Showing papers on "Blade element theory published in 2014"

On the Impact of Geometric Variability on Fan Aerodynamic Performance, Unsteady Blade Row Interaction, and Its Mechanical Characteristics

Abstract: The focus of the present study is to assess and quantify the uncertainty in predicting the steady and unsteady aerodynamic performance as well as the major mechanical characteristics of a contrarotating turbofan, primarily due to geometric variations stemming from the manufacturing process. The basis of this study is the optically scanned blisk of the first rotor, for which geometric variations from blade to blade are considered. In a first step, selected profile sections of the first rotor were evaluated aerodynamically by applying the 2D coupled Euler/boundary-layer solver mises. Statistical properties of the relevant flow quantities were calculated firstly based on the results of the nine manufactured blades. In a second step, the geometric variations were decomposed into their corresponding eigenforms by means of principal component analysis (PCA). These modes were the basis for carrying out Monte Carlo (MC) simulations in order to analyze in detail the blade's aerodynamic response to the prescribed geometric variations. By means of 3D-computational fluid dynamics (CFD) simulations of the entire fan stage for all the nine scanned rotor 1 blade geometries, the variation of the overall stage performance parameters will be quantified. The impact of the instrumentation will be discussed, here partly doubling the standard deviation of the major performance indicators for the instrumented blades and also triggering a premature laminar/turbulent transition of the boundary layer. In terms of the unsteady blade row interaction, the standard deviation of the resulting blade pressure amplitude shall be discussed based on unsteady simulations, taking advantage of a novel harmonic balance approach. It will be shown that the major uncertainty in terms of the predicted blade pressure amplitude is in the aft part of the front rotor and results from upstream shock/blade interaction. Apart from the aerodynamic performance, an analysis of the mechanical properties in terms of Campbell characteristics and eigenfrequencies was carried out for each of the scanned blades of rotor 1, reflecting the frequency scattering of each eigenmode due to geometric variability.

LDV measurement of boundary layer on rotating blade surface in wind tunnel

Abstract: Wind turbines generate electricity due to extracting energy from the wind. The rotor aerodynamics strongly depends on the flow around blade. The surface flow on the rotating blade affects the sectional performance. The wind turbine surface flow has span-wise component due to span-wise change of airfoil section, chord length, twisted angle of blade and centrifugal force on the flow. These span-wise flow changes the boundary layer on the rotating blade and the sectional performance. Hence, the thorough understanding of blade surface flow is important to improve the rotor performance. For the purpose of clarification of the flow behaviour around the rotor blade, the velocity in the boundary layer on rotating blade surface of an experimental HAWT was measured in a wind tunnel. The velocity measurement on the blade surface was carried out by a laser Doppler velocimeter (LDV). As the results of the measurement, characteristics of surface flow are clarified. In optimum tip speed operation, the surface flow on leading edge and r/R=0.3 have large span-wise velocity which reaches 20% of sectional inflow velocity. The surface flow inboard have three dimensional flow patterns. On the other hand, the flow outboard is almost two dimensional in cross sectional plane.

Time-accurate aeroelastic simulations of a wind turbine in yaw and shear using a coupled CFD-CSD method

Abstract: In the present study, aeroelastic simulations of horizontal-axis wind turbine rotor blades were conducted using a coupled CFD-CSD method. The unsteady blade aerodynamic loads and the dynamic blade response due to yaw misalignment and non-uniform sheared wind were investigated. For this purpose, a CFD code solving the RANS equations on unstructured meshes and a FEM-based CSD beam solver were used. The coupling of the CFD and CSD solvers was made by exchanging the data between the two solvers in a loosely coupled manner. The present coupled CFD-CSD method was applied to the NREL 5MW reference wind turbine rotor, and the results were compared with those of CFD-alone rigid blade calculations. It was found that aeroelastic blade deformation leads to a significant reduction of blade aerodynamic loads, and alters the unsteady load behaviours, mainly due to the torsional deformation. The reduction of blade aerodynamic loads is particularly significant at the advancing rotor blade side for yawed flow conditions, and at the upper half of rotor disk where wind velocity is higher due to wind shear.

