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What is NVH?
NVH stands for Noise, Vibration, and Harshness. It is a term mainly used when evaluating the noise and ride comfort of a vehicle. However, all vibration and noise phenomena that can occur in other mechanical systems are also generally referred to as NVH.
Noise refers to unwanted, unpleasant sounds that interfere with hearing and are perceived as disturbing by people. It occurs when vibrations are transmitted through a medium, such as air.
Vibration is a phenomenon in which an object undergoes repetitive motion relative to a reference position. For instance, when an object is held and released, the shaking it exhibits is classified as vibration.
Harshness is the subjective perception of noise and vibration resulting from irregular impacts.
Multibody Dynamics functions for NVH Simulation
There are 4 basic functions of Multi-Body Dynamics for NVH simulation.
- Modal analysis: Obtaining the mode shapes of the flexible body through simulation (mathematical analysis)
- System eigenvalue analysis: Obtaining the system’s natural frequencies
- FRA : Analysis of the response of the system under forced vibration conditions
- Dynamic analysis: Analysis of the resonance and transient vibration
RecurDyn, a Multibody Dynamics simulation software package, supports all 4 of these analysis methods.
Example of NVH Simulation Using Multi-Body Dynamics
With a 1-DOE shaker example,
let's explore how multibody dynamics simulation can be employed to analyze NVH problems.
Modal Analysis: the natural frequencies and mode shapes of the flexible plate can be obtained through simulation (mathematical analysis). This information provides valuable insights about the noise and vibration behavior of the mechanical system.
System eigenvalue analysis: vaious results can be obtained, including the number of modes and the natural frequencies of the system. While modal analysis is typically used for individual components, system eigenvalue analysis can be applied to mechanical systems or assemblies in motion. In the below example, the first mode of the system occurs at 47Hz, and the second mode occurs at 185Hz, and so on.
The natural frequencies of the system differ from those of individual components. This highlights the importance of NVH simulation for the mechanical system.
Dynamic Analysis: transient results can be examined. The video below shows the dynamic analysis of the system being excited with changing frequency over time. The graph in the video shows the excitation frequency at any moment. As the excitation frequency approaches 47 Hz, which is the first mode frequency of the system, the flexible plate shakes significantly. After that, as the frequency approaches 185 Hz, the second resonance occurs.
We can observe that the deformation and stress distribution change with increasing excitation frequency throughmultibody dynamics simulation.
The 6 animations at the bottom of the video show the deformed shapes that occur close to 6 of the resonant frequencies.
By utilizing multi-body dynamics simulation, you can perform an NVH analysis to confirm how much stress is generated and how the deformation shape changes over time.
How FRA is used for NVH Simulation?
FRA (Frequency Response Analysis) is an NVH simulation method that calculates the system responses due to excitation input while considering the system constraints. Using FRA, it becomes possible to directly identify the responses to specific forced vibrations.
For instance, when rotational excitation relative to the Z-axis is applied, the system exhibits a significant acceleration response at the first mode frequency of 3.9 Hz. The second and third modes do not contain much Z-axis rotation, causing the response to this excitation to be small.
Conversely, in the case of translational input, the second mode exhibits a substantial response at 52 Hz in the translational direction.
FRA can be very useful because it can help you develop a better understanding of the excitation conditions for a specific system. Also, FRA is especially useful when the excitation conditions of a system are already known.
