Forced Convection

This is being editted by jdeereanton to inform that this thread was split from a thread about hydration started by jdeereanton. We now continue with the regularly schedule program... Take it away Hammy -


I remember reading from the last issue of Iron Butt magazine that when you ride in temps of 93 degrees and above, that is the point where the air no longer cools your body but actually heats it up as well as your bike. In other words, at those temps, you'd probably actually benefit more from being shielded from the wind instead of trying to expose more of yourself to it (e.g. if you have an adjustable shield, you might want to raise it instead of keep it lowered).

If our core body temp is 98.6 degrees, how can a surface temp that is cooler (93 degrees) cause the body temp to increase?

I'd like to understand the physics of that phenomena.

I think it has to do with the amount of heat the body can deal with.
One assumes that 93º is the point at which the human body begins to struggle.
Just guessing.

That's a nice very colorful chart. Of course it does not answer the question - How can cooler ambient temperature cause the temperature of an already warmer object to increase (measureable, not voodoo)?

I get that the chart displays how hot it "feels". That is rather subjective, I'd like to discuss how hot it sounds?? Or maybe the smells like temperature! How does this chart address the reality of two people standing side by side and one "feels" hot and the other does not. Maybe they need a mauve color for the voodoo temps.

jdeereanton wrote:

That's a nice very colorful chart. Of course it does not answer the question - How can cooler ambient temperature cause the temperature of an already warmer object to increase (measureable, not voodoo)?

I get that the chart displays how hot it "feels". That is rather subjective, I'd like to discuss how hot it sounds?? Or maybe the smells like temperature! How does this chart address the reality of two people standing side by side and one "feels" hot and the other does not. Maybe they need a mauve color for the voodoo temps.

http://en.wikipedia.org/wiki/Heat_index

The human body normally cools itself by perspiration, or sweating, which evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which relates one temperature and humidity combination to another at a higher temperature in drier air.

So you agree mauve would be a good color for the "feels like" (voodoo) temps.

Still doesn't answer the question. Does inject more data points that are tangentially related to the question, always good on the road to confusion.

As a coach, we live by the heat index for "legal" reasons. It is a pretty good barometer. Another thing many people fail to do is to monitor their caffeine intake. Lots of Coke, Coffee, and Mountain Dew and you will dehydrate far quicker. Power drinks with electrolytes and water are best. Moisture wicking clothing is also helpful. I try to regularly drink some water at least every 2-3 hours to help keep and maintain a good amount in my body at any given time. If in the sun, more often still. If active, more often still. Heat is nothing to play around with when the heat index is high.

jdeereanton wrote:

If our core body temp is 98.6 degrees, how can a surface temp that is cooler (93 degrees) cause the body temp to increase?

I'd like to understand the physics of that phenomena.

The article in the magazine was pretty informative and explained the physics but the magazine's kinda lost somewhere in my room and I haven't found it yet. Suffice it to say that my good buddy, who just took part in the Iron Butt Rally 5000 this past August, rode in pretty high temps and essentially found that the hot air (it was in the low 100s in Kansas) actually made it quite unbearable and his Yamaha Majesty began to overheat as the air wasn't cooling but heating up the radiator. It was apparently so hot that the more he moved along, the more he heated up. I'd imagine that at some point, a bigger shield would help more than one might think?

I don't know the physics myself. All I can trust is that smarter folks who are riders I hold in high esteem have done the research and I go by what they tell me. I also agree with the other poster who stated that many desert dwellers go out swathed in robes -- they definitely know how to deal with the heat.

Hammy wrote:

... Suffice it to say that my good buddy, who just took part in the Iron Butt Rally 5000 this past August, rode in pretty high temps and essentially found that the hot air (it was in the low 100s in Kansas) actually made it quite unbearable and his Yamaha Majesty began to overheat as the air wasn't cooling but heating up the radiator. It was apparently so hot that the more he moved along, the more he heated up. I'd imagine that at some point, a bigger shield would help more than one might think?

I'd imagine that at some point it is apparently possible that riders and bikes will just burst into flames.:lol!: Cars too - odd that's not covered in the owner's manual.

I travelled for a good while in the desert area of Saudi Arabia / Kuwait / Iraq. It was hot, not warm, but HOT. Daily temps over 105 degrees, we drove our vehicles, wheeled and tracked daily. No flaming vehicles due to mystery combustion unless it was at the request of the FDC (fire direction controller) and then we didn't consider that a mystery - more like a victory.

This was a thread about hydrating. The article is located here: http://www.roadrunner.travel/wp/2010/09/09/touring-tip-staying-hydrated/

Sorry for hijacking the thread then. I thought it was related. I usually carry a Camelbak system when I'm on a ride. But feel free to delete the irrelevant info if you wish.

