Ideal car rear roofline angle and shape for low drag?

[AEOLUS]

Originally Posted by aerohead

All present low-drag concept cars happen to fit the ASTs. Including Think Flight's boat-tailed 2022 Subaru Impreza Wagon, of est. Cd 0.1764 ( configuration #4).
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Nice work!
What was orignal Cd of car and how you meassure 0.176?
How looks cofiguration #4?

https://www.youtube.com/watch?v=O-FY...el=ThinkFlight

[freebeard]

[QUOTE]What was orignal Cd of car...QUOTE]

You_Know_My_Name_(Look_Up_the_Number)

....and how you meassure 0.176?

"Think Flight's boat-tailed 2022 Subaru Impreza Wagon, of est. Cd 0.1764"

[AEOLUS]

@aerohead

tail 2 has higher drag compare to tail 1, did you install tail 1 back to car?

Also why is tail 2 + skirt + air dam compare to base model of 36.6mpg if first tail is compare to base model of 35.6mpg?

After you meassure second base model 36.6mpg, you return home and install tail 2+skirts+air dam at car in few hours?

[aerohead]

Originally Posted by AEOLUS

What is delta Cd at first graph,why delta?
And why negative numbers?

The graph depicts the range of change of drag to the original body design, for five (5) different aft-body lengths:
Each of the five designs indicate their representative drag 'bucket', at which Porsche's Hermann Burst referred to the 'reversal' point:
* Once the 'bucket' is reached for the specific downslope 'angle' any additional increase in downslope only 'INCREASES' drag again!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Many 'educators' fail to master this critical aspect, relating to boundary layer theory, of which Wolf H. Hucho was a student, studying under Professor Schlicting, as a graduate student. Dr. Hermann Schlicting literally 'wrote' the book on boundary layer theory!
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The delta-Cd is the measured change in drag coefficient for each of the five different rooflines, as a function of increased downslope angle.
You can see that, for a 'longer' roofline, flow can remain attached for a steeper angle, up to some 'reversal' point.
' When vehicles developed for the lowest drag coefficient are evaluated for Cd vs length, the correlation is discerned and expected trend is in fact confirmed.' W.H. Hucho

[aerohead]

Originally Posted by Cd

Whoa ! I am just now seeing this. I just posted a similar question Aerohead. Sorry 'bout that

Since you bring up ThinkFlight's design, you might notice that it falls out of the Template curve. ( Have a look at his second video )
This got my attention, because despite having too steep a curve at the rear, the design had great attached flow all the way to the tip of the tail, and even along the centerline.

1) The V1 tail follows the 1935 FKFS, 5-configuration, 'Kamm' test model, and W,A. Mair boat-tail contour. It's upper slope, for it's 'length, never exceeds 19-degrees, while the diffuser goes off reservation, @ 20.5-degrees ( impossible for flow attachment).
I estimate Cd 0.214 for the 40.528-mpg performance.
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2) The V2 tail, with airdam & rocker panel extensions, conforms to W. A. Mair's initial contour, and evolves into the 1981 Cd 0.14, VW Flow-Body( long-tail ) contour, 'peaking' @ Buchheim et al.'s 23-degrees at top ( classical limit for flow attachment ), and the diffuser has been relaxed down to 5-degrees.
This #4-configuration is estimated at Cd 0.1764, to explain the 43.097-mpg ( a 20.99% mpg increase).

[aerohead]

Originally Posted by AEOLUS

@aerohead

tail 2 has higher drag compare to tail 1, did you install tail 1 back to car?

Also why is tail 2 + skirt + air dam compare to base model of 36.6mpg if first tail is compare to base model of 35.6mpg?

After you meassure second base model 36.6mpg, you return home and install tail 2+skirts+air dam at car in few hours?

1) First of all, this project car belongs to Think Flights originator.
2) My mathematics implies that the second-generation tail and modifications, before before being 'sawed off, ' actually generated the greatest drag reduction, ( 37.57%) in order to explain the 20.909% mpg improvement ( 43.0971-mpg vs 35.644-mpg ).

[Cd]

So - Aerohead, what would happen if I created a nearly exact duplicate of the Aptera rear end from the end of my car's roofline ? ( 1993 Civic hatchback - same as the AeroCivic )
The curve of the Aptera swoops down out of the curve of the template and sweeps back again to the shape towards the end of the tail.
It would certainly shorten the overal length to use the Aptera shape.
While I have your attention, I'd also like to ask you how you had / have your front wheel covers installed on your truck.
It appears as though they have no rollers like the AeroCivic. Does the tire just rub against to backside ?
What keeps them in place ?
Have they been removed recently ? If so, why ?
- Thanks

[aerohead]

Originally Posted by AEOLUS

Nice work!
What was orignal Cd of car and how you meassure 0.176?
How looks cofiguration #4?

