setting a curved surface as a wall

Dear all

my question is about useing a curved surface as wall model in Lammps by using “fix wall/region command”. For example can any one tell me, how can i model fluid flow through a single tube while the wall is the side area (or lateral area) of the cylinder( = tube)? ( fluid enters from one end of the tube and exists from other one)

I my self find the way with steve guidance.

to construct such wall one have to create two cylinder one thicker from the other. then subtract the thinner one from the other. then use out side of the resulting region as wall region in “fix wall/region command”. it works successfully.