On The Effects Of Unsteady Flow Conditions On The Performance Of A Cross Flow Hydrokinetic Turbine

Hydrokinetic turbines convert the energy of flowing water into usable electricity. Axial flow and cross flow turbines are the most common forms of hydroNinetic turbine, however cross flow turbine performance and the impact of surface waves are not well understood. Tests were conducted to observe the effects of waves on the performance characteristics of a cross flow turbine promulgated by the Department of Energy’s Reference Model Project, specifically Reference Model 2. Testing of a 1:6 scale model was conducted in the large towing tank in the USNA Hydromechanics Laboratory. Baseline (no wave) turbine performance was compared to published data on the same model turbine. Additionally, tests were conducted with incident waves and at various turbine depths and various tow speeds. The average turbine performance characteristics improved slightly as depth decreased due to acceleration of the constricted flow near the surface. Waves did not significantly change the performance of the turbine when averaged over of an entire cycle and several wave periods. This was the case even though the test waves created a velocity shear across the entire span of the blade. The waves were found to impart cyclic signatures in the torque measurement which may have consequences for instantaneous blade loading and power output from the device. A computational model was developed to predict turbine performance and compares favorably to the experiment.

Cross-flow hydrokinetic turbines are a promising option for effectively harvesting energy from fast-flowing streams or currents. This work describes the dynamics of such turbines, analyzes techniques used to scale turbine properties for prototyping, determines and demonstrates the limits of stability for cross-flow rotors, and discusses means and objectives of turbine control. This involves a progression from the analysis of a laboratory-scale prototype turbine to the emulation of a field-scale commercial turbine under realistic control. Understanding of turbine and system component dynamics and performance is leveraged at each phase, with the ultimate goal of enhancing the efficacy of prototype testing and enabling safer, more advanced control techniques. Novel control strategies are under development to utilize low-speed operation (slower than at maximum power point) as a means of shedding power under rated conditions. However, operation in this regime may be unstable. An experiment designed to characterize the stability of a laboratory-scale cross-flow turbine operating near a critically low speed yields evidence that system stall (complete loss of ability to rotate) occurs due, in part, to interactions with turbulent decreases in flow speed. The turbine is capable of maintaining 'stable' operation at critical speed for short duration (typically less than 10 s), as described by exponential decay. The presence of accelerated 'bypass' flow around the rotor and decelerated 'induction' region directly upstream of the rotor, both predicted by linear momentum theory, are observed and quantified with particle image velocimetry (PIV) measurements conducted upstream of the turbine. Additionally, general agreement is seen between PIV inflow measurements and those obtained by an advection-corrected acoustic Doppler velocimeter (ADV) further upstream. Definitive evidence linking observable flow events to the onset of system stall is not found. However, a link between turbulent kinetic energy of the flow, the system time constant, and the turbine's dynamic response to turbulence indicates changes in the flow occurring over a horizon of several seconds create the conditions under which system stall is likely. Performance of a turbine at small (prototype) geometric scale may be prone to undesirable effects due to operation at low Reynolds number and in the presence of high channel blockage. Therefore, testing at larger scale, in open water is desirable. A cross-flow hydrokinetic turbine with a projected area (product of blade span and rotor diameter) of 0.7 m^2 is evaluated in open-water tow trials at three inflow speeds ranging from 1.0 m/s to 2.1 m/s. Measurements of the inflow velocity, the rotor mechanical power, and electrical power output of a complete power take-off (PTO) system are utilized to determine the rotor hydrodynamic efficiency (maximum of 17%) and total system efficiency (maximum of 9%). A lab-based dynamometry method yields individual component and total PTO efficiencies, shown to have high variability and strong influence on total system efficiency. The method of tow-testing is found effective, and when combined with PTO characterization, steady-state performance can be inferred solely from inflow velocity and turbine rotation rate. Dynamic efficiencies of PTO components can effect the overall efficiency of a turbine system, a result from field characterization. Thus, the ability to evaluate such components and their potential effects on turbine performance prior to field deployment is desirable. Before attempting control experiments with actual turbines, hardware-in-the-loop testing on controllable motor-generator sets or electromechanical emulation machines (EEMs) are explored to better understand power take-off response. The emulator control dynamic equations are presented, methods for scaling turbine parameters are developed and evaluated, and experimental results are presented from three EEMs programmed to emulate the same cross-flow turbine. Although hardware platforms and control implementations varied, results show that each EEM is successful in emulating the turbine model at different power levels, thus demonstrating the general feasibility of the approach. However, performance of motor control under torque command, current command, or speed command differed; torque methods required accurate characterization of the motors while speed methods utilized encoder feedback and more accurately tracked turbine dynamics. In a demonstration of an EEM for evaluating a hydrokinetic turbine implementation, a controller is used to track the maximum power-point of the turbine in response to turbulence. Utilizing realistic inflow conditions and control laws, the emulator dynamic speed response is shown to agree well at low frequencies with simulation but to deviate at high frequencies. The efficacy of an electromechanical emulator as an accurate representation of a fielded turbine is evaluated. A commercial horizontally-oriented cross-flow turbine is dynamically emulated on hardware to investigate control strategies and grid integration. A representative inflow time-series with a mean of 2 m/s is generated from high-resolution flow measurements of a riverine site and is used to drive emulation. Power output during emulation under similar input and loading conditions yields agreement with field measurements to within 3% at high power, near-optimal levels. Constant tip-speed ratio and constant speed proportional plus integral control schemes are compared to optimal nonlinear control and constant resistance regulation. All controllers yield similar results in terms of overall system efficiency. The emulated turbine is more responsive to turbulent inflow than the field turbine, as the model utilized to drive emulation does not account for a smoothing effect of turbulent fluctuations over the span of the fielded turbine's rotors. The turbine has a lower inertia than the demand of an isolated grid, indicating a secondary source of power with a similar frequency response is necessary if a single turbine cannot meet the entire demand. Major contributions of this work include exploration of the system time constant as an indicator of turbine dynamic response, evidence a turbine experiences system stall probabilistically, a reduced-complexity field performance characterization methodology, and demonstration of the effectiveness of electromechanical emulators at replicating turbine dynamics.

