# Better assessing the frequency response of a structure

*by N. Brie, EDF R&D / AMA*

A new operator, `DEFI_LIST_FREQ`, has been developped to compute more accurately the frequency response of a structure computed with the help of the `DYNA_VIBRA` operator (`TYPE_CALCUL=’HARM’`). This operator allows to define a discretized list of frequencies with an automatic refinement around the eigenfrequencies of the structure, obtained beforehand with a modal analysis.

The user simply has to provide the list of eigenfrequencies and some parameters regarding the refinement (number of points to add around each eigenfrequency, width of the refined intervals, etc). Interestingly, if the user provides the modal dampings of the eigenmodes : the list will be automatically refined around the magnitude resonance and phase resonance. The local *maxima* of the Frequency Response Function will therefore be well assessed.

To illustrate this improvement, we compute the frequency response of a concrete slab excited by an operating pump (figure 1). Figure 2 shows that without the refinement, the response of the slab would have been considerably underestimated (about 30%) if the pump rotational speed was close to one of the first two eigenmodes whose modal dampings are low.

Figure 1 : mesh of the concrete slab with pump location (all sides are clamped) |

Figure 2 : FRF at the pump location along the vertical axis |

This new operator `DEFI_LIST_FREQ` has been developed for harmonic analysis, but it is generic and can then be useful for any case where a refinement around pre-determined points is needed. We shall note that the inspiration behind this development came from the Open-Source community as this thread in the forum illustrates.

*Available in version 12.0.15 and on.*