Abstract: A turbulence-generating cementing sub adapted to be coupled in screw-threaded end-to-end engagement between two sections of a string of well pipe, casing or liner and run through a cased oil well and cemented therein at a selected depth. The cementing sub is a short section of pipe having two sets of rigid flow-directing rubs or vanes welded or machined on the outer surface thereof. One set of ribs are angled on the sub to change the direction of a flowing cement slurry causing it to swirl around the sub in one direction. The adacent set of ribs are angled to the opposite side of the sub axis to interrupt at least a portion of the swirling cement slurry and cause it to swirl around the sub in the opposite direction thus generating a zone of turbulence in the flowing cement slurry between two strings of well pipe or casing. The rigid ribs also act as centralizers for the inner pipe or casing.