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This work examines the dynamic stall process and resulting wake features of cross-flow turbines under confined configurations using two computational modeling approaches, Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES). Cross-flow turbines 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 their 360° rotation cross-flow turbine 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. The dynamic stall process causes an instantaneous loss in torque generation and unsteady force fluctuations which pose a challenge to accurate predictions of both the performance and the resulting unsteady flow field. This research compares RANS simulations to higher fidelity LES of a straight-bladed two-blade cross-flow turbine at a moderate Reynolds number (Rec = 45,000) in a confined configuration. The RANS model is shown to be very sensitive to confinement at the simulated tip speed ratio as it over-predicts power generation due to suppression of flow separation, while the flow field from LES matches well with the experimental validation. Results are compared with an unconfined configuration for which the RANS model successfully predicts a power curve; however, it displays significant differences in the evolution of flow structures such as premature shedding of the dynamic stall vortex and a lack of vortex diffusion during convection in the wake. [ABSTRACT FROM AUTHOR]

Cross-flow turbines convert kinetic power in wind or water currents to mechanical power. Unlike axial-flow turbines, the influence of geometric parameters on turbine performance is not well-understood, in part because there are neither generalized analytical formulations nor inexpensive, accurate numerical models that describe their fluid dynamics. Here, we experimentally investigate the effect of aspect ratio—the ratio of the blade span to rotor diameter—on the performance of a straight-bladed cross-flow turbine in a water channel. To isolate the effect of aspect ratio, all other non-dimensional parameters are held constant, including the relative confinement, Froude number, and Reynolds number. The coefficient of performance is found to be invariant for the range of aspect ratios tested (0.95–1.63), which we ascribe to minimal blade–support interactions for this turbine design. Finally, a subset of experiments is repeated without controlling for the Froude number, and the coefficient of performance is found to increase, a consequence of the Froude number variation that could mistakenly be ascribed to aspect ratio. This highlights the importance of a rigorous experimental design when exploring the effect of geometric parameters on cross-flow turbine performance. [ABSTRACT FROM AUTHOR]

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 this work, the performance of a two-turbine array in a recirculating water channel was experimentally optimized across 64 unique array configurations. For each configuration, turbine performance was optimized using tip-speed ratio control, where the rotation rate for each turbine is optimized individually, and using coordinated control, where the turbines are optimized to operate at synchronous rotation rates but with a phase difference. For each configuration and control strategy, the consequences of co- and counter-rotations were also evaluated. This is the first experimental cross-flow turbine array study to simultaneously address array geometry, control, and turbine rotation direction. Based on these results, we hypothesize how array configurations and control cases influence interactions between turbines and affect the performance of the array. [ABSTRACT FROM AUTHOR]

When experimentally evaluating the performance of a wind or water current turbine, one must impose a regulating torque on the turbine rotor by electrical or mechanical means. Some options limit this controlling torque to a purely resistive quantity, while servomotors and stepper motors allow torque to be applied in the direction of turbine rotation. Any control mode that results in net positive power for a turbine may be of interest for energy harvesting, and all of these are net "fluid-driven." Here, we present experiments that characterize the power, torque, and force coefficients of a cross-flow turbine operated at a constant rotational speed or under a constant imposed control torque. Time- and phase-average performance coefficients are largely equivalent for the two strategies although torque-regulated control is restricted to a narrower range of rotational speeds and the two strategies result in slightly different blade kinematics. [ABSTRACT FROM AUTHOR]

Cross-flow or vertical-axis turbines are flow energy conversion devices in which lift forces cause blades to rotate around an axis perpendicular to the flow. In marine currents, rivers, and some wind energy applications, cross-flow turbines are a promising alternative to more conventional axial-flow turbines. The performance implications of the choice of structure used to mount turbine blades to the central shaft are examined experimentally in a recirculating water flume. Turbine performance is found to be strongly dependent on the choice of the mounting structure. Power loss due to rotational drag on these structures is estimated experimentally by rotating the mounting structure without blades. Through a perturbation-theory approach, interactions between turbine blades and mounting structures are examined. Analytical models for the power loss due to mounting structure drag are introduced and shown to be consistent with experiments. To provide guidance for cross-flow turbine design, the models are re-formulated in terms of non-dimensional turbine geometric and operational parameters. Mounting blades solely at their mid-span is shown to decrease performance through multiple fluid effects. Using foil cross-section struts located at the turbine blade tips is found to result in the highest turbine performance. [ABSTRACT FROM AUTHOR]

