A book to buy and read

[JulianEdgar]

Originally Posted by aerohead

How do you explain the 100% atmospheric pressure acting on the nose of a 1968 Volkswagen Transporter?
I'm looking at Figure 6.8, Hucho.
How do you explain the 100% atmospheric pressure acting on the nose of the circa 1936, Jaray car, Figure 9.4, page 160, AERODYNAMIC DRAG, Horner, 1951. Hucho uses the 1960s edition of the book.
How do you explain the 100% atmospheric pressure acting on the nose of a L/D= 5.5, streamline body of revolution, depicted in the same book?
How do you explain the 100% atmospheric pressure acting on the nose of the body depicted in Figure 2.4, page 51, Hucho, ( which is 100% accurate for the first 85.5% of the body) ?

Figure 6.8, Hucho. I am seeing the stagnation pressure on the front face of the VW bus, and high pressure on the very slightly tilted windscreen. To suggest these forces are major forces offsetting lift is simply wrong - they are acting almost completely horizontally. Note how large the suction is as the airflow wraps around onto the roof, and subsequently, there is in fact no positive pressure anywhere along the whole rest of the vehicle!

Figure 2.4 Hucho we've already covered - as the caption says, it's a 2D schematic for a vehicle-shaped body in inviscid flow. That's not actually a real car in real air.

Basically, Aerohead's theories - or the way he applies them - are very frequently wrong, I am afraid.

[freebeard]

Note how large the suction is as the airflow wraps around onto the roof, and subsequently, there is in fact no positive pressure anywhere along the whole rest of the vehicle!

Show me your Hucho.



It depends. The slick-top bus looks pretty good. The hatch-top is worse, the Westy is worst (the wake creeps up to mid-wheelbase on top?). The high-top has the best Cd.

[Vman455]

Originally Posted by freebeard

Minor correction:
"there's no way it can "overcome" the suction acting over the hood, roof, and rear window.'

Doh! Brain fart--thank you. Just the roof, then, which is a nice, large, horizontal surface, where all the force created by the low pressure can act vertically.

[JulianEdgar]

Originally Posted by freebeard

Show me your Hucho.

[freebeard]

Thanks. Now salted away in my aero technical album.

[JulianEdgar]

Honestly, the one really good thing that Aerohead's posts do is make me check my facts. That is, why, I think to myself, am I so confident Aerohead is quite wrong, especially when he has good citations to (purportedly) back it up?

That's why, where I can, I go look at his citations.

So this was a good example.

Aerohead writes:

Originally Posted by aerohead

5) all the low pressure of the suction peak can be overwhelmed by the high local static pressure acting at the nose and tail.

...and I think, nah, I've never measured high downwards pressures on the nose of any normal car. And in published (and my measured stuff), downwards pressures on the rear of a car come only with a rear spoiler (or, in very unusual vehicles - like solar race cars - by fully attached flow).

So then Aerohead nominates Fig 6.8 in Hucho (second edition) as his evidence.

So maybe I am wrong? Better go take a look.



No, there it is.

The only downwards pressure occurs ahead of the windscreen of the Transporter - the peak (Cp of 1) - is in a very clear stagnation zone... so acting backwards, not up or down!

You need to look not only at the pressure, but also the direction in which it is acting. A big, flat, vertical surface on the front of a vehicle is going to develop high pressures, but that's all drag, not downforce!

Furthermore, the idea that "all the low pressure of the suction peak can be overwhelmed by the high local static pressure acting at the nose and tail" is obviously wrong when the area of the graph is far greater in the negative pressure zone (let alone that acting vertically) than the positive!

So, either Aerohead cannot read and understand the diagram, or he is confused, or his theory is wrong, or he is deliberately prosecuting an argument by being deceptive - you choose.

[aerohead]

Originally Posted by JulianEdgar

Figure 6.8, Hucho. I am seeing the stagnation pressure on the front face of the VW bus, and high pressure on the very slightly tilted windscreen. To suggest these forces are major forces offsetting lift is simply wrong - they are acting almost completely horizontally. Note how large the suction is as the airflow wraps around onto the roof, and subsequently, there is in fact no positive pressure anywhere along the whole rest of the vehicle!

Figure 2.4 Hucho we've already covered - as the caption says, it's a 2D schematic for a vehicle-shaped body in inviscid flow. That's not actually a real car in real air.

Basically, Aerohead's theories - or the way he applies them - are very frequently wrong, I am afraid.