Cross-flow turbines (CFT) harvest energy from wind or water currents via rotation about an axis perpendicular to the flow, and are a complementary technology to the more common axial-flow turbine. During the 360 degree rotation, the CFT blades experience a cyclical variation in the angle of attack and velocity relative to the oncoming flow, leading to flow separation and reattachment, otherwise known as dynamic stall. This causes an instantaneous loss in torque generation and unsteady force fluctuations which pose a challenge to accurate models and predictions of both the performance and the flow field. This work first examines the dynamic stall process and resulting wake features of CFTs under confined configurations applicable to river and tidal currents. High fidelity large-eddy simulations (LES) of a straight-bladed two-blade cross-flow turbine operating at a moderate Reynolds number are compared to unsteady Reynolds-averaged Navier-Stokes (RANS) simulations. The RANS model is shown to be sensitive to confinement at the simulated tip speed ratio as it over-predicts power generation due to suppression of flow separation. Results are compared with an unconfined configuration for which the RANS model successfully predicts a power curve, however displays significant differences in the evolution of flow structures. Next, the stall development is investigated via proper orthogonal decomposition (POD) of the velocity fields from LES. The POD modes' time development coefficients capture the trend of aerodynamic loads on the blade, along with critical events such as vortex formation and detachment. Flow curvature, history effects, and flow induction are identified as significant factors changing the blade aerodynamic loads as compared to pitching or plunging foils. Lastly, a previously optimized intracycle control of angular velocity that enhances power generation by 40% is compared to constant velocity control. This is enabled by a delay in the blade stall and an alignment of peak torque with the peak of blade angular velocity.

In the search for clean, renewable energy, the kinetic energy of water currents in oceans, rivers, and estuaries is being studied as a predictable and environmentally benign source. We investigate the flow past a cross flow hydrokinetic turbine (CFHT) in which a helical blade turns around a shaft perpendicular to the free stream under the hydrodynamic forces exerted by the flow. This type of turbine, while very different from the classical horizontal axis turbine commonly used in the wind energy field, presents advantages in the context of hydrokinetic energy harvesting, such as independence from current direction, including reversibility, stacking, and self-starting without complex pitch mechanisms. This thesis develops a numerical simulation methodology that applies the Reynolds Average Navier Stokes equations and the three-dimensional sliding mesh technique to model CFHTs. The methodology is validated against small scale experiments, available within NNMREC at the University of Washington and is used to investigate the efficiency of the energy capture and the hydrodynamic forces acting on the blades. First, we study the stationary turbine and conclude that the developed methodology accurately models the starting torque of a turbine initially in static conditions; some limitations are found, however, in predicting separated flow. The dynamic performance of the rotating turbine is predicted with reasonable accuracy using the sliding mesh technique. Excellent qualitative agreement with experimental trends is found in the results, and the actual predicted values from the simulations show good agreement with measurements. Though limitations in accurately modeling dynamic stall for the rotating turbine are confirmed, the good qualitative agreement suggests this methodology can be used to support turbine design and performance over a wide range of parameters, minimizing the number of prototypes to build and experiments to run in the pursuit of an optimized turbine. This methodology can also provide a cost-effective way of evaluating detailed full scale effects, such as mooring lines or local bottom bathymetry features, on both turbine performance and environmental assessment.