The near-wake of a vertical-axis cross-flow turbine was modeled numerically via blade-resolved k-ω shear stress transport (SST) and Spalart-Allmaras Reynoldsaveraged Navier-Stokes (RANS) models in two and three dimensions. The results for each case were compared with the experimental measurements of the turbine shaft power, overall streamwise rotor drag, mean velocity, turbulence kinetic energy, and momentum transport terms in the near-wake at one diameter downstream. It was shown that 2-D simulations overpredict turbine loading and do not resolve mean vertical momentum transport, which plays an important role in the near-wake's momentum balance. The 3-D simulations fared better at predicting performance, with the Spalart-Allmaras model predictions being closest to the experiments. The SST model more accurately predicted the turbulence kinetic energy, while the Spalart-Allmaras model more closely matched the momentum transport terms in the near-wake. These results show the potential of blade-resolved RANS as a design tool and a way to "extrapolate" experimental flow field measurements. [ABSTRACT FROM AUTHOR]

For economic reasons, above a certain water speed, it is desirable for current turbines to maintain a constant power output. This requires a control strategy to shed power. Here, such a strategy is evaluated through simulation and laboratory experiment for a helically bladed turbine with four blades and a straight-bladed turbine with two blades. These are contrasting cases because hydrodynamic torque produced by the straight-bladed turbine has a substantially more azimuthal phase variability than the helically bladed turbine. For practical implementation, a control algorithm is desired that requires only a time-average characteristic performance curve and estimates of angular velocity and control torque. Additionally, the transition between power maximizing and power tracking regimes should be smooth and automatic. The controller can be further constrained to only apply a resistive torque to the turbine. A control strategy satisfying these constraints is shown experimentally to achieve these objectives for both types of turbines at a variety of power set points. The power-tracking error (<3%) is primarily at the blade passage frequency for the straight-bladed turbine and at the rate of turbine rotation for the helical turbine. While partially a consequence of how the generator drive is tuned, comparison to simulation indicates that perfect power-tracking is not generally possible even under ideal conditions for a fixed-pitch, cross-flow turbine using purely resistive torque control. [ABSTRACT FROM AUTHOR]

In the planning of hydrokinetic turbine array deployment and for the performance prediction of its constituent turbines, the use of simplified turbine models is essential to alleviate the computational costs. The Effective Performance Turbine Model (EPTM) introduced in 2018 is a promising tool for that purpose, allowing array analysis and optimization. Its performance predictions scale with a local flow velocity characterization, which ensures to take into account inherently blockage effects and mean, local flow conditions. However, the characteristics of the local flow within an array also include different types of perturbation such as shear, large-scale temporal fluctuations, and turbulence. To ensure that the model is still reliable in those conditions, this paper presents a validation of the EPTM through the analysis of a large-scale tandem turbine configuration. In fact, three unsteady-Reynolds-averaged-Navier–Stokes simulations of a cross-flow turbine tandem configuration with a longitudinal spacing of six diameters have been conducted in this study, in addition to a simulation of a single turbine with turbulent ambient conditions. This set of simulations allows us to study independently the different types of perturbations associated with array deployment. Whereas the slow-varying large-scale upstream velocity fluctuations do not seem to affect significantly the turbine operation, the upstream non-uniform velocity distribution affects appreciably the extracted power. We find also that the value of the effective power coefficient associated with the EPTM needs to be adapted to the array simulation. Compared to the case of a single turbine in a uniform flow with a low turbulence level, we show that a smaller effective power coefficient value must be used in array simulations. An important result is that the different types of perturbations are found to yield similar effective performance coefficients, which suggests that the same set of values can be used throughout the array. With the appropriate set of values, we show that the EPTM succeeds to predict accurate downstream turbine performance. [ABSTRACT FROM AUTHOR]

This study focuses on the interaction between the water free surface, the riverbed, and some Darrieus-type hydrokinetic turbines deployed in river flows. As turbines offer a resistance to the flow, they affect the upcoming velocity, which in turn affects their performance. The proximity of the neighboring deformable free surface or rigid bed may also influence their power extraction. In this context, 2D and 3D URANS simulations of a cross-flow (H-Darrieus type) turbine are conducted with free-surface modeling and adapted boundary conditions allowing the capture of the interactions between the turbine and the resource. Different water depth immersions are considered in order to study local proximity effects. It is found, neglecting riverbed friction, that shallow immersion is detrimental to power extraction whereas bed proximity associated with deep immersion is favorable. This observation does not hold when considering a more realistic river with a velocity profile throughout the depth. Direction of rotation in high proximity cases also plays a role. Although the literature suggests a slight increase in power extraction with the Froude number, we find that when interaction with the resource is taken into account, the power extraction is rather independent of the Froude number for deep immersion or slightly decreasing for shallow immersion. Nonetheless, all the variations in power extraction reported in this study remain small compared to the ones associated with blockage effects. Finally, the shallow immersion case simulated in 3D behaves similarly to that simulated in 2D. Switching the orientation of the rotation axis from horizontal to vertical, despite changing the local interaction with the free surface, does not affect significantly the performance of the turbine. [ABSTRACT FROM AUTHOR]

