Quote:
Originally Posted by aerohead
Jim,I think it's just a scale relationship,or 'verisimilitude' as I think Don Sherman referred to it.
If the typical full-scale auto length/velocity @ 20 mph establishes the proper Reynolds number then,
*1/2-scale requires 40-mph
*1/4-scale 80-mph
*1/8-scale 160-mph
*1/16-scale 320-mph
*1/24-scale 420-mph
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Lower air velocities can be used and a 'fudge-factor' arrived at.
When GM uses the Guggenheim tunnel at Cal Tech the Cds come out as much as 30% lower than what will happen in full-scale.
As long as a relationship is determined between the model test and full-scale results,a 'confidence' can be arrived at when predicting full-scale numbers.
Paul Jaray wrote about this issue back in the 20s.He used 1/10-scale models.
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Without the correction factor you could end up with disappointing Cds as did Ledwinka with the Tatra,Tremulis with the Tucker,Breer with the Airflow,others I'm sure.  |
Thank you very much, Phil. My friend and I were thinking about testing his 1/4 scale clay model, and this information is very useful. I have a couple quick questions, though.
First, what size should the wind tunnel be? I imagine building one for a quarter scale model may be challenging if we can't find a dedicated space for it. Scaling that cardboard one up by a factor of six could take up an entire room. (The school has the space, but perhaps the CCS administration wouldn't appreciate that
) We would probably have to make it out of MDF or some kind of particle board.
Second, and this one is for onefstek too, how does one make the smoke and get it in that neat stream? I've seen the needle like tools they use in the full scale tunnels but don't really know where they get them or what the smoke is from.
Edit: does anyone know how fast box fans run? Edit edit:
2-8mph. I assume to speed the air up to 40mph you'd need at least a 3x3 array of fans on the back end of the tunnel, with quite the funnel on the other end. Perhaps there's a better solution?
Blah, questions keep popping into my head. I suppose it's pointless unless one can't record drag, but how does one figure out the "fudge factor"? All the stuff I'd like to try is either over my head or beyond my resources!
Thanks! Tyler