Usage of catch ratio tables in CHAMPS-BES simulations
To reduce the computational expense, the time consuming steady-state technique will only be employed for a limited number of wind speed and rainfall intensity couples, while intermediate results will be obtained by linear interpolation. Interpolation in a 2D U-Rh space provided by an area-weighted computation yields two catch ratio values (for two wind directions). Then, interpolation between these catch ratio values is performed to find the value for the wind direction. Nonlinear interpolation method for wind directions should be discussed!!
Since values for the horizontal rain intensity are given by discrete data points only, the temporal interpolation should be discussed, too. Step function or linear interpolation is usually used for interpolation of climate file data given on hourly basis. As shown below, the interpolation method has no influence on the total rain amount. Linear interpolation is recommended because avoiding jumps in boundary conditions result in better numerical performance of the simulation.
Numerical modeling of water uptake by rain events
By application of the modeling considerations above, the driving rain load normal to the wall surface can be calculated as function of time. The real water uptake flux can be different due to different saturation degrees of the material and its reduced ability the take up the rain water. Rain water not penetrating the wall is considered to run off.
The maximum water uptake flux is calculated from the material parameters (liquid water conductivity or diffusivity) and a gradient in capillary pressure or moisture content, respectively. For sake of convenience, a surface transfer coefficient can be introduced as shown below. The water uptake flux can be determined by comparison of the maximum water uptake flux and the driving rain intensity normal to the wall.
