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.