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-   -   Unsteady Simulation - Wierd results from Fluent (http://www.cfd-online.com/Forums/fluent/90684-unsteady-simulation-wierd-results-fluent.html)

nikhil July 18, 2011 12:17

Unsteady Simulation - Wierd results from Fluent
 
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?

sandisk July 18, 2011 17:23

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.

Mohankumarg12 July 19, 2011 07:31

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

mannudakamal October 3, 2011 07:13

Quote:

Originally Posted by nikhil (Post 316495)
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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