Schematic representation of the physical models used to compare our numerical results against analytical solutions and against previous calculations (see Sects. 3.3 and 3.4). In the upper panel the velocity field is set along magnetic-field lines and has a constant gradient of Λ₃ = 10⁻¹¹ s⁻¹. In the lower panel the velocity field is perpendicular to magnetic field lines and the velocity gradient in each direction is Λ₁ = Λ₂ = 10⁻⁹ s⁻¹.