Blade Element Momentum Analysis Research Articles

Investigating horizontal-axis wind turbine aerodynamics using cascade flows

The simplest aerodynamic model of horizontal-axis wind turbines is the blade element momentum theory, which assumes that the blades behave as airfoils, but a correct two-dimensional representation is an infinite cascade of lifting bodies. This study analyzes the conventional and impulse forms of the forces on cascades of airfoils at spacings and pitch angles typical of wind turbine applications. OpenFOAM software was used to simulate steady, incompressible flow at a Reynolds number of 6×106 through cascades of NACA 0012 airfoils. The force equations agree well (less than 1% error) with the forces determined directly from OpenFOAM for four spacing ratios. We concentrate on the “wake vorticity” term, which is ignored in blade element momentum analysis. At a pitch angle of 90°, this term balances the viscous drag when the angle of attack is zero. At zero pitch, which models the outer region of a wind turbine blade at a high tip speed ratio, the term can account for 27% of the axial thrust when the angle of attack is about 4°. The normal force equation, like the angular momentum equation for wind turbines, has no viscous term, which forces the body drag to contribute to the circulation in the wake. It is shown that the airfoil assumption is conservative in that cascade elements have higher lift-to-drag ratios than airfoils at the same angle of attack. An associated result is that separation occurs at higher angles of attack on a cascade element compared to an airfoil.

Aerodynamic Analysis of Propeller for Heavy Lifting Drone Applications Using Blade Element Momentum Analysis

Nowadays, drone technology is seen to be rapidly advancing in various fields and applications including photography, military, transportation, sports, and many more. Therefore, each drone designs require different aerodynamic requirements, which includes different types of propeller designs. By revolving and generating airflow, the propellers give drones or unmanned aerial vehicles (UAV) a lift force or thrust. This work examines the method to calculate the thrust force generated by propellers using the Blade Element Momentum Analysis which is programmed in MATLAB. This program is developed to perform the calculation of thrust and torque for a given propeller blade geometry. This investigation compares the thrust coefficient produced by different propeller designs a various rotation speeds and parameters using the extended blade element momentum theory. Five different types of propellers are analysed and simulated using the in-house MATLAB program. The effects of rotational speeds are then added to these databases. At low advance ratios compared to a generic blade element-momentum model, a considerable improvement in modelling accuracy is seen when results are compared to experimental ones. The calculation may overestimate performance by 5% to 10%.

Optimization of small wind turbine blades using improved blade element momentum theory

Typically, it is desired to operate the wind turbine at the maximum power point. However, in small wind turbines which have a storage system integrated with them, harvesting as much energy as possible is more crucial. This may be achieved by reducing the cut-in speed while maximizing the mechanical power. These two goals may be achieved by optimizing the turbine blades. In this article, the turbine blades are optimized using improved blade element momentum theory including Viterna-Corrigan stall model with the objective to yield low cut-in speed and high power level. Using the blade element momentum analysis, the power coefficient curves as functions of tip-speed ratio at various range of wind speeds are obtained for the optimized turbine. Using MATLAB/Simulink tool, a wind energy system, which consists of a wind turbine, permanent magnet synchronous generator and a resistive load, is simulated. The curves obtained from the blade element momentum analysis are used to emulate the wind turbine. The results obtained from the simulation are compared to experimental results. It is noticed that the wind turbine may be optimized to harvest more energy.

Insight into stall delay and computation of 3D sectional aerofoil characteristics of NREL phase VI wind turbine using inverse BEM and improvement in BEM analysis accounting for stall delay effect

In this paper, analysis of stall delay phenomenon for the NREL Phase VI wind turbine is done in order to improve Blade Element Momentum (BEM) method. The current study includes different proposals for extrapolation of 2D aerofoil characteristics for S809 aerofoil at higher angles of attack and their implementation in BEM analysis and comparison with experiment results. Also four existing stall delay corrections models are examined. In general BEM with no stall delay models did not account for stall delay and BEM with existing stall delay models result in over-prediction of lift coefficients at inboard sections of the blade at higher wind speed. Improved inverse BEM method is developed here to compute 3D aerofoil characteristics. In addition to five radial locations as described in the NREL/NASA Ames test analysis, thirteen additional radial locations are considered for a better understanding of stall delay. A new BEM model with the local radius effect for aerofoil characteristics (other than as a function of Reynolds number and angle of attack only) is proposed. Implementation of the new model showed a good agreement with aerofoil characteristics distribution along the blade span. It further revealed a good matching for overall thrust comparison and promising comparison in power computation.

