The built-in explicit symplectic integrator has been replaced by a
general framework supporting run-time selectable integrators.  Currently
the explicit symplectic, implicit Crank-Nicolson and implicit Newmark
methods are provided, all of which are 2nd-order in time:

Symplectic 2nd-order explicit time-integrator for 6DoF solid-body motion:

    Reference:
        Dullweber, A., Leimkuhler, B., & McLachlan, R. (1997).
        Symplectic splitting methods for rigid body molecular dynamics.
        The Journal of chemical physics, 107(15), 5840-5851.

    Can only be used for explicit integration of the motion of the body,
    i.e. may only be called once per time-step, no outer-correctors may be
    applied.  For implicit integration with outer-correctors choose either
    CrankNicolson or Newmark schemes.

    Example specification in dynamicMeshDict:
    solver
    {
        type    symplectic;
    }

Newmark 2nd-order time-integrator for 6DoF solid-body motion:

    Reference:
        Newmark, N. M. (1959).
        A method of computation for structural dynamics.
        Journal of the Engineering Mechanics Division, 85(3), 67-94.

    Example specification in dynamicMeshDict:
    solver
    {
        type    Newmark;
        gamma   0.5;    // Velocity integration coefficient
        beta    0.25;   // Position integration coefficient
    }

Crank-Nicolson 2nd-order time-integrator for 6DoF solid-body motion:

    The off-centering coefficients for acceleration (velocity integration) and
    velocity (position/orientation integration) may be specified but default
    values of 0.5 for each are used if they are not specified.  With the default
    off-centering this scheme is equivalent to the Newmark scheme with default
    coefficients.

    Example specification in dynamicMeshDict:
    solver
    {
        type    CrankNicolson;
        aoc     0.5;    // Acceleration off-centering coefficient
        voc     0.5;    // Velocity off-centering coefficient
    }

Both the Newmark and Crank-Nicolson are proving more robust and reliable
than the symplectic method for solving complex coupled problems and the
tutorial cases have been updated to utilize this.

In this new framework it would be straight forward to add other methods
should the need arise.

Henry G. Weller
CFD Direct

tutorials/multiphase/interDyMFoam/ras/floatingObject: Update to demonstrate MULES with Crank-Nicolson

sixDoFRigidBodyMotion: Add support to specify the centre for rotation independent of the centre of mass
via the point, line or plane constraints.

sixDoFRigidBodyMotion/sixDoFRigidBodyMotionSolver: Support restart by caching state in the uniform directory
sixDoFRigidBodyMotionSolver: use point-edge rather than face-cell wave to calculate point distance to surface.

Keep original case as floatingObjectOld

Added sixDoFRigidBodyMotionSolver using septernion interpolation for mesh-morphing