Experimental investigation of the upstream turbulent flow modifications in front of a scaled tidal turbine

Abstract

A 1:20 scaled three-bladed horizontal-axis tidal turbine is positioned in the wake flows of two wall-mounted obstacles: a square cylinder and a combination of a cube in front of a cylinder. The turbine's induction effects are examined with upstream flow measurements without and with the turbine operating at its optimal regime. In front of the turbine (at 0.07D upstream of the hub extremity, with D the turbine diameter), the mean axial and vertical velocities are modified by induction effects whatever the complexity of the flow is. In the other hand, the normal Reynolds tensor components are less impacted by the turbine's blockage. In the presence of an incoming uniform homogeneous Gaussian flow field, a mean axial velocity deficit of 30% is observed in front of the hub while this velocity deficit is around 10% at the rotor edge. Moreover, the turbine's blockage modifies the intermittency leading to non-Gaussian fluctuations. When low-frequency large-scale flow structures are embedded in the incoming turbulent flow, these large-scale flow structures are affected by the hub whereas the rotating blade's blockage effect is of minor contribution. In this case, the turbine does not affect significantly the intermittency.