ForewordLike many teachers of thermodynamics, I've always found the study of Joule's decompression to be very specific. "After a certain time waiting for equilibrium to return, the perfect gas returns to its initial temperature, reflecting the absence of additional internal energy. Before this decompression, as well as after this waiting time, the system is in a quasi-static situation, with randomly moving, isotropic and homogeneous particles".
In contrast, the transient stage has never been studied. It takes place just after expansion; the ephemeral, fleeting situation means that the system is anisotropic and non-homogeneous, with particles moving in a direction that is not completely random.
The temperature reflecting the random fluctuation energy density of the particles must therefore be lower, and only returns to its initial value, as...