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Opacity controlled requires that each boundary is adjacent to exactly one opaque domain. Opacity is controlled by the Opacity (Surface-to-Surface Radiation Interface) condition.
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Select Negative normal direction to specify that the surface radiates in the negative normal direction (un vector direction).
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Select Positive normal direction if the surface radiates in the positive normal direction (dn vector direction).
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Select Both sides if the surface radiates on both sides.
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Radiosity does not directly affect the boundary condition on the boundary where it is specified, but rather defines how that boundary affects others through radiation.
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When Wavelength dependence of radiative properties is set to Constant it defines J = eb(T) when radiation is defined on one side or Ju = eb(Tu) and Jd = eb(Td) when radiation is defined on both sides.
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When Wavelength dependence of radiative properties is set to Solar and ambient or Multiple spectral bands, it defines for each spectral band Ji = FEPi(T)eb(T) when radiation is defined on one side or Ji, d = FEPi, d(Td)eb(Td) and Ju = FEPi, u(Tu)eb(Tu) when radiation is defined on both sides.
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When radiation is defined on both sides, define the Surface emissivity, upside εu and the Surface emissivity, downside εd on the upside and downside, respectively. The surface radiosity on upside and downside is then defined by Ju = εueb(Tu) and Jd = εdeb(Td) respectively.
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When radiation is defined on one side for Bi spectral band, define the Surface emissivity εi to set Ji = FEPiεieb(T), or
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When radiation is defined on both sides for Bi spectral band, define the Surface emissivity, upside εi, u and the Surface emissivity, downside εi, d on the upside and downside, respectively. The surface radiosity on upside and downside is then defined by Ju = FEPBi(Tu)εBi, ueb(Tu) and Jd = FEPBi(Td)εBi, deb(Td), respectively.
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Set the surface emissivity to a number between 0 and 1, where 0 represents diffuse mirror and 1 is appropriate for a perfect blackbody. The proper value for a physical material lies somewhere in-between and can be found from tables or measurements.
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Several settings for this node depend on the Wavelength dependence of radiative properties setting, which is defined for the physics interface. See Radiation Settings.
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Upside and downside settings can be visualized by plotting the global normal vector (nx, ny, nz), that always points from downside to upside. Note that the normal vector (ht.nx, ht.ny, ht.nz) may be oriented differently.
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To define temperature dependencies for the user inputs (surface emissivity for example), use the temperature variable ht.T, that corresponds to the appropriate variable (upside, downside, or average temperature of a layer, wall temperature with turbulence modeling), depending on the model configurations. See Boundary Wall Temperature for a thorough description of the boundary temperature variables.
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