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Old 03-02-2012, 02:11 PM   #281 (permalink)
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I stopped babysitting the solver after a couple of nights I've logged over 70 CFD hours in the last week, 50 in the last few days. Watching the solver is actually nerve racking...like watching the ticker of your stock Just hoping it'll move the way you want it.

I finished the "template" prism. Instead of rotating the body, I extruded it. The side edges have a 5" radius.

LWH 241.1" x 78.74" x 55"
ground clearance (hG) 5"
A = 2.540 mē
v = 30 m/s

Cd = 0.161
Cz =0.083

I have observed snow falling on cars and creating sharp edges along the roof. I was curious if a sharp edge was better, and somehow impeded streamlines from wrapping up along the sides and creating vortices. nope. It seems the DP at sharp edges is highest at sharp edges, and wreaks havoc.

Cx = 0.175
Cz = 0.099

So it not only hurts drag, but lift as well (drag induced by lift, aka vortex drag). It's still only 150 lb of lift on a, say, 3500 lb vehicle; it's not a stability or safety issue.

Phil, I was perfectly happy with my simple extrusions and revolutions. Now you're telling me I need to work in surfaces?! Without at least 3 views it's not possible to model it accurately. Dimensions would help too.

When I go to SAE congress next month, I'll see if I can steal the boxfish solid model

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Old 03-02-2012, 03:02 PM   #282 (permalink)
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I'm sure I'm not the only one who view these 'aero' templates to be very narrow in their scope. The almost '2D' shapes are almost always tested and highly scrutinized with just straight head winds. Pragmatic 'optimal' forms will surely change when these shapes are evaluated with off-center cross winds. The relatively high & flat sides of some of these shapes will go through erratic yaw movements and have straight line instability. Wind tunnel researchers/junkies & especially HPV designers are always mindful of constantly changing cross winds' effect on their designs. I'd like to see wind tunnel smoke trails, tuft testing or CFD flow visualization art created by off-center sidewinds.
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Old 03-02-2012, 06:16 PM   #283 (permalink)
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Ertw

Quote:
Originally Posted by ERTW View Post
I stopped babysitting the solver after a couple of nights I've logged over 70 CFD hours in the last week, 50 in the last few days. Watching the solver is actually nerve racking...like watching the ticker of your stock Just hoping it'll move the way you want it.

I finished the "template" prism. Instead of rotating the body, I extruded it. The side edges have a 5" radius.

LWH 241.1" x 78.74" x 55"
ground clearance (hG) 5"
A = 2.540 mē
v = 30 m/s

Cd = 0.161
Cz =0.083

I have observed snow falling on cars and creating sharp edges along the roof. I was curious if a sharp edge was better, and somehow impeded streamlines from wrapping up along the sides and creating vortices. nope. It seems the DP at sharp edges is highest at sharp edges, and wreaks havoc.

Cx = 0.175
Cz = 0.099

So it not only hurts drag, but lift as well (drag induced by lift, aka vortex drag). It's still only 150 lb of lift on a, say, 3500 lb vehicle; it's not a stability or safety issue.

Phil, I was perfectly happy with my simple extrusions and revolutions. Now you're telling me I need to work in surfaces?! Without at least 3 views it's not possible to model it accurately. Dimensions would help too.

When I go to SAE congress next month, I'll see if I can steal the boxfish solid model
ERTW,your doing plenty good without me! I apologize for the incompleteness of the thread.
Since it was intended as an aft-body streamlining aid,I figured that folks would just work with what they had dimension wise, 'n try 'n make the most with what they had to work with.
There's been some interest in boat-tails and I was following Hucho's criteria for a minimum structure which still respected separation-free architecture.I settled on the 2.5:1 streamline body partly because Hucho showed it in one of his tables,it had Cd 0.04 (which is close to a theoretical minimum )in free air,which would translate into Cd 0.08 in mirror-image ground reflection ( after Prandtl/Rumpler).
When wheels are added to these 'pumpkin seeds' it is typical for the Cd to jump to at least 0.12.
It wasn't supposed to be a body-in-white.I added 'features' to the original drawing to make it look more like an automobile.
I have an underside view which is close,and an end view which is kind of like a wire frame model,depicting what bulkhead stations might be like if you were doing a mockup.
I'll try 'n get 'busier'!
The CFD is really impressive and I've got to tip my hat to you and others that can just wade in.The isobaric contours really help one visualize what the pressures are doing.And that's the name of the game.
Have a blast at SAE! That should be a riot.I hope they stagger presentations enough that you can take in all the sessions you're interested in.
Too much fun!
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Old 03-02-2012, 06:27 PM   #284 (permalink)
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templates

