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This paper concerns the structural stability of smooth cylindrical symmetric transonic flows in a concentric cylinder under helically symmetric perturbation of suitable boundary conditions. The deformation-curl decomposition developed by the second author and his collaborator is utilized to effectively decouple the elliptic-hyperbolic mixed structure in the steady compressible Euler equation. A key parameter in the helical symmetry is the step (denoted by $\sigma$), which denotes the magnitude of the translation along the symmetry axis after rotating one full turn. It is shown that the step determines the type of the first order partial differential system satisfied by the radial and vertical velocity. There exists a critical number $\sigma_{*}$ depending only on the background transonic flows, such that if $0<\sigma<\sigma_{*}$, one can prove the existence and uniqueness of smooth helically symmetric transonic flows with nonzero vorticity. |
