Literature Review
A review of the most relevant literature-to date is presented on work related to turbulent flow around a fixed model turbine blade. This includes looking into the most up-to-date experimental and analytical analysis carried out on this topic.
Experimental and Analytical Studies
This section highlights important findings carried out on previous projects looking into the effect of turbulent flow on tidal turbine blades, with respect to loads applied. Significant research into the field of turbulent flow measurement has been carried out.
In 2007, McCann [13] performed a parametric analysis on a 2MW horizontal axis, variable speed turbine to investigate the effect of turbulence intensity and wave state. He found that the blade root fatigue loads were sensitive to both. In 2008, McCann et al [22] used site specific data from EMEC to demonstrate the importance of understanding the nature of turbulence intensity and waves on blade loads. These two papers highlight the fact that turbulence intensity is a key parameter for characterising the inflow conditions, reiterating the importance of turbulence intensity within the project.
Milne et al’s 2010 paper [5] looked into dominant hydrodynamic drivers for tidal turbine blade loads of a 500kW horizontal axis turbine; studying sensitivity of blade loads to turbulence intensity, wave state, and other parameters. The analysis was performed numerically using the software Tidal Bladed by Garrad Hassan, which is currently undergoing validation.
The sensitivity analysis concluded that horizontal mean flow speed, longitudinal turbulence intensity, wave state and relative height of hub compared to channel depth are the dominant parameters influencing blade root fatigue and extreme loads. For fatigue loads, these parameters produce the largest increase in magnitude of loads in the high load, low cycle loading of the loading spectrum [5] [15]. The study also concluded that the mean velocity distribution has a small impact on the blade load lifetime damage [5].
In the same year, Milne et al [6] produced a parametric analysis investigating the sensitivity of the root out-of plane bending moment and turbine blade loads to turbulence intensity, the integral length scale. They also assessed the suitability of Von Kármán and Kaimal spectral models. This was carried out using the Tidal Bladed software, as used in [5]. As with previous analysis, it was found that the turbulence intensity was the primary fatigue load driver for horizontal-axis tidal turbines. It also concluded that no pronounced difference in the loads between simulations using the Von Kármán and Kaimal turbulence models were predicted. The image below shows the two modelling spectra for two length scale values.
In 2007, McCann [13] performed a parametric analysis on a 2MW horizontal axis, variable speed turbine to investigate the effect of turbulence intensity and wave state. He found that the blade root fatigue loads were sensitive to both. In 2008, McCann et al [22] used site specific data from EMEC to demonstrate the importance of understanding the nature of turbulence intensity and waves on blade loads. These two papers highlight the fact that turbulence intensity is a key parameter for characterising the inflow conditions, reiterating the importance of turbulence intensity within the project.
Milne et al’s 2010 paper [5] looked into dominant hydrodynamic drivers for tidal turbine blade loads of a 500kW horizontal axis turbine; studying sensitivity of blade loads to turbulence intensity, wave state, and other parameters. The analysis was performed numerically using the software Tidal Bladed by Garrad Hassan, which is currently undergoing validation.
The sensitivity analysis concluded that horizontal mean flow speed, longitudinal turbulence intensity, wave state and relative height of hub compared to channel depth are the dominant parameters influencing blade root fatigue and extreme loads. For fatigue loads, these parameters produce the largest increase in magnitude of loads in the high load, low cycle loading of the loading spectrum [5] [15]. The study also concluded that the mean velocity distribution has a small impact on the blade load lifetime damage [5].
In the same year, Milne et al [6] produced a parametric analysis investigating the sensitivity of the root out-of plane bending moment and turbine blade loads to turbulence intensity, the integral length scale. They also assessed the suitability of Von Kármán and Kaimal spectral models. This was carried out using the Tidal Bladed software, as used in [5]. As with previous analysis, it was found that the turbulence intensity was the primary fatigue load driver for horizontal-axis tidal turbines. It also concluded that no pronounced difference in the loads between simulations using the Von Kármán and Kaimal turbulence models were predicted. The image below shows the two modelling spectra for two length scale values.
Graph showing normalized auto-spectra, Von Kármán (blue), Kaimal (red) and =10m (solid), =30m (dash)
Picture Courtesy of Milne et al [6]
Picture Courtesy of Milne et al [6]
Finally in 2012, Faudot et al [23] produced a paper concerning the importance of developing models which could predict blade loads explicitly for tidal turbines; this was deemed important due to environmental differences relative to wind turbines, in terms of density, viscosity of fluid and flow perturbations [23]. The paper also looked into the relevance of including added mass of blades in a Blade Element Momentum theory (BEM) algorithm, for different sea states. The BEM was found to be accurate, both for mean loads and load variations [23]. During the research, a lab-scale model of a horizontal axis rotor was developed and analysed. To measure the wave loads, and therefore the loads acting on the blade, strain gauges were located at the root of the blade giving forces (Fx, Fy) and momentums. This paper justifies the use of strain gauges for force measurement for work associated in the topic of interest.