The Viscosity Package in MODFLOW 6 is used to account for the effects of solute concentration or temperature on fluid viscosity and thereby their effects on hydraulic conductivity and stress-package conductance. The VSC: Viscosity Package pane is on the MODFLOW Packages and Programs dialog box under Flow Packages.
If the Viscosity package is used, the Groundwater Transport process must also be used. In addition, the flow and transport models must be part of the same simulation. These options will be selected automatically when the Viscosity package is selected if they are not selected already.
The Viscosity package will adjust the conductances in the model based on the solute concentrations.
The user must specify a fluid reference viscosity used in the equation of state. In ModelMuse, it is set to 0.0008904 by default.
For each chemical species, the user specifies a reference concentration or temperature (cviscref) and dviscdc. Dviscdc is the slope of the line relating concentration or temperature to fluid viscosity.
The user may specify one species as the thermal species. The effect of the thermal species on viscosity can be simulated linearly or nonlinearly depending on THERMAL_FORMULATION. For the nonlinear treatment, the following term is added to the equation relating concentration to viscosity.
where A2, A3, and A4 are constants that are typically set to 10, 248.37, and 133.15 respectively.
Instead of calculated the viscosity of water entering the model through boundary conditions, the user can specify the viscosity directly by checking the Specify viscosity checkbox. If this is done, a special chemical species named "Viscosity" will be include in the Chem Species Pane. It is used to specify the fluid viscosity of water entering through the boundary condition in the General Head Boundary (GHB), River (RIV), Lake (LAK), Streamflow Routing (SFR), and Multiaquifer Well (MAW) packages.
If the Write viscosity option is selected, MODFLOW will write a binary file with the fluid viscosity at each cell each time model heads are written. The file has the same format as the binary head file. ModelMuse can import and display the viscosity file.
