Discussion
A shock wave is moving into still air in a shock tube. Which one of the following happens to the air?
Answer: B
The air's static temperature increases and its total temperature increases.
Across the shock the gas is compressed, so its static pressure and static temperature rise. In the shock tube the air was at rest and the shock sets it moving, so behind the shock it also has kinetic energy. The moving shock does work on the gas: in the laboratory frame the flow is unsteady, and total enthalpy is not conserved across a moving shock, so the total temperature rises too.
The total temperature is constant across a shock only in the frame moving with the shock, where the flow is steady; that is the trap in 'total temperature remains constant', in both options that say so. A fall in total temperature would need energy to be taken out of the gas, which a compression shock does not do.
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