# Relation between physical and computational time

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 August 8, 2005, 08:21 Relation between physical and computational time #1 Salman Guest   Posts: n/a hi, Can anyone comment in any relation between computational time step (e.g., used in Fluent in the form of CFL etc) and real life time step.Can we relate physical and computational time in any sense? In fact, I am simulating a flow through a cylinder with moving piston in it. If i simulate the flow with a time step of 1e-4 with 10000 iterations...Can i say that this will be the state of real life flow in 1 second? Regards, S Khan

 August 8, 2005, 08:54 Re: Relation between physical and computational ti #2 Jim_Park Guest   Posts: n/a One condition that must be met for the computational (or simulated) time to be equivalent to the physical time is that the boundary conditions you apply must match the physical time dependence of the system you're simulating. If you wish to predict the piston motion, you'll need a number of time steps. The language is important. An iteration is a pass through the equations for one time step and is directed at obtaining a correct solution for that one time step. The process is repeated for as many time steps as needed. Incidently, with this definition, the often-used direct steady state solution can be interpreted as one infinite time step! I believe that Fluent applies a number of iterations within each time step (10000 seems like a lot). You would first select a time step size so that the physical transient you wish to simulate is well resolved in time. Then you apply the number of iterations required for EACH of those time steps to get convergence for that time step.

 August 8, 2005, 09:17 Re: Relation between physical and computational ti #3 Vinod Dhiman Guest   Posts: n/a Yes Jim is right. Iteration and time step are related the way Jim has explained. There can be as many iterations for a single time step as are required to converge the solution for that time step, and then you increase the time. That's the way numerical simulation works. Computational time step and physical time steps are very same if your initial and boundary conditions are physically correct. Vinod Dhiman

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