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Re: [Question #695542]: Unstable advection simulaton for gas flow


Question #695542 on Yade changed:

    Status: Open => Answered

Robert Caulk proposed the following answer:
Hey Zoheir,

> The problem is the model becomes unstable.

What does that mean? Is there an error? If so, what is the error? If
not, what is happening?

> I used this example [1] and only modified the fluid properties to
represent vapor.

Are you sure? When I use those parameters and decrease the timestep
proportionally to viscosity, the model is stable. Please provide an MWE
[1] ;).

> I used fluid density of 0.013 kg/m3 and heat capacity of 1996 J/kgK
and viscosity of 0.00013 pa.s.

The stability (read maximum allowable timestep) of the coupled thermo-
hydro system depends on the viscosity indeed. In fact, assuming the
pressure field remains identical, a back of the napkin scale analysis
demonstrates that the time step is directly proportional to viscosity.
So in this case you should only need to decrease the timestep by 1 order
of magnitude. As I mention above, it is in fact stable by decreasing
timestep by 1 order of magnitude. If you change the particle size,
however, you are playing with a quadratic relationship between particle
size and maximum allowable timestep.

> I reduced the timestep by two orders of magnitude and still no luck.

Hence why I suspect you changed particle sizes or pressure field.

> One more question: How can I specify different thermal properties
(like conductivity) to different spheres in the model?

I am happy to answer this question but please open a separate thread.
The goal here is to maintain an organized knowledge base that is easy
for future yade users to search and parse information. Thanks for
helping us build this knowledge base :-). If you are interested in the
guidelines that we adhere to on launchpad, you are welcome to review
them here [2]


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