As the previous studies have made clear, a great deal of preliminary work has been conducted on scale-model turbines in uniform flow. However it is apparent that little in-depth experimental studies have been made in more realistic conditions. Therefore the aim of this this is to address this gap by completing a detailed study on turbine wake characteristics in a number of different flow conditions along with power and thrust measurements. Not only will these experiments provide physical insight, but they will represent a comprehensive benchmarking data set for comparison to numerical models. Using CFD, Mason-Jones (2010) calculated the power and thrust coef- ficients of a turbine, the one used in this thesis, with different numbers of blades and blade-pitch-angles and compared them to a limited experimental
profile, is produced in the UoL water-channel. Power and thrust coefficients of a three-bladed turbine with optimum blade-pitch-angle (6o) are taken in non-uniform and are compared to those in uniform flow.
The studies to date which have investigated the wakes of tidal stream turbines have mostly been concerned with the mean streamwise flow and turbulence intensity in single planes downstream of the turbine, with mea- surements confined to a centre-plane through the turbine or planes normal to the flow. In the experimental studies the number of locations at which the flow has been characterised has been fairly sparse or been concerned with the far-wake effects. The third objective is to produce a more detailed three-dimensional data set than has hitherto been reported. In this study the near-wake of a single turbine in uniform flow with a low TI is characterised by means of an ADV. Initially the near-wake velocities, at 5 different height up to 7D downstream, of a three-bladed turbine with optimum blade-pitch- angle are measured with upstream mean velocity of 0.9m/s. As a number of studies have been conducted at low Reynolds numbers such as Rose et al. (2011b), Stallard et al. (2013) and Chamorro et al. (2013), to investigate any Reynolds number effects, the near-wake of the turbine is measured with upstream mean velocities of 0.45m/s and 0.68m/s. These velocity measure- ments were taken through a centre-plane at a depth equal to the centre of the turbine and compared with the wake of the turbine at an upstream velocity
CHAPTER 1. INTRODUCTION 35 of 0.9m/s. Harrison et al. (2010) and Myers & Bahaj (2010) suggested that the thrust coefficient was the main influence on the wake structure. There- fore a further objective will be to investigate this hypothesis in section 3.3 by measuring the near-wake downstream of a two bladed-turbine with optimum (3o) and non-optimum (9o) blade-pitch-angles, therefore with differentλ,C
T, and CP.
After investigating the wake of the turbine in uniform flow conditions, another objective of this thesis is to investigate it in more complex flows. Although there have been some limited studies in more complex flows the conditions have not been well characterised and the wake measurements are sparse. Here, turbulence is generated in the channel, while keeping the mean flow uniform, by means of a upstream grid creating higher TI levels. The wake of the three-bladed turbine through a centre-plane is measured using ADV in this flow with higher TI. Additionally the velocities in the wake of the three-bladed turbine in non-uniform steady flow at five different heights up to 5D downstream is measured and compared with the wake in both uniform flow conditions.
Along with the work in this thesis, most studies investigating the flow field around a tidal turbine have used ADV (e.g. Maganga et al., 2009, Stallard et al., 2013 and Neary et al., 2013). Using this velocity measurement technique recently Khorsandi et al. (2012) found that some of the velocity fluctuations were overestimated compared to other measurement techniques. Therefore the final objective is to study the robustness of the use of ADV measurements to characterise the near-wake of a tidal turbine and in chapter 5 ADV measurements are presented with different probe orientations and compared to LDV measurements.
designed and built at Cardiff University (Mason-Jones, 2010), was tested and the experimental arrangement is detailed here.
2.1
Description of University of Liverpool Wa-
ter Channel
Testing was undertaken in the University of Liverpool high-speed re-circulating water-channel, a schematic of which is shown in figure 2.1. The channel uses a 75kW motor-driven axial-flow impeller to circulate 90,000 litres of water. The water flows into the working section which is 3.7m long by 1.4m wide and can provide a depth range of between 0.15m and 0.85m. This provides the possibility of a uniform velocity profile ranging between 0.03m/s and 6m/s. On leaving the impeller the flow is passed through the lower section, then up through brass honeycomb which straightens the flow. The flow is
CHAPTER 2. EXPERIMENTAL TECHNIQUE 37
Figure 2.1: Schematic of the University of Liverpool water channel
accelerated by the contraction into the working section providing uniform flow where all measured mean streamwise velocities agree to within 1%. The jet injection, which can be adjusted according to the mean flow speed, at the start of the working section adds flow to the surface to compensate for the velocity deficit at the free-surface which is caused by the boundary layer on the contraction wall. The splitter flap at the the end of the working section separates the topmost layer of the flow from the main flow, this is necessary to remove any air bubbles added to the flow in the working section, this splitter flap was kept constant throughout the testing at 76 on the scale. To reintroduce the separated flow there is another adjustable flap, set constant to 82 on the scale, further down the channel. More details of the high-speed water-channel can be found in Millward (2002).
The conditions under which the experimental tests were made were 0.8m
water depth with uniform velocity in the range 0.45 - 1.34m/swith an average turbulence intensity (TI) of 2% measured by an Acoustic Doppler Velocime-
Figure 2.2: Normalised blade profile
ter (ADV) throughout the channel. Where TI is defined in equation 1.8. The turbine was located with its centreline at a depth of 0.42m, giving a swept area blockage ratio of approximately 18% (Mason-Jones et al., 2012) and takes up 62% of the vertical water column.