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.
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This content will become publicly available on July 27, 2025
Relating Skewness and Fourier Harmonics in Low Reynolds Number Wake Flow
Simple, laminar wake flows require many Fourier modes to represent their dynamics, even though they are perfectly periodic with a single period. The spatial form of the Fourier modes alternate between having a maximum value in the center of the wake for odd harmonics and having a zero crossing in the center of the wake for even harmonics of the primary frequency. We demonstrate that the harmonic organization and the alternating shapes of the Fourier modes for simple wakes are direct results of the skewness of the wake, which changes sign at the center of the wake flow and also at different positions in the streamwise direction. Having a non-zero skewness guarantees that more than one Fourier mode is required to represent the dynamics, even for a perfectly periodic signal, and the spatial variation of the skewness explains the alternating structure of the Fourier modes’ shapes. We demonstrate these relationships through a one-dimensional analysis of how Fourier modes relate to skewness in a model problem and by examining the skewness and Fourier modes of a low Reynolds number flow past a flat plate at an angle of attack of 35 degrees.
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- Award ID(s):
- 2118209
- PAR ID:
- 10544976
- Publisher / Repository:
- American Institute of Aeronautics and Astronautics
- Date Published:
- ISBN:
- 978-1-62410-716-0
- Format(s):
- Medium: X
- Location:
- Las Vegas, Nevada
- Sponsoring Org:
- National Science Foundation
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