Effects of the spacing between two hydrokinetic turbines on the bedforms by numerical simulations

Abstract

Accelerating hydrokinetic renewable energy development towards endurance requires investigating interactions between the hydrokinetic turbine and its surrounding physical environment. Interactions between hydrokinetic turbines (HT) and mobile sediment bed are considered as a critical area of assessment, however limited research studies have been published to address this issue. Hill et al. (2015) have shown experimentally that the presence of either single or multiple turbines and the rotation of the blades affect the bed morphology. Musa et al. (2019) have investigated experimentally the local effect of streamwise aligned turbines on the bedload, they found as a result that the geomorphic effects are stronger with increasing shear stress due to the presence of the rotors, inducing an alternating scour-deposition phenomenon. Chen et al. (2017) have investigated the influence of rotor blade tip clearance (distance between blades and seabed) on the scour rate of pile-supported horizontal axis current turbine. The results suggest that the decrease in tip clearance increases the scour depth, hence more sediment transport. Recently, Khaled et al. (2021) have studied the impact of hydrokinetic turbine on erodible sand banks, they showed numerically a significant interaction between the confinement of the turbine and its impact on the near bottom.

In the present issue, we study the impact of two interacted turbines on the near bedform morphology. A modelling framework is derived to predict the significant transport induced by the turbines, such as the Euler-Euler (EE) multiphase model for sediment transport and the Blade Element Method (BEM) to model the forces generated by the turbines, using the open source platform OpenFOAM and the library SedFoam (Chauchat et al. 2017). A phase of validation is presented for the combined model (EE and BEM) using experimental results of Hill et al. (2015). The present study consists in considering one sediment class, sand of diameter of 0.25 mm, and two horizontal axis turbines with an axial flow direction corresponding to the riverine case. The approach is configured with four different axial inter-turbines distances. The wake distribution behind the second turbine is altered by the wake of the upstream turbine in all configurations (fig. 2). This interaction promotes the erosion under the turbines and the deposition along the axis of the turbines in the wake due to relation between the dynamics of ripples generation and the wake effects of turbines (fig. 3).