rain water absorption value
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James Rydock
- WUFI User
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- Joined: Tue May 30, 2006 12:24 am -1100
rain water absorption value
I wonder if you have any advice about using different rain water absorption values instead of the default value of 0.7 (for a wall)? Is this meant to be valid for any type of wall? If not, why are there no other typical values listed, as is the case for Sd values? Thanks for your help.
Re: rain water absorption value
Dear Mr. Rydock,
the rain water absorption factor will probably depend slightly on the surface texture of the wall, on wind speed and on the size spectrum of the rain droplets, but to my knowledge no systematic studies have yet been done to quantify this, so I'm afraid we cannot provide more detailed values. However, we expect only a relatively slight dependence of this factor on specific surface properties and meteorological conditions, so the uncertainty introduced by ignoring this dependence and always setting the factor to 0.7 will be much less than the uncertainty already present in measuring or computing the amount of driving rain.
For example, after a rain event, driving rain gauges installed in different sections of a wall will often indicate noticeably different amounts of rain, partly because of slightly different wind patterns at different wall sections (depending on height, ambient obstacles etc.) and partly because of the statistical nature of gusts of wind delivering different amounts of driving rain to different gauges. The effect of ignoring a possible variability of the rain absorption factor will be negligible in comparison.
Furthermore, by design the surfaces of most building components only have low water absorption capacity. In the majority of rain events, the surfaces will not be able to absorb all the incident rain water anyway, the surplus then simply running off. Under these circumstances, changing the amount of rain water available for absorption by adjusting the rain absorption coefficient (i.e. assuming that a different fraction of rain splashes away on impact) will usually still result in more rain than can be absorbed, and the calculation result will be the same. The calcultion result will only be affected in the case of surfaces which have high enough water absorption capacity, so that the amount of rain available for absorption and not the absorption capacity of the surface materials is the limiting factor deterimining the absorbed amount of water.
Regards,
Thomas
the rain water absorption factor will probably depend slightly on the surface texture of the wall, on wind speed and on the size spectrum of the rain droplets, but to my knowledge no systematic studies have yet been done to quantify this, so I'm afraid we cannot provide more detailed values. However, we expect only a relatively slight dependence of this factor on specific surface properties and meteorological conditions, so the uncertainty introduced by ignoring this dependence and always setting the factor to 0.7 will be much less than the uncertainty already present in measuring or computing the amount of driving rain.
For example, after a rain event, driving rain gauges installed in different sections of a wall will often indicate noticeably different amounts of rain, partly because of slightly different wind patterns at different wall sections (depending on height, ambient obstacles etc.) and partly because of the statistical nature of gusts of wind delivering different amounts of driving rain to different gauges. The effect of ignoring a possible variability of the rain absorption factor will be negligible in comparison.
Furthermore, by design the surfaces of most building components only have low water absorption capacity. In the majority of rain events, the surfaces will not be able to absorb all the incident rain water anyway, the surplus then simply running off. Under these circumstances, changing the amount of rain water available for absorption by adjusting the rain absorption coefficient (i.e. assuming that a different fraction of rain splashes away on impact) will usually still result in more rain than can be absorbed, and the calculation result will be the same. The calcultion result will only be affected in the case of surfaces which have high enough water absorption capacity, so that the amount of rain available for absorption and not the absorption capacity of the surface materials is the limiting factor deterimining the absorbed amount of water.
Regards,
Thomas
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James Rydock
- WUFI User
- Posts: 3
- Joined: Tue May 30, 2006 12:24 am -1100
Thanks Thomas,
Your last paragraph zeroes in on my problem. As it is true that many building components are designed to withstand the weather (wind-driven rain), they often have a low water absorption capacity. To the untrained eye, a water absorption coefficient of 0.7 would seem to imply that 70% of the water that impacts the wall will be taken up by the wall (assuming the material has not already absorbed to its capacity, i.e. is not saturated). This does not seem realistic to the unsophisticated user trying to model a water-repellent material. The natural inclination is to conclude that a factor of 0.7 is much too high. Therefore one would be tempted to reduce the water absorption coefficient drastically (say to 0.2) to reflect the fact that water-repellent materials in fact repel most of the water which strikes them and therefore do not have so much water available to be taken up. This type of change can of course have a noticeable effect on the results, particularly in the case of shorter periods of more intense wind-driven rain, as opposed to long episodes of gentle rain. It is hard to argue (to skeptical clients) that all materials are the same with regard to the water absorption factor (take for example a wall consisting of new untreated wood versus a wooden wall that has been sealed with paint).
Regards,
James.
Your last paragraph zeroes in on my problem. As it is true that many building components are designed to withstand the weather (wind-driven rain), they often have a low water absorption capacity. To the untrained eye, a water absorption coefficient of 0.7 would seem to imply that 70% of the water that impacts the wall will be taken up by the wall (assuming the material has not already absorbed to its capacity, i.e. is not saturated). This does not seem realistic to the unsophisticated user trying to model a water-repellent material. The natural inclination is to conclude that a factor of 0.7 is much too high. Therefore one would be tempted to reduce the water absorption coefficient drastically (say to 0.2) to reflect the fact that water-repellent materials in fact repel most of the water which strikes them and therefore do not have so much water available to be taken up. This type of change can of course have a noticeable effect on the results, particularly in the case of shorter periods of more intense wind-driven rain, as opposed to long episodes of gentle rain. It is hard to argue (to skeptical clients) that all materials are the same with regard to the water absorption factor (take for example a wall consisting of new untreated wood versus a wooden wall that has been sealed with paint).
Regards,
James.
Dear Mr. Rydock,James Rydock wrote:To the untrained eye, a water absorption coefficient of 0.7 would seem to imply that 70% of the water that impacts the wall will be taken up by the wall (assuming the material has not already absorbed to its capacity, i.e. is not saturated).
the rain water absorption factor of 0.7 displayed in the dialog for the surface transfer coefficients only indicates how much the amount of incident driving rain is reduced due to some of it splashing off on impact. So if the weather file indicates 1 Liter/m2h of driving rain and the rain water absorption factor is 0.7, only 0.7 Liter/m2h of rain will be available to the component for absorption.
How much of this is really absorbed depends on the water absorption coefficient of the surface material which is individually specified in the "material data" dialog of the respective materials (either directly as the water absorption coefficient or as a table of moisture-dependent liquid transport coefficients).
It is somewhat unfortunate that these different numbers have such similar names. I think there are plans to rename the rain absorption factor to something like "rain reduction factor".
Regards,
Thomas
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James Rydock
- WUFI User
- Posts: 3
- Joined: Tue May 30, 2006 12:24 am -1100