[luccy]

hi
I'm simulating the stepped spillway with multiphase flow in fluent.I have two inlet and one outlet (velocity inlet for water and pressure inlet for air and pressure outlet),
Why am I getting a negative net mass flow rate in water phase in my model?
(mass flow rate in inlet - mass flow rate in outlet = negative value in water phase )?????

[LuckyTran]

Is the difference small? You have to accept some small differences as it's unlikely your continuity residual is exactly 0. What about convergence? Your solution may not even be converged.

[flotus1]

Are you sure that you got the signs right?
When evaluating a mass flow rate at an outlet in fluent, the value is supposed to be negative as long as the flow velocity vector is pointing out of the computational domain.

[luccy]

Originally Posted by LuckyTran

Is the difference small? You have to accept some small differences as it's unlikely your continuity residual is exactly 0. What about convergence? Your solution may not even be converged.

Thanks for the reply.
i dont have any idea about convergence,I attached a picture,According to residual monitor i think it is converged.
pls help me

[luccy]

Originally Posted by flotus1

Are you sure that you got the signs right?
When evaluating a mass flow rate at an outlet in fluent, the value is supposed to be negative as long as the flow velocity vector is pointing out of the computational domain.

I do not know what you mean, I attached a picture,it shows a negative value for net mass flow rate!

[flotus1]

At the inlet, a mass flow rate of 17603 kg/s enters the domain. At the outlet, a mass flow rate of 17604 leaves the domain (the negative sign indicates this). The imbalance between the two values is 1kg/s, the relative imbalance is 5.7e-5. A rather low value, just as it should be.
Yet judging from the residuals alone, the solution does not seem to be converged sufficiently.

[luccy]

Originally Posted by flotus1

At the inlet, a mass flow rate of 17603 kg/s enters the domain. At the outlet, a mass flow rate of 17604 leaves the domain (the negative sign indicates this). The imbalance between the two values is 1kg/s, the relative imbalance is 5.7e-5. A rather low value, just as it should be.
Yet judging from the residuals alone, the solution does not seem to be converged sufficiently.

how can i understand of residuals that solution is converged sufficiently?
convergence criteria is 10^-3 and my residual monitor shows residual values has reached to 10^-5? Is it not enough? what can i do?

[flotus1]

The first thing that seems suspicious to me is that some of the residuals are rising instead of falling.

Someone here put quite some effort in explaining how to find out if a solution is converged: http://www.cfd-online.com/Forums/flu...nvergence.html

[luccy]

Originally Posted by flotus1

The first thing that seems suspicious to me is that some of the residuals are rising instead of falling.

Someone here put quite some effort in explaining how to find out if a solution is converged: http://www.cfd-online.com/Forums/flu...nvergence.html

thank u very much.