Driving rain is rain that is carried by the wind and driven onto the building envelope (fa?ades and roofs). It is a complex phenomenon of falling raindrops in a turbulent flow of wind around a building. It is one of the important climatological factors which determine long-term use and durability of building envelopes.

We started the present study because the adequacy of the measurement techniques for driving rain described in the literature (from 1937 onwards) was doubted, and because, since the beginning of the 1990s, computational fluid dynamics made simulations of the complex phenomenon of driving rain --and hence a better understanding of it-- possible.

We developed a driving rain gauge which we improved by addition of a turning wiper. We tested it in an international comparison experiment in full-scale on the west fa?ade of the Main Building of the Eindhoven University of Technology. The results indicated that our improved driving rain gauge is an accurate and reliable instrument, and that one may measure approximately half of the actual driving rain amount by an unimproved, traditional driving rain gauge. We formulated design rules for driving rain gauges.

The full-scale experiments resulted in a unique series of continuous wind, rain and driving rain measurements during 24 months. Raindrop spectra with a disdrometer were measured during 3 months too. The measurements are detailed (data at 5-minute intervals were provided) and are available for future research.

The measurements of driving rain on the west fa?ade of the Main Building showed much variation for all observed reference wind and rain quantities. A traditional empirical model (Lacy 1965) was applied to the data. We presented an improved empirical model, by which wind direction and position on the fa?ade are explicitly taken into account. Only with the improved model we could estimate quite accurate values of maximum driving rain intensities.

The wind at the experiment site was simulated by a standard K-epsilon model, and the results compared well with the measurements. The applied model of driving rain takes drop trajectories (including the drop dispersion due to wind turbulence) and raindrop spectra into account. The simulated driving rain intensities on the two positions on the west fa?ade compared rather well with our measurements of driving rain. We discussed the aspects which should be dealt with to obtain reliable simulation results.

Keywords: building envelope, fa?ade, durability, micro-climate, wind, rain, driving rain (= wind-driven rain), raindrop spectrum (= raindrop (size) distribution), driving rain gauge, computational fluid dynamics (= CFD), standard K-epsilon model.

ˇ­ˇ­..

• wind driven rain

1. Full-scale measurements and numeric simulation of driving rain on a building fasade
2. Measurements and simulations of driving rain on the main building of the TUE