Rayleigh number ( Ra) dependence in Rayleigh-Bénard (RB) convection has been studied by many investigators, but the reported power-law scaling expressions are different in these researches. Previous studies have found that when Rareaches a critical value, the flow patterns change and a transition appears in the scaling of Nu( Ra) (where Nurepresents Nusselt number) and Re( Ra) (where Redenotes Reynold number). The Grossmann-Lohse(GL) model divides the Ra-Pr(where Prrefers to Prandtl number) phase into several regions to predict the scaling expressions of Nu( Ra,Pr) and Re( Ra,Pr), indicating that the thermal dissipation behavior and kinetic dissipation behaviors are diverse in the different regions. Moreover, some physical quantities also show a transition and some structures in the flow fields, such as large scale circulation and boundary layer, change when Raincreases. In this work, we conduct a series of numerical simulations in two-dimensional RB convection with Raranging from 10 ⁷to 10 ¹²and Pr ranging from 0.25 to 100, which is unprecedentedly wide. The relationship between the maximum velocity and Rais investigated, and an unexpected drop happens when Rareaches a critical value Ra _c, and Ra _cincreases with Pr increasing. The Renumber, which is defined as a maximum velocity, also shows a plateau at Ra _c. Before and after Ra _c, the Rascaling exponent of Reremains 0.55, which gets smaller at very high Ra. Specially, under different Prvalues, the plateau appears at Re _c≈ 1.4 × 10 ⁴. In addition, a scaling Ra _c~ Pr ^1.5is found and the Rais compensated for by Pr ^–1.5to disscuss the relationship between Reand RaPr ^–1.5. It is interesting that the Re( RaPr ^–1.5) expressons at different Prvalues well coincide, indicating a self-similarity of Re( RaPr ^–1.5). The plateau appears at RaPr ^–1.5= 1 × 10 ⁹, meaning that Re _cwould reach 1.4 × 10 ⁴at any Prvalue when RaPr ^–1.5= 1 × 10 ⁹. To further investigate the plateau of Re, the flow patterns are compared with time-averaged velocity fields and we find that the large scale circulation (LSC) changes from ellipse to circle at Ra _c. In other words, the flow pattern will change into circular LSC at Re _cat different Prvalues, and Re _cis a constant as mentioned above. This finding can help us to distinguish the two flow patterns with given Raand Pr, and to predict the Rescaling in an appropriate range of Rawith different Prvalues.