Hydrodynamics, the study of liquids in motion, is a type of fluid dynamics, the other being the study of gas in motion, aerodynamics. Magnetohydrodynamics is the study of electrically conducting fluids (either liquid or gas) in motion in an electric field.
Hydrodynamics is sometimes considered to be the branch of fluid dynamics for incompressible fluids, i.e., no changes in density. Liquids such as liquid water approximate this. Gases may approximate this under some circumstances, in which case the normal hydrodynamic models can be used.
A simulations of fluid flows (Hydrodynamic Model or Hydrodynamical Model) can be used for modeling Earth oceans or Atmosphere, or the same for atmospheres of astronomical bodies.
Other subclasses of hydrodynamics, significant in some extreme situations (e.g., modeling, supernovae).
Navier-Stokes Equations (NS Equations)
Radiation Hydrodynamics (RHD)
Semi-Analytic Model (SAM)
Smoothed-Particle Hydrodynamics (SPH)