Refining the NVH of today’s automobiles can be as tricky as ever. NVH engineers determine how much noise is radiating from engines, drivetrains, and roads, and they track how that noise eventually gets to passengers and drivers. NVH travels through the vehicle structure, the air, or a combination of both. Small design changes can sometimes have a profound effect. One avenue to refining NVH cost-effectively is to use CAE simulation. “The objective of a lot of companies today is to move simulation earlier in the design cycle. You can run dozens of virtual tests with different configurations much more inexpensively than physical tests,” explains Lance Proctor, Senior Product Manager for MSC Software.
To further that goal, MSC Software added new NVH simulation tools to its existing suite of finite element tools in MD Nastran in its 2010 release.
The Equivalent Radiated Power (ERP) feature is a simple model to characterize structure borne noise, added at the request of automotive customers who were using their own versions. “What ERP does is gives you a good look at how panels impart sound and interact with one another with a model that is not as complex as a higher fidelity FEM,” explains Proctor. It does so by summing the square of the normal surface velocities of each model-defined panel. “The value of ERP is that one does not need to model the acoustic air with a finite element model or the transfer function.”
Energy Finite Element Analysis (EFEA) is a FEM for airborne noise for frequencies greater than 1 kHz. This is used when high frequency is required. Before EFEA, noise engineers used Statistical Energy Analysis (SEA) to simulate frequencies greater than 1 kHz. SEA is not a finite element approach and requires a different kind of model to be developed. How does EFEA work? “A traditional FEA technique uses displacement as the primary variable,” explains Proctor. “This requires enough elements to capture [the frequency] of a wavelength using displacements.” Higher frequencies mean smaller wavelengths and so smaller elements. Above 200 Hz, FEM models generally become too large to solve even with today’s computers. To illustrate, one modeling rule of thumb is to use eight to 10 elements per wavelength. “EFEA uses energy as the primary variable—not displacement—and allows one to use much coarser meshes. Using EFEA instead of a statistical method means one can re-use the FEM meshes instead of creating a statistical model.” MSC licensed the EFEA technology from Michigan Engineering Services. The company says it is good for analysis up to 10 kHz. Proctor points out that EFEA is good for analyzing interior airborne noise, while an equivalent Energy Boundary Element Analysis (EBEA) method is used for airborne noise effects on the exterior of the vehicle – say the transfer of tire noise from the road to the windshield.
Test-based Frequency Response Functions (FRFs) are an addition to more conveniently combine test data with simulations. Simulations combine individual component FRFs—say engine, driveline, transmission, and mounts—together into a complete system model. Now, with MD Nastran 2010, the total solution can be a mix of modeled and measured FRFs. “The nice thing about this is that there is a lot of test data available for road loads, engine idles, and other measured quantities,” explains Proctor. This allows re-use of that existing data. There are also structures that are difficult and expensive to characterize analytically—test data still has its place.
Other features include an improved Frequency Dependent Rigid absorber model, now accurate across a wide range of frequencies. This is used to model the energy absorption effect of carpets, trim pieces, or mastic more accurately. Proctor also points out there are improvements to the weld element called CWELD, an often-critical feature in correctly modeling structures. “We are constantly enhancing and improving our NVH capabilities,” recaps Proctor. “While allowing us to better characterize materials like elastomers, or virtual configurations, our goal is to not replace testing but enhance it. Ultimately, testing will be used to confirm designs rather than use test to develop designs.”