Rigid Blade Research Articles

Rotorcraft blade-vortex interaction noise prediction using the Lattice-Boltzmann method

The aim of this paper is to assess the accuracy, capabilities and computational performances of the Lattice-Boltzmann/Very Large Eddy Simulation Method to predict the unsteady aerodynamic loads, the rotor wake development and the noise radiation of helicopter rotors in strong Blade-Vortex Interaction conditions. The numerical flow solution is obtained by solving the explicit, transient and compressible Lattice-Boltzmann equation implemented in the high-fidelity CFD/CAA solver Simulia PowerFLOW®. The acoustic far-field is computed by using the Ffwocs-Williams & Hawkings integral solution applied to a permeable surface encompassing the whole helicopter geometry. The employed benchmark configuration is the 40% geometrically and aeroelastically scaled model of a BO-105 4-bladed main rotor tested in the open-jet anechoic test section of the German-Dutch wind tunnel in the framework of the HART-II project. In the present study, only the baseline operating condition of the experimental campaign, without Higher-Harmonic Control enabled, is considered. All simulations are performed by assuming a rigid blade motion, but a computational strategy based on a combination of a rigid blade pitching motion and a transpiration velocity boundary condition applied on the blade surface is employed to take into account the blade elastic deformation motion measured during the experiments. As expected, modeling the blade elastic deformation leads to more accurate predictions of control settings, unsteady air-loads and noise footprint. The effects of the computational grid on the aerodynamic and aeroacoustic prediction is documented as well.

Wave damping by flexible vegetation: Connecting individual blade dynamics to the meadow scale

Aquatic vegetation provides ecosystem services of great value, including the damping of waves, which protects shorelines and reduces resuspension. This study proposes a physically-based model to predict the wave decay associated with a submerged meadow as a function of plant morphology, flexibility, and shoot density. In particular, the study considers both the rigid (sheath) and flexible (blade) segments of the plant. Flexible plants reconfigure in response to wave orbital velocity, which diminishes wave decay relative to a rigid plant of the same morphology. The impact of reconfiguration on wave decay can be characterized using an effective blade length, le, which represents the length of a rigid blade that generates the same drag as the flexible blade of length l. The effective blade length depends on the Cauchy number, which defines the ratio of hydrodynamic drag to blade stiffness, and the ratio of blade length to wave orbital excursion. This laboratory study considered how the scaling laws determined for individual blades can be used to predict the wave decay over a meadow of multiple plants, each consisting of multiple blades attached at a rigid stem (sheath). First, the drag force on and motion of individual model blades (made of low-density polyethylene) was studied for a range of wave conditions to provide empirical coefficients for the theoretically determined scaling laws for effective blade length, le. Second, the effective blade length predicted for individual blades was incorporated into a meadow-scale model to predict wave decay over a meadow. The meadow-scale model accounts for both the rigid and flexible parts of individual plants. Finally, wave decay was measured over meadows of different plant density (shoots per bed area), and the measured decay was used to validate the wave-decay model. Wave decay was shown to be similar over meadows with regular and random arrangements of plants.

Numerical analysis of blade coating of a third-order fluid

In this study, the blade coating process of a third-order fluid has been investigated. Flow between the narrow channel formed by the rigid blade and the web is generated due to the web motion and the constant pressure gradient. The governing equations of motion are simplified using lubrication approximation theory. We obtained analytical solutions for velocity field, pressure distribution, pressure gradient and coating thickness using perturbation method. How the model’s dimensionless parameters affect velocity, pressure gradient, pressure and coating thickness are presented graphically and in tabulated form. We found that the third-order parameter provides a mechanism to control velocity, pressure, power input and the final coating thickness.

Critical influences of particle size and adhesion on the powder layer uniformity in metal additive manufacturing

The quality of powder layers, specifically their packing density and surface uniformity, is a critical factor influencing the quality of components produced by powder bed metal additive manufacturing (AM) processes, including selective laser melting, electron beam melting and binder jetting. The present work employs a computational model to study the critical influence of powder cohesiveness on the powder recoating process in AM. The model is based on the discrete element method (DEM) with particle-to-particle and particle-to-wall interactions involving frictional contact, rolling resistance and cohesive forces. Quantitative metrics, namely the spatial mean values and standard deviations of the packing fraction and surface profile field, are defined in order to evaluate powder layer quality. Based on these metrics, the size-dependent behavior of exemplary plasma-atomized Ti–6Al–4V powders during the recoating process is studied. It is found that decreased particle size (increased cohesiveness) leads to considerably decreased powder layer quality in terms of low, strongly varying packing fractions and highly non-uniform surface profiles. For relatively fine-grained powders (median particle diameter 17 μm), it is shown that cohesive forces dominate gravity forces by two orders of magnitude leading to low quality powder layers not suitable for subsequent laser melting without additional layer/surface finishing steps. Besides particle-to-particle adhesion, this contribution quantifies the influence of mechanical powder material properties, nominal layer thickness, blade velocity, as well as particle-to-wall adhesion. Finally, the implications of the resulting powder layer characteristics on the subsequent melting process are discussed and practical recommendations are given for the choice of powder recoating process parameters. While the present study focuses on a rigid blade recoating mechanism, the proposed simulation framework can be applied to systems using alternative recoating tools such as soft blades, rakes or rotating rollers.

