Tonight was extreemly humid and abnormally cold after the rain this afternoon. On the way home while driving down the highway I noticed a build up of moisture on the top and side of my bed through the side mirror. At first I said thats neet, but then I began to think why it would do that on part of the bed and not the rest. Aerodynamics was the answer to my question. I made sure to note in my head where the line was (at aprox 65-70). Heres the picture
(The red spot is where the humiudity was)

(dont mind the bed cover, its the only side pic i have)
After I got home I looked at the flow spilt over the edge of the bed, and tried to figgure out why it would spill there. The fenders on the inside of the bed direct the airflow upwards (light blue line indicates the side of the inside of the bed)

The GREEN arrow shows the air flow comming off of the roof. and building up between the front of the bed and the fender creating a high pressure vortex splitting the air comming off of the roof (purple arrow)(creating more drag).

With the pressure being built up at the front of the bed along the sides it will continue across the front of the bed from side to side creating a pocket of high pressure, but then drops into a valley in the center of the bed past the front of the fenders. This causes air to get trapped under the lip of the top of the bed making another high pressure area. (see diagram I made in the same picture, where the light blue is the high pressure area, the green arrow is the flow off of the roof, and the blue is the spill over the corners) Since the flow off of the roof was split by the high pressure pocket near the cab, the top portion of the flow cuts through the valley, and pushes the air thats stuck on the boundry layer, the pocket of air directly on the metal, up and in twards the center of the bed, but the pressure of the flow is greater than the displaced air slipping off so it is redirected out of the bed and down the sides (the reason that there was the humid spot on the side of my bed, the shape of the humid spot was determined by the speed at which I travled). I have a few ideas in mind to redirect airflow to reduce drag caused by certan components of the bed, as well as the drag caused by air pockets pressing against the truck, which I like to call second hand drag.

Tomorrow Ill be doing a full out test to determine if my theory on the air path is 100% accurate or not,though I am quite confident it is. (The test is taken from a test that aircraft designers use to determine when a wing of an aircraft is stalling, which is taking a bunch of streamers and attaching them in a grid pattern over the surface area of the tested area.) I will be taking a video of it so that its not just heresay or put to the test of the falicy of memory, so ill upload that when its done.

After I have the flow path determined I am going to start to create the flow redirectors, the first one being an elemination for the lip on the edge of the bed, and a second one to smooth the fenders into the bed, and a third one being a wedge or something of the like to eliminate the big pocket of air that builds up along the cab edge of the bed. The objective of the whole project is to redirect the flow so that the wind helps push (like when you try to pick up something wet that slips out of your finger tips) rather than holds back.Im going totry to make these redirectorsa quickly removable and installableprocess, so I can go to the track and test it run after run, so that I can keep the variables to a minimum.Not only willit help my track times, but I hope for better gas mileage if I decide tokeep these things in all the time.I guess you can call this a home made mythbusters.

 

Traditionally, little pieces of yarn are put on to the vehicle
and then a second vehicle drives along side
with someone making a movie with a video camera
showing the directions the yarn indicates the flows are moving.

http://autospeed.drive.com.au/cms/A_108656/article.html

note that there is a part II and part III to the above article.

There is a whole chapter on studying your own vehicle aero wise
in this book:

http://tinyurl.com/3xr88c

After getting the flows visualized in your mind
the 'bottom line' is to improve how easily
the truck slips through the wind.

You can test for this two different ways
either by coasting down a hill...this way avoids the 'flywheel effect' of energy stored up in your rotating wheels, driveshaft, transmission parts, etc.

The second way is to coast the truck down from about 60 mph on a level section of highway...hopefully without much wind, which is usually best around sun rise in the early morning.
This website has a little java application to help do the calculations:

http://tinyurl.com/3aompw

Improving the aero will make the truck accelerate better from 50 mph and up. With relatively simple changes this can be like adding 40+ horsepower to the engine...but what you are really doing is 'freeing up' 40 hp that had been used pushing against the wind and is now available to accelerate.

The best aero mods can improve MPG at 75 mph by at least 4.

Good luck with whatever experiments you try.

 

Thanks hank. You are always a welth of technical information that is infinately handy.

the yarn experiment is the experiment I am going to be conducting today.

 

I think you just made HankL's 'top ten favorite people of all time list'. Admirable experiment. Keep us posted.

It just so happens that I usually cruise at ~75mph on the highway. Was that 4mpg or 4%?

 

I am impressed that gas there is only $2.63 for regular

 

im going to postpone the test due to circumstances beyond my control, in part. I had two objectives for the day, neither were complete #1 install oil autometer oil pressure guage, and #2 the string exp. Well, if any of you have ever replaced the oil sending unit you may notice that there is a bolt hole in the manifold about 1" to the right of it that is the same size and thread count. My dumb self put the sending unit in there, and started it up to make sure there was no leaks. Obviously the oil came GUSHING out like from out of a hose. I spent most of the day cleaning up a good 3 quarts of oil from all over the street and my truck. I got it finished and not a second later it started pouring rain, so I buttoned it up and closed shop for the day. Both the guage and aero test will be completed hopefully monday when I have some free time.

 

Sorry to hear about that oil problem.

I guess Murphy's Law strikes once again.