# Physical Time step

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 September 24, 2022, 12:49 Physical Time step #1 New Member   saadoudi messaoud Join Date: Mar 2022 Posts: 4 Rep Power: 4 This is my second pub in the forum hope it's not break rules here, I'm doing a steady state simulation for centrifugal pump using ansy cfx with inlet and outlet extensions, I set total pressure in inlet and Mass flow rate at outlet as boundary conditions, So my questions are: 1. How should my time step value be, and am I able to use two times step in the same situation because I didn't get the convergence criteria and are the results applicable in this case (using two times step in same situation)? 2.my second question please, how should my inlet and outlet extensions length be? Please I need your help because I still beginner in CFD.

 September 25, 2022, 16:42 #2 Senior Member   Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27 I have done simulations on various centrifugal pumps with varying time steps. The choice for time step size strongly depends on the exact configuration and the question you are trying to answer when doing this. Can you elaborate? Nevertheless, in steady state, I always use auto timestep with timescale factor 1 but sometimes decrease to 0.1 or increase to 10. Large pumps have larger numbers, smaller ones have smaller numbers. And length at inlet and outlet...... that also depends. Are you trying to evaluate a test rig, or a real life situation. In other words: what question are you trying to answer when doing CFD......? ahmed abdelhamid likes this.

September 25, 2022, 18:06
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 Originally Posted by Gert-Jan I have done simulations on various centrifugal pumps with varying time steps. The choice for time step size strongly depends on the exact configuration and the question you are trying to answer when doing this. Can you elaborate? Nevertheless, in steady state, I always use auto timestep with timescale factor 1 but sometimes decrease to 0.1 or increase to 10. Large pumps have larger numbers, smaller ones have smaller numbers. And length at inlet and outlet...... that also depends. Are you trying to evaluate a test rig, or a real life situation. In other words: what question are you trying to answer when doing CFD......?
Thank you very much Gert-Jan for all clarification, the purpose of my CFD simulation is to validate my CFD results with Experimental
Concerned the first question about exact configuration, what configurations you want to know and I will elaborate them
Thank you so much

 September 25, 2022, 18:21 #4 Senior Member   Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27 Then your geometry should look like your experimental setup. Not only the pump as in a fancy brochure. The outlet is less important, just extend it to where your measurement is. Inlet is more important. This should be like in experiments. Inlet swirl and suction head might be important if NPSH needs to be calculated as well, if that is part of the experiments..... ahmed abdelhamid likes this.