Wind power is the renewable source with the most successful deployment over the past decade (2007-2016), growing from 93 GW to 490 GW of world-installed capacity during this time (source: “Renewable capacity statistics 2017”, International Renewable Energy Agency). A key role in this spectacular advance has been played by the increased understanding of the turbulent flow in the wind farms, thanks to ever-growing computational power and improved algorithms. In EoCoE-II, the ambition is to be able to simulate a full wind farm over complex terrain with up to 100 wind turbines. Ultimately this will require increases in the size of the problems to be solved and the amount of computational resources of at least two orders of magnitude. The main goal of the wind scientific challenge is to bring the flagship Alya code to Exascale in order to tackle this Large Eddy Simulation (LES) flow problem. Furthermore, the rotating wind turbine blades will be resolved to account for their effect on the flow via a full rotor model. Blade deformation may also be included via the use of sliding meshes. These combined tasks all pose daunting technical challenges for the kernel solvers and meshing algorithms in Alya, for which the EoCoE-II consortium is already well equipped.
Alya (https://www.bsc.es/research-development/research-areas/engineering-simulations/alya-high-performance-computational) is a high performance computational mechanics code that solves complex coupled multi-physics problems, mostly coming from the engineering realm. Alya will be deeply refactored in order to be able to address heterogeneous computing nodes. A very strong effort on linear algebra is planned for both the fluid and the solid problems handled in the two tasks foreseen in the wind scientific challenge.
waLBerla (http://www.walberla.net) is an exascale-enabled multi-physics software framework. In EoCoE-2 its Lattice Boltzmann module will be employed as alternative algorithm for modeling wind turbines and wind farms. WaLberla has shown scalability up to 2 million parallel threads and can be used for simulations with up to a trillion (1012) mesh cells that are partitioned as a forest of octrees.
waLBerla features advanced load balancing and performance tuning through advanced code generation and automatic code transformation technology.
The wind scientific challenge has two main tasks. The first one is related to the CFD simulation of flow over a wind farm, commonly known as microscale simulations in the wind community. This kind of simulations is performed by energy companies interested in developing a wind farm to assess the wind resource and help identify the best locations for the wind turbines. BSC has been collaborating with Iberdrola, one of the most prominent investors in wind energy, for more than five years. We have adapted our code, Alya so that it can be used directly by the wind experts in Iberdrola. The industry typically relies on RANS simulation for wind resources assessment which is robust and does not require substantial computational resources. However, it is well known that RANS has limitations for the simulation of separated flows that commonly occurs in complex terrain. Our first objective within EoCoE-II is to further develop the LES implantation in Alya so that it can be used for wind resource assessment over complex terrain. LES is more accurate than RANS but also significantly more computationally demanding. It is the logical tool to be used with the advent of Exascale Supercomputers. Figure 1 shows a volumetric rendering of the velocity obtained with a LES simulation using Alya over the Bolund cliff, one of the most well-known benchmarks for flow over complex terrain.
Our second objective is to perform LES simulations of the rotating wind turbine blades commonly known, as full rotor simulations. The simulation of the rotation of the blades is possible thanks to a sliding mesh approach that is being developed within EoCoE-II. The blade deformation will be taken into account by using Alya’s solids module and its Fluid-Structure Interaction capabilities. Figure 2 shows the Q-vorticity isosurfaces for the flow behind the rotating blades of an NREL-VI wind turbine. Despite such simulation could also have some interest for wind energy companies, we expect that the main interest for this technology should come from wind turbine manufacturers. EoCoE-II has obtained a support letter from Vestas, and we expect that we can provide the industry with a useful technology by the end of the project.
Videos with research results using Alya simulations on wind flows can be found here.
A CFD framework for offshore and onshore wind farm simulation. Avila, Matias; Gargallo-Peiró, Abel; Folch, Arnau; (2017)
Mesh generation, sizing and convergence for onshore and offshore wind farm Atmospheric Boundary Layer flow simulation with actuator discs. Gargallo-Peiró, Abel; Avila, Matias; Owen, Herbert; Prieto-Godino, Luis; Folch, Arnau; (2018)
A wind field downscaling strategy based on domain segmentation and transfer functions Barcons, Jordi; Avila, Matias; Folch, Arnau; (2018)
High-Performance Computing: Do’s and Dont’s. Houzeaux, Guillaume et. al. (2018)
Wall-modeled large-eddy simulation in a finite element framework. H. Owen, G. Chrysokentis, M. Avila, D. Mira, G. Houzeaux, R. Borrell, J. C. Cajas, O. Lehmkuhl. Int J Numer Meth Fluids. 2020;92:20–37. DOI: 10.1002/fld.4770.
All EoCoE-I and EoCoE-II publications are available here (openAIRE).