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In this work, an experimental analysis of the performance of a cross-flow turbine, commonly referred to as a Michell–Banki turbine (MBT), is carried out for small-scale hydropower production in rural areas located in developing countries to support their social and economic development activities. The study investigates how the efficiency of the MBT is influenced by the presence or absence of a nozzle, along with variations in the internal guide vane (GV) and its angle. The runner had 26 blades that were arranged symmetrically in the periphery between two circular plates. The designed MBT had the ability to generate a maximum of 100 W of power at a water flow rate and a head of 0.009 m3/s and 0.6311 m, respectively. The experimental tests were carried out using a hydraulic bench. The turbine efficiency without the inner GV was found to be higher than that of the turbine with the inner GV; i.e., it was found that the utilization of the GV did not enhance the efficiency of the MBT due to the occurrence of a choking effect. A maximum hydraulic efficiency of 85% was achieved in the turbine without an inner GV in comparison with the efficiency achieved (77%) with this device and an optimum opening angle of the GV of 24° (75% of opening). In this regard, the GV design should be carefully carried out to improve the MBT efficiency. Additionally, the effect of the GV shape on the MBT performance should be experimentally investigated to obtain a more general judgment regarding the role of this device. [ABSTRACT FROM AUTHOR]

In this work, the experimental characterization of a Michell-Banki turbine on a laboratory scale is carried out and recommendations are presented for an adequate experimental setup that allows measuring the variables involved in energy generation and adequately characterizing the turbine. The turbine's impeller consisted of 26 blades that were symmetrically arranged between two circular plates. The designed turbine could produce up to 100 W at a water flow rate and head of 0.009 m3/s and 0.6311 m, respectively. The turbine efficiency curve was determined on a hydraulic bench using a torque sensor with encoder. The maximum turbine efficiency was 85%. This technology could be of great help in promoting activities that require electrical energy, such as those related to agriculture, food production, education, and health in areas where access to electricity is limited or non-existent. [ABSTRACT FROM AUTHOR]

This paper presents a parametric study of H-Darrieus turbines in cavitating conditions. The objective of this work is to assess the impact of cavitation on turbine performances for a wide variety of configurations and under various operating conditions. A better insight on the most “cavitation-friendly” turbine is provided by determining the optimal design parameters in terms of solidity, deployment depth, number of blades and blade pitch angle. Such design parameters are studied here and help to identify and analyse the inception and effects of cavitation. Two-dimensional URANS numerical simulations are carried out within Star-CCM+ at a Reynolds number of 107 based on the turbine diameter. The cavitating conditions simulated in this paper correspond to near-surface deployments at free-stream velocities ranging from 2 to 3 m/s. In a first phase, the solidity is varied from 0.1 to 0.7 while the tip-speed ratio is varied accordingly for optimal power coefficient. It is shown that a higher solidity turbine operating at a lower tip-speed ratio is beneficial in cavitating conditions due to the low effective velocity on the blades. The chord-to-radius ratio is varied in the second phase of this study. Turbines with 1, 3, 6 and 9 blades for a constant solidity of 0.6 are simulated and the results show that a larger chord-to-radius ratio is detrimental in cavitating conditions. Thus, the high curvature effects are to be avoided by using a larger number of blades for a given turbine solidity. The third and final phase of this study simulates turbines where the blade pitch angle was varied from −6° to +2° for a medium turbine solidity of 0.3. The small negative blade pitch angles of −2° and −4° show promising results where cavitation is greatly reduced, showing an improvement on power coefficient of 45 % for a blade pitch angle of −4° compared to the no blade pitch angle case at a cavitation number of 35. Overall, this paper concludes on the fact that the blade’s effective velocity and angle of attack are the key parameters controlling cavitation and that a combination of the studied design parameters should be considered to completely avoid cavitation inception or limit its detrimental effect on performance.

Low/zero-head cross-flow turbines (LZH-CFT) possess vast potential in the field of renewable energy technology. This research presents a modified design of a cross-flow turbine specifically engineered to harness the untapped hydro-kinetic energy from low/zero-head water resources. Unlike traditional hydro turbines, the LZH-CFT leverages a cross-flow configuration to utilize the water flows of natural streams, making it an ideal solution for flow channels with low/zero head. The design is modified through ANSYS simulations by varying the rotor's blade angles (from 0 to 90°) and 60° angle is found optimum. The prototype model of modified design is prepared and tested, which represents 62.5% RPM efficiency and provides double power as compared to the typical cross-flow turbine. [ABSTRACT FROM AUTHOR]

The cost effective design of tidal turbines requires a good estimation of the loading cycles and their extrema that are related to the unsteady fluctuation of the current velocity. Apart from the ambient turbulence, the main source of velocity fluctuation is the presence of surface waves. In the present study, we analyse the effect of waves propagating against the current on the performance and the loads of a twin vertical axis tidal turbine by an experimental approach at a 1/20 scale. Overall, the results show little or no effect of the waves on the average power and loads compared to the conditions with current only, but a significant impact on their standard deviation that rises linearly with the amplitude of the waves. The drag, lift, and pitching moment show extended ranges up to 7.5 times higher and extreme values exceedance by 60 to 100% with irregular waves compared to the conditions without waves. That load and power fluctuation increase is totally due to the presence of waves as the coherence function between the rotor torque or the loads and the velocity spectra exceed 0.5 on the whole wave frequency bandwidth. The results also reveal a rotational sampling of the waves by the rotors that had also been observed on horizontal axis turbines. From a structural design point of view, the authors recommend conducting tests in irregular wave conditions as both the load ranges and extreme values are 1.5 to 2 times higher than those encountered with regular waves of the same significant height and period. [ABSTRACT FROM AUTHOR]

While studies on horizontal axis tidal turbines are plentiful, those on ducted twin vertical axis alike HydroQuest's turbines are lacking. For such a device, both the relative counter-rotation direction of the rotors and the tripod base geometry upstream are different between ebb and flood tide. Consequently, this paper analyses the effect of the two opposed flow directions on the hydrodynamic performance and on the wake of the turbine. The study is based on experimental measurements at a 1/20th scale in Ifremer's wave and current flume tank. The hydrodynamic performance of the model are characterised over a wide range of operating points with the turbine installed on a tripod and on a monopile base. In addition, the 3D wake of the turbine is thoroughly analysed in the two flow directions using 3-component laser Doppler velocimetry. Overall, the drag and the maximum average power coefficient are not affected by the current direction but the optimal tip speed ratio is 7 % lower during ebb with 1.5 times higher power fluctuations compared to flood tide. Besides, the wake of the two rotor columns interact differently depending on the flow direction, leading to a 30 % faster surface averaged velocity recovery in the flood tide configuration. The observed effect of flow direction provides a better knowledge of twin vertical axis turbine wake interactions and highlights the impact of the gravity base geometry on the development of the overall turbine wake. This paper also provides a wide experimental database for the validation of numerical models applied to ducted twin-vertical axis tidal turbines. • Ebb and flood tides modelled experimentally in a tank by turning the turbine around. • Twin vertical axis turbine (2-VATT) mean power is barely affected by the flow direction. • Gravity based 2-VATT power fluctuation and wake development depend on base geometry. • Rotors wake merging distance and recovery depend on the counter-rotation direction. [ABSTRACT FROM AUTHOR]

