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Unsteady Simulation - Wierd results from Fluent

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Old   July 18, 2011, 12:17
Default Unsteady Simulation - Wierd results from Fluent
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Nik
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Hello,

I am running an unsteady simulation with Mass flow inlet and pressure outlet on a 3D geometry.
Mass flow inlet is a transient boundary condition that updates with the time step of 0.0005 sec.

I am experiencing that for the same value of mass flow rates at different flow times, fluent is providing different and very very random values of static pressure at inlet.

For a particular mass flow rate value at inlet, there has to be a single static pressure value, isn't it? But, i am getting random values of pressure even though the convergence criteria is set to 1e-07.

Any comments why this is happening?
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Old   July 18, 2011, 17:23
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sunny
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HI Nikhil,
I am new to Fluent and not very experienced in transient simulation. I am simulating flow of slurry in pipes. I wanted to know what is the criteria behind the time step size and number of time step ? Should the time step size be set such that for every time step the convergence should be within 5-10 iterations. So how should we determine number of time steps ? Any help would be appreciated.Thanks.
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Old   July 19, 2011, 07:31
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Mohankumar.G
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Hi Nikhil

Have you a set a surface monitor for the static pressure at the inlet to check whether the pressure is getting constant there? even though the equations are converged some times the pressure at the inlet has yet to be converged.

Regards,
Mohan.G
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Old   October 3, 2011, 07:13
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mannu
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Quote:
Originally Posted by nikhil View Post
Hello,

I am running an unsteady simulation with Mass flow inlet and pressure outlet on a 3D geometry.
Mass flow inlet is a transient boundary condition that updates with the time step of 0.0005 sec.

I am experiencing that for the same value of mass flow rates at different flow times, fluent is providing different and very very random values of static pressure at inlet.

For a particular mass flow rate value at inlet, there has to be a single static pressure value, isn't it? But, i am getting random values of pressure even though the convergence criteria is set to 1e-07.

Any comments why this is happening?
Is there density change? (or have you found solution yet?)
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