head, [[i.e. to maintain a moving (convective) local inflectional instability similar to that naturally sustained at the head of turbulent bands,]{}]{} the forcing area is moved at the same speeds, i.e., $x_c=x_0 + c_x t$ and $z_c=z_0 + c_z t$, where $x_0$ and $z_0$ are the initial coordinates of the center of the forcing area. $R$ should be chosen such that the diameter of the forcing area is comparable with the streamwise width of turbulent bands at the head. The localisation parameter $B$ is chosen such that the forcing area is sufficiently localised and at the same time the force does not create a too steep speed variation at the boundary of the forcing area. In the results shown in this paper, we set $B=0.5$ (see Figure \[fig:localisation\_factor\]) unless explicitly stated.

![\[fig:forcing\_amplitudes\] (a) The effect of the forcing amplitude on the generation of bands at $Re=750$. The kinetic energy of the disturbances with respect to the parabola, integrated in the whole channel, is plotted. $A_x$ and $A_z$ denote the amplitude in the streamwise and spanwise directions, respectively. (b) The forces are switched off at $t=550$ and the following development of the flow is plotted as a thin red line. The 