Experimental and numerical investigation of turbulence and wake effects in tidal turbine arrays

Abstract

The work presented in this thesis investigates the hydrodynamic interaction and resulting wake characteristics of multiple axially aligned and staggered array sections consisting of up to four horizontal axis tidal turbines (HATTs) through experimental and numerical modelling. Experiments with single and multiple devices in array (1-2-1) and in-line configurations with varying longitudinal spacing were conducted in a low ambient turbulence circulating water channel and long, medium turbulence intensity flume respectively using particle image velocimetry for centreline wake characterisation and flow field recordings. Longitudinal variations between two and three turbines aligned in the flume with turbulence intensity of 10% have shown small differences in the wake velocity recovery. The effect of varying longitudinal and transverse spacing of HATT devices on the array wake recovery within and downstream of the array was investigated by varying the position of the middle row of turbines and the transverse spacing between the second row turbines. Numerical simulation of corresponding configurations in the open-source CFD software OpenFOAM are conducted using Reynolds-averaged Navier Stokes models for turbulence closure, namely k - ω SST and Reynolds Stress Model and dynamic mesh capabilities to account for the rotation of each turbine. The numerical simulations are validated against experimental wake characteristics and used for further investigation of the flow field within tidal turbine arrays. Good agreement between numerical and experimental wake characteristics is shown for large parts of the wake domain with biggest differences in areas where the PIV measurements experienced difficulties due to high velocity shear and turbine structures affecting the laser sheet. The wake recovery in closely spaced arrays is strongly determined by the transverse spacing in the second row with close spacing leading to entrapment of large volumes of slow moving fluid in a combined wake of multiple turbines while wide transverse separation increases wake recovery between adjacent wakes through increased mixing with accelerated ambient flow. This effect is more pronounced for increased longitudinal spacing as more mixing occurs for the centreline wake before passing through the second row of turbines. Open source software for numerical simulation and image based flow field measurements were successfully applied for investigation of multiple tidal turbine wake effects and further investigations of the complex flow fields within tidal turbine arrays.