Session J15: Energy: Water Power

Optimal Kinematics for Energy Harvesting using Favorable Wake-Foil Interactions in Tandem Oscillating Hydrofoils

Pitching and heaving oscillating hydrofoils for energy harvesting can be arranged in tandem configurations that may lead to increased system efficiency. Finding the optimal kinematics for maximum efficiency is necessary in order to make this technology viable for widespread use. The efficiency increase in tandem arrays results from favorable wake-foil interactions between the trailing foil and the wake behind the leading foil. The energy extraction performance of a tandem array of flat plate oscillating hydrofoils is studied experimentally in a water flume at Re = 30k. The leading foil's motion was prescribed to produce a wake within specific regimes, and force and flow measurements of the trailing foil were acquired for varying kinematics. Heave amplitude and inter-foil phase have significant influence on energy extraction, with optimum heave values adapting to structural changes in the wake from the leading foil. Performance is less sensitive to the trailing foil pitch. Velocity fields around the trailing foil, measured using PIV for different parameters are also measured and will be discussed.   

Confinement and 3D effects on the fully-passive oscillating-foil hydrokinetic turbine

In recent years, the fully-passive oscillating-foil turbine (OFT) has been extensively studied for unconfined and 2D flows, with efficiencies reaching up to 50% at high Reynolds number (Boudreau et al., 2020). Such high efficiencies were obtained for a specific set of structural parameters, which passively control the pitch and heave motions of the foil. However, in more realistic conditions where confinement and 3D effects are present, the fully-passive OFT has not yet been tested. In this study, we conduct 3D URANS numerical simulations in which the blade's span and the turbine's confinement level are varied while maintaining the same set of optimal structural parameters. As expected, the performances of finite-span turbines are lower than their 2D counterparts due to blade tips effects. To mitigate the impact on power extraction, a simple adjustment of the heaving damping coefficient modeling the turbine's generator, is proposed and tested. It is found that lowering the heaving damping coefficient increases the turbine's efficiency. An original procedure to quantify the required adjustment of the heave damping coefficient is proposed and validated.   

Wave Energy Absorption Spectrum and Energy Budget of an Oscillating Horizontal Cylinder Near Free Surface

In this talk we present control volume analysis from a series of experiments with a wave energy converter (WEC) consisting of a neutrally buoyant horizontal slender circular cylinder (diameter D=0.27m) fully submerged below the free water surface with the cylinder axis aligned in the direction perpendicular to the incident wave propagation. Results are presented for two different experimental configurations. The first experiment was performed in a small (120ft) USNA towing tank where the long axis of the cylinder extended across the full width of the tank and cylinder oscillated in a single degree of freedom (heave). In the second experiment, the same oscillating cylinder was tested in large (380ft) tow tank that has sufficient width to account for three-dimensional effects in both single and two degree of freedom (heave and surge) motions. Discussion will concentrate on the cylinder response and its wave energy absorption spectrum in a range of wave steepness 0.02 < kA < 0.2 where k is the wavenumber and A is incident wave amplitude and wave periods from 0.8 to 2.2 seconds. Energy budget results suggest that loss of wave energy flux through the control volume equals in magnitude to measured absorbed power by the cylinder plus viscous losses on the tank walls. However, at resonance conditions, this approach fails due to non-linear regime characterized by localized increased wave steepness and wave breaking occurring on the downstream side of the control volume.   

Optimal Power Extraction Conditions for Oscillating Foils Using Non-Sinusoidal Motions

We present novel experiments and simulations of the power extraction performance of heaving and pitching foils. Previous studies have suggested that the angle of attack (AoA) waveform is important for maximizing the performance rather than the heave or pitch motion. However, a prescribed sinusoidal AoA waveform may be achieved with two approaches: (1) sinusoidal heave with a non-sinusoidal pitch or (2) sinusoidal pitch with a non-sinusoidal heave. We compare both approaches for a wide range of AoA, Strouhal number, and dimensionless amplitude to determine whether the two approaches produce invariant performance or whether heave or pitch is preferred as a sinusoidal motion. We consider a three-dimensional NACA0015 airfoil with an aspect ratio of four and pitching about the one-third chord at a Reynolds number of Re = 10,000.   

