Relationship between body pressures on the rear panels and the base pressure

[JulianEdgar]

Originally Posted by aerohead

6) If flow remains attached all the way, it will be at its slowest velocity, highest pressure, and base pressure will mimic that at the separation line.

I just saw this.

As with Aerohead's comment on the thickness of the boundary layer being of no consequence, it is also wrong.

If you wish to see this for yourself (anyone) just measure some pressures on real cars on real roads. On many cars, as indicated by the experts quoted above (surprise, that!), base pressures do not "mimic that at the separation line".

Here's a real life example from my measurements (side/wake and then centreline/wake):





...and from car company measurements (centreline/wake):



It's a good example of Aerohead just repeating what he has said before, and utterly ignoring any credible information that doesn't match his existing beliefs.

I know it's quite silly of me, but I was actually looking forward to Aerohead commenting on this thread, and potentially building on the latest information from some of the best car aero experts in the world. Especially, when there is so little on this specific subject in the textbooks - and it so strongly relates to our number one topic of reducing drag.

But I should have guessed that instead he'd just repeat same-old, same-old, complete with mistakes. Then, subsequently, defend those mistakes to the death, irrespective of any evidence brought to the contrary.

[Shrug] For everyone else, look and learn - I certainly am.

(And from Adrian Gaylard today in response to a last question from me:

Classically we’d think of the pressure gradient driving the boundary layer, which is essentially the way adverse pressure gradient driven separation is described. But energy loss at the surface generates the boundary layer. There’s usually more than one thing happening.)

[aerohead]

Originally Posted by JulianEdgar

I just saw this.

As with Aerohead's comment on the thickness of the boundary layer being of no consequence, it is also wrong.

If you wish to see this for yourself (anyone) just measure some pressures on real cars on real roads. On many cars, as indicated by the experts quoted above (surprise, that!), base pressures do not "mimic that at the separation line".

Here's a real life example from my measurements (side/wake and then centreline/wake):





...and from car company measurements (centreline/wake):



It's a good example of Aerohead just repeating what he has said before, and utterly ignoring any credible information that doesn't match his existing beliefs.

I know it's quite silly of me, but I was actually looking forward to Aerohead commenting on this thread, and potentially building on the latest information from some of the best car aero experts in the world. Especially, when there is so little on this specific subject in the textbooks - and it so strongly relates to our number one topic of reducing drag.

But I should have guessed that instead he'd just repeat same-old, same-old, complete with mistakes. Then, subsequently, defend those mistakes to the death, irrespective of any evidence brought to the contrary.

[Shrug] For everyone else, look and learn - I certainly am.

(And from Adrian Gaylard today in response to a last question from me:

Classically we’d think of the pressure gradient driving the boundary layer, which is essentially the way adverse pressure gradient driven separation is described. But energy loss at the surface generates the boundary layer. There’s usually more than one thing happening.)

1) The Mercedes is not a 'full' body, with respect the 'entire' body it's derived from.
2) Your measurements, while representative of the given length of the Mercedes, would, in no way, reflect the context within which my comment was prefaced.
3) If 'half' of the Mercedes weren't missing, you'd be left with the thickest boundary layer and lowest drag, represented in the pressure profile of the missing body.
4) 'many' cars does not constitute 'all' cars. Measure a 'Kamm-back' and report back.( Renault Vesta-II, Mercedes-Benz Bionic Boxfish, Mercedes-Benz IAA, with tail extended etc..)
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Is Gaylard going to retract his ' flow separation due to the adverse pressure gradient' quote from page- 59 of his Doctoral Thesis?
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Viscous shearing forces, fed by momentum interchange from the local streamline does generate the TBL. That's classical boundary layer theory.
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An EcoModder's best dream come true would be to have the 'BIGGEST' boundary layer imaginable!

[JulianEdgar]

Originally Posted by aerohead

1) The Mercedes is not a 'full' body, with respect the 'entire' body it's derived from.
2) Your measurements, while representative of the given length of the Mercedes, would, in no way, reflect the context within which my comment was prefaced.
3) If 'half' of the Mercedes weren't missing, you'd be left with the thickest boundary layer and lowest drag, represented in the pressure profile of the missing body.
4) 'many' cars does not constitute 'all' cars. Measure a 'Kamm-back' and report back.( Renault Vesta-II, Mercedes-Benz Bionic Boxfish, Mercedes-Benz IAA, with tail extended etc..)

What, so it is actually a theory that applies only to three cars in the entire world? Sorry, I would have thought we're interested in theory that applies to the cars we're working on - you know, the ones we're modifying.

Yet again, an Aerohead theory that doesn't match reality.

[aerohead]

Originally Posted by JulianEdgar

What, so it is actually a theory that applies only to three cars in the entire world? Sorry, I would have thought we're interested in theory that applies to the cars we're working on - you know, the ones we're modifying.

Yet again, an Aerohead theory that doesn't match reality.

You used my material out of context and I was simply clarifying the situation, adding the proper context.
I just added SAE 2020-01-0673 in its own thread, which illustrates 'entire ' bodies, which begin in their entirety, afterwards truncated to a 'practical' length if so chosen.
Whenever I see a vehicle for the first time, this is the context in which I experience them.
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You may come to realize that many 'cars we're working on' respond very favorably to the 'theory.'

[JulianEdgar]

Rob Palin had a lot of interesting things to say regarding this topic (so now all my experts have got back to me).

