  Relaminarization of wall-bounded turbulent flows by means of external static
magnetic fields is a long-known phenomenon in the physics of electrically
conducting fluids at low magnetic Reynolds numbers. Despite the large
literature on the subject, it is not yet completely clear what combination of
the Hartmann ($M$) and the Reynolds number has to be used to predict the
laminar-turbulent transition in channel or pipe flows fed by upstream turbulent
flows free of magnetic perturbations. Relying upon standard phenomenological
approaches related to mixing length and structural concepts, we put forward
that $M/R_\tau$, where $R_\tau$ is the friction Reynolds number, is the
appropriate controlling parameter for relaminarization, a proposal which finds
good support from available experimental data.
