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Does Wind Chill Have Any Effect On Surfaces

Fri Dec 13, 2002 2:53 am

We all know the story, your watching the weather and the weatherperson say the temprature will be 4* but the high winds will make it feel like its below freezing. Does this only effect humans or does it "actually" lower the air temprature?
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RE: Does Wind Chill Have Any Effect On Surfaces

Fri Dec 13, 2002 3:35 am

"Wind chill" basically accounts for the increased rate of heat transport; It doesn´t actually lower the temperature.

When there is a cold wind, your skin will lose more heat in the same amount of time because the "boundary layer" of somewhat warmer air is blown away and replaced by colder air instead of staying put and forming a kind of buffer that shields you from the colder air. (Actually, with "wind chill" you feel the real air temperature; Without it the air appears to be warmer than it really is...)

The subjective effect is the same as if you´d be in still air of much lower temperature, although it´s not really the case.

The same idea is behind using a fan to cool the CPU of your computer: If the fan dies, the warm air isn´t replaced and continues to heat up; In the process, the CPU won´t be cooled properly and will shut down or burn out. So it´s the same basic principle for heat-producing pieces of equipment as for living organisms.
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RE: Does Wind Chill Have Any Effect On Surfaces

Sat Dec 14, 2002 12:48 pm

It can have an effect when deciding to de-ice/anti-ice as Klaus noted it speeds the rate of heat transportr, making it neccessary to prevent ice formation due to that. (this is when aircraft skin temp. is lower that outside air temps. due to the loss of heat.)

I may be wrong, but this is how I understood it.

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RE: Does Wind Chill Have Any Effect On Surfaces

Sat Dec 14, 2002 1:24 pm

The only time I can think of that wind chill will effect an aircraft surface is if you have heated wings or windshields for de-icing.

They get damm hot if there isn't an airflow over them. And it isn't unheard of them to burn out because of overheating when they are used on the ground.

They where designed to work with air hitting the surface at cruise speed and that air carrying away the heat they are producing.
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RE: Does Wind Chill Have Any Effect On Surfaces

Wed Mar 08, 2006 5:27 am

Of course if you get enough of the wind (ie 500kt TAS), you're going to be heating the surfeaces, not cooling them (compared the OAT).
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RE: Does Wind Chill Have Any Effect On Surfaces

Wed Mar 08, 2006 6:50 am

(this is when aircraft skin temp. is lower that outside air temps. due to the loss of heat

The temperature of the skin cannot drop below the temperature of the outside air. Heat transfer between the skin/air only takes place when there is a temperature difference between the two. Unless you add/remove heat from the skin with a device, the skin will always reach the outside air temp eventually, point at which heat transfer between the two ceases (yes stagnation points will raise the air temp locally depending on your airspeed, but let's keep it simple  Smile). If the skin temperature were colder than the outside air, then heat would flow from the air to the skin until both reached an equal temperature.

The problem with all these temperature/wind chill definitions is that human senses are simply terrible for determining temperature. If you stand outside in -10ºC weather and dip your hand in a stream of water, your brain will tell you that the water is much colder. Yet the water is probably near 0ºC, warmer than the outside air. But your body loses heat much faster to the flowing water and your brain interprets that as a lower temperature.

One experiment that was always fun to try on physics students was to get a hold of samples of several different materials (piece of styrofoam, wood, marble, and metal) and ask them to estimate the temperature of each by touching them. Then they were provided a thermocouple to take actual readings. All 4 were at room temperature, but by touch styrofoam and wood feel "warm" since they do not conduct heat away from your hand as well as the marble and metal pieces. The styrofoam and wood samples would always end up with marks where the students tried to drive the thermocouple in an attempt to get a "warmer" reading than the metal/marble samples as their senses had told them to expect  Smile
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RE: Does Wind Chill Have Any Effect On Surfaces

Wed Mar 08, 2006 8:35 am

To augment Klaus' excellent explanation, the increase in heat transfer is dependent on several factors. For example, I believe the vernacular "windchill" heard on television is for bare skin. Bare skin is not only heated, but it is also moist, a factor which increases the heat transfer over items of similar temperature but without any surface moisture.

Windchill even affects you when wearing clothing, but the effect is reduced because the surface temperature of the clothing is less than bare skin (which is why you wear it) and it tends to have lower surface humidity than bare skin.

So, the following factors tend to drive the impact of "windchill":

(1) Freestream velocity.
(2) Geometry of the surface subjected to the freestream velocity. This includes macro and micro effects. For example, the windchill on a fluffy fur coat will be different than that for bare skin, even if the coat happens to be shaped exactly as a human body and it is heated to the same temperature.
(3) Difference in temperatures between the freestream and the surface.
(4) Moisture content of the surface subjected to the freestream velocity. Surfaces with moisture will see a rather amplified "windchill" effect.

My heat transfer professor told a great story about a common question for Ph. D. candidates engaged in their oral defense. They pose a situation in which you have a steaming cup of coffee in a styrofoam cup, and the styrofoam is a perfect insulator. Blowing across the surface of the liquid increases the rate of heat transfer.

Given the following equation, which term is obviously affected by blowing across the surface?

Q = -hA(Ts – T)

He said quite a few of them struggle with the obvious answer: the heat transfer coefficient is the only thing changed. The flux area is the same, as is the temperature difference (we're assuming you're blowing for the first time). If A and deltaT are unchanged, then the answer MUST be a change in the coefficient since the heat transfer rate is obviously increased.

You gotta love those coefficients. They are an easy catch-all for factors obviously in play but which cannot easily (if at all) be quantified.
Christianity. Islam. Hinduism. Anthropogenic Global Warming. All are matters of faith!
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RE: Does Wind Chill Have Any Effect On Surfaces

Sun Mar 12, 2006 5:44 am

Good discussion about rates of cooling but would you believe that in an ambient outside temperature of -56C the skin temperature of the leading edges of airliners at Mach 0.80 warm up by as much as +25C from ambient?

Compressibility effects create not only increased skin friction but warmer than ambient surface temperatures on aircraft lifting surfaces. One of the monitoring functions we use in long range cruise at altitude is to monitor the fuel temperature. It should never be lower than the TAT or Total Air Temperature (Static Air Temperature corrected for compression effects) with the exception that after prolonged cruise at lower temperatures transitting to warmer temperatures, the fuel will take some time to warm up.

It is not uncommon to see airliners that have just landed after protracted flight at altitude on the ground after landing in humid environments with a LOT of ice on the upper surface of the wings as well as the normal hoar frost on the underside. The ice forms over time much as the condenser rings in a refrigerator can succumb to icing. This phenomenon is not an inflight hazard in the landing phase but presents distinct problems in subsequent takeoffs.

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