WUFI

Hello,

What is the difference between the "adhering fraction of rain" and the driving rain coefficient?

For example, with the default value of 0.7 adhering fraction, does that mean that 70% of the rain computed from driving rain coefficient will adhere to the wall? Both inputs seem quite similar, they both seem to be related to how much rain will land on a building.

I'm just concerned that I might be double counting or double excluding if I select values which result in very aggressive or relaxed parameters, or possibly cancelling each other out (ie. choosing tall building but then select user value 0.1 for adhering fraction).

Same question for the ASHRAE 160 optional parameters.

Hi Walter,

the driving rain coefficients determine how much rain will reach the surface. They include everything that happens before the water hits the surface.

For example, the "normal rain" is measured with respect to a horizontal receiving plane, the driving rain is measured with respect to a tilted (usually vertical) receiving plane. The geometrical factor converting between these different vector components is part of the rain coefficients.

Another factor is that the wind carrying the rain droplets will be forced to flow around the building since it cannot penetrate the facade. Depending on the wind speed and the size distribution of the droplets, some drops will follow this sudden change of flow direction without hitting the facade, the heavier ones will continue on their trajectory and still hit the building. These effects will be different in the center of the facade and close to the edges.

The wind speed close to the building may have been reduced by surrounding buildings, trees, etc, or some rain may have been intercepted by surrounding objects. On the other hand, a funnel effect of surrounding objects may increase the local wind speed.

The driving rain coefficients attempt to include these and similar influences in a single number (or in the case of the ASHRAE model, in a small set of numbers).

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The "adhering fraction of rain" describes what happens after the driving rain determined by the above method has hit the wall. Usually some of it splashes off and is not available for absorption any more. On horizontal surfaces the splash-off will fall back, and the adhering fraction will be close to one. Only what adheres after this kind of water loss is available for absorption.

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Since modern facades usually only have a small absorptivity for rain, most of the rain which has survived all this will run off anyway, without being absorbed. In these cases, the precise amount of rain available for absorption is not really decisive, and the simple models used by WUFI, with roughly estimated coefficients, are usually sufficient. If your simulation depends very sensitively on the amount of driving rain, you will need measurements anyway, rather than models.

Kind regards,
Thomas