1) CarsDirect.com published a Cd 0.29 for Subaru's Impreza Wagon.
2) To estimate a Cd capable of explaining how the Impreza's fuel economy could change from it's baseline mpg, to any different mpg, as the ONLY variable in the calculus, I used a thermodynamic energy balance protocol published by Chrysler Corporation in December, 1941, referencing the SAE Paper, aerodynamic engineering evaluation of Chrysler's Cd 0.244, 1934, DeSoto Airflow, aerodynamic test mule, by Carl Breer et al..
3) The 'conditions' for the calculus I used can be found at the 'Think Flight' thread at the other Forum here at EcoModder.com.

[aerohead]

Originally Posted by Cd

So - Aerohead, what would happen if I created a nearly exact duplicate of the Aptera rear end from the end of my car's roofline ? ( 1993 Civic hatchback - same as the AeroCivic )
The curve of the Aptera swoops down out of the curve of the template and sweeps back again to the shape towards the end of the tail.
It would certainly shorten the overal length to use the Aptera shape.
While I have your attention, I'd also like to ask you how you had / have your front wheel covers installed on your truck.
It appears as though they have no rollers like the AeroCivic. Does the tire just rub against to backside ?
What keeps them in place ?
Have they been removed recently ? If so, why ?
- Thanks

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1) First, the Morelli form, with 4-wheels, has no discernible aerodynamic advantage over a K-form, fastback, as championed by W.H. Hucho (It's Cd can never be lower than 0.16 ).
2) Only a '3-wheeled' Morelli form would have an edge ( 1992, MIT Aztec solar race car / 2022 Aptera ).
3) The Morelli form is derived from a 1947 NACA ( NASA) 2-dimensional, mathematical algorithm which Professor Alberto Morelli studied into the weeds at the Turin Polytechnic Institute.
4) No 'styling' is involved in the Morelli form.
5) No 'committees' were involved in the Morelli.
6) Only Morelli and Sergio Pininfarina had anything to say about the Morelli form.
7) Every detail of every square-millimeter of a Morelli form's surface is generated mathematically.
8) There are serious ground clearance constraints imposed on Morelli forms.
9) There are serious body 'camber' constraints imposed on Morelli forms.
10) Geometrically, a 1993, HONDA CIVIC hatchback forebody possesses none of the architectural parameters necessary as the prerequisites to create the 'SOURCE' flow which allows an Aptera's aft-body 'SINK' flow to exist.
11) Aerodynamically, this really is an 'apples & oranges' situation.
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12) Hucho's only recommendation for low-drag revolved around ' a half-body, which forms a complete body of revolution together with its mirror image-produced through reflection from the roadway.' (This IS, by definition, the AST ).
13) The lowest drag streamline body of revolution has a Cd 0.04, which is satisfied by the AST.
14) This body of revolution produces a half-body of Cd 0.08.
15) Adding wheels pushes the Cd to 0.13.
16) Integration of the wheels into the body allows for a Cd which approaches 0.08, in ground proximity ( Hucho, page- 201, 2nd-Edition ).
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Until someone can provide 'better' solutions to those DIY'ers without access to industry-grade CFD or a full-scale wind tunnel, I'll remain an advocate of the K-form, fastback, based on the 2.5:1 fineness ratio streamline body.
Goro Tamai, of MIT, wrote in his 'The Leading Edge', that those without access to CFD or wind tunnel, would want to limit ( solar race car ) aft-body contours to within a 'rule-of-thumb, 17-degrees, to 20-degrees of the free stream flow.
23-degrees is Buchheim et al.'s limit.
W.A.Mair advocated for 22-degrees.
Hucho did not dispute Mair.
EPA said 22-degrees.
All dimpled golf balls experience flow separation @ 22-degrees.
Douglas Aircraft Corporation ( US NAVY Aviator ground school text ) flow imaging revealed an attachment limit @ 22-degrees.
I'm a chicken when involving hundreds of man-hours of member's time on complex, compound-curve fabrication that, they'd probably like to get 'right' the first time, so I'll remain a 22-degree maximum advocate.

[Cd]

Phil, I was aware that the Morelli form would not fully adapt to the forebody shape of my Civic.
I was just curious about the roofline taper.
It swoops down steeply, and then corrects back to the template farter downwind.
I ask, because I have seen steep rooflines like this on other low drag cars that were not Morelli in shape.
( That's why I mentioned cars like the Tesla, the Lucid, and new Mercedes EV in some of my last questions to you )
I know that the airflow at the floor level of the car would not follow the Morelli form, as well as the sides.

I think a lot of non math types like me would love to see some "thermodynamic energy balance protocol " figures for other vehicles on this site.
For example, Darin's "Flea" with the boattail, and that Prius that is on this site " ChamPrius" with that full boattail.

- Thanks