when the Apollo Space shuttle Re Enter the earth it suffer by high temperature (11000 K) in atmosphere and speed will be almost 15 Mach But When i Ride the bike at 80 Km the air will cool the engine Fin.
From this i can understand that at low Speed, air friction cool the object and in high speed it will heat up the object. But i want to know that at which speed the air will start heating the object.
Vignesh,
Fast objects are not warmed by 'friction' with the air.
When moving quickly through air, the object will compress the air ahead of it. This causes the air to be warmed - and that warm air now flows over the object, heating it.
At very high speeds the air is compressed so much that it becomes incandescent, and radiation from this layer of air also then warms the object.
Air flow at low speed does not cool an object - if you have an inert object at the same temperature as the air, blowing air over it will not make it colder. But you are not inert. You are transpiring water - and by removing that water vapour faster than would otherwise be the case, you are removing heat - and that causes your temperature to drop.
Moving air over an object that is *hotter* than the ambient air flow - of course - will cool it by removing heat more effectively than simple convection would allow.
There is no simple answer to your question - because it depends on the nature of the object that is being cooled (does it transpire? is it warmer than the air flow? etc.)
Vignesh,
Fast objects are not warmed by 'friction' with the air.
When moving quickly through air, the object will compress the air ahead of it. This causes the air to be warmed - and that warm air now flows over the object, heating it.
At very high speeds the air is compressed so much that it becomes incandescent, and radiation from this layer of air also then warms the object.
Air flow at low speed does not cool an object - if you have an inert object at the same temperature as the air, blowing air over it will not make it colder. But you are not inert. You are transpiring water - and by removing that water vapour faster than would otherwise be the case, you are removing heat - and that causes your temperature to drop.
Moving air over an object that is *hotter* than the ambient air flow - of course - will cool it by removing heat more effectively than simple convection would allow.
There is no simple answer to your question - because it depends on the nature of the object that is being cooled (does it transpire? is it warmer than the air flow? etc.)
The speed at which things start changing is the speed of sound. Once you cross the speed of sound, then air is moving faster than the sound waves that alert the surrounding medium to adjust and shock waves form which lead to stagnation regions with higher temperature!! In the case of space shuttle, you have a bow shock ahead of the vehicle and a stagnation region in the nose area with very high temperatures.
when you are riding your bike you are nowhere near the speed of sound so the surrounding medium has plenty of time to adjust as the information moves much faster; I.e. Speed of sound. The cooling in this case is governed by Newton's law of cooling so the flow will extract heat from the surface.
In addition to Abdelkader Frendi, Air is compressible when moving at high speed, This mean that the kinetic energy and momentum of air particles are great as compared with incompressible flow. When such flow is in contact with object there will be heat generated on the surface due to friction. Some of kinetic energy will be dissipated on the surface of the object.
Shear flow creates friction, It leads to heat over an body... air Molecules interaction over an body.
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