Why does The Template have to be a half body of revolution?

[freebeard]

Edit: I may be misunderstanding your question. All three of the shapes you are proposing are "flat bottomed", right?

Yes. I put 'half body of revolution' in the title and showed full bodies in the illustration. In my defense I posted at 4 am. I realized at 2 am I wasn't going to sleep, so I got up and prepared the models. By 4 am, I was sleepy and didn't truncate them properly. I thought I scaled the square sectioned one in all three dimensions, but the length looks the same, so ??????

What I was getting at was if the frontal area is the same, and there is no air movement except out and then back into the center (i.e., no lateral movement that would lead to vortexes), would compound curves be necessary? Or would the simplified construction of the (half) square section carry an aerodynamic penalty?

What strikes me is that even though the superellipse is half way between the square and circle, it looks closer to the circle. That may be because I didn't subdivide the square section equivalently. There are a whole class of superelliptic curves, some would be very close to a square with radiused edges.

I understand that the underbody is a whole separate can of worms, until they re-merge at the rear.

dang you! I came on here to post my pics of my latest CFD testing.

ERTW -- Thanks for sharing. That is precisely on-topic. Consider it a preview and do your own separate post if you like.

Is the line at the bottom a ground plane or are they in free air? And if so, why do you think the stream-lines from the top cross the stream-lines from the bottom in the side view? Wasn't that a no-no in Ghostbusters?

Does anyone have a reference for the study done on a body shape with a pronounced arch to the underbody? I know I've seen it referenced here but I didn't save and can't find it now. My recollection is an Italian-sounding name in the 1960s. I wanted that for MTrenk's underbody thread, and the 'Aero concept car -- Bugatti Stratos' thread the mods spun out of that one. TIA

[freebeard]

Did someone say car shaped like a fish?

[Frank Lee]

http://ecomodder.com/forum/showthrea...-pdf-1028.html

pininfarina banana car

[NeilBlanchard]

My memory is that the computer model of the boxfish was a Cd of 0.06:



The blue quarter scale model has a Cd of 0.095 and the diesel powered concept car has a Cd of 0.19:




Edit: here's a series of screen captures I got from a video about the Boxfish, that show many more angles of the early model:

CarBEN EV Concept :: Mercedes Bionic/Boxfish clay model picture by NeilBlanchard - Photobucket

[aerohead]

Originally Posted by freebeard

My understanding is that The Template is a half body of revolution to insure that air doesn't want to move laterally because of pressure differentials between the top and sides, which leads to vortex generation.

If that's the case, wouldn't a half-square cross section serve as well, given equal cross-sectional area? If air is not wrapping across the edge, would no vortexes be created?

Here are three aeroforms, a cylinder, a superellipse and a square. Would their Cd be equivalent?


(These are not dimensionally highly accurate; I just eyeballed the proportions against The Template)

The next question: If the height/width proportion changes, what happens to the respective tapers? If the half revolution form is 7' wide, then it is 3 1/2' high. If the height is proportionally greater, does the side taper increase or relax?

TIA

The streamline bodies of revolution are of the lowest drag 3-dimensional bodies known.
And their half-bodies lend themselves okay to a 2-door coupe or sedan and such.
Today's Bochum University', Cd 0.14 solar car would be a recent example.
There has been a tremendous interest in the 1948 NACA mathematical algorithm which became the 1976 Morelli shape as used by Aptera,although,with ground proximity and wheels,the Morelli form has failed to demonstrate lower drag than a half-body 'pumpkin seed.'
If you'll look at the full-boat-tail trailer thread there should be some pictorial drag tables which demonstrate the drag increase when side radii are lost on the streamline half-body.
An example would be the square fuselage of the 'Spirit of St.Louis' with Cd 0.247 and the 'round' fuselage of the Arado 'Smooth' of similar fineness ratio,at Cd 0.06.
You will never see a high performance,low drag fish,bird,airship,fuselage,submarine,torpedo,etc., with square edges.
There are a lot of graphical drag coefficient tables in the public domain from which to compare architectures.
I believe that if you will exhaust all avenues,you will come to the realization that you're not going to beat the 'Template.'I've been actively looking since 1973.

