Application of actuator surface concept in LES simulations of the near wake of wind turbines

Abstract

During the last decades wind energy became a crucial part of the renewable energy mix seeking to satisfy the growing energy demands in a sustainable manner. While current state of the art wind turbine blades are close to their aerodynamical optimum, the challenge lies today in the viability of concepted wind farms. Hence wind farm designs aim for denser turbine arrangements and topographically more complex terrains. Therefore the interaction between turbines namely through the wind turbine wakes become an important aspect of the layout of a wind farm. Theses wakes are characterized by an elevated turbulence level and a noticeable velocity deficit. While the wind energy industry prefers more approximative solutions for the sake of computational costs, the non-linear character of the phenomenon is best captured by the Navier-Stokes equations. As there exists no analytical solution, computational fluid dynamics is used to tackle this equation. Depending on the turbulence modelling different levels of accuracy can be achieved. The objective of this work is to examine the near wake of a wind turbine for different kinds of ambient turbulences. The rotor is represented by the actuator line method implemented in the widely popular wind energy framework SOWFA from NREL, USA. In order to evaluate the here used method a non-turbulent test case is validated against another popular wind energy framework EllipSys3D and then verified with the results from the open jet wind tunnel experiments MEXICO and NEW MEXICO. By adapting the distribution width of the force within the computational domain this approach is then extended towards an actuator surface. An optimal distribution width depending on the grid resolution is found empirically for the here presented case. After the base case is established and successfully tested the rotor is immersed in homogeneous isotropic turbulence for which a custom boundary condition for the OpenFOAM framework is conceived. Then the synthetic turbulence field obtained by the Mann algorithm is imposed on the large-eddy simulation with dynamic Lagrangian model for the sub-grid scales. Finally the rotor is exposed to a shear layer turbulence using an external library allowing to impose a synthetic turbulent field obtained from the Mann algorithm on a sheared flow. By analyzing the vortex properties and the energy spectra in both cases the influence of the ambient turbulence on the near wake are deduced. Another interesting aspect is the way how the energy spectra gives insight in how far downstream the effects of the distinct blades are noticable.

Date	19 Mar 2018
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Christian Masson (Supervisor) & Louis Dufresne (Co-supervisor)