Abstract
This paper investigates the effect of surface waves on the hydrodynamic torque fluctuations of vertical axis tidal turbines. Wave-induced orbital velocities and accelerations are shown to generate periodic variations of the incoming flow, leading to significant unsteady torque loads. A reduced-order torque model inspired by Morison-type formulations is here developed to study wave-induced effects on vertical axis tidal turbines. The model is derived and expressed in terms of the instantaneous wave-modified tip-speed ratio and decomposes the torque response into three main parameters: a mean shift term, a linear sensitivity coefficient linking torque fluctuations to tip-speed ratio variations and an inertial coefficient proportional to the orbital acceleration. Phase-averaged analysis reveals the relative importance of velocity-driven and acceleration-driven mechanisms and the wave-induced torque variability is interpreted as a structured perturbation of the stationary performance curve. The results obtained under irregular waves demonstrate that the proposed Morison-type model is able to reproduce the wave-induced contribution to torque fluctuations with a limited number of parameters identified in monochromatic waves. The use of conservative coefficients derived from monochromatic wave tests appears as a promising strategy to bound torque fluctuations under a given sea state. Such an approach would provide a compact and operational modelling framework, positioned between purely linear spectral transfer-function methods and computationally intensive fully unsteady numerical models. This model offers practical perspectives for structural load assessment, performance optimisation and the development of control strategies for VATT operating in wave-exposed sites.