This experimental study is focused on quantifying the effect of trailing edge flexibility on the performance of a three straight bladed vertical axis wind turbine, with a chord-to-diameter ratio c/D=0.16at a moderately high Reynolds number (based on diameter) ReD=4⋅105. The blades consist of NACA-0015 profiles that are fixed with a pitch angle β=6∘toe-out, and allow interchangeable trailing edges in the last 17% of their chord length. The research presented here provides a proof of concept for the improved performance of vertical axis wind turbines, due to the effect of flexibility at the trailing edge of their blades. We show that blades with semi-flexible trailing edge, can extend the range of rotor operating regimes, leading to an increase of approximately 10% in the performance of the turbine. An excess of flexibility results in diminished efficiencies.

Both scientific and industrial communities have a growing interest in marine renewable energies. There is a wide variety of technologies in this domain, with different degrees of maturity. This study focuses on two models of a mast-free vertical axis Darrieus tidal turbine with the objective of characterizing the effect of vertical confinement, rotor configuration, and fluid–structure interactions on their performances in free-surface flows. The first model comprised four straight rigid blades maintained by circular flanges on both ends of the rotor and the second model is equipped with free-ended interchangeable blades attached to a single upper flange. Two configurations of the second model mounted with either rigid or flexible blades were used, first for comparison with the dual-flange turbine, then in order to address the effect of fluid–structure interactions on the turbine performances. While the single-flange models exhibit a significantly lower efficiency at all operating points, it is observed that the use of flexible blades tends to enhance turbine performances at low Reynolds numbers. The flow topology obtained from PIV measurement at selected operating points is discussed with respect to the performance of each turbine model in order to highlight the role of the dynamic stall and blade–vortex interactions. [ABSTRACT FROM AUTHOR]

The energy potential of low-head water can be exploited by using the horizontal axial flow to produce rotation in the turbine, which is then converts it into electricity in the generator. The types of blades that are commonly used are propeller types, cross-flow types, and Kaplan types. In this research, a prototype of micro-hydro power plant with a capacity of 20 W will be developed, which utilizes the potential of water flow with a head < 0.5 m. This research begins with observing water flow, including flow characteristics and parameters such as speed, head, and available discharge. The next step is to design the power plant, which includes the design of a very low head horizontal axial turbine, turbine structure and accessories, turbine blade rotor, power transmission system, generator selection, and control system. The manufacture of the main components of the turbine, reservoir, diffuser, etc., is carried out before integrating the entire micro hydropower system. The result shows the physical model of the very low-head turbine prototype produces power at the highest Head (0.4 m) of 11.02 W, while at the same Head, the computational fluid dynamic (CFD) results produce the most significant power with 13.55 W. [ABSTRACT FROM AUTHOR]

In the last few decades, hydropower production has been moving toward a new paradigm of low and diffused power density production of energy with small and mini-hydro plants, which usually do not require significant water storage. In the case of nominal power lower than 20 kW and ultra-low head H (H < 5 m), Archimedes screw or Kaplan type turbines are usually chosen due to their efficiency, which is higher than 0.85. A new cross-flow type turbine called Ultra-low Power Recovery System (UL-PRS) is proposed and its geometry and design criteria are validated in a wide range of operating conditions through 2D numerical analysis computed using the ANSYS Fluent solver. The new proposed solution is much simpler than the previously mentioned competitors; its outlet flow has a horizontal direction and attains similar efficiency. The costs of the UL-PRS turbine are compared with the costs of one Kaplan and one cross-flow turbine (CFT) in the case study of the main water treatment plant of the city of Palermo in Italy. In this case, the UL-PRS efficiency is estimated using a URANS 3D numerical analysis computed with the CFX solver. [ABSTRACT FROM AUTHOR]

Low/zero-head cross-flow turbines (LZH-CFT) possess vast potential in the field of renewable energy technology. This research presents a modified design of a cross-flow turbine specifically engineered to harness the untapped hydro-kinetic energy from low/zero-head water resources. Unlike traditional hydro turbines, the LZH-CFT leverages a cross-flow configuration to utilize the water flows of natural streams, making it an ideal solution for flow channels with low/zero head. The design is modified through ANSYS simulations by varying the rotor’s blade angles (from 0 to 90°) and 60° angle is found optimum. The prototype model of modified design is prepared and tested, which represents 62.5% RPM efficiency and provides double power as compared to the typical cross-flow turbine.

The cost effective design of tidal turbines requires a good estimation of the loading cycles and their extrema that are related to the unsteady fluctuation of the current velocity. Apart from the ambient turbulence, the main source of velocity fluctuation is the presence of surface waves. In the present study, we analyse the effect of waves propagating against the current on the performance and the loads of a twin vertical axis tidal turbine by an experimental approach at a 1/20 scale. Overall, the results show little or no effect of the waves on the average power and loads compared to the conditions with current only, but a significant impact on their standard deviation that rises linearly with the amplitude of the waves. The drag, lift, and pitching moment show extended ranges up to 7.5 times higher and extreme values exceedance by 60 to 100% with irregular waves compared to the conditions without waves. That load and power fluctuation increase is totally due to the presence of waves as the coherence function between the rotor torque or the loads and the velocity spectra exceed 0.5 on the whole wave frequency bandwidth. The results also reveal a rotational sampling of the waves by the rotors that had also been observed on horizontal axis turbines. From a structural design point of view, the authors recommend conducting tests in irregular wave conditions as both the load ranges and extreme values are 1.5 to 2 times higher than those encountered with regular waves of the same significant height and period.

