About Solar Radiation in a Thermal Flow Simulation

You can include the thermal effects of solar radiation in the fluid physics of a simulation.

See Also
Defining Solar Radiation in Fluid Physics

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.