This is the idea I have for version 3 of my aerocap.
As far as I can determine within the limitations of Micro$oft Excel, there are no rearward flow angles that exceed 15 degrees, and most flow angles were designed to be around 12 degrees. That creasing you see in the second picture is an artifact of the 3D graph, and will not actually be a feature of the aerocap.
The idea behind this cap is to simultaneously transition the shape from the cab outline at the front of the cap to a semicircular outline at the rear, while dropping down from front to rear. Transitioning the shape appears to account for the vast majority of rearward angle changes.
I hope to see about a 13% or so improvement in FE over the baseline of no aerocap.
For comparison, here are versions 1 and 2.
Version 1 - destroyed - about 5% improvement in FE
Version 2 - currently in use - about 10% improvement in FE
Last edited by t vago; 03-19-2012 at 03:43 AM..
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Do you see any weird vortexes off the corner of the bed from the current version?
With the version 2 aerocap, I've seen numerous times a nice strong vortex out my driver's side mirror in the rain and snow. Obviously, the vortex is more easily seen in snow, but the rain also makes it plainly visible. I'm not as able to examine the passenger side mirror as well as I can with the driver's side mirror, but I can see the effects of a vortex there, too.
They appear to form because the straight rearward moving air on the bed sides of the truck are interacting with the angled air being made to move at an angle because of the sides of the bedcap.
Quote:
Originally Posted by Sven7
The new one looks very nice- a great compromise between aero and volume, while reducing vortexes and side wind resistance with that semicircle.
Thanks. I'm still not sure what to think about the fact that it's quite a bit larger than any other aerocap I've seen. Might take some getting used to.
Quote:
Originally Posted by Sven7
I've never thought about "modeling" in Excel. Is it difficult?
It can be. Certainly, measuring the cab rear shape was a challenge. A lot of computation is tedious, too, which is ideal for Excel.
I modeled the rear cab shape as a series of points with lines connecting them. The shape was then processed over and over and over again using an arc transform function that I coded into Excel that was combined with a trig function, and the result was again shrunk by another trig function.
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what is the "frontal" area at the back of the cap which is flat for versions 2 and versions 3?
I actually like version 2 better. My opinion - the minor vortexes created around the square edges are insignificant compared to the turbulence at the back of the car once the body ends.
I like it. Version 3 eliminates almost all concavities. Very close to a concept in geometry called convex hull. Rock on!
Heh. It seems like it'd be simple to fabricate, too. At least, compared to version 2. The ribs are going to be fun to cut out, but laying the skin ought to be a lot easier.
Quote:
Originally Posted by drmiller100
what is the "frontal" area at the back of the cap which is flat for versions 2 and versions 3?
It's calculated out to be about 4.8 square feet for the version 3 model, and about 2.1 square feet for version 2.
Quote:
Originally Posted by drmiller100
I actually like version 2 better. My opinion - the minor vortexes created around the square edges are insignificant compared to the turbulence at the back of the car once the body ends.
The streamlines for version 2 don't work so well. Currently, some of the air from the top is actually going over the sides of the cap, which is causing some of the vortex interference I'm seeing. The vortex effects cause my wake to be approximately twice as wide as my truck, which is not insignificant.
I've played around a bit with the design, and come up with the suggestion that it's not necessary to have the cap be a perfect arch at the rear. It can approach, say, 80% of an arch, still be round enough, still allow a maximum flow angle of no more than 15 degrees, and narrow the rear face area to around 3.5 square feet (correction from last night).
Last edited by t vago; 03-21-2012 at 02:37 PM..
Reason: corrected estimated rear face area - again! Don't drink and compute, folks.
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I've played around a bit with the design, and come up with the suggestion that it's not necessary to have the cap be a perfect arch at the rear. It can approach, say, 80% of an arch, still be round enough, still allow a maximum flow angle of no more than 15%, and narrow the rear face area to around 2 square feet.
I took a queue from your revision and made similar changes to my design that I did a while back. The rear fascia is larger than on the old one; the darker area on the back is the face from the previous design.
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