A DIFFUSE-INTERFACE DESCRIPTION OF INTERNAL WAVES IN A NEAR-CRITICAL FLUID

We present a diffuse-interface treatment of the internal gravity waves which have been observed experimentally by Berg et al. in xenon near its thermodynamic critical point. The results are compared with theore tical predictions by Berg et al. that were obtained using separate mod els above and below the critical temperature T-c. The diffuse-interfac e model applies both above and below T-c, and is formulated by using t he density as an order parameter. The diffuse interface is represented as a transition zone of rapid but smooth density variation in the mod el, and density gradients appear in a capillary tensor, or Korteweg st ress term, in the momentum equation. We obtain static density profiles , compute internal wave frequencies and compare with the experimental data and theoretical results of Berg et al. both above and below the c ritical temperature. The results reveal a singularity in the diffuse-i nterface model in the limit of incompressible perturbations.