# 4.10.8. Composite

The composite boundary condition is used with the 1D composite sub-model to simulate a burning and off-gassing composite. It requires a separate material definition for the composite material (Composite Material Properties) and parameters for the initial composite composition, thickness, and backside thermal boundary condition.

    begin composite boundary condition on surface surface_4
      composite material = new_composite

      composite nodes = 10
      composite thickness = 0.002  # 2 mm

      activate backside convection
      activate backside radiation
      composite backside_convection_coefficient = 10.0
      composite backside_reference_temperature = 800.

      # Initially hot to accelerate ignition for regression test
      composite initial_temperature = 300.
      composite initial_mass_fraction Carbon = 0.6724  # 0.6 volume fraction
      composite initial_mass_fraction Epoxy  = 0.3276  # 0.4 volume fraction
      composite initial_mass_fraction Char   = 0.0

      # Composition of outgassed mixture
      turbulent_kinetic_energy = 2.5e-5
      soot_mass_fraction = 0.0
      soot_nuclei_mass_fraction = 0.0
    end

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

## 4.10.8.1. 1D Composite Model

The details of the 1D composite model are described in the Theory chapter in One-Dimensional Composite Fire Boundary Condition. A pseudo-1D domain is created on the boundary nodes of the composite boundary with the number of nodes and thickness of the domain specified by `composite nodes` and `composite thickness`. The composite material defines a series of solid decomposition reactions that produce gas (and other solids like char). An ODE solver evaluates an energy balance and the decomposition ODEs at each composite node, and the net gas produced is applied as a mass injection term at the composite wall (similar to a mass inject boundary condition).

## 4.10.8.2. Composite Interface

In some cases, you may want to use a more complicated 3D composite model in Sierra/Aria. Fuego supports this using the `composite interface` boundary, which expects transfers of composite decomposition products from Aria instead of solving them directly.

    begin composite interface boundary condition on surface surface_4
      external field for composite_species_mass_rate of CH4 = bc_species_diff_adv_flux_normal_CH4
      external field for composite_species_mass_rate of O2 = bc_species_diff_adv_flux_normal_O2
      external field for composite_species_mass_rate of N2 = bc_species_diff_adv_flux_normal_N2
    end