posted on 2017-01-11, 05:35authored byJerome Rowcroft
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.
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.
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.
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.