To set the parameters governing the update of turbulent viscosity, select the turbulent viscosity solver node in the Solvers node as shown below. Two properties are available in the Properties window.
The Maximum Ratio property sets the maximum allowable ratio of turbulent to laminar viscosity.
The Under-Relaxation Factor property governs what portion of the newly computed turbulent viscosity field is used to update the turbulent viscosity. The default value of 1 implies that the turbulent viscosity will be entirely computed from the new field.