5.10 Horizontal Mixing

5.10.1 Command Status

Required.

5.10.2 Description

Horizontal turbulent mixing is specified for the entire model domain by using the Momentum Mixing Model command and two ‘Global’ commands in combination. Supported momentum mixing model implementations are summarised in Table 5.16, with links to the relevant implementation sections below. Commands are listed in Table 5.17. Global commands set parameters that are spatially constant over the entire model domain. For momentum mixing this may be a spatially constant coefficient or eddy viscosity value depending on the selection of the Momentum Mixing Model command. Global values can be overridden locally through use of material specific values (Section 5.14).

Horizontal momentum mixing model science is provided in Appendix B.7.

Table 5.16: Horizontal Mixing Implementations

Model Implementation	Description
None	No horizontal momentum mixing.
Constant	Spatially constant momentum mixing value.
Smagorinsky	Smagorinsky turbulence closure for momentum mixing.
Wu	Wu momentum mixing formulation.

Table 5.17: Horizontal Mixing Commands

Command	Description
Momentum Mixing Model	Required - Selects the momentum mixing model for horizontal sub-grid scale turbulent mixing.
Global Horizontal Eddy Viscosity	Conditional - Required when using the Constant, Smagorinsky or Wu Momentum Mixing Model. Assigns the eddy viscosity coefficient or constant value.
Global Horizontal Eddy Viscosity Limits	Optional - Sets a minimum and maximum limit on computed eddy viscosity when using the Smagorinsky or Wu Momentum Mixing Model.

5.10.3 None

This model is the default and excludes the simulation of momentum mixing. Using none is useful for analytical cases and laboratory experiments. It is not recommended for project modelling and an alternative momentum mixing model implementation should be adopted.

! Momentum Mixing Disabled
Momentum Mixing Model == None ! {None} | Constant | Smagorinsky | Wu

5.10.4 Constant

This model applies a spatially constant eddy viscosity value to all cells in the model domain.

! Constant Model
Momentum Mixing Model == Constant ! {None} | Constant | Smagorinsky | Wu
Global Horizontal Eddy Viscosity == 1.0 ! {0.0} Eddy viscosity (m2/s)

5.10.5 Smagorinsky

This model calculates eddy viscosity on a cell by cell basis using local horizontal velocity gradients and cell size. This method is recommended for ocean, coastal, estuarine and lake studies or for models that are likely to be extended to 3D hydrodynamics during a project. The argument to the command Global Horizontal Eddy Viscosity is interpreted as the Smagorinsky coefficient in this model.

! Smagorinsky Model
Momentum Mixing Model == Smagorinsky ! {None} | Constant | Smagorinsky | Wu
Global Horizontal Eddy Viscosity == 0.5 ! {0.0} Smagorinksy coefficient
Global Horizontal Eddy Viscosity Limits == 0.05, 99999. ! {0.0} Minimum eddy viscosity (m2/s), {99999.} Maximum eddy viscosity (m2/s)

5.10.6 Wu

This model calculates eddy viscosity on a cell by cell basis using local velocity gradients and water depth. Wu is recommended for 2D only simulations in riverine and floodplain areas where cell size can become small relative to the water depth. The argument to the command Global Horizontal Eddy Viscosity is interpreted as the Wu coefficient in this model.

! Wu Model
Momentum Mixing Model == Wu ! {None} | Constant | Smagorinsky | Wu
Global Horizontal Eddy Viscosity == 4.0 ! {0.0} Wu coefficient
Global Horizontal Eddy Viscosity Limits == 0.05, 99999. ! {0.0} Minimum eddy viscosity (m2/s), {99999.} Maximum eddy viscosity (m2/s)