Propeller-Wing Interaction Prediction for Early Design

Abstract: In the current work a methodology to quickly estimate propeller slipstream effects on a finite wing, for early design stage, is developed. The aerodynamic model consists of a Truckenbrodt 3D lifting surface method, coupled with an airfoil numerical solver for taking into account viscosity and camber effects, and a blade element theory for the formulation of the propeller aerodynamics. Results of the proposed method are validated against RANS (Reynolds Averaged Navier Stokes) data for a conventional four-propeller transport aircraft and a novel Contra-Rotating Open Rotor (CROR) aircraft. A good agreement with the high fidelity solutions is obtained. The low computational cost makes the method attractive and suitable for performing extensive studies such as Multi-Disciplinary Optimisation (MDO) or to explore several alternative designs during the conceptual and preliminary design phases.

Development of Free Vortex Wake Method for Aerodynamic Loads on Rotor Blades

Abstract: The aerodynamics of a wind turbine is governed by the flow around the rotor, where the prediction of air loads on rotor blades in different operational conditions and its relation to rotor structural dynamics is crucial for design purposes. One of the most important challenges in wind turbine aerodynamics is therefore to accurately predict the forces on the blade, where the blade and wake are modeled by different approaches such as the Blade Element Momentum (BEM) theory, the vortex method and Computational Fluid Dynamics (CFD). A free vortex wake method, based on the potential, inviscid and irrotational flow, is developed to study the aerodynamic loads. The results are compared with the BEM method, the GENUVP code and CFD.

Unsteady Flow Simulations of an Over-the-wing Propeller Configuration

Abstract: The aerodynamic integration effects of an embedded over-the-wing propeller at take-off conditions are discussed based on steady and unsteady Reynolds-averaged Navier-Stokes flow simulations. In contrast to the rotating blade and hub geometry, the steady computations utilized an actuator disk model with blade element theory enhancement to investigate the mutual influnce between installed propeller and wing with sufficient accuracy. A simplified high-lift geometry of this channel wing concept is compared to a conventional tractor configuration. While the general overthe-wing integration effects, such as lift-to-drag ratio improvement and deteriorated propeller efficiency, are already captured by inexpensive steady simulations, only unsteady computations with full propeller geometry reveal some important flow details. The most striking unsteady effect is the interaction of the blade tip vortex with the boundary layer of the wing which only occurs at the channel wing due to the close coupling. As a consequence the low momentum fluid detaches above the flap leading to a comparatively low lift coefficient.

On the inverse problem of blade design for centrifugal pumps and fans

Abstract: The inverse problem of blade design for centrifugal pumps and fans has been studied The solution to this problem provides the geometry of rotor blades that realize specified performance characteristics, together with the corresponding flow field Here a three-dimensional solution method is described in which the so-called meridional geometry is fixed and the distribution of the azimuthal angle at the three-dimensional blade surface is determined for blades of infinitesimal thickness The developed formulation is based on potential-flow theory Besides the blade impermeability condition at the pressure and suction side of the blades, an additional boundary condition at the blade surface is required in order to fix the unknown blade geometry For this purpose the mean-swirl distribution is employed The iterative numerical method is based on a three-dimensional finite element method approach in which the flow equations are solved on the domain determined by the latest estimate of the blade geometry, with the mean-swirl distribution boundary condition at the blade surface being enforced The blade impermeability boundary condition is then used to find an improved estimate of the blade geometry The robustness of the method is increased by specific techniques, such as spanwise-coupled solution of the discretized impermeability condition and the use of underrelaxation in adjusting the estimates of the blade geometry Various examples are shown that demonstrate the effectiveness and robustness of the method in finding a solution for the blade geometry of different types of centrifugal pumps and fans The influence of the employed mean-swirl distribution on the performance characteristics is also investigated

Flow field studies on a micro-air-vehicle-scale cycloidal rotor in forward flight

Abstract: This paper examines the flow physics and principles of force production on a cycloidal rotor (cyclorotor) in forward flight. The cyclorotor considered here consists of two blades rotating about a horizontal axis, with cyclic pitch angle variation about the blade quarter-chord. The flow field at the rotor mid-span is analyzed using smoke flow visualization and particle image velocimeV are compared with flow fields predicted using 2D CFD and time-averaged force measurements acquired in an open-jet wind tunnel at three advance ratios. It is shown that the experimental flow field is nearly two dimensional at μ = 0.73 allowing for qualitative comparisons to be made with CFD. The incoming flow velocity decreases in magnitude as the flow passes through the retreating (upper) half of the rotor and is attributed to power extraction by the blades. A significant increase in flow velocity is observed across the advancing (lower) half of the rotor. The aerodynamic analysis demonstrates that the blades accelerate the flow through the lower aft region of the rotor, where they operate in a high dynamic pressure environment. This is consistent with CFD-predicted values of instantaneous aerodynamic forces which reveal that the aft section of the rotor is the primary region of force production. Phase-averaged flow field measurements showed two blade wakes in the flow, formed by each of the two blades. Analysis of the blades at several azimuthal positions revealed two significant blade-wake interactions. The locations of these blade-wake interactions are correlated with force peaks in the CFD-predicted instantaneous blade forces and highlight their importance to the generation of lift and propulsive force of the cyclorotor.

