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The characteristics of the incoming wind, especially the turbulence intensity (TI), greatly
impact the performance of a Vertical Axis Wind Turbine (VAWTs). Vertical axis wind turbines
are omnidirectional i.e. they can be directed in different directions and can be employed in
highly variable flows, which makes them a very suitable candidate for complex flow
environments like that of Urban landscapes. However, the impact turbulence has on the
performance of VAWT still needs a lot of investigation, particularly when it comes to
comparing a high turbulence intensity area like urban environments to a low turbulence
intensity open area [1], [2]. The aim of the study is to examine the influence of the Turbulence
Intensity on the aerodynamic behavior and the performance of a VAWT under these two
contrasting environmental conditions.
In order to do this, a series of Computational Fluid Dynamics (CFD) simulations were
performed that involved modelling a standard VAWT that was subjected to varying inflow
conditions. Turbulence intensities were varied to closely reflect a rural and urban environment.
The employed approach was a time-resolved, two-dimensional (2D) Unsteady Reynolds
Averaged Navier Stokes (URANS) model with a Shear Stress Transport (SST) k-ω turbulence
model. The reason behind choosing this model was its ability to capture the unsteady
aerodynamic phenomenon and its transient wake dynamics [3]. The key parameters that would
be analyzed are Torque, power coefficient, pressure distribution, magnitude of velocity, and
wake structure.
The results indicate a dual role of turbulence in the performance of a VAWT. Under high TI
conditions like those of an urban environment, turbulence promotes a higher degree of mixing
and enhanced local vorticity near the rotor region. As a result of that, we have a higher degree
of momentum transport, which acts to reenergize the boundary layer on the blades, thus
delaying flow separation. Thisin turn, leads to improved torque generation at a low wind speed.
This also contributes to a more effective startup behavior and reduces the turbine’s cut-in speed,
which is essential for urban environments where wind is intermittent and is interrupted by the
buildings, particularly skyscrapers, that act as obstructions. With an increase in turbulence
intensity, the velocity magnitude contours indicate a greater degree of vortex shedding and
coherent structures forming around the blade, which contributes to the turbulent mixing and
energy recovery within the wake.
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Contrary to that, in an open terrain, with a lower value of TI, the wind inflow remains more
stable and uniform. As a result of this, the aerodynamic loading is much more predictable and
the efficiency is much higher, specifically at rated wind speeds. The downside is the reduction
in the turbine’s ability to harness fluctuating gusts. We see an early separation of the flow
around the blade, particularly at lower rotational speeds. Wake recovery is also slower due to
reduced mixing.
The study thus concludes that a higher value of turbulence intensity can be beneficial for the
performance of a VAWT, especially in urban environments, as it enhances flow reattachment,
improves the starting behavior, and provides an increased power output under fluctuating
conditions. Despite these benefits, higher turbulence intensity results in increased unsteady
loading, which can impact the long-term structural integrity of the turbine. In contrast, the
deployment of VAWT in an open terrain offers higher efficiency and a smoother operation, but
may require a higher value of wind thresholds to initiate effective power generation.