# Unsteady Simulation - Wierd results from Fluent

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 July 18, 2011, 12:17 Unsteady Simulation - Wierd results from Fluent #1 New Member   Nik Join Date: Mar 2010 Location: Michigan Posts: 25 Rep Power: 16 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?

 July 18, 2011, 17:23 #2 New Member   sunny Join Date: Jan 2011 Posts: 29 Rep Power: 15 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.

 July 19, 2011, 07:31 #3 Member   Mohankumar.G Join Date: Sep 2010 Location: Pune,India Posts: 44 Rep Power: 16 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

October 3, 2011, 07:13
#4
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mannu
Join Date: Aug 2011
Posts: 3
Rep Power: 14
Quote:
 Originally Posted by nikhil 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?)