Turbulent wake flows behind helical- and straight-bladed vertical axis wind turbines (VAWTs) in boundary layer turbulence are numerically studied using the large-eddy simulation (LES) method combined with the actuator line model. Based on the LES data, systematic statistical analyses are performed to explore the effects of blade geometry on the characteristics of the turbine wake. The time-averaged velocity fields show that the helical-bladed VAWT generates a mean vertical velocity along the center of the turbine wake, which causes a vertical inclination of the turbine wake and alters the vertical gradient of the mean streamwise velocity. Consequently, the intensities of the turbulent fluctuations and Reynolds shear stresses are also affected by the helical-shaped blades when compared with those in the straight-bladed VAWT case. The LES results also show that reversing the twist direction of the helical-bladed VAWT causes the spatial patterns of the turbulent wake flow statistics to be reversed in the vertical direction. Moreover, the mass and kinetic energy transports in the turbine wakes are directly visualized using the transport tube method, and the comparison between the helical- and straight-bladed VAWT cases show significant differences in the downstream evolution of the transport tubes. [ABSTRACT FROM AUTHOR]

The present paper reports findings of the transient thermography studies of three-phase resistive system protection load (RSPL) or diversion dump load used for safeguarding the wind turbines during extreme wind conditions. RSPLs with coil-type and plate-type resistive elements made of Nichrome 80 and SS 304, respectively, were studied. The first set of studies was conducted on the permanent magnet synchronous generator (PMSG) test bench with 24 and 48 V PMSGs as power sources. The second set of studies was conducted on the auto-transformer-based test setup. For both the studies, surface temperature distributions over the resistive elements of the RSPLs were captured under the different test conditions using transient thermography and compared. Quantitative comparison of the variation in maximum temperature of RSPL heating elements and PMSG's rotational speed for a given power rating for both the RSPLs was carried out. Thermographs revealed the presence of localized hotspots in RSPLs with Nichrome 80 based coil-type heating elements. Similar localized hotspots were absent in RSPLs with SS 304 based plate-type heating elements. Maximum surface temperatures of RSPLs with coil-type resistive elements were found to be about 8.25% higher than the other RSPL. Based on these observations, it was inferred that as against the conventional approach of using RSPLs with a coil-type heating element made of Nichrome 80, RSPLs with a plate-type heating element made of SS 304 offered better safety for the wind turbines. [ABSTRACT FROM AUTHOR]

Health monitoring devices are critical for the safe and reliable operation of water supply systems, but their chemical batteries are easily depleted. Thus, this paper developed a micro-vertical axis water turbine that converts part of the water head into electrical energy and supplies power for the monitoring device. The performance of four groups of micro-turbines was investigated in detail through experimental testing. Different from our previous study, the entire performance curves of micro-turbines at different rotation speeds and flow velocities were obtained in order to consider the wide range of flow velocities inside the real water pipeline. The feasibility of the designed turbines in the power supply system was verified by on-site testing. The experimental results show that different types and opening sizes of blocks have a distinct influence on the output power of micro-turbines. Each type of block has the corresponding optimal opening size to gain the highest performance. Within a certain range, the maximum output power varies linearly with the increase in flow velocity. In a specific flow velocity, the optimal rotational speed of the micro-turbine generated the maximum output power can be predicted by the power curve. In addition, through the experimental data, it is found that among all the measured turbines, the 12-blade hollow shaft turbine with a 90% eye-shaped block is the best one, with an output power of 88.2 W and an efficiency of 15.76%. Finally, the results of an on-site test are consistent with that of the experiment properly, which verifies the reliability of the measured data in the laboratory. [ABSTRACT FROM AUTHOR]

We experimentally explored the effect of a series of low-level-jet (LLJ) velocity profiles on the energy entrainment of a single and a pair of counter-rotating Giromill–Darrieus vertical-axis wind turbines (VAWTs) models under two types of rotations. Planar particle image velocimetry (PIV) was used to obtain the mean flow and turbulence statistics in the wake of the model wind turbines. Incoming LLJ profiles had peaks coincident with the turbines midspan and top tip; complementary flow characterization with a canonic turbulent boundary layer is also included for comparison. Results show that the positive shear region of the LLJ velocity profiles increased the wake asymmetry due to higher vertical velocity gradients. The positive shear of the LLJ contributed to the enhancement of energy entrainment in the wake compared with a standard turbulent boundary layer profile by increasing the mean kinetic energy advection into the wake in the single and a pair of counter-rotating VAWTs. Comparatively, high vertical velocity gradients may be responsible for this phenomenon; it promoted stronger cross-flow and counter-rotating vortices in the wake. [ABSTRACT FROM AUTHOR]

