Harmonic (FRF) Analysis Description

The harmonic analysis in the frequency domain consists in performing sequentially the harmonic analysis for successive frequency values in a selected range. In the software, an FRF (Frequency Response Functions) analysis case is a composed case including subcases. Each of subcases has a solution to the harmonic analysis with a specified frequency.

To define a case of the harmonic analysis in the frequency domain you need to change an analysis type from a static case. First you have to define a static case with loads or case combinations, then change the type of case to the harmonic analysis in the frequency domain. NOTE: It is possible only when the load case is preceded by the modal analysis case (i.e. the modal analysis case was defined earlier). You can also define a case of the harmonic analysis in the frequency domain by defining it as a new case and next, by defining loads as for a static case.

To perform the harmonic analysis in the frequency domain, you need results for the modal analysis case (structure eigenvibrations) that you have defined previously. The software solves the FRF analysis using the modal decomposition method. You need to choose a sufficient number of eigenvibration modes in the modal analysis preceding the FRF case, to obtain reliable results for the FRF harmonic analysis.

The harmonic analysis in the frequency domain is needed to study vibration receptance of the structure. The purpose of this analysis is to obtain a Frequency Response Function (FRF) for a selected node of the model. The Frequency Response Function expresses the response of the structure to given harmonic vibrations in the frequency domain. A plot of the function indicates for which frequency the influence of vibrations on a structure is maximal. You can continue such analysis as the time history analysis for a selected, critical frequency.

Frequency Response Functions are most often expressed in values of displacement, velocity or acceleration. In addition, they can be transformed to express these values in relationship to force.

The results of the harmonic analysis in the frequency domain are typical displacement and force values.

From 2019 release on:

• The results of the standard harmonic analysis with non-zero damping correspond to the composed load case with 360 components corresponding to 360 phase angle steps during one period of harmonic excitation. See Harmonic Analysis Assumptions.
• The extreme values of different result components can occur at a different moment in time. In case of the FRF analysis with non-zero damping, only the extreme values of all results for the given harmonic frequency are available. The values can differ depending on a moment in time, that is a different phase angle. To see the results for a given moment in time along one period of harmonic excitation, you need to define a standard harmonic analysis with such period or frequency.