Distribution of the mean radial velocity, ⟨V_R⟩ in the (x − y)-plane, computed at different times when the model rthick0.0 harbours a strong bar and prominent spirals. Dashed black lines denote the contours of constant surface density. The magenta points denote the spatial distribution of the phase-angle of the m = 4 Fourier moment (φ₄). Within the bar region, φ₄ remains constant whereas in presence of prominent spirals, φ₄ shows a characteristic modulation and this trend holds for all bar+spiral scenarios shown here.