Morelli Shape

[Jim Bullis]

RobertSmalls,

Your comment about the tandem Aptera is noted, but this is why the real Morelli paper is needed since it explains why the Aptera is shaped as it is, and once that is clear, you will see why a back seat is not so easy to fit in. Notice that the Aptera is already plenty long without a rear seat at all.

I think that that old Popular Science article really did a disservice by focusing on the car that came out of the Morelli studies, and really ignored the shape that he investigated that looked like the Aptera does now. Perhaps this is one reason why the whole industry largely ignored the thing I call the 'Morelli shape'.

So all this has led me to the view that it would be worth while to actually elevate a relatively thin, ideally shaped, body of revolution and then do the things necessary to make that work. That looks like a way to get into a whole new league for efficiency.

[Jim Bullis]

RobertSmalls,

I would like to hear more about your CFD work. How much does it cost to use ANSYS FLUENT, and does it take a cluster of 500 computers to get anything done?
What kind of CAD files does it take?

I think it would be useful to simply run a CFD on a USS Akron ZRS-4 shape, and if we could get good results in free flow, we could then test sensitivity to ground proximity. Any suggestions?

[RobertSmalls]

Ansys Fluent is free, if you can get access to your alma mater's engineering computer lab. I told you I was low-budget.

CPU time is mostly a function of mesh size, but I would expect the kind of simulation you're doing to run for hours, not weeks, on a modern home PC.

CAD files? It can import from PRO/Engineer and .dxf, among others.

Starting with a validation case, then increasing proximity to ground, sounds like the way to go. Ansys say they've run a number of validation cases themselves, so you can expect a known degree of accuracy if you've created a mesh that accurately represents the problem.

You should post on the forums at eng-tips.com. The folks there are very knowledgeable.

[3-Wheeler]

Originally Posted by Jim Bullis

....I argue for two feet clearance based on Morelli data in the book I gave reference for a couple comments back, as well as a variety of less direct indications from other publications.....

I am not an aero expert, but does seem to me that two feet is not enough clearance for the ground effect to be minimized.

Why??

Ever been on a narrow two lane country road, and have an opposing car go by? Notice the large blast of air that pushes the car sideways as both cars pass by each other?

The clearance between the the two cars from the centerline of the road to each car is about two or three feet at best, yet not enough to diminish this effect.

I would think that based on this phenomena, that something closer to 8 feet would be the minimum, just from observation.

Jim.

[Jim Bullis]

3-wheeler, (and aerohead is mentioned here)

The effect does not have to go away altogether, but just be substantially reduced. So your observation is not exact, though relevant. The bigger consideration is the volume of the passing car, and really, the cross sectional area for the most part.

Compare the blast from an 18-wheeler compared to that from a motorcycle to get a rough sense of this relationship. It also has something to do with shape, and I maintain that a flat sided box will create more of a blast than a rounded cylinder. This gets a little more complicated, but think about the way air gets entrapped under a large flat surface compared to the way it only partly is entrapped under a rounded body. This is a particularly damaging effect of double wide seating, since it really forces car underbodies to be mostly flat.

Again the rounded underbody of the Aptera seems representative of this principle, though they still manage to cram in two people. I could do that also, but partly bearing in mind the kind of thinking from aerohead about safety, I would rather reserve side space for side impact defense. Aptera talks a lot about this but I tend to think their design is oriented to the NHTSB simple test of a flat ram hitting the car flat on the side rather than to generalized random chaos when one really gets clobbered from the side.

[Jim Bullis]

Robert Smalls,

Thanks for the info on Ansys Fluent. And I would rather work with the dxf files than the strange stereo files that Cham Phoenics required. My low budget CAD is actually very capable in this regard, and I was able to communicate with the Cham Phoenics, but getting into things at a level where fine details could be sorted out was not possible, at least not in a reasonable time. I am trying to avoid getting too bogged down with analysis, so I have not been able to work this through adequately. And the expectation of useful CFD results is not encouraging anyway.

I have never been particularly impressed with anything I have seen by CFD since nobody has shown results for very fundamental objects and compared them with wind tunnel measurements. Even relatively large Cd forms do not seem to get validated in a way that I would consider appropriate, so getting really good results in the Cd = .05 range seems unlikely. I would be glad to be proven wrong on this, such that I could get past seat of the pants extrapolation based on Morelli and based on the Akron data.

[NeilBlanchard]

Hello Jim,

Even 2 feet above the ground is hard to achieve -- the only way is like the car I posted earlier.

Having standard size wheels and motors and batteries in a portion of the car near the ground with the passenger pod up on struts -- *still* has the ground effect because the wheels and motors and batteries are near the ground.

[Jim Bullis]

Hi Neil,

I know of two ways to achieve the two feet above the ground height.

And two feet is the requirement, sort of, for a thin body of revolution.

Yes, there will be separate aerodynamic entities near the ground, but these are very low profile so that the drag on these, including ground effect associated with these, is not large compared to the main aerodynamic entity which is the thin upper body.

So the two ways I know to achieve the two feet are attached with thin struts designed also to make the main body work as it would in free flow. These will not be as close to the ideal as the supports used in wind tunnel measurements, but the objective is to get as close as possible to this condition.

[Bicycle Bob]

The Morelli shape is an excellent example of how to modify a pure shape to work close to the ground. I don't think there is any problem with this approach until you squeeze the boundary layer, and even then, it is not always bad.

[Jim Bullis]

Bicycle Bob,

Yup, though even Morelli did not squeeze the boundary layer. His analysis assumed laminar flow, and dealt only with that part of the problem.

There are two problems with the Morelli shape (1) the camber requirement forces the underside to rise so as to cut into room behind where the front seat would be, and (2) (not discussed here yet) is that there is a well expressed desire of people to not ride puny, meaning riding so low that vision is blocked and the sense of being immently crushed becomes a nerve racking problem.

I realized some time ago that the popularity of pick-ups and raised vehicles had some rational basis, which is that it really is quite pleasant to ride with a high visual perspective. I even surmise that no car will really be successful in the market that is low slung; I came to this conclusion and it was supported by a study of the 100 year history of the automobile. There have been quite a few that actually were very efficient, these include Isetta, Messerschmitt, and all the way back to the 'Cycle cars' of the 1910-1920 era. All were quickly done in by big and relatively high cars.

And we should not get confused by the roadsters and their great appeal for driving on scenic mountain roads, as with Grace Kelly driving and so on. On freeways, the sports car definitely gives a different set of sensations, as you ride along looking into truck wheel wells from below.