EcoModder Forum Flow Illustrator - a simple windtunnel simulator

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 07-21-2008, 04:46 PM #81 (permalink) EcoModding Lurker   Join Date: Dec 2007 Location: Wichita Posts: 53 Thanks: 0 Thanked 0 Times in 0 Posts Ok, I reworked the numbers and came up with 494,000 for 60mph. My discrepancy is that we have been using a diameter for characteristic length. That is ok for tubes, but for flow over a flat plate, the characteristic length is the car's length. Am I wrong?
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07-21-2008, 05:23 PM   #82 (permalink)
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apgrok1 -

Quote:
 Originally Posted by apgrok1 Ok, I reworked the numbers and came up with 494,000 for 60mph. My discrepancy is that we have been using a diameter for characteristic length. That is ok for tubes, but for flow over a flat plate, the characteristic length is the car's length. Am I wrong?
Can you put it in more context? Is the 494,000 for your car or "all" standard-issue passenger cars? If it is special for your car, what would other numbers be for other cars of different length?

Edit: Can you show the whole equation?

CarloSW2
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07-21-2008, 05:46 PM   #83 (permalink)
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From an earlier example:

Quote:
 Originally Posted by co_driver Numbers to use: L* = effective diameter of your car [1990 Firebird: (72.4" (width) * 49.8" (height) )^0.5 = ~60" = 1.53 m] v = velocity [50 mph = 22.35 m/s] 'nu' = 1.6*10^-5 m^2/s Thus: Re = 1.53 * 22.35 / 1.6*10^-5 = ~2,100,000 (*) Note: A better reference would be the effective diameter rather than its length, as length is used for flat plates. ...
Quote:
 Originally Posted by apgrok1 Ok, I reworked the numbers and came up with 494,000 for 60mph. My discrepancy is that we have been using a diameter for characteristic length. That is ok for tubes, but for flow over a flat plate, the characteristic length is the car's length. Am I wrong?
As I believe that we may consider practically all vehicles "bluff bodies", an effective length should be considered as the square root of the frontal area. Only in extreme cases could one start to consider a car as an aerofoil, in which case the length of the aerofoil would actually be an effective length for use in the Reynolds Number. Remember, the Re is meant for modelling full-size objects to a smaller scale in order to simulate its characteristics.

BTW: a sub-500k Re @ 60 mph is a very small frontal area (3.2 sqft)...

 07-21-2008, 05:51 PM #84 (permalink) Pokémoderator     Join Date: Dec 2007 Location: Southern California Posts: 5,864 1999 Saturn SW2 - '99 Saturn SW2 Wagon Team Saturn 90 day: 40.49 mpg (US) Thanks: 439 Thanked 528 Times in 355 Posts co_driver - Thanks, that's the equation I was using. So, my 2,500,000 Re is still ok. CarloSW2 __________________ What's your EPA MPG? Go Here and find out! American Solar Energy Society
 07-21-2008, 11:54 PM #85 (permalink) Sequential     Join Date: Jun 2008 Location: Kansas Posts: 177 The Truck - '00 Chevy S10 Extended Cab 90 day: 22.47 mpg (US) Thanks: 0 Thanked 22 Times in 7 Posts cfg83 was not trying to second guess you - more worried about the simulation itself. even if the Reynolds numbers different it would just mean we are simulating the wrong speed a bit. If the simulation math is wrong we could be simulating flow on Saturn or worse - flow in a cartoon world that is completely unattached to reality. I guess I was expecting somebody with a kamm back to get flow like below. even the snow cone looks like the flow is trying to separate down the back. I just thought of something Could just drop this pic in the simulator and try it and see how it compares That is about as close to a head to head against reality as we may be able to get not sure i will get it done tonight though Attached Thumbnails   __________________ Concrete Start where you are - Use what you have - Do what you can.
07-22-2008, 02:54 AM   #86 (permalink)
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Concrete -

Quote:
 Originally Posted by Concrete cfg83 was not trying to second guess you - more worried about the simulation itself. even if the Reynolds numbers different it would just mean we are simulating the wrong speed a bit. If the simulation math is wrong we could be simulating flow on Saturn or worse - flow in a cartoon world that is completely unattached to reality. I guess I was expecting somebody with a kamm back to get flow like below. even the snow cone looks like the flow is trying to separate down the back. I just thought of something Could just drop this pic in the simulator and try it and see how it compares That is about as close to a head to head against reality as we may be able to get not sure i will get it done tonight though
I am always confounded by aero because I know it works, but I don't feel comfortable with *how* to prove it works beyond testing it in the real world. This is the *closest* thing we've had to a usable free CD tester. The better thing would be a 3D freeware simulator, but then we have to spend time modeling and/or buy models that may also be lacking.

If I were you, I would do scale multiples of the 256x128 silhouette in 2-bit black and white. I am partial to the orientation of my car bitmaps because you can see more of the wake :

I have been watching for a pitch black wall to take a more representative picture of my car.

CarloSW2
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 07-22-2008, 02:57 AM #87 (permalink) Pokémoderator     Join Date: Dec 2007 Location: Southern California Posts: 5,864 1999 Saturn SW2 - '99 Saturn SW2 Wagon Team Saturn 90 day: 40.49 mpg (US) Thanks: 439 Thanked 528 Times in 355 Posts Hello - Hrmmmmm, maybe we should as Professor Chernyshenko to comment. CarloSW2 __________________ What's your EPA MPG? Go Here and find out! American Solar Energy Society
 07-22-2008, 04:00 AM #88 (permalink) Sequential     Join Date: Jun 2008 Location: Kansas Posts: 177 The Truck - '00 Chevy S10 Extended Cab 90 day: 22.47 mpg (US) Thanks: 0 Thanked 22 Times in 7 Posts streamline test okay it looks pretty close to the streamline picture check it out gotta say this maybe a good tool for a 2d starting point at least for a streamlined body (turbulent flow is tuff even for the pros) CarloSW2, I think it would be great if Professor Chernyshenko would help us out especially in knowing how "artistic" & how realistic the tool is but I will leave that to someone who can talk aero talk __________________ Concrete Start where you are - Use what you have - Do what you can.
 07-22-2008, 04:25 AM #89 (permalink) EcoModding Lurker   Join Date: Jun 2008 Location: South Africa Posts: 5 Thanks: 0 Thanked 0 Times in 0 Posts feel very dumb to come in this late in this thread and ask such a basic question ...but how do you upload your pic? I just know that when I have posted this I will see how and feel even dumber!!!
 07-22-2008, 09:58 AM #90 (permalink) EcoModding Lurker   Join Date: Jun 2008 Location: Nottinghamshire England Posts: 19 Thanks: 0 Thanked 0 Times in 0 Posts Hi there This may be of use if you have a 3d mesh of your car ? Dolfyn: Examples found here :- http://www.cfd-online.com/Wiki/Codes#Free_codes regards paul

 Tags air dam, air flow, simulation, spoiler, wind tunnel