Flow Illustrator - simple air flow tool

[thirdman]

After beating my brains out looking for a free 2-d wind tunnel simulator, I stumbled across this:

http://chernyshenko.sesnet.soton.ac.uk/FIHome.htm

aka "flow illustrator". It will let you upload a bitmap and do a ROUGH simulation of airflow around it.

I uploaded a silhouette of a festiva and got some fun results. Check it out... it is free, I am not affiliated in any way...

(see attached for my silhouette and a screen shot from the resulting movie)

[Pu241]

You "The Man"!
Can we take a pic and bit map it into a more realistic/detailed sihouette?
Was thinking of a side profile with wheels.
The side profile gives us top to bottom and front to back flow.
I was thinking of a top profile to see flow around the vehical left to right.
Will it provide estimates of C sub d, coefficent of air resistance?
Be nice to see if it comes close to the reference provided by Festyboy.
Which I beleive to be as acurate an estimate of C sub d for the festiva we are likely to get.
If so then we can use this tool to model modifications to the body to reduce C sub d.
I beleive it is within our reach to reduce C sub d from approx 0.4 to 0.2.
Without structural modifications requiring welding and loss of fuctionality.
This should then provide a nice increase in MPG or alternatively better "high" speed operation.

[thirdman]

To answer all of your questions: dunno...

It is VERY primitive... but should give some ball-park estimates for gross changes. I was playing around with spoiler positions, and basically found out that the only hope for a spoiler to sufficiently "spoil" the airflow is if it is flat, thin, even with the roof, and has a sizable air gap between it and the back of the car.

See the two attached images to see what I mean.

Also, I had troubles with any BMP bigger than 256x128 and/or movies more that 25 seconds long. YMMV.
(The 25 second limit is annoying as it take 15-20 seconds for it to "settle down")

[thirdman]

Oh, and attached is a cleaner .BMP silhouette.

[JPT]

Reducing C sub d (I'm taking this to mean drag coefficient) to 0.2 is going to take some work. I'm not saying it's not possible, but it won't be easy. For example, a BMW 520i has a drag coefficient of 0.31 Compared to that, our cars are rolling bricks.

[thirdman]

Yeah, there is only so much you can do.

I am tentatively impressed with the spoiler...

[Pu241]

I've posted this guy's work before.
But he has done such a great job with the aero mods it is really worth the time it takes to read.
By his calculations he has acheived a C sub d of 0.17 or somewhere in that area.
He doen't claim to be the expert.
He does a lot of testing and his MPG results speak to a low C sub d. (70+ MPG)
I don't think the mods are that radical.

http://ecomodder.com/forum/showthread.php/5th-generation-civic-hatchback-improving-aerodynamics-312.html

This is another one, except his mods are more extreme:

http://ecomodder.com/forum/showthread.php/aerocivic-how-drop-your-cd-0-34-0-a-290.html

[JPT]

I've posted this guy's work before.
But he has done such a great job with the aero mods it is really worth the time it takes to read.
By his calculations he has acheived a C sub d of 0.17 or somewhere in that area.
He doen't claim to be the expert.
He does a lot of testing and his MPG results speak to a low C sub d. (70+ MPG)
I don't think the mods are that radical.

http://ecomodder.com/forum/showthread.php/5th-generation-civic-hatchback-improving-aerodynamics-312.html

This is another one, except his mods are more extreme:
http://ecomodder.com/forum/showthrea...4-0-a-290.html

The wheel skirts on the first one are kind of cool. The front air dam is HUGE though! I know that it's not for looks, but it makes the car look nose heavy.

The second one is indeed extreme, but the MPG can't be argued with

I wonder what kind of MPG improvement wheel skirts and full wheel covers would make on a Festiva?

[Pu241]

FestivaFan,
Wheel covers should help some and do look cool for the rear wheels, but the fronts seem to be a bit awkward.
Thinking of looking into that coast down test they talk about and get a baseline and work my way from the small mods to the more exotic stuff!
Agreed that front air dam look big and ugly but apparently does provide a benefit.
Would hope I could make something a less obtrusive and more conducive to air flow.
Still think 0.2 C sub d is more easily "doable" than it first appears.
Just have to get started!

[Axlander9289]

The first Civic reminds me of an early 50's Nash. This aerodynamic stuff is interesting. I may have to join ecomodder out of curiosity...

[thirdman]

Ok, I have fiddled with that profile all day yesterday, and here is what I have concluded so far:

1) Reducing air flow under the car is a good thing. This can be accomplished with an air dam and/or lowering. An air dam is helped by using skirts.
2) A smooth underside helps, but reducing air flow seems to help more.
3) A spoiler helps A LOT... BUT: it must be thin, parallel to the ground, level with the roof and have a largish air gap between it and the car.
4) Effective boat tails are HARD to design. Getting one that works well destroys the "festiva-ness" of the car.

The summary of my summary:
In my ever-so-humble opinion, the best bang-for-the-buck aero mods are to get a properly designed spoiler and reduce flow under the car by lowering or using a simple air dam. This jives with the common-sense recommendations that I have heard elsewhere, but it is sure nice to see WHY and HOW it works.

[thirdman]

regarding the wings noted above, here is the comparison I did:

Wing providing "downforce"
(note massive red behind wing)




Wing/Spoiler attached (with no air gap):
(note: it's as bad or worse than nothing at all)





Wing pushing more air down, therefore causing "lift":
(note: least drag for the wing, but more overall drag, plus it would "lighten" the rear end, which is undesirable)




Proper spoiler:
(note: some drag by wing, but least overall drag (to my eye))




Original Car:

[Pu241]

Thirdman,
Good work!
But in this simulation isn't the underside of the car represented by a stright line?

Wonder if something similar to a spoiler can be attached to the lower bumper to detach the low pressure region?

I can confirm lowering the car helps.
I have lowering springs on the front only of my other fester and going down a large hill everyday coming home from work the lowered one will coast up to 75 mph while the stock one will barely reach 65 mph!
Idea - can you angle the front of the fester down, representing lowering springs on the front and see how that effects the rear air flow?

"Effective boat tails are HARD to design. Getting one that works well destroys the "festiva-ness" of the car."
I agree it will not be easy but the payoff could be significant.
But I disagree about the "festiva-ness", under all the mods it will always be a fester.
I was thinking of something to attach to/hard mounted to the rear hatch as they are easy to come by and easy to detach.
Two bolts(if you don't have a defrost and wiper). So the mod can evolve over time and keep the time the vehical is not road worthy to a minimum.

[thirdman]

Yeah, I cheated on the underside.

I am a tiny bit of a purist, so adding a boat tail kinda feels "wrong" to me.

I wonder if you could rig something up that fit in-between a slightly opened hatch and the car to effectively create a boat tail. Then you could even have the hatch still open-able.

I will try a run with the nose down to see what happens...

[Pu241]

I wonder if you could rig something up that fit in-between a slightly opened hatch and the car to effectively create a boat tail. Then you could even have the hatch still open-able.

That's the kind of idea I was hoping for!
Great thought!
That would be easier to integrate into the body too!
How difficult do you think it would be to simulate a festiva's hatch at various open angles and fill in the gap to make it solid to the flow?
Another thing about this idea is you then don't have to relocate the tail lights!
Still have to do something about the lisence plate though.