In this article, we study the gas expansion problem by turning a sharp corner into a vacuum for the twoâdimensional (2âD) pseudosteady compressible Euler equations with a convex equation of state. This problem can be considered as the interaction of a centered simple wave with a planar rarefaction wave. To obtain the global existence of a solution up to the vacuum boundary of the corresponding 2âD Riemann problem, we consider several Goursatâtype boundary value problems for 2âD selfâsimilar Euler equations and use the ideas of characteristic decomposition and bootstrap method. Further, we formulate 2âDâmodified shallow water equations newly and solve a damâbreakâtype problem for them as an application of this work. Moreover, we also recover the results from the available literature for certain equations of states that provide a check that the results obtained in this article are actually correct.