Cross-flow turbines harness kinetic energy in wind or moving water. Due to their unsteady fluid dynamics, it can be difficult to predict the interplay between aspects of rotor geometry and turbine performance. This study considers the effects of three geometric parameters: the number of blades, the preset pitch angle, and the chord-to-radius ratio. The relevant fluid dynamics of cross-flow turbines are reviewed, as are prior experimental studies that have investigated these parameters in a more limited manner. Here, 223 unique experiments are conducted across an order of magnitude of diameter-based Reynolds numbers (≈ 8 × 1 0 4 – 8 × 1 0 5 ) in which the performance implications of these three geometric parameters are evaluated. In agreement with prior work, maximum performance is generally observed to increase with Reynolds number and decrease with blade count. The broader experimental space clarifies parametric interdependencies; for example, the optimal preset pitch angle is increasingly negative as the chord-to-radius ratio increases. As these experiments vary both the chord-to-radius ratio and blade count, the performance of different rotor geometries with the same solidity (the ratio of total blade chord to rotor circumference) can also be evaluated. Results demonstrate that while solidity can be a poor predictor of maximum performance, across all scales and tested geometries it is an excellent predictor of the tip-speed ratio corresponding to maximum performance. Overall, these results present a uniquely holistic view of relevant geometric considerations for cross-flow turbine rotor design and provide a rich dataset for validation of numerical simulations and reduced-order models. [Display omitted] • Experimental parameter space unifies narrower prior work on cross-flow turbines. • Optimal chord-to-radius ratio decreases as diameter-based Reynolds number increases. • Optimal preset pitch angle becomes more negative as chord-to-radius ratio increases. • Turbines with more blades have lower efficiency, but more uniform power and forces. • Turbines with the same solidity do not always have the same maximum efficiency. [ABSTRACT FROM AUTHOR]

Crisis electricity was a crucial issue in the rural area. Crossflow turbine (CFT) in pico in pico scale is the best option for electricity provider for rural areas. Due to its usefulness and development of computer technology, computational fluid dynamics method application for CFT study becomes increasingly frequent. This paper compiles the implementation of the computational fluid dynamic (CFD) approach for CFT on a pico scale. Based on the literature, the Renormalization Group (RNG) turbulence model is recommended to predict the flow field that occurs in CFT because its error is lower than others turbulence models, the RNG error of 3.08%, standard of 3.19%, and transitional SST of 3.10%. Furthermore, six-degrees of freedom (6-DoF) is recommended because it has an error of 3.1% than a moving mesh of 9.5% for the unsteady approach. Thus, based on the review, the RNG turbulence model and 6-DoF are recommended for the CFT on the pico scale.

• A novel inverted cross-flow turbine and its design method for the low and ultra-low head hydraulic condition were proposed. • The four-step parametric studies were conducted to achieve the optimized turbine design. • The turbine achieved higher than 60 % efficiency range in the ultra-low head condition (under 3 m water head) by using a simple spiral nozzle and without a guide vane. • The optimized turbine maximum efficiency was achieved 72.92 % efficiency at the turbine speed 170 RPM with 8.01 m of water head. This study aimed to address the challenges of micro-hydropower generation in low-head environments to expand the proportion of hydropower in renewable energy and achieve a carbon neutrality policy. To achieve these goals, various factors affecting the micro-hydro systems, including geographical constraints, construction costs, and grid connectivity, were reviewed and assessed. The results concluded that a novel type of cross-flow turbine with inverted structures, improved the economic feasibility, productivity, and installability of micro-hydropower plants by reducing head losses in open channels or small streams. The parametric studies consisted of four steps: first, designing the initial inverted cross-flow turbine based on the traditional design rules; for the second and third steps, a parametric study for comparing the runner diameter ratio and the number of blades is conducted; and in the last step optimized turbine was designed. The best efficiency point of the optimized inverted cross-flow turbine was 72.92 % at a runner rotational speed of 170 RPM with a 0.75 runner diameter ratio and 35 blades. The optimized turbine was improved by 7.8 % in power and 7.35 % in efficiency, higher than the initial turbine. From the hill chart comparison between the initial and optimized turbine, operation range expansion was confirmed. Overall, this research demonstrates the potential of the inverted cross-flow turbine under low-head conditions. [ABSTRACT FROM AUTHOR]

Both scientific and industrial communities have a growing interest in marine renewable energies. There is a wide variety of technologies in this domain, with different degrees of maturity. This study focuses on two models of a mast-free vertical axis Darrieus tidal turbine with the objective of characterizing the effect of vertical confinement, rotor configuration, and fluid–structure interactions on their performances in free-surface flows. The first model comprised four straight rigid blades maintained by circular flanges on both ends of the rotor and the second model is equipped with free-ended interchangeable blades attached to a single upper flange. Two configurations of the second model mounted with either rigid or flexible blades were used, first for comparison with the dual-flange turbine, then in order to address the effect of fluid–structure interactions on the turbine performances. While the single-flange models exhibit a significantly lower efficiency at all operating points, it is observed that the use of flexible blades tends to enhance turbine performances at low Reynolds numbers. The flow topology obtained from PIV measurement at selected operating points is discussed with respect to the performance of each turbine model in order to highlight the role of the dynamic stall and blade–vortex interactions.

In the last few decades, hydropower production has been moving toward a new paradigm of low and diffused power density production of energy with small and mini-hydro plants, which usually do not require significant water storage. In the case of nominal power lower than 20 kW and ultra-low head H (H < 5 m), Archimedes screw or Kaplan type turbines are usually chosen due to their efficiency, which is higher than 0.85. A new cross-flow type turbine called Ultra-low Power Recovery System (UL-PRS) is proposed and its geometry and design criteria are validated in a wide range of operating conditions through 2D numerical analysis computed using the ANSYS Fluent solver. The new proposed solution is much simpler than the previously mentioned competitors; its outlet flow has a horizontal direction and attains similar efficiency. The costs of the UL-PRS turbine are compared with the costs of one Kaplan and one cross-flow turbine (CFT) in the case study of the main water treatment plant of the city of Palermo in Italy. In this case, the UL-PRS efficiency is estimated using a URANS 3D numerical analysis computed with the CFX solver.

Hydraulic turbines for energy recovery in wastewater treatment plants, with relatively large discharges values and small head jumps, are usually screw Archimedes or Kaplan types. In the specific case of a small head jump (about 3 m) underlying a rectangular weir in the major Palermo (Italy) water treatment plant, a traditional Kaplan solution is compared with two other new proposals: a Hydrostatic Pressure Machine (HPM) located at the upstream channel and a cross-flow turbine (CFT) located in a specific underground room downstream of the same channel. The fluid mechanical formulations of the flow through these turbines are analyzed and the characteristic parameters are stated. Numerical analysis was carried out for the validation of the HPM design criteria. The efficiency at the design point of the CFT and HPM are estimated using the ANSYS CFX solver for resolution of 3D URANS analysis. The strong and weak points of the three devices are compared. Finally, a viability analysis is developed based on several economic indicators. This innovative study with a theoretical formulation of the most suitable turbomachine characterization, the potential energy estimation based on hydraulic energy recovery in a real case study of a wastewater treatment plant and the comparison of the three different low-head turbines, enhancing the main advantages, is of utmost importance towards the net-zero water sector decarbonization. [ABSTRACT FROM AUTHOR]