A Numerical Investigation of the Friction Contact of an Unfilled Styrene Butadiene Rubber by a Blade Sliding Indentation

Rubber is mostly modeled as a hyperelastic material and as a consequence, large deformation occur along friction contact against a rigid counterface. In general, Coefficient of friction (COF) of a contact surface consists of two components, i.e. adhesion and deformation (hysteresis). However, it is difficult to investigate the deformation component of COF analitically on the rubber sliding. By means of a rigid blade sliding indentation technique, this paper studies the friction contact phenomena on Unfilled Styrene Butadiene Rubber (SBR-0) numerically by using a Finite Element Analysis (FEA) in plane strain mode. By a given sliding speed, the FEA simulation is carried out with the various adhesion COF and sliding depth. The presented simulation output are stress, deformation and reaction forces. Results show that the deformation COF strongly depends on the sliding displacement. Finally, the overall COF highly increases and then decreases with respect to the sliding displacement and tends to indicate stick-slip phenomena.

Fluid Structure Interaction Modelling of Tidal Turbine Performance and Structural Loads in a Velocity Shear Environment

Tidal Current Turbine (TCT) blades are highly flexible and undergo considerable deflection due to fluid interactions. Unlike Computational Fluid Dynamic (CFD) models Fluid Structure Interaction (FSI) models are able to model this hydroelastic behavior. In this work a coupled modular FSI approach was adopted to develop an FSI model for the performance evaluation and structural load characterization of a TCT under uniform and profiled flow. Results indicate that for a uniform flow case the FSI model predicted the turbine power coefficient CP with an error of 4.8% when compared with experimental data. For the rigid blade Reynolds Averaged Navier Stokes (RANS) CFD model this error was 9.8%. The turbine blades were subjected to uniform stress and deformation during the rotation of the turbine in a uniform flow. However, for a profiled flow the stress and deformation at the turbine blades varied with the angular position of turbine blade, resulting in a 22.1% variation in stress during a rotation cycle. This variation in stress is quite significant and can have serious implications for the fatigue life of turbine blades.

Y-type three-blade bluff body for wind energy harvesting

A three-blade bluff body for wind energy harvesting is proposed and designed. The bluff body with a Y-type cross-section is formed by three rigid thin blades and fixed at the free end of a piezoelectric cantilever beam. Simulations and experiments confirmed that this three-blade structure can achieve much higher energy output than a square prism. The output voltages of horizontal harvesters with different half-angles between the two front blades of the bluff body were measured first. The half-angle dramatically affected the performance of the energy harvester. Half-angles between 60° and 80° are found to be the optimal values for generating a high output voltage. In addition, the performance of the harvester can be enhanced when the length ratio of the rear blade to the front blade is in the range of 4/3–5/3. Interestingly, the output voltage of the vertical three-blade harvester was higher than that of the horizontal one and the optimal half-angle in this case was also between 60° and 80°.

Aeroelastic analysis of membrane blade via panel-BEM coupling

This paper aims at introducing a methodology for aeroelastic analysis of membrane blades using panel-BEM coupling. The proposed methodology is used for evaluating the performance of a membrane blade with the baseline platform similar to the NASA-Ames Phase VI blade. The performance of the membrane blade is compared with the rigid baseline blade. The studied membrane blade has certain aerodynamic advantages over its rigid counterpart. For the studied blade and at higher wind speeds, the axial induction factor for membrane blade is nearer to the Betz optimum value of 1/3 and as a result the membrane blade is more efficient in producing power. The performance of the membrane blade is further examined via high fidelity FSI analysis. The same trend as in panel-BEM coupling is also observed in the high fidelity model, namely for higher wind speeds the studied membrane blades is more efficient than its rigid counterpart.

