Flexible Blades Research Articles

Comparison and Analysis of Heat Transfer Enhancement for Wastewater Heat Exchanger in Wastewater Source Heat Pump System

In order to improve the heat transfer performance of the heat exchanger and solve the problem of descaling and anti-fouling, the wastewater heat exchanger was improved on the basis of previous research through inserting flexible single helix rod, flexible double helix rod and flexible helical blade into the heat exchange tube, respectively. Helical pitch and rotating speed were the key factors to compare the heat transfer performance of the models. The simulation results show that in terms of helical pitch, the tube with the built-in rotating helical blade while the helical pitch is 10 mm should be chosen as the optimum heat exchange tube; in terms of rotating speed, the tube with the built-in rotating helical blade while the rotating speed is 40 rad/s should be chosen as the optimum heat exchange tube.

Multi-fidelity fluid–structure interaction analysis of a membrane blade concept in non-rotating, uniform flow condition

Abstract. In order to study the aerodynamic performance of a semi-flexible membrane blade, fluid–structure interaction simulations have been performed for a non-rotating blade under steady inflow condition. The studied concept blade has a length of about 5 m. It consists of a rigid mast at the leading edge, ribs along the blade, tensioned edge cables at the trailing edge and membranes forming the upper and lower surface of the blade. Equilibrium shape of membrane structures in the absence of external loading depends on the location of the supports and the prestresses in the membranes and the supporting edge cables. Form-finding analysis is used to find the equilibrium shape. The exact form of a membrane structure for the service conditions depends on the internal forces and also on the external loads, which in turn depend on the actual shape. As a result, two-way coupled fluid–structure interaction (FSI) analysis is necessary to study this class of structures. The fluid problem has been modelled using two different approaches, which are the vortex panel method and the numerical solution of the Navier–Stokes equations. Nonlinear analysis of the structural problem is performed using the finite-element method. The goal of the current study is twofold: first, to make a comparison between the converged FSI results obtained from the two different methods to solve the fluid problem. This investigation is a prerequisite for the development of an efficient and accurate multi-fidelity simulation concept for different design stages of the flexible blade. The second goal is to study the aerodynamic performance of the membrane blade in terms of lift and drag coefficient as well as lift-to-drag ratio and to compare them with those of the equivalent conventional rigid blade. The blade configuration from the NASA-Ames Phase VI rotor is taken as the baseline rigid-blade configuration. The studied membrane blade shows a higher lift curve slope and higher lift-to-drag ratio compared with the rigid blade.

Aeroelastic stability of a flexible ribbon rotor blade

This paper describes the static and dynamic aeroelastic behavior of a thin ribbon that is used as an extremely flexible helicopter rotor blade. The non-dimensional torsional stiffness of this rotor blade is three orders of magnitude lower than that of a conventional helicopter rotor blade. As a result, the rotor blade undergoes large torsional deformation and its static and dynamic behavior are dominated by centrifugal forces. An aeroelastic analysis is developed based on Euler–Bernoulli beam theory including large twist angles and unsteady aerodynamics including the effect of returning wake. The flow is assumed to be attached at all times, and only classical divergence and flutter stability are evaluated. The analysis is validated with deformation measurements of a 23cm diameter rotor with ribbon blades. Divergence and flutter stability boundaries are identified, and the effects of rotational speed, rotor diameter, location of blade center of gravity and blade pitch are discussed. The analysis can be used as a design tool for flexible ribbon rotors in a variety of missions.

Effect of linear and non-linear blade modelling techniques on simulated fatigue and extreme loads using Bladed

There is a trend in the wind industry towards ever larger and more flexible turbine blades. Blade tip deflections in modern blades now commonly exceed 10% of blade length. Historically, the dynamic response of wind turbine blades has been analysed using linear models of blade deflection which include the assumption of small deflections. For modern flexible blades, this assumption is becoming less valid. In order to continue to simulate dynamic turbine performance accurately, routine use of non-linear models of blade deflection may be required. This can be achieved by representing the blade as a connected series of individual flexible linear bodies - referred to in this paper as the multi-part approach. In this paper, Bladed is used to compare load predictions using single-part and multi-part blade models for several turbines. The study examines the impact on fatigue and extreme loads and blade deflection through reduced sets of load calculations based on IEC 61400-1 ed. 3. Damage equivalent load changes of up to 16% and extreme load changes of up to 29% are observed at some turbine load locations. It is found that there is no general pattern in the loading differences observed between single-part and multi-part blade models. Rather, changes in fatigue and extreme loads with a multi-part blade model depend on the characteristics of the individual turbine and blade. Key underlying causes of damage equivalent load change are identified as differences in edgewise- torsional coupling between the multi-part and single-part models, and increased edgewise rotor mode damping in the multi-part model. Similarly, a causal link is identified between torsional blade dynamics and changes in ultimate load results.

