CFD analysis of unsteady hydrodynamic loading on Horizontal Axis Tidal Turbine (HATT) blades

• Main Author: Wang, Xue
• Published: University of Strathclyde 2015
• Subjects: 621.406
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.694571

Horizontal Axis Tidal Turbines (HATTs) can experience amplified, time varying hydrodynamic loads during operation due to dynamic stall. Elevated hydrodynamic loads impose high structural loads on turbine blades, thus appreciably shortening machine service life. An improved characterization of the unsteady hydrodynamic loads on tidal turbine blades is therefore necessary to enable more reliable predictions of their fatigue life and to avoid premature failures. This thesis reports on a Computational Fluid Dynamics (CFD) analysis of the unsteady blade loading of a scale-model HATT taking dynamic stall into account. Numerical simulations are performed both in two-dimensional (2-D) and three-dimensional (3-D) using the commercial CFD solver ANSYS Fluent. After a brief description of the theories and methods involved, the behaviour of flow at low Reynolds number around a NACA-0012 aerofoil pitching in a sinusoidal pattern that induces dynamic stall is studied firstly to validate the numerical method and the choice of turbulence models. Then full 3-D computations of a rotating scale-model HATT rotor are presented for steady and periodic unsteady inflow situations, respectively. The reliability of the 3-D numerical method is evaluated by comparing the blade loads, especially the out-of-plane blade-root bending moment (defined as being about an axis normal to the rotor axis), with measurement data obtained from experimental tests conducted at the University of Strathclyde’s Kelvin Hydrodynamics Laboratory towing tank. Analyses in the steady velocity study are documented for a broad range of rotor speeds and flow velocities. Furthermore, investigations of 3-D flow separation and scale effects on blade loads are also performed. The periodic unsteady velocity study aims to examine the out-of-plane blade-root bending moment response to harmonic axial motion, deemed representative of the free-stream velocity perturbations induced by the unsteady flow. Parametric tests on oscillatory frequencies and amplitudes are carried out in order to analyse the HATT blade hydrodynamic behaviour under different flow patterns. Detailed flow field data is analysed to understand 3-D dynamic stall from a modelling perspective. It is concluded that the results by the present study provide significant insights into the flow physics occurring around the HATT rotor blades under various flow conditions. The CFD method can be used for designing more advanced HATT rotors, it also can be used to fine tune the computationally faster lower order Blade Element Momentum (BEM) methods for parametric design studies where experimental data is not available, particularly at the challenging rotor operating conditions involving flow separation and dynamically varying hydrodynamic behaviours.