How to... define precision of simulation and the criterion of convergence Ansys Fluent

 An important step in modeling is to define the precision of calculations and the criterion of convergence. 

How to define precision of solver in Fluent

The figure above shows the definition of static analysis based on the Courant number

In the case of static analyzes, the size of the residual equations will determine the end of the analysis. When it comes to transient simulations, the size of the defined imbalance equations will determine the number of steps performed in each subsequent iteration.

However, the definition of the precision of calculations determines the length of the simulations. For static analyzes, the precision can be defined using the Courant number or quasi-transient analysis (pseudo time step size definition).

For simulations based on the time of the analyzed phenomenon, the main factor influencing the precision of the analysis is the determination of the size of the time step.

In addition to all the above-mentioned factors, it is invariably important to carry out a sensitivity analysis for  each simulation, examining the effect of the adopted time step size (static - courant number) and the size of the finite element of the model mesh.

Window where U can define residuals in Fluent 

The figure above shows the definition of the precision criterion of the performed analysis. The definition was to determine the size of the residual equations.

In the case of CFX, the precision of calculations is usually defined from the Solver Settings level (residual avg or max) with one value. Also when it comes to defining the size of a step or iteration - Fluent offers more possibilities, especially when it comes to manual control of subsequent stages of the analysis.