Flexible Blades Research Articles

Aero-structural rapid screening of new design concepts for offshore wind turbines

The development of large-scale wind turbines is challenged by loads, mass, and cost growth, and more flexible blades that impact power and tower clearance. Further, the long-duration design iterations with standard-sequential-multidisciplinary processes challenge the development of new timely and robust designs. To overcome these limitations and answer open questions of large-scale wind turbines, we develop and demonstrate an aero-structural rapid screening (ASRS) design approach. ASRS uses optimization techniques, emulates a baseline controller, produces detailed blade, tower, and monopile models, and provides fast aerodynamic, structural, and economic results. Blade deflection is introduced as a design variable to achieve load-aligned designs as a function of wind speed for passive load alleviation. To illustrate the approach, a large set of 25 MW rotors for offshore and fixed-bottom wind turbines are studied for variations in blade pre-cone, load alignment via blade deflection, and upwind vs. downwind configurations. These designs vary in number of blades, blade length, axial induction factor, and airfoil family. We demonstrate that blade deflection can be designed to maximize energy capture and minimize turbine mass. ASRS is demonstrated to identify trends and trade-offs that are useful to down-select to options to be studied further using aero-servo-elastic simulations with high-fidelity control design.

Influence of Blade Flexibility on the Dynamic Behaviors of Monopile-Supported Offshore Wind Turbines

At present, monopile-supported offshore wind turbines (MOWTs) are widely used in offshore wind farms. The influence of blade flexibility on the dynamic behaviors of MOWTs excited by waves and earthquakes was investigated in this study. Numerical analysis models were established for 5 MW and 10 MW MOWTs, incorporating flexible and rigid blade configurations. The modes and natural frequencies of the full system were compared between these two numerical models, and their dynamic responses were evaluated under wave-only and earthquake-only excitations. It was revealed that the influence of blade flexibility on the first- and second-order modes of the system can be neglected. The dynamic response of these MOWTs under wave excitation can be predicted by the rigid blade model, where the maximum relative difference is less than 5%. However, higher-order modes of the system are significantly affected by the blade flexibility. Under high-frequency excitations, these higher-order modes of the system are remarkably stimulated. Additionally, a large relative difference, exceeding 50%, is detected when the rigid blade model is used to predict the seismic response of the two MOWTs. Consequently, the blade flexibility should be adequately modeled when predicting the dynamic response of OWTs.

Including Blade Elasticity into Frequency-Domain Propeller Whirl Flutter Analysis

Whirl flutter as an aeroelastic instability can be a limiting factor in the design and certification of turboprop aircraft configurations, especially for the engine suspension. Whirl flutter prediction for these configurations is currently done in the frequency domain using rigid propeller aerodynamic derivatives. Blade flexibility is neglected in this process, although it is known to have an impact on whirl flutter predictions. This paper uses frequency-domain transfer matrices for the propeller hub loads identified from a time-domain multibody simulation model of an isolated turboprop propeller and included into a frequency-domain flutter analysis to study the impact of blade elasticity on propeller whirl flutter. Results demonstrate a significantly stabilizing effect of blade elasticity on propeller whirl flutter due to a reduction of the destabilizing pitch-yaw cross-coupling moment. The method demonstrated in this paper is applicable to arbitrary time-domain propeller models and compatible with standard frequency-domain flutter processes, allowing for increased fidelity in the flutter prediction process.

Passive aeroelastic study of large and flexible wind turbine blades for load reduction

In this paper, passive aeroelastic coupling of wind turbine blades for load reduction is realized by introducing bending-twist coupling (BTC) and geometric sweep coupling (GSC). The BTC is implemented by composite fiber off-axis of the blade spar cap, and the GSC is achieved by defining sweep curves of the blade reference axis. To investigate the passive coupling effect and load reduction of the large and flexible blades, the IEA 15 MW blades are considered. BTC are introduced into the three-dimensional finite element (FE) models and the residual stiffness coefficient is obtained. Meanwhile, geometrically exact beam theory (GEBT) is used to build the flexible blade. Both BTC and GSC are introduced into the beam model, and the effect elastic deformation on blade load is also considered. Multi-objective optimization (MOO) process of rotating blades for load reduction is proposed under steady-state rated conditions. The results show that the highest coupling efficiency can be obtained when the fiber off-axis angle is between 12° and 15°. The BTC of the blade can reduce the load of blade root with a small fiber off-axis angle. Meanwhile, the GSC can further reduce the axis torque. Compared with the initial design of IEA 15 MW steady-state rotating blade, the maximum thrust, flapwise moment and axis torque of the blade can be reduced by 8.8 %, 13.4 % and 22.9 % respectively.

