Flow Illustrator - a simple windtunnel simulator

[tjts1]

I'm not sure what happened with this one. I think this Passat went supersonic about 15 seconds into the video.

YouTube - passat fart

[HyperMiler]

hahaha

Does that Passat have a jet engine?

[adrive7]

Add more white area ahead of the car and run it again. Check your numbers too. Re=2,500,000

[tjts1]

Originally Posted by adrive7

Re=2,500,000

Why are we using 2,500,000?

[adrive7]

Originally Posted by tjts1

Why are we using 2,500,000?

Because I said so apparently. But I said so because it is about right for an automobile. All that really matters is that the Re is in the millions range. Variations within that range shouldn't affect the flow.

But if you want a more official source,

The Reynolds number

As for the white area, the flow needs some room to develop before it hits your car. Otherwise you will get bad results.

[Sergei]

Hello everyone,

Thirdman told me about this forum and this thread. Naturally, I was happy to see the interest in Flow Illustrator, and also I noticed a lot of recent activity on the server, which is now explained.

I would be gratefull for your comments, as this may improve Flow Illustrator. If, say, the functionality of it is fixed, what can be improved?

Another thing: I read all the posts, and, rather naturally, the level of the background knowledge of aerodynamics and CFD (Computational Fluid Dynamics) issues, well, varies. I would be happy to explain things, but there is just no way to explain aerodynamics in one sitting, and it would be nice not to need to repeat the same explanations again and again. May be I should concentrate them somewhere. Would there be much interest, do you think, to something like 'Aero without math' blog (run by a professor of the Department of Aeronautics of the Imperial College London, which, as you surely know, is one of the best universities in the world :-))?

[tasdrouille]

Hi Sergei,

I'm glad you joined the discussion. The informaion in the forum is a bit spread out, but if you digg a bit in the aero forum you will find that there are some well written posts on aerodynamics, and we have a few very knowledgeable folks too.

That being said, I would be very interested, along with many others I'm sure, to read an aero without math blog run by you.

Would you happen to have experience with road vehicles areodynamics? It might be nice to start a new thread where people could ask questions. I know there are some people very interested in aerodynamics here which would enjoy reading your take on some specific aspects of road vehicles aerodynamics. I know I am

I complied a list of papers (research, SAE and others) I read and found to contain useful information in regards to fuel economy, in a sticky post in the general efficiency forum which contains some papers dealing with aerodynamics. Those papers however are just what I was able to find freely available on the Internet. If you know of other aero papers freely available (research papers preferred) that could apply to road vehicules, I would very much like to read them.

[lunarhighway]

i for one would be very interested in learning some 'Aero without math' basics, i know it's complex matter, but it would be nice to have some educated view on certain questions and assumptions raised here. that would give me a vague reference to understand why certain things work, rather than just have to copy things without understanding what they do....

also could someone post a list of input numbers for the program and what speeds they correspond to? i thought i had it, but it seems i got a few things wrong, would be nice to be able to test different configurations at different speeds so see when they start makeing a differnece

[Sergei]

Originally Posted by lunarhighway

... also could someone post a list of input numbers for the program and what speeds they correspond to? i thought i had it, but it seems i got a few things wrong, would be nice to be able to test different configurations at different speeds so see when they start makeing a differnece

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_am_socket]

Sergei, thanks for joining the discussion! Your input is much appreciated.

I am not the best with math so anything that can benefit my aerodynamic knowledge and its application without manual number crunching is a major plus