Using solar radiation in a simulation includes radiative energy from the
sun, which can be significant for geometry designs that are in direct or
indirect sunlight. The simulation of solar radiation consists of three main
parts: the solar calculator, the solar load calculation, and the inclusion in
the energy equation. The app performs the first two parts before the CFD solver
is launched, and the last part is performed in the solver.
- Solar Calculator
- The first part of the simulation determines the sun direction from
the geometry and the amount of energy being emitted, both directly and
indirectly. You can specify these values directly or use the solar calculator
to find them. The app determines the sun direction and irradiance using
formulas developed and printed in the ASHRAE handbook and in papers from the
National Renewable Energy Laboratory (NREL). These formulas require the time,
date, and location; and for transient simulations, the solar calculator finds
multiple sun directions and irradiances to account for the sun sweeping across
the sky. You can specify the frequency of these positions.
- Solar Load Calculation
- With the sun's location (or locations) in the sky and the direct
and indirect irradiance now known, the app creates a plane above the geometry
and emits and traces rays to the scene below. The absorptivity, reflectivity,
transmissivity, and diffuse fraction of each surface determine whether a ray
terminates on the surface or continues and how it continues. For example, a
surface with an absorptivity of 0.4, transmissivity of 0.5, and reflectivity of
0.1 will have 40% of the rays absorbed by the surface, 50% go directly through
unaffected, and 10% reflected. (The sum of these three fractions is always
1.0.) The diffuse fraction determines the nature of the reflection. In the same
example, for a diffuse fraction of 0.6, 60% of the reflected rays will reflect
in any direction with equal probability, and 40% will reflect in a mirror-like
fashion. When all rays have terminated, the total energy associated with each
ray is accumulated and the app writes the solar loads to a file.
- Inclusion in the Energy Equation
- The final part of the simulation includes these solar loads in the
energy solver. The CFD solver reads in these solar loads as heat fluxes,
interpolates them for transient cases, and adds them to the boundary conditions
in the energy equation.