Top row: snapshots showing contour plots of the modulus of the velocity |v| (in units of 10⁶  cm   s^-1; left panel) and the hydrogen mass fraction (right panel) for the 3D model hifexp.3d.f at t ~ 13   300   s in the meridional plane (φ = 0°). Bottom row: same quantities but for the 2D model hifexp.2d.f. The dotted line separates helium- and hydrogen-rich layers (H-shell), and the dashed lines mark the boundaries of the computational domain.