INTRODUCTION
The wind turbine industry is growing significantly with the increasing
annual installation of wind turbines worldwide. Wind turbines are essential for the
transformation of wind energy into electrical power. Therefore, wind turbine
modelling is an important aspect of the continuous development of the wind
industry. Wind turbine modelling revolves around determining the aerodynamic
performance
of
a
turbine.
With aerodynamic
performance
determined,
manufacturers can improve component design and optimize the overall wind
turbine system.
Over the years, numerous numerical simulations have been developed to
study wind turbine behaviour. Computational Fluid Dynamics (CFD) has become a
popular tool for solving problems in fluid mechanics, including modelling wind
turbine behaviour. However, numerical simulations sometimes may not produce
accurate results due to the complexity of the problem. For this reason,
experimental measurements in wind tunnels remain vital to validate CFD
simulation results. This paper reviews the various CFD wind turbine modelling
approaches against wind tunnel measurements.
CFD WIND TURBINE MODELLING
CFD involves the mathematical modelling of turbulent fluid flow at different
scales using the Navier-Stokes equations. The equations are challenging to solve,
so simplifications are made, including the assumption of steady, in viscid flow. As
for wind turbines, CFD provides a way to evaluate the performance of a turbine's
blades and their interaction with the flow field.
Wind Turbine CFD Simulation
CFD modelling covers different methods, such as actuator disc and actuator line
models, which vary depending on the level of detail needed. Actuator disc models
are the most straightforward CFD models, providing simplified yet practical
information about a wind turbine. These models have a disc of negligible thickness
situated within the wind turbine's rotor plane, where flow is forced through the
blades. The models use theoretical results to provide a representation of the flow
field through the disk's thickness.
In contrast, actuator line models account for flow separation by modelling
individual blades considering surface roughness, the angle of attack, and wing
loading. Actuator line models are computationally more expensive than actuator
disc models, but they provide better accuracy.
VALIDATION OF CFD MODELS
Proper validation of the CFD models is essential to guarantee accuracy. CFD
models validation is done using experimental data obtained through wind-tunnel
testing or field measurements. Wind tunnel tests are the standard experimental test
method for measuring wind turbine aerodynamics. Wind tunnel experiments using
full-scale turbines are expensive, so researchers have employed non-dimensional
scaling to reduce costs. Scaling is essential in data analysis to ensure consistency
from small, low-speed environments to large, high-speed turbines.
The coefficient of power (Cp) is a fundamental measure of a wind turbine's
efficiency, and it serves as a basis for comparing experimental and CFD results. Cp
represents the ratio of the power extracted by the turbine to the total power of the
wind energy that flows through the rotor disk. Cp is defined as:
Cp = P / (0.5*rho*A*V^3)
Where:
P = power
rho = air density
A = rotor swept area
V = wind speed
COMPARISON OF CFD MODELS AND WIND TUNNEL MEASUREMENTS
There is no one-fit-all CFD model as different types of models are suitable
for different situations. However, comparisons of CFD simulations with
experimental data from wind tunnel testing are necessary to validate the models.
Various studies have compared CFD models with experimental wind tunnel data.
A numerical simulation of the 6MW NREL wind turbine by Bacaloni et al. (2020)
used both actuator disc and actuator line models. The study compared numerical
simulations of the Cp of the wind turbine with experimental data from a wind
tunnel test. Bacaloni et al. concluded that actuator line modelling gave better
results than actuator disc modelling in predicting the flow field's effects on power
output.
In another study, Perry et al. (2019) compared the performance of the NREL
5MW wind turbine using CFD models and experimental wind tunnel data. Three
different CFD models were tested: pure actuator line model, coupled actuator
line/actuator disc model, and the actuator/surface panel method. Perry et al. (2019)
found that the coupled actuator line/actuator disc model gave the most accurate
results compared to the previous two CFD models tested. Also, Perry et al.
concluded that CFD results showed little sensitivity to wind tunnel Reynolds
number.
Li et al. (2018) compared experimental data with numerical simulations of flow
velocity around a vertical axis wind turbine using actuator disc and line models for
different tip speed ratios (TSR). The study concluded that both CFD models agreed
well with experimental data, with actuator line models providing better results. The
study also concluded that the level of agreement between CFD simulations and
experimental data depends on the TSR. For TSRs < 1.0, the results agree well, but
for TSRs > 1.0, the CFD simulations deviated significantly from the experimental
results.
Conclusion
CFD simulations have become an essential tool for modelling wind turbine
behaviour. Different types of CFD models are suitable for different applications,
but the models require validation through wind tunnel measurements.
Experimental data has been used to validate many CFD models, and the results
have shown that some models have better accuracy than others. Actuator line
models have been found to provide better results than actuator disc models.
However, the level of agreement between CFD simulations and experimental data
can vary depending on the model and tip speed ratio. Therefore, it is essential to
choose a CFD model that not only considers the level of detail needed but also
produces accurate results.