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Old 07-15-2014, 05:35 PM   #71 (permalink)
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on the rear

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Originally Posted by chefdave View Post
Hi freebeard again
can these half rounds be used on rear to help reduce drag. Do you need to use 4" or can i go up to 6"
Thanks
dave
There's not a lot of published data which addresses your question.
Here is a drag table for bus type vehicles.You can see that softening the rear vertical edges DOES lower drag.

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Old 07-16-2014, 12:26 AM   #72 (permalink)
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Quote:
Originally Posted by aerohead View Post
There's not a lot of published data which addresses your question.
Here is a drag table for bus type vehicles.You can see that softening the rear vertical edges DOES lower drag.
That is interesting ...

Is it the same for cars ?
I was told (nope, didn't red it) radius at the back is bad ...
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Old 07-16-2014, 02:10 AM   #73 (permalink)
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Box vans are a special case, shown in the upper right. Car's shapes tend to promote vortexes. Convex, flat and concave truncations all have a drag vector that points straight back and are pretty equivalent. Sharp edges are used, however, to control separation (seen on the 3rd Gen Prius back bumper.

May aerohead correct me if I'm wrong.
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Old 07-16-2014, 08:12 AM   #74 (permalink)
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Originally Posted by chefdave View Post
Hi renault megane dci
do i need to make the rear vertial from top to bottom or from bottom to where the rear body sticks out the furtherest. having trouble posting photo.
Apparently I was wrong as seen in drawings shared, round is not bad.

Based on figures shown, the biggest question is : how much overhang can you tolerate ?

(aim for F)
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Old 07-16-2014, 01:28 PM   #75 (permalink)
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I'm curious about D and unsure about Bb.

Here's a solution:
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Old 07-16-2014, 03:16 PM   #76 (permalink)
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same for cars

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Originally Posted by renault_megane_dci View Post
That is interesting ...

Is it the same for cars ?
I was told (nope, didn't red it) radius at the back is bad ...
On cars we need all the elongation we can get.If you soften the rear of a car,you kill off a little area which might have continued to produce pressure recovery.This is why Fachsenfeld,Kamm,and Morelli recommended just slicing the back off like a loaf of bread.
Lately,a Chrysler aero guy talked about 'burst,' where a chop would prevent any 'hunting' of the Cd,although,above 20-mph,Cds are constant,and it's not really a problem.
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Old 07-16-2014, 03:42 PM   #77 (permalink)
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sharp edges

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Originally Posted by freebeard View Post


Box vans are a special case, shown in the upper right. Car's shapes tend to promote vortexes. Convex, flat and concave truncations all have a drag vector that points straight back and are pretty equivalent. Sharp edges are used, however, to control separation (seen on the 3rd Gen Prius back bumper.

May aerohead correct me if I'm wrong.
The only thing I have,other than the drag table depicted already,which specifically addresses boat tail sharp edges is for semi-trailer vans.
It's from Project Tailwind,at Texas Tech.Graduate student Mark Funderbergh (sp?) made a comment that sharp edges created drag-inducing vorticity.
On the cover photo for the article,they depict the GM OPTIMUM boat tail.It has taper on top,bottom,and sides,as well as progressively increasing edge rounding as the tail progresses rearward.It's about 10.5-feet overall length.
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In his second edition,Hucho cautions that body designers pay close attention to pressure gradients below,besides,and on top,so as not to induce vorticity,as high pressure would be induced to flow towards low,creating the spanwise shearing and rotation.
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Paul Jaray's 1922 'pumpkin seed' half-body car body transitioned from semicircular section,to rectangular section,then back to semicircular for the boat tail.I've seen this done hundreds of times in commercial/industrial HVAC ductwork applications.
The semicircular section has 16.6% lower drag than a pure rectangular section (as seen on Charles Lindbergh's Ryan 'Spirit of St.Louis' ) according to Fachsenfeld and Kamm's research at FKFS.
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Old 07-16-2014, 03:47 PM   #78 (permalink)
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D and Bb

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Originally Posted by freebeard View Post
I'm curious about D and unsure about Bb.

Here's a solution:
Here's Ahmed's work which covers a few other shapes.Everything is hard edges.

Here's (I think) Fachsenfeld's inflated tail of the mid-30s.It would be the ultimate for low drag,but most complicated for fabrication.
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Last edited by aerohead; 07-16-2014 at 03:51 PM.. Reason: add data
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Old 07-17-2014, 07:35 AM   #79 (permalink)
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Sorry but I can't see too well the numbers.

Is the best drag setup 25 top, 10 bottom, extra 25% length and unquoted angle sides ?
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Old 07-18-2014, 04:48 AM   #80 (permalink)
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Hi
know we are discussing the rear aero but have smoothed underbelly where i can and have netted a 10% improvement. looking to make under belly even smoother have room for more improvements.
At the rear thinking of how to fit a rear diffuser to match up to above diagrams to gain max benefits.

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