bcbigfoot wrote:
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.
You can buy a cheap ASI for about $135 from Aircraft Spruce, indicates 0-80 mph. This might be more convenient to use. To test your setup, try taping the open end of the tube into the wind as far out ahead of the truck as possible, in the free flow. I would expect the airflow anywhere close to the surfaces to be not all that fast, except maybe around the edges and corners where the streamlines compress. (Air itself is considered incompressible in low speed flow dynamics). A truck/camper is a very bumpy and irregular shape, there is going to be almost no "laminar flow" around it in the sense that term is used in aerodynamics. A large slow moving bubble dragged along with the truck seems like a good possibility. Bugs are dense compared to air and will tend to travel straight on when the air turns. But the point made above about the being seemingly more bugs at the camper bulkhead than anywhere else is an interesting observation.
For the open end of the tube, a pipe of perhaps a foot long might help with accuracy. A real pitot tube has a rounded end with a small hole in it - this is to reduce the errors that can occur due to the flow not being aligned perfectly with the pipe. Another possibility is that the truck cab is not a good static pressure source. For example if the flow through vents happen to be in a low (or high) pressure area of the flow, it will offset the readings by that amount.
As you know, 1 inch of water is almost no pressure difference, 1 psi is approximatly 2 ft. How large a tube was it? In water levels, sometimes a small amount of alcohol is added to reduce the surface tension in the water, to keep it from "sticking" in the tube. If you got the pitot out in the free flow, and the cab is a proper static source, 55 mph would give you about 15 ft!
I am quite interested to see some actual data taken....
"One accurate measurement is worth a thousand expert opinions" - Admiral Grace Hopper
