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some1 April 14, 2002 23:47

another VOF doubt
 
Hi all!! I'm simulating a 2 phase flow using VOF in (very)laminar conditions. I am interested in obtaining linear streamline functions but when I include the surface tension source term, the streamline functions are highly deformated.

Is it a inherent property of the method?

can it be my comercial code?

shall I let the simulations run for longer time?

Any information is highly appreciated. thanks

some1

edward g. cruz April 15, 2002 01:52

Re: another VOF doubt
 
did you follow my suggestions before? i would have assumed you've got your simulation running. with regard to your questions: 1) if you try all the available methods,

you'll probably get the same result. 2. Fluent??? it's pretty good, and if you try another package, you'll end up where you are now. 3) yes!!!, time interval settings in unsteady simulations is critical, also the total number of iterations. does your solution converge? i.e. on your iteration plot, does all the plotted variables converge into a flat line as the # of iteration increases? if not, run the simulation for more iterations. remember to try out default settings first before trying out by trial and error. keep on it and go with the flow, Edward

some1 April 15, 2002 09:40

Re: another VOF doubt
 
Thanks Edward.

I've followed your suggestions. The solutions converge but the time step is around 0.1 microsecond (10-7). The variables converge to a flat line except the continuity equations that oscilates depending on the nature of the liquid. I'm using the default settings as well as the suggestions found in the manual, ie, PISO for the pressure-velocity coupling, and underrelaxation factors around 1.

I'll keep on that.

thanks again.

New1

mukhopadhyay April 15, 2002 22:51

clarification on VOF
 
Single phase momentum equation(s) and a continuity equation accommodating the density (weighted average) are the basis of VOF - am I correct in my understanding ? Suppose I am modeling the surface disturbance in a liquid (say water) exposed to air, using VOF. How do I take care of the viscosity ? Is it the weighted average too ? Is that a right proposition? Viscosity is intensive property - can I do that? Has there been any attempt to evaluate/estimate the prospective numerical diffusion ? Else, what are we predicting ? At this stage, I am not talking of Newtonian behavior or not.

edward g. cruz April 15, 2002 23:07

Re: another VOF doubt
 
-new1 or some1; I think your time step is too fast for the flow you're considering! I'm assuming that this simulation is the one with the cylinders with liquid flowing on top of it and air flowing on top of the liquid(the picture you sent me).

if you did a total of 100 iterations, for example with your current time step, that's only a 10 microsec time window for your flow! I think the fluid flowing on top of your cylinders will not flow that fast or even the air on top of it! clue: estimate the amount of time for the flow to convect from the inlet to outlet boundaries and double it(or multiply by a factor of your choice).

also, I think your residual plot is incorrect(the plot that shows you the convergence of your solution). With my experience with unsteady flows, the solutions alway converges to a nearly flat sinusoidal line(all variables).

i think if you fix the time step problem, you'll have your simulation done correctly. forget about the coupling and relaxation factors for now because it only affect the rate of convergence and it's not your headache for now.

I hope this helped you out. Go with the flow, Edward

some1 April 16, 2002 00:00

Re: clarification on VOF
 
Thanks Edward I will follow your advice and let you know if I succed.

Your help is really appreciated.

some1

mohammadi April 16, 2002 06:52

Re: another VOF doubt
 
HI I WANT TO SIMULATE THE MOULD FILLING. I WANT TO USE FLAIR METHOD IN APPROXIMATION FREE SURFACE OF MELT I DEVELOPE A CODE THAT MOVES A SQUAR IN X DIRECTION AND Y DIRECTION WELL .BUT IN XY DIRECTION IT MOVE BADLY. I WANT TO KNOW WATH IS MY WRONG. THANKS


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