Session L37: Water Power

Unsteady Fluid-Structure Interactions in Semi-Passive Oscillating Hydrofoil Turbines

When generating power from riverine flows, oscillating foil turbines (OFTs) are an attractive solution for their high efficiency, depth adaptability, performance resistance to bio-fouling, and low environmental impact. We present new fluid-structure interaction simulations that model the unsteady flows of OFT systems that include an oscillating foil, springs, and a generator. The foil's pitch motion is fully prescribed and its heave translation is determined by solving the equations of motion of the foil-spring-generator system. Since the heave kinematics and, consequently, the angle of attack profile cannot be prescribed, the trajectory is controlled and optimized for maximum efficiency by adjusting structural and generator properties such as the pitching frequency to natural frequency ratio, and the linear and nonlinear generator damping coefficients. It is demonstrated that the frequency ratio can alter the heave waveform significantly, and damping is utilized to get the optimal balance of phase difference and heave amplitude. This study aims to uncover the key fluid-structure interactions and unsteady flow characteristics of semi-passive OFT systems.   

Quantifying the impact of the free surface on the performances of a fully passive oscillating-foil turbine via numerical simulations

This presentation aims to quantify the impact of the free surface on the performances of a fully passive oscillating-foil turbine operating in a horizontal configuration. Since the movement of such turbine is passive, it is sensitive to operating conditions and flow variations. A numerical approach to evaluate the impact of the free surface on the performances of the turbine in realistic conditions is presented along with the associated design guidelines. The turbine's mathematical model with gravity was implemented within a fluid-structure coupling algorithm. Simulations were conducted under different flow conditions and installation depths. Efficiencies of over 50 % are obtained with a turbine undergoing a stall-flutter instability, whereas in previous studies turbines undergoing a coupled-flutter instability were reported to be better. The improvement of the stall-flutter configuration is achieved by taking advantage of the free surface and the confinement it provides. The results also show that these turbines are more robust to changes in the flow than one operating through a coupled-flutter instability. Indeed, the former is much less sensitive to gravitational and buoyancy effects, whereas the fundamental principles of the coupled-flutter instability make it not suitable for operation with a horizontal axis when these effects are important.   

Performance of a two-body wave energy converter with an annular heave plate

We theoretically examine the performance of a two-body wave energy converter (WEC) featuring a floating sphere and a submerged annular heave plate, connected by a power take-off (PTO) system. Utilizing linear wave theory, we derive the system's frequency-domain response to regular plane waves and analyze the impact of varying disk porosity on power generation. Our results suggest that annular disks can enhance power extraction efficiency in various cases compared to solid heave plates. Additionally, permeable plates can broaden operational conditions by reducing oscillation amplitudes and decreasing the mechanical strain on the PTO system without substantially compromising the power conversion efficiency. Overall, our findings provide valuable insights for optimizing WEC designs to improve energy capture, emphasizing the potential hydrodynamic advantages of using porous reaction bodies.   

Surface Wave Effects on Intracycle Blade Dynamics of Cross-Flow Turbines

This project computationally investigates the effect of waves on marine energy cross-flow turbines (CFT) operating close to the water surface. CFTs extract energy from moving water with the axis of rotation perpendicular to the freestream velocity. Compared with the more common axial flow turbine, blade level dynamics of a CFT change throughout one cycle due to a transient airfoil orientation. These dynamics offer opportunities to optimize performance through control strategies and geometry changes. Previous research utilizes a steady uniform freestream velocity to understand CFT performance, but many turbines are installed in locations of non-uniform flow or close to a free surface. To understand these effects, a numerical model is developed for a turbine in uniform flow with surface wave conditions. The model uses the volume of fluid method to simulate and track the air-water interface. A two-bladed CFT is placed horizontally in the water channel. Simulations are conducted with and without the presence of waves under various operating conditions and wave parameters. Differences in conditions are investigated using flow field visualization. Turbine performance and unsteady forces on the blades are reported and results are compared with baseline turbine simulations. This framework is then used to characterize intracycle effects on dynamic stall and power production.   

DYNAMIC INTERACTION OF A MODEL SCALE TIDAL TURBINE IN NON-HOMOGENEOUS SHEARED TURBULENCE

Tidal turbines face considerable challenges due to elevated turbulence and velocity shear in tidal environments. The fluctuations in the turbine power due to the inflow turbulence enhance fatigue loading and pose stability issues for the electrical grid, necessitating costly synchronizers. Mimicking such inflow conditions in controlled laboratory settings allows a detailed understanding of the dynamic interaction of the inflow with the turbine. The current study generates a range of inflow conditions from quasi-laminar to elevated turbulence with and without shear and coherent structures using an active grid in a water tunnel. We analyze the response of a model-scale turbine to these conditions and establish their power spectral characteristics. The results showed that despite the anticipated increase in loading with elevated turbulence and shear, turbine performance diminishes under these conditions for optimum tip-speed ratios compared to quasi-laminar cases. The inflow scales primarily influence the shape and trends of the power spectra up to the turbine frequency, beyond which rotor dynamics dominate. The power spectra exhibit strong correlations with low-frequency large-scale fluctuations, confirmed by generating larger integral length scales, while correlations decrease for high-frequency fluctuations. Additionally, the turbine responds to coherent structures in the inflow, with notable energy peaks potentially intensifying loading on the devices. A marked correlation exists between turbine power fluctuations and these coherent structures in the inflow. Lastly, our study demonstrates that current turbine power spectral models can replicate the shape of the spectra, albeit with some limitations.

