Mechanism of boundary-layer turbulence at the onset of ultimate convection
In plain words
The theory of the ultimate regime assumes that the thin layers of fluid next to the plates become turbulent. What triggers this, and whether the turbulent layers have the predicted logarithmic profiles, is not established.
Precise statement
Identify the mechanism that renders the thermal and kinetic boundary layers turbulent in Rayleigh-Benard convection near $\mathrm{Ra}*$: shear-driven subcritical transition under the large-scale wind, plume impacts, or another process, and determine whether the mean temperature and velocity profiles become logarithmic in distance from the plate with Ra-independent slopes, as some experiments and DNS report. Answer: a mechanism with quantitative predictions for onset and profile slopes.
What would settle it
Resolved near-plate velocity and temperature measurements or DNS across Ra* showing the onset sequence and the profile slopes.