Pico-hydropower attracts much attention as a clean, reliable, and cost-efficient technology to generate electricity. Pico-hydropower systems are widely used in small rivers, aquacultures, irrigation systems, water supplies, and sewage systems. However, few studies have been conducted to investigate how to harness the excess energy in water supply systems (WSSs) in cities and towns. Previously, our research group developed several inline pico-hydropower systems for WSS which can supply power for monitoring sensor networks of water pipelines and maintenance services. In this paper, we have focused on the water turbine with a vertical 50% block water baffle for further analysis and optimization. We have carried out a combination of numerical and experimental investigations with the objective of improving the performance of this turbine at a velocity of 1.5 m/s–3.5 m/s and a rotational speed of 100 RPM–1000 RPM. The numerical results have been validated by the test results. A further optimization has been conducted through the numerical methods for the angle of water baffle and blade diameter. Also, we have presented the flow field in and around the water turbine to understand the power generation mechanism. [ABSTRACT FROM AUTHOR]

The emergence of tidal energy as a key renewable energy source requires the development of computational design models for accurate prediction of turbine performance and wake effects whilst also being computationally efficient. In this paper, we develop and validate a three-dimensional CFD model for vertical axis turbines, which achieves high accuracy. We also investigate the limitations of two-dimensional models and present a blockage correction for improved prediction. The two-dimensional blockage correction model is potentially attractive for preliminary design studies due to its computational advantage over three-dimensional models. [ABSTRACT FROM AUTHOR]

An optimization methodology for a stall regulated, fixed pitch, horizontal axis hydrokinetic turbine is presented using a combination of a coupled hydro-structural analysis and Genetic Algorithm (GA) based optimization method. Design and analysis is presented for two different designs: a constant chord, zero twist blade, and a variable chord, twisted blade. A hybrid approach is presented combining Blade Element Momentum (BEM), GA, Computational Fluid Dynamics (CFD), and Finite Element Analysis (FEA) techniques. The preliminary analysis is performed using BEM method to find the hydrodynamic performance and flap-wise bending stresses in turbine blades. The BEM analysis used for the current study incorporates effect of wake rotation, hub loss and tip loss factors, and effect of Reynolds number on hydrodynamic data. A multi-objective optimization is then performed to maximize performance and structural strength of turbine. The results of optimization for a constant chord, zero twist blade design are validated with detailed three dimensional CFD and finite element analysis. The fluid domain is coupled with the structural domain through one way coupling and fluid-structure interaction analysis is carried out to find the effect of blade geometry and operating conditions on the stresses developed in the blades. The hydrodynamic performance of a constant chord turbine was found to be limited by the high stresses developed in turbine blades. Hence, in an effort to reduce the stresses in turbine blades, a variable chord, twisted blade design was developed; and a multi-objective optimization is presented for the variable chord twisted blade turbine for hydro-structural performance improvement. The final-optimized variable chord, twisted blade design was found to improve the power coefficient by 17% and resulted in lower overall stresses. [ABSTRACT FROM AUTHOR]

The hydrodynamic performance of horizontal axis hydrokinetic turbines (HAHkTs) under different turbine geometries and flow conditions is discussed. Hydrokinetic turbines are a class of zero-head hydropower systems which utilize kinetic energy of flowing water to drive a generator. However, such turbines very often suffer from low-efficiency which is primarily due to its operation in a low tip-speed ratio (≤4) regime. This makes the design of a HAHkT a challenging task. A detailed computational fluid dynamics study was performed using the k-ω shear stress transport turbulence model to examine the effect of various parameters like tip-speed ratio, solidity, angle of attack, and number of blades on the performance HAHkTs having power capacities of ∼12 kW. For this purpose, a three-dimensional numerical model was developed and validated with experimental data. The numerical studies estimate optimum turbine solidity and blade numbers that produce maximum power coefficient at a given tip speed ratio. Simulations were also performed to observe the axial velocity ratios at the turbine rotor downstream for different tip speed ratios which provide quantitative details of energy loss suffered by each turbine at an ambient flow condition. The velocity distribution provides confirmation of the stall-delay phenomenon due to the effect of rotation of the turbine and a further verification of optimum tip speed ratio corresponding to maximum power coefficient obtained from the solidity analysis. [ABSTRACT FROM AUTHOR]