Numerical modelling of cross-flow vertical-axis turbines
Close-packed contra-rotating vertical-axis turbines have potential advantages for wind and hydrokinetic power generation. It is believed that such turbines maximise the fraction of flow passage swept, allowing generation well above the Betz limit for rotors in channels (Salter & Taylor, 2007). The design concept of contra-rotation, following Buntine and Pullin (1989), is based on two vortices of opposite-sign cancelling each other out, and thus conditioning the flow through the turbine while lowering turbulent kinetic energy in the wake. To determine the feasibility of large-scale marine hydrokinetic application of such devices, a numerical model is required. This thesis describes the development of a numerical model of vertical-axis turbines with torque-controlled and contra-rotating systems using an actuator line model (ALM). This parallelised model, coupled with the open-source OpenFOAM computational fluid dynamics (CFD) code, is used to examine the characteristics of turbulent flow behind vertical-axis turbines. The numerical model is first validated against experimental measurements from a two-bladed H-type wind turbine, where the flow field containing the turbine is simulated by solving the unsteady Reynolds-averaged Navier-Stokes (URANS) equations with a k-ω SST turbulence model. Turbine loading is predicted, and the vorticity distribution is investigated in the vicinity of the turbine. Good overall agreement in terms of the amplitude and profile is obtained between numerical predictions and measured data on thrust coefficients. The rotor is demonstrably driven by the blade-generated lift, which is counteracted by the torque that accelerates the blades and turns the generator. The model captures important three-dimensional flow features that contribute to wake recovery behind a vertical-axis turbine, and enables predictions of the dynamic response of practical vertical-axis turbines to unsteady flow. Close-packed contra-rotating vertical-axis turbines are then implemented for a small-scale case involving four two-bladed wind turbines. The downstream wake evolution behind the turbines undergoes fast velocity recovery in the near-wake region behind the group of rotors. The lower level of turbulent kinetic energy in the wake, owing to the contra-rotation system, has a slight effect on the turbine performance of the present limited test cases in this thesis. However, it is still recommended to have further investigation to show the contributions to enhanced energy extraction in future research. The numerical model is also validated against experimental measurements from a three-bladed vertical-axis tidal turbine. A parameter study of a pitch control system is carried out to solve the problem of the dynamic stall at low tip speed ratios, and to examine the effect of active pitch on wake turbulence. Tip vortices caused by adjacent foils at different angles are assumed to be suppressed using the turbine spoked-ring wheel (Salter & Taylor, 2007).