Analysis of the momentum recovery in the wake of aligned axial-flow hydrokinetic turbines

Abstract

Large-Eddy Simulations are reported, dealing with an axial-flow hydrokinetic turbine operating in the wake of an upstream one. Computations were conducted on a cylindrical grid consisting of 3.8 × 109 points, using an Immersed-Boundary methodology. The performance of the downstream turbine was negatively affected by the wake of the upstream one and substantially dependent on its distance. Results demonstrated a faster wake development, compared to the case of the same turbine operating in isolated conditions within a uniform flow, due to the faster instability of the tip vortices, induced by the perturbation of the inflow conditions by the wake of the upstream turbine. In contrast with the turbine performance, the process of wake recovery was found rather insensitive to the distance from the upstream turbine. In comparison with the case of the isolated turbine, the role of radial turbulent transport just downstream of the instability of the tip vortices was found especially important in accelerating the process of wake recovery at the outer radii, providing a significant contribution together with radial advection. Further downstream, the contribution by turbulent transport was verified reinforced also within the wake core, where instead momentum replenishment by radial advection was rather limited.