Hi Harish Kamath ,

For URF, it's better to start the calculation with the default value then check the convergence. If the convergence isn't good enough, you can reduce it. But in that case, I personally preferred to reduce the time step size instead of URF.

For RMS convergence, 0.01 is small enough for system coupling if the solutions in fluent and structural have good convergence. So the convergence criteria in Fluent is more important and should be check during the calculation.

At the end of the simulation, check your results. If the results weren't accurate by these settings, you could reduce time step size in system coupling.

To conclude, I have to say that the RMS convergence target in fluent and structural is more important than in system coupling. If the solution in fluent and structural were accurate, the default value for system coupling is good for simulation. The main reason I emphasize convergence in Fluent is that FSI simulation is very complicated and takes much time, and by changing the settings in system coupling, the amount of calculations increases exponentially. Therefore try to change and improve the setup in Fluent and structural.

Dear/Harish

The following answer is copied from Franco Concli dated July 20, 2015 which was given 19 recommendations:

Relaxation and Convergence Criteria

Numerical methods used to solve the equations for fluid flow and heat transfer most often employ one or more iteration procedures. By their nature, iterative solution methods require a convergence criteria that is used to decide when the iterations can be terminated.

In many cases, iteration methods are supplemented with relaxation techniques. For example, over-relaxation is often used to accelerate the convergence of pressure-velocity iteration methods, which are needed to satisfy an incompressible flow condition. Under-relaxation is sometimes used to achieve numerically stable results when all the flow equations are implicitly coupled together.

Choosing Relaxation Criteria

The amount of over or under-relaxation used can be critical. Too much leads to numerical instabilities, while too little slows down convergence. Similarly, a poorly chosen convergence criteria can lead to either poor results (when too loose) or excessive computational times (when too tight).

Selecting proper relaxation and convergence criteria can be a difficult and frustrating experience for users of computational fluid dynamics software. The criteria depend on the specifics of the problem being solved, which may change during the evolution of a problem. Unfortunately, there are no universal guidelines for selecting criteria because they depend not only on the physical processes being approximated, but also on the details of the numerical formulation. Many CFD programs have a standard set of recommended criteria, but users must often resort to trial-and-error adjustments to get good results.

Hopefully, it will be valuable with my respect towards Franco Concli

thank you Ahmed Abdelhameed Sir and Ashkan Ghafari Sir for your valuable inputs.

Ashkan Ghafari Sir, if I am doing transient analysis then as you suggested I can reduce the time step size. But sir, if I have to do steady-state analysis then what supposed to do.

Thank you

Harish Kamath , Do you mean one-way FSI? As far as I know, steady-state analysis is defined as one-way FSI, and system coupling is no longer needed. In these simulations convergence is controlled by pressure-velocity coupling scheme and URF.

Ashkan Ghafari Sir, in two way FSI