Performance characteristics of an experimental cross-flow turbine array at high confinement

Cross-flow turbines show great promise for extracting power from river and tidal currents due to their ability to achieve high blockage. As a turbine occupies more of the channel cross-sectional area, its efficiency and structural loading increase since both kinetic and potential energy in the freestream are converted to mechanical power. Here, we characterize the performance of a laboratory-scale two-turbine array at various levels of confinement in a recirculating water channel. The array blockage ratio is varied from 30% to 60%—the upper end of what might be realizable in a natural channel—while other important non-dimensional parameters are held constant. The cross-flow turbines were tested across a range of tip-speed ratios under a counter-rotating coordinated constant speed control scheme, wherein the turbines rotate at the same speed but in opposite directions, and with a constant angular phase offset between their cycles. At the higher end of confinement, we observe performance coefficients exceeding unity and force coefficients substantially higher than in conventional array designs. This has substantial implications for the design and control of full-scale arrays at high confinement.

    

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

The effect of free-stream turbulence on a four-bladed H-Darrieus tidal turbine was investigated through a series of experiments conducted in a water tunnel.  Two-dimensional particle image velocimetry (PIV) and direct measurements of the torque applied to the shaft of the turbine were used to obtain phase- and ensemble-averaged flow measurements in the wake of the turbine and to quantify its power extraction performance under varying turbulent intensities on the inflow. The inflow turbulence levels were varied by employing fractal grids upstream of the test section. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) techniques were used to extract coherent structures and dynamic modes in the wake of the turbine. The wake flow characteristics in conjunction with the directly measured torque applied to the shaft of the turbine indicate that an increase in the free-stream turbulence leads to a decrease in turbine performance and provide an insight into the underlying physics.  The increase in free-stream turbulence also resulted in the increased turbulent fluctuations in the near-wake of the turbine.

    

Modeling the Effects of Surface Waves on Power Generation of Cross-Flow Turbines

Cross-flow turbines are distinct from the more common axial flow turbines, as they extract energy from moving water with the axis of rotation perpendicular to the freestream velocity. Previous research utilizes a steady uniform freestream velocity to understand performance, but many turbines are installed in locations where free surface effects may be important. To understand these effects, we develop a computational fluid dynamics model combining steady uniform freestream flow and surface wave conditions in which we place a two bladed turbine horizontally in the channel. We conduct simulations with and without the presence of waves under various operating conditions and wave parameters. We report turbine performance and unsteady forces on the blades, which are also compared with experimental data and baseline turbine simulations. This framework is then extended to multiple turbine and surface wave configurations. This research will be able to explore the effects of deep- vs shallow-water waves, turbine distance from the free surface, and other setup parameters on power generation. The results will help inform cross-flow turbine installation to maximize power generation in channels with surface waves.   

Simulation of dual cross-flow turbines under confinement

Cross-flow turbines are an alternative to the more commonly known axial flow turbines, and distinguish themselves by rotating on an axis perpendicular to the oncoming flow, as opposed to rotating on an axis coincident with the flow. The efficacy of cross-flow turbines can be increased by placing them in narrow river channels to take advantage of the natural confinement that augments the power conversion efficiency. This project explores the unsteady flow mechanisms of power augmentation by conducting simulations of multiple turbine configurations. Preliminary results indicate that downstream counter-rotating cross-flow turbine pairs in a fenced arrangement with no phase offset show the most promise for effective power generation, with 40% more average power per turbine than a single turbine operating under the same conditions. Staggered turbine arrangements motivate investigation by displaying successful turbine interaction through augmentation of the downstream turbine power generation.   

