NewsDate: 09-01-2018 by: Tiffany Won
SIMULATING ROLLING NOISE FROM A TRAIN FOR SILENT TRAVEL
When you’re a kid, you want to hear every bump, squeal and chugalug while riding the train rushing towards a far-off adventure with your folks.
When you’re all grown up, however, you probably want silence while riding the train to some far-off design meeting with your boss. And don’t get me started on how trains can be a noisy neighbor. Engineers clearly need to do something to fix the problem of noise.
“The focus of onboard, in-carriage noise on trains is an important customer satisfaction criteria,” said Trevor Edwards, vibro-acoustics global business development manager at ESI Group. “Onboard noise manifests itself to rail designers in the form of acoustic targets at strategic points in the carriage. The focus on exterior noise can sometimes be legislative. For example, laws restricting pass-by-noise (PBN) of rail or other vehicles that pass near housing.”
Engineers have their work cut out for them to limit these noises as much as possible. Unfortunately, much of the noise is caused by the rails and the environment, which are beyond the control of the train designer.
This makes it ever more important to use simulation tools, such as ESI VA One, to help optimize everything you can to ensure the comfort of drivers, operators, passengers and the general public sleeping through the night.
In every train, there are numerous sources of noise. Engineers need to assess how these will affect the comfort and experience of a passenger in transit. (Image courtesy of ESI Group.)
How Engineers Can Measure Vehicle Noise
The main sources of noise from a train include its equipment, rolling noise, aerodynamics and the bogie. For an effective acoustic design, each source of sound needs to be assessed and limited.
Some of the tools within ESI VA One that can help engineers design noise-conscious vehicles. (Image courtesy of ESI Group.)
Robert Fiedler, vibro-acoustics specialist at ESI Group, noted that the noise created from the imperfections and roughness of the wheel/rail interface is of particular interest due to its significant contribution. It is true that engineers can’t control the rails, but they can design the wheels, their housing and their assemblies in ways that reduce vibration. However, first they need to measure or characterize the sound.
“You can account for the radiated noise by measuring it with acoustic antennas, which consist of an array of microphones,” Fiedler said. “But this is difficult, as you require an expensive measuring device and it’s hard to separate the equipment and environmental noises on the recording.”
Another method Fiedler suggests is to record the noise using a set of microphones close to the underbelly of the car. This is easier to perform, but will also result in the measurement capturing equipment and environmental noise.
“The third approach is to characterize the radiated power coming from the wheel/rail noise with simulation,” Fiedler added. “The method I used was the Remington approach. This includes an analytical approach where you split up the rail and wheel into a mesh of squares with inertia, mass and spring bodies. Or you could use the FEM/BEM (finite element method/boundary element method) approach.”
In other words, to characterize component noises quickly, accurately and in an affordable manner, engineers require computer-aided engineering (CAE) software capable of performing noise, vibration and harshness (NVH) simulations.
Sergej Italjancev CAD designer specialist, Mechanical Projects of Škoda Transportation explains that when they first designed a metro train for a bid his organization was unable to complete the noise criteria of the tender. However, after applying VA One and the design changes the software led them towards, they had won the tender.
Currently they are supplying eight train units each with six wagons.
HOW TO SIMULATE NVH OF A WHEEL/RAIL INTERFACE
Using simulations, engineers can optimize their rail component designs to reduce vibration and noise propagation both inside and outside of the train.
To simulate these noises, the engineer must first characterize the interaction between the wheel and rail.
“Earlier studies indicated that noise could be attributed to small-scale roughness on the running surface of the wheel and rail exciting both into vibrations,” Fiedler said.
Schematic representation of the wheel/rail interaction. These imperfections initiate the mechanism of noise generation. (Image courtesy of ESI Group.)
In other words, as the wheel moves over the rail, they are pressed together under the weight of the train and its cargo. This load will force local deformations in both the rail and wheel.