Numerical Calculation of Internal Flow Field of the New Type Rigid Blade Motor of the Down-hole Dynamic Drilling Tool

Along with the development of ocean oil and gas exploration to the deep-water, the down-hole temperature is more and more higher with the drilling depth increasing. The down-hole hydraulic dynamic drilling tool does not adapt to the needs of the offshore deep-water drilling. Therefore, development of high resistant temperature, excellent oriented performance down-hole motor is imminent. The author designed a new type rigid blade down-hole power drilling tool, its slipping blade belongs to the rigid blade, which has the advantages of large output torque and high reliability compared with the domestic and foreign down-hole dynamic drilling tool. This paper mainly carried on the numerical simulation calculation to the 178mm down-hole motor. The calculation model was established based on characteristics of the new type blade dynamic drilling tool. The fluent software was used to calculate different pressure and velocity distribution at the different flow rate and different speed of rotator. The calculation results show that when the drilling fluid flow rate from 30 to 35L/s and the rotator speed is 180rpm, the dynamic drilling tool can achieve the best efficiency, and the abrasion of the motor blade and other parts is minimum. The results can provide the basis for the optimization of the structure parameters and working parameters of the new type down-hole dynamic drilling tool.

Full-Annulus Simulation of Nonsynchronous Blade Vibration Excitation of an Axial Compressor

A high speed 1–1/2 axial compressor stage is simulated in this paper using an unsteady Reynolds-averaged Navier–Stokes (URANS) solver for a full-annulus configuration to capture its nonsynchronous vibration (NSV) flow excitation with rigid blades. A third-order weighted essentially nonoscillatory scheme for the inviscid flux and a second-order central differencing for the viscous terms are used to resolve nonlinear unsteady fluid flows. A fully conservative rotor/stator sliding boundary condition (BC) is employed with multiple-processor capability for rotor/stator sliding interface that accurately captures unsteady wake propagation between the rotor and stator blades while conserving fluxes across the rotor/stator interfaces. The predicted dominant frequencies using the blade tip response signals are not harmonic to the engine order, which is the NSV excitation. The simulation is based on a rotor blade with a 1.1% tip-chord clearance. Comparison with the previous 1/7 annulus simulations show that the time-shifted phase-lag BCs used in the 1/7 annulus are accurate. For most of the blades, the NSV excitation frequency is 6.2% lower than the measurement in the rig test, although some blades displayed slightly different NSV excitation frequencies. The simulation confirms that the NSV is a full annulus phenomenon. The instability of the circumferential traveling vortices in the vicinity of the rotor tip due to the strong interaction of incoming flow is the main cause of the NSV excitation. This instability is present in all blades of the rotor annulus. For circumferentially averaged parameters like total pressure ratio, NSV is observed to have an effect on the radial profile, particularly at radial locations above 70% span. A design with a lower loading of the upper blade span and a higher loading of the midblade spans is recommended to mitigate or remove NSV.

Aeroelastic stability of full-span tiltrotor aircraft model in forward flight

The existing full-span models of the tiltrotor aircraft adopted the rigid blade model without considering the coupling relationship among the elastic blade, wing and fuselage. To overcome the limitations of the existing full-span models and improve the precision of aeroelastic analysis of tiltrotor aircraft in forward flight, the aeroelastic stability analysis model of full-span tiltrotor aircraft in forward flight has been presented in this paper by considering the coupling among elastic blade, wing, fuselage and various components. The analytical model is validated by comparing with the calculation results and experimental data in the existing references. The influence of some structural parameters, such as the fuselage degrees of freedom, relative displacement between the hub center and the gravity center, and nacelle length, on the system stability is also investigated. The results show that the fuselage degrees of freedom decrease the critical stability velocity of tiltrotor aircraft, and the variation of the structural parameters has great influence on the system stability, and the instability form of system can change between the anti-symmetric and symmetric wing motions of vertical and chordwise bending.

A biomimetic wind turbine inspired by Dryobalanops aromatica seed: Numerical prediction of rigid rotor blade performance with OpenFOAM®

