What causes car drag?

[JulianEdgar]

Originally Posted by aerohead

Automotive drag is about pressure drag.
Pressure drag is an artifact of flow separation on the aft-body.
Separation is a function of aft-body shape.
An introduction to known shapes, devoid of separation would be fundamental to any meaningful understanding of 'car drag.'

I do wish you'd stop spreading incorrect information: pressure drag is not just an artefact of flow separation.

(But then again, if you don't look at the diagrams shown in the cited videos, and numerous other ones that are available, you'd keep on saying that.)

But honestly, it's just such a basic mistake to keep on making.

[aerohead]

Originally Posted by JulianEdgar

I do wish you'd stop spreading incorrect information: pressure drag is not just an artefact of flow separation.

(But then again, if you don't look at the diagrams shown in the cited videos, and numerous other ones that are available, you'd keep on saying that.)

But honestly, it's just such a basic mistake to keep on making.

Take it up with Hucho. It's his quote. It's the entire premise for road vehicle aerodynamic study.
Total drag can be reduced to surface friction drag in the absence of separation.
80% - 90% of total kinetic energy can make it to the rear stagnation point on a streamlined car.
I'm familiar with diagrams. I'm familiar with context also.

[JulianEdgar]

Originally Posted by aerohead

Take it up with Hucho. It's his quote. It's the entire premise for road vehicle aerodynamic study.
Total drag can be reduced to surface friction drag in the absence of separation.
80% - 90% of total kinetic energy can make it to the rear stagnation point on a streamlined car.
I'm familiar with diagrams. I'm familiar with context also.

You completely missed my point. I don't know why I am bothering, but...

1. Honda Insight Gen 1 - attached flow over rear window. (Tuft test your own car if you want to take issue with that.)

2. Measured pressures:



3. Draw in the force vector for -80 on the hatch. (Pressure forces act at right-angles to the panel.)

4. Resolve the vector into lift and drag elements (use the triangle of forces - engineering 101).

5. Note that attached flow is, in the case, causing drag (and lift).

As I said, if you think that pressure drag can be caused only through separation, you are making a very basic mistake.

[aerohead]

Originally Posted by JulianEdgar

You completely missed my point. I don't know why I am bothering, but...

1. Honda Insight Gen 1 - attached flow over rear window. (Tuft test your own car if you want to take issue with that.)

2. Measured pressures:



3. Draw in the force vector for -80 on the hatch. (Pressure forces act at right-angles to the panel.)

4. Resolve the vector into lift and drag elements (use the triangle of forces - engineering 101).

5. Note that attached flow is, in the case, causing drag (and lift).

As I said, if you think that pressure drag can be caused only through separation, you are making a very basic mistake.

There's no dispute about your measurements.
Others are recording hundreds of pressure readings over the entire surface of the vehicle to ascertain the average, mean, and force vector resultants. Or simply recording the drag and lift from the strain-gauge load cells in the tunnel. And everything is normalized to SAE Standard atmosphere.
If the Insight can be Cd 0.09, and it's recording Cd 0.25, what percentage of that Cd differential would be attributed by the resultants down the centerline, versus everywhere else? That's delta- 0.16. We would need to parse it out. A daunting proposition.

[JulianEdgar]

Originally Posted by aerohead

There's no dispute about your measurements.
Others are recording hundreds of pressure readings over the entire surface of the vehicle to ascertain the average, mean, and force vector resultants. Or simply recording the drag and lift from the strain-gauge load cells in the tunnel. And everything is normalized to SAE Standard atmosphere.
If the Insight can be Cd 0.09, and it's recording Cd 0.25, what percentage of that Cd differential would be attributed by the resultants down the centerline, versus everywhere else? That's delta- 0.16. We would need to parse it out. A daunting proposition.

Did you ignore the point deliberately, or you missed it?

My point is that attached flow can cause drag. You state that only separated flow can cause drag. You are clearly wrong, as even a moment's thought about what I have written above will show.

[aerohead]

Originally Posted by JulianEdgar

Did you ignore the point deliberately, or you missed it?

My point is that attached flow can cause drag. You state that only separated flow can cause drag. You are clearly wrong, as even a moment's thought about what I have written above will show.

And even so, the lowest drag forms have no separation, in spite of those resultants.
A car's got to be 3-dimensional in order to get inside. The air must be displaced as it passes. Some things are inescapable. And all that said, the achievable target is still on the order of Cd 0.09.
I'm just taking about the end game. The exit strategy for really low drag.
Ya gotta get a little wet in order to drink.

