Lightweight construction and NVH

LeichtFahr research project

Together with partners from industry and science, Novicos is researching new calculation methods for predicting acoustic properties in lightweight construction.

Acoustic properties of vehicles play a central role in the customer's purchasing decision. Typically, noise and vibration phenomena inside the vehicle tend to be perceived as annoying, but some, such as engine noise, are desired as acoustic feedback or are even perceived as a quality feature. In this context, it is becoming increasingly important to evaluate and influence acoustic behavior at an early stage of vehicle development.

At the same time, there is a clear trend in the automotive industry towards high-strength steel and lightweight constructions. The aim of lightweight construction concepts is to reduce the energy consumption of vehicles and thus ensure compliance with EU emissions regulations. However, as the resulting vibrations are less reduced by the lighter structure, the use of lighter materials also has a significant impact on the vibroacoustic behavior of the vehicle. A key question is therefore how lightweight construction can be reconciled with NVH.

Predicting vibroacoustic behavior - FEM, BEM, SEA

Determining this usually requires very complex acoustic models. Numerical methods such as the Finite Element Method (FEM) or the Boundary Element Method (BEM) are widely used to predict vibroacoustic behavior at low to medium frequencies or in the time domain. At higher frequencies and for large and complex technical systems, the wavelength is short compared to the overall system under consideration. The density of the eigenmodes also increases considerably. In order to be used effectively, the FEM requires a large number of finite elements in this case, which also drives up calculation costs.

Lower costs can be achieved through the use of BEM. Here, 3D structures are reduced to 2D surface models and thus simplified. A major drawback, however, is the smaller scope of possible numerical solutions due to the great simplification. In addition, both FEM and BEM can react extremely sensitively to parameter deviations. For this reason, statistical methods such as Statistical Energy Analysis (SEA) are predominantly used for simulations in the high-frequency range.

For SEA, a system is divided into several coupled subsystems and the acoustic behavior of each individual subsystem is described by a defined number of equations. Although the number of equations to be solved in SEA is comparatively small, the corresponding models cannot be derived directly from the CAD data and the modeling requires a high level of application-specific expertise. In addition, SEA does not provide any information on the spatial energy distribution and therefore effects such as damping or structural excitation cannot be described locally.

Calculation based on energy density - EFEM

The energy flow analysis (EFA) was developed as an alternative approach. This describes the energy distribution in terms of the area-average energy density. The central energy balance of the EFA was subsequently transformed into a partial differential equation in which a similarity between the propagation of acoustic energy and heat conduction is utilized. On this basis, the energy density can now be calculated using existing FEM methods - the EFA becomes the energy-based finite element method (EFEM).

While conventional FEM is based on displacements, EFEM is based on time- and location-averaged energy densities. This means that calculations can also be carried out in the higher frequency range with a relatively high level of accuracy. Due to the low discretization effort, it is possible to simulate even large and complex structures such as complete vehicles or ships and still take local effects into account.

In contrast to SEA, EFEM does not require the damping between subsystems or the coupling strength to be limited when defining the subsystems. This makes it possible to carry out detailed analyses, for example for the precise definition of external loads, the consideration of any distributed damping or the analysis of frequency-dependent and spatially distributed results.

The underlying energy equations are set up on an element basis, analogous to FEM. As these elements or subsystems are significantly smaller than in SEA, they allow finer modeling and a more detailed prediction of the energy flows and distributions within the structure under investigation. Due to the energy-based approach, however, a much coarser discretization is possible compared to conventional FEM/BEM, so that larger structures can also be investigated in the high-frequency range. Various application examples with promising results show the great potential of EFEM for the analysis of large structures.

The EFEM reaches its limits when calculating very small components. In principle, the shortest distance of the component under consideration should correspond to at least 2.47 times the wavelength in order to achieve a reliable result. If smaller components are treated as part of a larger structure, modeling that deviates from the EFEM standard must be used.

Information on the LeichtFahr project can be found on the LeichtFahr website.