What defines a fluid? No “preferred” shape, deforms to fit container, always flows when force is applied, not spring-based (internal forces depend on velocities, not displacements)
- Types: incompressible, compressible, viscous, inviscid (ideal), turbulent flow, laminar (streamline) flow, Newtonian, non-Newtonian
- Newtonian fluids: smoke, fire, granular flow (snow and sand)
- Non-Newtonian fluids have viscoelastic flow: blood, honey
- Boundary conditions: consideration of the interactions of the fluid surface. There’s three types of surfaces: solid wall, free surface (fluid is adjacent to nothing), other fluid
Compressibility
To have compressible flow means the volume of the fluid can be easily changed. Fluids with incompressible (Divergence-free) flow1 include liquids like water, and fluids with compressible flow include gasses like air and steam.
Also see Incompressibility condition.
Footnotes
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In reality there are no fluids that are 100% incompressible, but for the purposes of simulation it’s easier to pretend some are. ↩