Authors: Jiayi Liu (University of Hawaii), Xudong Sun (University of Hawaii), Peter Schuck (NASA Goddard Space Flight Center), Sarah Jaeggli (NSO), Brian Welsch (University of Wisconsin-Green Bay)

Delta solar active regions (ARs) sometimes host fast photospheric flows along the central magnetic polarity inversion line (PIL). Here we study the unusual Doppler signature in the delta-penumbra of AR 12673. SDO/HMI observations indicate highly sheared, strong magnetic fields up to 4 kG near the PIL. Large Doppler shifts up to 4 km/s appeared during the formation of the delta-penumbrae and persisted for about 10 hours. Applying the new DAVE4VMwDV velocity estimator to HMI data, we find fast shearing motion along the PIL. The velocity vectors are largely (anti)parallel to the inclined magnetic fields, suggesting that the observed Doppler shift may be explained as the projection of field-aligned flows. A  inversion of the Hinode/SP observations further hints at gradients of magnetic field along the line of sight. We discuss the implications on the delta-spot magnetic structure and the flow driving mechanism.