HMS Beagle wrote:
bjbear wrote:
At this point, I am thinking that dampening the Bernoulli effect might have more of an effect than the displacement of air to the sides (like your barge analogy). Although displacing the air sideways also reduces the air volume and therefore velocity!
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I am going to work this some more, but for now, I will hold off on installing my air tabs until I understand the physics a little better. :? Where is Sheldon Cooper when I need him!! :D
You might want to actually test that hypothesis. It would not be that difficult, using an Air
speed Indicator and a primitive pitot. Lacking an ASI, you could make one (manometer type) out of a clear plastic tube and some water. The dynamic pressure at 55 mph is around 7.8 psi, at 60 it is 9.2, a 1.4 lb difference. That would give you around 2.3 feet difference in your water column - pretty easy to see. The interior of the cab is probably a good enough static source.
If you are right, then the air tabs might be working just by slowing the air. Adding a lot of vorticity is going to make the boundary layer very thick, once it is half the thickness of the gap then the hypothesized jet of air ceases - or at least is substantially reduced.
Yesterday I run a couple of quick, rough, and dirty tests with manometer, to check vacuum/pressure between truck rear window and camper gap, and ahead of deflector.
Test 1: I had manometer in cab of truck with one end of manometer out the rear slider approx. one foot, rear slider was closed except 1/4 in. I saw no changes in pressure between 0 to 70 mph. I opened side truck window at 60 mph and saw a .4 in. water column vacuum in truck cab.
Test 2: Check pressure ahead of air deflector approx. 4" ahead of leading edge of deflector V. Again manometer indicator in cab of truck windows rolled up. At 60 mph there was approx. 1" water column of pressure, at 70 mph approx. 1.2" of water column.
My manometer is a simple water filled hose strapped to a 2x10" board, very basic. This was a spur of the moment test and if anyone has a idea for a good/better way of performing test, let me know.
I was surprised that in test 2 that I didn't have higher pressures as I placed the hose end pointing into the wind on one run then turned around 180 degrees. On both runs I only saw approx. 1" water column. It was not what I was expecting, I thought air would be traveling up the window and then hitting the deflector with high velocity, this may not be happening. Perhaps there is a large bubble air ahead of deflector (not really a deflector haha). I have noticed that I don't even get a lot of bugs on the windshield. I hadn't associated this to the air deflector, but will need to rethink. I'm going to try to move the tube around and see where laminar air flow returns.
Here is a picture of my deflector.
When temps come down a little I may also paste a number of short strings to the TC in order understand the air flow better as described in the article.
http://www.autospeed.com/cms/A_3058/article.html
