Simulating Flow Induced Noise Propagation through Solids and Fluids
Shawn Wasserman posted on June 15, 2016 |
Wave6 from CD-adapco helps to simulate vibro-acoustics and aero-vibro-acoustics.
Wave6 applications for automotive design.

Wave6 applications for automotive design.

When automotive engineers simulate how flow affects the acoustics of their designs, it isn’t always as simple as looking at computational fluid dynamics (CFD) results in the time domain.

For instance, when looking at the wave propagation of sounds through a solid, and over a broad frequency range, engineers need to look past CFD into the frequency domain.

This is where CD-adapco’s wave6 technology comes into play. This acoustics computer-aided engineering (CAE) technology, announced at STAR Global Conference 2015 and highlighted at CD-adapco’s North American Vehicle CFD Conference, can simulate the wave propagation of sound though objects across the entire audible frequency range.

Wave6 has many applications in the automotive industry. For starters, it can characterize side-glass wind noise, acoustic flanking of door seals, tailpipe noise and much more.

The Tools You Need to Simulate Various Types of Acoustics

Definitions of aero-acoustics, FSI, vibro-acoustics and aero-vibro-acoustics. (All images taken at CD-adapco’s North American Vehicle CFD Conference).

Definitions of aero-acoustics, FSI, vibro-acoustics and aero-vibro-acoustics. (All images taken at CD-adapco’s North American Vehicle CFD Conference).

To dig deeper into flow induced noise and vibration simulations, it is best to understand acoustic terminology, namely: aero-acoustics, fluid-structure interactions (FSI), vibro-acoustics and aero-vibro-acoustics.

Aero-acoustics describes applications in which fluid flows over a rigid object, which then causes noises at a distance. The region at which the noise is received is called the far field.

An example of aero-acoustics is when an airplane’s landing gear might cause noises for people on the ground.

However, aero-acoustics shouldn’t be confused with the interactions of fluids and structures in the phenomena known as FSI. Here, flow can create large amplitude motions of a structure. An example would be something fluttering in the wind.

“Traditionally, we would use time domain methods to model aero-acoustics and fluid structure interactions,” explained Phil Shorter, vice president of CD-adapco.

“Those methods are very well suited to a lot of problems, but not to the prediction of noise and vibration across a broad frequency range,” he added. “Instead we typically use frequency domain methods for vibro-acoustic applications and aero-vibro-acoustic applications.”

Vibro-acoustics occur when noise and vibration are transmitted by waves propagated through structures, fluids, foams and fibers across multiple transmission paths. Aero-vibro-acoustics, on the other hand, occur when you have flow over something, similar to aero-acoustics, but you are interested in the interior or near-field noise. An example of this is the internal noise in a car due to flow around the outside of the car. In this case, fluctuations in the surface pressure will create wave propagation throughout the car which is propagated to the passenger’s ears.

“The side-glass of a vehicle acts like a spatial and spectral filter and preferentially transmits certain components of the exterior pressures to the vehicle interior” said Shorter. “You might think that a single flush mounted microphone on the exterior of the side glass would fully characterize interior noise but it absolutely doesn’t. In order to understand which designs produce the least interior noise you need to characterize the transmission through the structure.”

CD-adapco’s Portfolio for Noise and Vibration

Wave6 from CD-adapco covers aero-, vibro- and aero-vibro-acoustics problems.

Wave6 from CD-adapco covers aero-, vibro- and aero-vibro-acoustics problems.

CD-adapco’s acoustics technology was created to fulfill requests from their customers to help simulate flow induced noise and vibration. The simulation company has tested its noise and vibration technology, both wave6 and its leading CFD platform STAR-CCM+, with its customers.

“CD-adapco continues to get requests to model flow-induced noise and vibrations,” said Shorter. “Over the last year and a half, STAR-CCM+ has added technology to accurately capture unsteady flows on the exterior of vehicles and to simulate the exterior acoustics generated by that flow.”

“But that isn’t enough by itself as our clients are interested in interior noise and we cannot characterize that with CFD alone,” added Shorter. “So over the past three years we have developed new methods for predicting the transmission of noise and vibration through vehicles to predict interior noise.”

The method Shorter is talking about is wave6. It is a standalone noise and vibration CAE platform that seamlessly integrates with STAR-CCM+. What sets wave6 apart from the STAR-CCM+ CFD platform is that the acoustics simulation software focuses on noise and vibration applications in situations in which engineers will need more than just CFD to assess the problem. For instance, when waves propagate through objects.

“CFD is great for characterizing sources,” said Shorter, “but when it comes to the transmissions, we often use other methods.”

Wave6 uses finite element analysis (FEA), statistical energy analysis (SEA), and boundary element method (BEM) models to simulate the transmissions of broadband, or low- and high-frequency, noise. Using CD-adapco’s “power session” licensing model, users can access these methods and connect them as needed. Engineers can also seamlessly integrate their CFD results from STAR-CCM+ and use it as load inputs for wave6.

Wave6 vibro-acoustic analysis methods.
Wave6 vibro-acoustic analysis methods.
The need for all of these acoustic analysis methods lies in the broad range of frequencies that humans can hear in comparison to the scale of the system being assessed. Because the wavelengths vary largely, often different or multiple methods are needed to truly simulate the situation.

“The wavelengths within structural components can be hundreds of meters long at low frequencies. As you go up in frequency, the wavelengths get shorter and shorter and can often be of the order of a few millimeters at the top of the audible range,” explained Shorter. “You have this massive variation in wavelength across the audible frequency range and, because of that, noise and vibration simulation requires different methods depending on the frequency range [and] depending on the objects of interest.”

 “Noise and vibration modeling uses different methods depending on how big an object is compared to the wavelength,” added Shorter. “If you are looking at low frequency or small things, you use methods like finite elements and boundary elements. If you have something with many wavelengths in it, it’s more efficient to use statistical wave propagation methods like SEA.”

To find out more about wave6, read “FEA Solver, Results Viewing Tool and Vibro-Aeroacoustic comes to STAR-CCM+.”

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