  Delta ($\delta$) sunspots sometimes host fast photospheric flows along the
central magnetic polarity inversion line (PIL). Here we study the strong
Doppler shift signature in the central penumbral light bridge of solar active
region NOAA 12673. Observations from the Helioseismic and Magnetic Imager (HMI)
indicate highly sheared, strong magnetic fields. Large Doppler shifts up to 3.2
km s$^{-1}$ appeared during the formation of the light bridge and persisted for
about 16 hours. A new velocity estimator, called DAVE4VMwDV, reveals fast
converging and shearing motion along the PIL from HMI vector magnetograms, and
recovers the observed Doppler signal much better than an old version of the
algorithm. The inferred velocity vectors are largely (anti-)parallel to the
inclined magnetic fields, suggesting that the observed Doppler shift contains
significant contribution from the projected, field-aligned flows.
High-resolution observations from the Hinode/Spectro-Polarimeter (SP) further
exhibit a clear correlation between the Doppler velocity and the cosine of the
magnetic inclination, which is in agreement with HMI results and consistent
with a field-aligned flow of about 9.6 km s$^{-1}$. The complex Stokes profiles
suggest significant gradients of physical variables along the line of sight. We
discuss the implications on the $\delta$-spot magnetic structure and the
flow-driving mechanism.
