Flow Illustrator Tryout

[ChazInMT]

Cfg Bud...There is a "More Information" page on the website. He is not real clear on some things, but, he is clear on the Re. Check it out yourself. 250,000 is not a number I think he'd agree was useful. 250,000 may indeed be the Re for real life calculations (and to be honest, I have no idea how I'd use it)

I was going to say that your model size was too big to see what was happening, and to prove my point I made 3 movies using 3 images with different sizes, and it looks like relative size isn’t that important. Only real difference is how much wake you see to study.

See these:

Small Drop

Medium Drop

Large Drop

Your 40 second image seems like a long time, if you set the dt to equal .02, it is like a .01dt but in half the time. The dt is simply a function of like film speed, it does not effect the flow, it just speeds things up so the steady state flow image is arrived at sooner. I have found dt = .02 and 10 second image time to be more than enough without taxing the resource too much. (and you get the movie back much quicker)

I use a 840 X 420 image size, it makes the cars look smoother. This I think makes the computation run longer, so it may be my way of Bogarting the resource by having a picture that is too large.

[cfg83]

ChazInMT -

Yes, I read that :

Flow Illustrator More Info page

In other places I have read that the professor said something akin to "100000 and 2500000 are going to give the same result". The numbers I picked came out of the original Flow-Illustrator thread from July 2008 :

http://ecomodder.com/forum/showthrea...ator-3709.html

The author of the Flow Illustrator joined our forum (with the username of Sergie) and made 3 posts in the above thread. Here is one :

Originally Posted by Sergei

All right.

For dt, the smaller dt the more accurate the result is, but the movie becomes slower as dt decreases. Use the default value for a start.

For Re, there are two answers, one easy, one hard.

The easy one: Flow Illustrator is not a predictive tool. Whatever parameters you submit, you cannot rely on the results to be valid. If you know what the correct answer should look like, you can play with the parameters until the movie looks right. As I wrote in the info section: treat it as artist impression, the artist is you. The only hard part of this is to know what is right :-)

The hard one:

Flow Illustrator uses the bitmap as a grid. The grids corresponding to bitmaps of the size for which calculation time is reasonable are nowhere near what is needed to resolve turbulence, and anyway 2D calculations cannot do it in principle. Flow Illustrator does not use any tubulence modeling, the Reynolds number corresponds to the laminar viscosity. Qualitatively, in the calculation the diffusion effects (read viscosity) is the sum of the laminar viscosity (determined by Re parameter) and the viscous-like effects of errors caused by the grid being not fine enough. Reynolds number based on grid viscosity has an order of magnitude of the number of the bitmap points across the picture (since the finite difference scheme is of the first order). Say, one has 800x600 bitmap, then the Reynolds number based on the grid viscosity is of order 600. So, once the Re one prescribes is much greater than this value, almost all the viscosity is the grid viscosity: the movies for Re=100,000 and Re=2000,000 will most likely look the same because the flow behaviour will be determined by the grid viscosity (there is, however, an issue of numerical stability into which I would not go at this point: one can recognise instability by relatively fine wiggles and other small-scale irregularities of the picture).

Still, as turbulence is not resolved, one has to do something about it. Turbulence is usually modeled by adding turbulent viscosity to laminar viscosity. The problem is, turbulent viscosity varies from point to point, while laminar viscosity is the same everywhere. In particular, turbulent viscosity is larger in mixing layers and smaller in boundary layers. It also strongly varies across boundary layers. So, what can one do with the Flow Illustrator? Select Re in the region 100-1000 if you want to model better the mixing layers, or in the region of 1000-10,000 if you are more after boundary layers, and hope for the best: anyway this will not be accurate. Why these values? -Too long a story, and, anyway, one would have to start from the very beginning.

One nice thing is, however, that flows modeled by many people here are separated. Many features of separated flows are determined primarily by the position of the separation points, provided the rest is modeled not too wrongly. In real flows and in the Flow Illustrator alike, if Re is large enough, the flow always separates from sharp corners. Note, it may also separate from a smooth wall, but from a corner it always separates. So, if the shape of the body is such that there is no separation from smooth walls but there are corners, even Flow Illustrator might serve as a predictive tool. One only needs to know a lot of aerodynamics to say if the body is such that there will be no separation from the smooth walls ...

Enough for this post, I think.

I'm not saying that your numbers are not the right ones. It's just that I haven't seen a consensus on what the best numbers would be for this tool.

CarloSW2

[ChazInMT]

Carlos, Sorry, I see now where you interacted with Dr. C in the intial thread. If you read the paragraph you're quoting from, he is simply making a point that once yer above Re 100,000, It's all gonna look the same. The paragraph below this he is saying to model mixing layers, use Re 100-1,000, and 1,000-10,000 for boundary layers. (which again, I'll admit, if a mixing layer ran me over with a bulldozer right now, I couldn't tell you what the heck it was) I just don't see where he recommends 250,000 anywhere to model flow. Anyway, none of this is really very accurate science as he himself states.

Thanks Carlos.

[cfg83]

ChazInMT -

Egg-zactly. Aero-stuff is a black art for me because the co$t of doing an accurate job is enormous. That's why I'm saying that your recommendations may be spot-on. I just don't know.

CarloSW2

[Arragonis]

Sorry guys, I can hear a wooshing way above my head

Are my basic settings OK to begin with and I assume the red/pinky bits behind the car are drag so its good to minimise those ? I know overall its more complicated but is this a good starting point ?

[dmac257]

Originally Posted by Arragonis

Just for fun I tried the Fabia in the flow illustrator - not very neat pictures (I don't do drawing very well). One with rear spoiler

Then I took the spoiler off


Then put on an unscientific kamm-back


I also did a 'basjoos'd version but it won't upload. Might have another play later.

What is the flow illustrator you refer to?

Don

[cfg83]

dmac257 -

This is the first thread on the simulator :

http://ecomodder.com/forum/showthrea...ator-3709.html

CarloSW2