Documentation/UserGuide/Viscosity

Both isotropic (Navier-Stokes) and anisotropic (Braginskii) viscosity can be added. In either case, configure the code with

    % configure --enable-viscosity


Both are added at first-order via operator splitting.

The update is explicit in time, so that a very restrictive CFL constraint on the time step will be used.

# Navier-Stokes viscosity

Enable by setting a value for the coefficient of kinematic viscosity nu_iso in the problem generator. For example, to read a value from the <problem> block in the input file, or to set a default value of zero if a value is not specified in the input file, add the line

    nu_iso = par_getd_def("problem","nu_iso",0.0);


anywhere in the problem generator. If the code detects nu_iso > 0, then the viscous stresses will be added. Currently nu_iso must be a constant.

# Braginskii viscosity

In this case, viscous stresses are applied only in the direction parallel to the magnetic field lines. MHD must be enabled to use Braginskii viscosity.

Enable by setting a value for the coefficient of parallel kinematic viscosity nu_aniso in the problem generator. For example, to read a value from the <problem> block in the input file, or to set a default value of zero if a value is not specified in the input file, add the line

    nu_aniso = par_getd_def("problem","nu_aniso",0.0);


anywhere in the problem generator. If the code detects nu_aniso > 0, then the viscous stresses will be added. Currently nu_aniso must be a constant.

To add both non-zero parallel and isotropic viscosities, simply set values for both nu_iso and nu_aniso in the problem generator.

If both nu_iso and nu_aniso are zero, and viscosity is enabled, the code will terminate with an error message.