In this paper, we report on the numerical simulation of a multiphysic electromagnetic/mechanical problem, as a further step toward the development of microwave circuits based on phonon propagation at micro and nanoscale. In the future, this kind of circuits is likely to integrate optomechanically pumped phonon sources and detectors, as well as phonon processing components (waveguides, splitters, and memories) to process information by means of phonons. In particular, we propose a rigorous approach for the solution of the electromagnetic/mechanical system of equations governing light behavior in optomechanical cavities, with the help of the transformation optics method. Such a kind of calculation allows the development of an efficient generation of microwave coherent phonon sources, by engineering their propagation or coupling with phonon waveguides.