Reducing lift on different body shape cars

[aerohead]

Originally Posted by JulianEdgar

Which one?

As clearly spelled out in the header, the relationship between boundary layer thickness and separation.

[JulianEdgar]

Originally Posted by aerohead

Are we to expect that your second-edition will have your rules-of-thumb edited out?

There are very few rules of thumb in the book, and where they are used, they are always qualified or, when numerical, use ranges not specific figures.

I think there is only one exception to that, and it was in a direct quote from a professional aerodynamicist.

Same in my engine management tuning book, suspension book, electronics book - rules of thumb are not a good way of modifying cars.

[JulianEdgar]

Originally Posted by aerohead

As clearly spelled out in the header, the relationship between boundary layer thickness and separation.

Barnard, Aerodynamics of Road Vehicles, 3rd edition, Page 12 is the first reference I could find in 30 seconds of looking that supports this.

[aerohead]

Originally Posted by JulianEdgar

Barnard, Aerodynamics of Road Vehicles, 3rd edition, Page 12 is the first reference I could find in 30 seconds of looking that supports this.

And you're going to briefly say something about it?

[Vman455]

Originally Posted by aerohead

1) Mair's model consisted of an elliptical nose, of 1,3-D in length, A cylindrical main body of 3-D in length, and the boat-tailed section of 1.9-D, which creates a total length of 6.2-diameters.
2) All values reflect zero viscosity effects, only pressure drag. Nothing to do with boundary layer thickness.
3) The take-away for aero modders is the 'lead- in' profile, leading to the final downslope angle. It's a known quantity.
4) Without the boat-tail we're looking at Cd 0.204 for Mair's model with skin friction.
5) With the boat-tail, Cd 0.066.
6) This 'lead-in' profile basically exists on all PGA Regulation golf balls.
7) 'Compound-curvature' roof modifications can entail 800-man-hours of construction time. Some form of Go NoGo might save many hundreds of hours of fabrication time. This would make something like Mair's profile quite
valuable. It may not be perfect, but short of a few hundreds of thousands of dollars for laboratory R & D, it might be handy for some with a more meager budget.
8) This was the original premise for sharing it. And perhaps why Hucho and others have presented it.

Here's what Hucho actually presented when talking about Mair's research (emphasis in original text!):

Fig. 4.43 shows the extent to which the drag of a body of revolution can be reduced by tapering. The optimal tapering angle of 22 degrees given in this diagram should be taken only as indicative; the specific optimal angle depends on the upstream history of the flow.

(Hucho, W.H. "Aerodynamic Drag of Passenger Vehicles." In Aerodynamics of Road Vehicles, 4th edition, W.H. Hucho, ed. [Warrendale: SAE, 1998], 164).

Note the caveats; he doesn't give a blanket "go/no go" because there is no blanket "go/no go"--it depends on the shape and features of the particular car in question, not some hypothetical ideal.

[JulianEdgar]

Originally Posted by aerohead

And you're going to briefly say something about it?

The book is, like $7 secondhand.

Last time it was nominated as a good book to buy you said something like, "Why bother?"

Maybe you should bother?

[JulianEdgar]

Originally Posted by Vman455

Here's what Hucho actually presented when talking about Mair's research (emphasis in original text!).

(Hucho, W.H. "Aerodynamic Drag of Passenger Vehicles." In Aerodynamics of Road Vehicles, 4th edition, W.H. Hucho, ed. [Warrendale: SAE, 1998], 164).

Note the caveats; he doesn't give a blanket "go/no go" because there is no blanket "go/no go"--it depends on the shape and features of the particular car in question, not some hypothetical ideal.

This is very important because, as you have indicated in this case, I have found that time and time again Aerohead misquotes references.

It's only when you go and check that you find that to be the case - the quotes sound quite authoritative...

[aerohead]

Originally Posted by Vman455

Here's what Hucho actually presented when talking about Mair's research (emphasis in original text!):


(Hucho, W.H. "Aerodynamic Drag of Passenger Vehicles." In Aerodynamics of Road Vehicles, 4th edition, W.H. Hucho, ed. [Warrendale: SAE, 1998], 164).

Note the caveats; he doesn't give a blanket "go/no go" because there is no blanket "go/no go"--it depends on the shape and features of the particular car in question, not some hypothetical ideal.

1) so remove the constant-diameter cylindrical section.
2) join the nose and tail section to form the streamlined body of revolution.
3) use Hucho's drag table from Hoerner to ascertain the Cd of the streamlined body you've just created.
4) subtract out the skin friction no longer present from the elongated body.
5) bifurcate what's left, longitudinally, to create the half-body.
6) double the Cd of the streamlined body to get the drag of the half-body.
7) now add wheels, and wheel drag, as Jaray and Buchheim did, and tell me what you come up with.
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Once the detail optimization is accomplished with the forebody, the flow there is 'saturated', no additional drag reduction is left.
The rest of the story involves the aft-body, given the caveats listed by Hucho which qualify the boat-tailing performance, and for exactly the reasons Hucho spelled out.

[aerohead]

Originally Posted by JulianEdgar

The book is, like $7 secondhand.

Last time it was nominated as a good book to buy you said something like, "Why bother?"

Maybe you should bother?

I believe the context of that comment had to do with measuring pressures. Something you've yet to make a compelling case for.

[JulianEdgar]

Originally Posted by aerohead

1) so remove the constant-diameter cylindrical section.
2) join the nose and tail section to form the streamlined body of revolution.
3) use Hucho's drag table from Hoerner to ascertain the Cd of the streamlined body you've just created.
4) subtract out the skin friction no longer present from the elongated body.
5) bifurcate what's left, longitudinally, to create the half-body.
6) double the Cd of the streamlined body to get the drag of the half-body.
7) now add wheels, and wheel drag, as Jaray and Buchheim did, and tell me what you come up with.

[jaw drops].

Now you're just off in fantasy land, I am afraid. Why not add the calories of what you had for lunch?

Seriously, this is just absolute rubbish.