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

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Discussion Overview

The discussion revolves around modeling the temperature of a windshield during a clear night when the air temperature is slightly above freezing, specifically focusing on how to choose the appropriate sink temperature for convection and radiation in Abaqus FEA simulations. Participants explore the physical phenomena that lead to the windshield cooling below the ambient air temperature.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests setting the sink temperature for radiation in Abaqus to -271°C, referencing space conditions, or possibly a much lower temperature than the air temperature at higher altitudes.
  • Another participant argues that the sink temperature for both convection and radiation should be set to the ambient air temperature of 1°C.
  • A later reply questions the reasoning behind the windshield cooling below 0°C, suggesting that evaporation might play a significant role in this phenomenon.
  • One participant notes that measurements from an IR thermometer indicate temperatures between -35°C and -50°C, which they state depend on humidity levels in the atmosphere.
  • Another participant discusses the influence of radiation loss to space and the effects of starlight and moonlight on the windshield temperature, while also mentioning that cloudy conditions tend to reduce frost formation.
  • A personal anecdote is shared about experiencing heavy dew freezing instantly on a motorcycle seat, which adds a real-world example of temperature effects in similar conditions.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate sink temperature for radiation and convection, with no consensus reached on the best approach to model the windshield temperature. The discussion remains unresolved regarding the exact mechanisms causing the windshield to cool below the ambient air temperature.

Contextual Notes

There are unresolved assumptions regarding the influence of various environmental factors, such as humidity and radiation, on the windshield's temperature. The discussion also highlights the complexity of accurately modeling such phenomena in simulations.

Wille
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TL;DR
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
 
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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.
 
FEAnalyst said:
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?
 
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 ?
 
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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.
 
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Wille said:
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.
 
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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
 
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Tom.G said:
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.
 

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