The incompressible fluid revisited: vortex-sound interactions

Understand

We study the dynamics of a nearly incompressible fluid via (classical) effective field theory.

• In the kinematical regime corresponding to near incompressibility (small fluid velocities and accelerations), compressional modes are, by definition, difficult to excite, and can be dealt with perturbatively.
• We systematically outline the corresponding perturbative expansion, which can be thought of as an expansion in the ratio of fluid velocity and speed of sound.
• Our analysis is fully relativistic, but, if desired, the non-relativistic limit can be taken at any stage.