The cross-flow turbines cover from the point of view hydraulic power the running domain of some well-known turbines such as Pelton, Francis or Kaplan. This type of turbine has a simple construction, long life and low execution cost, which makes it very suitable for on and off grid small to medium hydro power plants. It is quite difficult to establish an exact theoretical dynamic model for this type of turbines, due to the complex flow phenomenon (bi phase flow water and air). In order to obtain the exact dynamic behavior of the hydraulic machine, experimental dynamic identification will be done. In automation, the dynamic properties, represent the fundamental characteristic of the object which must be regulated. When the dynamic properties of the regulated object are obtained experimentally, we analyze the characteristics of the transient regime, which appears because of the application at the system inlet of some stochastic or deterministic signals (sine waves for our case). The hydraulic turbine is modeled as an informational quadrupole having the inlet parameters the movement of the wicket gate and the turbine head and outlet parameters the torque and the speed. In this paper it will be presented the frequency modelling of the cross-flow turbine and the validation of the mathematical model through experimental dynamic identification. [ABSTRACT FROM AUTHOR]

The small hydroelectric power plants (SHPP) are implemented in non-interconnected zones (NIZ) of developing countries. In which, the provision of electrical energy from the national interconnected system is not economically feasible. Therefore, in the literature, hydroelectric generation technologies have been implemented taking advantage of the energy available in the rivers. One of these technologies is the Michell-Banki type cross-flow turbines (MBT), which, despite having lower efficiencies than turbines such as Pelton and Francis, maintain their efficiency although fluctuations in site conditions. For this reason, different studies have been made to increase the efficiency of the MBT by making geometric modifications to both the nozzle and the rotor. The purpose of this study is to determine numerically the effect of the geometry of the blades that form the runner on the efficiency of Michell-Banki Turbine (MBT). For this, two (2) geometries were studied corresponding to a circular sector of a standard tubular profile and an airfoil NACA 6512 modified in curvature profile and chord length, according to the profile of the standard tubular blade. For this study, transient simulations for multiphase water-air flow were implemented using a k--ε turbulence model in the Ansys 2020R1® CFX software. The two (2) turbine models were configured to the same hydraulic conditions of head and volumetric flow corresponding to 0.5 m and 16.27 L/s, respectively. Variations in rotational speed were configured between 100 and 200 RPM with 20 RPM steps. It was found that using the modified 6512 hydrodynamic profile, at 140 RPM increased efficiency by 6 %, compared to the conventional tubular type blade geometry. [ABSTRACT FROM AUTHOR]

In this study, the computational and experimental examination of the hydraulic cross-flow turbine (CFT) is presented. The experimental studies are conducted at the turbine laboratory of Gebze Technical University. The commercial Computational Fluid Dynamics (CFD) software, Star CCM+, is employed to calculate three-dimensional, incompressible, steady Reynolds-Averaged Navier–Stokes (RANS) equations for the flow inside the CFT. In the numerical model, the rotational motion of the blades is modeled by means of the moving reference frame approach. The correlation between the turbulent viscosity and the velocities is acquired using the standard k-ɛ turbulence model. The computational results are obtained in terms of the torque, power, and efficiency of the turbine. The outcome of this research reveals that the CFD software used in this study can reliably be used to predict experiments since the numerical model can accurately estimate the experimental values of the torque, power, and efficiency of the CFT. [ABSTRACT FROM AUTHOR]

Cross-flow turbines have recently been proposed for energy recovery in aqueducts when the outlet pressure is greater than zero, owing to their constructive simplicity and good efficiency within a large range of flow rates and head drops. In the case of high head drop (higher than 150 m) and relatively small discharge (lower than 0.2 m3/s), the traditional design of these turbines leads to very small widths of the nozzle and the runner; as a consequence, friction losses grow dramatically and efficiency drops down to very low values. Standard Pelton turbines require zero outlet pressure and cannot be used as alternatives. A new counter-pressure hydraulic turbine for high head and low flow rate, called the High Power Recovery System (H-PRS) is proposed. H-PRS presents a different geometry to reduce friction losses inside the nozzle and the runner by widening the two external walls. Several curved baffles are proposed to guide the fluid particles inside the nozzle and to guarantee the right velocity direction at the inlet surface of the runner. Computational Fluid Dynamics (CFD) 3D transient analyses are carried out to measure H-PRS efficiency for different operating conditions and to compute its characteristic curve for different positions of the regulating flap.

Cross-flow turbine is one of the leading turbines to harness the hydropower potential in micro-hydro power schemes. In spite of having a simple structure, low maintenance, and low initial investment, this turbine suffers the problem of low efficiency due to unguided flow in the runner cavity. In the present study, an attempt has been made to enhance the performance of cross-flow turbine by attaching hydrofoils shaped (symmetrical and unsymmetrical) guiding mechanism. The effectiveness of the placement of guide vane was examined numerically in terms of turbine efficiency and flow field distributions. The performance of the modified turbine was compared with the turbine having no guide vane. The position and placement angle of the guide vanes were also investigated to find the suitable position of the guide vane in the runner cavity. The turbine with symmetrical and unsymmetrical guide vane yields its maximum efficiency corresponding to vane angle of 55° and 45°, respectively, placed at the 'right' position (toward nozzle) for both types of guide vanes. The maximum efficiency of the turbine was observed as 80.1% and 79.9% corresponding to symmetrical and unsymmetrical guide vanes, respectively. Further, the flow field distribution inside the turbine runner and in the vicinity of the guide vane has been observed. It has been visualized that placement of the guide vane minimizes the water recirculation with the streamlined flow for smooth entry to the second stage of the runner. The reduction in turbulence and vortices generation results in the improvement of turbine efficiency for similar operating conditions. [ABSTRACT FROM AUTHOR]

Using delayed detached-eddy simulations, the objective of this paper is to show the effects of some design parameters on the wake recovery of vertical-axis turbines. To that end, the wake dynamics of a single-blade turbine is analyzed and compared for two different positions of blade attachment point and for two designs of blade support structures (struts). The results show that the chordwise position of the blade attachment point affects the temporal evolution of the blade circulation during the turbine revolution. More precisely, we demonstrate that an attachment point positioned near the trailing edge of the blade leads to a temporal vorticity distribution that is detrimental to the wake recovery. Moreover, we show that the use of tip struts, that are highly beneficial to the turbine efficiency, affects the spatial distribution of the vortex structures shed by the turbine as well as the wake recovery. The results presented in this paper suggest that the velocity recovery in the wake is highly dependent on the very near-wake vortex dynamics. Therefore, we can infer that any geometric parameter affecting the temporal and spatial distributions of the vorticity shed by the blades can significantly alter the wake recovery of vertical-axis turbines. [ABSTRACT FROM AUTHOR]

