Modelling the aerodynamic response of a wind turbine blade passing in front of the tower

authored by: Wolf Gerrit Früh, Jörg Seume, Alejandro Gomez
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.
Organisation(s): Institute of Turbomachinery and Fluid Dynamics
External Organisation(s): Heriot-Watt University
Type: Conference contribution
Pages: 2955-2964
No. of pages: 10
Publication date: 2008
Publication status: Published
Peer reviewed: Yes
ASJC Scopus subject areas: Energy Engineering and Power Technology, Renewable Energy, Sustainability and the Environment
Sustainable Development Goals: SDG 7 - Affordable and Clean Energy