- variableThe name of the variable that this object applies to
C++ Type:AuxVariableName

Description:The name of the variable that this object applies to

- PorousFlowDictatorThe UserObject that holds the list of PorousFlow variable names
C++ Type:UserObjectName

Description:The UserObject that holds the list of PorousFlow variable names

- componentThe spatial component of the Darcy flux to return
C++ Type:MooseEnum

Description:The spatial component of the Darcy flux to return

- gravityGravitational acceleration vector downwards (m/s^2)
C++ Type:libMesh::VectorValue

Description:Gravitational acceleration vector downwards (m/s^2)

# PorousFlowDarcyVelocityComponentLowerDimensional

Darcy velocity on a lower-dimensional element embedded in a higher-dimensional mesh. Units m^{3.s}-1.m^{-2, or m.s}-1. Darcy velocity = -(k_ij * krel /(mu * a) (nabla_j P - w_j)), where k_ij is the permeability tensor, krel is the relative permeability, mu is the fluid viscosity, P is the fluid pressure, a is the fracture aperture and w_j is the fluid weight. The difference between this AuxKernel and PorousFlowDarcyVelocity is that this one projects gravity along the element's tangent direction. NOTE! For a meaningful answer, your permeability tensor must NOT contain terms that rotate tangential vectors to non-tangential vectors.

This `AuxKernel`

records the Darcy velocity within a lower-dimensional element living in a higher-dimensional mesh. For instance, to study flow within a fractured material, you might have created a 3D mesh with its own 3D subdomains (blocks of elements representing different aquifers and aquitards, for example) and within that 3D mesh you might have included 2D elements to represent the fractures. Those 2D elements must share nodes with the 3D elements for the MOOSE model to make sense. The 2D elements belong to a different set of subdomains, and those subdomains typically have different Material properties assigned to them (for example, high permeability and porosity). If you want to measure Darcy velocity within those lower-dimensional subdomains, then use this `AuxKernel`

.

This `AuxKernel`

calculates the *x*, *y*, or *z* component of the Darcy velocity for fluid phase , where and are projected onto the tangent direction of the lower-dimensional element. All parameters are defined in the nomenclature.

Notice that the denominator of this expression includes , which is the fracture aperture. If the user doesn't want to divide by , the default value of should be used. However, it is likely that when using this `AuxKernel`

the dimensions of permeability will be m, because of a pre-multiplication by the fracture aperture. The denominator includes so that the unconventional units of permeability may be accounted for if desired.

As this `AuxKernel`

uses material properties, only elemental (`Monomial`

) `AuxVariables`

can be used. The `AuxVariables`

must be defined on the lower-dimensional subdomain only.

For the result to make sense, the permeability tensor, , must not rotate tangential vectors to non-tangential vectors. For instance, an isotropic permeability tensor is sensible.

## Input Parameters

- aperture1.0Aperture of the fracture
Default:1.0

C++ Type:std::vector

Description:Aperture of the fracture

- execute_onLINEAR TIMESTEP_ENDThe list of flag(s) indicating when this object should be executed, the available options include NONE, INITIAL, LINEAR, NONLINEAR, TIMESTEP_END, TIMESTEP_BEGIN, FINAL, CUSTOM, PRE_DISPLACE.
Default:LINEAR TIMESTEP_END

C++ Type:ExecFlagEnum

Description:The list of flag(s) indicating when this object should be executed, the available options include NONE, INITIAL, LINEAR, NONLINEAR, TIMESTEP_END, TIMESTEP_BEGIN, FINAL, CUSTOM, PRE_DISPLACE.

- boundaryThe list of boundary IDs from the mesh where this boundary condition applies
C++ Type:std::vector

Description:The list of boundary IDs from the mesh where this boundary condition applies

- fluid_phase0The index corresponding to the fluid phase
Default:0

C++ Type:unsigned int

Description:The index corresponding to the fluid phase

- blockThe list of block ids (SubdomainID) that this object will be applied
C++ Type:std::vector

Description:The list of block ids (SubdomainID) that this object will be applied

### Optional Parameters

- control_tagsAdds user-defined labels for accessing object parameters via control logic.
C++ Type:std::vector

Description:Adds user-defined labels for accessing object parameters via control logic.

- enableTrueSet the enabled status of the MooseObject.
Default:True

C++ Type:bool

Description:Set the enabled status of the MooseObject.

- seed0The seed for the master random number generator
Default:0

C++ Type:unsigned int

Description:The seed for the master random number generator

- use_displaced_meshFalseWhether or not this object should use the displaced mesh for computation. Note that in the case this is true but no displacements are provided in the Mesh block the undisplaced mesh will still be used.
Default:False

C++ Type:bool

Description:Whether or not this object should use the displaced mesh for computation. Note that in the case this is true but no displacements are provided in the Mesh block the undisplaced mesh will still be used.

### Advanced Parameters

## Input Files

- modules/porous_flow/examples/flow_through_fractured_media/coarse.i
- modules/porous_flow/test/tests/aux_kernels/darcy_velocity_lower.i
- modules/porous_flow/examples/flow_through_fractured_media/fine_transient.i
- modules/porous_flow/test/tests/aux_kernels/darcy_velocity_lower_2D.i
- modules/porous_flow/examples/flow_through_fractured_media/coarse_3D.i
- modules/porous_flow/test/tests/aux_kernels/darcy_velocity_lower_except.i