When this method is adopted, it is usual to prescribe a uniform value for the whole fluid field.
A suitable value is of the order of:
0.005 * typical velocity * typical cross-stream distance.
In PHOENICS, setting ENUT to the required value in the Q1 file is all that it is necessary to do.
It should be noted that the value whch is set is the kinematic viscosity. This is automatically multiplied by the local fluid density, within the PHOENICS solver, so as create the dynamic viscosity when it is needed.