A novel vertical augmentation channel housing a direct-drive cross-flow turbine, with nozzles on both the sides of the turbine, was designed and an optimized configuration was obtained. The geometries of the guide nozzle and the front nozzle were optimized under steady flow conditions. The performance of the cross-flow turbine was analyzed using commercial computational fluid dynamics (CFD) code ANSYS-CFX. The optimized design was then evaluated as a wave energy converter both experimentally and computationally. The waves in the numerical wave tank (NWT) were generated using a piston type wave-maker. The optimized design gave a maximum output power of 13.2 W and an efficiency of 48.31% at a wave height of 0.2 m and wave period of 2.75 s for a rotational speed of 35 rpm. The difference between numerical and experimental efficiencies was within 3.5%. In addition to this, particle image velocimetry was used to study the flow characteristics in the augmentation channel and the turbine. The results show that the CFD code captures the flow in the augmentation channel and around the turbine accurately. The optimized design, which occupies less space than other wave energy converters, can be used to efficiently harness energy from the waves. [ABSTRACT FROM AUTHOR]

Cross-flow turbines have recently been proposed for energy recovery in aqueducts when the outlet pressure is greater than zero, owing to their constructive simplicity and good efficiency within a large range of flow rates and head drops. In the case of high head drop (higher than 150 m) and relatively small discharge (lower than 0.2 m3/s), the traditional design of these turbines leads to very small widths of the nozzle and the runner; as a consequence, friction losses grow dramatically and efficiency drops down to very low values. Standard Pelton turbines require zero outlet pressure and cannot be used as alternatives. A new counter-pressure hydraulic turbine for high head and low flow rate, called the High Power Recovery System (H-PRS) is proposed. H-PRS presents a different geometry to reduce friction losses inside the nozzle and the runner by widening the two external walls. Several curved baffles are proposed to guide the fluid particles inside the nozzle and to guarantee the right velocity direction at the inlet surface of the runner. Computational Fluid Dynamics (CFD) 3D transient analyses are carried out to measure H-PRS efficiency for different operating conditions and to compute its characteristic curve for different positions of the regulating flap. [ABSTRACT FROM AUTHOR]

The impact of the blade support structures (the struts) on the performance of vertical-axis turbines at a high Reynolds number is investigated here. Numerical simulations of a single-blade turbine with different strut configurations are carried out and the results are compared to a reference hypothetical turbine without strut. The results show that the struts are less detrimental to the turbine efficiency if they are located at the tips of the turbine blade, rather than at other intermediate positions along the blade span. Moreover, for struts located at the blade tips, it is shown that using rounded junctions between the struts and the blade can lead to a significant increase in the turbine efficiency. Indeed, the best turbine configuration presented in this paper has an efficiency that is more than 20% larger than that of the reference turbine without strut. This important increase in the turbine efficiency can partly be attributed to the fact that the tip struts with rounded junctions act as efficient winglets, leading to a significant decrease in the induced drag on the turbine blade. The results presented show that well-designed blade support structures can be very beneficial to the performance of vertical-axis turbines. • The spanwise position of the struts affects the vertical-axis turbine efficiency. • Struts located at the blade tips are less detrimental to the turbine efficiency. • Rounded blade-strut junctions are highly beneficial to the turbine efficiency. • Rounded blade-strut junctions allow to decrease the induced drag on the blades. [ABSTRACT FROM AUTHOR]

This paper introduces a new methodology for designing and optimizing the performance of hydraulic Cross-Flow turbines for a wide range of operating conditions. The methodology is based on a one-step approach for the system-level design phase and a three-step, successive numerical analysis approach for the detail design phase. Compared to current design methodologies, not only does this approach break down the process into well-defined steps and simplify it, but it also has the advantage that once numerical simulations are conducted for a single turbine, most of the results can be used for an entire class of Cross-Flow turbines. In this paper, after a discussion of the research background, we explain the design process used and the ANSYS®-based CFD model of the turbine in detail. The design process consists of three steps. First, designing nozzle geometry; second, optimizing runner parameters; and third, enhancing turbine performance by analyzing various load conditions. A turbine designed using this process in a simulation case study achieves a peak hydraulic efficiency of 91% and peak overall efficiency of 82% that is maintained for volume flow rates as low as 14% of the nominal value and water head variations up to 30% of the nominal value. • A novel design methodology for hydraulic Cross-Flow turbines is proposed. • The two-dimensional steady state simulations were performed in ANSYS CFX. • Numerical results were validated with experimental data. • The proposed design methodology was used in a case study to design a new turbine. • The designed turbine maintains above 80% efficiency for a range of load conditions. [ABSTRACT FROM AUTHOR]

Hydraulic turbines for energy recovery in wastewater treatment plants, with relatively large discharges values and small head jumps, are usually screw Archimedes or Kaplan types. In the specific case of a small head jump (about 3 m) underlying a rectangular weir in the major Palermo (Italy) water treatment plant, a traditional Kaplan solution is compared with two other new proposals: a Hydrostatic Pressure Machine (HPM) located at the upstream channel and a cross-flow turbine (CFT) located in a specific underground room downstream of the same channel. The fluid mechanical formulations of the flow through these turbines are analyzed and the characteristic parameters are stated. Numerical analysis was carried out for the validation of the HPM design criteria. The efficiency at the design point of the CFT and HPM are estimated using the ANSYS CFX solver for resolution of 3D URANS analysis. The strong and weak points of the three devices are compared. Finally, a viability analysis is developed based on several economic indicators. This innovative study with a theoretical formulation of the most suitable turbomachine characterization, the potential energy estimation based on hydraulic energy recovery in a real case study of a wastewater treatment plant and the comparison of the three different low-head turbines, enhancing the main advantages, is of utmost importance towards the net-zero water sector decarbonization.

