Airfoil, other "V" wind deflectors, and one lead duck

While I think that turbulance is a source of noise and
I think that laminar (smooth) flow usually eliminates noise....

We also have Noise that begins with laminair flow, i.e., laminar flow over windows, antennas, mirrors, bumpper guards, camper jacks... which causes turbulance... some noise and reduced fuel economy.

06Fargo wrote:
So with today's modern fabrics and materials, I wonder if a guy could resurrect the old whoopee cushion style inflatable donut roof cushion people used to put between the cab roof and the camper overhead, and enclose it in a soft fabic that won't scratch... and have a blocker?

I don't think it's the donut that does the scratching.... it is just the sanding pad.

Dirt is the grit.... that can polish, or grind... or even cut.

You can actually cut case hardened steel with string and dirt.

And road dirt has a lot of rock dust, metals, etc.

sleepy

So with today's modern fabrics and materials, I wonder if a guy could resurrect the old whoopee cushion style inflatable donut roof cushion people used to put between the cab roof and the camper overhead, and enclose it in a soft fabic that won't scratch... and have a blocker?

HMS Beagle wrote:

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

Thanks HMS Beagle for the great idea's. I may have something wrong then, I used a 3/8 clear plastic hose that was laying around, and has likely been used for siphoning gas, antifreeze yada,yada. I hadn't thought about the surface tension in the tube.

First thing this morning I went out with the TC and stuck the hose straight into the wind a foot ahead of the mirror and only got 2" WC at 55 mph. I will try some new tubing and a little alcohol, metal tube as suggested and see if I can get accurate data. If not I will look around for better test equip..

bcbigfoot... you might want to borrow some instruments from an airconditioning tech.... 20 minutes on the road and you'd have your info.

I have been using Dwyer Magnehelic Gauges
for years. They will measure positive, negative or differential air pressures.

I even have one measuring the air through my furnace filter... one port to the room side of the filter... the other inside the duct. As the filter plugs the pressure change shows....I have the point to change the filter marked on the face of the magnehelic.

They come in quite a few ranges...

I have a box full of them left over from past experiments...

Janet isn't going to like having them mounted on the dash of our truck. :M

I'll have to see if my iPhone has an app...

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

So door dings aid in mileage? If proven, I will have to start parking closer to others.

I can just see the bumper stickers now:

"Ding me, I need the mileage!"

Bcbigfoot I think that your windsheild is an intregal part of your wind deflector... it is at the optimal 45 degrees (like a talus cone... any more verticle angle the wind pressure will build up... and if the windheild is any more horzontal the wind will accelerate into the gap between the Cab and cabover.

Here is the picture of your truck and deflector.



BCbigfoot...As far as noise... I think that your 45 degree deflector would function better if its most forward point was at the top of the trucks windsheild. Looking at the bernoulli effect below you can see that a large part of your trucks roof is in the highest pressure area from the accelerated wind and from the very high speed compressed air.


BJBear I am using some parts of your excellent post:

Your thoughts on the Bernoulli effect are extremely important.

Especially when you asked me: Have you considered the Bernoulli effect resulting from the alignment of the cab and camper overhang??

You made me try to explain it in a way that makes sense and your illustrations are essential.



you go on to say:

I am thinking that this effect will greatly intensify the force exerted on the camper bulkhead behind the cab. Probably several times higher that that exerted on the front of the camper overhang and truck itself. This is a result of the dynamic pressure (kinetic energy) which is a function of velocity squared.

Is it possible that the air foil works in part more by slowing the volume of air between the cab and camper overhang and therefore reducing the velocity and thereby the dynamic pressure? This should enhance the improved fuel economy being experienced.

and you commented... 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!

I recently watched a episode of Myth Busters dedicated to hypermiling. They apply a skin to the surface of car much like a golf ball. I found this product by doing a search, sound interesting. http://www.treehugger.com/cars/biomimicry-shark-inspired-skin-for-cars-claims-to-improve-mpg.html

subconsulting: Steve...I've alwas wanted to do something to cover all of the non-aerodynamic clutter on my vamper roof. I like the looks of the trucks in your link.

Now... I'd have to make mine out of a bunch of "bowed" PV Solar panels... because I couldn't live without the ones I have and wouldn't cover them.



06Fargo: I have to ask Janet...I hear very little... I am stone deaf in the right and have less tha 60% in my left (workplace explosion in 1982)

She says that there are infrequent times that she hears wind noise on the the forward door(s)(none on the rear doors)...

Out top speed is less than 62 mph... usually we loaf along at 58 mph on cruise control. We avoid the interstate highways as much as possible.

Please ask for more... Janets hearing is perfect.And like most good wives her memory is long.

sleepy