Illustration of the coordinate system used in our model, as described in Section 2.1. The upper panels show face-on and observer’s views of the disk surfaces, along with both the disk-plane coordinates (l, m, n) and the sky-plane coordinates (x, y, z). The lower panels present velocity fields for counterclockwise (ϵ = 0) and clockwise (ϵ = 1) rotations. We adopt PA_geo = 7π/4.