  The flow of an electrically conducting fluid driven by a traveling magnetic
field imposed at the endcaps of a cylindrical annulus is numerically studied.
At sufficiently large magnetic Reynolds number, the system undergoes a
transition from synchronism with the traveling field to a stalled flow, similar
to the one observed in electromagnetic pumps. A new type of boundary layer is
identified for such electromagnetically-driven flows, that can be understood as
a combination of Hartmann and Shercliff layers generated by the spatio-temporal
variations of the magnetic field imposed at the boundaries. An energy budget
calculation shows that energy dissipation mostly occurs within these boundary
layers and we observe that the ohmic dissipation Db always overcomes the
viscous dissipation Dv, suggesting the existence of an upper bound for the
efficiency of electromagnetic pumps. Finally, we show that the destabilization
of the flow occurs when both dissipations are nearly equal.
