Trebuchet03's JettKamm (2000 VW Jetta)

[trebuchet03]

I've had this on the back burner for awhile now. The key issue being, how do I mount this thing?

Well, I've had the idea breakthrough I've been waiting for, so I'm now moving on to the next phase of designing/building a kamm back for my Jetta



Design
The design will extend from my roof until the projected line extending from my trunk back. The sides will follow the side of my car - potentially pulling towards the center line of my car. My initial CAD model suggests that the top panel curvature should be progressive to keep an aesthetically pleasing shape (and it even matches a nice foil). I'm not completely against other design features, such as vents to allow some flow over the rear glass (to clear off dew and such).

I'll be using CFD to barometrically compare designs. Basically, run CFD and then watch flow trajectories and local fluid velocities. Each CFD run takes my machine ~12 hours, so this will take some time Hopefully, I can get meaningful results from this that will equate to a more optimized design.

Things to test

• Slope/Progressive Slope - try a variety
• Benefits/Drawbacks of vents to allow some flow over rear glass for morning dew purposes
• extending the trailing edge slightly further and
• Rounding off trailing edge?
• Zig Zag trailing edge (skeptical, but I'll follow the evidence here)
• Flow Fence(s) - to keep left/right flow from interacting
• Spoiler? (I'm curious)
• ??? Throw me some ideas

Mounting

How will I mount? The plan is to conformal mold the fiberglass sides to slip into my trunk. When the trunk closes, it should press the structure between the trunk lid and body of the car - I'll add rubber feet to fill in the gap and allow for a nice grip.

The top will use a material called VHB. VHB stands for "Very High Bond" and is used in the automotive industry (among many others) for things like attaching emblems, rub strips, etc. Transportation: Trailer, Truck Body & Emergency Vehicle, Bus & Rail, Marine, Aerospace. It's super strong; I have no doubt that it can handle the forces at highway speeds AND, with a little effort, can be removed (pdf).


Operation

The top leading edge will be mounted with VHB and will have a hinge mounted to it. The rest of the kamm will be mounted to the hinge. The hing will allow full use of the trunk which, when open is higher than the roof

Construction
The plan, as it stands now is to construct the kamm from cardboard paper (not corrugated), luan plywood (unless I can find corrugated plastic) and fiberglass. I'll need too find a stainless steel hinge which will be pop riveted on the "permanent side" and screwed on the removable side with thread bosses.

Potentially considering a Lexan section on the top panel to increase rear visibility.

Cost
As cheap as possible, of course I'm looking to spend about the same as Metro, circa $100.

Some pretty pictures from an invalid run (forgot to scale up, so fluid flow is actually something like 2mph instead of 25)







Thoughts, comments, etc.?

[Funny]

What program are you running in order to get the 2D wind tunnel information? Is it open source, and can I have it please?

Seriously, you are putting a tremendous amount of effort into theoretical models. I laud you on this, but there is a point when theory breaks down and one must make a physical representation of ones work. I can't wait to see a finished product.

[trebuchet03]

It's Floworks - a part of the Solidworks suite (yay College ) - it's actually doing a 3D analysis, but I'm restricting it to a thin area.

While it may seem I'm putting a lot of time, really, I'm setting up a model and hitting run before I go to bed. The model took me about 15 minutes to build off a photograph and then another 5 minutes to setup the CFD suite.

Theory vs. physical is a valid point... You can have a sound theory, but physical application doesn't work... But, to waste cycles on building something that doesn't work on the theoretical level is not something I'm interested in. Concept -> theory -> optimize -> compromise -> refine -> implement is more or less the linear flow I'm following. Besides, what's good enough to the auto mfr's, Lockheed, Siemens, Boeing, Airbus, Nasa, etc. etc. etc. is good enough for me - Analyze first, then build They didn't build the 787 before modeling and analyzing it

[MetroMPG]

Drat! I was planning to mock up a cardboard Kammback on my dad's Corolla and ABA test it on the road this fall, but I ran out of time. I was hoping it would encourage someone in the sedan fleet to take the plunge.

But, yay! You're taking the plunge.

Thoughts:

1) That black & white picture is by far the coolest;

2) You shouldn't have dew problems to solve on the OEM rear glass if it's shaded by the Kammback;

3) Definitely consider lexan/clear for part of the top portion if the horizon as viewed through your inside rearview mirror is higher than the lowest point of the Kammback;

4) Don't leave anything on the table by just following the existing side curvature; I'd change "potential" plan taper to "definite" That will put "shoulders" in the design, complicating construction, aesthetics & clearance for opening the trunk, but it's likely worth doing.

