Model for description of essentually subsonic flows of real one component gas with twoparametric equation of state is presented. In the model changing of density and pressure under gravitation are taking into account. In case of complex nonlinear equation of state the stratification can be considered in linear approximation. With the presented model heat and mass transport in a side-heated isolated square cavity are calculated. There is a near-critical van der Waals gas in the cavity. Comparison with results by other athors shows that the presented model correctly "filters' process of propagation of acoustic perturbations and dosen't lose accuracy of other processes description.