Building a robust path for virtual wind turbine design
Leading wind turbine manufacturer partnered with LMS Engineering Services to sharpen capability to master wind turbine acoustics
The large physical size and characteristic acoustic radiation of wind turbines make it a real challenge to accurately simulate wind turbine acoustics early in development. A leading wind turbine manufacturer and LMS Engineering Services joined forces to meticulously build a hybrid vibro-acoustic simulation model, and validate the wind turbine model through operational measurements executed 100 meter above the ground. The resulting full-scale acoustic wind turbine model enabled engineers to predict far-field wind turbine acoustics with adequate accuracy and efficiency. Collaboration, commitment and expertise helped the team to overcome the project’s extreme modeling and testing challenges and allowed the wind turbine manufacturer to establish a robust virtual path for mastering wind turbine acoustics. Advanced testing efforts were deployed to create and validate the vibro-acoustic simulation model, and additionally allowed modeling challenges to be better understood in facilitation of future wind turbine developments.
Capitalizing on clean, indigenous and inexhaustible energy
In many countries, governments tend to increase the share of renewable power generation, leaving the choice between a limited number of options that are economically viable. With ambitious European ecologic targets and resolute choices for energy that is clean, indigenous and inexhaustible, wind energy is predicted to meet approximately 25% of Europe’s power demand in 25 years time. On a worldwide scale, wind turbine markets are also growing fast in the United States, India, Japan, China, Canada, Australia and New Zealand.
When standing in front of a wind turbine, its impressive size is striking. The manufacturer’s largest wind turbines are 120 meter tall and feature a 110-meter bladed rotor diameter, translating into a swept air area of approximately 8,000 square meter! The drivetrain of wind turbines, consisting of large gearbox and generator assemblies, converts the wind power into bladed rotor mechanical torque and subsequently into electrical power. The wind turbine control system together with the power converter guarantee a clean and steady output voltage at constant frequency, irrespective of varying wind conditions.
Optimizing wind turbine acoustics through virtual simulation
The noise wind turbines generate is influenced by many factors, including blade size and design; drivetrain operation as well as the orientation, force and turbulence of the wind. Roughly spoken, a megawatt wind turbine generates a relatively flat 45-55 dBA broadband noise spectrum at a distance of 130-150 meters. At average wind speed, wind turbine noise only drowns out wind turbulence, vegetation and/or traffic noise that are present in the background by approximately 10-15 dBA. Specific tonal noise components occur as a result of dynamic forces that come into play inside the gearbox (teeth meshing), the generator (electro-mechanical poles interaction), and system hydraulics equipment. These dynamic forces cause local housing surface vibrations, which distribute the noise to the surrounding area through radiation. The noise generated by driveline rotating machinery also propagates directly through structural noise paths.
To accelerate efforts to reduce the noise of its comprehensive range of wind turbines, the manufacturer contracted LMS Engineering Services to run a number of joint hybrid vibroacoustic modeling and simulation projects. "Our motivation to engage in these projects relates to the capability of acoustic simulation in identifying design improvements up-front in the development process," Laurent Bonnet, Leader of Acoustic & Vibration Engineering at the manufacturer in Germany, stated. "The advanced modeling expertise acquired through these projects represents the foundation for building accurate wind turbine models, and enables us to predict the acoustic performance of multiple design variants. Acoustic simulation insight is most helpful in tracing individual noise sources and adapting the design for enhanced acoustic performance early on in the process."
For the initial project, the manufacturer selected its 1.5 Megawatt wind turbine platform, which is currently in operation at a large international install base. LMS engineering consultants helped develop a validated vibro-acoustic model of the full-scale wind turbine, using a method combining structural FE (Finite Element), acoustic BE (Boundary Element) and ATV (Acoustic Transfer Vector) modeling and simulation.
Challenging tests to create/ validate wind turbine FE model
In the process of building a complete, accurate and full-scale FE model of the wind turbine, the engineering team faced the challenge of characterizing all principal components that make up a wind turbine, such as the blades, hub, rotor, tower, gearbox, brake, bedplate and nacelle. An exceptional modal testing experience was undoubtedly the detailed structural characterization of a 37- meter rotor blade. Such a blade almost entirely consists of a complex laminated composite construction with various curvature geometrical topology with relatively low initial structural damping. Other complex subsystems that were characterized include the large oil-cooled gearbox assembly and the 160-tons and 100-meter tall carbon-steel wind turbine tower.
In addition to transfer functions (Frequency Response Functions – FRFs) that were acquired for modal analysis and general dynamic assessments, engineers measured and validated FRFs to characterize the interface between various wind turbine parts. This testing approach enabled them to properly define appropriate stiffness and contact area representations. After modeling all individual components and interfaces, they grouped a number of related components into partial system assemblies, such as the rotor hub in combination with the three rotor blades. For each subassembly that was considered, they updated the overall FRF analysis and junction transfer functions by specific measurement blocks. Subsequently, the project team performed FRF analysis on the complete wind turbine assembly in order to update the structural full-system FE model and to validate all cross transfer functions.