Effect of free-stream turbulence on the hydrodynamic performance and wake structure of an H-Darrieus tidal turbine

The effect of free-stream turbulence on a 1/10^th scale H-Darrieus tidal turbine was investigated through a series of experiments conducted in a water tunnel at a diameter-based Reynolds number (Re_D) = 0.5 x 10⁶. The inflow turbulence level was varied between < 1%, 5%, and 10% by the installation of fractal grids upstream of the test section. The performance of the turbine was characterized across a range of tip-speed ratios (TSR) from 1.0-to-3.4 using continuous measurements of the torque applied to the shaft of the turbine’s rotor. Particle image velocimetry (PIV) was employed to obtain ensemble- and phase-averaged velocity measurements in the wake at the optimal TSR of 2.65 for each turbulence level. Aside from increasing torque transients, the phase-resolved performance data reveals that an increase in free-stream turbulence delays the onset of stall and accelerates the reattachment of the boundary layer, effectively impacting the azimuth angle at which the turbine interacts with the free-stream. The performance data further reveals that turbulence intensity alone is not sufficient in quantifying the power extraction performance of the turbine, while the integral length scale aids in classifying the torque transients and the net power coefficient. The obtained wake flow fields at the optimal TSR show that increased free-stream turbulence reduces the span-wise velocity deficit on the upstream side of the turbine. Additionally, increasing free-stream turbulence decreases the total stream-wise momentum transfer and increases the cross-stream advection. These results offer practical implications in the context of tidal turbine design and implementation. The added non-periodic loading on the turbine blades may require a more robust turbine design depending on the free-stream turbulence characteristics of the tidal environment. Furthermore, the change in momentum transfer rate in the wake of the turbine will require extra consideration for optimal tidal array spacing.   

Gradient-Based Design Optimization of a 5kW Ducted Hydrokinetic Turbine Using RANS CFD

Building upon our previous study on ducted hydrokinetic turbine optimization, this study focuses on enhancing the performance of a 5kW ducted hydrokinetic turbine under real-world constraints. Starting with a foil-shaped duct and a turbine featuring a bulky hub to house a generator, we optimize the duct, hub, and blades to maximize efficiency. The geometry is represented by a CAD-based parametrization using an open-source Engineering Sketch Pad. The gradient-based optimization process is performed, evaluating the turbine performance using a RANS-based flow solver and computing the gradients for the optimization using the adjoint method. The optimized design is subsequently re-evaluated using a higher-fidelity flow solver, yielding approximately 50% hydrodynamic efficiency, higher than an unducted counterpart.   

CFD model of hydrokinetic turbine power generation

Hydrokinetic power is directly linked to the velocity of the water flow. Previously, companies relied on finding fast-moving water to place the turbines. A way to increase the speed of the water is by adding a nozzle or concentrator to the inlet of a hydrokinetic turbine. Both physical and computational fluid dynamics (CFD) models were developed on a 1/64th scale. A transient rotating mesh CFD model was used to analyze the work of the turbine. Physical testing on the model was used to measure speed, torque, and power for verification of the CFD model. Different blade designs, triplex and helical, are used to test the efficiency of the turbine. The results of velocity increase, available power, and efficiency are measured and compared in the CFD and physical models. The efficiency is also compared to benchmark products such as MeyGen's tidal power turbines.   

Numerical Study of Cavitation for Horizontal Axis Hydrokinetic Turbine

Horizontal axis hydrokinetic turbines are numerically studied to assess the deployment and operating conditions for which cavitation may play an important detrimental role to both turbine performances and its structural integrity. For some critical, yet practical conditions, the turbine performances may be dramatically decreased due to cavitation. The effects of cavitation are studied with 3D URANS numerical simulations using the Volume of Fluid approach and the Schnerr-Sauer cavitation model. Two different 3-bladed turbine designs are considered at large and small scales in different inflow conditions and blockage. The turbines differ with their blade profile and chord distribution. The present research is mainly focused on the effects of the cavitation number, tip-speed ratio, confinement level and Reynolds number. These parameters are shown to impact the inception and extent of the region of cavitation which in turn may affect more or less strongly the turbine performances. The presentation also discusses some design choices that can be made to restrain the vapor region, especially near the blade tip where cavitation usually takes place.   

Impact of Downstream Centrifugal Reverse Osmosis Module on Tidal Turbine Performance

This study investigates the performance of tidal turbines with downstream Centrifugal Reverse Osmosis (CRO) module through Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations. Mesh independence study is conducted with mesh sizes of 6 million (6M), 10 million (10M), and 17 million (17M) cells by comparing the power coefficient (Cp) and thrust coefficient (Ct) of the turbine. The difference in Cp and Ct  between the 6M and 17M cases is approximately 3%, while the difference between the 10M and 17M cases is around 1%, leading to the selection of the 10M mesh for further simulations. The simulations employing the k-ω SST turbulence model are validated against laboratory experiments (1:20 scale) without the CRO module downstream. The same mathematical and numerical models are adopted for the full-scale tidal turbine simulations. The study explores the influence of the CRO module's diameter, varying between 0.1D_t and 0.5D_t (where D_t is the turbine diameter of 5.5 meters), and the module’s axial position from the rotor, ranging between 0.25D_t and 1D_t. This comprehensive analysis aims to enhance the understanding of the interactions between tidal turbines and downstream flow modifications, contributing to the shape and position optimization of downstream objects.