Comparison of Non-Linear and Linearized CFD Analysis of the Stator-Rotor Interaction of a Compressor Stage

Abstract: The prediction of resonance amplitudes due to stator-rotor interactions is still an important task within the design process of turbomachinery bladings.In this paper the stator-rotor interaction of a compressor stage which consists of an inlet guide vane and a rotor blade is studied with a non-linear and a linearized CFD code. First, a quasi-3D-study of a section close to the tip region is considered. The passing of the wake of the inlet guide vane over the rotor is studied for six different vibration mode shapes of increasing complexity (first bending mode up to 4th chordwise bending mode).Whereas for low rotor speeds the comparison between linearized and non-linear calculations is quite good, large differences are found for high rotor speeds. It is shown that an acoustic interaction between the two stages with a cut-on mode is the cause for the large differences, leading to much higher unsteady pressure amplitudes on the rotor blade. This in turn leads to different aerodynamic work on the rotor blade for the different mode shapes.The extension of the investigations to 3D shows essentially the same effects.Copyright © 2014 by ASME

Blade geometry transformation in optimization problems from the point cloud to the parametric form

Abstract: We consider the issues of transforming the blade geometry, which is conventionally defined as a set of points over several cross-sections, to parametric form that is necessary for the computer-aided solution of optimization problems. The method for coordinated movement of blade profile points under deformation within the optimization problem and the method of coordinated change of the profile parameters over the blade height, are proposed. An algorithm for the analytic representation of the profile specified discretely without loss of accuracy is described.

Semi-empirical modeling of fuselage–rotor interference for comprehensive codes: the fundamental model

Abstract: The flow field around the isolated HART II fuselage is computed by a computational fluid dynamics code. Velocities normal to the rotor rotational plane are extracted in a volume around the rotor as a data basis. A simple semi-empirical analytical formulation of the fuselage-induced velocities, based on parameter identification from computational fluid dynamics or measured data, is developed for use in comprehensive rotor codes. This model allows the computation of fuselage–rotor interferences on the rotor blade element level. It also allows the prediction of the rotor wake geometry deformation due to the presence of the fuselage in both prescribed wake and free-wake codes. Its impact on rotor thrust, power and trim is evaluated analytically using blade element momentum theory and by DLR’s comprehensive rotor code.

Modeling and optimal design of small HAWT blades for analyzing the starting torque behavior

Abstract: This paper presents a computational framework for an optimal aerodynamic blade design. Analysis of the wind turbine rotor is conducted using blade element momentum (BEM) theory. The geometric parameters of the wind turbine also have been calculated by BEM theory. The methodology and design approach is discussed in detail throughout this paper. A small three-bladed horizontal axis wind turbine (HAWT) is designed to improve its starting torque behaviour. The study of the starting torque behaviour, its analysis, and a new approach to smoothen it, is the important part of this work. The simulation results show that a turbine starts rotating with a smooth rising torque, and after ten seconds it gains a constant torque and a constant angular velocity which seems to be quite encouraging thus reducing the mechanical vibration of the system. The MATLAB is used to get the blade geometry parameters and aerodynamic forces along the blade span, the Pro/ENGINEER is used for the solid structure modeling of the wind turbine and ADAMS is being used for the simulation purpose.

Numerical investigation on effects of rotor control strategy and wind data on optimal wind turbine blade shape

Abstract: Recently, the horizontal axis rotor performance optimizer (HARP_Opt) tool was developed in the National Renewable Energy Laboratory, USA. This innovative tool is becoming more popular in the wind turbine industry and in the field of academic research. HARP_Optwas developed on the basis of two fundamental modules, namely, WT_Perf, a performance evaluator computer code using the blade element momentum theory; and a genetic algorithm module, which is used as an optimizer. A pattern search algorithm was more recently incorporated to enhance the optimization capability, especially the calculation time and consistency of the solutions. The blade optimization is an aspect that is highly dependent on experience and requires significant consideration on rotor control strategies, wind data, and generator type. In this study, the effects of rotor control strategies including fixed speed and fixed pitch, variable speed and fixed pitch, fixed speed and variable pitch, and variable speed and variable pitch algorithms on optimal blade shapes and rotor performance are investigated using optimized blade designs. The effects of environmental wind data and the objective functions used for optimization are also quantitatively evaluated using the HARP_Opt tool. Performance indices such as annual energy production, thrust, torque, and roof-flap moment forces are compared.