From Wind to the Electric Grid: Comprehensive Modeling of Wind Turbine Systems

A detailed model of a full-conversion wind turbine (WT) is presented in this article. The developed model uses cosimulation by several freeware WT simulation tools-such as the turbulent wind simulator (TurbSim) and Fatigue, Aerodynamics, Structures, and Turbulence (FA ST), both from the National Renewable Energy Laboratory (NREL)-and commercial software to perform a detailed analysis of all WT components. An optimal level of model complexity is achieved to allow the design, analysis, and simulation of the controls and subsystem interactions within a turbine under various conditions. The article introduces a maximum power point tracking (MPPT) control for a variable-speed WT with stall power regulation and analyzes its influence on flexible blades and the drivetrain. The proposed simulation tools and the developed model reveal some stability issues and subsystem interaction complexities that are not visible in simpler models. In this article, modeling results are presented that detail the performance of the WT system.

Aeroelastic simulation of multi-MW wind turbines using a free vortex model coupled to a geometrically exact beam model

The current paper investigates the aeroelastic modelling of large, flexible multi- MW wind turbine blades. Most current performance prediction tools make use of the Blade Element Momentum (BEM) model, based upon a number of simplifying assumptions that hold only under steady conditions. This is why a lifting line free vortex wake (LLFVW) algorithm is used here to accurately resolve unsteady wind turbine aerodynamics. A coupling to the structural analysis tool BeamDyn, based on geometrically exact beam theory, allows for time-resolved aeroelastic simulations with highly deflected blades including bend-twist, coupling. Predictions of blade loading and deformation for rigid and flexible blades are analysed with reference to different aerodynamic and structural approaches. The emergency shutdown procedure is chosen as an examplary design load case causing large deflections to place emphasis on the influence of structural coupling and demonstrate the necessity of high fidelity structural models.

Open Access Wind Tunnel Measurements of a Downwind Free Yawing Wind Turbine

A series of free yawing wind tunnel experiments was held in the Open Jet Facility (OJF) of the TU Delft. The ≈ 300 W turbine has three blades in a downwind configuration and is optionally free to yaw. Different 1.6m diameter rotor configurations are tested such as blade flexibility and sweep. This paper gives a brief overview of the measurement setup and challenges, and continues with presenting some key results. This wind tunnel campaign has shown that a three bladed downwind wind turbine can operate in a stable fashion under a minimal yaw error. Finally, a description of how to obtain this open access dataset, including the post-processing scripts and procedures, is made available via a publicly accessible website.

The impact of geometric non-linearities on the fatigue analysis of trailing edge bond lines in wind turbine rotor blades

The accurate prediction of stress histories for the fatigue analysis is of utmost importance for the design process of wind turbine rotor blades. As detailed, transient, and geometrically non-linear three-dimensional finite element analyses are computationally weigh too expensive, it is commonly regarded sufficient to calculate the stresses with a geometrically linear analysis and superimpose different stress states in order to obtain the complete stress histories.In order to quantify the error from geometrically linear simulations for the calculation of stress histories and to verify the practical applicability of the superposition principal in fatigue analyses, this paper studies the influence of geometric non-linearity in the example of a trailing edge bond line, as this subcomponent suffers from high strains in span-wise direction. The blade under consideration is that of the IWES IWT-7.5-164 reference wind turbine. From turbine simulations the highest edgewise loading scenario from the fatigue load cases is used as the reference. A 3D finite element model of the blade is created and the bond line fatigue assessment is performed according to the GL certification guidelines in its 2010 edition, and in comparison to the latest DNV GL standard from end of 2015.The results show a significant difference between the geometrically linear and non-linear stress analyses when the bending moments are approximated via a corresponding external loading, especially in case of the 2010 GL certification guidelines. This finding emphasizes the demand to reconsider the application of the superposition principal in fatigue analyses of modern flexible rotor blades, where geometrical nonlinearities become significant. In addition, a new load application methodology is introduced that reduces the geometrically non-linear behaviour of the blade in the finite element analysis.

