For an HBM cyclic analysis, the dynamics equations of a cyclically symmetric structure are transformed to cyclic coordinates (see The General Cyclic Symmetry Analysis Equations) leading to decoupled system matrices.
The structure is subjected to a traveling wave excitation whose fundamental harmonic has engine order (EO), , which is referred to as the fundamental engine order, with higher time harmonics, , having the engine order . This excitation may be represented as [9]:
(1–19) |
where the force applied on the sector is given by [9]:
(1–20) |
where:
is time |
is the time harmonic of the excitation |
is the number of time harmonics needed to describe the excitation |
is the excitation amplitude for the time harmonic, , applied on the first sector of the physical structure |
is the sector index |
is the engine order (EO) of the excitation. It may be positive for a backward traveling wave or negative for a forward traveling wave. |
is the time frequency of the excitation, which is related to the rotational velocity of the excitation, , by: |
(1–21) |
Substituting Equation 1–20 into Equation 1–19 we obtain:
(1–22) |
where:
is the applied excitation amplitude in cyclic coordinates |
(1–23) |
where:
is the primitive root of unity. |
Note that is equal to some scalar multiple of a column of the complex Fourier matrix. Thus, is the unique spatial harmonic associated with the time harmonic, , of the traveling wave excitation, whose fundamental harmonic has engine order .
In HBM analysis, the displacement solution and the internal forces are assumed to be harmonic with:
(1–24) |
(1–25) |
Substituting into Equation 1–5 and equating coefficients of , the equation of motion in the frequency domain in physical coordinates for time harmonic, , is obtained:
(1–26) |
Because the time harmonic, , is associated with only the spatial harmonic, , it is isolated by pre-multiplying the equation above by :
(1–27) |
Due to the block-circulant structure of the matrices of a cyclic symmetric structure [10] and the properties of columns of a Fourier matrix, the nonzero rows of the above equation reduce to:
(1–28) |
which is the equation of motion in cyclic coordinates.
is the complex dynamic stiffness matrix in cyclic coordinates associated with time harmonic, , which may be obtained from partitions of real cyclic system matrices [10] and the equivalent direction of the traveling wave component: |
(1–29) |
(1–30) |
where:
Subscripts and represent the DOFs of the base and duplicate sectors |
are system matrices obtained by applying cyclic constraints to base and duplicate sectors with the aliased spatial harmonic index : |
(1–31) |
Note that the direction of the traveling wave component for harmonic, , in the equation above (for and ) is reversed from the direction of the fundamental EO if .
may be further reduced by using reduction techniques such as component mode synthesis.