Achieving High‐Efficiency Wind Energy Harvesting Triboelectric Nanogenerator by Coupling Soft Contact, Charge Space Accumulation, and Charge Dissipation Design

AbstractTo enhance the durability of a triboelectric nanogenerator (TENG), soft contact is an effective approach due to its flexible and elastic contact mode. However, soft contact is hard to obtain with a large charge density by triboelectrification, resulting in low power output. Herein, a novel blade soft contact TENG (BSC‐TENG), coupling flexible functional blades with shielded electrodes on the rotor, charge accumulation, and charge dissipation design on the stator, is proposed. Extra polishing blades and debris storage grooves are adopted in the BSC‐TENG to further ensure high durability. A remarkable charge density of 328 µC m−2 is achieved, setting a new record for soft contact TENGs. Besides, the output charge remains at 100% even after 200 000 cycles. The wind‐driven BSC‐TENG not only can power 3840 green LEDs and 80 parallel hygrothermometers but also can drive electronic devices for smart farms, establishing self‐powered sensing systems. This work provides a novel strategy for enhancing soft contact TENG output and durability.

Enhancing Lower Extremity Defect Coverage: High Viability Ultra-Thin Split-Thickness Skin Grafts Obtained from the Scalp.

Repairing lower extremity defects presents challenges due to the scarcity of available local tissue. Skin grafting is a widely employed technique for addressing non-healing ulcers, improving the quality of life of patients and minimizing discomfort. However, using traditional donor sites, such as the thigh, can hinder mobility and result in noticeable scarring and pigmentation changes. This study aims to assess the effectiveness of a novel approach utilizing autologous ultra-thin split-thickness skin grafts (STSGs) harvested from the scalp using a disposable, commercially available razor blade named DermaBlade. Fifteen patients (median age: 72 years, eight males and seven females) with diverse lower limb lesions, including carcinomas and ulcers of varying etiologies, were prospectively enrolled. Donor sites included the sideburn extending to the hairy temporal skin (nine cases) and hairy occipital skin (six cases). Ultra-thin skin strips (<0.2 mm thick) were obtained from the scalp through the use of the disposable flexible blade DermaBlade. The strips were positioned over the receptor area with no sutures in most cases and secured using dressings. A substantial majority of patients (90%) achieved successful graft take with no complications. Swift re-epithelialization occurred within a median of 12 days for the donor site and 24 days for the receptor site. No hair transfer or alopecic scars were noted. In contrast to traditional grafting methods, DermaBlade-assisted scalp grafting yields highly viable STSGs that adhere to wound beds without the need for sutures. Notable advantages of this technique encompass rapid wound healing, minimal complications, and superior cosmetic outcomes. Furthermore, it avoids scarring and alopecia, making it a promising approach for addressing lower extremity defects.

A new criterion for the instability threshold of a square tube bundle subject to an air–water cross-flow

In this paper, we investigate the fluid-elastic instability of a square tube bundle subject to two-phase cross-flow. A dimensional analysis is carried out, leading to a new criterion of instability. This criterion establishes a direct link with the instability thresholds in single-phase flows. In parallel to the dimensional analysis, experimental work is carried out to i) determine the instability thresholds in single-phase flows (new relation between the Scruton, Stokes and Reynolds number), ii) to test the validity of the two-phase flow instability criterion, derived from the dimensional analysis. The experiments are carried out on a square tube bundle (pitch ratio of 1.5) consisting on 5 rows of 3 tubes (plus two end-rows of half tubes). The central tube is mounted on two flexible blades allowing a vibration in the transverse direction only, whereas all the other tubes are rigid. The instability threshold in single-phase (water) flow is obtained from a method of direct measurement of the fluid-elastic forces, in which the motion of the central tube is imposed. The instability threshold in two-phase (air–water) flow is obtained from a method of indirect measurement, with an active system stability control, of the fluid-elastic forces, in which the central tube vibrates freely. Three sets of blades with different stiffnesses are tested to investigate the stability of the central tube. The criterion of instability is in very good agreement with air–water experiments, predictive for all homogeneous void fractions, and so for all flow regimes (identified in our experiments with an optical probe). This new criterion is of theoretical interest for the understanding of complex two-phase flow excitations, as well as of practical significance for the predictive analysis of industrial components.

