# 4.10.3. Wall

A wall boundary is a boundary through which no advective mass flux is allowed. The wall itself can be stationary, or have a specified velocity tangential to the flow. It can be isothermal, or have a specified `wall_temperature`, for simulations including an enthalpy equation.

    begin wall boundary condition on surface surface_4
        wall_temperature = 400
    end

If you specify a value on the wall for a mass fraction, mixture fraction, or progress variable there will be a dirichlet boundary condition applied on the wall nodes for that scalar, which means that there will be diffusion of that scalar through the wall. If this is not desired, do not specify a value on the wall boundary condition.

The complete list of available commands for the wall boundary condition and the syntax for the different options can be found in Wall Boundary Condition On Surface.

## 4.10.3.1. Turbulent Wall Modeling

Wall-modeled (as opposed to “wall-resolved”) turbulent simulations use a boundary layer model at the wall. The choice of whether a wall model is used is defined in the `Turbulence Model` block in `Solution Options`. There are two wall-model related options one can specify per boundary. They are `Use equilibrium production model`, which sets the rule that turbulent production and dissipation are equal at the wall, and `law_of_wall_roughness_parameter`, which sets a dimensionless roughness parameter used in the wall models (, default is 9.8). For more details, see the wall modeling section in the theory chapter.

    begin wall boundary condition on surface surface_4
        # ...
        Use equilibrium production model
        law_of_wall_roughness_parameter = 9.8
    end

## 4.10.3.2. Conjugate Heat Transfer

When a wall provides a conjugate heat transfer interface to Sierra/Aria, you can indicate this with the `interface boundary` command. This signals to Fuego that `wall_temperature` will be supplied via a transfer from an external region.

    begin wall boundary condition on surface surface_4
        # ...
        Interface boundary
    end

## 4.10.3.3. Convection Coefficients

By default Fuego calculates linearized flux terms (`flux_linearization_coefficient` and `flux_linearization_temperature`) on the wall in a form that resemble a convection equation, but using a near-wall temperature instead of a far-field reference temperature. This means that the coefficient here will be mesh-dependent, and will not correspond to an expected convection coefficient from a correlation.

(4.29)

For comparison with a conventional correlation for one would want something of the form

(4.30)

Fuego has a special post-processor for calculating in this manner on the wall boundaries, with a user=specified . This will produce a `wall_convection_coefficient` nodal field that can be output to Exodus or used in other post-processors.

    begin wall boundary condition on surface surface_4
        # ...
        Calculate Convection Coefficient using Tref = 350
    end