Large Eddy Simulations and Low-order Models of Tethered Coaxial Turbines

Tethered Marine Hydro Kinetic devices reduce the specific cost of energy production, while tapping the renewable energy potential of oceans. Specifically, tethered, coaxial-counter rotating turbines offer the advantage of producing zero net torque to avoid tether twisting and entanglements. The optimization of single and multiple coaxial devices requires insights into wake behavior, which can be obtained from numerical simulations under varying operating conditions and inlet turbulence. However, the turbines will be deployed in ocean current turbine (OCT) farms rather than as individual units, necessitating the development of low-order models to characterize their wake behavior. We have developed a low-order wake model based on the concept of an Equivalent Single Rotor (ESR), defined by its induction factor, that extracts the same relative power from the mean flow as a dual-rotor coaxial turbine, while injecting the same amount of turbulence into the wake. Validation for this concept and corresponding low-order models is achieved by comparing the wake structure from large eddy simulations (LES) of a coaxial turbine with results from a single rotor simulation with a turbine induction factor corresponding to the ESR condition. Analysis of turbulence is also presented, as inlet turbulence is varied in order to observe its effect on peak turbulence, transition point from near wake to far wake, and decay rate of turbulence for the coaxial configuration. The insights gathered via LES can be extended to capture wake interactions between multiple turbines in an OCT farm.   

Near Wake Dynamics of a two Cross-flow Turbine Array

Cross-flow turbines, also known as vertical-axis turbines, convert the kinetic energy in moving fluid to mechanical energy using blades that rotate about an axis perpendicular to the incoming flow. In these experiments, the performance and wake of a two-turbine array in a fence configuration (side-by-side) were characterized. The turbines were operated under coordinated control, characterized by synchronous rotation rates with a mean phase difference. Measurements were made with turbines co-rotating, counter-rotating with the blades advancing upstream at the array midline, and counter-rotating with the blades retreating downstream at the array midline. From the performance data, we found individual turbine and array efficiency to depend significantly on rotation direction and phase difference. Similar variations are observed in the wake and, using these data, we hypothesize how rotation direction and phase influence interactions between adjacent turbines.   

Axial-flow hydrokinetic turbine performance prediction and wake reproduction in turbulent flow conditions using a simplified turbine model

Aiming at obtaining good performance prediction for each individual turbine in farm at a practical computational cost, a simplified model for hydrokinetic axial-flow turbine, the Effective Performance Turbine Model (EPTM-AFT), has been previously developed based on the non-uniform actuator disk concept, which has been proven to correctly reproduce the mean drag and power of a turbine in uniform and clean inflow (Bourget et al., 2018). However, turbines deployed in farms with realistic flow conditions are generally exposed to different types of flow perturbations. Particularly, small-scale inflow fluctuations significantly promote the tip vortex instability and accelerate the wake recovery, which further affects the performances of downstream turbines. In this study, the EPTM-AFT model is tuned and tested in a characteristic turbulent flow environment using 3-D Reynolds-Averaged Navier-Stokes simulations. The comparison with blade-resolved Delayed Detached-Eddy Simulation shows that the EPTM-AFT model produces accurate predictions for mean turbine performances and wake properties.   

Compliant membranes to augment the performance of oscillating foil energy harvesting systems

Oscillating Foil Turbines (OFT) take advantage of the strong transient lift forces generated by vortices formed at the leading edge when the hydrofoil operates at a large pitch angle. Membrane wings inspired by flying and gliding mammals such as bats and flying squirrels have been shown to stabilize leading edge vortices and here we explore the use of membranes to augment the performance of OFTs. Compliant membrane wings with varying Aeroelastic number - the ratio of inertial to elastic stresses - were tested in a open surface water tunnel, and their performance is compared with that of inextensible membranes and rigid foils over a range of pitching frequencies, heave and pitch amplitudes.