Asking the right questions on Rayleigh–Bénard turbulence

The paper by Castaing et al. ( J. Fluid Mech. , vol. 204, 1989, pp. 1–30) on turbulent Rayleigh–Bénard convection has been one of the most impactful papers on the subject – not by giving the right and complete answers but by developing versatile concepts and by asking the right questions, namely: (i) What is the overall flow organization? (ii) What is the dependence of the Nusselt number ${\textit {Nu}}$ (the dimensionless heat transport) on the Rayleigh number ${\textit {Ra}}$ (the thermal driving strength)? (iii) What is the ultimate state of turbulence for extremely large ${\textit {Ra}}$ ? Thanks to Castaing et al. having asked the right questions, the field has made tremendous progress over the last 35 years.