The continuing demand for light weight car body structures and the increasing awareness of acoustic comfort is a conflict of objectives. Excitations introduced by the driveline or the drivetrain can not be avoided. The body structure will radiate sound to the passenger compartment. Consequently, reducing the radiated sound power in car body structures develops towards a main topic in the car body development process.
The common approach to stiffen the body structure parts by introducing beads using an empirical approach is not sufficient for reducing the sound pressure level. Stiffening, that decreases the radiated sound power at certain frequencies, often leads to increased radiation at other frequencies. To improve the overall acoustic behaviour the complete frequency response of the structure has to be taken into account. Amplitudes of radiated sound power have to be decreased over the complete frequency range of interest.
State-of-the-Art optimisation methods can help to find solutions. Presently in this context optimisation is normally used with shell thickness parameters to increase eigenvalues or to decrease frequency dependent accelerations of dedicated points in the structure. However, the gradient methods used are inadequate for the optimisation of body structures concerning equivalent radiation power.
The reasons for the poor performance of gradient methods are highly nonlinear objective functions when minimising the equivalent radiation power. To find a topography of the body structure which radiates significantly less sound a global optimisation has to be run.
The introduced method changes the topology of the structure by morphing the FE-Mesh. A genetic algorithm with discrete variable representation is applied for global optimisation. In order to perform successful optimizations with genetic algorithms a large number of function evaluations is required. To reduce the overall computation time the use of substructuring and parallelisation is necessary. The acoustic behaviour of the complete car body is taken into account.
The example shows a successful optimization of the topography of a car body part. The equivalent radiation power can be decreased significantly over the complete frequency range of interest.