Tuesday, July 22, 2008

Incidentally..

Fluent does the same.....

Let me give the background. I'm talking about a fluid dynamics problem of simulating Bingham-plastic fluid in an annulus. People who know about the physical situation will easily recognise the problem that the center part of the fluid should move like a slug (a solid).

To recognise the exact nature of the problem it took me a working discussion with my friend. Let me explain the complexity of it.

Let us assume a filter cake on (bingham plastics are typical suspensions) some wall.. Till certain gradient that filter cake is a solid.. after that it should be a fluid. We can talk in terms of stresses also same way. The important point to understand here is we do not have any clue for the stress strain relations of the solid portion (It can not be treated as bingham fluid for obvious reasons). So we can get the stresses very near to the solid cells (cell is a typical computing unit if CFD) and check whether it is fluid or not. But it is not the information in the solid portion which makes the solid portion to be untouchable by.. fluid gradients. What it tells...

is once you identify some part as solid, it remains as solid to the end.

All this is because of the way we are treating the solid. If we say it is rigid solid (no internal gradients) it will behave as a rigid one. But in reality solids do follow some machanics and there by having some internal gradients (after crossing certain magnitude.. it will turn into fluid) which we do not know.

So.. what I did is, for the sake of simplicity, assumed it as a high viscos Newtonian fluid, which keeps the gradients inside to ALMOST (but stills have them and will evolve) zero and there by allowing us to manipulate it through solid to fluid transition. (Fluid to solid transition is easy as we know the detail of the bingham plastic fluid).

All the garbage is over.. I've sent a mail explaining all this boring stuff to my lead. Guess what..??
he came and asked me... Is this the way fluent does this.. ?? I have seen it asking an effective viscosity which is very large..... blah..... blah.......

love this....

PS#1: fluent solver is much more^n stable.. but it seems I got my logic correct..

PS#2: It is always been difficult for me to keep the discussion at other's pace.. Some can really do it..
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