A new correlation on the MEXICO experiment using a 3D enhanced blade element momentum technique

Abstract: The blade element momentum (BEM) theory is based on the actuator disc (AD) model, which is probably the oldest analytical tool for analysing rotor performance. The BEM codes have very short processing times and high reliability. The problems of the analytical codes are well known to the researchers: the impossibility of describing inside the one-dimensional code the three-dimensional (3D) radial flows along the span-wise direction. In this work, the authors show how the 3D centrifugal pumping affects the BEM calculations of a wind turbine rotor. Actually to ascertain the accuracy of the analytical codes, the results are compared with rotor performance, blade loads and particle image velocimetry measurements of the model experiment in controlled conditions. A reliable agreement with the measurement is obtained. A good improvement is gained when the blade stall state modified aerofoil data instead of the original aerofoil data are used in the calculations.

An Iterative Method to Optimize the Twist Angle of a Wind Turbine Rotor Blade

Abstract: Maximizing the power output of a wind turbine is possible by ensuring the airfoils along the blade operate at an optimal angle of attack. However, blades deflect under loading but the torsional deflection of wind turbine blades is normally ignored in the blade design process. Since the power output is dependent upon the blade twist angle, an optimized blade shape must take into account the torsional deflection of the blade. The torsional deflection changes the loading of the blade. This necessitates iterative methods to determine the equilibrium loading and deflection and to optimize the blade shape. This paper explains a method to determine the equilibrium torsional deflection of the blade and a method to determine the optimal static blade twist angle to maximize power production. The results are the blade twist angle distribution that will deform to an optimal angle of attack under load, thereby changing the peak of the coefficient of power (CP) curve to a chosen wind speed.

Advanced modelling of helicopter nonlinear dynamics and aerodynamics

Abstract: The work presented here provides a comprehensive dynamic and aerodynamic helicopter model. The possible applications of this work are wide including, control systems applications, reference and trajectory tracking methods implementation amongst others. The model configuration corresponds to a Sikorsky helicopter; a main rotor in perpendicular combination with a tail rotor. Also, a particular model of unmanned aerial vehicle has been modelled as part of collaboration with the La Laguna University (Spain). The modelling tool is VehicleSim, a program that builds rigid body systems, solves the nonlinear equations of motion and generates the time histories of the corresponding state variables of the vehicle under study. VehicleSim is able to provide the linearised equations of motion in a Matlab file and the symbolic state-space model. This is useful when control systems are to be designed. The main rotor model accounts for flap, lag and feather motions for each blade as well as for their nonlinear dynamic coupling. The tail rotor is modelled including the flap-feather coupling via delta three angle. The main and tail rotors' angular velocities are implemented by PID controllers. Main rotor linear and nonlinear equations are derived and validated by comparison with the theory. Main rotor flap and lag degrees of freedom are validated using frequency domain approaches in the absence of external forces. Also, fuselage-main rotor interaction is studied and validated by using modal analysis and root locus methodology. Vibrations originated at the main rotor are simulated and their effects on the fuselage are examined by a Short Time Fourier transformation. The aerodynamic model uses blade element theory on the main-tail rotors. Hover, climb, descent and forward flight conditions are simulated and they allow the helicopter to follow certain trajectories. Finally, the ensuing vibrations when an external perturbation is applied to the main rotor are investigated.

Application of new body-force concept to the free surface effect on the hydrodynamic force and flow around a rotating propeller

Abstract: The effect of free surface on the flow around a rotating propeller and the open water characteristics are studied by varying the propeller immersion depth to investigate the applicability of the new body-force method. A simplified quasi-steady blade element theory (BET) with the infinite-bladed propeller model is coupled with the Reynolds averaged Navier-Stokes (RANS) code CFDSHIP-IOWA to calculate the flow around the propeller near the free surface. Propeller open-water characteristics are simulated in still water for different immersion depths for the Methodical-AU type fixed-pitch propeller. The results show that the wake structure is heavily affected by the free surface. The propeller open characteristics are compared with experiment.