LES simulation study of wind turbine aerodynamic characteristics with fluid-structure interaction analysis considering blade and tower flexibility

As wind turbine blades become longer and more flexible, aerodynamic characteristics become increasingly complex and require complete investigations. In this study, a two-way fluid-structure interaction (FSI) model is proposed to analyze the effects of inflow conditions, including uniform and atmospheric boundary layer (ABL) winds, as well as blade and tower flexibility, on the aerodynamic characteristics of wind turbine blades. A 4.5 MW real wind turbine with the rotor diameter of 152 m and the hub height of 94 m is considered. Results show that the edgewise blade motion can be disregarded, while the motion of the tower and the flap-wise motion of blade significantly influence the aerodynamic characteristics of blades. The separation flow at the blade root suction side is more distinct and becomes less significant with an increase in spanwise distance, with stagnation points primarily distributed along the flow separation boundary at 0.45–0.95L (L is the blade length). The three-dimensional rotational effect of the blade induces notable fluctuations in the pulsating pressure coefficient, particularly within this region. Notably, a prominent peak is observed at the location of 0.65L. Consequently, it is recommended to allocate additional focus on this specific region to mitigate potential fatigue loads.

The Impact of Bend–Twist Coupling on Structural Characteristics and Flutter Limit of Ultra-Long Flexible Wind Turbine Composite Blades

Flutter is an instability phenomenon that can occur in wind turbine blades due to fluid–structure interaction, particularly for longer and more flexible blades. Aeroelastic tailoring through bend–twist coupling is an effective method to enhance the aeroelastic performance of blades. In this study, we investigate the impact of bend–twist coupling on the structural performance and flutter limit of the IEA 15 MW blade, which is currently the longest reference wind turbine blade, and determine the optimal layup configuration that maximizes the flutter speed. The blade is modeled by NuMAD and iVABS, and the cross-section properties are obtained by PreComb and VABS. The accuracy of the blade model is verified in terms of stiffness and frequency. The bend–twist coupling is implemented by changing the fiber angle of the skin and spar cap considering symmetric and asymmetric layups. The flutter limits of both the baseline and the bend–twist coupled blade are evaluated based on HAWC2. The results show that the angle of spar cap carbon fiber has a greater effect on the blade’s structural properties and flutter speed than the skin fiber. Varying the spar cap carbon fiber angle increases the flutter speed, with the effect being more significant for the symmetric layup, up to 9.66% at a fiber angle of 25 degrees. In contrast, the variation in skin fiber angle has a relatively small impact on flutter speed—within ±3%.

Design of a high fidelity Fluid–Structure Interaction solver using LES on unstructured grid

Despite promising experimental works, high fidelity numerical simulations of chordwise flexible blade are useful to better understanding. However, such simulation remains a challenging problem as it requires a Fluid–Structure Interaction (FSI) solver capable of accurately predicting the stall dynamics, while computing the deformation of a solid with complex geometry. In this paper, the authors propose a LES-based FSI solver using 3D solid elements for the solid and unstructured grid for fluid and solid solvers. This approach aims at being universal and is based on a partitioned coupling scheme, allowing low density ratios of the structure to the fluid. It uses an original pseudo-solid method for the mesh movement solving, specifically developed for this work. Besides, this solver is suited for massively parallel computing and can perform Dynamic Mesh Adaptation to be able to take into account any solid movement. Both fluid and solid solvers are validated independently before validation of the FSI solver against a 2D laminar benchmark, including mesh convergence study. The entire methodology is then successfully applied to experimental 3D complex case with high Reynolds number, confirming the potential of the FSI solver for its intended use, without geometry restriction. This is finally illustrated with a simulation of an experiment involving a chordwise flexible blade with large deformation, which has never been reproduced with a 3D LES approach in literature so far.

