Ultra low drag vehicles

[JulianEdgar]

Originally Posted by freebeard

Fineness ratio?

Has nothing to do with Reynolds Numbers.

[aerohead]

Originally Posted by JulianEdgar

That's seriously going from the sublime to the ridiculous. It seems you're now just making stuff up - no expert, reference or paper supports this nonsense.

I never said I like it. I'll dig through my rat's nest and see if I can trace it down.
I've made transcription errors before.
Thanks for flagging it.

[Vman455]

Originally Posted by aerohead

Characteristic 'length' for automobiles is derived from frontal area.

I suppose you could make an argument that frontal area and length are correlated, but even that is only generally true. Take a car like the Smart Fortwo (A = 21.0 square feet) and the original Honda Insight (A = 19.8 square feet) and it falls apart; the Smart is only 106" long while the Honda is 155".

In vehicle aerodynamics we're restricting Reynolds number calculations to incompressible flow; once you introduce things like compressibility and heat flux there are many, many other ways to calculate it. Perhaps you're thinking of one of those?

[JulianEdgar]

Originally Posted by aerohead

I never said I like it. I'll dig through my rat's nest and see if I can trace it down.
I've made transcription errors before.
Thanks for flagging it.

It just seems very strange to me that you endlessly quote Hucho 2nd edition in support of your theories, but get wrong something as basic as how Reynolds Numbers are calculated.

Hucho 2nd ed page 49:

[The Reynolds number] is a function of the speed of the vehicle, the kinematic viscosity of the fluid and a characteristic length of the vehicle eg its total length as in Figure 2.2.

(And Fig 2.2 shows vehicle length exactly as we'd expect - front of vehicle to back of vehicle.)

No mention of area, and certainly no mention of working out length from area.

Incidentally, for people testing and modifying the aero of their car, Reynolds Numbers can basically be ignored. They only become relevant when different size models are being used. The only time I've ever had to pay attention to Reynolds Numbers is when calculating wing downforce / lift from wind tunnel aerofoil data.

[aerohead]

1) Cd is based upon frontal area
2) Drag minimum occurs @ CdA minimum
3) 'Laminar' profile Cds are Rn-dependent
4) Rn is 'size', 'velocity', and 'kinematic viscosity'- specific
5) The metric for Rn-related ' size' is length
6) The 'length' of any given 'laminar' profile is a derivative of Af ( scaling-factor)
7) Competition 'laminar' bodies are 'shrink-wrapped' over, and constructed to fit around the body of a single individual team member chosen for 'pilot'
8) The smallest Af = the shortest length
9) The shortest length = smallest RN
10) The smaller the Rn, the longer the delay in reaching transition to super-critical Rn
11) Super-critical Rn brings a concomitant transition to TBL
12) TBL renders a 'laminar' profile impotent
--------------------------------------------------------------------------------------
A curiosity in research reporting is, that for the Rocketail Wing.
The investigators at the University of Glasgow derived their Rn of 5.3 X 10-to the 5th, upon the 18-wheeler's height, rather than length. First I can recall, ever.

[Vman455]

1) cD is based on frontal area only for convenience' sake. We could relate cD to plan area, as in aeronautics, or wetted area, or any other reference area we choose. However, those are all much harder to measure or calculate on complex three-dimensional bodies such as cars.
6) "Scaling factor" in aeronautics and aerodynamics refers to the necessity of obtaining the same Re in the flow around a model as the full-size object, as in this paper written by some researchers working down the street from me. It appears you are conflating "scaling factor" with "fineness ratio," as freebeard suggested. But, since production and concept cars have widely varying fineness ratios this isn't an accurate method of predicting Re for cars based on height or frontal area, as I pointed out in a post above. (It might work for 18-wheelers, which have standardized trailer lengths, typically 53').

[aerohead]

Originally Posted by Vman455

1) cD is based on frontal area only for convenience' sake. We could relate cD to plan area, as in aeronautics, or wetted area, or any other reference area we choose. However, those are all much harder to measure or calculate on complex three-dimensional bodies such as cars.
6) "Scaling factor" in aeronautics and aerodynamics refers to the necessity of obtaining the same Re in the flow around a model as the full-size object, as in this paper written by some researchers working down the street from me. It appears you are conflating "scaling factor" with "fineness ratio," as freebeard suggested. But, since production and concept cars have widely varying fineness ratios this isn't an accurate method of predicting Re for cars based on height or frontal area, as I pointed out in a post above. (It might work for 18-wheelers, which have standardized trailer lengths, typically 53').

