• # Question: If an object starts moving from standstill through the air, getting faster and faster, its initial temperature will start to drop, but as the speed increases to the point where air molecules cannot get out of the way quick enough the objects temperature will start to increase. Is there a name for the point where this happens, and would the shape of the object make any difference?

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Asked by pmason to Olivia on 24 Nov 2017.
• Olivia Stodieck answered on 24 Nov 2017:

Challenging question! I had to look this one up, since it’s been a while since I had to deal with thermodynamics… So, I believe that the statement “if an object starts moving from standstill through the air, getting faster and faster, its initial temperature will start to drop” is only true if the air around the object is much colder than the object itself. Some energy will always be lost through friction in the boundary layer between the object and the free flow, and this will actually generate heat, rather than dissipate it. But the friction effect is small at low speeds. The point where “air molecules cannot get out of the way quick enough” is called the the lower critical Mach number of the object – in effect, the flow becomes supersonic locally (Mach=1), but is subsonic (M1), the air becomes compressible in that supersonic region, which is called a supersonic expansion fans. In these regions, the air is expanded and this causes the temperature to go down (see ideal gas law). That is why you can see condensation cones around fast flying fighter aircraft sometimes. But where the flow slows down again, a shock wave occurs and this generates heat as the gas is compressed again. When object moves even faster, the shock wave starts to form in-front of the object, and this is called the upper critical Mach number. For aircraft wings this usually happens at free flow mach numbers of around M=1.2. (thanks for Wikipedia!!) Hope this makes sense?