Quote:
Originally Posted by botsapper View Post
I'm sure I'm not the only one who view these 'aero' templates to be very narrow in their scope. The almost '2D' shapes are almost always tested and highly scrutinized with just straight head winds. Pragmatic 'optimal' forms will surely change when these shapes are evaluated with off-center cross winds. The relatively high & flat sides of some of these shapes will go through erratic yaw movements and have straight line instability. Wind tunnel researchers/junkies & especially HPV designers are always mindful of constantly changing cross winds' effect on their designs. I'd like to see wind tunnel smoke trails, tuft testing or CFD flow visualization art created by off-center sidewinds.
I doubt that there's any risk that we'll see any pumpkin seed cars on the road.They are the ideal,but billions will find fault with them.There are too many reasons not to consider ideal automotive forms for daily transportation.
Anyone who might be interested in exploring truly low drag may find few alternatives.
For a door-slammer which you can get into and drive away,see to park,and negotiate real roads,streets,'n highways there is some wiggle room for Cd available.
If you ever want to see the high side of 47 mpg in a 1/2-ton pickup,I recommend 'em.
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Old 03-02-2012, 07:48 PM   #285 (permalink)
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Sometimes people get it beautifully right, on the first try. http://www.saab-hk.com/images/saab_w.../museum_03.jpg
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Old 03-03-2012, 02:01 PM   #286 (permalink)
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Saab

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Originally Posted by botsapper View Post
Sometimes people get it beautifully right, on the first try. http://www.saab-hk.com/images/saab_w.../museum_03.jpg
Somewhere there is an artists rendering of SAAB's first car before its design was locked-in for production.I think you'd prefer it over the production version.
It's very tasty!
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Old 03-03-2012, 02:24 PM   #287 (permalink)
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new images

I have two images I tried to post with other images but it didn't take,so I'll try here.
One is an underside plan-view depicting wheels with canoe fairings,and the other a rear elevation illustrating how the semi-elliptical cross-section morphs into a semicircle as the body projects rearward.
These are just working drawings from last year.
ERTW has been kind enough to spend some valuable time doing a CFD analysis of the 'Template,' and I hope these extra views will indicate the direction my brain was going
----------------------------------------------------------------------------
Sorry! No good here either.I get the same error message.We reduced the file size but apparently it's not enough.
I'll post 'em when I can.
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Old 03-03-2012, 10:17 PM   #288 (permalink)
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Quote:
Originally Posted by ERTW View Post
I stopped babysitting the solver after a couple of nights I've logged over 70 CFD hours in the last week, 50 in the last few days. Watching the solver is actually nerve racking...like watching the ticker of your stock Just hoping it'll move the way you want it.

I finished the "template" prism. Instead of rotating the body, I extruded it. The side edges have a 5" radius.

LWH 241.1" x 78.74" x 55"
ground clearance (hG) 5"
A = 2.540 mē
v = 30 m/s

Cd = 0.161
Cz =0.083

I have observed snow falling on cars and creating sharp edges along the roof. I was curious if a sharp edge was better, and somehow impeded streamlines from wrapping up along the sides and creating vortices. nope. It seems the DP at sharp edges is highest at sharp edges, and wreaks havoc.

Cx = 0.175
Cz = 0.099

So it not only hurts drag, but lift as well (drag induced by lift, aka vortex drag). It's still only 150 lb of lift on a, say, 3500 lb vehicle; it's not a stability or safety issue.

Phil, I was perfectly happy with my simple extrusions and revolutions. Now you're telling me I need to work in surfaces?! Without at least 3 views it's not possible to model it accurately. Dimensions would help too.

When I go to SAE congress next month, I'll see if I can steal the boxfish solid model
Makes perfect sense. If the top slopes down but the sides don't taper in then lower pressure above tends to suck air up

and over

resulting in vortices.


This is FloWorks? Are you using professional or academic version? TIA
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Old 03-04-2012, 09:54 AM   #289 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Yes, looking at that rear view makes me wonder if a flattened "beaver tail" or a vertical "fish tail" would result in a lower drag shape?
I agree, and think Morelli would too.

Nice work ERTW, this exercise of yours is helping me sort out what I want to do next on my own 2-part design concept.
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Old 04-11-2012, 08:50 AM   #290 (permalink)
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I have a question about the template, when comparing it to a lifting body aircraft similar to the NASA reentry craft of the 1960's and 1970's.

Commercial application:
US-SpacePlane Systems - Associations

Automobile 2 - Odds and Ends pictures by kach22i - Photobucket


From the image above (flipped the sunrise picture), you can see several deviations from the template. Considering this form is supposedly optimized, how can this be explained.

I'm assuming full airflow attachment considering claims like the one below.

US-SpacePlane Systems - Associations
Quote:
During its two and one half year test period this remarkable aircraft has perform numerous runway takoffs and landings, stall recovery, aerobatic manuvers and water landings. Its jet fighter performance has paved the way for even more advanced lifting body crafts to be built by USSPS in the near future........

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1977 Porsche 911s Targa
1998 Chevy S-10 Pick-Up truck
1989 Scat II HP Hovercraft

Chin Spoiler:
http://forums.pelicanparts.com/off-t...effective.html

Rear Spoiler Pick Up Truck
http://forums.pelicanparts.com/off-t...xperiment.html

Roof Wing
http://ecomodder.com/forum/showthrea...1-a-19525.html

Last edited by kach22i; 04-11-2012 at 08:56 AM..
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