Rayleigh-Taylor-like instability in near-critical pure fluids

The hydrodynamic stability; of a thermodiffusive interface in a near-superc ritical fluid is studied. The Navier-Slokes equations written for a van der Waals gas above its critical point are solved by means of a finite volume numerical method. The growth rate of the fluctuations shows that there exis ts a cutoff wave number beyond which the short wavelengths are stabilized b y diffusion. The good agreement between the obtained values and recent theo ries for miscible fluids confirms that a near-critical fluid subjected to a thermal gradient may develop a gravitational instability for which the den sity gradient is driven by thermal diffusion and large compressibility.