A better understanding of the flow field of the cross-flow turbine (CFT) will be useful in its design and operation. As far as is known, no comprehensive study carried out relating to the effect of Reynolds number to turbulent shear stress, shear wall, energy kinetic turbulent, dissipation rate and Reynolds stress, and the occurrence of vortices around the runners of the CFTs. This study was designed to investigate the flow field in the nozzle and runner of the CFT using computational fluid dynamics (CFD) method. CFD methods were chosen because they can visualize detailed flow patterns that other methods cannot. The setups used in the CFD method such as two-dimensional unsteady simulations, six degrees of freedom features, shear stress transport k-ω turbulent model, and pressure-based solver. Based on results, for the nozzle, the shape of the velocity profile shows that the highest momentum flux occurs at the end of the nozzle, near the runner. Distribution of shear wall was highest at the base and tip of the nozzle; it was lowest at the centre. The turbulent kinetic energy profile at the nozzle was proportional to the turbulent boundary layer profile, Reynolds stress and eddy viscosity. This indicated that nozzle shape affects the momentum flux; therefore, good nozzle geometry can transfer the maximum water energy into the blade. The nozzle's optimum geometry can be achieved by discharge and direction, optimizing velocity magnitude. This minimizes energy loss due to friction between the stream, vortex and mass of wasted fluid. For the runner, the highest turbulent kinetic energy, dissipations rate and Reynolds stress were located at the runner. Not all the water's energy converted into mechanic energy because the part of that energy was used in mixing between water and air. The establishment of lift force on the active blades was not caused by the flow field that crosses the upper part of the blade, but by the momentum of water that hit the lower part of the blade. A vortex formed due to separation of the flow from the blade significantly affected the runner's performance rather than rotational flow (air phase) in the CFT. [ABSTRACT FROM AUTHOR]

Michell-Banki turbines, also known as cross-flow turbines, are hydraulic machines used for the production and generation of of small-scale hydroelectric power. Since its creation, the Michell-Banki turbine has been the subject of multiple investigations focused on improving efficiency in order to obtain the maximum use of the available hydraulic resource. This work presents the main characteristics of the turbine, its components and its operating principle. Subsequently, a description of the most relevant research and contributions of each study. Then an analysis is made of the use and application of the Michell-Banki turbine and the opportunities for use in the Colombian territory with the aim of providing the energy resource to areas that lack the service and that have water resources. Finally, the most important considerations to take into account for the design of this type of turbines are described. [ABSTRACT FROM AUTHOR]

The cross-flow turbine is an attractive technology for small-scale hydropower generation thanks to its low capital cost and relatively high efficiency even under partial discharge operating conditions. It has been proposed that this kind of turbine can be manufactured from standard steel pipe sections given its simple geometry, an alternative that would further decrease the turbine's capital cost compared to highly engineered cross-flow designs. This article explores the trade-offs encountered during the design of cross-flow turbines under the constrains imposed by the discrete set of dimensions available for commercial steel pipes. First, the computational model is presented, analyzed in terms of its convergence behavior, and validated with experimental data. Then, a parametric analysis is performed to understand the relative importance of the design variables and their optimum value in regard to the turbine efficiency. Based on the design guidelines derived from the parametric analysis, an example cross-flow turbine design is presented and thoroughly characterized, demonstrating that it is possible to engineer a cross-flow turbine of competitive efficiency out of commercial steel pipes. These guidelines may encourage the use of cross-flow turbines as an appropriate technology for off-grid regions. • A cross-flow turbine design methodology is presented. • The numerical model is characterized and then validated with experimental data. • The influence of the main design parameters on the efficiency is investigated. • Design guidelines and insight are drawn based on the parametric analysis results. • A cross-flow turbine design example is characterized throughout its operating range. [ABSTRACT FROM AUTHOR]

Understanding the effect of tidal turbines on local erosion of the estuarine bed is crucial for design and maintenance of turbines with stable foundations and assessment of their environmental impacts. This report describes the results of flume experiments on clear-water scour caused by a single cross-flow turbine in steady flow conditions. The turbine investigated is a Momentum Reversal Lift (MRL) turbine originally designed in collaboration with the University of Exeter. Results show that the turbine can cause significant bed scour, particularly when it was not spinning and in a particular orientation of blades. This is opposite to the previous findings for axial flow turbines. The bottom plate of the turbine, although increasing scour depth, was found to increase the turbine performance and reduce adverse effects on the downstream flow. The findings highlight the importance of regular monitoring and taking immediate repair actions for a tidal installation. • Flume experiments on clear-water scour for MRL of cross-flow turbine. • The MRL turbine produces significant downstream scour. • Regular monitoring of the turbine to ensure it is operating properly is important. • Turbine runs slower without the bottom plate. • Temporal evolution of scour reduces the rotational speed of the turbine. [ABSTRACT FROM AUTHOR]

In laboratory experiments involving wind or water turbines, it is often desirable to correct measured performance for the effects of model blockage. However, there has been limited experimental validation of the analytical blockage corrections presented in the literature. Therefore, the objective of this study is to evaluate corrections against experimental data and 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 non-dimensional parameters, such as the free-stream 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. We used these experimental data to assess the effectiveness of multiple analytical blockage corrections for both turbine types. Of the corrections evaluated, the two based on measured thrust performed best. 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. • Flow confinement augments the performance of wind and water turbines. • Analytical blockage corrections account for some of the effects of flow confinement. • Thrust-based corrections perform best and are recommended for future use. • Reynolds dependence may impact the effectiveness of blockage corrections. • Assumptions that underpin momentum theory introduce some error into corrections. [ABSTRACT FROM AUTHOR]

In the present work, Delayed Detached-Eddy Simulations (DDES) are conducted to evaluate the performance of an H-Darrieus vertical-axis turbine with detached end-plates, namely stationary end-plates that are not in contact with the turbine blades. More precisely, the force and the power coefficients of the turbine are analyzed for two different geometries of detached end-plates. The turbine wake dynamics is also presented and the main mechanisms responsible for the wake recovery rate are assessed. The results show that the presence of detached end-plates significantly increases the efficiency of the vertical-axis turbine considered. In addition to this improved efficiency, a semi-annular shape of detached end-plate leads to an improved recovery of the mean streamwise velocity in the turbine wake. The important wake recovery rate in the near-wake of the turbine with semi-annular detached end-plates is mainly related to the transport of momentum by the mean spanwise velocity field, which is also known to be the dominant contribution to the wake recovery rate of a turbine without detached end-plates. The results presented in this paper confirm that detached end-plates have a great potential to improve the performance of H-Darrieus vertical-axis turbines used both individually and within turbine farms. • The use of detached end-plates improves the efficiency of vertical-axis turbines. • Detached end-plates affect importantly the vorticity dynamics in the turbine wake. • The use of semi-annular detached end-plates improves the turbine wake recovery rate. • Transport of momentum by the mean spanwise velocity governs the wake recovery rate. [ABSTRACT FROM AUTHOR]

