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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.