This paper documents the predictive capability of rotating blade-resolved unsteady Reynolds averaged Navier-Stokes (URANS) and Improved Delayed Detached Eddy Simulation (IDDES) computations for tidal stream turbine performance and intermediate wake characteristics. Ansys/Fluent and OpenFOAM simulations are performed using mixed-cell, unstructured grids consisting of up to 11 million cells. The thrust, power and intermediate wake predictions compare reasonably well within 10% of the experimental data. For the wake predictions, OpenFOAM performs better than Ansys/Fluent, and IDDES better than URANS when the resolved turbulence is triggered. The primary limitation of the simulations is under prediction of the wake diffusion towards the turbine axis, which in return is related to the prediction of turbulence in the tip-vortex shear layer. The shear-layer involves anisotropic turbulent structures; thus, hybrid RANS/LES models, such as IDDES, are preferred over URANS. Unfortunately, IDDES fails to accurately predict the resolved turbulence in the near-wake region due to the modeled stress depletion issue.

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Thrust, power and intermediate wake predictions obtained using resolved rotating blade with sliding mesh simulations for a hydrokinetic turbine (HKT) are assessed using the open-source flow solver OpenFOAM. Single- and two-phase URANS and DES computations are performed for three-blade, 0.5m diameter (D) turbine mounted on a stanchion that intersects the free surface with a tip-speed ratio λ = 6.15. The thrust and power predictions compare within 5% of the experimental data. Results show that the thrust predictions are dominated by the pressure distribution on the blades, whereas the shear stress plays a significant role in the power predictions. The turbine performance showed unsteadiness with amplitudes around 3% of the mean, due to the disruption of the flow each time a blade passed in front of the stanchion. The wake recovery is primarily due to the growth of shear layers (originating from the blade tips) towards the turbine axis, which are primarily caused by the cross-plane turbulent velocity. The shear layer growth is enhanced by the turbulence produced by the stanchion. Predictions of the mean wake profile compared within 10% of the experimental data, which is significant improvement over previous Fluent predictions that showed large errors of 22%. The improved predictions in OpenFOAM is attributed to better turbulence predictions. Two-phase results show that the interaction between the wake and free-surface is initiated by the interaction of stanchion with the free-surface. The free-surface creates a blockage effect that accelerates the flow in the upper bypass region and enhances the wake recovery.

This study evaluates the capability of Navier–Stokes solvers in predicting forward and backward plunging breaking, including assessment of the effect of grid resolution, turbulence model, and VoF, CLSVoF interface models on predictions. For this purpose, 2D simulations are performed for four test cases: dam break, solitary wave run up on a slope, flow over a submerged bump, and solitary wave over a submerged rectangular obstacle. Plunging wave breaking involves high wave crest, plunger formation, and splash up, followed by second plunger, and chaotic water motions. Coarser grids reasonably predict the wave breaking features, but finer grids are required for accurate prediction of the splash up events. However, instabilities are triggered at the air–water interface (primarily for the air flow) on very fine grids, which induces surface peel-off or kinks and roll-up of the plunger tips. Reynolds averaged Navier–Stokes (RANS) turbulence models result in high eddy-viscosity in the air–water region which decays the fluid momentum and adversely affects the predictions. Both VoF and CLSVoF methods predict the large-scale plunging breaking characteristics well; however, they vary in the prediction of the finer details. The CLSVoF solver predicts the splash-up event and secondary plunger better than the VoF solver; however, the latter predicts the plunger shape better than the former for the solitary wave run-up on a slope case.