Evaluation of structure-borne NVH performance of a passenger car using virtual simulation process in the low frequency range

In today's automotive industry, making vehicles lighter every year has become one of the primary goals. Along with the weight reduction, there may be adverse effects on vehicle performance. While there are negative points such as lowering global stiffness and weakening of local connection stiffness, significant gains can be obtained in terms of fuel performance and the vehicle can be positioned at a competitive position in its class. One of the problems that can occur with the lightening of the vehicle is vibration and noise problems. Especially in the low-frequency structure-borne vibration region where the modal density is low and global stiffness is effective on vibration and noise, a significant problem may occur due to the decrease of body stiffness. Significant costs can be spent on isolators and damping materials, characterization and laboratory testing of these materials to regain the vehicle's vibration and noise performance, and these materials can reduce vehicle weight reduction. On the other hand, a vehicle with superior Noise, Vibration, and Harshness (NVH) performance has a significant impact on customer perception. In addition, various factors that determine the NVH properties also determine the deformation resistance and fatigue behavior of the vehicle. In this respect, numerical methods such as Finite Element Method (FEM), Boundary Element Method (BEM), Statistical Energy Analysis (SEA) are the most commonly used methods to improve the vibration and noise characteristics of vehicles during the development phase. Among these, the FEM method is the most well-known method to solve low frequency NVH problems. In this thesis, during the development of a vehicle body design, the necessary steps to be followed in order to evaluate the structure-borne NVH in the low frequency region were examined. First, static and dynamic behaviors of the vehicle model were examined by body stiffness and modal analyses. Structural modes, acoustic modes and strain energy outputs were discussed. Body sensitivities were calculated with the help of metrics (Point Mobility, Vibration Transfer Function (VTF), Noise Transfer Function (NTF), Dynamic Stiffness) which determine vibration and noise sensitivity. Then, structure modes and panels with the highest contribution to the calculated NTF were determined by using the Modal Participation Analysis and Panel Participation Analysis within the scope of NVH diagnostic studies. In addition to these analyses, the compliance of the obtained results with the generic targets determined with the help of the literature was evaluated. Finally, design changes were applied to reduce the NTF amplitude that occurs at the passenger ear level. As a result of repeated analysis, the effect of design changes on the results was examined.