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Old 11-17-2014, 07:56 PM   #1 (permalink)
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Aero rear angle

I noticed that the Ev1 rear glass (according to the aero template) is too steep,
how can it be too steep and yet have such low drag?

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Old 11-17-2014, 10:15 PM   #2 (permalink)
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If they had built it shaped like the XL-1 (VDub), it would have had an even better Cd.

Just because it is not "Ideal" does it mean that it's awful, it's not all black & white. In looking at the shape of the EV1 from the top, we see the sides really flow together more than most shapes. I think this helps to balance the "faster than the curve shape" of the side view taper that you refer too. By balance, I mean the pressures are all equalizing at the same time so you are not generating turbulence as you would if the top was dropping fast and the sides stayed flat.

So, not ideal, supported by other aero design elements, still very efficient.
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Old 11-18-2014, 06:13 PM   #3 (permalink)
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rear




*With the stock aft-body,the EV1 comes in at what GM now says,Cd 0.21.
*With boat-tailing and other tricks she drops to around Cd 0.15.
*If we take the aft-body and loft it up closer to the 'Template' we've got a shot at Cd 0.10.
*So while very good,the EV1 is still good for a 50% drag reduction.
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Old 11-18-2014, 06:50 PM   #4 (permalink)
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Thanks! I was also noticing that the template slopes down, but not up, is air from under the car not able to gently slope up and stay attached? Or is it just a lesser amount? Or does the template just not show this?
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Old 11-18-2014, 07:04 PM   #5 (permalink)
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slope up

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Originally Posted by Hypermiler1995 View Post
Thanks! I was also noticing that the template slopes down, but not up, is air from under the car not able to gently slope up and stay attached? Or is it just a lesser amount? Or does the template just not show this?
The 2nd-gen 'Template' has a 2-1/2-degree diffuser to mimic Paul Jaray's original 'pumpkin seed' of 1922.Here's a link:http://ecomodder.com/forum/showthrea...rt-c-9287.html

Here is Jaray's 1922 original

Jaray appears to have invented the diffuser,along with so much more!
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Old 11-18-2014, 08:35 PM   #6 (permalink)
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If one had 3D models of, oh say, the EV-1, the Volkhart-Sagitta, and the VW XL-1 that all had the same polygon counts, with equivalent vertices along the hard edges (wheel wells and rocker panels to start); then it would be possible to morph among the three to define an envelope of possible design solutions. Then turn evolutionary algorithms loose on the data set and see what falls out.
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Old 11-19-2014, 05:05 PM   #7 (permalink)
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IF/Then

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If one had 3D models of, oh say, the EV-1, the Volkhart-Sagitta, and the VW XL-1 that all had the same polygon counts, with equivalent vertices along the hard edges (wheel wells and rocker panels to start); then it would be possible to morph among the three to define an envelope of possible design solutions. Then turn evolutionary algorithms loose on the data set and see what falls out.
I don't think that day is too far off.
CFD is coming on like gang busters,and wind tunnel results show increasing accuracy with respect to the numerical models.
With an 'archive' of body vectors of known aerodynamic performance,as with EV-1,Volkhart,etc.,seems like the algorithms could solve for variations which remain within some geometric boundary,analyzing for bounded pressure and velocity distributions known to protect a turbulent boundary layer.
Any feature could be tweaked,then the computer would solve for appropriate changes to the rest of the body in order to satisfy the aero requirements.
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Old 11-19-2014, 05:55 PM   #8 (permalink)
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Originally Posted by aerohead View Post
I don't think that day is too far off.
CFD is coming on like gang busters,and wind tunnel results show increasing accuracy with respect to the numerical models.
I talked to a MechE PhD student and he told me CFD actually has a long way to go because the algorithms are just too slow (which means, to my surprise as a mathematician, that there is a very very important practical use for PDE research!). Evolutionary algorithms would not be hard to implement for this purpose because you could just have the program randomly add bits and pieces to the shape and see where that goes but since you're running the CFD computations millions of times over you'd need to have a better algorithm for doing that.

On the bright side, the current best algorithms are like O(e^(e^(e^n))) or something hideous like that so cutting it down to say a double-exponential would make this kind of thing feasible on a supercomputer.
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Old 11-19-2014, 07:07 PM   #9 (permalink)
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slow

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Originally Posted by serialk11r View Post
I talked to a MechE PhD student and he told me CFD actually has a long way to go because the algorithms are just too slow (which means, to my surprise as a mathematician, that there is a very very important practical use for PDE research!). Evolutionary algorithms would not be hard to implement for this purpose because you could just have the program randomly add bits and pieces to the shape and see where that goes but since you're running the CFD computations millions of times over you'd need to have a better algorithm for doing that.

On the bright side, the current best algorithms are like O(e^(e^(e^n))) or something hideous like that so cutting it down to say a double-exponential would make this kind of thing feasible on a supercomputer.
This student that you talked to,is he working with the full Navier-Stokes equation for 3-d flow?
I understand it to be THE numerical tool for analyzing 3-D bluff body flow.
There is a Kappa-Epsilon turbulence model ancillary software which is dovetailed into FLUENT or something else which gets the Reynolds numbers/boundary layer conditions correct.
And I understand that it does take a supercomputer to take this on,but what if,say,we start with the Ahmed body,run a change on it,then make that data available to the next (and any) investigator who then runs their change,each iterations data available to any interested investigators,so there's never any duplication of work.Parallel processing as is being done with astronomy?
Seems like you could dial in a form for verification in a tunnel with a high certainty of performance beforehand.
Just thinking out loud.
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Old 11-19-2014, 09:13 PM   #10 (permalink)
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I wasn't aware there was any other way to do this than Navier-Stokes in 3d? I am an algebra guy so I have no idea how PDEs work nor do I know how this kind of stuff is implemented, but to do an optimization where the algorithm figures out a shape that works would be like running the model millions of times. If you made changes and then gave the data to someone else to modify, that would be the same as what people are doing now wouldn't it?

Seems like some of the millions going to wind tunnels should be funnelled towards research grants for figuring out a better implementation of the numerical solution to Navier-Stokes! 10 million dollars is good for like, 20 postdocs + faculty advising for 5 years. Between Toyota, Honda, Ford, Ferrari, GM, etc. + Boeing, Lockheed, etc. I bet they wouldn't have any trouble coughing up at least several times that number, and considering the cost of wind tunnel testing, I bet it would pay off.


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