Structural -Acoustic Optimization using CMS Super Elements

It is very important for automobile manufacturers to design automobiles with low levels of noise in the passenger compartment. Noise is generated from the tires, engine, transmission, and exhaust system and transmitted through the body structure to the air in the passenger compartment. Coupled structure -acoustic frequency response analysis is used to dete rmine the sound levels at the driver’s and passenger’s ears at different driving speeds and engine speeds. The passenger compartment noise can be reduced by correct sizing of structural components such as suspension bushings and dampers, motor mounts, exh aust system hangers, etc., as well as by applying sound absorbing materials to the metal structures of the automobile. In addition, the structural design itself can be modified to change vibration load paths to reduce the noise levels. These design decis ions are usually made by trial and error. In this work we look at applying optimization design concepts to the reduction of the noise level in the passenger cabin. Topology Optimization as well as sizing and material optimization can be used to reduce th e sound levels to specified levels at different frequencies. A single coupled structure -fluid frequency response analysis of a typical automobile finite element model containing over 10 million DOF can take many hours on a modern computer. The significan t new aspect of this work is the use of a sub structuring technique to reduce the optimization run time. With these techniques, the non design portion of the structure does not have to be reanalyzed during the design iterations. Substructuring is perform ed to create a combined structure -fluid (air) Super Element using the Component Mode Synthesis (CMS) method. The use of these Super Elements can reduce the run times from hours to minutes.