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 Ra reaches a critical value, the flow patterns change and a transition appears in the scaling of Nu(Ra) (where Nu represents Nusselt number) and Re(Ra) (where Re denotes Reynold number). The Grossmann-Lohse(GL) model divides the Ra-Pr(where Pr refers 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 Ra increases. In this work, we conduct a series of numerical simulations in two-dimensional RB convection with Ra ranging from 10⁷ to 10¹² and Pr ranging from 0.25 to 100, which is unprecedentedly wide. The relationship between the maximum velocity and Ra is investigated, and an unexpected drop happens when Ra reaches a critical value Ra_c, and Ra_c increases with Pr increasing. The Re number, which is defined as a maximum velocity, also shows a plateau at Ra_c. Before and after Ra_c, the Ra scaling exponent of Re remains 0.55, which gets smaller at very high Ra. Specially, under different Pr values, the plateau appears at Re_c ≈ 1.4 × 10⁴. In addition, a scaling Ra_c~Pr^1.5 is found and the Ra is compensated for by Pr^–1.5 to disscuss the relationship between Re and RaPr^–1.5. It is interesting that the Re(RaPr^–1.5) expressons at different Pr values well coincide, indicating a self-similarity of Re(RaPr^–1.5). The plateau appears at RaPr^–1.5 = 1 × 10⁹, meaning that Re_c would reach 1.4 × 10⁴ at any Pr value when RaPr^–1.5 = 1 × 10⁹. 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_c at different Pr values, and Re_c is a constant as mentioned above. This finding can help us to distinguish the two flow patterns with given Ra and Pr, and to predict the Re scaling in an appropriate range of Ra with different Pr values.