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Experiments with a three-bladed, constant chord tidal turbine were undertaken to understand the influence of free surface proximity on blockage effects and near-wake flow field. The turbine was placed at various depths as rotational speeds were varied; thrust and torque data were acquired through a submerged sensor. Blockage effects were quantified in terms of changes in power coefficient and were found to be dependent on tip speed ratio and free surface to blade tip clearance. Flow acceleration near turbine rotation plane was attributed to blockage offered by the rotor, wake, and free surface deformation. In addition, particle image velocimetry was carried out in the turbine near-wake using time- and phase-averaged techniques to understand the mechanism responsible for the variation of power coefficient with rotational speed and free surface proximity. Slower wake propagation for higher rotational velocities and increased asymmetry in the wake with increasing free surface proximity was observed. Improved performance at high rotational speed was attributed to enhanced wake blockage, and performance enhancement with free surface proximity was due to the additional blockage effects caused by the free surface deformation. Proper orthogonal decomposition analysis revealed a downward moving wake for the turbine placed in near free surface proximity. 

In light of the 2018 special report on climate change compiled by the United Nations, there is a renewed urgency to the rapid adoption of renewable energy technologies. A key roadblock to the large-scale/commercial conversion of tidal energy is the question concerning the operational efficiency of existing technologies in the non-homogeneous, turbulent and corrosive marine environment. A thorough understanding of the aforementioned aspects of full-scale deployment is vital in developing robust and cost-effective turbine designs and farm layouts. The current experimental work at Lehigh University aims to better the understanding of turbine performance and near-wake statistics in homogeneous and non-homogeneous turbulent flows, similar to actual marine conditions. A 1:20 laboratory scale tidal turbine model with a rotor diameter of 0.28m is used in the experiments and an active grid type turbulence generator, designed in-house, is employed to generate both homogeneous and non-homogeneous turbulent inflow conditions. To the knowledge of the authors, this is the first experimental study to explore the effects of non-homogeneous inflow turbulence on tidal turbines. From the data collected it was observed that the non-homogeneous inflow condition led to a considerable drop (15-20%) in the measured thrust coefficient. They also resulted in larger torque and thrust fluctuations on the rotor (~40% under the tested conditions). The effect of inflow non-homogeneity was evident in the asymmetric near-wake characteristics as well. Turbulence intensity and Reynolds stresses measured in the wake of the rotor were found to adapt quicker to inflow non-homogeneity than the wake velocity deficit and integral length scales. 

A yaw misalignment to the inflow for tidal current turbines are known to result in performance degradation and deflection of the downstream wake. A comprehensive analysis of the wake behavior under yaw is thus essential to provide insights to marine energy developers for optimizing farm layouts. A detailed understanding of wake deflection and propagation by a yawed turbine is crucial, as, with this knowledge, the wake can be steered away from the downstream turbine. Wake path can be ascertained by tracking the center of the wake and is expected to meander both horizontally and vertically. Several methods are used to determine the center of the wake, most common of which are Gaussian-like fit, Center of mass, and mean available specific power. The variability in these definitions acts as a source of uncertainty in evaluating the wake center at downstream locations. In this paper, we aim to discuss the various methods and evaluate the usefulness of each technique based on the fidelity of the data set that is available. To this effect, we will use results from a three-dimensional transient computational fluid dynamics analysis for a tidal turbine subjected to 0°, -15° and +15° yaw cases. Change in wake shape was observed for γ ≠ 0° yaw cases, where the wake adapts an elliptical shape as it propagates downstream. The center of the mass technique is considered to be the best center of wake estimation technique as it takes into account change in wake shape for yawed flows. 

In tidal streams and rivers, the flow of water can be at yaw to the turbine rotor plane causing performance degradation and a skewed downstream wake. The current study aims to quantify the performance variation and associated wake behavior caused by a tidal turbine operating in a yawed inflow environment. A three-dimensional computational fluid dynamics study was carried out using multiple reference frame approach using κ-ω SST turbulence model with curvature correction. The computations were validated by comparison with experimental results on a 1:20 scale prototype for a 0° yaw case performed in a laboratory flume. The simulations were performed using a three-bladed, constant chord, untwisted tidal turbine operating at uniform inflow. Yaw effects were observed for angles ranging from 5° to 15°. An increase in yaw over this range caused a power coefficient deficit of 26% and a thrust coefficient deficit of about 8% at a tip speed ratio of 5 that corresponds to the maximum power coefficient for the tested turbine. In addition, wake propagation was studied up to a downstream distance of ten rotor radius, and skewness in the wake, proportional to yaw angle was observed. At higher yaw angles, the flow around the turbine rotor was found to cushion the tip vortices, accelerating the interaction between the tip vortices and the skewed wake, thereby facilitating a faster wake recovery. The center of the wake was tracked using a center of mass technique. The center of wake analysis was used to better quantify the deviation of the wake with increasing yaw angle. It was observed that with an increase in yaw angle, the recovery distance moved closer to the rotor plane. The wake was noticed to meander around the turbine centerline with increasing downstream distance and slightly deviate towards the free surface above the turbine centerline, magnitude of which varied depending on yaw.