Rounded corners vs. Straight edges on kammback

[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.

On the sides and top of the Kammback, there will be no vortex created if the air was perfectly proportioned between sides and back. However, we all agree the odds of that are about zero.

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.

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.

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.

[euromodder]

Originally Posted by Varn

Take a good look at the Prius, almost all folded corners. Is it ideal? Probably not, but it has been tested and it has low drag.

But you'd need a windtunnel to get it right.

The simple aerodynamics of yesteryear can be more easily applied and have good effects.

[ChazInMT]

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.

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.

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

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".

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.

[aerohead]

Originally Posted by 2000neon

I am really considering a much more dramatic, full kammback for my Neon. Allowing the sides to taper in as much as possible without interfering with opening the trunk. Also, I am considering making a hinge system on the top, to allow me to fold it back and use the trunk. My issue is with building rounded corners on the top-to-side transition.

I know that rounded corners are preferred aerodynamically speaking, ease of construction is another story. To the best of my understanding, this is because with just a straight, creased edge, the airflow "trips" over the edge, causing vortices which will increase drag and severely limit the drag reduction potential, am I correct?

So how drastic of a rounded corner does it need to be? I assume the more rounded the better, but is there a known minimum amount of curve that is required?

Finally, I know people can compare the amount of roundness by talking about the size of it, for example a 3/4" radius. So does that mean that if the curve was a whole circle, it would be 1.5" in diameter? Is that sufficient for avoiding air tripping over the edge? I am just trying to get my head around what kind of curve I need, to figure out the best way to build it.

Thanks for any input, you guys have already helped me out so much in building my Neon to where it is, and I appreciate it. This site is great!

I think that I'm going to need some help with respect to "full".Will it extend to the original length of the car,converting from a notch back to K-form roof,or are you planning to extend it beyond the bumper,combined with boat-tailing to go for really low drag?
If you maintain factory length you could mimic the architecture Chrysler did (as GM has done with today's VOLT),maintaining the same roof section curvature and greenhouse tumblehome,intersecting with an identical crease as on the standard car.
If you go longer,you could gently increase the roof/greenhouse edge radii as you progress back.
The 'Template' curvature is meant to prevent radical pressure variations which lead to the bleeding into the low pressure area,which creates the viscous shearing forces which sets up the nasty attached longitudinal vortices.
Over at the full-boat-tail trailer thread there are some pictorial drag tables.One of them demonstrates the difference edge radii can play with respect to drag coefficient.(It will show a bus of different configurations,with corresponding Cd).
The really low drag cars are circular,or semicircular when viewed from behind (this is excepting solar race cars with their PV arrays).

[freebeard]

While it's entertaining to watch people come at each other on Internet, I have been wondering about why nobody invokes the Coanda Effect, named by Theodore von Kármán after Henri 'my tails on fire' Coanda: Coand

Maybe it's because it can't be invoked passively; the main aero example being blown flaps. If only there was a car with handy supply of hot, pumped engine cooling air in the tail.

[freebeard]

Sorry for using 'invoked' in two different senses in a single post.

As penance I will refute myself: There is such a car, the Renault R8.



I thought the cooling air exhaust created an invisible spoiler that flipped air downward with a vortex. But as the vented air probably bleeds straight back, the reported gains [citation needed] might be due to the Coanda effect.

[drmiller100]

great study, and totally supports my argument on the angles.
if you look at page 75, they ran a CFD with 18.77 degrees, and a hard cut angle at the front, and hard angles at the sides, and the
FINAL RESULT IS EXACTLY AS I PREDICTED.
The minimum possible drag with no vortices.

Regards,
Douglas Robert Miller

[COcyclist]

Originally Posted by freebeard

I have been wondering about why nobody invokes the Coanda Effect, named by Theodore von Kármán after Henri 'my tails on fire' Coanda: Coand

Hijack?? I believe this has been discussed? This thread is about rounded corners on a Kamm tail.

[2000neon]

Thank you all very much for your input, it is all greatly appreciated. If it helps, my name is Steve, it might be easier than "2000neon". You are definitely right that when (not if) I build this, I want to aim for as close to perfect as my skills and materials will allow for, so that's exactly why I have been doing a ton of reading on 3 wheelers tail extension, sven's probe, the aerocivic, weather spotter's tail, the new aerofocus, the cargaero trailer, and of course as much reading as I can on everything from Phil, your truck, and you yourself are a huge asset and inspiration. I'm sure that I am still leaving some people out too.

If you look at my other thread (which I just realized that I have to update), I have already gotten most of the simple mods done to this car already. I am now also planning on bringing the rear wheel skirts farther down to cover more of the rear tire, but that is for a different time.

I will try to address as many questions as I can.

Aerohead: What I mean by "full", is that I will be aiming to build it to the existing length of the car. I am debating whether or not to end it flush with the trunk lid, or take advantage of the extra few inches I can gain rearward of the trunklid, out to where the bumper is. I called it a full kammback because I have already done a small kammback, just over the top 2/3rds of the window. Although the side transitions were too sharp and definitely limiting my potential gains. Although I did have some small gains with it. This will be it's big brother version, and hopefully fight the itch to build a boattail (for now).

Based on a lot of the reading from other's build threads, I am going to try and aim for 12-15 degrees on the top. I am going to try and maintain access to my trunk, so the side taper will be limited to how far I cam bring it in without blocking the trunk lid. I'm going to try to keep everything as smooth as I can to prevent any significant pressure variations like you mentioned.

ChazInMt: I really appreciate the input, and the nice words. Same to you Dr Miller, and Sven and everyone else. It is actually incredibly helpful and motivating to have the support and input when going about something like this, especially detail stuff like construction and angles.

[2000neon]

As I was saying before, the build plan I am aiming for will be an aluminum bar structure, mounted longitudinally, with 2, 3 or 4 vertical ribs, with some perpendicular bars attaching them to eachother. The exact number and placement will be determined when I can get a cardboard mockup built on the car. I am hoping to be at least that far by the weekend. I usually average between 55-62 hours of work a week, plus time for my girlfriend, so free time is hard to come by.

When I first started building my Neon I was very hesitant, even a grill block had me worried. Now that I have slowly become desensitized to even some of the more drastic modifications, I am actually really looking forward to taking this next step. It has gone from simple money savings, to a whole lot of fun and a great way to let my creativity come to life. I can't give enough thanks to everyone here for your build threads, discussions and input you're giving me.