Satyajeet Padhi
Ansys Employee

For a transient simulation, the number of iterations per time steps indicate how many times the flow equations will be solved per time step before moving on to the next time step. Generally this number is 20. You may increase or decrease this number depending on your application. In the residuals plot, you will notice a saw-tooth pattern, which indicates the iterations per time step. The residual curve will start from a high at 1st iteration, keep dropping if everything is fine until 20th iteration. This completes the first time step and then the residual will rise again and drop for the next 20 iterations and so on. 

Along with the residuals, I recommend having some monitor points in the domain where you can monitor the flow. For example, you can create a monitor point along the axis of the channel sufficiently far away from the entrance and monitor axial velocity there. For transient flows, you would obtain either of the following two:

1. Steady State Solution: The flow variables such as velocity flatten out (not change) after some time.

2. Periodic solution: The flow variables fluctuate with a repeating pattern.

It is recommended to begin with or initialize your flow field with a steady state solution, if possible. This way you are providing the best possible initial conditions for the model. If not, then the initial few time steps may not converge completely. You can then use a smaller time step in this case for the first few iterations. The time step size should be small enough to resolve time-dependent features. It should not be too large, else the transient changes will not be captured. However, using a very small time step will slow down the simulation significantly. The Courant number is used to estimate a time step. It is the product of characteristic flow velocity and time step divided by the cell size. This gives the number of mesh elements the fluid passes through in one time step. You can use values in the range of 1-10, but in some cases higher values are acceptable. You can experiment with some numbers to appreciate the significance of setting the right Courant number and time step size. 


But most importantly, the accuracy of any CFD simulation will depend on the boundary conditions and the initial conditions. If these two inputs are physically correct along with a nice mesh, you would obtain a nice converged solution.