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A Professor of Fluid Mechanics a told that the static temperature is the temperature observed when the relative speed between observer(thermometer) and fluid is zero. I have trouble understanding as to how will the temperature of fluid depend on the reference frame since temperature is a quantity which should be a function of Static Pressure and not Pressure itself. How can the temperature of a body depend on its speed?

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Moving fluid has kinetic energy which will be converted to internal energy if the flow changes. This is straight-up first law.

So you might consider air blowing through a venturi for instance; if you were to insert a thermometer at different points along the airflow, you would measure different temperatures as the airflow accelerates and decelerates.

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  • $\begingroup$ But if the fluid motion is assumed uniform and steady, and two thermometers, one moving with same speed as fluid, other stationary with respect to ground. Then shouldn’t both thermometers read the same temperature? $\endgroup$ – Varun Patel Apr 25 '18 at 1:10
  • $\begingroup$ No! How do you measure temperature? You allow the relative motion of one set of molecules to come into equilibrium with the relative motion of another. That will change if the relative motion changes, whether that's macroscopic or microscopic. This is precisely why water is colder at the top of a waterfall. $\endgroup$ – Maury Markowitz Apr 25 '18 at 13:06

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