Okay -

I did an odd thing I split the stuff that was kind of wild and took it out of the staying hydrated topic.

Why is this topic titled Forced Convection? Because, that is the term, according to my friend the Physicist (Phd), a genuine rocket scientist, that best describes what Hammy was obliquely referring to. Can't swing a cat in this town without hitting a rocket scientist. Anyway, Dr. Blackwood is a great guy and was quick to understand what was being described in this thread.

So the difference between the body temp and 93 degrees is a paltry 5 degrees. None the less it is impossible for a lesser ambient temp to heat an object that has a higher temp, unless that object with the higher temp is moving.

Not so fast. Little pun there, the higher temp object has to be moving fast enough through the medium of the lower temp (air in this case) for friction to act upon the surface of the object. That speed? Would have to be travelling faster than 150 mph. Although it is highly unlikely that you would be able to ride you Honda SilverWing at that speed and you would have to ride at that speed for a sustained period of time depending on materials and actual drag.

Now then, a larger surface area (more screen) means more resistance thus you could become a fire ball at a lower speed, maybe somewhere in the vicinity of 150 mph.

Remember the engine runs as a result of many, many little fire ball like explosions, these create heat. The faster you go the more explosions, thus more heat. If the Majesty were truly over heating it is more likely that it had some mechanical or fluid problem than metal surfaces were heating due to ambient temps. Remember your body may have a hard time regulating the temp as it is exposed to direct sunlight and such, but the engine is mechanical and therefore not subject to the whims and same type of fatigue as the human body. If the engine was overheating and was being operated within the manufacturers tested parameters - there is a mechanical problem. The human experience is a bit more subjective.

Dale:
The only reason that the human body averages an internal temperature of approximately 98.6 degrees F is that it has the ability to cool itself through perspiration. If for some reason the body loses the ability to cool itself, then heat that is generated by cell metabolic processes and muscle activity will cause the body temperature to rise and eventually the heat will build up to a point that it will damage the body. Does that make sense?

Oh, it absolutely does make sense to me. I understand that. But it was not necessarily being presented that way. It was a straight forward at a given temp, less than the average internal temp, the lesser ambient temp will begin to heat the objects immersed in that ambient medium.

I will try to find that article and just put up what it wrote. For all I know, I'm remembering it wrong. It just stuck in my head that there was a specific temp above which the moving air no longer cools you down but heats you up.

Hammy:
What you are describing is also correct. When air moves extremely fast across a surface it causes a small amount of friction. It is this condition that causes the outside of the space shuttle to heat up as it re-enters the earths atmosphere.

The article was quite informative and had a whole bunch of diagrams. I think it also figured that the rider would be wearing full gear, so maybe that might have been why that temp was chosen.

jdeereanton wrote:

It was a straight forward at a given temp, less than the average internal temp, the lesser ambient temp will begin to heat the objects immersed in that ambient medium.

Evaporation cooling is not consistent a given temperature.

http://science-edu.larc.nasa.gov/SCOOL/psychrometer.html

If the surrounding air is dry, more moisture evaporates from the wick, cooling the wet-bulb thermometer more so there is a greater difference between the temperatures of the two thermometers. If the surrounding air is holding as much moisture as possible - if the relative humidity is 100% - there is no difference between the two temperatures.

And, again, Bill, information on relative humidity was not provided in the original statement so I proceeded without that.

What I hope has occurred during this back and forth is that each of you like me were forced to think critically about the things posted and not just accept off hand comments. Facts and science matter and so they should matter to us. We humans have a tendency to not consider all of what goes into an idea and casually accept the surface premise if it reinforces our belief(s).

Very true. I will clearly refrain from adding anything I've come across that I thought was useful if I don't completely understand the science behind it. Again, I apologize for my "off hand comment". There are plenty of people on here who are more informed than I am and I am glad to learn from them. There's no need for anyone to be confused by me.

The Scootist wrote:

Hammy:
What you are describing is also correct. When air moves extremely fast across a surface it causes a small amount of friction. It is this condition that causes the outside of the space shuttle to heat up as it re-enters the earths atmosphere.

Absolutely! And how fast is the shuttle moving when it begins heating up? And to take it a step further - the air temps where this begins are very cold, what is the surface temp of the shuttle when the heating begins?

Hammy not at all - How many may have read the initial premise and wandered away from it with a misconception? We'll never know and the impact is most likely very small to miniscule.

Maybe I'm just yanking the chain... so I'm going to flush me out of this.

Humor is difficult to figure out on this type of interaction. Probably why email type psychotherapy hasn't caught on! No worries. I just wish I can find that dang magazine... there were pictures explaining what you're asking, stuff which goes way over my head, hence why I typically defer to those who are better informed.