I am having some confusion in computation fluid dynamics. navier-stokes equation is the differential equation for fluid motion, but does this equation apply to gas as well? to my understanding gas qualifies as a fluid but I am having trouble seeing how the same equation applies to both liquid and gas motion. liquid has attraction force between each element while gas attempts to occupy the entire shape of container and therefore only has repelling force rather than attraction force, but how is this reflected in the navier-stokes equation? because I am using the introductory form of navier-stokes, assuming the fluid to be incompressible, there are only 3 terms contributing to the acceleration, change in pressure, change in speed for viscosity, and external force. just reading the equation itself, I am guessing if fluid pressure pushes each element apart, then the viscosity acts as damper so the motion reaches stops once the shape pressure force is below the damping force. therefore gas motion would have very low viscosity so the motion keeps expanding until the gas occupies the entire space.