[esp-r] Re: high longware flux

[Marco Manzan]

> Date: Tue, 25 May 2010 17:42:16 +0800
> From: ??? <luckygrass100 at hotmail.com>
> Subject: [esp-r]  high longware flux
> To: <esp-r at lists.strath.ac.uk>
> Message-ID: <COL109-W54EF9F3F8282C3CBA0DD6BE2E80 at phx.gbl>
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>
> Dear everyone,
>
>         Have you met the similar warning that ?a longware flux of  
> -496 Watts has been calculated for wall-4:6-2.This is considered  
> high and a correlation will be used? when simulating?What gives rise  
> to so high longware flux possibly?Will the correlation cause wrong  
> simulation results?So complicated!
>
>
>
>
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> ------------------------------
>
> Message: 3
> Date: Tue, 25 May 2010 13:45:45 +0100
> From: Jon Hand <jon at esru.strath.ac.uk>
> Subject: [esp-r] Re: high longware flux
> To: ??? <luckygrass100 at hotmail.com>,	"esp-r at lists.strath.ac.uk"
> 	<esp-r at lists.strath.ac.uk>
> Message-ID:
> 	<2C3CF455489D6B4F95AFAB84847FA413E1049B640D at E2K7-MS2.ds.strath.ac.uk>
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>
> The longwave flux warning is triggered if the longwave exchange is  
> over a certain value.
> The value is based on calculations which are sensitive to the solar  
> angle and humidity
> and the properties of the surface.  This is not an issue if it  
> happens a few times in a simulation.
>
> If it happens several hundred times then it may be that there is  
> some questionable data
> in your climate file.  The warning tends to happen with surfaces  
> that mostly face the sky.
>
> The other factor is the thermophysical properties of the  
> construction.  A very thin
> metal foil over a thick insulation could result in this type of  
> warning.  Running at a
> shorter timestep can help to make the estimate of long wave radiant exchange
> more accurate.
>
> -ESRU

This is a useful message indicating that you are working with a too  
high timestep for the outer layer of your construction. A said by John  
Hand you can reduce the time step, but if it is excessively short you  
can also fictitiously increase the capacitance of the the outer layer.  
You can compare the results between the two methods and if the results  
of the second are not very different from those obtained with a  
shorter timestep you can proceed with the larger one.
The error occurs because the low capacitance outer layer receives  
energy from the sun therefore increases its temperature resulting in  
an excessive shortwave radiation exiting the layer. In the subsequent  
timestep its temperature is too low, sometimes  causing a  longwave  
radiation entering the layer and therefore an higher temperature in  
the next step and so on in a wavy pattern, increasng the capacitance  
helps stabilizing the wavy pattern.

Marco

-- 
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|Marco Manzan               Email: manzan at units.it            |
|Tel:   (+39) 040 5583506   Fax  : (+39) 040  572033          |
|Dipartimento di Ingegneria Navale, del Mare e per l'Ambiente |
|Universita' degli Studi di Trieste                           |
|piazzale Europa 1, 34127 TRIESTE                             |
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