[aerohead]

Originally Posted by NeilBlanchard

My memory is that the computer model of the boxfish was a Cd of 0.06:



The blue quarter scale model has a Cd of 0.095 and the diesel powered concept car has a Cd of 0.19:




Edit: here's a series of screen captures I got from a video about the Boxfish, that show many more angles of the early model:

CarBEN EV Concept :: Mercedes Bionic/Boxfish clay model picture by NeilBlanchard - Photobucket

It would have been really instructive had Mercedes published a plan-view image of the fish.This decides the flow path and the side view makes it as difficult to understand as if they offered a photo of panties when speaking of brassieres.

[ERTW]

It's not called a bodyofrevolution fish . Mb was surprised at the low Cd. They also mention that it's very stable and maneuvrable. We can't say for sure what the most streamlined animal is until we test them. In the meantime we just have to drive cigar shaped cars.

I was at the big al's the other day and noticed the shark's head is elliptical. The body is almost triangular.

Imo as long as the air on the top and side is the same, sharp edges won't hurt. It's when there is a pressure differential that sharp edges will accentuate it and create vortices. In that case a round edge will allow a gradient, and not feed vortices

[freebeard]

Aerohead -- Thanks for you thoughts. Have you posted your Bonneville stories anywhere? I have a post at Project the third that I would appreciate your comments on.

...the Morelli form has failed to demonstrate lower drag than a half-body 'pumpkin seed.' ... I believe that if you will exhaust all avenues,you will come to the realization that you're not going to beat the 'Template.'I've been actively looking since 1973.

I'm not looking so much to beat it, as to find some way to accommodate it into my life. Life full of compromise, if you take the half body of revolution and hang a license plate on it, you've already compromised.

An example would be the square fuselage of the 'Spirit of St.Louis' with Cd 0.247 and the 'round' fuselage of the Arado 'Smooth' of similar fineness ratio,at Cd 0.06.

Now that right there is what I was looking for. One would expect the superllipse to have Cd ~0.15.


ERTW -- This is how I'm seeing it:

Imo as long as the air on the top and side is the same, sharp edges won't hurt. It's when there is a pressure differential that sharp edges will accentuate it and create vortices. In that case a round edge will allow a gradient, and not feed vortices

So we have a preferred shape (or shapes), with the wheels spats and departure angles; we know that surface dirt and duct tape are trivial. That leaves the scale from drip rails and door handles up to rear view mirrors to think about. Maybe air scoops behind the rear view mirror to eat its vortex?

[freebeard]

Aerohead -- I found your Bonneville post. I also rediscovered this picture this morning. Perhaps it is what started my thought process. It out-boxes the boxfish.

[aerohead]

Originally Posted by freebeard

Aerohead -- I found your Bonneville post. I also rediscovered this picture this morning. Perhaps it is what started my thought process. It out-boxes the boxfish.

It would be nice if Honda published a Cd for the pictured car,then we'd know what we were 'looking' at.Have you seen the Honda P-NUT?
Some of the current minis are running on the order of Cd 0.3.
If you wanted lower drag you'd have to extend the rear.
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If you'd like to depart from the 'Template,' you might want to find out all you can about Wolfgang Klemperer's 'minivan' of Cd 0.16 (in naked model form) which he tested in 1921.Klemperer was a doctoral student working under Paul Jaray at the Zeppelin Werke.This 'minivan' is what VW used as the basis for the VW 2000 concept car.
I might be more excited about it if there was more data available on it.After extensive shaping in their new wind tunnel,VW got a 'production-ready' model to about Cd 0.25.
If you weren't interested in going below Cd 0.16 it would be a good way to go.