ABSTRACT

The present work deals with the assessment of variable fidelity simulation models to analyze the complex phenomenology of a turbine operating in a tidal stream. In the framework of an Unsteady Loading Tidal Turbine Benchmarking Study by Supergen ORE HUB [Tucker Harvey et al.(2021)], performances of an Horizontal Axis Tidal Turbine (HATT) are extensively analised. A general purpose finite volume solver based on the solution of the unsteady Navier-Stokes equations for multi-block structured grids is considered [Dubbioso et al.(2019), Gregori et al.(2020)]. The turbine has been simulated over a wide range of tip-speed ratios, by means of unsteady RANS simulations, in the frame of reference fixed to the rotating turbine, in uniform onset flow and calm water conditions. Since the work is also focused on the assessment of design-oriented hydrodynamics models, cross-validation studies between URANS and BIEM (Boundary Integral Equation Model) results are presented. The wake-field by URANS is presented and analised with the aim to validate results by lower fidelity models. The wake-field has been processed and geometrical parameters of the trailing wake geometry were derived, allowing, in principle, the improvement of simplified wake models as those adopted by the BIEM.

INTRODUCTION

The exploitation of the kinetic energy of marine currents has been making great progress in recent years, and represents one of the most promising technology in the marine renewables sector, partly driven by the developers' experience of the accelerating capability by the application of Computational hydrodynamics models in the turbine design process.

The variety of computational models depends mainly on the assumptions, that influence the nominal accuracy of the model but also its computational burden. Depending on the accuracy level, models are considered as high-fidelity and low-fidelity models. The terminology ‘Computational Fluid Dynamics’ (CFD) usually denotes high-fidelity models, based on the solution of the Navier-Stokes equations, such as Direct Numerical Simulation (DNS, [Nakhchi et al.(2022)]), Large Eddy Simulation (LES) or Reynolds Averaged Navier Stokes (RANS) ([Afgan et al.(2013)]), with different levels of accuracy (time and spatial scales resolved) and, according to it, of different computational burden. These CFD solvers are characterized by the need to provide a computational mesh of the fluid region of interest. By neglecting viscosity effects, the governing equations are simplified and lower-fidelity potential flow models are formulated. A classical computational modelling approach is based on the Boundary Integral Equation Method (BIEM), with the problem solved at the solid boundaries of the fluid region, [Baltazar (2015)]. When the flow is treated according to the momentum theory, loads exerted by lifting bodies' sections are evaluated on the basis of the local flow-field. Numerical methods are indicated as ‘Blade Element Momentum Theory’ (BEMT or BEM, [Allsop et al.(2016)]) and are based on the a-priori knowledge of the aerodynamic characteristics of the sections.

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