Vibroacoustic analysis of a submarine-propeller system under self-propulsion condition from the perspective of approximate configurations

As a particular instance of the complete acoustic radiation problem for an underwater structure, structure-borne noise radiated from a combined submarine-propeller system under self-propulsion condition is investigated. The analysis procedure applies a set of numerical methods to capture different aspects of the problem: unsteady computational fluid dynamics to express the hydrodynamic excitation, finite element analysis to obtain the dynamic characteristics of the combined elastic system, and boundary element method to assess the radiated noise owing to structural vibrations. The structural and acoustic responses rely on the modal descriptions. The study has three objectives: employing the methodology in a realistic setting, investigating the validity of the proposed approximate configurations, and studying the influence of propeller blade flexibility on noise radiation. The DARPA-SUBOFF submarine and INSEAN E1619 propeller models are adopted for the application. It is found that approximate configurations provide accurate estimations for limited frequency bands and should be employed with caution. In addition, the flexibility of the propeller plays a major role in noise generation, not only by yielding higher noise levels but also by increasing the amplitude of the peak responses in the low- and high-frequency regions.

Numerical investigation of wave-induced flexible vegetation dynamics in 3D using a coupling between DualSPHysics and the FEA module of Project Chrono

Vegetation meadows in coastal waters are a key constituent of a future green defense package due to the ecosystem services they provide and the potential to attenuate wave energy. To numerically describe the vegetation dynamics under wave action, this paper presents a novel application of a numerical coupling for solving fluid–elastic structure interactions (FSI) problems involving ultra-thin elements in a 3-D environment. The extended two-way coupling employed in this work combines the mesh-free Smoothed Particle Hydrodynamics (SPH) method in the DualSPHysics code to solve the fluid flow, and the Finite Element Analysis (FEA) structural solver in Project Chrono to solve the structural dynamics. To represent the vegetation, a flexible structure based on the Euler–Bernoulli beam model is used. The beam element is embedded into the SPH domain using an envelope subdomain that is discretized using dummy boundary particles. As such, this dummy envelope serves as a decoupling interface for the geometrical properties of the structure, allowing for ultra-thin structures smaller than the initial inter-particle distance (dp). The numerical approach is validated against an experimental setup including a flexible blade swaying under the action of an oscillatory flow. The results demonstrate that the numerical model is able to resolve the wave–vegetation interaction problem. Furthermore, additional insights into the blade dynamics reveal that the swaying velocity increases linearly along the length, with the upper part swaying at a speed comparable to the fluid velocity while the stem remains relatively stationary. Additionally, the findings indicate that rigid vegetation experiences higher forces per unit length, and in systems with substantial swaying motion, energy dissipation predominantly occurs around the lower base of the vegetation.

Experimental study for the use of Na2SO4.10H2O as a PCM with fixed blades for temperature and efficiency parameters of photovoltaic panel

This study used sodium sulfate decahydrate (Na2SO4.10H2O) with different masses as a PCM. Thermal conductivity was improved by using rigid blades and heated sinks instead of flexible blades behind the panel. The results show a positive effect of fixed blades and heatsinks on temperature parameters and panel efficiency. The test was performed in the radiation range of 800 W/m2 at an angle of 35° to the horizon. During the last 70 min of the test, the panel containing phase change material and fixed blades, even after the test was over, had the same temperature as the standard panel. The temperature difference was 11.5 °C compared to the standard panel. In addition, the maximum efficiency difference between the panel with the material of the phase change and the panel with a fixed blade of the phase-changing material with a conventional panel was 3.05% and 6.17%, respectively. Furthermore, mass analysis of PCM reveals that the melting point increases by 50% of the initial mass of the phase material, resulting in a maximum temperature difference of 7.5 °C on the panel surface, a maximum efficiency difference of 2.34%.

