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   Absorbed radiation from an incremental area.
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   Form:
   q = AbsDa( alpha, dA, S )
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   ------
   Inputs
   ------
   alpha       (1,1)    Absorptivity
   dA          (1,1)    Incremental area
   S           (1,:)    Heat flux

   -------
   Outputs
   -------
   q           (1,:)    Absorbed heat flux (watts)

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