by Cassandra Burgess
Computation Fluid Dynamic models are used to investigate the influence of rotating wind and tidal energy generator turbines on the surrounding environments. Johnson et al. (2014), compared current analytical and numerical models and experimental findings to a new computational fluid dynamic (CFD) model, and found that the CFD model agreed well with a simple conservation of momentum model, but did not closely match the experimental and numerical findings on reactions to the spinning turbines. This result was especially pronounced far from the turbine. The numerical and experimental findings predict much more turbulence downstream from a turbine, and larger changes in velocity.
This research is meant to serve as a benchmarking study to determine the accuracy of models and the best model to use in a variety of situations. However, this study is limited to a specific category of turbine. It assumes incompressible flow, implying that the fluids in question are moving at relatively slow speeds. This assumption is most likely to be valid for any wind or marine situation, as the upper limit of Mach 0.3 indicates an upper speed limit of 102 meters per second. Most of the systems considered are moving at speeds far below this. The model also assumes an open channel with no other obstacles disrupting flow. Finally, the discs that compose the turbines were modeled as infinitely thin and as having a constant load applied. This does not match the conditions of the real world.
The major differences between the models studied and the experimental data occurred in the definition of the changing momentum due to the turbine. In the models this was defined to be constant, and in only one direction. Because of this the model tended to predict a lower velocity and turbulence than the experimental data, particularly in the region near the turbine. The models also predicted that turbulence peaked further downstream than the experimental data. Based on these results, future analysis of turbine environmental impacts must take into account that the most common models underestimate the resulting turbulence. The difference in predicted turbulence and location of turbulence could have significant impacts on the optimal placement of turbines.
Johnson, B., Francis, J., Howe, J., Whitty, J., 2014. Computational Actuator Disc Models for Wind and Tidal Applications. Journal of Renewable Energy 2014.