Quote:
Originally Posted by JulianEdgar
1. I've already quoted the best reference that I am aware of on subsonic wind tunnel testing, and what it says about blockage factors.
'Low Speed Wind Tunnel Testing' (Barlow/Rae/Pope):
2. To suggest that car manufacturers, F1 teams (etc) build huge wind tunnels when there is no need to do so is simply not credible. |
Many readings of Hucho's chapter on wind tunnels helped me to understand the issue.
* On page 403 Hucho writes that the value of 5% is the limit for ' aircraft aerodynamics', not automobiles.
* On page 404, Hucho writes that ' condiderably higher blockage ratio is permissible for automotive aerodynamics.'
* On page 405, Hucho write that ' with streamlined walls the same result could be obtained with blockage of 20% as in test section with parallel walls with blockage of only 5%.'
* On page 407, Hucho writes that ' with 'adaptive-wall' tunnels with up to 30% blockage is fine.
* Later he writes that all these conditions are predicated upon the ability for yaw-testing, which is a different way to say that,with only zero-yaw measurements, even higher than 30% is okay.
* With a universal calibration model, Pininfarina, one of the smallest tunnels, returned values well within the standard deviation for a dozen or so tunnels tested worldwide.
* Adaptive-wall tunnels develop perfectly acceptable quanta, with blockage ratios of 30%. And this includes yaw testing.
* Curved-wall tunnels, as A2 and DARKO can provide perfectly fine numbers with even higher blockage ratios than 30%, due to the fact that everything is measured at zero yaw. There is no implication that results reflect anything but zero-yaw conditions.
* All this is predicated upon blockage correction factors developed from calibration testing in larger tunnels.
An example I gave you was the Toyota Prius.
1) We have a Cd for it from Toyota's wind tunnel
2) We have a Cd for it from A2. ( it was in the CAR and DRIVER 'Drag Queens' work which Don Sherman honcho'd ).
3) And we have a Cd for it from DARKO.
The Prius values, as a calibration model, allow us to 'adjust' measured coefficients between tunnels. An excepted practice within the industry.
4) Another wrench in the works is the fact that 'streamlined' cars upset the wind tunnel flow less than any other type during testing. They have a default safety margin designed in, as far as tunnel flow perturbation is concerned.
5) The other thing is that, the absolute values aren't as important as the trends in measurements, when limiting testing to a single tunnel as we did with Spirit. I could have done it all at A2. If I lived in Japan, I could have done all the work with the Toyota facility.
6) When you take all the conditions as a whole, a different picture emerges than one demanding a 5% blockage ratio. It's just not a universal absolute.
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F1 is a multi-billion Euro / Dollar entertainment enterprise. It's a culture unto itself. Every trick in the book will be used to cheat within the rule book, to pull off a win. They're not going to leave any hypotheses untested. However that doesn't necessarily mean that there'll be any trickle-down technology useful in a passenger car. The tax code allows all expenses to be written off. And with millions in sponsor funding, it's skies the limit!