Existence of Global Steady Subsonic Euler Flows through Infinitely Long Nozzles

In this paper, we study the global existence of steady subsonic Euler flows through infinitely long nozzles without the assumption of irrotationality. It is shown that when the variation of Bernoulli's function in the upstream is sufficiently small and mass flux is in a suitable regime with an upper critical value, then there exists a unique global subsonic solution in a suitable class for a general variable nozzle. One of the main difficulties for the general steady Euler flows, the governing equations are a mixed elliptic-hyperbolic system even for uniformly subsonic flows. A key point in our theory is to use a stream function formulation for compressible Euler equations. By this formulation, Euler equations are equivalent to a quasilinear second order equation for a stream function so that the hyperbolicity of the particle path is already involved. The existence of solution to the boundary value problem for stream function is obtained with the help of the estimate for elliptic equation of two variables. The asymptotic behavior for the stream function is obtained via a blow up argument and energy estimates. This asymptotic behavior, together with some refined estimates on the stream function, yields the consistency of the stream function formulation and thus the original Euler equations.