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ChazInMT
Aero Deshi

Join Date: Jan 2010
Location: Vero Beach, FL
Posts: 1,063

MagMetalCivic - '04 Honda Civic Sedan EX
Last 3: 34.25 mpg (US)
Thanks: 430
Thanked 664 Times in 355 Posts
If you want to openly contradict me on a post in the future, be prepared for some major push back on my part.

So let's break down what you posted last.

Quote:
 Originally Posted by drmiller100 he is talking the tail of the car, right? so the pic of the van is misleading. we can argue about the transition from the body to the kammback, but suffice it to say if we are going from straight to 20 degrees, I'm willing to bet a 4 inch radius woudl be great, and if you had no radius, just a straight angle, you would lose a very small amount of efficiency.
We weren't even discussing the transition from roof to top of Kamm, you seem to be off track here. We were talking about where the sides of the Kamm itself meet with the top of the Kamm.

But while you're on the subject, going from straight to 20° would have dire consequences. I base this on the chart available to you here in this paper.

Drag Reduction of a Pickup using Add-On Devices

Look on page 85, what I see is that when you approach 20° you really lose the effectiveness of the addition and you start to generate a ton of lift. Is it still a net gain? Yes it is. Stuffing the wake is a net gain. But why go to 20° when for the same effort you could use 10°-12° and have better results? This is why I said what I did.

Feel free to back up your "Just put it in at 20° and it'll work great" statement, I'd love to see data on it.

Quote:
 Originally Posted by drmiller100 However, if the top and sides are both at 20 degrees to the centerline of the car, then there will not be any huge pressure differentials in the air. What there will be is minor pressure differentials.
How do you know this??? Without knowing what is upstream, you can't possibly make this statement. If the roof line was already substantially tapered to where it was at 10° prior to the added Kamm, but the sides are still flat, when you make the 20° all sides addition you speak of, there will be a serious low pressure area created on the sides with a higher pressure on the top. This will set the air spinning in a large undesirable vortex.

Anyway, your poo-pooing the vortex problem fails to recognize how significant it can be. You seem to think it a 5% impact on a bad day where you have created a monster vortex. Fact is it can be a 140% negative impact on a cars aerodynamics, and even a minor error would render an otherwise good design bad due to failing to consider this Vortex Generation. Radiusing the corner goes a long way towards reducing this effect.

Page with lots of Vortex discussion

Quote:
 Originally Posted by drmiller100 If there are minor "holes" in the air being turbulently attached to the car, a small vortex will help the air "fill the holes", just like a golf ball's dimples.
This statement here seems to illustrate that you shoot from the hip and talk in pseudoscience terms. What "Holes" do you even refer to? I mean seriously? Would you please show me something, somewhere, that backs up what you're saying here? It sounds to me like you've got vortex generator babble mixed up with parasitic vortex creation and now you think any vortex is a potentially good thing. Again, support with something other than "Cause I say so." or "Cause I've built a million vehicles, so I know what I'm talking about" or "Here's what the Vortex Generator Support Group claims".

Quote:
 Originally Posted by drmiller100 You and your strawman are of course welcome to disagree, but I again applaud the OP for DOING something and TRYING something where most of the rest of us are nitpicking his great efforts.
OK, here's the deal, go back and carefully read the first post written by the "OP" Let's call him 2000neon. You'll notice he is asking for advise and doesn't really have a preconceived notion of what he wants. So we gave him our best advise straight up, and he seems to appreciate it.

So far, it looks as though 2000neon likes what he hears in here.

Everyone here is proud of 2000neon for making an effort to do something. I for one am super heartened by the fact that he wants to get the best advice possible before he settles on a design, he wants his first effort to be a good one and not waste his time designing something which is not optimal. 2000neon is a frickin genius in my book.

The only "nitpicking", was against you. Until you educate yourself better and post things which are based on science, I think you can pretty much expect this is going to happen to you from time to time. I notice a number of people have spent a serious amount of time trying to discuss things with you rationally. I know I have been a bit terse here, but I just want to put you on notice, if you openly contradict what I say, you better have some facts and research to back it up.

Last edited by ChazInMT; 08-27-2012 at 03:12 PM.. Reason: Was 2 Mean B4

 The Following 3 Users Say Thank You to ChazInMT For This Useful Post: 2000neon (08-23-2012), Frank Lee (08-22-2012), serialk11r (08-22-2012)