Solitary waves in the Madelung's fluid: Connection between the nonlinear Schrodinger equation and the Korteweg-de Vries equation

An investigation to deepen the connection between the family of nonlinear Schrodinger equations and the one of Korteweg-de Vries equations is carried out within the context of the Madelung's fluid picture. In particular, under suitable hypothesis for the current velocity, it is proven that the cubic nonlinear Schrodinger equation, whose solution is a complex wave function, can be put in correspondence with the standard Korteweg-de Vries equation, is such away that the soliton solutions of the latter are the squared modulus of the envelope soliton solution of the former. Under suitable physical hypothesis for the current velocity, this correspondence allows us to find envelope soliton solutions of the cubic nonlinear Schrodinger equation, starting from the soliton solutions of the associated Korteweg-de Vries equation. In particular, in the case of constant current velocities, the solitary waves have the amplitude independent of the envelope velocity (which coincides with the constant current velocity). They are bright or dark envelope solitons and have a phase linearly depending both on space and on time coordinates. In the case of an arbitrarily large stationary-profile perturbation of the current velocity, envelope solitons are grey or dark and they relate the velocity u(0) with the amplitude; in fact, they exist for a limited range of velocities and have a phase nonlinearly depending on the combined variable x - u(0)s (s being a time-like variable). This novel method in solving the nonlinear Schrodinger equation starting from the Korteweg-de Vries equation give new insights and represents an alternative key of reading of the dark/grey envelope solitons based on the fluid language. Moreover, a comparison between the solutions found in the present paper and the ones already known in literature is also presented.