Papers by James McNaughton
An effective way of using computational fluid dynamics (CFD) to simulate flow about a rotating de... more An effective way of using computational fluid dynamics (CFD) to simulate flow about a rotating device-for example, a wind or marine turbine-is to embed a rotating region of cells inside a larger, stationary domain, with a sliding interface between. This paper describes a simple but effective method for implementing this as an internal Dirichlet boundary condition, with interfacial values obtained by interpolation from halo nodes. The method is tested in two finite-volume codes: one using block-structured meshes and the other unstructured meshes. Validation is performed for flow around simple, isolated, rotating shapes (cylinder, sphere and cube), comparing, where possible, with experiment and the alternative CFD approach of fixed grid with moving walls. Flow variables are shown to vary smoothly across the sliding interface. Simulations of a tidal-stream turbine, including both rotor and support, are then performed and compared with towing-tank experiments. Comparison between CFD and experiment is made for thrust and power coefficients as a function of tip-speed ratio (TSR) using Reynolds-averaged Navier-Stokes turbulence models and large-eddy simulation (LES). Performance of most models is good near the optimal TSR, but simulations underestimate mean thrust and power coefficients in off-design conditions, with the standard k-" turbulence model performing noticeably worse than shear stress transport k-! and Reynolds-stress-transport closures. LES gave good predictions of mean load coefficients and vital information about wake structures but at substantial computational cost. Grid-sensitivity studies suggest that Reynolds-averaged Navier-Stokes models give acceptable predictions of mean power and thrust coefficients on a single device using a mesh of about 4 million cells.
identify the quality of the predictions. Finally, detailed modelling of the turbulence and veloci... more identify the quality of the predictions. Finally, detailed modelling of the turbulence and velocity in the near and far wake is presented. The SST and LRR models are able to identify tip vortex structures and effects of the mast as opposed to the standard − model.
This paper presents the predicted effect of realistic velocity profiles on a 1MW tidal stream tur... more This paper presents the predicted effect of realistic velocity profiles on a 1MW tidal stream turbine (TST) using computational fluid dynamics (CFD). A baseline study using a uniform onset velocity and low turbulence intensity is first performed. The effects of realistic flow conditions typical of Flood and Ebb tides are then modelled using velocity profiles based on field measurements recorded at the EMEC test site. The effect of these different flow conditions are assessed through instantaneous and mean quantities of power and thrust coefficients. Both coefficients are related to average velocity across the turbine's swept area, which is 2% and 3% greater than the uniform flow for the flood and ebb profiles respectively. An effect of the velocity profiles is that the power coefficient is increased, by 1% and 2.7% for the flood and ebb velocity profiles respectively. To assess the influence of the mast and depth-varying velocity on individual blades the instantaneous loading is investigated. For the baseline uniform flow study the mast causes some variation in the instantaneous blade loads over a range that is 4% of the mean. Increased levels of turbulence are shown to have little effect on the mean power and thrust coefficients whilst causing the immediate wake to recover much more rapidly.
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Papers by James McNaughton