Role of damping in NVH CAE procedures

Abstract:   Automotive manufacturers currently face a challenge with expeditious enhancement of the vibro-acoustic properties of their vehicles. A major reason for this setback is the limited design information available during initial development stages added with limited knowledge of damping within complex structures. It is now well established that CAE studies of vibration energy flow show good correlation between power flowing into trimmed body and the interior noise produced. Aim of the dissertation is to harness this "good" correlation between power input and interior noise, by learning about the changing behaviour of system in different suspension damping scenarios. It investigates how the mechanical power input to body from suspension, interior road noise produced, and their relation is affected by changing the way damping is modelled into suspension. This is being done to make stronger design decisions from NVH point of view during the concept phases of vehicle development. The investigation is for vehicle programs during early development phases, and hence a simplified vehicle CAE model was chosen, that contains a trimmed body with cavity fluid, and wheel suspension to capture all relevant effects of varying damping. Then, a detailed flowchart of suspension and trimmed body connections was prepared to understand how power flows into the trimmed body through suspension. Using results of power flow study, the most relevant paths and their frequency ranges were identified (to reduce the number of parts in study, yet results relevant and easily extrapolatable to a larger system). Lastly, responses are analyzed for various damping cases of suspension and trimmed body.  Results obtained show a reducing trend in mechanical input power and interior noise values with increasing damping in system. Whereas, for good correlation between power and noise, a great inclination towards structural damping localized into bushings is observed compared to other damping cases. Additionally, a strong dependency of noise, active power and reactive power is observed on trimmed body and cavity fluid damping. Active power is reduced when trimmed body damping is decreased to zero, and more so when cavity fluid damping is put to zero. On the other hand, noise and reactive power have an exact opposite correlation compared to active power and noise. These results suggest that although active mechanical input power is the cause of interior noise, their correlation starts to deteriorate with reducing damping within the system, and instead it is the reactive power that starts to correlate better at very low damping values. But, it is physically impossible to have no damping or very low damping, so a modelling of damping within suspension that provides relatively better correlation between (active) input power and noise is when structural damping is localized within connectors.