Pascal Weihing

The present study investigates flow dynamics in the hub region of a wind turbine focusing on the influence of nacelle geometry on the root aerodynamics by means of Reynolds averaged Navier–Stokes simulations with the code FLOWer. The turbine considered is a generic version of the Enercon E44 converter incorporating blades with flat-back-profiled root sections. First, a comparison is drawn between an isolated rotor assumption and a setup including the baseline nacelle geometry in order to elaborate the basic flow features of the blade root. It was found that the nacelle reduces the trailed circulation of the root vortices and improves aerodynamic efficiency for the inner portion of the rotor; on the other hand, it induces a complex vortex system at the juncture to the blade that causes flow separation. The origin of these effects is analyzed in detail. In a second step, the effects of basic geometric parameters describing the nacelle have been analyzed with the purpose of increasing the aerodynamic efficiency in the root region. Therefore, three modification categories have been addressed: the first alters the nacelle diameter, the second varies the blade position relative to the nacelle and the third comprises modifications in the vicinity of the blade–nacelle junction. The impact of the geometrical modifications on the local flow physics are discussed and assessed with respect to aerodynamic performance in the blade root region. It was found that increasing the nacelle diameter deteriorates the root aerodynamics, since the flow separation becomes more pronounced. Possible solutions identified to reduce the flow separation are a shift of the blade in the direction of the rotation or the installation of a fairing fillet in the junction between the blade and the nacelle.

In this study, which was part of the project AssiSt, numerical analyses of wind turbine power, loads and wake development in the extreme wind conditions found on a complex terrain site in northern Spain have been performed. The turbine is located near the edge of a characteristic cliff. Realistic atmospheric inflow conditions are prescribed by propagation of highly resolved meteorologic data of the LES code PALM in the aerodynamic solver FLOWer. The global features of the flow field in the complex terrain site were investigated showing significant inclination angles, high turbulence intensities and a characteristic mixing mechanism of high and low momentum velocity layers. Further, the wake development and its deflection and deformation was analyzed. Finally, the turbine's load response was investigated, showing severe fatigue loading as consequence of the highly unsteady inflow conditions.

This article gives an overview of the results of the wind energy research project AssiSt. Results of the four work packages include flow in complex terrain, wind energy converters (WEC) in complex terrain subject to atmospheric inflow, laminar-turbulent transition, generator cooling, hub aerodynamics, and passive flow control devices. Four different flow solvers (PALM, FLOWer, THETA, OpenFOAM) are in use during the course of the project depending on the corresponding problem requiring specific solver features. Key achievements of the project are the coupling of atmospheric LES (PALM) and URANS simulations of the complete WEC (FLOWer as well as THETA) in order to impose the external turbulent flow fields to the inflow of the WEC for physics resolved load simulations, numerical replication of complex flows over blades with vortex generators using OpenFOAM and efficiency-augmented pressure loss simulations on very complex industrial geometries of ENERCON's direct drive WEC generators for precise cooling analyses. Industrial validity of all methods, model and process developments were key objectives of this research project.

In the present study the actuator line method is used to simulate the wake of a wind turbine and ways to characterize its impact on the flight of a helicopter model through the wake are evaluated. The helicopter within the simulation setup is represented by a dynamic actuator line which was implemented into the flow solver FLOWer. In order to evaluate the loads on the helicopter the thrust force distribution is compared in different areas of the wake. For further improvement of the actuator line implementation different methods to evaluate the angle of attack are compared. This was examined as the angle of attack is a highly important parameter for the actuator line and helps to optimise the accuracy and performance of the actuator line method. As there is no experimental data available and the model of the helicopter comes with distinct simplifications the loads are compared to values without the wake of the wind turbine, thus in free flight.

In the present paper the actuator line method is compared with fully resolved wind turbine simulations in offshore and complex terrain applications. In such flow fields, which are characterized by non-homogeneous and unsteady velocity distributions in the rotor plane, unsteady aerodynamic effects are likely and it is unclear how these characterize the wake development and load behavior of the wind turbine. The wake properties and loads are therefore compared for the case of a 5 MW wind turbine operating in a typical maritime atmosphere and a 2.4 MW onshore turbine located at a complex terrain site downstream of an escarpment. It was found that the actuator line predicts the wake structure, wake deflection and wake deficit in good agreement with the fully resolved simulation. However, an overestimation of velocity fluctuations was observed.

This paper deals with the Delayed-Detached-Eddy-Simulations (DES) of a generic 2.4 MW wind turbine in a complex terrain site facing a turbulent atmospheric boundary layer. The boundary layer is generated based on measurement data derived at the complex terrain site. Further, the process of data preparation as well as the numerical setup are described. In the results the impact of complex terrain on the flow field is shown and estimations on the influence on the turbine performance are made. Afterwards, simulations of the turbine facing atmospheric inflow in flat and complex terrain are presented. An increase of loads resulting from a speed-up caused by the terrain as well as a clear change in the power spectrum of the turbine become visible in complex terrain compared to flat terrain. This finding is in agreement with the estimations derived previously. Moreover, the impact of inclined inflow caused by the local terrain slope can be seen in the load distribution vs. the azimuth angle, amongst others.