CARD33: Analyzing Off-Design Point Operation for LVADs Using Flexible Rotor Blades

Introduction: The use of left ventricular assist devices (LVADs) has exhibited clinical success in both destination and bridge-to-transplant therapy for patients suffering from end stage heart failure (HF). However, the operating speeds of the LVAD are often tuned to accommodate patient variability which might lead to operating the LVAD at off design points. This could cause a potential loss in efficiency and an increased risk of thrombosis/hemolysis. The flexible rotor blades will deform in response to the complex inflow conditions, and with an appropriate geometry and flexibility generate a flatter performance curve. This would not only improve the preload sensitivity of the LVAD but also offer a wider range of operation with optimal efficiencies. In this study, we investigate the hydraulic performance of varied impeller geometries for a centrifugal LVAD with flexible rotor blades at pediatric and adult pressure/flow conditions. Methods: The flexible rotor blades are manufactured by casting polyurethane resin (Easton, PA) of shore hardness 60A in a four-piece mold. These mold pieces were 3D printed using a J850 Pro (Stratasys Ltd, MN, USA). The inlet angle of the blades was set at 20o and the outlet angle was varied from 10o to 70o. The blade thickness was 1.2 mm and the blade height was 7 mm. These flexible rotor blades were evaluated on a hydrodynamic test setup with 40%wt glycerin solution as the working fluid. This setup consisted of a motor to drive the rotor, a flow probe, pressure transducers upstream and downstream of the rotor and a pinch valve to vary the downstream resistance. The mounted rotor is driven at 3000 RPM, and the flow produced by the rotor is measured as the pinch valve incrementally increases downstream resistance. The hydrodynamic results were then scaled to achieve pediatric and adult pressure/flow conditions of 70mmHg-2L/min and 90mmHg-5L/min respectively. To analyze the off-design point operation, the gradient of the pressure/flow curve at two operating conditions was calculated using the equation in Figure 1. Results: For a flexible rotor blade with an outlet angle of 30o, the operating speed for pediatric and adult pressure/flow conditions was found to be 2550 RPM and 3230 RPM respectively. The gradient at the pediatric and adult operating conditions was – 6.14 mmHg/ (L/min)-1 and – 11.12 mmHg/ (L/min)-1. Results for the remaining flexible rotor blades is currently being generated and will be presented at the conference. Conclusion: We successfully analyzed the hydraulic performance of a centrifugal LVAD with flexible rotor blades. The flexible rotor blades proposed in this study will be a positive step towards improving the off-design point operation of LVADs and thereby, reducing the blood damage associated with it.Figure 1. Equation used to calculate the gradient of the pressure/flow curve at two operating conditionsFigure 2. Hydrodynamic Test Setup SchematicFigure 3. Hydrodynamic Performance Results for Flexible Rotor Blade with 30o Outlet Angle

A study of Stall-Induced Vibrations using Surrogate-Based Optimization

Stall-Induced Vibrations (SIV) are an important design consideration for wind turbine blade design, especially for large, modern, wind turbines with highly flexible blades. Their severity depends on both the inflow and the structural characteristics of the blades. Studying SIV has a high computational cost, because it requires high-fidelity aeroelastic simulations, and potentially a large number of input variables. In an effort to reduce the computational cost of the domain exploration, in this work we have adopted a Surrogate-Based Optimization (SBO) framework. This way, the combination of input variables that lead to SIV can be explored with the minimum number of aeroelastic simulations. The proposed SBO framework can use any type of surrogate model, and leverages Delaunay triangulation to iteratively select samples to refine the surrogate model. The occurrence and severity of SIV on the IEA 10MW turbine is studied in a five variables space consisting of: wind speed, yaw angle, vertical wind shear, wind veer, and atmospheric temperature. A well-trained surrogate model is developed and used to predict the damping ratio of the first blade edgewise mode in the entire inflow space at a reduced computational cost. Sensitivity analysis of the predicted damping ratio shows that yaw angle is the most influential variable, while temperature is the least influential variable in terms of inflow conditions that can lead to the occurrence of SIV. Inflow conditions with a moderate yaw angle (around 10–25 deg), high wind speeds, and moderate to high negative veer are found to lead to severe SIV. This study should serve as a guiding tool to decide the scope of the more computationally expensive simulations such as high-fidelity CFD-based aeroelastic simulations which can provide a more accurate description of SIV.

