The motion of fluids and gases are described with the Navier-Stokes
equation and the equation of
These two equations describe the motion of fluids in both two- and three-dimensional systems.
is the velocity field. denotes the derivative of a quantity when you follow the flow (as opposed to ) and can be expanded to . is the density of the fluid. p is the pressure. is the kinematic viscosity and is the second viscosity. The viscosity of a fluid can under most circumstances, including the experiments presented in this thesis, be considered constant. F represents external forces such as gravity and the Coriolis-force.