Title: Transonic shocks for steady Euler flows with an external force in an axisymmetric perturbed cylinder

Abstract: We concern the structural stability of transonic shocks for the steady Euler system with an external force in an axisymmetric perturbed cylinder. For a class of external forces, we first prove the existence and uniqueness of the transonic shock solution to the one-dimensional steady Euler system with an external force, which shows that the external force has a stabilization effect on the transonic shock in the flat cylinder and the shock position is uniquely determined. We then establish the existence and stability of the transonic shock solution under axisymmetric perturbations of the incoming supersonic flow, the nozzle boundary, the exit pressure and the external force. Different from the transonic shock problem in two-dimensional nozzles, there exists a singularity along the symmetric axis for axisymmetric flows. We introduce an invertible modified Lagrangian transformation to overcome this difficulty and straighten the streamline. One of the key elements in the analysis is to utilize the deformation-curl decomposition to effectively decouple the hyperbolic and elliptic modes in the steady axisymmetric Euler system with an external force. Another one is an equivalent reformulation of the Rankine-Hugoniot conditions so that the shock front is uniquely determined by an algebraic equation.