Surprising solutions to the isentropic Euler system of gas dynamics

In a recent paper, jointly with Elisabetta Chiodaroli and Ondřej Kreml we consider the Cauchy problem for the isentropic compressible Euler system in 2 space dimensions, with initial data which assume two different constant values and have a discontinuity across a line. If we consider selfsimilar solutions we then encounter a classical 1-dimensional Riemann problem for the corresponding hyperbolic system of conservation laws. We show that for some suitable choice of the pressure and of the initial data there exist infinitely many bounded admissible solutions which are not selfsimilar and indeed are genuinely 2-dimensional. We also show that some of these Riemann data are generated by a 1-dimensional compression wave. Our theorem leads therefore to Lipschitz initial data for which there are infinitely many global bounded admissible weak solutions. Each of these solutions coincide as long as the classical (Lipschitz) solution exists and they differentiate themselves immediately after the first blow-up time. Our approach is heavily influenced by a work of László Székelyhidi which provides a similar result in the case of the classical vortex-sheet problem for the incompressible Euler equations.