Lightweight construction and NVH

Research project LeichtFahr

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 purchase 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 structures. Lightweight design concepts pursue the goal of reducing the energy consumption of vehicles and thus ensuring that the exhaust emission regulations stipulated by the EU are met. However, since 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 central question is therefore how lightweight construction can be reconciled with NVH.

Predicting Vibroacoustic Behavior - FEM, BEM, SEA

To determine 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 engineering systems, the wavelength is short compared to the overall system under consideration. The density of eigenmodes also increases considerably. To be used effectively, the FEM in this case requires a high number of finite elements, which also drives up computational costs.

Lower costs can be achieved by using BEM. Here, 3D structures are reduced to 2D surface models and thus simplified. However, a major drawback is the smaller scope of possible numerical solutions due to the strong simplification. In addition, both FEM and BEM can be extremely sensitive to parameter deviations. Therefore, mainly statistical methods like Statistical Energy Analysis (SEA) are used for simulations in the high frequency range.

For SEA, a system is divided into several coupled subsystems and the acoustic behavior of each 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 high and application-specific expertise. In addition, SEA does not provide information on the spatial energy distribution and thus effects such as damping or structural excitation cannot be described locally.

Calculation based on energy density - EFEM

As an alternative approach, the energy flow analysis (EFA) was developed. This describes the energy distribution with respect to the area-average energy density. The central energy balance of the EFA was transformed in the further course into a partial differential equation, in which a similarity of the propagation of acoustic energy to heat conduction is exploited. 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. Thus, calculations can be performed with a relatively high accuracy even in the higher frequency range. Due to the low discretization effort, it is possible to simulate even large and complex structures like complete vehicles or ships and still consider local effects.

In contrast to SEA, it is not necessary to limit the damping between the subsystems and the coupling strength when defining the EFEM. This allows the execution of detailed analyses, for example for the point-exact definition of the external loads, the consideration of an arbitrarily distributed damping or the analysis of the frequency-dependent and also spatially distributed results.

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

EFEM reaches its limits when calculating very small components. In principle, the shortest distance of the considered component 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, a modeling deviating from the EFEM standard must be resorted to.

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