Highly resolved Large-Eddy Simulation of wind turbine wakes

An horizontal axis wind turbine placed in a free stream develops a wake behind its rotor. Inside this wake, complex vortical instabilities are developing and can lead to turbulent structures generation1 . In order to predict performances and loads of wind turbines in wind farms, it is essential to accurately characterize these wakes and their impact on the downstream turbines. Large-Eddy Simulation (LES) is well adapted to this problem as the considered flow is 3D, complex and strongly unsteady. The state-of-the-art approach for taking into account the effect of the turbine on the flow is the Actuator Line (AL) method2,3. This method enables the use of Cartesian grids, which brings numerous advantages but prevents space adaptivity and limits the shape of the flow domainWind Energ

Large-Eddy Simulation of wind turbines wakes including geometrical effects

Accurate simulation of wind turbine wakes is critical for the optimization of turbine efficiency and prediction of fatigue loads. These wakes are three-dimensional, complex, unsteady and can evolve in geometrically complex environments. Modeling these flows calls thus for high-quality numerical methods that are able to capture and transport thin vortical structures on an unstructured grid. It is proposed here to assess the performances of a fourth-order finite-volume LES solver to perform massively parallel scale-resolving simulations of wind turbines wakes. In this framework, the actuator line method that takes the effect of the wind turbine blades on the flow into account is implemented. It is demonstrated that both near and far parts of the turbine wakes are accurately modeled as well as geometrical details. The methodology is assessed on two different test cases and validated with experimental results. It is demonstrated that the flow predictions are of equivalent quality on both structured and unstructured grids. The influence of the geometrical details (e.g. nacelle and tower) on the wake development as well as the influence of the discretization scheme are also investigated.Wind Energ

Simulating the helix wake within an actuator disk framework: verification against discrete-blade type simulations

Dynamic flow control strategies are raising interest for wake mitigation purposes. Among the different strategies, the so-called helix one relies on individual pitch control (IPC). The numerical simulation of the helix is thus readily performed by means of discrete-blade capturing methods. Yet, if this control strategy is considered at the scale of wind farms, the resolution required by such methods becomes prohibitive and actuator disk (AD) models should be envisioned. It is however not trivial to translate IPC strategies to an AD framework which by definition considers rotor-averaged effects. This work assesses the ability of an AD method to simulate the helix strategy by comparing it to a higher fidelity approach relying on a discrete-blade capturing model. Results show that the disk-type approach supplemented with a disk-adapted IPC scheme is able to capture both the forced motion of the wake at low turbulence and the faster wake recovery at moderate turbulence. From a quantitative perspective, the disk-type approach predicts bigger power gains, compared to those foreseen by the discrete-blade type approach, for a downstream turbine in the wake of a helix-operated one. Team Jan-Willem van Wingerde

Direct Numerical Simulation and Large-Eddy Simulation of wake vortices: Going from laboratory conditions to flight conditions

This paper aims at presenting DNS and LES as applied to the simulation of vortex wakes: in laboratory conditions (moderate to medium Reynolds numbers) and up to real aircraft conditions (high to very high Reynolds numbers). Only incompressible flows are considered. DNS and LES are able to capture complex 3-D physics provided one uses high quality numerical methods: methods with negligible numerical dissipation (i.e., methods that conserve energy in absence of viscosity and/or subgrid modelling) and with low dispersion errors (to properly transport complex vortical structures). Methods that can do that are: spectral methods, high order finite difference methods, and vortex-in-cell (VIC) methods. As the problems of interest are of large spatial extent and contain vortices with small cores, it is also essential that the methods be efficiently parallelized. As to LES of wake vortex flows, this require subgrid scale (SGS) models that are essentially inactive during the gentle, well-resolved, phases of the flow and within the vortex cores, and that become active only during the complex turbulent phases of the flow. The recent multiscale models, that act solely on the high wavenumbers modes of the LES, are seen to be most appropriate. We present some illustrative examples of DNS and LES results that were obtained within our group