Distribution of the mean radial velocity, ⟨V_R⟩ in the (x − y)-plane, calculated at the end of the simulation run (t = 9 Gyr), for all thin+thick models considered here. The dashed black lines denote the contours of constant surface density. The cyan dashed circle denotes the bar length, R_bar, and the magenta dashed circle denotes the extent of the quadrupole feature, R_quadrupole. The magenta points denote the spatial distribution of the phase-angle of the m = 4 Fourier moment (φ₄). Top row: rthickS models. Middle and bottom rows: rthickE and rthickG models, respectively. The thick-disc mass fraction (f_thick) varies from 0.1 to 0.7 (from left to right panels). A prominent quadrupole feature is present in all thin+thick models considered here.