  Nonlinear convection structures are investigated in quantitative detail as a
function of Rayleigh number for several negative and positive Soret coupling
strengths (separation ratios) and different Lewis and Prandtl numbers
characterizing different mixtures. A finite difference method was used to solve
the full hydrodynamic field equations in a range of experimentally accessible
parameters. We elucidate the important role that the concentration field plays
in the nonlinear states of stationary overturning convection (SOC) and of
traveling wave (TW) convection. Structural differences in the concentration
boundary layers and of the concentration plumes in TW's and SOC's and their
physical consequences are discussed. These properties show that the states con-
sidered here are indeed strongly nonlinear, as expected from the magnitude of
advection and diffusion in the concentration balance. The bifurcation behaviour
of the states is analysed using different order parameters such as flow
intensity, Nusselt number, a newly defined mixing parameter characterized by
the variance of the concentration field, and the TW frequency. For further
comparison with experiments, light intensity distributions are determined that
can be observed in side-view shadowgraphs. Structural analyses of all fields
are made using colour coded isoplots, vertical and lateral field profiles, and
lateral Fourier decompositions. Transport properties of TWs are also discussed,
in particular the mean lateral concentration current that is caused by the
phase difference between concentration wave and velocity wave and that is
roughly proportional to the TW frequency. This current plays an important role
in the structural dynamics and stability of spatially-localized traveling-wave
convection (cf. accompanying paper).
