Trebuchet03's JettKamm (2000 VW Jetta)

[Deezler]

Most advice I've seen for VG placement on a passenger car is for them to be about 100 mm in front of the start of the major rear angle. So on a Jetta, I was thinking right in front of the antenna. This is quite a bit further forward than at the glass line (or on the glass for that matter). I think the VG effect takes a bit of distance to occur, perhaps depending on vehicle speed....?

The deck lid extension results thus far don't look great. But I'll stay optimistic for now.

[trebuchet03]

Originally Posted by Deezler

Most advice I've seen for VG placement on a passenger car is for them to be about 100 mm in front of the start of the major rear angle.

That advice comes from ignorant users.... I don't mean the word ignorant as an insult, however

If you're going to re-energize the boundary layer - it needs to be placed in the transitional zone. Essentially, where the BL starts becoming too thick, on the verge of being called turbulent. Where is this zone, physically? You're not going to know by just looking at a car The only way to know is by either physically testing, which is expensive and time consuming... Or analytically testing (cfd).

The reason they tell you 100mm in front is because that's what Mitsubishi did - read up here: http://www.mitsubishi-motors.com/cor...004/16E_03.pdf - it's a pdf.

From the paper






If your car is a Mtisubishi Lancer - 100mm is correct.... Otherwise, it depends on the car, where flow starts to separate etc.

For example - the Fusion 999.... I don't necessarily see why it should be added....

[trebuchet03]

As requested - a streamlined shape

The length is approximately 100 inches.... Tunnel velocity was 55mph.

The shape is a NACA 0012 symmetric foil

[tasdrouille]

Maybe you could try something like this in place of the decklid extension.

Green Car Congress: Novel Spoiler Design Reduces Fuel Consumption for Minivans, SUVs

[trebuchet03]

Originally Posted by tasdrouille

Maybe you could try something like this in place of the decklid extension.

Green Car Congress: Novel Spoiler Design Reduces Fuel Consumption for Minivans, SUVs

I must admit that I'm wasn't optimistic going into this - but keeping an open mind, I did it anyway.





Is it helping? I can't say... I bet the shape is highly referenced to the shape of the vehicle.... I haven't checked to see if I have access to the journal, so I haven't read the paper yet...


So, I'm tired of not being able to compare these things any way but subjectively by visualization.... So I'm changing modes. For now, I'm going to continue with the 2D models - but now run them as 2D in floworks. I'm adding a total force goal and am going to record total force... Maybe, I'll add a cD equation in there (purely academic, but seems to be a good metric that is understood by many ).

I'll work on compiling a table of models and the corresponding forces (max, average and minimum).

[Deezler]

Originally Posted by trebuchet03

That advice comes from ignorant users.... I don't mean the word ignorant as an insult, however

If your car is a Mtisubishi Lancer - 100mm is correct.... Otherwise, it depends on the car, where flow starts to separate etc.

For example - the Fusion 999.... I don't necessarily see why it should be added....

Ok, well, yeah....
Without the tools that you have here one can only make an educated guess. The Lancer has a very similar body style to a Jetta, so I don't see why this would be too much different. But you are absolutely right, it depends on the car and where the flow begins to separate. So, hopefully we learn it from your simulations! Perhaps you can run a couple trials to see which placement best sends some flow down the rear glass. What do you place in the model shape to simulate this "trip"? Is it just a shape input, or does the software know its meant to trip the flow?

Did you mention that the fusion has a 20 deg. rear glass angle? That is still quite a bit more than the supposed 12 degree maximum thats so often quoted on this site. If the VGs can trip the flow to help minimize wake, they may benefit drag more than their own frontal area and turbulence negate.....?

[tasdrouille]

I am really looking forward to the total force results of the models tested so far.

[trebuchet03]

Originally Posted by Deezler

Ok, well, yeah....
Without the tools that you have here one can only make an educated guess. The Lancer has a very similar body style to a Jetta, so I don't see why this would be too much different. But you are absolutely right, it depends on the car and where the flow begins to separate. So, hopefully we learn it from your simulations! Perhaps you can run a couple trials to see which placement best sends some flow down the rear glass. What do you place in the model shape to simulate this "trip"? Is it just a shape input, or does the software know its meant to trip the flow?

Did you mention that the fusion has a 20 deg. rear glass angle? That is still quite a bit more than the supposed 12 degree maximum thats so often quoted on this site. If the VGs can trip the flow to help minimize wake, they may benefit drag more than their own frontal area and turbulence negate.....?

