How wide must Rayleigh–Bénard cells be to prevent finite aspect ratio effects in turbulent flow?

We employ direct numerical simulations to investigate the heat transfer and flow structures in turbulent Rayleigh–Bénard convection in both cylindrical cells and laterally periodic domains, spanning an unprecedentedly wide range of aspect ratios $0.075 \leqslant \varGamma \leqslant 32$ . We focus on Prandtl number ${Pr}=1$ and Rayleigh numbers ${{Ra}}=2\times 10^7$ and ${{Ra}}=10^8$ . In both cases, with increasing aspect ratio, the heat transfer first increases, then reaches a maximum (which is more pronounced for the cylindrical case due to confinement effects), and then slightly goes down again before it finally saturates at the large aspect ratio limit, which is achieved already at $\varGamma \approx 4$ . Already for $\varGamma \gtrsim 0.75$ , the heat transfers in both cylindrical and laterally periodic domains become identical. The large- $\varGamma$ limit for the volume-integrated Reynolds number and the boundary layer thicknesses are also reached at $\varGamma \approx 4$ . However, while the integral flow properties converge at $\varGamma \approx 4$ , the confinement of a cylindrical domain impacts the temperature and velocity variance distributions up to $\varGamma \approx 16$ , as thermal superstructures cannot form close to the sidewall.