Normal to is mathematician's speak for at right angles to. A generatrix (also called a generator according to the Penguin Dictionary of Maths) is any of a set of straight lines that make up a surface. If your condotta was a smooth cylinder, you could create its surface by moving a straight line around the axis of the pipe. Your condotta is not smooth but corrugated (ondulato). For structural strength a pipe can be given corrugations that would each be a circumferential ring, like the neck of a person with many double chins.
The Italian says these waves are normal to the generatrix - which incidentally I am puzzled by, because it seems to me ambiguous: to say the waves are normal to something does not (I think) tell you which way you are looking at the waves. However, even if the Italian is ambiguous, the same ambiguity is recreated in the English.