Let's take the simplest case of an ideal gas undergoing adiabatic expansion against a vacuum of finite volume, without any heat and work transfer into or out of the system.
The gas moves in bulk to fill the space, while its internal energy and temperature drop.
Is part or all of this ΔU converted to kinetic energy? When equilibrium is reached and the gas becomes homogeneous in temperature and density once more, does this kinetic energy revert back into internal energy? If it does, will this process cause the temperature to increase?
Or, does the gas expand evenly and gradually 'brakes' down consuming kinetic energy as it approaches the final volume, and eventually naturally stop by itself the very moment the total volume is reached?
How is momentum conserved in either case?