Semi-Empirical Physics-Based Modeling of Fuselage-Rotor and Fuselage-Wake Interferences for Comprehensive Codes

Abstract: A semi-empirical analytical formulation of the induced velocities generated by the fuselage shell of the HART II wind tunnel model in the volume around the rotor is derived from computational fluid dynamics velocity data. The operational conditions investigated range from vertical descent, shallow descent, level flight and climb to vertical ascent and from forward to quartering flight. The analytical induced velocity model can directly be used to account for the inflow at the blade elements and also allows for analytical or numerical integration of rotor wake convection to compute the associated displacements of rotor blade tip vortices travelling downstream within this velocity field. The model’s influence on the major flow field variables around the rotor disk and the wake trajectory is demonstrated. Since the method is generic it is anticipated that the effect of any kind of fuselage can be accounted for this way and it may be used within any comprehensive code for blade element aerodynamics, within prescribed wake codes and even free-wake codes for influencing the wake convection.

A discussion of measured static and dynamic rotor loads during testing of the ERICA tilt-wing rotorcraft configuration in DNW-LLF wind tunnel

Abstract: A heavily instrumented 1:5 scaled model of the ERICA tilt-wing configuration has been tested in the DNW-LLF wind tunnel as part of the EU co-funded NICETRIP project. Tests were made for a variety of conditions ranging from pure helicopter and conversion corridor cases up to a low speed aircraft mode, with appropriate changes in tilt rotor and outer wing pitching angles. In total over 400 test conditions were measured. Rotor forces and moments have been measured with rotor balances. Blade bending moment and torsion as well as rotor shaft bending and torque were measured with calibrated strain gauge sensors. Measurements were made for fully trimmed and for non-trimmed conditions. The four bladed rotors are counter-rotating, are rigid in plane and allow cyclic blade pitch control through a remote controlled swash plate. Since cyclic pitch control caused damage to the blade pitch bearings, most of the test conditions had to be done without cyclic blade pitch control, leading to a large in-plane moment in the rotor plane and high bending moments in the rotor shaft. Some test cases were both measured with and without cyclic pitch control. The present paper analysis the rotor induced forces, moments and nacelle vibrations for the trimmed conditions. Both steady and dynamic contents of the signals are analyzed. It is found that with cyclic blade pitch control the time mean rotor in-plane forces and moments and the 1/rev rotor shaft and blade bending moments are effectively suppressed, but the dynamic components of the rotor in-plane forces and moments become larger than without cyclic blade pitch control. Without cyclic blade pitch control the 1/rev blade and rotor shaft bending moments become quite large and, rather surprisingly, for some of these test conditions also a large 2/rev blade bending moment is observed. Careful analysis of the data suggests that the 2/rev flap bending is caused by a 1/rev excitation (in the non-rotating system) of both nacelles, caused by the proximity of the nominal rotor rpm frequency to symmetric and anti-symmetric nacelle torsion eigen-frequency and probably also in combination with other nacelle related eigen-modes. The measured data are well suited to validate or verify existing semi-empirical or CFD methods to predict the steady and unsteady loads of the rotors. A correlation between rotor dynamics and (outer) wing dynamic loading and the influence of the outer wing and/or flaperon setting on the rotor forces and moments is left for future anal-ysis. Introduction

A case study on the air flow characteristics of the Hirobo-FALCON 505 controllable helicopter's main rotor blade

Abstract: The aerodynamics of the helicopter rotor is one of the most elating and exigent tribulations faced by the aerodynamicists today. Study through flow visualization process plays a key role in understanding the airflow distinctiveness and vortex interaction of the helicopter main rotor blade. Inspecting and scrutinizing the effects of wake vortices during operation is a great challenge and imperative in designing effective rotor system. This study aimed to visualize the main rotor airflow pattern of the Hirobo-FALCON 505 controllable subscale helicopter and seek for the vortex flow at the blade tip. The experimental qualitative data is correlated with quantitative data to perform scrupulous study on the airflow behavior and characteristics along with its distinctiveness spawned by the main rotor blade. Simulation using design software is performed in analogous stipulations to endow with comparability between the flow visualization results. Throughout the blade span several dissimilar flow patterns have been identified. The main rotor hub has turbulent flow at its center due to low energy of air amassed in this region whereas in the middle portion of the rotor blade, the air encompasses high kinetic energy with a clear straight streamline pattern.