Extensions of hyperbolic systems of balance laws

An algorithm is presented for the extension of a hyperbolic system of balance laws to a system of higher dimension, in such a manner that weak solutions of the original system form a subset of the weak solutions of the extended system. Hyperbolicity and the symmetry group of the original system survive the extension, and some information on the characteristic speeds of the extended system is obtained. Applying the method to isentropic fluid flow, a new form of model for two-fluid flow is obtained.