Investigation into wind flow over complex topography and escarpments for wind turbine siting

Flow over cliffs is examined in a wind tunnel to determine how to improve wind turbine micro-siting. The research builds on existing research into flow over forward facing steps. It highlights the role of turbulence intensity in controlling the size of the mean recirculation bubble. Complexity is added to the forward facing step geometry by considering the effect of wind direction. The natural ruggedness associated with cliffs is then investigated, first by applying a sawtooth lateral variation to the crest of the forward facing step, and then by applying a sinusoidal lateral variation. <br> <br> In considering wind direction, flow visualisation and pressure probe measurements demonstrate the presence of the separation region at the crest of the step, as well as the ejection of vortices from the separation region. Such features could adversely affect wind turbine performance. Based on the flow structure that was observed, it was recommended that the optimal site for a wind turbine should be half a step height downstream of the step, with the blades passing no lower than half a step height above the surface. Such a location would subject the wind turbines to maximum wind speeds, whilst minimising fluctuating loads and unbalanced loads across the wind turbine rotor. <br> <br> Considering ruggedness, the amplitude to wavelength ratio of the sawtooth and sinusoidal lateral variations is varied. The development of the flow topology is observed using surface shear stress visualisation, pressure probe measurements and surface pressure tap measurements. The sawtooth cases are in good agreement with computational work performed. Delta wing style vortices were observed in all the cases where ruggedness was modelled. Sawtooth cases exhibit topological development as the amplitude to wavelength ratio is increased. With corresponding increases in amplitude to wavelength ratio, the flow topology in the sinusoidal cases remains constant. It was concluded that the curvature of the crest in the sinusoidal cases controls the development of secondary vortex structures that are induced by the delta wing vortices, keeping the secondary structures from increasing in size, thus causing the flow topology to remain equivalent as a function of the amplitude to wavelength ratio of the sinusoid. <br> <br> Pressure probe measurements indicate that the vortex structures remains close to the surface over the rugged protrusions, but increase significantly in height with distance downstream, indicating that if wind turbines are to be sited in the vicinity of rugged cliffs, they ought to be sited on the centre of the protrusions, with the wind turbine rotors passing above a minimum height of half a step height