On the Variation of Turbulence in a High-Velocity Tidal Channel

Charles Greenwood,Vengatesan Venugopal,Arne Vogler

doi:10.3390/en12040672

Charles Greenwood, Vengatesan Venugopal + Show 1 more

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https://doi.org/10.3390/en12040672

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Abstract

This study presents the variation in turbulence parameters derived from site measurements at a tidal energy test site. Measurements were made towards the southern end of the European Marine Energy Centre’s tidal energy test site at the Fall of Warness (Orkney, Scotland). Four bottom mounted divergent-beam Acoustic Doppler Current Profilers (ADCPs) were deployed at three locations over an area of 2 km by 1.4 km to assess the spatial and temporal variation in turbulence in the southern entrance to the channel. During the measurement campaign, average flood velocities of 2 ms−1 were recorded with maximum flow speeds of 3 ms−1 in the absence of significant wave activity. The velocity fluctuations and turbulence parameters show the presence of large turbulent structures at each location. The easternmost profiler located in the wake of a nearby headland during ebb tide, recorded flow shielding effects that reduced velocities to almost zero and produced large turbulence intensities. The depth-dependent analysis of turbulence parameters reveals large velocity variations with complex profiles that do not follow the standard smooth shear profile. Furthermore, turbulence parameters based on data collected from ADCPs deployed in a multi-carrier frame at the same location and time period, show significant differences. This shows a large sensitivity to the make and model of ADCPs with regards to turbulence. Turbulence integral length scales were calculated, and show eddies exceeding 30 m in size. Direct comparison of the length scales derived from the streamwise velocity component and along-beam velocities show very similar magnitudes and distributions with tidal phase.

Highlights

As tides pass through narrow channels and around headlands, high-velocity flows are produced.Depending on site-specific characteristics, the turbulence of these flows varies in time and space.When these turbulent flows encounter a tidal stream turbine (TST), the severity of turbulence affects the turbine energy extraction capability by altering blade performance
The streamwise velocity vector for all sensors shows a periodic fluctuation as a result of the flood and ebb tide, where one complete tidal cycle is composed of two neighbouring peaks in the streamwise velocity
This study investigates the spatial and temporal differences in turbulence in the southern entry to the Fall of Warness tidal energy test site in Orkney

Summary

Introduction

Depending on site-specific characteristics, the turbulence of these flows varies in time and space. When these turbulent flows encounter a tidal stream turbine (TST), the severity of turbulence affects the turbine energy extraction capability by altering blade performance. This results in reduced rotor thrust and torque [1], which causes a detriment in turbine performance of over 10%. Prolonged exposure to high turbulent flows increases fatigue load cycles and blade bending [2], thereby reducing the expected lifespan of a turbine. The quantification of turbulence remains an important factor in the optimization of energy extraction and turbine durability and successful tidal stream developments

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Discussion

Conclusion