Undershot cross-flow water turbines are characterized by their being easily applicable to open channels with various widths and depths by changing their blade width and/or the outer diameter of the runner. On the other hand, as this water turbine operates in a free-surface flow field, it is expected that when the flow rate changes, the flow field around the runner including the water depth changes dramatically, and that the turbine performance also changes. Thus, the objective of this study is to elucidate the effect of flow rate on the performance and flow field of an undershot cross-flow water turbine. By changing the flow rate, we conduct experiments and free surface flow analysis on the water turbine, as well as torque component analysis. The result showed that, among the torques in the respective flow regions, that torque resulting from the second stage cross-flow was dominant. Furthermore, it was found that among the respective dynamic components of the torque, the most dominant component was the gravitational component for low flow rates and the angular momentum component for medium-to-high flow rates. We expect these results to contribute to the establishment of higher performance design guidelines, such as optimum shapes for the flow rates. • The undershot cross-flow water turbine is applicable to open channels of various widths and depths. • The effect of flow rate on the performance of the present water turbine were elucidated. • The flow field was elucidated at different flow rates. • The breakdown of the torque into its flow rons and dynamic components were elucidated. [ABSTRACT FROM AUTHOR]

— A better understanding of the flow field of the cross-flow turbine (CFT) will be useful in its design and operation. As far as is known, no comprehensive study carried out relating to the effect of Reynolds number to turbulent shear stress, shear wall, energy kinetic turbulent, dissipation rate and Reynolds stress, and the occurrence of vortices around the runners of the CFTs. This study was designed to investigate the flow field in the nozzle and runner of the CFT using computational fluid dynamics (CFD) method. CFD methods were chosen because they can visualize detailed flow patterns that other methods cannot. The setups used in the CFD method such as two-dimensional unsteady simulations, six degrees of freedom features, shear stress transport k-ω turbulent model, and pressure-based solver. Based on results, for the nozzle, the shape of the velocity profile shows that the highest momentum flux occurs at the end of the nozzle, near the runner. Distribution of shear wall was highest at the base and tip of the nozzle; it was lowest at the centre. The turbulent kinetic energy profile at the nozzle was proportional to the turbulent boundary layer profile, Reynolds stress and eddy viscosity. This indicated that nozzle shape affects the momentum flux; therefore, good nozzle geometry can transfer the maximum water energy into the blade. The nozzle’s optimum geometry can be achieved by discharge and direction, optimizing velocity magnitude. This minimizes energy loss due to friction between the stream, vortex and mass of wasted fluid. For the runner, the highest turbulent kinetic energy, dissipations rate and Reynolds stress were located at the runner. Not all the water’s energy converted into mechanic energy because the part of that energy was used in mixing between water and air. The establishment of lift force on the active blades was not caused by the flow field that crosses the upper part of the blade, but by the momentum of water that hit the lower part of the blade. A vortex formed due to separation of the flow from the blade significantly affected the runner’s performance rather than rotational flow (air phase) in the CFT. [ABSTRACT FROM AUTHOR]

Marine hydrokinetic turbines are a promising source of renewable energy, but the costs of existing cross-flow turbine designs are too high. In order to develop new designs with reduced costs, there exists a need for a low-order computational model that is simultaneously fast, accurate, and robust. Herein, we present a novel theoretical model for the analysis of cross-flow propellers and turbines based on the vortex lattice method (VLM). In order to overcome the limitation that the VLM ignores viscous effects (such as trailing-edge flow separation), we present a novel method to account for viscous-thickness-load coupling (VTLC). In order to overcome the difficulty that wake passages cause the VLM to be unstable, we present the panel-averaged induced velocity for the influence of wake vortices on the blades. Herein, we describe our model and provide verification versus RANS CFD. Results show that our VLM + VTLC model predictions are three times more accurate than prior low-order models (i.e. three times lower root mean square error to high-fidelity RANS results) yet 10,000 times faster than RANS over a wide range of operating conditions. Image 1 • Cross-flow turbomachines are modeled via vortex lattice method (VLM). • Viscous-thickness-load coupling (VTLC) is derived to model viscous effects. • VTLC accurately models loss in lift due to trailing-edge flow separation. • VLM + VTLC yields fast and accurate predictions of cross-flow turbomachine performance. [ABSTRACT FROM AUTHOR]

Cross-flow hydrokinetic turbines have sparked interest among fluid dynamicists for their potential for power enhancement in paired configuration. Following the first part of a laboratory-scaled turbine wake measurement project, this second part presents a monoscopic particle image velocimetry measurement of the near-wake of two cross-flow hydrokinetic turbines in six different counter-rotating configurations. The turbines operated in a small water flume at an average diameter-based Reynolds number of 2×104 with the incoming streamwise velocity of 0.316 m/s. The six configurations included two turbine separation distances, two turbine phase angles differences, and two different relative incoming flow angles. Similar to the observation of the single turbine configurations in part I, a correlation between flow field structures and the corresponding power output was observed. Effects of each parameter of the counter-rotating configurations are further discussed, which suggest guidelines for setting up multiple devices in a power farm. This article is accompanied by all full numeric data sets and videos of the results.

An open-source 2D Reynolds-averaged Navier–Stokes (RANS) simulation model was presented and applied for a laboratory-scaled cross-flow hydrokinetic turbine and a twin turbine system in counter-rotating configurations. The computational fluid dynamics (CFD) model was compared with previously published experimental results and then used to study the turbine power output and relevant flow fields at four blockage ratios. The dynamic stall effect and related leading edge vortex (LEV) structures were observed, discussed, and correlated with the power output. The results provided insights into the blockage effect from a different perspective: The physics behind the production and maintenance of lift on the turbine blade at different blockage ratios. The model was then applied to counter-rotating configurations of the turbines and similar analyses of the torque production and maintenance were conducted. Depending on the direction of movement of the other turbine, the blade of interest could either produce higher torque or create more energy loss. For both of the scenarios where a blade interacted with the channel wall or another blade, the key behind torque enhancement was forcing the flow through its suction side and manipulating the LEV.

Recent developments in marine hydrokinetic (MHK) technology have put the cross-flow (often vertical-axis) turbines at the forefront. MHK devices offer alternative solutions for clean marine energy generation as a replacement for traditional hydraulic turbines such as the Francis, Kaplan, and Pelton. Following previous power measurements of laboratory-scaled cross-flow hydrokinetic turbines in different configurations, this article presents studies of the water flow field immediately behind the turbines. Two independent turbines, which operated at an average diameter-based Reynolds number of approximately 0.2×105, were driven by a stepper motor at various speeds in a closed circuit water tunnel with a constant freestream velocity of 0.316 m/s. The wakes produced by the three NACA0012 blades of each turbine were recorded with a monoscopic particle image velocimetry technique and analyzed. The flow structures with velocity, vorticity, and kinetic energy fields were correlated with the turbine power production and are discussed herein. Each flow field was decomposed into the time averaged, periodic, and random components for all the cases. The results indicate the key to refining the existed turbine design for enhancement of its power production and serve as a baseline for future comparison with twin turbines in counter-rotating configurations.

