Left: zones of predominances for dissipative mechanisms. In the pink area b), ζ^therm ≫ ζ^visc: the dissipation is mainly due to thermal diffusion. In the white area a), it is led by viscous friction. The transition zone corresponds to , where ζ^visc ~ ζ^therm. Right: asymptotic domains with the predominance zones. Low Prandtl-number areas of Fig. 7 are divided in sub-areas corresponding to the locally predominating dissipation mechanism. In a), b), and f), ζ^therm ≪ ζ^visc while in c), d), and e), ζ^therm ≫ ζ^visc.