

Last edited by JulianEdgar; 06-06-2020 at 04:33 PM..

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 06-06-2020, 06:18 PM #42 (permalink) Master EcoModder     Join Date: Aug 2012 Location: northwest of normal Posts: 20,176 Thanks: 5,659 Thanked 6,595 Times in 5,318 Posts Am I correct in thinking that the action is longitudinal rather than normal to the surface? Even though pressure diagrams use normals for magnitude the resultant is closer to toward the wake? __________________ ."Once men turned their thinking over to machines thinking this would set them free. This only allowed other men with machines to enslave them." Frank Herbert ____________________ Sadhguru: Yesterday isn't good or bad, it's gone. ____________________
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Quote:
 Originally Posted by freebeard Am I correct in thinking that the action is longitudinal rather than normal to the surface? Even though pressure diagrams use normals for magnitude the resultant is closer to toward the wake?
Of what, vortices? It doesn't matter what happens to them after the car - they can only act on the car through surface pressures. Therefore, measuring pressures at the centre of trailing vortices (not that I have done this specifically), and looking at the angle of the panel, will tell you the force direction.

But I might add that vortices is a very tricky area, one I couldn't get agreement on from my experts when I was writing the book. The disagreement was so strong that I did a panel on it (Page 19). I am at the moment in communication with a professional aerodynamicist about lift/downforce and vortices, and he is adding to my knowledge on this topic as we speak!

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attached

Quote:
 Originally Posted by JulianEdgar That's right: did you actually read what you wrote in the quote? The flow remains attached... Not that 'the flow only looks like it remains attached because of what the tufts show - but they're not right', but the flow is attached. So, let's go back to the Porsche - initially, the flow is largely attached, as indicated by the tufts. The placing of the spoiler reduces flow attachment ie increases separation. And the placing of the spoiler reduced lift. Therefore, lift was reduced by increasing separation. It seriously isn't hard!

1) had the flow actually 'wrapped' the 911's aft-body, it would actually be at its highest-possible pressure, zero rear lift, and lowest drag.( This won't mean anything to you until you study boundary layer theory and Bernoulli ).
2) the flow separated when the inclination exceeded 23-degrees.That's the law. It's in Hucho, just where I said it was.
2) the high drag, Cd 0.40, is produced by low base pressure and attached, counter-rotating longitudinal vortices, which 'induce' a 'downwash' (this is not the same as attached flow) which is responsible for the alignment of the tufts. Hucho might have clarified this better. Seeing shouldn't be believing!
3) the tufts indicate a small wake when, in reality, there's an enormous turbulent wake and vortical flow, following for hundreds of feet behind the 911. Only smoke will reveal this.
4) The spoiler(s) reach upwards, through the separated flow, attempting to harvest kinetic energy from the overlying inviscid flow by reattachment (Hucho page 281). You've got the physics completely inverted.
-----------------------------------------------------------------------------------
A) drag comes from the aft-body.
B) the drag is pressure drag.
C) pressure drag is a function of flow separation
D) the 911's high drag cannot be explained by ' air wrapping over the long curve.' Bernoulli's Theorem will explain this.
E) Spoilers do not trip flow, they're there to capture,or over-capture flow and deflect it.
----------------------------------------------------------------------------------
* In 1968, while the 2,360-pound, Cd 0.40, 911 was plying the Autobahn at 132-mph, the 1968, 1,240-pound, Cd 0.27, Porsche 907, long-tail was plying the world's race courses at 180-mph,without spoilers or wings. A look at the difference between the two car's aft-bodies gives a clue about rear design, drag, and lift. And ironically, the 911 could have been Cd 0.27 in 1968, but it would have impacted the car's silhouette. Law of the Paris Dressmaker.
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the action

Quote:
 Originally Posted by freebeard Am I correct in thinking that the action is longitudinal rather than normal to the surface? Even though pressure diagrams use normals for magnitude the resultant is closer to toward the wake?
Think of it as a resultant vector force, with both axial, vertical, and perhaps periodic,cyclic transverse components, depending upon the local Mach number. Very complicated !
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Quote:
 Very complicated !
In 3D graphics, Blender specifically, you have Displacement, which acts normal to the surface, and Vector Displacement, which can act at any angle to the surface. It's used to procedurally generate overhangs.

Assuming the air molecules adjacent to the surface are immobile [locally], displaced air molecules up through the boundary layer act at a resultant angle. Therefore, there exist shear forces as well as normal forces. And the vehicle is 'stuck' in the hole left by activities in the atmospheric shell. Shapes it's own destiny, as it were.

