Abaqus FEA - how to choose temperature for the sky on a clear night

[Wille]

TL;DR Summary

A surface facing only the clear sky during night (a windshield of a car), how should the zink/ambient temperature for radiation (*sradiate) be chosen in Abaqus?

Hi,

I want to model the phenomena that a windshield gets covered in ice during a night with clear sky even tough the air holds a temperature of a few degrees above zero Celsius (at which water freezes for sea level pressure). Clearly the windshield gets a temperature below zero Celsius, and this is due to the radiation. In Abaqus I must choose a zink temperature for both the convection and the radiation. For the convection I have set 1 degree Celsius (i.e. the air holds 1 Celsius). But what value should the zink temperature be for the radiation? Should it be about -271 Celsius (i.e. about 2 Kelvin) as in space? Or maybe 60 minus degrees C which is the air temperature a little bit higher up in the atmosphere (at 30 000 feet)?

Thanks

 

[FEAnalyst]

Sink temperature for both convection and radiation is defined as ambient temperature. So in both cases you should use the temperature of surrounding air, i.e. 1 degree celsius.

 

[Wille]

Sink temperature for both convection and radiation is defined as ambient temperature. So in both cases you should use the temperature of surrounding air, i.e. 1 degree celsius.

Ok, but what makes the windshield go below zero degrees C then?

 

[FEAnalyst]

Ok, I admit that this is very unusual case for simulation and may require non-standard methods. You can try with ambient temperature value corresponding to outer space and see if the results are reasonable. However I think that there might be another important phenomenon that causes this effect in real life - evaporation.

Have you found any articles describing similar research for reference ?

 

[Baluncore]

When I aim an IR thermometer upwards on a clear night it shows a temperature between -35°C and -50°C. That temperature will be dependent on humidity at lower levels in the atmosphere.

 

[256bits]

Ok, but what makes the windshield go below zero degrees C then?

Temperature of the upper atmosphere in the grand scheme of things. Of course, some radiation would be lost to space directly, and you gain some from the starlight, moonlight, surroundings such as from trees and buildings. But why complicate things too much.
On cloudy days the chances of getting frost is less due to a higher temperature of clouds.

PS.
And the conduction/ convection heat transfer gain would have to be lower than the radiation heat loss.

 

[Tom.G]

Only moderately related:
Many years ago my wife and I would go to Edwards Air Force Base in the Mojave Desert to watch the Space Shuttle landings. Since that was a several hour trip, we would often spend the night at her sisters place, 4000ft. elevation in the Tehachapi Mountains. We were usually on a motorcycle.

On one trip we got up in the early morning to find heavy dew on the motorcycle seat. When I went to wipe it off, the dew instantly froze solid as soon as I touched it!

At least it was a little easier to get ice off the seat!

Cheers,
Tom

 

[anorlunda]

On one trip we got up in the early morning to find heavy dew on the motorcycle seat. When I went to wipe it off, the dew instantly froze solid as soon as I touched it!

That's cool (no pun intended). I've seen supercooled water in other circumstances, but nothing like that.