Numerical investigations are carried out for natural convection within domains with curved geometry by
using an immersed-boundary method. Influence of the Prandtl number (0.71e0.07) on the flow structure
are investigated through the flow structure map in the RaePr plane. By reducing the Prandtl number, the
flow experiences transition from steady single thermal-plume state, steady double thermal-plume state
and then to unsteady state for Ra 2 106. However at elevated Rayleigh number (Ra > 2 106), the
transition is directly from steady single-plume state to unsteady state without going through doubleplume
regime. The variations of the threshold Pr during transition from single-plume to doubleplume
or to unsteady regime at different Rayleigh numbers are also addressed.