If you are performing a steady-state calculation, the governing equations for the pressure-based solver do not contain time-dependent terms. For steady-state flows,
Section 25.3 describes control-volume-based discretization of the steady-state transport equation (see Equation 25.2-1).
Under-Relaxation of Variables
The under-relaxation of variables is used in all cases for some material properties, in the NITA solver for solution variables, and in the pressure-based coupled algorithm where this explicit under-relaxation is used for momentum and pressure.
Because of the nonlinearity of the equation set being solved by FLUENT, it is necessary to control the change of . This is typically achieved by under-relaxation of variables (also referred to as explicit relaxation), which reduces the change of produced during each iteration. In a simple form, the new value of the variable within a cell depends upon the old value, , the computed change in , , and the under-relaxation factor, , as follows:
Under-Relaxation of Equations
The under-relaxation of equations, also known as implicit relaxation, is used in the pressure-based solver to stabilize the convergence behavior of the outer nonlinear iterations by introducing selective amounts of in the system of discretized equations. This is equivalent to the location-specific time step.
The CFL number is a solution parameter in the pressure-based coupled algorithm and can be written in terms of :