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5.2  Normal mode models

The spectral decomposition is a key notion for the resolution of linear differential equations and the characterization of system dynamics. Predictions of the vibrations of structures are typically done for linear elastic structures or, for non-linear cases, refer to an underlying tangent elastic model.

Spectral decomposition applied to elastic structures leads to modal analysis. The main objective is to correctly represent low frequency dynamics by a low order model whose size is typically orders of magnitude smaller than that of the finite element model of an industrial structure.

The use of normal modes defined by the spectral decomposition of the elastic model and corrections (to account for the restricted frequency range of the model) is fundamental in modal analysis.

Associated models are used in the normal mode model format

   
 [[Is2 + [Γ] s + [Ω2]]{p(s)} = [φT b]{u(s)} 
  {y(s)} = [cφ] {p(s)}
    (5.4)

where the modal masses (see details below) are assumed to be unity.

The nor2res, nor2ss, and nor2xf functions are mostly based on this model form (see nor2ss theory section). They thus support a low level entry format with four arguments


ommodal stiffness matrix Ω2. In place of a full modal stiffness matrix om, a vector of modal frequencies freq is generally used (in rad/s if Hz is not specified in the type string). It is then assumed that om=diag(freq.^2). om can be complex for models with structural damping (see the section on damping ).
gamodal damping matrix Γ (viscous). damping ratios damp corresponding to the modal frequencies freq are often used instead of the modal damping matrix ga (damp cannot be used with a full om matrix). If damp is a vector of the same size as freq, it is then assumed that ga=diag(2*freq.*damp). If damp is a scalar, it is assumed that ga=2*damp*diag(freq). The application of these models is discussed in the section on damping ).
pbmodal input matrixj}T[b] (input shape matrix associated to the use of modal coordinates).
cpmodal output matrix [c]{φj} (output shape matrix associated to the use of modal coordinates).

Higher level calls, use a data structure with the following fields


.freqfrequencies (units given by .fsc field, 2*pi for Hz). This field may be empty if a non diagonal nor.om is defined.
.omalternate definition for a non diagonal reduced stiffness. Nominally om contains diag(freq.^2).
.dampmodal damping ratio. Can be a scalar or a vector giving the damping ratio for each frequency in nor.freq.
.gaalternate definition for a non diagonal reduced viscous damping.
.pbinput shape matrix associated with the generalized coordinates in which nor.om and nor.ga are defined.
.cpoutput shape matrix associated with the generalized coordinates in which nor.om and nor.ga are defined.
.dof_inA six column matrix where each row describes a load by [SensID NodeID nx ny nz Type] giving a sensor identifier (integer or real), a node identifier (positive integer), the projection of the measurement direction on the global axes (if relevant), a Type.
.lab_inA cell array of string labels associated with each input.
.dof_outA six column matrix describing outputs following the .dof_in format.
.lab_outA cell array of string labels associated with each output.

General load and sensor definitions are then supported using cases (see section 4.5.3).

Transformations to other model formats are provided using nor2ss (state-space model), nor2xf (FRFs associated to the model in the xf format), and nor2res (complex residue model in the res format). The use of these functions is demonstrated in demo_fe.

Transformations from other model formats are provided by fe2ss, fe_eig, fe_norm, … (from full order finite element model), id_nor and res2nor (from experimentally identified pole/residue model).


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