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Old 07-21-2008, 04:46 PM   #81 (permalink)
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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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Old 07-21-2008, 05:23 PM   #82 (permalink)
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apgrok1 -

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Originally Posted by apgrok1 View Post
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?

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

Quote:
Originally Posted by co_driver View Post
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.
...
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Originally Posted by apgrok1 View Post
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)...
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Old 07-21-2008, 05:51 PM   #84 (permalink)
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co_driver -

Thanks, that's the equation I was using. So, my 2,500,000 Re is still ok.

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Old 07-21-2008, 11:54 PM   #85 (permalink)
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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
Click image for larger version

Name:	streamline.jpg
Views:	64
Size:	29.8 KB
ID:	1206  
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Old 07-22-2008, 02:54 AM   #86 (permalink)
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Concrete -

Quote:
Originally Posted by Concrete View Post
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 :

Name:  saturn_profile.05.a.BMP
Views: 363
Size:  4.1 KB

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

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Old 07-22-2008, 02:57 AM   #87 (permalink)
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Hello -

Hrmmmmm, maybe we should as Professor Chernyshenko to comment.

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Old 07-22-2008, 04:00 AM   #88 (permalink)
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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
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Old 07-22-2008, 04:25 AM   #89 (permalink)
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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!!!
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Old 07-22-2008, 09:58 AM   #90 (permalink)
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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

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