In short, he took elements from what they other experts had said, and then fleshed them out in more detail.

My summary is this:

1. There is no simple, pat answer as to what influences base pressure on different cars.

2. Therefore, without testing, you're working very much in the dark as to the effectiveness of modifications.

3. Higher pressures on the side of the car, especially towards the rear, will likely increase base pressure. (And my testing clearly shows that higher side pressures occur with better flow attachment.)

4. Thick boundary layers towards the back of the car will likely increase base pressure.

[aerohead]

Originally Posted by JulianEdgar

Rob Palin had a lot of interesting things to say regarding this topic (so now all my experts have got back to me).

In short, he took elements from what they other experts had said, and then fleshed them out in more detail.

My summary is this:

1. There is no simple, pat answer as to what influences base pressure on different cars.

2. Therefore, without testing, you're working very much in the dark as to the effectiveness of modifications.

3. Higher pressures on the side of the car, especially towards the rear, will likely increase base pressure. (And my testing clearly shows that higher side pressures occur with better flow attachment.)

4. Thick boundary layers towards the back of the car will likely increase base pressure.

1)
A) ' it is possible to relate geometry to flow pattern clearly for the fastback and the squareback.' Hucho, page- 155
B) ' On a prismatic body, the base pressure in real flow is dependent upon the fineness ratio.' Hucho, page- 140
C) ' The base pressure also depends upon the angle at which the flow separates from the contour.' Hucho, page-141
D) 'With a long diffuser, a notable reduction in drag can be achieved with a very small angle.' Hucho, page- 144
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2)A) ' The shape for minimum drag can be calculated for a given fineness ratio and volume, and the theory can be used to translate results from scale models to full-size bodies.' Hucho, page- 106
2)B) ' Basic bodies with drag figures of less than 0.15 can be ... used as initial shapes for shape optimization.' Hucho, page- 198-9
2)C) It's conceivable that a template may allow for zero ' changing the detail in question until the flow no longer separates around it.' Hucho, page- 124.
So in regards to your comment, I could not disagree more.
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3) absolutely!
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4) If the aft-body cross-section contracts progressively in the flow direction such that only a 'moderate' pressure increase is produced, the boundary layer will be protected from separation, flow will remain attached for the entire length of the existing body, turbulent boundary layer will be the thickest, and pressure will be the highest possible, when separation does finally occur at the trailing edges; yielding the highest base pressure.

[JulianEdgar]

Originally Posted by aerohead

1)
A) ' it is possible to relate geometry to flow pattern clearly for the fastback and the squareback.' Hucho, page- 155
B) ' On a prismatic body, the base pressure in real flow is dependent upon the fineness ratio.' Hucho, page- 140
C) ' The base pressure also depends upon the angle at which the flow separates from the contour.' Hucho, page-141
D) 'With a long diffuser, a notable reduction in drag can be achieved with a very small angle.' Hucho, page- 144
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2)A) ' The shape for minimum drag can be calculated for a given fineness ratio and volume, and the theory can be used to translate results from scale models to full-size bodies.' Hucho, page- 106
2)B) ' Basic bodies with drag figures of less than 0.15 can be ... used as initial shapes for shape optimization.' Hucho, page- 198-9
2)C) It's conceivable that a template may allow for zero ' changing the detail in question until the flow no longer separates around it.' Hucho, page- 124.
So in regards to your comment, I could not disagree more.
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3) absolutely!
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4) If the aft-body cross-section contracts progressively in the flow direction such that only a 'moderate' pressure increase is produced, the boundary layer will be protected from separation, flow will remain attached for the entire length of the existing body, turbulent boundary layer will be the thickest, and pressure will be the highest possible, when separation does finally occur at the trailing edges; yielding the highest base pressure.

I looked up each of your quotes and they're generally either taken out of context, refer to 'shapes' (not real cars), are a bit simplistic or are rather tortuously applied to support your theories.

All the professional car aerodynamicists that I asked to comment on the topic of 'what influences base pressures?' on real cars said there was no single answer, and the influences of various mechanisms were poorly understood.

My summary thus remains:

1. There is no simple, pat answer as to what influences base pressure on different cars.

2. Therefore, without testing, you're working very much in the dark as to the effectiveness of modifications.

3. Higher pressures on the side of the car, especially towards the rear, will likely increase base pressure. (And my testing clearly shows that higher side pressures occur with better flow attachment.)

4. Thick boundary layers towards the back of the car will likely increase base pressure.

[aerohead]

Originally Posted by JulianEdgar

I looked up each of your quotes and they're generally either taken out of context, refer to 'shapes' (not real cars), are a bit simplistic or are rather tortuously applied to support your theories.

All the professional car aerodynamicists that I asked to comment on the topic of 'what influences base pressures?' on real cars said there was no single answer, and the influences of various mechanisms were poorly understood.

My summary thus remains:

1. There is no simple, pat answer as to what influences base pressure on different cars.

2. Therefore, without testing, you're working very much in the dark as to the effectiveness of modifications.

3. Higher pressures on the side of the car, especially towards the rear, will likely increase base pressure. (And my testing clearly shows that higher side pressures occur with better flow attachment.)

4. Thick boundary layers towards the back of the car will likely increase base pressure.

Thanks for the response. The clocks about to run out here, and no time to cover all the items. I'll be back Friday, with a fresh 'clock' and will get back into it. Thanks again, see you Friday.