The Two-Fluid Method is capable of modeling large-scale (i.e., lab scale or larger) multiphase (particle-fluid) flows by treating both the fluid and particle phase as interpenetrating continua and solving mass and momentum balances for each phase. To solve for the flow of the solids phase the momentum balance requires constitutive relations in the form of normal and shear stresses ± i.e., pressures and viscosities. However, the stresses that account for frictional contacts in dense particle systems, and are relevant to this work, are empirically based. A study of the effects of adjusting the frictional model formulation (the empirical parameters of the model), by changing the overall frictional stress magnitude and the relative magnitude of the frictional viscosity to the frictional pressure, on the behavior of the bed is presented here. It was found that the magnitude of the frictional viscosity relative to the frictional pressure affects the crater growth prediction almost as much as the magnitude of the overall frictional stress. Additionally, a frictional model formulation is validated for sand particles, and predictions are compared with existing experimental data for the crater formation of a sand bed under a vertical, impinging jet of gas (Metzger et al. J Areo Eng (2008) v22, p24-32). In the low jet velocity regime (subsonic, turbulent jet), the model predicts the salient features previously measured for the growth rate of the crater of time, the profile of the crater, and the response of the crater to turning the jet off. In the high jet velocity regime (compressible, near sonic jet flow) the prediction agrees qualitatively with prior experimental observations.
This paper presents computational fluid dynamics (CFD) simulations of the flow around a horizontal axis hydrokinetic turbine (HAHT) found in the literature. The volume of fluid (VOF) model implemented in a commercial CFD package (ANSYS-Fluent) is used to track the air-water interface. The URANS SST k-ω and the four-equation Transition SST turbulence models are employed to compute the unsteady three-dimensional flow field. The sliding mesh technique is used to rotate the subdomain that includes the turbine rotor. The effect of grid resolution, time-step size, and turbulence model on the computed performance coefficients is analyzed in detail, and the results are compared against experimental data at various tip speed ratios (TSRs). Simulation results at the analyzed rotor immersions confirm that the power and thrust coefficients decrease when the rotor is closer to the free surface. The combined effect of rotor and support structure on the free surface evolution and downstream velocities is also studied. The results show that a maximum velocity deficit is found in the near wake region above the rotor centerline. A slow wake recovery is also observed at the shallow rotor immersion due to the free-surface proximity, which in turn reduces the power extraction.
The water level above a hydrokinetic turbine is likely to vary throughout the season and even along the day. In this work, the influence of the free surface on the performance of a three bladed horizontal-axis turbine is explored by means of a three-dimensional, transient, two-phase flow computational model implemented in the commercial CFD software ANSYS Fluent 19.0. The k – ω SST Transition turbulence model coupled with the Volume of Fluid (VOF) method is used to track the air-water interface. The rotor diameter is D = 0 8m. Two operating conditions are analyzed: deep tip immersion (0.55D) and shallow tip immersion (0.19D). Three tip speed ratios are evaluated for each immersion. Simulation results show a good agreement with experimental data reported in the literature, although the computed torque and thrust coefficients are slightly underestimated. Details of the free surface dynamics, the flow past the turbine and the wake near the rotor are also discussed.
This paper challenges the standard wind turbine design numerically assessing the wake and aerodynamic performance of two and three bladed wind turbine models implementing downwind and upwind rotor configurations, respectively. The simulations are conducted using the Actuator Line Model (ALM) coupled with a three-dimensional Navier Stokes solver implementing the k-omega SST turbulence model. The sensitivity of the ALM to multiple simulation parameters is analyzed in detail and numerical results are compared against experimental data. These analyses highlight the most suitable Gaussian radius at the rotor to be equal to twice the chord length at 95% of the blade for a tip-speed ratio (TSR) of $10$, while the Gaussian radius at the tower and the number of actuator points have a low incidence on the flow field computations overall. The numerical axial velocity profiles show better agreement upstream than downstream the rotor, while the discrepancies are not consistent through all the assessed operating conditions, thus highlighting that the ALM parameters are also dependent on the wind turbine's operating conditions rather than being merely geometric parameters. Particularly, for the upwind three-bladed wind turbine model, the accuracy of the total thrust computations improves as the TSR increases, while the least accurate wake predictions are found for its design TSR. Finally, when comparing both turbine models, an accurate representation of the downwind configuration is observed as well as realistic power extraction estimates. Indeed, the results confirm that rotors with fewer blades are more suitable to operate at high TSRs.
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