The local roughness in the spinning wheel will cause these local deformations and the contact forces to change quickly creating vibrations.
“To express the deformation, engineers could substitute wheel and rail parts with frequency dependent springs,” Fiedler explained. “The dynamic characteristics of the springs and local point mobilities could be evaluated using either an analytical 1D approach or a 3D FEM/BEM approach.”
Fiedler continued to explain that engineers modeling the deformation with the 3D approach will have more control of the boundary conditions. These engineers will also be able to model any wheel or track shape they require. This is important as different part geometries could experience different damping.
With ESI VA One, using FEM/BEM, engineers can model any wheel shape they need to, such as this tram wheel. Using simplified analytical simulations, this is not possible. (Image courtesy of ESI Group.)
Additionally, the use of a 3D model will open up the chance to explore pre-stressed modes that experience axle loads or centrifugal forces.
“Modal behavior expresses how the structure behaves under certain loads at certain frequencies,” Fiedler noted. “When applying the correct force, the engineer will see the real structural vibrations in the frequency domain. This is known as a Modal approach.”
The next step in the simulation is to calculate the radiated power that is coming from the wheel. This is done by modeling the air around the wheel.
“In BEM simulations, the user will create a surface envelope around the wheel geometry and specify which surface side is wetted by the air,” Fiedler instructed. “After the solve, the engineer has access to the acoustic radiated power.”
This acoustic radiated power can then be used as a yard stick to compare the performance of different wheel geometries.
A rail acoustic simulation workflow for VA One. (Image courtesy of ESI Group.)
Additionally, the acoustic radiated power can be used in a systems model via statistical energy analysis (SEA). This tool helps predict the interior noise within the train by accounting for all noise sources in the system.
After the acoustic radiated power of the wheel is determined, similar decompositions can be assessed from other noise sources. These sources can include the heating, ventilation and air conditioning (HVAC) system as well as extruded or composite structures under vibration. Once all the noise sources are assessed, they can then be integrated within the SEA model to evaluate the noise propagation as a whole.
“The method described helped us early in the project phase to identify which structural components of the tram contribute most to interior noise,” said Petr Cuchý, lead researcher, Field Research of Škoda Transportation. “It is important to focus on acoustically sensitive components and avoid solution of parts that have a minor effect on overall noise and to subsequently avoid adding additional mass and cost to these acoustically insensitive components. Equally important for us was to have an estimate of expected interior noise levels.”
How to Simulate Interior vs. Exterior Noise
So, now you have a model that calculates the vibration and noise from the source at a component or system level. What next?
“VA One calculates and ranks all structural and acoustic paths from the source to a selected point,” said Edwards. “For example, a cavity in a train [can be selected] so that a designer can easily establish the most important noise contributions at that location and take remedial action.”
Fiedler explains the importance of knowing the seat lay-out as they influence and absorb interior noise. (Image courtesy of ESI Group.)
NVH software simulates sound propagation using a source, path and receiver concept. The idea is that the vibrations caused by the source inject energy into the system, which is passed through the structure, substructures and air to the receiver. This is defined as the transmission path. This receiver is typically defined by a specific location within or external to the source.
Edwards explained that it can be quite challenging to determine the interior noise within the cabin due to the fittings and fixtures inside the space.
“The main difference is that interior noise is in a confined space, whereas exterior noise is in an open environment,” Edwards said. “The same source, path, receiver model is used, but different frequencies of interest will require different tools in VA One to perform the calculation.”
For instance, depending on the frequency of the sound, something as simple as a seat could absorb the noise, while complex paths contained in structures such as walls and open spaces can diffract, echo or perpetuate the noise.
Using VA One, engineers can use these tools to assess the sound levels at a receiver’s location, and then determine how features within an acoustic space such as carpets and curtains can absorb the sound.
As a result, engineers can use this information from VA One to optimize features within the acoustic space to limit noise. This should be very important when designing the train’s interior to optimally limit any whisper of noise propagation.