Point groups in three dimensions the finite subgroups of SO(3), are: the cyclic groups "C" (the rotation group of a canonical pyramid), the dihedral groups "D" (the rotation group of a uniform prism, or canonical bipyramid), and the rotation groups "T", "O" and "I" of a regular tetrahedron, octahedron/cube and icosahedron/dodecahedron. In particular, the dihedral groups "D", "D" etc. are the rotation groups of plane regular polygons embedded in three-dimensional space, and such a figure may be considered as a degenerate regular prism. Therefore, it is also called a "dihedron" (Greek: solid with two faces), which explains the name "dihedral group". The rotation group of
Two-dimensional flow Two-dimensional flow Fluid motion can be said to be a two-dimensional flow when the flow velocity at every point is parallel to a fixed plane. The velocity at any point on a given normal to that fixed plane should be constant. Considering a two dimensional flow in the formula_1 plane, the flow velocity at any point formula_2 at time formula_3 can be expressed as – Considering a two dimensional flow in the formula_5 plane, the flow velocity at a point formula_6 at a time formula_3 can be expressed as – Vorticity in two dimensional flows in the formula_1 plane can