## Abstract

It is recognised that even upwind turbines feel the presence of the supporting tower in a pulse of the output known as 3p oscillations. Here we investigate the aerodynamics of this phenomenon by modelling the upwind perturbation of the free wind through the tower as a time-dependent boundary condition on a Computational Fluid Dynamics (CFD) model of a turbine blade section.

To model the origin of the 3p oscillation, the perturbation of the free wind upstream of a cylindrical tower is modelled as the ideal flow solution of the flow around a cylinder. In the second step of the analysis, the local relative velocity field around a turbine blade section is found as a function of the free wind, the tower dimensions, the blade overhang, and the time-dependent position of the section in that flow as the blade rotates around the hub. The resulting velocities and pressure variations are then used as time dependent boundary conditions of a 2D-CFD model using the software package NUMECA based on the S809 aerofoil section. Two basic turbulence models were tested, the one-equation Spalart-Allmaras model and the two-equation k- model.

The analysis of the perturbation flow field shows that the tower shadow not only causes a short pulse of the effective velocity but also an even sharper change of the effective angle of attack of around 10%. The results suggest that the overall effect is a moderate reduction of the time-averaged torque coefficient compared to the simple flow over the aerofoil section. Over the range of wind speeds investigated, the magnitude of this effect appears to be largely unaffected by the distance between the blade and the tower. However, at high effective angles of attack, the results indicate that the onset of stall is delayed if the distance between the blade and the tower is stall. It appears that the increasing pulse caused by the passing in front of the tower acts as a mechanism to re-attach the flow which would be fully detached in the free case.

To model the origin of the 3p oscillation, the perturbation of the free wind upstream of a cylindrical tower is modelled as the ideal flow solution of the flow around a cylinder. In the second step of the analysis, the local relative velocity field around a turbine blade section is found as a function of the free wind, the tower dimensions, the blade overhang, and the time-dependent position of the section in that flow as the blade rotates around the hub. The resulting velocities and pressure variations are then used as time dependent boundary conditions of a 2D-CFD model using the software package NUMECA based on the S809 aerofoil section. Two basic turbulence models were tested, the one-equation Spalart-Allmaras model and the two-equation k- model.

The analysis of the perturbation flow field shows that the tower shadow not only causes a short pulse of the effective velocity but also an even sharper change of the effective angle of attack of around 10%. The results suggest that the overall effect is a moderate reduction of the time-averaged torque coefficient compared to the simple flow over the aerofoil section. Over the range of wind speeds investigated, the magnitude of this effect appears to be largely unaffected by the distance between the blade and the tower. However, at high effective angles of attack, the results indicate that the onset of stall is delayed if the distance between the blade and the tower is stall. It appears that the increasing pulse caused by the passing in front of the tower acts as a mechanism to re-attach the flow which would be fully detached in the free case.

Original language | English |
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Publication status | Published - 2008 |

Event | European Wind Energy Conference 2008 - Brussels, Belgium Duration: 31 Mar 2008 → 3 Apr 2008 |

### Conference

Conference | European Wind Energy Conference 2008 |
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Country/Territory | Belgium |

City | Brussels |

Period | 31/03/08 → 3/04/08 |