“…Recently, L. L. Du et al [12,14] used this variational approach to study impinging subsonic jets and collision of two subsonic flows. There are also many works on irrotational and rotational subsonic flows past profiles or in nozzles, which are formulated as fixed boundary problems, and we refer to [5,6,11,13,15,16,18,28,34,35,36] and the references therein. Continuous subsonic-sonic flows in convergent nozzles was studied in [29,30,31,32,33], where the flows are governed by free boundary problems of a degenerate elliptic equation, and the sonic curve is a free boundary where the flow velocity is along the normal direction.…”