Wind Turbine Performance in Controlled Conditions: BEM Modeling and Comparison with Experimental Results

Predictions of the performance of operating wind turbines are challenging for many reasons including the unsteadiness of the wind and uncertainties in blade aerodynamic behaviour. In the current study an extended blade element momentum (BEM) program was developed to compute the rotor power of an existing 4.3 m diameter turbine and compare predictions with reported controlled experimental measurements. Beginning with basic blade geometry and the iterative computation of aerodynamic properties, the method integrated the BEM analysis into the program workflow ensuring that the power production by a blade element agreed with its lift and drag data at the same Reynolds number. The parametric study using the extended BEM algorithm revealed the close association of the power curve behaviour with the aerodynamic characteristics of the blade elements, the discretization of the aerodynamic span, and the dependence on Reynolds number when the blades were stalled. Transition prediction also affected overall performance, albeit to a lesser degree. Finally, to capture blade finite area effects, the tip loss model was adjusted depending on stall conditions. The experimental power curve for the HAWT of the current study was closely matched by the extended BEM simulation.

Design methods of horizontal axis wind turbine rotor blades

Horizontal axis wind turbines (HAWTs) development and utilisation is growing rapidly. No such a unique methodology in literature describes the design of HAWT rotor blades. Each researcher usually adopts one of the design methods known to his best knowledge. In this paper, we outline the HAWT design analysis using the blade element momentum (BEM) theory. A total of ten different methods for the calculation of the chord length distribution along the rotor blade, with and without considering wake rotation, are presented. The performance of various methods for the correction of the axial induction factor is also presented. An important outcome of this work is the elimination of the discrepancy encountered when trying to select among the methods of blade design as we found that a few of the published methods are actually different faces of the same coin and they lead to exactly the same final output yet through a different procedure. Several methods based on BEM analysis for optimum chord length and flow angle are found to produce very close blade geometry along the outer half of the blade length. The inclusion of drag in the calculation of the optimal chord length is found to be insignificant.

Aerodynamics performance of continuously variable speed horizontal axis wind turbine with optimal blades

For conventional HAWT (horizontal axis wind turbines), the rotor speed is maintained constant while the blade tip speed changes continuously. This reduces considerably the power performance of the wind turbine particularly at high wind speeds where the tip speed ratio is small. With growth of variable speed generators, a compact BEM (blade element momentum) analysis is derived to design optimal blades for continuously variable speed HAWTs. First, a generalized quadratic equation on the angular induction factor is introduced which is related to local axial induction factor, blade local speed ratio, and drag to lift ratio. Second, the optimal blade geometry is obtained for which the maximum power coefficient is calculated at different design tip speed ratios and drag to lift ratios by assuming variable operational speed. Third, it is demonstrated that the power performance of the variable speed wind turbine is significantly higher than the conventional constant speed wind turbines. In addition, the present BEM modeling may be useful to reduce the computational effort of iterative numerical methods used in determining off-design power performance of conventional wind turbines with constant speed.

Magnus type wind turbines: Prospectus and challenges in design and modelling

One of attracting concepts has been the use of Magnus effect to produce lift from rotating cylinders in various engineering applications. With emerging innovative Magnus type wind turbine technology, it is important to determine power performance and characteristics of such generators as correctly as possible. As stressed by Seifert, there is lack of theories in design and modelling of using Magnus force in engineering which is particularly noticed for the horizontal axis Magnus type wind turbines. In this study, the importance of research carried out for determining lift and drag forces of rotating circular cylinders is highlighted and reviewed. Then, the theoretical methods used in designing commercial aerofoil type wind turbines are extended to apply on the Magnus types. New formulation is presented for potential flow around the Magnus blades. The blade element momentum (BEM) theory is formulated for the Magnus wind turbines. A cubic function for angular induction factor is found from the BEM analysis which is strongly dependant on the drag to lift ratio. It is also observed that the relative wind incidence angle and the local power coefficient of the Magnus cylinder are independent functions of spin ratio.

