Pascal Weihing

CFD Studies on Wind Turbines in Complex Terrain under Atmospheric Inflow Conditions

This article presents various detached eddy simulation (DES) results of a commercial wind turbine under multiple inflow conditions in complex and flat terrain. Challenges regarding the meshing process of wind turbines in complex terrain are described and an approach to overcome those is presented. The main focus of the evaluation is blade load and power response to inflow turbulence and terrain effects, e.g. the change of the inclination angle or the speed-up due to a hill. To separate the different influences, the complexity of the simulation setup is increased stepwise. Starting with a baseline simulation in flat terrain and uniform inflow over adding atmospheric turbulence to a complex terrain simulation of a fully meshed rotating 3D wind turbine under atmospheric inflow.

Investigations of the inflow turbulence effect on rotational augmentation by means of CFD

The present studies are addressed to gain more insights into the inflow turbulence effect on rotational augmentation using computational fluid dynamics. Three different cases were simulated and analysed focusing on the three-dimensional effects in the inboard blade region of a 10 MW generic wind turbine rotor. The evidence of rotational augmentation was presented and compared to two-dimensional simulations of the blade sections at consistent inflow conditions. Inflow turbulence has a very strong impact on the instantaneous blade loads and standard deviations, but the effect on the mean values is small. The amplitudes of the blade load fluctuations are amplified under turbulent inflow conditions and these are related to the blade passing frequency and the specified turbulence length scale at the inlet. Detailed examinations of these phenomena were performed and are presented in the present manuscript.

Effects of ambient turbulence on the near wake of a wind turbine

Developments of the near wake behind the Avatar research turbine (radius of 102.88 m) in ambient turbulence are investigated using high fidelity numerical simulations. A moderate level of background turbulence with a wide range of scales, which has not been considered in the previous studies is applied. With ambient turbulence, a significant impact on the near wake development is observed. The mean velocity profile becomes Gaussian after 450 m distance downstream, which is a demarcation between the near and the far wake. From the spectral analysis of the wake, clear peaks in the spectra are observed at the blade passing frequency, but the distributions of the peak extend into a wide range of frequency domain. Such aspects provide useful information in classifying periodic and stochastic fluctuations, and their contributions to the momentum mixing in the wake.

Hybrid RANS/LES Capabilities of the Flow Solver FLOWer—Application to Flow Around Wind Turbines

The compressible block-structured flow solver FLOWer of the German Aerospace Center (DLR) has been extended towards state of the art detached-eddy simulation (DES) methods in order to conduct hybrid RANS/LES simulations of flow around rotary wings. The large-eddy simulation (LES) capabilities of the code are demonstrated for decaying isotropic turbulence. Excessive numerical dissipation is avoided by using the fifth-order WENO scheme and an appropriate low-Mach number correction. The DES implementations are validated first for the well documented test cases backward facing step and NACA0021 airfoil at \(60^\circ \) incidence, before increasing the complexity by simulating the flow around the MEXICO model wind turbine operating in stalled conditions and comparing with experimental data. From the latter, recommendations on the numerical settings are derived to successfully set up eddy resolving simulations for wind turbine or helicopter applications.

Evaluation and Control of Loads on Wind Turbines under Different Operating Conditions by Means of CFD

This article presents results from Computational Fluid Dynamics (CFD) simulations of wind turbines performed within the project WEALoads. The project is devoted to the unsteady load response of wind turbines under realistic environmental conditions and to mechanisms to control these loads. An extract of the latest research done on these topics is presented in this article. Based on previous studies of wind turbines in complex terrain done by the authors the effects of the terrain on the load response is tried to break down to several single events. Some of these are the shear of the wind profile, turbulence intensity of the inflow, inclined inflow or yawed inflow. In this article the effect of yawed inflow on the loads and aerodynamics of a wind turbine is described in further detail. The second part of the studies is dedicated to the control of wind turbine loads which is one of the key factors in the current turbine design processes. An example to reduce load fluctuations caused by tower blockage is shown. This can be seen as a prove of concept for the method applied. All of the simulations were performed using the flow solver FLOWer from DLR (German Aerospace Center). For the studies of the first part a Detached Eddy Simulations (DES) approach was used whereas for the second part URANS methods were applied. Afterwards, a newly implemented DES method giving a more realistic prediction of the flow field around the turbine and consequently the loads is described in more detail. The general outcome of the article is that a load reduction under yawed inflow can be observed as well as a wake deflection which is of high importance in case of wind park development and control. Moreover, a bridge between the yaw results, the load reduction method and the new high fidelity DES methods is build to give an outlook to future works.

CFD Performance Analyses of Wind Turbines Operating in Complex Environments

This paper presents results from CFD simulations of wind turbines performed within the project WEALoads. The focus of this project is devoted to the unsteady load response of wind turbines under realistic environmental conditions, as for example operation inside of a wind farm or in complex terrain which are both subject of this paper. The first case shall investigate the behavior of a wind turbine operating half in the wake of an upstream turbine, in order to derive the dominant interference effects between wind turbines. Secondly, a wind turbine shall be analyzed which is sited on a hill to elaborate the main effects arising from the interaction of the atmospheric boundary layer with the hill and finally the wind turbine. Both simulations were performed using the flow solver FLOWer from DLR (German Aerospace Center) and the Detached Eddy Simulations (DES) approach. Results of the flow fields are shown in terms of wake development, as well as turbulence intensity. Regarding the case of the turbine sited in complex terrain, a site assessment study has been performed, in order to find designated positions where maximum power output of the wind turbine can be expected. Finally, for both cases, blade load evaluations showed significant influence of the operating environment. For the case of the interacting turbines the load response of the shadowed turbine showed a massively asymmetric loading of the entire rotor. For the turbine located on the hill, significant augmentation of the entire load level could be observed.

Hybrid RANS/LES Simulations of the Three-Dimensional Flow at Root Region of a 10 MW Wind Turbine Rotor

Numerical computations using the Unsteady Reynolds Averaged Navier-Stokes (URANS) and Delayed Detached-Eddy Simulations (DDES) approaches are carried out to investigate the complex three-dimensional flow in the root region of a generic 10 MW wind turbine rotor. Preliminary studies regarding the time step size and the number of rotor revolution required for the time averaging procedure are conducted. In the blade outer region, URANS is sufficient to predict the general flow characteristics, but small discrepancies are observed in the blade root area where the flow is massively separated.

Results of the research project AssiSt

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 ENERCONs direct drive WEC generators for precise cooling analyses. Industrial validity of all methods, model and process developments were key objectives of this research project.