1) the topic was high pressure acting on the nose.
2) any vertical force component will be acting at the maximum distance ahead of the front axle. The 'aerodynamic lever arm' as Hucho describes.
3) a very light load acting on your outstretched hand/ arm, can easily overwhelm a very large load applied to your upper arm, close to your shoulder.
4) I gave you 4-examples, counterfactual to your thesis.
5) suction peak near the windshield header is separate topic.
6) pressure distribution downstream of the suction peak is a separate topic.
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* The streamlines for Figure 2.4 are exactly correct.
* The velocity profile based upon those streamlines will be exactly correct.
* The pressure distribution based upon the velocity profile will be exactly correct.
* And according to Dr. Dietrich Hummel's conditions, and your own metric, the pressure distribution in 'real' flow will be exactly correct for the first 85.5 % of body, and within 91% of depicted pressure for the last 14.5% of body length.
* It doesn't matter if the car is real or not. Mathematics of the numerical, 2-D model doesn't make a distinction between the two. That's the whole point of numerical models.
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*They're not my theories. It's fluid mechanics. If you'd actually studied Dr. Hummel's chapter on fluid mechanics, or anyone elses, you'd already understand.
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Your task is to identify any post-1986, SAE, or other source document which overturns fluid mechanics as of 1986.

[aerohead]

Originally Posted by freebeard

Show me your Hucho.



It depends. The slick-top bus looks pretty good. The hatch-top is worse, the Westy is worst (the wake creeps up to mid-wheelbase on top?). The high-top has the best Cd.

Let's save those for a discussion on suction peak.

[freebeard]

Start your own thread?

[aerohead]

Originally Posted by JulianEdgar

Honestly, the one really good thing that Aerohead's posts do is make me check my facts. That is, why, I think to myself, am I so confident Aerohead is quite wrong, especially when he has good citations to (purportedly) back it up?

That's why, where I can, I go look at his citations.

So this was a good example.

Aerohead writes:



...and I think, nah, I've never measured high downwards pressures on the nose of any normal car. And in published (and my measured stuff), downwards pressures on the rear of a car come only with a rear spoiler (or, in very unusual vehicles - like solar race cars - by fully attached flow).

So then Aerohead nominates Fig 6.8 in Hucho (second edition) as his evidence.

So maybe I am wrong? Better go take a look.



No, there it is.

The only downwards pressure occurs ahead of the windscreen of the Transporter - the peak (Cp of 1) - is in a very clear stagnation zone... so acting backwards, not up or down!

You need to look not only at the pressure, but also the direction in which it is acting. A big, flat, vertical surface on the front of a vehicle is going to develop high pressures, but that's all drag, not downforce!

Furthermore, the idea that "all the low pressure of the suction peak can be overwhelmed by the high local static pressure acting at the nose and tail" is obviously wrong when the area of the graph is far greater in the negative pressure zone (let alone that acting vertically) than the positive!

So, either Aerohead cannot read and understand the diagram, or he is confused, or his theory is wrong, or he is deliberately prosecuting an argument by being deceptive - you choose.

1) the transporter's front bumper is essentially a splitter. I own one.
2) full local static pressure of stagnation is acting directly on top of it.
3) and as quite often, there's never a mention of the pressure distribution under the vehicle, nor lift-related palliatives known in the public domain, as if it's not part, nor could ever be a part of the calculus.
4) and the wider picture in which Hucho has claimed that for 'passenger' vehicles are concerned, just target 'neutral,' neither positive or negative lift.
5) 'Normal' cars aren't designed for downforce.
6) I suppose you missed the part about the Nissan LEAF, with the 'template' roofline, having the lowest Coefficient of Rear Lift, measured in the 'Drag Queens' article in CAR and DRIVER. Fully-attached flow up to the truncation, no rear spoiler necessary.
7) Hybrid-notchback, Mercedes-Benz CLA 250, having the highest rear lift of the five cars measured.
8) This is part of the prima facie evidence validating the claim that a 'streamlined' body can produce zero-lift.
9) do not make any association between the VW Transporter and the 'template'. It's an invalid comparison.
And let's quit with the 7-year-old's attempt at profiling and clinical psychology. You don't know me. You don't know what's in my mind.
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'seems to me that your greatest concern would be falling from grace as an aerodynamic guru, directly threatening book sales, should the buying public ever get wise to you, realizing that you're out of your depth in many things aerodynamics.
Perhaps you thought that, by including us in your book, that we'd be willing to submit to your dominance rituals, and peck corn from your feces.