A new frozen-in field w (generalizing vorticity) is constructed for ideal magnetohydrodynamic flow. In conjunction with the frozen-in magnetic field h, this is used to obtain a generalized Weber transformation of the MHD equations, expressing the velocity as a bilinear form in generalized Weber variables. This expression is also obtained from Hamilton's principle of least action, and the canonically conjugate Hamiltonian variables for MHD flow are identified. Two alternative energy-type variational principles for three-dimensional steady MHD flow are established. Both involve a functional R which is the sum of the total energy and another conserved functional, the volume integral of a function Φ of Lagrangian coordinates. It is shown that the first variation δ1R vanishes if Φ is suitably chosen (as minus a generalized Bernoulli integral). Expressions for the second variation δ2R are presented.

The self-propulsion of unicellular algae in still ambient fluid is studied using a previously reported laser-based

The paper is devoted to the studying of a dynamical system 'solid + fluid' in the presence of vibrations using the Van-der-Pol-Krylov-Bogoliubov (VPKB) averaging method. The main result of the paper is the discovery of a close similarity between a classical pendulum and a system of 'inhomogeneous solid + fluid' in the presence of vibrations. First, we consider the celebrated example of the Stephenson-Kapitza pendulum using the least action formulation of the VPKB averaging method. The method directly exploits the least action principle, in which an averaging procedure appears most naturally and conservation laws follow automatically. Its main advantage is a substantial decrease of the required amount of analytical calculations, which are typically cumbersome for the VPKB averaging method. Then, we consider the dynamics of a rigid sphere in an inviscid incompressible fluid, which fills a vibrating vessel of an arbitrary shape. The sphere can be either homogeneous or inhomogeneous in density. The results provide a full model for the averaged (or 'slow') motions, which includes the 'slow Lagrangians', the 'slow potential energy', and the 'vibrogenic' force, exerted by a surrounding fluid on a solid. We outline our calculations, present results in general forms, and briefly discuss related examples, properties, and conjectures. (2000). 76M45 (76B99, 70K65). Mathematics Subject Classification

The techniques developed in Part 1 of the present series are here applied to two-dimensional solutions of the equations governing the magnetohydrodynamics of ideal incompressible fluids. We first demonstrate an isomorphism between such flows and the flow of a stratified fluid subjected to a field of force that we describe as ‘pseudo-gravitational’. We then construct a general Casimir as an integral of an arbitrary function of two conserved fields, namely the vector potential of the magnetic field, and the analogous potential of the ‘modified vorticity field’, the additional frozen field introduced in Part 1. Using this Casimir, a linear stability criterion is obtained by standard techniques. In §4, the (Arnold) techniques of nonlinear stability are developed, and bounds are placed on the second variation of the sum of the energy and the Casimir of the problem. This leads to criteria for nonlinear (Lyapunov) stability of the MHD flows considered. The appropriate norm is a sum of the magnetic and kinetic energies and the mean-square vector potential of the magnetic field.