The unexpected response of near‐critical fluids to low‐frequency vibrations

The response of a slab‐shaped container filled with a near‐critical fluid and subjected to mechanical vibrations is studied by means of matched asymptotic expansions applied to the Navier–Stokes equations. The different characteristic regimes of the fluid’s vibration are explored and described. In particular, a specific low‐frequency regime is highlighted, in which the mechanical response couples with thermo‐acoustic convection (piston effect). In this particular regime, the bulk part of the fluid behaves almost like a solid which bounces back and forth between two highly compressible thermal boundary layers. Such a response is entirely specific to near‐critical fluids and is never witnessed in perfect gases.