This paper presents the performance study of a rigid biomimetic horizontal axis wind turbine (HAWT) rotor blade inspired by Dryobalanops aromatica seed with open source computational fluid dynamics (CFD) software, OpenFOAM®. The power coefficient, CP, thrust coefficient, CT and blade root bending stresses of the proposed biomimetic wind turbine were predicted and compared to a tapered and twisted blades wind turbine from Krogstad and Lund (2012). The simulation results show that the proposed biomimetic wind turbine has fair maximum CP which is 0.386 at tip speed ratio (TSR) of 1.5 in free stream velocity, U∞ of 10 m/s. The proposed biomimetic wind turbine is expected to have better self-starting ability as its starting torque at TSR = 0, is 772% higher than Krogstad and Lund (2012) wind turbine. The proposed biomimetic wind turbine is also expected to work in low wind speed condition due to its ability to generate higher torque during rotation. The preconing of the proposed biomimetic wind turbine blades enable the utilisation of centrifugal forces induced from rotation to reduce flapwise bending stresses. The performance of the scaled biomimetic wind turbine which have similar solidity as Krogstad and Lund (2012) wind turbine was also tested and it was able to exhibit the features of the original biomimetic wind turbine. This preliminary study will give insight on the potentials of this newly proposed biomimetic wind turbine in the wind power industry.

Towing tank testing of passively adaptive composite tidal turbine blades and comparison to design tool

Abstract Passively adaptive bend-twist (BT) tidal turbine blades made of non-homogeneous composite materials have the potential to reduce the structural loads on turbines so that smaller more cost effective components can be used. Using BT blades can also moderate the demands on the turbine generator above design conditions. This paper presents experimental towing tank test results for an 828 mm diameter turbine with composite BT blades compared to a turbine with geometrically equivalent rigid aluminum blades. The BT blades were constructed of a graphite-epoxy unidirectional composite material with ply angles of 26.8° to induce BT coupling, and an epoxy foam core. For steady flow conditions the BT blades were found to have up to 11% lower thrust loads compared to rigid blades, with the load reductions varying as a function of flow speed and rotational speed. A coupled finite element model-blade element momentum theory design tool was developed to iterate between the structural (deformation and stresses) and hydrodynamic (power and thrust loads) responses of these adaptive composite blades. When compared to the experimental test results, the design tool predictions were within at least 8% of the experimental results for tip-speed ratios greater than 2.5.

Some Results on the Vulnerability Assessment of HAWTs Subjected to Wind and Seismic Actions

The spread of the wind energy industry has caused the construction of wind farms in areas prone to high seismic activity. Accordingly, the analysis of wind turbine loading associated with earthquakes is of crucial importance for an accurate assessment of their structural safety. Within this topic, this paper presents some preliminary results of a probabilistic framework intended to be used for the estimation of the probability of failure of Horizontal Axis Wind Turbine-supporting structures when subjected to the wind and seismic actions. In particular, the multi-hazard fragility curves of the wind turbine-supporting structure were calculated using Monte Carlo simulations. A decoupling approach consisting of aerodynamic analysis of the rigid rotor blade model and subsequent linear dynamic Finite Element analyses of the supporting structure, including aerodynamic damping, was used. The failure condition of the tower structure was estimated according to the stress design procedure proposed by EC3 for the buckling limit state assessment. Finally, the vulnerability assessment of HAWTs to wind and seismic actions was evaluated in terms of fragility curves describing the probability of failure of the supporting tower structure as a function of the Peak Ground Acceleration (PGA) for each parked and operational wind condition. In particular, the results highlight a probability of failure larger than 50% for high levels of seismic action (PGA greater than 0.7 g) combined with the rotor in parked condition (wind speed of 3 m/s) or in operational rated condition (wind speed of 11.4 m/s).

Modelling and Analysis of Aeroelastic Tailoring Blade Wind Turbine Systems

Modelling and performance analysis of wind turbine control systems with aeroelastic tailoring blades (ATBs) are investigated. An industrial scale horizontal axis wind turbine (HAWT) model with rigid blades is firstly developed as the baseline model using the blade element momentum (BEM) theory. Designed twist angle variation distributions along blades are then introduced to the baseline model to characterise the ATB nature. The developed ATB wind turbine models are analysed by employing a baseline control system. The performances of the ATB wind turbine systems are compared with that of the baseline turbine using both nonlinear models and linearised models at selected wind speeds. The impacts of ATB design can be clearly observed from the simulation studies. Preliminary results suggest that with ATB design in wind turbines, the blade fatigue loads and the pitching activities can be reduced for large turbines without compromising the energy capture performance.

