[有限元模态分析-ANSYS.pptVIP

Modal Analysis Module 2 Module 2Modal Analysis A. Define modal analysis and its purpose. B. Discuss associated concepts, terminology, and mode extraction methods. C. Learn how to do a modal analysis in ANSYS. D. Work on one or two modal analysis exercises. Modal AnalysisA. Definition Purpose What is modal analysis? A technique used to determine a structure’s vibration characteristics: Natural frequencies Mode shapes Mode participation factors (how much a given mode participates in a given direction) Most fundamental of all the dynamic analysis types. Modal Analysis… Definition Purpose Benefits of modal analysis Allows the design to avoid resonant vibrations or to vibrate at a specified frequency (speakers, for example). Gives engineers an idea of how the design will respond to different types of dynamic loads. Helps in calculating solution controls (time steps, etc.) for other dynamic analyses. Recommendation: Because a structure’s vibration characteristics determine how it responds to any type of dynamic load, always perform a modal analysis first before trying any other dynamic analysis. Modal AnalysisB. Terminology Concepts General equation of motion: Assume free vibrations and ignore damping: Assume harmonic motion ( i.e. ) The roots of this equation are ?i2, the eigenvalues, where i ranges from 1 to number of DOF. Corresponding vectors are {u}i, the eigenvectors. Modal Analysis… Terminology Concepts The square roots of the eigenvalues are wi , the structure’s natural circular frequencies (radians/sec). Natural frequencies fi are then calculated as fi = wi /2p (cycles/sec). It is the natural frequencies fi that are input by the user and output by ANSYS. The eigenvectors {u}i represent the mode shapes - the shape assumed by the structure when vibrating at frequency fi. Modal Analysis… Terminology Concepts (cont.) Mode Extraction is the term used to describe the calculation of eigenvalues and eigenvectors. Mode