Search

Reliably predicting turbine/wake interactions in arrays of tidal stream turbines is paramount to reducing losses of energy yield by optimizing array layout at the design stage. This study focuses on the analysis of turbine performance and rotor wakes based on Navier-Stokes computational fluid dynamics, discussing modeling aspects associated with the method used for incorporating turbine hydrodynamics and turbulent flow effects in simulations. Important factors considered herein are a) the impact of laminar-to-turbulent transition of the blade boundary layers on rotor performance, and b) the sensitivity of the computed wake evolution on the turbine modeling approach. For these analyses, the results of rotor resolved and generalized actuator disk solutions are compared to flume tank measured data of a model turbine performance and wake. The overall agreement of all computed solutions and measured data is good. In the presented Reynoldsaveraged Navier-Stokes analyses, the rotor-resolved analysis predicts the measured rotor wake up to about four rotor diameters behind the turbine well, and seemingly better than the actuator disk model, as expected; thereafter, however, the rotor resolved simulation dissipates the wake more slowly than observed in the experiment, whereas the wake recovery rate predicted by the actuator disk analysis is closer to measured data. The cross-comparison of the geometry resolved analyses and measured data indicate that the blade boundary layers are likely to be fully turbulent, which has a significant detrimental impact on the turbine power. It is also found that the time-averaged solution of the time-dependent analysis and that of the steady flow analysis of the turbine resolved flow field, differ negligibly, as noted in other recent studies.

An increasing number of vertical-axis wind turbine prototypes have reached the step in which the theoretically predicted performance needs to be validated in order to move to the next steps of a real commercial project. This step often faces the significant challenges posed by their airfoil aerodynamics that are more complex than those of conventional horizontal-axis wind turbines, and it has also to deal with the lack of fundamental experimental data for robust validation. In this context, an accurate prediction of the real turbine operation is important and the use of computational fluid dynamics (CFD) is imposing itself as the most suitable tool to characterize the unsteady phenomena that are difficult to detect by means of experimental measurements. In the current work, two-dimensional numerical simulations of an H-type three-blade Darrieus turbine have been performed in a wide range of tip-speed ratios (TSRs) from TSR = 1.8 to TSR = 5.0. Unsteady CFD simulations were compared with unique experimental data collected in the field in terms of normal aerodynamic forces acing on the blades during the revolution. Generally, nice agreement was found between simulations and experiments, especially at medium-high tip-speed ratios. The influence of operating conditions on the performance prediction capability of the numerical model was also discussed. This is one of the key points of study since the lack of detailed experimental data often makes numerical analyses doubtful or scarcely effective. Finally, the simulation results were exploited in order to analyze the phenomena occurring during the revolution and to correlate them with the experimental findings.

Tidal stream turbines fixed on the seabed can harness the power of tides at locations where the bathymetry and/or coastal geography result in high kinetic energy levels of the flood and/or neap currents. In large turbine arrays, however, avoiding interactions between upstream turbine wakes and downstream turbine rotors may be hard or impossible, and, therefore, tidal array layouts have to be designed to minimize the power losses caused by these interactions. For the first time, using Navier-Stokes computational fluid dynamics simulations which model the turbines with generalized actuator disks, two sets of flume tank experiments of an isolated turbine and arrays of up to four turbines are analyzed in a thorough and comprehensive fashion to investigate these interactions and the power losses they induce. Very good agreement of simulations and experiments is found in most cases. The key novel finding of this study is the evidence that the flow acceleration between the wakes of two adjacent turbines can be exploited not only to increase the kinetic energy available to a turbine working further downstream in the accelerated flow corridor, but also to reduce the power losses of said turbine due to its rotor interaction with the wake produced by a fourth turbine further upstream. By making use of periodic array simulations, it is also found that there exists an optimal lateral spacing of the two adjacent turbines, which maximizes the power of the downstream turbine with respect to when the two adjacent turbines are absent or further apart. This is accomplished by trading off the amount of flow acceleration between the wakes of the lateral turbines, and the losses due to shear and mixing of the front turbine wake and the wakes of the two lateral turbines.