Extension of the Non-Linear Harmonic method for the study of the dynamic aeroelasticity of horizontal axis wind turbines

In this paper an innovative methodology for the study of horizontal axis wind turbines dynamic aeroelasticity is presented. It can be understood as an extension of the Non-Linear Harmonic (NLH) method, an efficient computational approach for the analysis of unsteady periodic flows. A linearized model of the structure consisting of a set of mode shapes and natural frequencies was included. The aeroelastic equilibrium was ensured through a set of equations linking the structural displacements and the fluid loads for both the time-averaged and the harmonic contributions. First, the developed methodology is tested in the framework of a 2D cylinder mounted on a single degree of freedom elastic system and undergoing Vortex Induced Vibrations (VIV). The results are compared with previous experimental and computational studies, revealing the potential of the method for the prediction of both the shedding frequency and the aeroelastic response. Secondly, the dynamic aeroelasticity of the complete DTU 10MW RWT wind turbine (i.e. including the tower) is assessed. A nominal operating point is studied, and the rotor flexibility is considered via a blade structural model. The results of this Fluid–Structure Interaction (FSI) simulation are compared with two additional computations, both assuming rigid blades, that modeled the isolated DTU 10MW RWT rotor and the complete machine. This allowed to distinguish the impact of the blade flexibility on the rotor performance from the potential effects associated to the presence of the tower. In particular, the consideration of the aeroelasticity led to a decrease of the predicted time-averaged rotor loads and the corresponding amplitudes of oscillation. For its application on the DTU 10MW RWT, the developed methodology was found to be one order of magnitude faster than a standard time marching approach.

Seagrass blade motion under waves and its impact on wave decay

AbstractThe hydrodynamic drag generated by seagrass meadows can dissipate wave‐energy, causing wave decay. It is well known that this drag depends on the relative motion between the water and the seagrass blades, yet the impact of blade motion on drag and wave‐energy dissipation remains to be fully characterized. In this experimental study, we examined the impact of blade motion on wave decay by concurrently recording blade posture during a wave cycle and measuring wave decay over a model seagrass meadow. We also identified a scaling law that predicts wave decay over the model meadow for a range of seagrass blade density, wave period, wave height, and water depth scaled from typical field conditions. Blade flexibility led to significantly lower drag and wave decay relative to theoretical predictions for rigid, upright blades. To quantify the impact of blade motion on wave decay, we employed an effective blade length, le, defined as the rigid blade length that leads to equivalent wave‐energy dissipation. We estimated le directly from images of blade motion. Consistent with previous studies, these estimates showed that the effective blade length depends on the dimensionless Cauchy number, which describes the relative magnitude of the wave hydrodynamic drag and the restoring force due to blade rigidity. As the hydrodynamic forcing increases, the blades exhibit greater motion. Greater blade motion leads to smaller relative velocities, reducing drag, and wave‐energy dissipation (i.e., smaller le).

Coupled CFD/CSD simulation of the helicopter main rotor in high-speed forward flight

The helicopter main rotor in forward flight is considered in this paper. The results for rigid blades and elastic blades are compared by the method of coupled simulation. The influence of the structural damping coefficient on the blade in-flight deformation is also considered.

Numerical Investigation of a Vertical Axis Tidal Turbine with Deforming Blades

The vertical axis tidal turbines are initially constructed by using simple rigid blades. As an alternative to the conventional rigid blades, the proposition of the present work is to deform the blade leading edge via a sinusoidal smoothed curve. A deforming mesh involving both sliding and remeshing techniques is used to control and update the mesh throughout the system motion. Examinations of the shed vortices interactions and the accompanying pressure contours indicate that the correction of the leading edge shape gives rise to a vortex with large size at an advanced stage than the conventional blade. This vortex promotes in turn, the blades suction zone which alters the pressure distribution along the blade’s surfaces. The turbine efficiency is boosted by 35% relative to the uncontrolled case.

Dynamic response analysis under atmospheric disturbances for helicopters based on elastic blades

A flight dynamics model based on elastic blades for helicopters is developed. Modal shape analysis is used to describe the rotating elastic blades for the purpose of reducing the elastic degrees of freedom for blades. The analytical result is employed to predict the rotor forces and moments. The equilibrium equation of the flight dynamics model is then constructed for the elastic motion for blades and the rigid motion for other parts. The nonlinear equation is further simplified, and the gradient descent algorithm is adopted to implement the trim simulation. The trim analysis shows that the effect of blade elasticity on the accuracy of rotor forces and moments is apparent at high speed, and the proposed method presents good accuracy for trim performance. The time-domain response is realized by a combination of the Newmark method and the adaptive Runge-Kutta method. The helicopter control responses of collective pitch show that the response accuracy of the model at a yaw-and-pitch attitude is improved. Finally, the influence of blade elasticity on the helicopter dynamic response in low-altitude wind shear is investigated. An increase in blade elasticity reduces the oscillation amplitude of the yaw angle and the vertical speed by more than 70%. Compared with a rigid blade, an elastic blade reduces the vibration frequency of the angular velocity and results in a fast return of the helicopter to its stable flight.