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DEFINE_UDS_UNSTEADY for Multiphase,apu,su meaning

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Old   December 7, 2006, 13:22
Default DEFINE_UDS_UNSTEADY for Multiphase,apu,su meaning
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Hello Everybody!

Do you have experience with DEFINE_UDS_UNSTEADY for _multiphase_ flows?

In the FLUENT UDF manual, paragraph 2.7.4,

we see the approximaton for the unsteady term for UDS:

[unsteady term] = = -( (rho*phi)^(n) - (rho*phi)^(n-1) )/delta_t * delta_V = = apu + su,


apu = -rho * phi^(n) * delta_V / delta_t, su = -rho * phi^(n-1) * delta_V / delta_t.

^ here means the time step, not the power, of course.

on the other hand, the natural idea to discretize the unsteady term is:

apu = -rho^(n) * phi^(n) * delta_V / delta_t, su = -rho^(n-1) * phi^(n-1) * delta_V / delta_t.

FLUENT's default is

apu = -rho^(n-1) * phi^(n) * delta_V / delta_t, su = -rho^(n-1) * phi^(n-1) * delta_V / delta_t,

though. The question is why does FLUENT take density values for calculating apu from the previous time step and not from the current time step?

Thank you very much in advance!!

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Old   December 11, 2006, 11:14
Default Re: DEFINE_UDS_UNSTEADY for Multiphase,apu,su mean
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okay, i've found it out by myself. This is addressed in "FLUENT defects", ID 38858.

It says it is an error in the FLUENT and it is fixed in version 6.3.1.


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