[JulianEdgar]

Originally Posted by aerohead

And even so, the lowest drag forms have no separation, in spite of those resultants.
A car's got to be 3-dimensional in order to get inside. The air must be displaced as it passes. Some things are inescapable. And all that said, the achievable target is still on the order of Cd 0.09.
I'm just taking about the end game. The exit strategy for really low drag.
Ya gotta get a little wet in order to drink.

So your oft-repeated point, that drag is caused only by flow separation, is actually quite wrong?

Pity the people you have so long misled - some of whom have contacted me amazed to find that attached flow can cause drag.

[aerohead]

Originally Posted by JulianEdgar

You completely missed my point. I don't know why I am bothering, but...

1. Honda Insight Gen 1 - attached flow over rear window. (Tuft test your own car if you want to take issue with that.)

2. Measured pressures:



3. Draw in the force vector for -80 on the hatch. (Pressure forces act at right-angles to the panel.)

4. Resolve the vector into lift and drag elements (use the triangle of forces - engineering 101).

5. Note that attached flow is, in the case, causing drag (and lift).

As I said, if you think that pressure drag can be caused only through separation, you are making a very basic mistake.

1) on the Insight, the trailing edge of the backlight is 24.5mm below the AST-II profile. Okay according to Buchheim et al.. No argument from me. Check!
2) so your measurements reflect those of a 'streamlined' roofline. Check!
3) whether or not each discrete resultant possesses a horizontal component will be reflected in the coefficient of drag.
4) the lion's share of drag will be determined between forward and rear stagnation point.
5) if the body is to be three-dimensional, capable of containing occupants, we must live with the consequences of resultants.
6) the final analysis is Cd.
7) if the Insight is capable of Cd 0.09, and it's Cd 0.25, then, by definition, the Insight has Cd 0.16 designed in.
8) this is what I've been interested in since 1973.
9) this is why I went to college.
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10) and then the big, ugly question rears its head. Even if we make all these measurements, what in the world do we do with them, which gets us to Cd 0.09?
11) Hucho told us in 1987.
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QUESTION :
at what velocity were the Insight measurements obtained? I've been adjusting all your values to 70-mph, as other automotive messengers are reporting figures obtained at 70-mph.
Second question. In other reporting, forward stagnation pressures = to local density altitude / station pressure are indicated, normalized to SAE standard = 101,327-Pascals.
When I adjust the 50-mph and 43-mph data to 70-mph, none the stagnation pressures indicate even a fraction of that.

[aerohead]

Originally Posted by JulianEdgar

So your oft-repeated point, that drag is caused only by flow separation, is actually quite wrong?

Pity the people you have so long misled - some of whom have contacted me amazed to find that attached flow can cause drag.

1) ' pressure drag is the largest component in the aerodynamic drag. Its minimization is the true objective of motor vehicle aerodynamics.'
Hucho, page- 110, top of page.
2) ' The main contributions to the drag force originate from the tear part of the body,' Hucho, page- 61
3) ' low drag can only be achieved when the separation at the rear is eliminated.' Hucho, page- 16.
4) ' Optimum shapes are 'streamlined' bodies having a very slender rear part.'
Hucho, page-61.

[JulianEdgar]

Originally Posted by aerohead

1) on the Insight, the trailing edge of the backlight is 24.5mm below the AST-II profile. Okay according to Buchheim et al.. No argument from me. Check!
2) so your measurements reflect those of a 'streamlined' roofline. Check!
3) whether or not each discrete resultant possesses a horizontal component will be reflected in the coefficient of drag.
4) the lion's share of drag will be determined between forward and rear stagnation point.
5) if the body is to be three-dimensional, capable of containing occupants, we must live with the consequences of resultants.
6) the final analysis is Cd.
7) if the Insight is capable of Cd 0.09, and it's Cd 0.25, then, by definition, the Insight has Cd 0.16 designed in.
8) this is what I've been interested in since 1973.
9) this is why I went to college.
-----------------------------------------------------------------------------------
10) and then the big, ugly question rears its head. Even if we make all these measurements, what in the world do we do with them, which gets us to Cd 0.09?
11) Hucho told us in 1987.
-----------------------------------------------------------------------------------

Sorry, but absolutely nothing you have written here is relevant to the point being made: that attached flow can cause drag.

So I'll say it again.

1. Honda Insight Gen 1 - attached flow over rear window. (Tuft test your own car if you want to take issue with that.)

2. Measured pressures (refer to above pic of measured pressures on Insight):

3. Draw in the force vector for -80 on the hatch. (Pressure forces act at right-angles to the panel.)

4. Resolve the vector into lift and drag elements (use the triangle of forces - engineering 101).

5. Note that attached flow is, in the case, causing drag (and lift).

As I said, if you think that pressure drag can be caused only through separation, you are making a very basic mistake.