The performance of a current turbine is influenced by numerous variables related to the geometry of the turbine and channel, the fluid properties, and the external forces acting on the system. These variables can be non-dimensionalized to form parameters that affect the dimensionless performance of a turbine. If these parameters are held constant between geometric scales, a smaller turbine model can exactly represent a much larger prototype. This method of testing scale models is frequently used to reduce the time and costs associated with the early stages of design. However, not all parameters can be easily matched between scales or maintained within experiments. These limitations prevent models from achieving complete similarity with full-scale prototypes and make it challenging to isolate the effects of individual parameters on turbine performance. Furthermore, the influence of certain parameters on turbine hydrodynamics and performance is not fully understood. Therefore, the aim of this work is to investigate the effects of certain scaling parameters on the hydrodynamics and performance of laboratory-scale current turbines. Three specific objectives are addressed. The first objective is to characterize the effects of the blockage ratio, Reynolds number, and Froude number on turbine performance and flow dynamics with the goals of better understanding the relative influence of these parameters and improving the quality of laboratory-scale testing. The second objective is to assess several analytical corrections intended to account for the influence of blockage on turbine performance. A better understanding of the effectiveness of these corrections will enable data collected under confined conditions to be accurately extrapolated to other environments. The third objective is to investigate the effects of blockage on the wake of a cross-flow current turbine. Better understanding these effects will inform the design of arrays that can exploit blockage to augment turbine performance. To characterize the effects of the blockage ratio, Reynolds number, and Froude number on turbine performance, a cross-flow current turbine was tested in a laboratory flume. The turbine's power and thrust coefficients were measured under a set of baseline operating conditions, then each parameter was increased while the others were maintained at their baseline values. We additionally measured the local channel depth directly upstream and downstream of the turbine to quantify the deformation of the free surface. We found that all three parameters significantly influenced turbine performance, with the power coefficient most sensitive to changes in the Reynolds number and least sensitive to changes in the Froude number. Furthermore, free surface deformation was affected by the Froude number but remained relatively unchanged from baseline values when the blockage ratio and Reynolds number were varied. Because all three parameters significantly affected the turbine's power and thrust coefficients, they should be carefully controlled in experiments where scale similarity is desired. In addition, further research is needed to determine the underlying fluid mechanisms that cause the observed change in turbine performance with Froude number. Because scale models are frequently tested at relatively high blockage ratios, it is desirable to correct measured performance for blockage effects. However, there has been limited experimental validation of the analytical blockage corrections presented in the literature. This work evaluated corrections against experimental data to recommend one or more for future use. For this investigation, we tested a cross-flow turbine and an axial-flow turbine under conditions of varying blockage with other dimensionless parameters, such as the Reynolds and Froude numbers, held approximately constant. Increasing blockage improved turbine performance, resulting in higher thrust and power coefficients over a larger range of tip-speed ratios. Of the analytical corrections evaluated, the two based on measured thrust performed best. Unexpectedly, these corrections were more effective for the cross-flow turbine than the axial-flow turbine. We attribute this result to changes in the local Reynolds number caused by increasing blockage, an effect not captured by the analytical theory. For both turbines, the corrections performed better for thrust than power, which is consistent with the assumptions that underlie the analytical theory. The potential to increase turbine performance through the use of high blockage arrays has inspired recent interest in array design. Arrays are typically composed of multiple rows of turbines, with downstream turbines operating in the wake of upstream turbines. To inform the design of arrays, the effects of blockage on the wake of a cross-flow current turbine were evaluated. Velocity data were collected downstream of the turbine under two different blockage conditions. As before, to isolate blockage effects, other dimensionless parameters that affect turbine performance were held approximately constant. The turbine was operated at the tip-speed ratio corresponding to peak power for each blockage ratio. Increasing the blockage caused faster streamwise flow speeds through and around the turbine, a decreased overall wake size, elevated turbulent kinetic energy, and an increased viscous dissipation rate. These results suggest that higher blockage could increase the power output and reduce the physical footprint of current turbine arrays due to faster wake mixing. However, these benefits must be weighed against the potential for high blockage arrays to reduce a turbine's "basin efficiency", which is an important ecological parameter. Furthermore, we observed that decreasing the width of the experimental channel while holding the depth constant decreased the extent of the wake in the lateral direction only. The wake was unaffected in the vertical direction, which suggests that lateral and vertical blockage have independent effects on turbine wakes. Consistent with prior studies, we also observed significant wake mixing in the vertical (i.e., spanwise) direction and negligible wake mixing in the lateral direction for both blockage conditions.

A computational fluid dynamics study was performed for a cross-flow marine hydro-kinetic turbine. The analysis was done in three dimensions and used the unsteady Reynolds averaged Navier-Stokes solver in the commercial code STAR-CCM+. The base turbine configuration is the RivGen® Turbine, designed by the Ocean Renewable Power Company (ORPC). A convergence and uncertainty analysis was performed for both the spatial and temporal discretization; this was done using the base configuration which features support struts and helical foils. The proposed study aims to compare the impact of the struts on both power performance and blade loading for helical and straight blades.