Running operational measurements from inside the nacelle
"Besides experimental modal analysis, testing efforts also included operational full-turbine measurements to be able to qualify acoustic sound levels, vibrations and forces," Laurent Bonnet stated. "The test crew for this extraordinary test campaign consisted of test personnel from both the wind turbine manufacturer and LMS. Overall, it deployed 6 LMS SCADAS III front-end stations – totaling nearly 400 measurement signals – to acquire deflection, vibration and acoustic responses in, on and around the wind turbine." Members of the crew used the LMS Test.Lab software suite to control synchronous data acquisition, and to perform any data analysis action they required. An extra measurement system, equipped with a wireless LAN, operated strain gauge measurements on the rotating rotor blades. In addition to providing lots of data in support of accurate acoustic simulations, Operation Deflection Shapes (ODS), for example, immediately provided valuable insight into the structural operation of various wind turbine components.
It took a lot of energy and perseverance from the testing crew to successfully complete this challenging testing assignment, which lasted several weeks. Installing sensor instrumentation required acrobatic skills rather than any other specialty. To equip strain gauge sensors inside the blades, the operator needed to exit through the nacelle roof, climb inside the rotor and enter the interior of the blade, which for the occasion, was positioned horizontally. During the time of intensive measurement, the crew, packed inside the compact nacelle, faced harsh winter time weather circumstances. On the coldest days, nearly-frozen testing professionals relied on LMS SCADAS front-ends with dripping icicles to fulfill their duties faithfully.
Accurate acoustic simulation and flexible design optimization
Based on the structural full-system FE model created earlier on, engineers derived a Boundary Element (BE) model of the wind turbine through a dedicated "skinning" procedure. The acoustic BE model makes it possible to simulate the acoustic power generated by the wind turbine through local surface vibrations of various system parts. This information serves as input for the innovative LMSproprietary ATV method that is integrated into the LMS Virtual.Lab software suite, which accurately and effectively translates the acoustic power into farfield noise emissions.
"The ATV method demonstrates the feasibility of reaching our ultimate goal, which is reliably determining the noise radiation of the entire wind turbine configuration through simulation," Laurent Bonnet commented. "The satisfactory level of correlation between the simulated and measured acoustic radiation proves that the new vibroacoustic simulation method lives up to our high expectations."
One of the major advantages of this deterministic acoustic simulation approach is that it supports different kinds of analyses that provide detailed insight into particular noise sources. Through post-processing, engineers are able to trace the modal contribution of specific system parts, or analyze the effect of individual panels and loads on overall noise radiation. The engineering information resulting from these investigations is vital for driving development improvements and new wind turbine development. Deterministic acoustic simulations were performed up to a frequency of 200 Hertz, which allowed significant structure-borne noise phenomena to be traced and tackled with sufficient reliability. To keep the massive processing workload that is involved in vibro-acoustic simulation within acceptable levels, multiple processing stations were used to crunch data in parallel.
In parallel with this hybrid vibro-acoustic simulation approach, the wind turbine manufacturer additionally performed hybrid SEA (Statistic Energy Analysis) and far-field acoustic holography. Its engineers used hybrid SEA to model the wind turbine and investigate nondeterministic noise and vibration sources. Far-field acoustic holography, a second high-frequency modeling method, was deployed to qualify noise emissions in the far field and to extract statistically significant acoustic phenomena.
Developing wind turbines with superior acoustic performance
The deployment of the hybrid vibroacoustic simulation in future wind turbine development processes will help the company in cascading structural and acoustic targets for the complete wind turbine down to subassembly and component level. "This new innovative approach strengthens the capability of our engineering teams in efficiently identifying the most promising design concepts and in developing the most effective component variants," Laurent Bonnet concluded. "As such, the engineering consultancy project provides us a head start in tackling the root causes of wind turbine noise and in designing countermeasures that further reduce radiated noise levels. Advanced testing efforts provide detailed insight into mechanical wind turbine operation, help create/validate vibro-acoustic simulation models, and allow modeling challenges to be better understood in facilitation of future wind turbine development. For a large part, the success of this innovative engineering project was founded on the specialized skills and experience of LMS engineering consultants and our own engineers, which really made the difference."
Currently, the wind turbine manufacturer and LMS are executing a similar subsequent modeling project that further builds on the acquired vibro-acoustic expertise. This follow-on project is being performed on one of the company’s larger wind turbine variants. Simulation accuracy is further improved by applying driveline forces to the model at interior gearbox and generator locations instead of gearbox mounting locations underneath the driveline machinery. In the meantime, simulation helped engineers identify several potential design enhancements for improved acoustic performance, which they will verify using full-scale prototype testing. A number of the proposed design changes are likely to find their way into new wind turbine revisions. This strategy proves the added value of a robust vibro-acoustic simulation process in developing wind turbines that offer superior acoustic performance.(end)
