Hydrodynamical Helicity
In fluid mechanics, helicity is the extent to which helix-like motion occurs. If a parcel of fluid is moving, undergoing solid body motion rotating about an axis parallel to the direction of motion, it will have helicity. If the rotation is clockwise when viewed from ahead of the body, the helicity will be positive, if counterclockwise, it will be negative.
Formally, helicity is defined as :
Where is the three dimensional velocity vector of the fluid particles.
The concept is interesting because it is a conserved quantity: is unchanged in a fluid obeying the Euler equations (i.e. zero viscosity) for incompressible fluids. This is analogous to the conservation of magnetic helicity.
Helicity is a useful concept in theoretical descriptions of turbulence.
Read more about Hydrodynamical Helicity: Meteorology