Flow reversal control in Rayleigh–Bénard convection by a pair of iso-concentration boundary bands

Recently, flow-reversal mechanisms in Rayleigh–Bénard (RB) convection and controlling strategies via modifying local temperature boundaries have received increasing attention due to the impact on heat-transfer efficiency and extreme eruption events. We consider an alternative possibility of altering fluid density: an added scalar field that induces double-diffusive convection, implemented by imposing local iso-concentration bands on the horizontal plates. In addition to the Rayleigh number (Ra) and Prandtl number (Pr), the system is governed by the Lewis number (Le), buoyancy ratio ($Br$), normalised bandwidth (δ) and normalised band-centre-to-midline distance (c). We examine the influence of δ and c on flow reversal at Ra=5 × 10⁷, Pr=2, Le =1 and Br=1.5. Paired bands effectively reduce reversal frequency, with stronger suppression for larger δ; the optimal band position is c=0.2. Fourier mode analysis reveals a previously underappreciated role of the 3 × 3 roll structure in reversal suppression, whose mean energy correlates positively with the single-roll structure. In standard RB convection, turbulence destabilises the symmetric $2\times 2$ roll configuration, causing frequent reversals owing to competition with asymmetric (1, 1) and (3, 3) modes. The concentration bands enhance the (1, 1) and (3, 3) modal energies, especially at the optimal band position, producing a steady mean flow structure comprising a large-scale circulation and four corner rolls. Despite the local boundary modification, scaling laws for the response parameters (Nusselt number (Nu) and Reynolds number (Re)) remain close to standard RB convection: Nu ~ Ra^1/3 and Re ~ Ra^4/9 Pr^-2/3. These findings demonstrate an effective approach to suppress flow reversal and alter heat transfer efficiency.