Managing vortexes off the back of a partial boattail

[Sven7]

I've been thinking about prototyping the boattail (it's literally a matter of time) and would like to bounce some ideas off those of you who actually studied aerodynamics or dropped cash for the Hucho book.



The liftback Probe has attached flow across the rear window and a typically boxy bottom end. It would probably be best described as Ba on the chart.



Illustrated are two vortexes forming on the C Pillars and rolling inward toward the center line. Even considering the attached flow, these should exist because of the pressure differential.

Since I plan on extending the rear end two feet or so at a 15* angle top and sides (and verifing it with tuft testing), there may be a unique opportunity. I seem to recall reading somewhere that two counter rotating vortexes on each side could neutralize each other. Piwoslaw posted something about counter rotating "eddies" but I'm not sure if that would technically be the same thing.

http://ecomodder.com/forum/167791-post25.html

That graph and explanation suggests that a rear belly pan angled 15 degrees upward from horizontal would in effect cancel out the eddies coming off the liftback form.

If that is true, is there anything else I could do or should think about in shaping the boattail to help neutralize this pressure differential without making it worse? Anything to look out for?

Thank you all very much.

[aerohead]

From Hucho,Buchheim's research took two directions,one the simple hard-edged model work,and curved surface research.
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On the hard-edged bodies the optimum angle was a direct function of the length of the slanted portion of the body.Angles can only be taken in the context of the slant length.
Cars with the largest slant length can sustain the greatest angle.
*a 9% aft-body min is @ 13.2-degrees
*a 18% aft-body is 15.3-degrees
*a 27% is 17.4-degrees
*a 36% is 18.5-degrees
*a 45% is 22.5-degrees ( halfway between Mair's 22-degree limit and Hucho's 23-degrees.
When you look at the minimum drags for the data presented,they always occur when at 'Template' tangent angles.
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For the curved body Buchheim's research demonstrated minimum drag also at 'Template' angles.His Fig.4.60,at drag minimum is a dead match to the 'Template.'
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I would recommend that all your contour be based on the 'Template.' You have less to work with on the sides,so you'd just pick up where Ford left off with respect to plan taper.
For the roofline I would stay as close to the 'Template ' as you can.22-23 degrees of slope is the maximum or you're looking at separation.
I wouldn't sweat the vorticity,it becomes a real problem at over 30-degrees of slope.You won't be going anywhere near there.
If you had a notchback you might do things a little different.

[Sven7]

Thanks, Phil. I won't worry about it then. It's nice that the wake area is pretty small on the Probe compared to a notchback or what have you, because a couple feet of boattail at 15*-20* will bring it down to next to nothing!

[botsapper]

Like that dazzle camo. A great solution for the different-color aero 'study' panels and other additions...and fool the wind.

[aerohead]

Originally Posted by Sven7

Thanks, Phil. I won't worry about it then. It's nice that the wake area is pretty small on the Probe compared to a notchback or what have you, because a couple feet of boattail at 15*-20* will bring it down to next to nothing!

I found a couple of road tests CAR and DRIVER did on the Probe,with some pretty good photos.
It looks like the stock car,in elevation,has about 41% aft-body and around 39% of 'Template.'
Length/Height = 3.453 which is very good for a production car.The ideal is 5.0.
The rear is kinda wide (it's a wedge form) so the wake is also wide,but flow very strong,and should provide nice flow onto the tail.
With an extra 24",your L/H will be 3.916 which exceeds many concept cars.
You're already sub-Cd 0.3, and with the new tail you should easily see in the 50mpg range.Sweet!

[Sven7]

Awesome! Ford says Cd 0.308 for the LX version (mine's a GL). Can you make a wild guess at what the Cd will be with the current mods, a full belly pan and a 24" boattail? I'm so excited!

[aerohead]

Originally Posted by Sven7

Awesome! Ford says Cd 0.308 for the LX version (mine's a GL). Can you make a wild guess at what the Cd will be with the current mods, a full belly pan and a 24" boattail? I'm so excited!

I'll program my single synapse and see where it goes.Actually,I'll look back over the mod data list 'n see if I can glean an educated SWAG.

[Sven7]

I made a quick visual

Basically the whole top half will need to be clear so people can see the tail lights. Look at how small that wake area is!


Boat Tail Virtual Mockup by Tyler Linner, on Flickr


Boat Tail Virtual Mockup by Tyler Linner, on Flickr


Boat Tail Virtual Mockup by Tyler Linner, on Flickr

[Tango Charlie]

Go longer!

[aerohead]

On my tail I cut in fenestrations to allow the existing lights to be seen from astern and covered with clear plex.
I figured,the more visibility the better.Just a thought.
Love the 'golden' light in the image! Sunrise or sunset?