Flow-reversal phenomena in a classical two-dimensional (2D) Rayleigh-Bénard convection, in a square enclosure, are usually explained through growth and merging of diagonally opposite counterrotating corner rolls. To probe further the corner-roll growth dynamics, we have altered the square enclosure edges by additional slanted conduction walls, so that the enclosure resembles an octagonal shape. We have performed a series of 2D numerical simulations by varying the slanted wall inclination angle ($\alpha$) from $0^{\circ }$ to ${45}^{\circ }$, to construct a detailed flow map in thermal convection in a range $5\times {10}^5\le \text{Ra}\le {10}^8$ and $0.8\le \text{Pr}\le 2.0$, where $\text{Ra}$ is the Rayleigh number and $\text{Pr}$ is the Prandtl number. Depending on $\text{Ra}$, $\text{Pr}$, and $\alpha$, flow features in the octagonal enclosure can exist in the form of a uniform circulation, a two-roll, a mixed, a periodic, a quasiperiodic, or multiple flow states superimposed on each other. The flow reversals in the octagonal enclosure take place in several ways, for example, by the ejection of mushroom-shaped plumes alternatively from the opposite slanted walls at low $\text{Ra}$ ($\approx {10}^5$) and high $\text{Pr}$ ($\approx 2$), by the corner-roll growth at high $\text{Ra}$ ($\approx {10}^8$) and low $\text{Pr}$ ($\approx 1.2$), and by the dipole at high $\text{Ra}$ ($\approx {10}^8$) and high $\text{Pr}$ ($\approx 2$). Strikingly, the dimensionless flow-reversal frequency scales linearly with an increase in $\alpha$, and the slope varies from 1.04 at low $\text{Ra}=5\times {10}^5$ to 0.328 at high $\text{Ra}={10}^8$. We have shown the flow reversals are a consequence of competition between the dipole (a two-roll state where a cold roll sits above a hot roll) and the quadrupole (the four corner rolls) modes with the monopole mode. A uniform circulation with flow-reversal results if the quadrupole mode wins, and a two-roll state with a reversal results, if the dipole wins. At high $\text{Ra}(\ge {10}^8)$, the dipole strengthens, and the core bulk region shows hydrodynamic instabilities in the form of turbulent-like engulfments. We have uncovered the mechanism responsible for the observed engulfments due to the increase in turbulence production in the core bulk region by the buoyancy. As a result, we have observed that total heat transport also increases up to $14\%$ when $\alpha$ is varied from $0^{\circ }$ to ${45}^{\circ }$.

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• Convection
• Rayleigh-Bénard convection