Why does the YF-75D nozzle material have such a strong spiral pattern?
Is it a result/biproduct of a manufacturing process only, or does it have some specific function?
edit: The last, cropped images that have enhanced contrast and sharpening show a bead-like appearance around the bottom edge of the nozzle. This gives the impression that the nozzles is fabricated from fused rods or "ropes" of metal. This is consistent with them being nearly vertical at the top, and increasingly spiraled as the bell shape widens.
Could these have been made by some additive process?
above: YF-75D engine cropped from here.
above: YF-75D engine cropped from Chinaspaceflight. (In Chinese)
above: YF-75D engine from Chinaspaceflight. (In Chinese)
above: YF-75D engine nozzle detail, cropped from Chinaspaceflight. (In Chinese)