Dynamic stability and nonlinear vibration analysis of a rotor system with flexible/rigid blades

In this paper, the primary resonances of a coupled flexible rotor with rigid disk and flexible/rigid blades are investigated. The Euler-Bernoulli beam theory is used to model the blade and shaft. The equations of motion are derived with the aid of the extended Hamilton principle. To simplify the equations of motion, the Coleman and complex transformations are used. The multiple scales method is used to analyze the primary resonances of the system. The influences of mass eccentricity and the damping of the surrounding medium on the steady state responses of the system are studied. It can be seen that rotor's damping values that guarantee the stability of system with flexible blades are higher than those that impose stable conditions in system with rigid blades. In addition, the system with rigid blades becomes completely stable in higher values of the mass eccentricity compared to the system with flexible blades.

Flow control for the vertical axis wind turbine by means of flapping flexible foils

An active flow control mechanism is proposed to improve the efficiency of the energy extraction for the vertical axis wind turbine. The proposed system consists of a vertical axis wind turbine with flexible blades. The conception is inspired from the vortex control mechanism utilized by the aero-/aqua animals to improve their performance via the flexion of their fins. The viscous non-stationary flow around the turbine is simulated using the ANSYS-FLUENT 15 software. The complex flapping motion is reproduced using a dynamic mesh technique and a user-defined function. The results show that, with this strategy of control, the turbine generates a higher moment coefficient due to the increase in the peaks of lift force caused by a better difference in the pressure between the two sides of the blade due to the flexure motion. The turbine power coefficient can reach 38 % enhancement for the optimal flow control conditions.

Numerical simulation on a straight-bladed vertical axis wind turbine with auxiliary blade

To improve the starting performance of the straight-bladed vertical axis wind turbine (SB-VAWT) at low wind speed, and the output characteristics at high wind speed, a flexible, scalable auxiliary vane mechanism was designed and installed into the rotor of SB-VAWT in this study. This new vertical axis wind turbine is a kind of lift-to-drag combination wind turbine. The flexible blade expanded, and the driving force of the wind turbines comes mainly from drag at low rotational speed. On the other hand, the flexible blade is retracted at higher speed, and the driving force is primarily from a lift. To research the effects of the flexible, scalable auxiliary module on the performance of SB-VAWT and to find its best parameters, the computational fluid dynamics (CFD) numerical calculation was carried out. The calculation result shows that the flexible, scalable blades can automatic expand and retract with the rotational speed. The moment coefficient at low tip speed ratio increased substantially. Meanwhile, the moment coefficient has also been improved at high tip speed ratios in certain ranges.

Impact of current speed on mass flux to a model flexible seagrass blade

AbstractSeagrass and other freshwater macrophytes can acquire nutrients from surrounding water through their blades. This flux may depend on the current speed (U), which can influence both the posture of flexible blades (reconfiguration) and the thickness of the flux‐limiting diffusive layer. The impact of current speed (U) on mass flux to flexible blades of model seagrass was studied through a combination of laboratory flume experiments, numerical modeling and theory. Model seagrass blades were constructed from low‐density polyethylene (LDPE), and 1, 2‐dichlorobenzene was used as a tracer chemical. The tracer mass accumulation in the blades was measured at different unidirectional current speeds. A numerical model was used to estimate the transfer velocity (K) by fitting the measured mass uptake to a one‐dimensional diffusion model. The measured transfer velocity was compared to predictions based on laminar and turbulent boundary layers developing over a flat plate parallel to flow, for which and , respectively. The degree of blade reconfiguration depended on the dimensionless Cauchy number, Ca, which is a function of both the blade stiffness and flow velocity. For large Ca, the majority of the blade was parallel to the flow, and the measured transfer velocity agreed with laminar boundary layer theory, . For small Ca, the model blades remained upright, and the flux to the blade was diminished relative to the flat‐plate model. A meadow‐scale analysis suggests that the mass exchange at the blade scale may control the uptake at the meadow scale.

