  Hub-filament systems (HFSs) are potential sites of protocluster and massive
star formation, and play a key role in mass accumulation. We report JCMT POL-2
850 $\mu$m polarization observations toward the massive HFS SDC13. We detect an
organized magnetic field near the hub center with a cloud-scale "U-shape"
morphology following the western edge of the hub. Together with larger-scale
APEX 13CO and PLANCK polarization data, we find that SDC13 is located at the
convergent point of three giant molecular clouds (GMCs) along a large-scale,
partially spiral-like magnetic field. The smaller "U-shape" magnetic field is
perpendicular to the large-scale magnetic field and the converging GMCs. We
explain this as the result of a cloud-cloud collision. Within SDC13, we find
that local gravity and velocity gradients point toward filament ridges and hub
center. This suggests that gas can locally be pulled onto filaments and overall
converges to the hub center. A virial analysis of the central hub shows that
gravity dominates magnetic and kinematic energy. Combining large- and
small-scale analyses, we propose that SDC13 is initially formed from a
collision of clouds moving along the large-scale magnetic field. In the
post-shock regions, after the initial turbulent energy has dissipated, gravity
takes over and starts to drive the gas accretion along the filaments toward the
hub center.