Aeroelastic model of flexible blades of wind turbines under complex wind speed profiles

Aeroelastic model of flexible blades of wind turbines under complex wind speed profiles

Deformation error compensation in 5-Axis milling operations of turbine blades

The precision and performance of machined flexible parts are under influence of deformation errors during end milling operations. Thus, prediction and compensation of deformation errors during milling operations of flexible parts can provide a key tool in accuracy enhancement of part production. In this study, an improved virtual machining system is proposed in order to assess and compensate deformation errors caused by cutting temperature and forces in 5-axis milling operations of flexible parts. The improved Johnson–Cook model is utilized to investigate the cumulative impact of strain rate and deformation temperatures on flow stress during milling operations of turbine blade. To estimate deformation errors caused by cutting forces and temperature on the workpiece and cutting tool, the finite element analysis is then applied. As a result, volumetric vectors of deformation error at each cutting location along the machining pathways are then generated in order to be compensated within new compensated machining tool paths. Thus, the deformation error created by cutting forces and temperature on the workpiece and cutting tool are compensated in order to enhance accuracy during 5-axis milling operation of flexible turbine blades. Experiments are carried out using a 5-axis CNC machine tool and errors are quantified using a CMM to verify the developed strategy in the study. As a consequence, precision of machining operations on flexible turbine blades can be enhanced by employing the developed virtual machining system in the study.

Numerical simulations of the wake and deformations of a flexible rotor blade in a turbulent flow

We present, for the first time, the mean deflection evolution of a flexible rotor blade using a coupled model based on Navier–Stokes equations, for the fluid flow, and linear elasticity equations for the blade deformation. Three turbulence models are tested to reach Reynolds numbers as high as 8 104. The absolute tip speed ratios are in the range [0,25]. The numerical results are validated by comparisons with available tip displacements from experiments. For the parameter ranges, above mentioned, the elastic behavior of the flexible rotor is characterized, and the vorticity field is compared with results obtained for a rigid rotor. The effects of the pitch, the tip speed ratio (or frequency), and its sign on the blade deformation are reported. Typically, the blade deforms in the downstream direction, and it is shown that this deformation is a non-monotonic function of the rotation frequency and the pitch angle. Furthermore, it is found that, for particular values of the frequency and pitch angle, the blade is subject to deformations in the upstream direction. It is shown also that the flexible rotor could develop a vortex ring state, but not the rigid one, under the same conditions. It is found that there is a supercritical frequency associated with the apparition of this vortex ring state and this frequency occurs for negative pitches only, for the considered blade. The vorticity field revealed, as well, that the tip vortex changes sign with that of the blade deflection. Finally, we present the effect of the pitch and frequency on the twist angle of the blade and characterize its evolution along the span.

Computational Fluid‐Structure Interaction towards Simulating Large Wind Turbines with openFOAM and deal.II Coupled via preCICE

AbstractWind energy is an essential part of the Green Deal. The trend to increase the size of wind turbines, especially offshore, introduces additional dynamic effects at the long and flexible blades. Embedded in the CRC 1463, DFG, we are working on the fluid‐structure interaction to avoid dynamic stall and investigate flutter effects and blade breathing of ultra‐slim blades [1,2]. This requires an accurate numerical setup that reliably captures the fluid‐structure interactions due to the highly turbulent flow and large deformations of the blades. In preliminary work, the Unsteady Reynolds‐averaged Navier‐Stokes method (URANS) in openFOAM [3] was used to simulate the flow around rotating helicopter blades with a changing angle of attack. [4] successfully predicted the distinct dynamic stall hysteresis with moderate computational effort and captured extreme values (load peaks) within the experimental uncertainties. This aerodynamic solver is to be coupled with a structural solution, for which deal.II [5] provides the linear elastic blade model. The fluid and the structure solvers are coupled via the software preCICE [6] and solved with a staggered approach. Numerical results are presented for a simplified 2D cross‐section of a rectangular solid of carbon‐fiber‐reinforced polymers and a steady inflow velocity. Key challenges for the coupling of the solvers are discussed and the future work is outlined.

Effect of Vertical Confinement and Blade Flexibility on Cross-Flow Turbines

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.