I disagree that the lancer is similar.... The regions we're trying to minimize (turbulent zones) are chaotic (not random, chaotic) - which means small changes in input may result in large changes in output. 20 versus 19 degrees can make a large difference. 20 degrees may be more than optimal - but that doesn't change weather or not it's better than 25 degrees. That said, the lancer appears to have a much steeper angle than the Jetta (without measuring, just looking).

Optimally we want to taper down as fast as possible without separating flow. This results in less surface area (skin friction) with all the benefits of attached flow Ideally, skin friction should be higher than pressure losses (flow detachment, induced vortecies, etc.). What that angle is somewhat depends on application. As mentioned by aerohead, height off the ground will play a role - in addition to the similar length/height ratio, angle of attack, etc.

For leisure reading
Race Car Aerodynamics - Designing for Speed *-* Bentley Publishers - Automotive Books and Repair Manuals Particularly this tidbit which will turn a lot of what you know on it's head

As the slant angle is increased from zero, a positive lift will develop, which increases up to ? = 30°. At slant angles larger than 10° the rearward projection of this negative pressure causes quite a large increase in drag, as shown in this Figure. The most interesting feature of this data is that above a critical angle (close to ? = 30°) the vortex structure breaks down and the drag and lift contribution of the slanted surface is much smaller. This fact has an effect on hatchback automobile design, where rear window inclination angle should be more than 35° or less than, say, 25°. Also, note that in this case, the basic body (with ? = 0°) has negative lift due to ground effect, similar to the case with the ellipsoid, shown in Fig. 2.22.

Yes, it said the drag for angles greater than 30 degrees actually goes down.... Explanation of this phenomena I will leave for another time (I'm not sure how to explain without introducing a fair bit of jargon). If someone else wants to explain (nudge nudge, aerohead ) - have at it BUT, I will say that this phenomena explains why "clean" separation for hatchbacks is effective (hence small lip spoilers and protrusions)

Here's the related image



The book (Race Car Aerodynamics by Katz) is a great read - I highly recommend it for aerophiles Not everything is directly applicable, so keep your scope (car traveling road speeds) in mind.

[modmonster]

i built my first kammback! im really pleased with it. i'll post pictures and more later when im home.

[Deezler]

Treb, thanks. Good Info. I will not deny that I'm a total aero newb.

But intuitively your post makes total sense. We either want to keep the flow attached as best we can, or break it cleanly. The rear angle defines this situation dominantly.

You seem to imply that the Lancer, with its steeper angle, relies more on the latter. Being to steep for flow attachment, they were going for a clean break, so to speak. However, they apply VG's to trip flow down the rear glass! So my logic is this: If you have any flow detachment, and you can positively offset this with a simple trick like VG's, why not? I would think your ability to run simulations to examine this could be a perfect test of this idea.
The Jetta angle is less, but from your own simulations we still see that the speed of air decreases as you move down the rear glass. Perhaps the Lancer angle being more severe made a trick like this necessary where it could have little benefit on a Jetta.

While its getting hard to keep track off all the pretty pictures thus far, a simple goal, in my mind, could be this: Minimize the total amount of blue behind the vehicle, and replace the green on the trunk lid with yellow.
I don't know the scales on the Mitsu pics as compared to yours, but they seem to have done just that with the application of the VG's.

Now, regarding the deck lid extensions for the Jetta. It seems clear that the flat lid is not helping. Only extending the wake, and keeping it larger at that. However, the angled extension (same as trunk angle) looks to have a similar wake to the stock vehicle. I have my fingers crossed that VG's (or flow trip, whatever the jargon is actually supposed to be called) when placed in the right spot can bring down some yellow and perhaps replace all the green on the trunk lid and down the extension. This would lessen the wake size by far! (I hope).

I don't mean to trivialize this investigation into a simple color analysis, and I second the anticipation by tas for the force results!

Whats up with the fusion model? Can you normalize that one to those done for the Jetta (i.e. same background flow color)? It seems to have perfect flow over the entire vehicle. Regarding the Ford aero points slide, perhaps they just really simplified things. They must have also done an undertray, and that is also not listed as any benefit. Nor are the moon discs on the wheels. We know these two things would help... but they're not on the list either. My hope is that anything unlisted was just too small an aero gain to make the top 5 list on that slide. (wait, the things listed add up to the total aero gain, doh). hmmm...