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Old 11-24-2009, 05:46 PM   #129 (permalink)
MetroMPG
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Location: 1000 Islands, Ontario, Canada
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Blackfly - '98 Geo Metro
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Tuft testing round 1: boat tail bottom, no undertray

I set up a 5 x 5 grid of tufts, minus 2 directly behind the exhaust. I got some video as well, which I'll post eventually.

First, below, here's how I dealt with the exhaust (2 layers heavy tin foil):



Below: 5 rows of 5 tufts, except in 2 rows behind the muffler:



Below: web cam set up, looking forward. (Not shown: I also attached it to the driver's side mud flap to capture video looking rearward.)



Observations:
  • obvious separation behind the wheels (no surprise there!)
  • turbulence everywhere else, but not reversed flow. It seems more turbulent toward the outsides (effect of the wheels and control arms), less worse toward the centerline
  • Correction: alternating between "sometimes separated / sometimes turbulent" for the two tufts closest to the control arms
I lent my Hucho to Ben, so I'm having a little trouble remembering my definitions. I believe "separation" is characterized by reverse flow (eg. behind the wheels).

The flow down the middle 3 of 5 columns (parallel to centerline) appears turbulent, but there does not appear to be any reverse flow, so I don't think it's separated.

That's a good sign for 2 reasons: first, I believe it means the bottom angle isn't too steep. And next, it suggests that adding the full belly pan will help reduce turbulence & minimize wake off the rear. (If there had been separation due to the angle, the belly pan wouldn't have helped.)

The outermost tufts in the last 3 rows (inline with, but furthest away from the wheels) are also in turbulence, but not separated flow.

Next in the boat tail evaluation: I'm going to reinstall the full belly pan and re-tuft test the underside.
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