SPECIFIC ACOUSTIC-WAVES REFLECTION RULES NEAR THE CRITICAL-POINT

The reflection of linear acoustic waves propagating in a very compress ible, low heat diffusing, supercritical fluid (CO2) is studied by solv ing the non-linear Navier-Stokes (NS) equations written for a near-cri tical van der Waals gas. It is shown that close enough to the critical point, the reflection rules of acoustic waves on thermostated boundar ies are inverted compared to those usually observed in ideal gases. Th is phenomenon has been put in evidence by numerical simulations and it s mechanism is shown to be due to the strong contraction or expansion of a hypercompressible thermal adaptation boundary layer. The asymptot ic analysis of the equations gives the order of magnitude of the dista nce 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 reache s its lower limit (i.e. the acoustic typical time).