Lumbar Neuroforaminal Decompression with a Flexible Microblade Shaver System: Results of a Cadaveric Study

The lumbar neural foraminal stenosis still is a challenging condition in minimally invasive spine surgery. Because of the anatomic situation a complete decompression of the nerve root often leads to a subtotal facetectomy associated with potential instability and the need for additional instrumentation of the decompressed segment. The iO-Flex system was introduced to address this problem by using a minimally invasive wire-guided microblade shaver to increase the neuroforaminal space by reducing the stenosis from intraforaminal while sparing bigger parts of the facet joint. In this study, we evaluated the feasibility and the surgical and radiological success in relation to the experience of the surgeon. We performed decompression of the neuroforamen in 10 lumbar levels of 2 fresh-frozen human cadavers. Before and after decompression, we obtained high-resolution computed tomography data to evaluate the diameter of the neural foramen. The mean foraminal width (7.88-10.94 mm, P<0.0001) and area (123.27-149.18 mm2, P < 0.003) increased significantly after the decompression, whereas the facet joints area (131.9-107.51 mm2, P < 0.005) and width (16.4-13.75 mm, P < 0.001) indeed decreased significantly but with an overall reduction of facet joint width by 16% and facet joint area by 18%. No complications such as nerve root damages or dural tears were observed. The flexible micro blade shaver system is feasible with a steep learning curve and achieves sufficient decompression of the neuroforamen in this cadaveric study.

Numerical Analysis of the Three-Dimensional Nonstationary Flow of Ideal Gas in the Last Stage of Turbine Machine Taking into Consideration the Nonaxisymmetric Exhaust Pipe Branch

A problem related to the forecast of the aeroelastic behavior and aeroelastic instability of blades (in particular self-oscillations, flutter, and resonance vibrations) becomes of great importance for the development of high-loaded compressor and vent rows and the last turbine stages whose long and flexible blades can be exposed to such phenomena. The solution of this problem requires the development of new models for the nonstationary three-dimensional flow, the use of contemporary numeric methods and the comparison of theoretical and experimental data. This scientific paper gives the data of numerical simulation of the 3-D flow of ideal gas passing through the last stage of turbine machine taking into account the flow nonuniformity caused by guide blades and nonuniform pressure distribution in the exhaust pipe branch and also nonstationary effects caused by blade vibrations. The numerical method is based on the solution of combined aeroelastic problem for the 3-D flow of ideal gas passing through the turbine stage and the nonaxisymmetric exhaust pipe branch including the annular diffuser. To solve the combined problem a partially integral method was used that includes integral equations of gas dynamics (Euler equations) and vibrating blade dynamics (modular approach) at each time step with the information exchange. The given method of the solution of combined aeroelastic problem allows us to predict the amplitude-frequency spectrum of blade vibrations in the three-dimensional flow of ideal gas including forced self-excited vibrations and self-vibrations to increase efficiency and reliability of the blade units of turbine machines.

Dynamic modelling and experimental validation of small-size wind turbine using flexible multibody approach

The wind energy is observed as an essential and powerful energy resource for the socio-economic development. It is proposed that the small-size wind turbines can demonstrate the innovative solution for the wind energy conversion for low speed regions. An innovative design, control, and monitoring processes require accurate and validated dynamic model of such turbines. In this investigation, the flexible multibody dynamics approach is used to extend the traditional method of dynamic modeling of small-size wind turbines. A systematic approach is developed based on floating frame of reference formulation (FFR) that includes the dynamics of the flexible blades as well as the aerodynamic loads. Beam element is used to model the blade structure with variable twist angle. In order to counteract the effect of the geometric stiffening, which comes due to high speed rotations, the coupling terms in the expression of axial strain energy are taken into account. In the other hand, the circulation theory is used to calculate the pressure coefficient difference along the blade, and the generalized aerodynamic forces are established. An experimental test-rig equipped with wind generator is used for the FFR model validation. The equipment gives a maximum discharge flow of 10 (m/s) in a 500 (mm) diameter duct. The dynamic effect of the twist angle of 30 (cm) diameter of rotor blades is studied based on the measured rotor speed of the wind turbine. The comparison of experimental results and numerical solution shows a very good agreement and consequently the wind turbine model obtained is suitable for stress analysis, structural and control design. It is concluded that the FFR formulation is best suited for large rotation and small deformation problems, which coincide with the operational conditions of small-size wind energy systems.

