pan material down z axis regardless of object orientation

Would like to pan a material down the z-axis of an object… but be able to rotate that object in the scene - and the texture will still pan down.

Same principle as water running down an object. On the top. flat surface it would appear not to move… on a slope it would be moving down, with the speed of movement determined by the slope.

Can’t help thinking this is material basics, but my searches are not throwing up the solution.

Thanks for any tips.