In Grasshopper the Solver component bundles all input geometry and settings and executes the simulation. The Builder output is connected to BLDGS. By holding down the shift key multiple Builder outputs can be attached to this input. BLK takes all blockage geometry and AMB all other ambient shading geometry. The black “Settings” button allows to specify the simulation parameters.
In the settings pop-up a project name and working directory can be specified. All relevant files such as sensor points, rad scene files, daysim input files and illuminace files will be written to this directory + the projcet name. Further the climate file [ collection of epw files is stored under C/UD/LIB ] and the simulation time frame can be specified. Start is the first analysis hour – Stop is the last. A year has 0 – 8760 hours.
The simulation steps check-boxes allow you to deactivate certain modules. “Files” writes out additional sensor point data and hourly illuminance profiles for the interior in the Daysim format. This is mainly for validation purposes. The “Rtrace” controls the DAYSIM simulation. “Rtrace” can be skipped when you re-run a simulation and you have not changed any geometry. “Impulse” controls the light-solver.
For a detailed description on the Radiance Parameters refer to the Radiance manual. For urban scenes the values shown in the screen shot might be a good starting point.