Trebuchet03's JettKamm (2000 VW Jetta)

[trebuchet03]

Originally Posted by Bicycle Bob

I'd try to copy the slope of the rear window on the Prius, more or less. The steeper and longer you can make it, the smaller the wake from the kamm back. Provided it is not so steep that you get separation, which will depend on how much turbulence is generated farther forward. It might be worth leaving the new rear window adjustable for a few runs to find the sweet spot. A kamm back is not a substitute for a long tapered tail, it is just a better compromise than a short taper or rounded tail.

I'm going to try two things...

First, I'm going to connect the roof line with the trailing edge of the trunk.
Second, follow the Prius curvature no matter what.

After that, I'm moving on to the 3D model... Computing time will be much higher, but I'll get results that are more reasonable.

[tasdrouille]

This thread is really interesting. Care to give a quick shot at just a small 45 degrees lip spoiler that would be as high as the boundary layer is thick (edit: on the baseline model) at the rear edge of the trunk?

[trebuchet03]

Originally Posted by tasdrouille

This thread is really interesting. Care to give a quick shot at just a small 45 degrees lip spoiler that would be as high as the boundary layer is thick at the rear edge of the trunk?

I'll put it in cue There's one or two more models that will be processed first.

That said, I'm open for similar requests I've got to computers at the moment, one has become the dedicated CFD monkey

[trebuchet03]

Promising Results


Baseline


RoofKamm


Taking measurements.... Separation, as I see it beginning) occurs about 5 inches back from the trailing edge of the kamm (measuring parallel to ground). BUT, at that point, the boundary layer is less than half an inch thick... At the trailing edge of the kamm, the BL is only a tad thicker than 1 inch.

Red indicates 55mph (or more) air flow, blue indicates 0mph (or less). My model is half scale, so double all measurements.




I'm somewhat pleased, so I'm running a refined version of this model.... Of course, always content - but never satisfied

[Cd]

I'm glad that you managed to open the 3D model !
It looks as if you have a lot of backwards facing polys and it is really faceted. Can you edit the model in FloWorks ?
What format are you having sucess with ?

Here is an image of the Bonneville Ford Fusion ( .21 Cd ) and its extended decklid as seen from above. Notice how that the sides of the spoiler 'pinch' inwards ?
I was wondering how much that this sort of thing helps to reduce the wake size.
Also, in your case a few vortex generators just below the roofline* on the back glass might be a good experiment to do ( * so as not to add frontal area )
I know VGs get "poo-pood" a lot, but I have seen at least one case where they made a noteworthy difference ( An article posted here linking to an Australian magazine that tested a 1st gen Prius with VGs )
I wonder if the wake size could be further reduced if the decklid extention had a downward slope to it ( it seemed to work pretty well on the Dodge ESX trunklid )

Personally, I like the LOOK of the Kamm-back.

[Cd]

I noticed that the bottom of the car also has 'fences' that 'pinch' the airflow.

[Deezler]

Cd, perhaps you meant to provide this link?:
AutoblogGreen
(doh, you were busy posting the photos)

The fact that this Fusion measured at 0.20 tells me that the same simple collection of aero mods should have great benefit on a Jetta also.

That is actually exactly what I am planning for my own VW Jetta. I hope to get it built this holiday season. But I'll start my own thread for discussion so as not to hi-jack this one.
p.s. I plan on mounting VGs on the roof, just for fun.

[Cd]

I'd like to see if lowering a Jetta could help as much as it did with the Fusion.
( see posted image above )
Glancing at the underside of a stock Fusion, it doesn't seems as if it would cause that much drag.

( Besides it just looks '*****n')

Edit : What ... you can't say the word for a female dog on this forum ?


O.K. It looks .... hmmm Bad donkey

[trebuchet03]

First time using Gimp



I aligned the top trailing edge of the trunk to the top rear trailing edge of the hatch. Very promising. I tried to scale them to be close....

I know VGs get "poo-pood" a lot, but I have seen at least one case where they made a noteworthy difference ( An article posted here linking to an Australian magazine that tested a 1st gen Prius with VGs )

Browser Warning There's also a paper published by Mitsubishi

VG placement is very far from arbitrary. Too far back, they won't do anything. That said, a VG is a band aid for design that isn't good in the first place - these aren't my words, these are the words of aerodynamicists I've worked with. Wake fill? Puh-lease

I haven't seen that angle I actually have been looking at the fusion




Interesting that deck lid gets no points.

[trebuchet03]

Rear end... Prius esque?



(ignore colors)



And before someone says it... I'm well aware that there are other aerodynamic features