1) Frontal area-based drag coefficient has been the industry standard basically all along.
2) I don't mention 'aeronautics' just as I never mentioned 'aerodynamics' with respect to static 'lift'. Please read for comprehension and pay close attention to actual language!
3) Any 'profile' will be profoundly affected by fineness-ratio. Any given frontal area demands a matching length to preserve the fineness ratio.
4) For road vehicle aerodynamics, scaling-factor has to do with dynamic similarity, verisimilitude, precisely related to Reynolds number. It's an absolute necessity with respect to scale-model wind tunnel testing.
5) I don't 'conflate.' The science speaks for itself.
6) A 'life-size' automobile, above 20-mph, will be at super-critical Reynolds number, and constant drag coefficient up to transonic velocity.
7) We don't need to concern ourselves with RN effects of a 'real' car.
8) The context of modern 'competition laminar' bodies is exactly related to frontal area, as it undergirds every other dimension of the body architecture.
One cannot understand the 'context' of a 'laminar' body without this understanding.
That was the condition of the discussion.

[Vman455]

Originally Posted by aerohead

1) Frontal area-based drag coefficient has been the industry standard basically all along.
2) I don't mention 'aeronautics' just as I never mentioned 'aerodynamics' with respect to static 'lift'. Please read for comprehension and pay close attention to actual language!
3) Any 'profile' will be profoundly affected by fineness-ratio. Any given frontal area demands a matching length to preserve the fineness ratio.
4) For road vehicle aerodynamics, scaling-factor has to do with dynamic similarity, verisimilitude, precisely related to Reynolds number. It's an absolute necessity with respect to scale-model wind tunnel testing.
5) I don't 'conflate.' The science speaks for itself.
6) A 'life-size' automobile, above 20-mph, will be at super-critical Reynolds number, and constant drag coefficient up to transonic velocity.
7) We don't need to concern ourselves with RN effects of a 'real' car.
8) The context of modern 'competition laminar' bodies is exactly related to frontal area, as it undergirds every other dimension of the body architecture.
One cannot understand the 'context' of a 'laminar' body without this understanding.
That was the condition of the discussion.

I'm not even going to bother to respond to the rest of this because I'm not nearly as patient as some of the other posters here, but re: 7) One thing Hucho did write to me was an explicit warning to be careful in testing devices on my car since front and rear spoilers can be sensitive to Re effects.

[JulianEdgar]

Perfect example of Aerohead's approach.

1. Aerohead made a mistake in a post (associating the calculation of Reynold's Numbers with frontal area, not length).

2. The mistake was pointed out by Vman455, with a simple and clear correction.

3. Aerohead immediately doubles down, reiterating the mistake.

4. A person mislead by the Aerohead's posts appears, trying to make sense of what Aerohead is saying (rather than simply seeing it as wrong) and bringing in another issue of no relevance.

4. I also point out that what Aerohead is saying is wrong (but I am less polite than Vman455, because I can see yet another example of Aerohead breeding confusion on this subgroup - see above).

5. Aerohead agrees to consult his references to find out how he arrived at his error (an error he hasn't yet admitted, despite it being as clear as saying that force = mass * velocity)

6. Aerohead comes back with his 'wall of noise approach', an approach that mixes irrelevancies, illogical jumps and correct definitions in another amazing mish mash that attempts to justify what is a simple error on his part.

7. Vman455 tries to correct the new errors that Aerohead has introduced in his response.

8. Aerohead replies with another wall of noise, but this times moves the goal posts right away from normal road cars into laminar flow bodies, further sowing confusion.

9. Vman455 understandably starts to give up in his corrections.

Yet another example of where something as simple as this equation:

Reynolds Number = (density * speed * length) / viscosity

...has under Aerohead's guidance become an incomprehensible mix of rubbish, errors, confusion and irrelevancies.

[freebeard]

4. A person mislead by the Aerohead's posts appears, trying to make sense of what Aerohead is saying (rather than simply seeing it as wrong) and bringing in another issue of no relevance.

Are we talking about Permalink #30. If so, then get stuffed.

I was pushing back against The Template long before you were around. The difference is I attack the concept, not the person. You mis-read a request for clarification.

Has nothing to do with Reynolds Numbers.

Did I push back on that? I think there is some correlation.

[The Reynolds number] is a function of the speed of the vehicle, the kinematic viscosity of the fluid and a characteristic length of the vehicle eg its total length as in Figure 2.2.

So what is fineness ratio again?