Ram Air
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Ram Air
Long time reader, new member, cheeky question.....
I have a reasonable understanding of high speed flight, but still don't quite understand why ram air rise never seems to warm me up when it gets windy every winter. The weather man tells me it will cool me down. Is he wrong again?
Edited to incorporate the mention of "wind chill factor."
I have a reasonable understanding of high speed flight, but still don't quite understand why ram air rise never seems to warm me up when it gets windy every winter. The weather man tells me it will cool me down. Is he wrong again?
Edited to incorporate the mention of "wind chill factor."
It would if you came on 'Down Under'...........
Going for 40C across the 'Top End' and the 'Centre' today.......with strong Easterlies across the 'Top'....
Going for 40C across the 'Top End' and the 'Centre' today.......with strong Easterlies across the 'Top'....
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There are two competing effects when a human stands in the wind.
One is that the air flow does indeed make the air hotter, and the total air temperature (TAT), which includes both the "static" air temperature (SAT) and the kinetic heating (or ram) effect, will be therefore slightly higher than the static temperature. However, it's a tiny effect at typical wind speeds:
A 66 knot wind at sea level (which is a LOT of wind!) would create a temperature rise of about 0.2%, or about half a degree Celsius.
The more important effect for a human sensing temperature in the presence of wind is the effect that the wind has on the effectiveness the air has in heating up or cooling down the skin - the effect normally called windchill.
Higher wind speeds make the air more effective at cooling - or heating - the body. Because your body is typically at close to 40 deg C, whenever the air is cooler than this it is, in fact, cooling you down. So the harder it blows the more it cools you. This effect is more powerful than the kinetic temperatuire rise due to the wind speed, so overall, a wind chills, or cools, the body.
Note that for an object which is not heated, or temperature regulated (which is more or less what the human body does) the windchill effect will bring the object to the stable temperature faster - but that temperature will be the TAT, not the SAT. (And definitely not the "windchill" temperature, which is a fictional "feels like" temperature)
One is that the air flow does indeed make the air hotter, and the total air temperature (TAT), which includes both the "static" air temperature (SAT) and the kinetic heating (or ram) effect, will be therefore slightly higher than the static temperature. However, it's a tiny effect at typical wind speeds:
A 66 knot wind at sea level (which is a LOT of wind!) would create a temperature rise of about 0.2%, or about half a degree Celsius.
The more important effect for a human sensing temperature in the presence of wind is the effect that the wind has on the effectiveness the air has in heating up or cooling down the skin - the effect normally called windchill.
Higher wind speeds make the air more effective at cooling - or heating - the body. Because your body is typically at close to 40 deg C, whenever the air is cooler than this it is, in fact, cooling you down. So the harder it blows the more it cools you. This effect is more powerful than the kinetic temperatuire rise due to the wind speed, so overall, a wind chills, or cools, the body.
Note that for an object which is not heated, or temperature regulated (which is more or less what the human body does) the windchill effect will bring the object to the stable temperature faster - but that temperature will be the TAT, not the SAT. (And definitely not the "windchill" temperature, which is a fictional "feels like" temperature)
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Agree (and hopefully add)
I agree that the ram temperature increase is modest (no more than the specific kinetic energy of the air).
There are a coupe of extra factors on skin though, none of which should change as dramatically with windspeed as the perceived windchill discomfort forecast numbers.
If you had a metal skin, being blown by dry air, there would be heat loss to the air by conduction only. As long as the air was blown quickly, so the boundary layer would not reach an equilibrium temperature, the speed of the flow shouldn't have a strong effect on cooling. I reckon this is why there's no significant `windchill' through clothes.
Being blown by damp air, condensation and maybe ice formation can take place. Initially this deposits a small amount of latent heat energy. However, the water/ice coating then cools the skin by conduction much faster than the air. If the delivery rate of water controls the condensation rate, then speed matters.
