  We consider the Nambu and Hamiltonian representations of Rayleigh-Benard
convection with a nonlinear thermal heating effect proportional to the Eckert
number (Ec). The model we use is an extension of the classical Lorenz-63 model
with 4 kinematic and 6 thermal degrees of freedom. The conservative parts of
the dynamical equations which include all nonlinearities satisfy Liouville's
theorem and permit a conserved Hamiltonian H for arbitrary Ec. For Ec=0 two
independent conserved Casimir functions exist, one of these is associated with
unavailable potential energy and is also present in the Lorenz-63 truncation.
This Casimir C is used to construct a Nambu representation of the conserved
part of the dynamical system. The thermal heating effect can be represented
either by a second canonical Hamiltonian or as a gradient (metric) system using
the time derivative of the Casimir. The results demonstrate the impact of
viscous heating in the total energy budget and in the Lorenz energy cycle for
kinetic and available potential energy.