A coupled hydro-structural design optimization for hydrokinetic turbines

An optimization methodology for a stall regulated, fixed pitch, horizontal axis hydrokinetic turbine is presented using a combination of a coupled hydro-structural analysis and Genetic Algorithm (GA) based optimization method. Design and analysis is presented for two different designs: a constant chord, zero twist blade, and a variable chord, twisted blade. A hybrid approach is presented combining Blade Element Momentum (BEM), GA, Computational Fluid Dynamics (CFD), and Finite Element Analysis (FEA) techniques. The preliminary analysis is performed using BEM method to find the hydrodynamic performance and flap-wise bending stresses in turbine blades. The BEM analysis used for the current study incorporates effect of wake rotation, hub loss and tip loss factors, and effect of Reynolds number on hydrodynamic data. A multi-objective optimization is then performed to maximize performance and structural strength of turbine. The results of optimization for a constant chord, zero twist blade design are validated with detailed three dimensional CFD and finite element analysis. The fluid domain is coupled with the structural domain through one way coupling and fluid-structure interaction analysis is carried out to find the effect of blade geometry and operating conditions on the stresses developed in the blades. The hydrodynamic performance of a constant chord turbine was found to be limited by the high stresses developed in turbine blades. Hence, in an effort to reduce the stresses in turbine blades, a variable chord, twisted blade design was developed; and a multi-objective optimization is presented for the variable chord twisted blade turbine for hydro-structural performance improvement. The final-optimized variable chord, twisted blade design was found to improve the power coefficient by 17% and resulted in lower overall stresses.

High-Incidence Airfoil Lift and Drag Estimates

VARIOUS numerical tools are available to predict the lift and drag behavior of airfoils for preliminary design purposes. To be representative, these codes are typically limited to maximum incidence values close to stall [1,2]. For most applications, these predictions are satisfactory. However, with the large scale interest in and implementation of wind turbines, predictions far beyond stall are required. The need for such estimates are driven by the major numerical tools used for turbine performance estimates, e.g., Aerodyne, WTPerf, etc. [3,4]. These methods implement blade element momentum (BEM) theory, which requires two-dimensional (sectional) airfoil data. BEM analysis may, however, require airfoil data for incidences from 180 to 180 deg. Inaccurate airfoil force coefficient data are generally considered to contribute the largest error in BEM turbine power estimation. To meet the need for highincidence force coefficient estimation, various methodologies have been devised, e.g., those of Viterna and Janetzke [5], Spera [6], and Lindenburg [7]. These methods are all fully empirical with essentially little/no flow physics or theory underpinning their formulation. They are based on large scale experimental observations of the limited data available for high-incidence airfoil and plate testing. The methods are generally composed of an equation set that is implemented using statements conditional upon specified airfoil incidence, etc. Note that this is not implied as a criticism, but an indication of the complexity of the problem. It would be valuable to the aeronautics (rotorcraft) and renewable energy community to have an estimation method that consists of a single (as opposed to piecemeal) expression to estimate airfoil lift and drag from low to extreme incidence. Additionally, using a single unified equation would negate the requirement for tabular lookup data as well as interpolation within this data, as required for many BEM codes. This would greatly simplify the BEM input files and potentially improve accuracy, but would require some recoding of these applications. Consequently, such an approach is presented in this Note. The developed method combines established theory with empirical observation to yield performance estimates. It is not implied that the present method is not empirical; the inherent complication and variability of the flows to be modeled virtually necessitate empiricism. The approach is not suggested to provide exact solutions, but suitable engineering approximations for wind turbine power modeling or high-incidence airfoil aerodynamics. As such it is seen as complementary to the existent methods [5–7]. Following development, the method is compared with experimental data for validation.