Evaporation from your non-metallic skin is a big factor (as stated above) - into dry air, energy is lost from your skin to latent heat energy. The evaporation rate should increase a bit with wind speed, I suspect more by reducing the capacity of your hairs to trap a protective warm layer than by enabling a higher evaporation rate.
I suggest that radiative cooling is less important for skin, but does have a small effect, independent of wind speed.
There are a coupe of extra factors on skin though, none of which should change as dramatically with windspeed as the perceived windchill discomfort forecast numbers.
If you had a metal skin, being blown by dry air, there would be heat loss to the air by conduction only. As long as the air was blown quickly, so the boundary layer would not reach an equilibrium temperature, the speed of the flow shouldn't have a strong effect on cooling. I reckon this is why there's no significant `windchill' through clothes.
Being blown by damp air, condensation and maybe ice formation can take place. Initially this deposits a small amount of latent heat energy. However, the water/ice coating then cools the skin by conduction much faster than the air. If the delivery rate of water controls the condensation rate, then speed matters.
Evaporation from your non-metallic skin is a big factor (as stated above) - into dry air, energy is lost from your skin to latent heat energy. The evaporation rate should increase a bit with wind speed, I suspect more by reducing the capacity of your hairs to trap a protective warm layer than by enabling a higher evaporation rate.
I suggest that radiative cooling is less important for skin, but does have a small effect, independent of wind speed.
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I once carpooled with an engineer who was a heat transfer specialist. Or so he claimed.
Yet he could not absorb the concept of wind chill - he had the same confusion that exdroopnosepilot expressed! I guess he was what you would call an overeducated dunce.
And thanks, MFS, for setting the story straight.
Yet he could not absorb the concept of wind chill - he had the same confusion that exdroopnosepilot expressed! I guess he was what you would call an overeducated dunce.
And thanks, MFS, for setting the story straight.
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But wind does not affect TAS nor Mach number.
I don't see how wind could affect RAT, unless you are on the ground, of course.
Only way to warm up in flight is increasing Mach and or descending to a warmer level.
Or I missed something in the question.
Regarding the windchill effect on a body, it increases the rate at which the bodie's temperature decreases, but not the final temperature that it will reach. If there is kinetic heating, this temperature is higher than in still air, of course. As MSF says.
By the way, I think that human body perception for heat or cold is rather of heating or cooling rate. If you get into a swimmingpool at 20ºC you feel cold. But if you get into the same water just after getting out of a 10ºC water, you can even feel heat, for a while. Getting out of a warm swimmingpool, say 35º, in a hotter air, say 45ºC should mean you feel hotter, but if humidity is very low and there is wind, you actually feel cold... At 45ºC!
The human body has a limit in its ability to maintain a safe temperature, I guess. If it cools or heats too fast, its temperature will go beyond limit. So it is a natural alarm.
Dersu Uzala wonderful movie comes to my mind...
I don't see how wind could affect RAT, unless you are on the ground, of course.
Only way to warm up in flight is increasing Mach and or descending to a warmer level.
Or I missed something in the question.
Regarding the windchill effect on a body, it increases the rate at which the bodie's temperature decreases, but not the final temperature that it will reach. If there is kinetic heating, this temperature is higher than in still air, of course. As MSF says.
By the way, I think that human body perception for heat or cold is rather of heating or cooling rate. If you get into a swimmingpool at 20ºC you feel cold. But if you get into the same water just after getting out of a 10ºC water, you can even feel heat, for a while. Getting out of a warm swimmingpool, say 35º, in a hotter air, say 45ºC should mean you feel hotter, but if humidity is very low and there is wind, you actually feel cold... At 45ºC!
The human body has a limit in its ability to maintain a safe temperature, I guess. If it cools or heats too fast, its temperature will go beyond limit. So it is a natural alarm.
Dersu Uzala wonderful movie comes to my mind...
Last edited by Microburst2002; 3rd Nov 2009 at 17:50.
