A variational principle for fluid mechanics (Q1368287)

A variational principle for fluid mechanics is derived without calling for any additional postulates in any ad hoc way. In the principle derived, the Lagrangian is the sum of kinetic and heat energy transferred to the fluid, less the sum of its internal and potential energy, less the work done on its exterior (similar to the enthalpy concept), rather than the difference between only kinetic energy and internal energy. The principle so obtained is applicable to the investigation of oscillatory motions of fluids and in the investigation of the stability of a given configuration. It also gives a condition for the possibility of evolution from one chosen initial state to another chosen final state.