# Uniform Sources/Sinks

Refer to Physical Processes for general information about processes and their implementation.

The uniform sources add a constant value $S$ to the rate of change of a quantity $q$. This contribution is uniform in space and therefore only affects the $(0, 0)$ wavenumber pairs. If $S$ is negative, the term acts as a sink rather than a source.

## Source/Sink with Constant Strength

Source term for a scalar quantity $q$ with source strength $S$ that is constant in space and time, i.e.

\[\frac{∂q}{∂t} = … + S \,.\]

The process is discretized with

\[f_i(\hat{q}^{00ζ}) = S \,.\]

`BoundaryLayerDynamics.Processes.ConstantSource`

— Type`ConstantSource(field, strength = 1)`

Source term for a scalar quantity $q$ with source strength $S$ that is constant in space and time.

**Arguments**

`field::Symbol`

: The name of the quantity $q$.`strength::Real`

: The source strength $S$.

## Source/Sink to Maintain Constant Mean

`BoundaryLayerDynamics.Processes.ConstantMean`

— Type`ConstantMean(field, mean_value = 1)`

Source term for a scalar quantity $q$ with a source strength that is constant in space but dynamically adjusted in time to maintain a constant mean value $Q$ for $q$.

**Arguments**

`field::Symbol`

: The name of the quantity $q$.`mean_value::Real`

: The mean value $Q$ that is maintained.

`ConstantMean`

is currently only implemented for $ζ_C$-nodes.

## Contributions to Budget Equations

Contribution to the instantaneous momentum equation:

\[\frac{\partial}{\partial t} u_i = … + f_i\]

Contribution to the mean momentum equation:

\[\frac{\partial}{\partial t} \overline{u_i} = … + \overline{f_i}\]

Contribution to the turbulent momentum equation:

\[\frac{\partial}{\partial t} u_i^\prime = … + f_i^\prime\]

Contribution to the mean kinetic energy equation:

\[\frac{\partial}{\partial t} \frac{\overline{u_i}^2}{2} = … + \overline{u_i} \overline{f_i}\]

Contribution to the turbulent kinetic energy equation:

\[\frac{\partial}{\partial t} \frac{\overline{u_i^\prime u_i^\prime}}{2} = … + \overline{u_i^\prime f_i^\prime}\]

For the `ConstantSource`

process, $\overline{f_i}=S$ and $f_i^\prime=0$, but for the `ConstantMean`

process this may depend on the exact definition of the averaging operation.