Dynamic analysis

Harmonic frequency response is a method of dynamic analysis.

Use harmonic frequency response analysis to evaluate the effects of vibration at specific harmonic frequencies as external energy is introduced to a system that was at equilibrium. The objective is to determine the optimal steady-state response to a dynamic series of oscillations at those frequencies. Oscillatory loading is sinusoidal in nature. In its simplest case, this loading is defined as having amplitude at a specific frequency. The steady-state oscillatory response occurs at the same frequency as the loading.

By looking at the dynamic response level at each frequency, such as vibration displacement and chatter, you can identify important frequency ranges that must be mediated to prevent structural or mechanical failure. Mediation can include adjusting your design, redistributing weight, changing material properties, and adding damping mechanisms.

Harmonic frequency response is an extension of normal modes analysis, in that you can use the results of a normal modes study (the frequencies at which an object resonates naturally, before external forces are applied) as input to a harmonic response study.

• To use this form of dynamic analysis, choose the Harmonic Response study type on the Create Study dialog box. For more information, see Harmonic response studies.

• For an example, see Define a harmonic frequency response study.

Harmonic frequency response analysis is an alternate numerical approach to computing the frequency response of a structure. This method uses the mode shapes of the structure to reduce the size and uncouple the equations of motion. The mode shapes are usually calculated in an earlier normal modes analysis to determine the behavior of the structure. For this reason, harmonic frequency response is a natural extension of a normal modes analysis, making it a very efficient solving method.

The results of harmonic frequency response analysis are in the displacement component plots, constraint force plots, and strain component plots that are produced. You also can Graph nodal analysis results.

An important aspect of a frequency response analysis or a transient heat analysis is the definition of a dynamic loading function. In a frequency response analysis, the force can be defined as a function of frequency. In a transient heat analysis, the thermal load can be defined as a function of time.

There are two important aspects of dynamic load definition. The location of the loading on the structure must be defined in physical space; this is called the spatial distribution of the loading. Next, the dynamic load references a frequency function or a time function, which is created using the Create Function command. The frequency-varying or time-varying portion of the load is known as the temporal distribution of the load. A complete dynamic loading is a product of spatial (for instance, actual forces on nodes and pressures on elements) and temporal distributions of the function.

Functions allow you to create general X vs. Y tables of information. They can be used for time- or frequency-dependent loads, or to attach nonlinear information to material properties.

For more information, see Using functions.