Helicity invariants in 3D : kinematical aspects

Abstract

Exact, degenerate two-forms on time-extended space R X M which are invariant under the unsteady, incompressible fluid motion on 3D region M are introduced. The equivalence class up to exact one-forms of each potential one-form is splitted by the velocity field. The components of this splitting corresponds to Lagrangian and Eulerian conservation laws for helicity densities. These are expressed as the closure of three-forms which depend on two discrete and a continuous parameter. Each two-form is extended to a symplectic form on R X M. The subclasses of potential one-forms giving rise to Eulerian helicity conservations is shown to result in conformally symplectic structures on R X M. The connection between Lagrangian and Eulerian conservation laws for helicity is shown to be the same as the conformal equivalence of a Poisson bracket algebra to infinitely many local Lie algebra of functions on R X M.