HYBRID PARALLELISATION OF AN ALGORITHMICALLY DIFFERENTIATED ADJOINT SOLVER

This research has been supported by the European Commission under the HORIZON 2020 Marie Curie fellowship (grant no. 642959

Performance and wake development of vertical axis wind turbines: a LES study using a vortex particle-mesh method

A Vortex Particle-Mesh (VPM) method with immersed lifting lines (ILL) has been developed and validated, in a HPC framework. The vorticity-velocity formulation of the Navier-Stokes equations is treated in a hybrid way: particles handle advection while the mesh is used to evaluate the differential operators and for the fast Poisson solvers (here a Fourier-based solver which enforces unbounded directions and inlet/outlet boundaries simultaneously); both discretizations communicate through high order interpolation. The sources of vorticity are accounted for through an immersed lifting line approach, based on airfoil polars. Each blade line handles the attached and shed vorticity contributions in a Lagrangian manner; its immersed treatment efficiently captures the development of vorticity from thin sheets into the three-dimensional field. A multiscale subgrid-scale (SGS) model, that was specifically developed for the vorticity-velocity formulation, is also used: it enables large-eddy simulations (LES) with detailed small-scales dynamics of the ensuing turbulent flow, while not artificially diffusing the coherent vortex cores. LES of Vertical Axis Wind Turbine (VAWT) flows are performed, for two competing designs: straight blades H-type design versus swept blades design, at the same diameter D and height H of the rotor (thus the same aspect ratio) and the same horizontal chord c_h of the blade profile (thus the same solidity). Cases with uniform wind and with realistic synthetic turbulent wind (using Mann's algorithm) are considered. Comparisons are made on the power performance coefficient, Cp, versus tip (in fact ``blade'') speed ratio, TSR, and on the cyclic blade loading. With such VPM-ILL, the wake complex development is also captured with much detail: from the blades to the near wake coherent vortices, to the transitional ones, to the fully developed turbulent far wake. Mean flow statistics in various planes (horizontal, vertical and cross) are also obtained, revealing a wake development that, even in the mean, is very different from that of Horizontal Axis Wind Turbine (HAWT) flows, and is also more complex (non-zero side force, re-circulation zone, faster decay rate)