# 4.10.2. Inflow

An inflow boundary is a specified velocity surface that flows into the domain. The properties of the conserved quantities flowing in are calculated using the specified primitive variables on the boundary.

    begin inflow boundary condition on surface surface_4
        x_velocity = 1.2
        y_velocity = 0
        z_velocity = 0
        temperature = 300
        mass_fraction He = 0.0
        mass_fraction O2 = 0.233
        mass_fraction N2 = 0.767
    end

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

## 4.10.2.1. Use Fluxes

Note

Using the `Use Fluxes` command is a recommended best practice, and may become the default behavior in a future version.

By default, the inflow properties are applied as a dirichlet condition on the nodes on the inflow boundary. If you add the `Use Fluxes` argument, the boundary condition is applied as a mass infusion into the control volumes along the inflow.

    begin inflow boundary condition on surface surface_4
        Use Fluxes
        # ...
    end

To further understand the difference, consider the following inflow boundary condition

    begin inflow boundary condition on surface surface_4
        x_velocity = "(t<5) ? 0 : 1.2"
        y_velocity = 0
        z_velocity = 0
        temperature = 300
        mass_fraction He = 1.0
        mass_fraction O2 = 0.0
        mass_fraction N2 = 0.0
    end

In this, we have a flow of pure Helium that turns on at 5 seconds. Prior to 5 seconds, the inflow velocity is 0. When using the `Use Fluxes` command, the flow will be un-affected by this boundary condition until 5 seconds. However, using the default form the mass fraction of Helium will be fixed to 1 immediately and Helium will diffusion into the domain before the 5-second activation.

This is also important for other conditions, such as a slow jet in a cross-flow. With the `Use Fluxes` command the concentration of Helium at the boundary nodes will be computed as a balance between the cross-flow into those control volumes and the boundary condition inflow. However, using the dirichlet form that balance will be discarded and those nodes will be fixed at a Helium mass fraction of 1.