Wake characteristics comparison between isolated and pair configurations of marine hydrokinetic crossflow turbines at low Reynolds numbers

Wake characteristics and power coefficient of laboratory scaled marine hydrokinetic cross flow turbines were studied both experimentally and numerically. Single and turbine pair configurations were experimentally tested in a 3.5 m long, 0.3 m wide and 0.15 m deep water channel facility. Each turbine was built with three straight NACA0012 blades with chord 2.54 cm and 6.828 cm diameter, corresponding to a solidity of 1.2. The Reynolds number associated with the velocity given by the water pump and blade chord oscillated near 7000. Empirical power measurement was obtained multiplying the average torque by average rotational velocity. These measurements were obtained with a magnetic hysteresis brake utilized as control system and a Hall effect sensor used as speed transducer, respectively. Wake velocity profile was obtained by image processing of Particle Image Velocimetry (PIV) measurements at different positions. The empirical results were contrasted with numerical computational fluid dynamics (CFD) simulations carried out with Salome and OpenFoam. The computational model solved the Unsteady Reynolds Averaged Navier-Stokes (URANS) equations in 2 dimensions using the turbulence models k-? Shear Stress Transport (SST) and Spallart-Allmaras (SA). The experimental and numerical results show a clear difference of power coefficient and wake shape for both turbine configurations. This influence of two nearby blades on the flow can be exploited to obtain higher ratios of power per land area, leading to an increase of the overall generation of a power plant by a careful arrangement.