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THE EFFECT OF TURBULENT FLOW ON WIND TURBINE LOADING AND PERFORMANCE

Mahmoodilari, Mahyar

[Thesis]. Manchester, UK: The University of Manchester; 2012.

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Abstract

Wind turbines are widely used for electricity generation. Typically turbines aredeployed in farms located either on-shore or off-shore. In these arrangementsthe flow onto a turbine may be turbulent due to the disruption caused by turbineslocated further upwind. At onshore locations, turbines are typically smallerbut will often be located downwind of structures or terrain which will causethe incident flow to be turbulent. Although wind turbines have been employedcommercially for several decades, design tools are based on assumptions of quasi-steadyflow and the effect of turbulence on turbine performance is not fully understood.In this study the effects of turbulent flow on wind turbine loadingand performance were investigated by means of some sophisticated experimentalmethods in conjunction with numerical predictions. With this intention, theatmospheric boundary layer was simulated using conventional methods withinthe wind tunnel in the University of Manchester. The characteristics of the flowwere established using cross hot-wire anemometry. The maximum thickness forthe simulated atmospheric boundary layer that was produced by an arrangementof a combination of vortex generators, a barrier wall and a group of cubeswas found to be over 0.7m. This combination sustained the turbulence intensityto between 3% and 23% and the turbulence length scale between 150mmand 210mm for the downstream flow. Meanwhile, the grid turbulence generatorproduced a turbulent flow at a cross section a distance of five mesh sizes downstream,with 16% turbulent intensity and with 35mm turbulent length scale acrossthe entire cross section. These flow fields were experienced by a designed 2-Dfoil (chord=60mm,span=400mm,40000<Re<75000) with the profile NACA4705alongside a reference flow with no upstream element. These flow conditions wereemployed to quantify the effect of turbulence characteristics on lift and drag coefficientsof the aerofoil prior to implementing the test case in a rotating frame.In addition, numerical simulations were conducted in order to corroborate theresults obtained in the 2-D experiment. Further to this, the experiments werecarried out on a rotating frame to observe how the turbulent characteristics ofthe flow might alter the performance of the miniature wind turbine. The bladeReynolds number in the rotor experiments is less than 105 and so considerablyreduced from the Reynolds of a full-scale wind turbine. However, since theboundary layer is turbulent the effect of onset turbulence is expected to be representative.The turbine performance was then supported by implementing theBlade Element Momentum theory in the MATLAB environment. In conclusion,the results confirmed that the unsteadiness in the upstream flow associated withthe high level of turbulent intensity can enhance the power coefficient of the turbineas a result of increasing the ratio of lift over drag coefficients. However, thelarge turbulent length scale can substantially diminish the power coefficient.