Trebuchet03's JettKamm (2000 VW Jetta)

[olmon]

I just registered and haven't perused all the topics, but in the reading I have done, I have noticed a total lack of reference to two aero tricks I have used since the 70s on various vehicles for both performance and economy gains. Vortec generators and ducting. Vortec generators are very easily made & installed and, in my experience, quite effective . Ducting is a bit more work, but is even more effective for relieving hi-pressure areas & moving the air to low-pressure areas. A combination of these two tricks can make a blatantly un-aero vehicle more slippery then anyone would believe it could be. On a 1990 Mitsubishi Montero, using small vortec generators on the front edges of the hood & frt fenders, Windshield posts and at the rear of the roof & r/quarters, I was able to consistently obtain 19/20MPG on cross country trips between Michigan & southern California running at speeds up to 100 mph & rarely less then 75. I was also pulling a small trailer at the time.

[aerohead]

Olmon, the Montero's definitely got that 'brick' thing going on,but Mitsubishi,as an aerospace corporation also,has probably optimized the front end just enough to get attached flow,so I'm a bit suspicious of the role of the turbulators.
With respect to the trailer though,I suspect that it is helping to fill and enhance the enormous wake behind the Montero, and it's highly likely that she wouldn't do as well WITHOUT it.
Some of the early pioneering work involving full-tail cars demonstrate that even mediocre filling of the wake can reduce drag and it's my opinion that this is what is happening with your rig.

[jimprince]

I would be very curious how the fiero notch back would do in a similar model. Does the rear wing/spoiler help or would an extension ot the roof line be better?

[Pendragon]

I am late to this discussion so please forgive me if I overlap some things.

There are several things that would be interesting to run through the CFD program. Number on would be an undertray and rear diffuser to a baseline model to see what difference it makes and then to the optimized upper surface model.

My observation about the use of vortex generators on aircraft is that they are positioned at a point near to or slightly forward of the point on a wing where laminar flow separates. The diagram I saw in the thread had them positioned too far aft I think. See what the CFD program says moving them forward incrementally until an optimal position is achieved. They can be added on to the actual car and tufts of yarn used to visualize the flow to see how well the full size model (the real car) compares to the CFD model.

I suspect that side fences along the fender line would have a larger than commonly expected effect upon the quality of the airflow along the sides of the car as well as allowing some potentially simpler mods to deal with the airflow over the top of the roof.

The shelf style 'spoiler' has been used in a number of applications which suggest that it might be worth exploring as a shorter (and therefore street legal) modification to a car than an extended boat tail or kamm tail which obscures the existing tail lights.

Additionally, it would be interesting to see what the computer has to say about modifying the airflow at the front of the vehicle with a 'bug deflector'. The reason they work to keep bugs off the windshield is that they deflect the airflow up over the main area of the windshield that the driver looks through. Obviously there may be some drag issues, but it could be interesting.

As to the Kamm back, the Cobra Daytona Coupe is an interesting example which was designed more by intuition than anything else and tested by shooting movie footage of yarn tufts placed all over it. Ultimately, a fairly simple spoiler was added at the rear to balance the car without much testing due to time constraints. You may also recall that the profile of the roof had the highest point farther aft than was or is today common to delay separation of laminar flow.

It should also be noted that the addition of a long tail could adversely affect vehicle stability in cross winds due to the movement of the center of lateral pressure to the rear. This should probably be run through the computer as well before beginning prototype construction.

This is a very interesting project you have undertaken and I am anxious to see how it develops.

Regards

[coolhand297]

Finally got to the last page of this and see nothing new in a long time as a jetta owner I am very interested.

[Pendragon]

Originally Posted by eeeehaw

I beg to differ. I am constantly building antennas for amateur radio, and I start with a modeling program (3 actually), because it saves a TON of time. Yes, I agree that modeling programs are imperfect, and that there is nearly always empirical "tweaking" of the finished design needed to get it "perfect". Meanwhile, I know others that are wasting money, time, and materials (read: feeding landfills & producing more toxins & CO2) experimenting with trial & error designs.

I think it's pretty cool that he's modeling on a computer first, ie starting with known science to predict outcomes. As my interest grows in aerodynamics, I'm now looking for free or inexpensive fluid modeling programs.

The F1 CFD design teams are able to test many aero variations and then do modeling of a few of the most promising ones in the wind tunnel. Prior to all the testing restrictions, the teams would then test some of them in full size at the track. Now much of the Friday time is devoted to testing that previously was done before a race. In any event, computer modeling is an advance which allows the consideration of more ideas in less time without having to fabricate a model.

[SwamiSalami]

so...I've looked at virtually every page on this thread and still no kammback. A lot of talk about aerodynamics that amounted to nothing, but no kammback.

[Bill in Houston]

Yeah, that was my summary too. Trebuchet is a brilliant and capable guy. I'm guessing life just got busy.

[SwamiSalami]

guess so

[aerohead]

Originally Posted by dieselbeetle

so...I've looked at virtually every page on this thread and still no kammback. A lot of talk about aerodynamics that amounted to nothing, but no kammback.

Remember,this Jetta is a 'K-form' already.It's simply at its original factory truncation point.
Yes,if it were to extend further back it would lower the Cd.