applications


the annual energy produced (AEP) by the farm. Te soſtware compared the wind flow and power production for multiple layouts, including different wind directions, and the effects of terrain and multi-turbine wakes. Te model optimised the final wind farm


design, with the modified design improving the AEP by 8.5 per cent, compared with the original, proposed design. Te simulation has since proved its worth with a closely matched projected – versus measured – AEP, representing an accuracy in the simulation of between two and four per cent. Such high accuracy simulations are a necessity


when designing a structure that needs to be right first time. Ryan said: ‘You cannot move a wind turbine once it’s in the ground. You need to simulate every angle the wind could possibly come from and what effects such changes have on the power production. You also want to know what happens if there are doldrums where the wind goes away and, on the flip side, if the wind goes beyond a threshold considered dangerous.’ ‘Wind simulation is about studying different


scenarios, not just one, and doing it fast, reliably and repeatedly,’ he added. Te data from a wind forecasting survey alone


does not tell engineers what the wind profile will look like in the presence of a wind farm, so complex aerodynamics and CFD models are also needed to complete the forecast. Tese simulations must take on board a wide


range of variables to be assessed by a wide range of specific users – from aerodynamic engineers profiling the wind, to mechanical engineers designing the mechanical aspects of the turbines and their blades and the electrical engineers who are concerned with the power electronics and the connection of the turbines within the farm. Communication between different engineering


disciplines is vital, as Dudgeon added: ‘One of the fundamental ways to reduce the risk of developing a wind farm site is making sure those interdisciplinary teams can communicate their engineering information very effectively.’ Vestas Wind Systems has taken a different


approach from the conventional met mast method, instead using historical data from met offices around the world to create a wind simulation of the entire planet. Te data is made up of 35,000 measurements and creates a virtual atmosphere, which can model wind environments down to a square kilometre resolution. When evaluating the best wind farm location,


which offers cost savings – in terms of manpower and the mast expenditure – historical data is based on 15 years of data, reducing the likelihood of anomalous results due to freak weather patterns that may occur in the 18 month stint for a met mast, according to Vestas. Anders Rhod Gregersen, chief specialist at Vestas Wind Systems, said: ‘We can tell what


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the weather is like in your back garden, for example, using this data without any physical measurements ever being done there. In real terms, you can now evaluate any site around the world in a few minutes and at a fraction of the cost.’ Gregersen added: ‘It is very different from


doing a simple extrapolation – this modelling technique involves complex weather modelling based on thousands of differential equations.’ Te complexity of any wind farm simulation,


including those based on historical data and the met mast method, continues to grow as turbines from around the world gather more data, and this new information needs to be coupled to the existing data to improve the accuracies of the simulations further. Gregersen said: ‘Previously, it was like we were driving a fast car on the motorway but we were steering by looking


MULTIPHYSICS CAN


CREATE MODELS CLOSE TO THE COMPLEXITIES OF THE REAL WORLD


through the rear-view mirror. Now we are looking out at the road ahead and steering not just for what we see, but what is over the horizon.’ Tis is achieved by harnessing the simulation


power of a HPC environment to manage the ‘tsunami of data coming at us’, according to Gregersen, who sees data handling capabilities as the biggest challenge facing the wind energy simulation space. Gregersen added: ‘Te discussion is becoming fact-based so we know what will happen when we set up the wind turbines, before we actually do it.’


Silent systems Noise is a primary concern for those living near a proposed site. Detailed simulations are required to reduce this impact, and to keep wind farms


operating within reasonable noise and vibration levels. While noise is more of a concern to the general public, vibration effects can, for example, adversely affect sensitive scientific equipment. Xi Engineering Consultants specialises in


complex noise and vibration issues. Dr Brett Marmo, technical director at Xi Engineering, said: ‘Te key challenge we have when modelling the acoustic part of the model is the mesh size. Te size of mesh elements is dependent on the wavelength of the sound being modelled – as the frequency of the sound of interest increases, the wavelength and mesh element size reduces.’ ‘At high frequencies the number of mesh


elements required to model the air surrounding the turbine becomes very large (tens of millions) making the computation very difficult. Furthermore, as the size of the turbine increases the mesh size also increases,’ Marmo added. Xi Engineering recently simulated the grinding


of teeth within the gearbox of a wind turbine, as this component can cause vibrations, that are perceived as excessive noise. Studying the design data, Xi’s engineers found the gearbox was the source of the noise and vibration, and that it was being amplified by the tower’s steel skin. Using a Comsol Multiphysics model of the


structure, they were able to locate hot spots where the noise was being amplified and test various solutions. Te solution was to use a specific material to coat the inside of the tower and reduce the amplification. Due to the material’s high cost, the simulations determined the least amount of material necessary to keep the noise levels below the required standards.


Moving offshore An obvious way to eradicate noise concerns is to site wind farms in isolated locations, such as offshore positions. Fluid flow models become even more important for an offshore environment, as CD-adapco’s Jim Ryan explained: ‘We see even greater value proposition for the ➤


Large eddy simulations of wind flow over terrain and the wake effect of wind turbines @scwmagazine FEBRUARY/MARCH 2016 31


Vestas Wind Systems


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