where m is a
circumferential mode number (or
azimuthal mode number) that can only have integer values to obey the axially symmetric nature of the corresponding 3D problem, that is, there is an azimuthal symmetry
ϕ =
ϕ + 2
π. Thus,
The static prestress solution u0 can be obtained using a standard static analysis in 2D axially symmetric geometry; and the circumferential wave number
km = m/
R can be introduced to describe the circumferential modes.
The displacement vector u1 can have nonzero values in all three components, which are functions of the radial and axial positions. For a given circumferential mode number
m, the displacement vector
u1 can be found using an eigenfrequency analysis in a 2D axisymmetric geometry. Hence,
The solution u =
u0 +
u1 represents eigenmodes in the corresponding 3D structure, which can be computed assuming certain constraints on the axis and possible static prestress and independent of the position along the axis.