Modelling unsteady hydrodynamic gust loading on tidal turbine blades

Abstract

This study investigates the blade loads on a model tidal turbine subject to unsteady gust forcing in the form of uniform small-amplitude oscillations in the axial inflow velocity. The validity of industry-standard 2D strip-theory models for calculating unsteady hydrodynamic loading on 3D rotor geometries is evaluated by comparing the 2D results to 3D simulations, both Reynolds-Averaged Navier Stokes (RANS) simulations and 3D inviscid vortex lattice modelling (VLM). The results show that the 2D function captures neither the trends nor the magnitudes of the unsteady turbine loads, which exceed the quasi-steady loads. The inviscid VLM corresponds more closely to unsteady RANS simulations, suggesting that 3D wake effects are a primary driver of the unsteady loads. A key non-dimensional parameter determining the unsteady load magnitudes is identified as the ratio of gust frequency to blade passing frequency. Finally, it is demonstrated that applying conventional tip-loss corrections to 2D unsteady hydrodynamic load models can in some circumstances lead to severely under-predicted blade loads. These outcomes have implications for the evaluation of peak and lifetime loads on tidal devices, and for any rotor application which relies on 2D strip-theory methods for unsteady load evaluation.