> So it only works when you can't compress the fluid, the fluid is not sticky like honey or oil, had as uniform-density, and is an enclosed body (bubbles, not s

> So it only works when you can't compress the fluid, the fluid is not sticky like honey or oil, had as uniform-density, and is an enclosed body (bubbles, not seas). Right, most (non-gaseous) fluids are just about incompressible. This is a pretty safe approximation for tons of applications, although if you were simulating sound wave propagation through water or something you would need compressibility. The lack of viscosity is a bit more limiting and more of an approximation, as even water is fairly viscous. I'm not sure about the enclosed body part, I'd have to read the paper in detail. > There seems to be two ways of looking at fluid motion. The Lagrangian way is following an individual parcel through time and space. The Eulerian way is looking at a specific location and watch fluid flow through that location. Exactly, although the Lagrangian/Eulerian views aren't just limited to fluid dynamics, they are common throughout continuum mechanics. Both approaches have tradeoffs, and hybrid techniques do exist. > Advection is the transport of a property by the fluid due to the fluids bulk motion. So the speed of fluid might compress the fluid, and that should be prevented. An incompressible velocity field is one that is 'divergence free,' as divergence roughly maps to how much material is entering or leaving an infinitesimal volume element. For an incompressible fluid simulation, you thus need to maintain a divergence free velocity field. This leads to the next point... > Orthogonal projection is representing a 3D object in two dimensions. In two dimensions there can be no divergence if I understand it correctly. I think here they are referring to a projection from a general velocity field to a velocity field that is divergence free. See for example [1]. [1] https://en.wikipedia.org/wiki/Projection_method_(fluid_dynamics) https://en.wikipedia.org/wiki/Projection_method_(fluid_dynam...