In this paper we are concerned with a class of double phase energy functionals arising in the theory of transonic flows. Their main feature is that the associated Euler equation is driven by the Baouendi-Grushin operator with variable coefficient. This partial differential equation is of mixed type and possesses both elliptic and hyperbolic regions. After establishing a weighted inequality for the Baouendi-Grushin operator and a related compactness property, we establish the existence of stationary waves under arbitrary perturbations of the reaction. A description of a related transonic flow model can be found in G.-Q.G. Chen, M. Feldman (2015), Philos. Trans. Roy. Soc. A 373:20140276 (arXiv:1412.1509.Here, the potential f = f (x, ξ) : Ω × R N ×N → R is assumed to be a quasiconvex function with respect to the second variable; we refer to Morrey [30] for details.Ball [4,5] was interested in potentials given bywhere det ξ denotes the determinant of the N × N matrix ξ. It is also assumed that g and h are nonnegative convex functions satisfying the growth hypotheses g(ξ) ≥ c 1 |ξ| p and lim t→+∞ h(t) = +∞,