Abstract During the boreal summer of 2015, a full-scale hydrokinetic turbine was deployed in the Kvichak River (Alaska), delivering electricity to the village of Igiugig. Here, quantification and analysis of the hydrodynamic modifications in the river caused by the turbine are presented. Field observations are used to produce a unique three-dimensional data set of fluid velocities in the vicinity of the turbine before and after turbine deployment. Three dynamic regions are distinguished in the wake. There is an induction zone just upstream of the turbine, where velocities decrease and turbulence increases. There is a near wake just downstream of the turbine, where the reduced velocities recover slightly and the elevated turbulence decays rapidly. Finally, there is a far wake well beyond the turbine, where reduced velocities are persistent and turbulence remains elevated. The results are used in a coarse energy budget for the river, including quantifying the total energy dissipated by turbulence in the near wake. This wake dissipation is found to be almost as large as the energy extracted for electricity generation, even when the turbine is not operational. Highlights • Unique data set of flow conditions around a full-scale hydrokinetic turbine. • Persistent turbine wake observed, with no recovery downstream of the turbine. • Similar operational and non-operational turbine wakes. • River energy loss in the turbine wake is comparable to the energy delivered to the grid by the turbine. [ABSTRACT FROM AUTHOR]

Harbour porpoise echolocation activity within and outside of the FORCE tidal turbine demonstration area in Minas Passage of the upper Bay of Fundy, Nova Scotia, has been monitored by up to eight bottom-moored (30-80 m depth) C-POD click train detectors across 1,210 days between May 2011 and May 2018. Detections occurred on almost every day (98.9% of days), averaging 86.5% across C-PODs, at a median of seven detection positive minutes per day and for a maximum of 44 minutes per day, noting that porpoise detection probabilities using buoy-mounted C-PODs should be considered minimum estimates. No confirmed dolphin click detections were documented. A statistical (Generalized Additive Model using GEE, or GAM- GEE) approach with a logit link function confirmed porpoise presence varied significantly by time of year (peak period mid-June and a lower secondary peak early November), by tidal current speed and tidal height (preference for 0-2.5 m/s ebb tides), by time of day (higher click activity at night), and across the lunar cycle. Seasonal and diel variability may be partly driven by prey accessibility. C-POD monitoring performance (termed Percent Time Lost) varied spatially and temporally. The Percent Time Lost was associated with non-biological sediment movement-related clicks causing the C-POD's maximum click detection memory buffer to be exceeded, notably during very high (e.g., spring tide) tidal speeds. The effects of these covariates were controlled for within the subsequent GAM-GEE modelling process to assess the effects of the installation and 130 days of operations of a 16 m diameter 2 MW OpenHydro tidal turbine. Importantly, despite the winter turbine deployment period, the C-POD data highlighted that porpoises were not excluded by turbine installation and operations over the mid-field study area, with click detections occurring on 98.5% of days, averaging 82% across C-PODs, with a median of three minutes per day. However, GAM-GEE modelling identified a significant decrease in porpoise click activity when the turbine was operational; this effect varied spatially, with a significant decrease in porpoise activity at the two sites closest (200-230 m) to the operating turbine berth of the five C-POD monitoring sites assessed. C-PODs at two sites that were more than a kilometre away did not show a significant turbine effect, while a third site, at 1,690 m from the turbine and in deeper water to the south, showed an increase in porpoise click detections. A longer time series of turbine deployment is believed required before robust conclusions can be drawn on turbine effects. Nevertheless, it was clear that after turbine removal, porpoise click activity rates returned to pre-installation baseline rates. [ABSTRACT FROM AUTHOR]

Hydrokinetic turbines, which convert the kinetic energy of flowing water into electrical energy, are increasingly being deployed in arrays, much like multiple wind turbines are commonly organized into a wind farm. Deploying multiple devices in an array can increase the amount of energy produced from a site and tends to make project economics more favourable. Despite growing interest in hydrokinetic turbine arrays, relatively few researchers have studied arrays and limited information exists concerning the character of the turbulent flows within arrays and methods for optimizing array layout, particularly for cross-flow turbines. This paper presents results from a set of experiments conducted in a current flume in which the wakes produced by a single cross-flow turbine and pairs of identical cross-flow turbines were measured in detail. Two different ambient turbulence intensities were studied so that the effect of turbulence level on wake recovery could be quantified. The velocity data has been analyzed to identify the mean velocity and the turbulence intensity in the turbine wake(s). These experiments form one component of a larger study that aims to develop numerical methods and guidelines to help project developers optimize the design of turbine arrays. [ABSTRACT FROM AUTHOR]

Abstract Three-dimensional steady-state RANS simulations of the flow around a stationary finite-span blade have been carried out. The analysis is directed toward the vorticity dynamics, the forces acting on the blade and their spanwise distributions when using end-plates or detached end-plates, namely end-plates that are not in contact with the blade tips. The objective is to understand the physics at play in the vicinity of the blade tips with an emphasis on the detached end-plates, as the use of such devices could be beneficial for optimizing the performance of vertical-axis turbines. This paper also aims at providing simple guidelines regarding the use of end plates and detached end-plates. The results show that the presence of detached end-plates leads to an important increase of the blade lift coefficient in addition to a significant reduction of its drag coefficient. This is explained by the fact that the detached end-plates modify the path of the vorticity lines shed by the blade, thus affecting the vorticity distribution and the resulting circulation along the blade. This work also shows that the forces on the blade are more sensitive to the dimensions of the detached end-plates in the transverse direction than to any other geometric parameter. Highlights • Both end-plates and detached end-plates strongly affect the forces on a blade. • Detached end-plates alter the distribution of the vorticity shed in the wake. • Detached end-plates do not affect the total circulation of the trailing vortices. • The forces are mainly sensitive to the transverse dimensions of the end-plates. [ABSTRACT FROM AUTHOR]

Oscillating hydrofoils were installed in a water tunnel as a surrogate model for a hydrokinetic cross-flow tidal turbine, enabling the study of the effect of flexible blades on the performance of those devices with high ecological potential. The study focuses on a single tip-speed ratio (equal to 2), the key non-dimensional parameter describing the operating point, and solidity (equal to 1.5), quantifying the robustness of the turbine shape. Both parameters are standard values for cross-flow tidal turbines. Those lead to highly dynamic characteristics in the flow field dominated by dynamic stall. The flow field is investigated at the blade level using high-speed particle image velocimetry measurements. Strong fluid–structure interactions lead to significant structural deformations and highly modified flow fields. The flexibility of the blades is shown to significantly reduce the duration of the periodic stall regime; this observation is achieved through systematic comparison of the flow field, with a quantitative evaluation of the degree of chaotic changes in the wake. In this manner, the study provides insights into the mechanisms of the passive flow control achieved through blade flexibility in cross-flow turbines.