Influence of blade deformation and yawed inflow on performance of a horizontal axis tidal stream turbine

For a better design of tidal stream turbines operated in off-design conditions, analyses considering the effects of blade deformation and yawed inflow conditions are necessary. The flow load causes deformation of the blade, and the deformation affects the turbine performance in return. Also, a yawed inflow influences the performance of the turbine. As a validation study, a computational fluid dynamics (CFD) simulation was carried out to predict the performance of a horizontal axis tidal stream turbine (HATST) with rigid blades. The numerical uncertainty for the turbine performance with blade deformation and a yawed inflow was evaluated using the concept of the grid convergence index (GCI). A fluid–structure interaction (FSI) analysis was carried out to estimate the performance of a turbine with flexible composite blades, with the results then compared to those of an analysis with rigid blades. The influence of yawed inflow conditions on the turbine performance was investigated and found to be important in relation to power predictions in the design stages.

Structural and Functional Characterizations of the Mechanosensitive Piezo Channel

Piezo proteins are evolutionarily conserved and functionally diverse mechanosensitive cation channels that play critical roles in various mechanotransduction processes. However, the overall structural architecture, ion permeation and gating mechanisms of Piezo channels have remained unknown. Here we determine the cryo-electron microscopy structure of the full-length (2,547 amino acids) mouse Piezo1 (Piezo1) at a resolution of 4.8 A. Piezo1 forms a trimeric propeller-like structure (about 900 kilodalton), with the extracellular domains resembling three distal blades and a central cap. The transmembrane region has 14 apparently resolved segments per subunit. These segments form three peripheral wings and a central pore module that encloses a potential ion-conducting pore. The rather flexible extracellular blade domains are connected to the central intracellular domain by three long beam-like structures. This trimeric architecture suggests that Piezo1 may use its peripheral regions as force sensors to gate the central ion-conducting pore. Importantly, by characterizing the structurally revealed featured domains, we provide functional evidence to support this hypothesis. Taken together, these findings significantly advance our understanding of the structure-function relationship of this novel class of mechanosensitive cation channels.

Performance Enhancement of Tilt-Body Micro Air Vehicle by Use of Orthotropic Laminated Proprotors

A passive twist control is considered as an adaptive way to maximize the overall efficiency of a proprotor developed for convertible Micro Air Vehicles (MAV). In this paper, adaptation of the proprotor geometry in accordance to flight configurations is achieved by induced twist generated by the inherent structural coupling effect in anisotropic composite material and centrifugal force emanating from the tip load. Beam Finite Element Model based on Rotating Timoshenko Theory is used to predict structural loads, while Blade Element Momentum Theory is employed to predict the aerodynamic performance of adaptive proprotor as applied on Micro Air Vehicles (MAV). The iterative process of combination of aerodynamic model and structural model is used to compute the steady-state deformation of the flexible laminated proprotor blade due aerodynamic loads. Finally, the optimal design of lamina blade material is carried out to investigate the potential of flexible blade in the proprotorperformance enhancement.

Saturation Control for a Rotating Thin-walled Composite Beam Structure

A nonlinear saturation control method applied for reduction of oscillations of a rotating composite beam is presented in this paper. The flexible blade is treated as a thin-walled structure attached to a rigid hub driven by an external periodic torque. The beam model, derived in [1,2], takes into account a strong coupling between the flexural and torsional deformations due to the assumed specific reinforcing fibers perimeter orientation. The proposed saturation control method is used for the reduction of coupled torsional-flexural beam oscillations around the third natural frequency of the system. It is shown that for assumed structural parameters it is possible to find gains of the controller which may essentially reduce vibrations of the beam near the resonance zone.

Stall Flutter Control of a Smart Blade Section Undergoing Asymmetric Limit Oscillations

Stall flutter is an aeroelastic phenomenon resulting in unwanted oscillatory loads on the blade, such as wind turbine blade, helicopter rotor blade, and other flexible wing blades. Although the stall flutter and related aeroelastic control have been studied theoretically and experimentally, microtab control of asymmetric limit cycle oscillations (LCOs) in stall flutter cases has not been generally investigated. This paper presents an aeroservoelastic model to study the microtab control of the blade section undergoing moderate stall flutter and deep stall flutter separately. The effects of different dynamic stall conditions and the consequent asymmetric LCOs for both stall cases are simulated and analyzed. Then, for the design of the stall flutter controller, the potential sensor signal for the stall flutter, the microtab control capability of the stall flutter, and the control algorithm for the stall flutter are studied. The improvement and the superiority of the proposed adaptive stall flutter controller are shown by comparison with a simple stall flutter controller.