5) I'm just WAGging, but I'd say fences would be less effective than just going with a very generous radius at the top/side transition.

Are you planning to enclose the transom, or leave it open?

[COcyclist]

Your design looks like the Kammback profile I want to add over the hatch of my TDI Golf. As for vents and dew, in my experience dew and frost mostly settles downward onto the car. I am guessing that covering the back glass from above will prevent dew from forming most of the time. Last week I had 2 cars parked outside, one under canvas carport with no sides and one right behind it but not sheltered. On a cold, humid night the one under the shelter had clear windows but I had to scrape and scrape the other.

I can't wait to see how this progresses.

[Tango Charlie]

Very cool. I think you would be better off NOT rounding any trailing edges. GM has reduced drag on the Volt by putting a sharp edge around the periphery of the trunk. See here.
I'm talking about the very trailing edge, not the 'shoulders' like MetroMPG was addressing. I agree with him that a radius 'shoulder' would be beneficial.

[trebuchet03]

Originally Posted by MetroMPG

Drat! I was planning to mock up a cardboard Kammback on my dad's Corolla and ABA test it on the road this fall, but I ran out of time. I was hoping it would encourage someone in the sedan fleet to take the plunge.

But, yay! You're taking the plunge.

Thoughts:

1) That black & white picture is by far the coolest;

2) You shouldn't have dew problems to solve on the OEM rear glass if it's shaded by the Kammback;

3) Definitely consider lexan/clear for part of the top portion if the horizon as viewed through your inside rearview mirror is higher than the lowest point of the Kammback;

4) Don't leave anything on the table by just following the existing side curvature; I'd change "potential" plan taper to "definite" That will put "shoulders" in the design, complicating construction, aesthetics & clearance for opening the trunk, but it's likely worth doing.

5) I'm just WAGging, but I'd say fences would be less effective than just going with a very generous radius at the top/side transition.

Are you planning to enclose the transom, or leave it open?

1. I'm not exactly sure what I'm looking at when I use that color profile, I assume it's something akin to pressure waves (but that profile is showing velocity)

2. Good point - another potential benefit worth testing is if I keep the back open, will having some flow in there increase pressure inside the kamm

3. I think it will look cool too Plus keep any zealous officers from trying to claim it's not possible to see

4. The whole trunk thing is the biggest concern.... I'll put some thought into it and see what the ramifications of shoulders on the trunk. I'll also need to work on a better model to get something worthwhile. Maybe I can take a picture of the top of my car from the roof of my house

5. Not sure - it'd be an afterthought in any case. I can always make 'em, tape 'em and then test I was primarily curious of one single fence down the centerline as seen in some cars from the 30's and VW 1L (it might not do anything for me given the curvature of the 1L's body where the fence was put in place).




Open or Closed.... That is the question I just finished my next model, which is open. I've also optimized my model a bit, so hopefully I can cut down on computing time. I've added a rolling road, some random gaps under the car and modeled only the profile I'm interested in (center line - so no wheel arches).

Open: As you no doubt know - easier to make - cheaper too
Closed: Arguable better aero - causes complications for opening trunk (more hinges, methods of sealing, etc.)

I forgot to mention, one of my design criteria is that I can continue to use my bike rack... It's used rather frequently (at least once per week)

[trebuchet03]

Originally Posted by Tango Charlie

Very cool. I think you would be better off NOT rounding any trailing edges. GM has reduced drag on the Volt by putting a sharp edge around the periphery of the trunk. See here.
I'm talking about the very trailing edge, not the 'shoulders' like MetroMPG was addressing. I agree with him that a radius 'shoulder' would be beneficial.

I'll run the straight termination first.... I'm still going to run the curved edge, if even purely academic, I think it would be interesting to see

I'm also considering straightening out the last two inches or so - to make flow parallel to the ground... Lift considerations and all

[MetroMPG]

I thought that feature on the 1L was intended more as a spoiler for minimizing yaw in cross wind situations, rather than for segregating flow in "normal" circumstances. Just a guess though.

Good idea taking the bike rack into consideration. Obviously, I can't use my hatchbackrack any more. But I can still stuff my bike inside easily enough.

[tasdrouille]

I've had this idea in the back of my mind for a while too. I even had formed sheet metal parts that bolted into the holes used to receive the clips holding the carpet under the trunk lid. It was tricky to get the shape right and there's not much of a gap there. It made very solid anchor points once the lid was closed though. I used the brackets to test an extension and planned a kammback (would have been a single lexan sheet on an alu frame), but never go around to building it as I could not find an easy solution to mount a hinge at the roof line without drilling the car.