INVERSION OF ACOUSTIC WAVES REFLECTION RULES IN NEAR CRITICAL PURE FLUIDS

The reflection of linear acoustic waves propagating in a very compressible, low heat diffusing, supercritical fluid (CO2) is studied by solving the nonlinear Navier–Stokes (NS) equations written for a near‐critical van der Waals gas. It is shown that close enough to the critical point, the reflection rules of acoustic waves on thermostatted boundaries are inverted, compared to those usually observed in ideal gases. This phenomenon has been put in evidence by numerical simulations, and its mechanism is shown to be due to the strong contraction or expansion of a hypercompressible thermal adaptation boundary layer. The asymptotic analysis of the equations gives the order of magnitude of the distance to the critical point where the inversion occurs. This transition corresponds to the transition to the so‐called Acoustically Saturated Regime, in which the characteristic time scale of heat transfer reaches its lower limit, which is the acoustic typical time.