Hi WUFI forum
I am analyzing a ventilated timber frame construction, built up very similar to the example given in WUFI Tutorial - Handling of typical constructions.
I am looking at the insulation layer. There is no cloud data and no counter radiation data in the weather file.
If I simulate with explicit radiation balance turned on, I get this warning:
"Selected climate for left surface does not contain information about long-wave atmospheric counter radiation. User-defined cloud index 0,66 will be used."
The water content in the insulation layer is slightly higher, when turned on.
Should I simulate with explicit radiation balance turned on or off when I don’t have the weather data needed?
Thanks
Cloud index
Re: Cloud index
Hi Marcus,
when the option "explicit radiation balance" is activated, WUFI simulates short-wave (solar) and long-wave (thermal) radiation exchange separately, and it explicitly calculates the long-wave radiation exchange between the component surface and the surroundings. During the night, this long-wave radiation exchange often results in subcooling of the component surface: When the surface loses more thermal radiation towards the surroundings than it receives, the surface temperature will drop below the ambient air temperature.
This subcooling has two consequences: The overall temperature of the component is lowered, reducing its drying potential. And when the surface temperature drops below the ambient dew point temperature, dew deposition will occur.
* If you intend to study dew deposition, you must activate the option "explicit radiation balance", because otherwise no subcooling and no dew deposition will occur. Furthermore, the frequency of dew deposition and the amount of deposited dew will depend very sensitively on the exact amounts of radiation exchanged and you need very accurate data for the atmospheric and/or terrestrial counterradiation in the climate file. Modeled or estimated counterradiation data will usually not be accurate enough, the best option is to use measured data.
* If dew is of no interest (as seems to be the case in your simulation), the calculation with activated radiation balance will still be slightly more realistic because in general it results in a slightly more realistic temperature level of the component. Usually, the precise amount of the temperature shift will not be very important for the simulation, and it may be sufficient to use modeled or estimated counterradiation data.
The higher water content in your insulation layer probably reflects the reduced drying potential of the construction when the option is turned on. It is probably preferable to take this effect into account, just don't expect it to be modeled very precisely if you have only counterradiation data which have been estimated assuming a constant cloud cover. But since the temperature shift resulting from the activated option will only be a minor "correction" to the simulation result, even a relatively large uncertainty in this correction will still be only a minor factor in the overall result.
Just to make sure that your construction doesn't depend very sensitively on inaccuracies in the counterradiation data, you should do some test runs where you apply variations (within reason) of the assumed constant cloud cover. You will see variations in the simulation results, but if the extent of these variations is so small that it does not affect the assessment of the results for your purposes, the uncertainty in the counterradiation data will be acceptable. On the other hand, if your construction turns out to depend very sensitively on the exact amount of counterradiation, there is probably no way around acquiring weather data with sufficiently accurate counterradiation.
Regards,
Thomas
when the option "explicit radiation balance" is activated, WUFI simulates short-wave (solar) and long-wave (thermal) radiation exchange separately, and it explicitly calculates the long-wave radiation exchange between the component surface and the surroundings. During the night, this long-wave radiation exchange often results in subcooling of the component surface: When the surface loses more thermal radiation towards the surroundings than it receives, the surface temperature will drop below the ambient air temperature.
This subcooling has two consequences: The overall temperature of the component is lowered, reducing its drying potential. And when the surface temperature drops below the ambient dew point temperature, dew deposition will occur.
* If you intend to study dew deposition, you must activate the option "explicit radiation balance", because otherwise no subcooling and no dew deposition will occur. Furthermore, the frequency of dew deposition and the amount of deposited dew will depend very sensitively on the exact amounts of radiation exchanged and you need very accurate data for the atmospheric and/or terrestrial counterradiation in the climate file. Modeled or estimated counterradiation data will usually not be accurate enough, the best option is to use measured data.
* If dew is of no interest (as seems to be the case in your simulation), the calculation with activated radiation balance will still be slightly more realistic because in general it results in a slightly more realistic temperature level of the component. Usually, the precise amount of the temperature shift will not be very important for the simulation, and it may be sufficient to use modeled or estimated counterradiation data.
The higher water content in your insulation layer probably reflects the reduced drying potential of the construction when the option is turned on. It is probably preferable to take this effect into account, just don't expect it to be modeled very precisely if you have only counterradiation data which have been estimated assuming a constant cloud cover. But since the temperature shift resulting from the activated option will only be a minor "correction" to the simulation result, even a relatively large uncertainty in this correction will still be only a minor factor in the overall result.
Just to make sure that your construction doesn't depend very sensitively on inaccuracies in the counterradiation data, you should do some test runs where you apply variations (within reason) of the assumed constant cloud cover. You will see variations in the simulation results, but if the extent of these variations is so small that it does not affect the assessment of the results for your purposes, the uncertainty in the counterradiation data will be acceptable. On the other hand, if your construction turns out to depend very sensitively on the exact amount of counterradiation, there is probably no way around acquiring weather data with sufficiently accurate counterradiation.
Regards,
Thomas
Re: Cloud index
Hi Thomas
Thank you very much for your reply.
Regards Marcus
Thank you very much for your reply.
Regards Marcus