Am I provably wrong again?
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normal

Quote:
 Originally Posted by freebeard In 3D graphics, Blender specifically, you have Displacement, which acts normal to the surface, and Vector Displacement, which can act at any angle to the surface. It's used to procedurally generate overhangs. Assuming the air molecules adjacent to the surface are immobile [locally], displaced air molecules up through the boundary layer act at a resultant angle. Therefore, there exist shear forces as well as normal forces. And the vehicle is 'stuck' in the hole left by activities in the atmospheric shell. Shapes it's own destiny, as it were. Am I provably wrong again?
For laminar flow, you can presume that any particular streamline filamant would displace normal to the 'wall', 'boundary,' however a vortice 'particle's' spatial location will vary temporally as it spirals from origin,to final position, perhaps hundreds of meters behind the vehicle, where, as with wake turbulence, viscous attrition has robbed it of all kinetic energy.
Only the air immediately adjacent to the 'wall' is at rest. Everything above, within the TBL is turbulent, with violent shear / mixing as it's bombarded by laminar flow at its interface. And only if separation is nor occurring.
Local atmospheric pressure will maintain constant normal force, surface friction heating will be robbing around 12% of all energy, if there's zero separation, then that's all the air loses. With separation, the induced turbulence wastes all its kinetic energy to atmospheric heating.You can never recover it ( 2nd Law of Thermodynamics).
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Quote:
 Originally Posted by aerohead 1) had the flow actually 'wrapped' the 911's aft-body, it would actually be at its highest-possible pressure, zero rear lift, and lowest drag.( This won't mean anything to you until you study boundary layer theory and Bernoulli ). 2) the flow separated when the inclination exceeded 23-degrees.That's the law. It's in Hucho, just where I said it was. 2) the high drag, Cd 0.40, is produced by low base pressure and attached, counter-rotating longitudinal vortices, which 'induce' a 'downwash' (this is not the same as attached flow) which is responsible for the alignment of the tufts. Hucho might have clarified this better. Seeing shouldn't be believing! 3) the tufts indicate a small wake when, in reality, there's an enormous turbulent wake and vortical flow, following for hundreds of feet behind the 911. Only smoke will reveal this. 4) The spoiler(s) reach upwards, through the separated flow, attempting to harvest kinetic energy from the overlying inviscid flow by reattachment (Hucho page 281). You've got the physics completely inverted. ----------------------------------------------------------------------------------- A) drag comes from the aft-body. B) the drag is pressure drag. C) pressure drag is a function of flow separation D) the 911's high drag cannot be explained by ' air wrapping over the long curve.' Bernoulli's Theorem will explain this. E) Spoilers do not trip flow, they're there to capture,or over-capture flow and deflect it. ---------------------------------------------------------------------------------- * In 1968, while the 2,360-pound, Cd 0.40, 911 was plying the Autobahn at 132-mph, the 1968, 1,240-pound, Cd 0.27, Porsche 907, long-tail was plying the world's race courses at 180-mph,without spoilers or wings. A look at the difference between the two car's aft-bodies gives a clue about rear design, drag, and lift. And ironically, the 911 could have been Cd 0.27 in 1968, but it would have impacted the car's silhouette. Law of the Paris Dressmaker.
You know, if what can actually be measured on real cars matched your theory, you could well be right.

But there's a major problem.

On a range of cars, here's what doesn't match your theory:

- surface body pressure measurement
- tuft testing showing separated/attached flow
- manufacturer's stated coefficients of lift.

I think if I had theories that were so far from reality, I might look pretty carefully at what I was saying...

The simplest way of you realising the size of the errors you are spreading would be to make some on-road measurements for yourself.

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Quote:
 Originally Posted by myself Am I correct in thinking that the action is longitudinal rather than normal to the surface?
Bucky Fuller used to say that an epiphany was just the answer coming back to a question you'd forgotten you asked yourself. In this case it was Youtube:

My answer is at 7:47. The last 2:30 is an ad [for baby wipes?]
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theory

Quote:
 Originally Posted by JulianEdgar You know, if what can actually be measured on real cars matched your theory, you could well be right. But there's a major problem. On a range of cars, here's what doesn't match your theory: - surface body pressure measurement - tuft testing showing separated/attached flow - manufacturer's stated coefficients of lift. I think if I had theories that were so far from reality, I might look pretty carefully at what I was saying... The simplest way of you realising the size of the errors you are spreading would be to make some on-road measurements for yourself.
They're not 'my' theories, they come from the same fluid mechanics Hucho told you about on the very first page of his book.
You're disinterest, or inability to comprehend them does not constitute a failure to understand them on my part. I'm formally trained. Re-read Chapter-2.

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