Arrow directions and lengths show their velocity direction and absolute values, respectively. Although each side of a mesh cell has a parameter of velocity, which expresses the absolute value of velocity in vertical direction to the side, only representative arrows are drawn to simplify the figure
There are some exemptions. For example, the Arbitrary Lagrangian Eulerian (ALE) method moves and deforms the mesh.
An element means substance whose volume is very small. Although elements are not handled directly in FDM, we need to consider the equation of momentum for every fluid element because fluid consists of microscopic elements.
Also called Lagrangian derivative. A time derivative of a physical quantity. The time derivative is evaluated by following a designated substance. That is to say, the evaluation point is moved with the designated substance