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Initial Condition for a transient problem

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Old   June 7, 2016, 17:46
Default Initial Condition for a transient problem
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Krishna Sandeep Prata
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Hi all,

The problem that I am simulating has the continuity, momentum conservation and species conservation equations. I am using the RANS form of the equations with Transient SST as turbulence closure. At time t=0, the entire geometry has a static field of Argon. Absolutely no motion at t=0. At t>0, there are two inlets one of which has CO2 flowing in and the other with Nitrogen. There are two outlets maintained at the same pressure as the initial pressure. The Reynolds number based in the hydraulic diameter of the inlets is about 3000 for each inlet. The velocity at the inlets is about 5 m/s while the volume of the geometry is 1.81e-02 m3.

In my simulation, I am starting at the physical initial condition (zero x,y and z velocities, mass fraction of Ar=1, mass frac of N2=mass frac of CO2=0, turbulent quantities all initialized to zero, and the initial value of the pressure. I am using a time step of 0.05s with 40 iterations per each time step. I am using the iterative transient solver, SIMPLE coupling scheme, second order discretization for the momentum and the species equations.

My residual convergence check values are 1e-04 for the continuity and the momentum equations, 1e-03 for the turbulence equations and 1e-05 for each of the species equations.

The issue is that the residual values are not dropping below the stipulated values within each time step. I read in fluent manual that the normalized residuals should drop three orders of magnitude within each time step. It is not happening in my case. Will this pose a problem if I am interested in the transient behaviour of the problem?

Please feel free me to ask any more details of the case.

Thanks.
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