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Old   March 16, 2014, 12:34
Question Convergence parameter CFX Solver
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Chirag Trivedi
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Hi,
Few Days back I have started a steady state simulation (Hexahedral mesh, node = 35 million) of a Penstock using ANSYS CFX Solver. Boundary condition are mass flow inlet (34000 kg/s) and pressure outlet (225 kPa absolute). Convergence criteria was 0.001 MAX. Residual for all
I have observed following points during the simulation.
(1) First I set automatic timescale, the solution was stabilized at 0.01 MAX Residual and automatic time scale was 0.7 s.
(2) Then I selected Physical time scale of 0.1 s the solution converged below than the automatic timescale residual value but not converged.
(3) Further I was continuously changed the physical timescale to 1E-9 s, then immediately the maximum residual reached to 0.001 and the solution converged.

The point is selected physical timescale was far below than the actual residence time in the domain. The residence time was 151 s (Checked in CFX Post streamline Timescale Min=0 and Max=151 s).

Mass imbalance <0.01%.

Is there something wrong with the solution since residence time (i.e 151 s)is far away from the selected timescale (i.e. 1E-9 s)?
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Old   March 16, 2014, 18:00
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Yes, I would be cautious of this result. When you need to use a microscopic time scale to get it to converge you can get into positions where the residuals appear tight but convergence has not been achieved.

If you have to use such a small time scale for convergence I would look at your simulation and improve the numerical stability - that means things like better mesh quality, double precision numerics or better initial condition.
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Old   March 17, 2014, 01:45
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Thank You Glenn for suggestions.

(1) Quality of the hexahedral mesh is above 0.7 in ICEM CFD.
(2) For initial guess, I used result file which was obtained based on automatic timescale.
(3) It is always double precision, high resolution for all solving equations including turbulence numeric (k-epsilon).

More importantly, When I run this simulation as transient using time step of 0.001 s, it converges fast with just three coefficient loop iterations. I do have limitations of RAM I could not continue with transient simulation and switched to steady state.

(1) Will another turbulence model make difference in this case, i.e. k-omega
(2) Will I try another boundary condition, i.e. Pressure inlet and pressure outlet
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