Flatback Airfoils Research Articles

Experimental Study of Drag-Reduction Devices on a Flatback Airfoil

Various trailing-edge drag-reduction devices, including a new flap device, were examined experimentally on a flatback airfoil in a wind tunnel. The tests concerned a 30% thick airfoil with 10.6% thick trailing edge. Pressure, hot wire, and stereo particle image velocimetry measurements were performed at a chord Reynolds number of . Results show that the best-performing devices decrease drag, increase the vortex shedding frequency, and reduce flow variation downstream of the wing trailing edge. The best-performing device was a combination of the flap with an offset cavity plate. Further investigation is required for the optimization of the new device to examine its effects on noise reduction, load mitigation, and control.

CFD aerodynamic analysis of non-conventional airfoil sections for very large rotor blades

The aerodynamic performance of flat-back and elliptically shaped airfoils is analyzed on the basis of CFD simulations. Incompressible and low-Mach preconditioned compressible unsteady simulations have been carried out using the k-w SST and the Spalart Allmaras turbulence models. Time averaged lift and drag coefficients are compared to wind tunnel data for the FB 3500-1750 flat back airfoil while amplitudes and frequencies are also recorded. Prior to separation averaged lift is well predicted while drag is overestimated keeping however the trend in the tests. The CFD models considered, predict separation with a 5° delay which is reflected on the load results. Similar results are provided for a modified NACA0035 with a rounded (elliptically shaped) trailing edge. Finally as regards the dynamic characteristics in the load signals, there is fair agreement in terms of Str number but significant differences in terms of lift and drag amplitudes.

Aeroacoustic simulations of a blunt trailing-edge wind turbine airfoil

We demonstrate the effectiveness of semi-empirical Brooks, Pope and Marcolini model and hybrid large eddy simulations in calculating the blunt trailing edge wind turbine noise at higher Reynolds number conditions. The 4 million element meshes of sharp and blunt trailing edge airfoils were tested at a Reynolds number of 3.2 million and an angle of attack of 4 degrees. The predicted airfoil self-noise by the modified semi-empirical formula with a low frequency directivity function and an additional term for large thickness ratio was compared to the experiments. The sound pressure level spectra from the hybrid large eddy simulation show that the predictions agree well with experimental measurements at the same observer location in the peak frequencies of the blunt trailing edge noise and sound pressure level rates of change at lower frequencies are also similar to experiments. The modified semi-empirical formula and the hybrid large eddy simulation can be considered as promising tools for high vorticity flow problems, especially for flatback airfoils for use on large wind turbines.

Numerical simulation of flatback airfoil aerodynamic noise

The present paper presents a possible path for developing a large eddy simulation (LES) applicable to high Reynolds-number complex turbulent flows and the performance of the coupling of LES with statistical turbulence models around the flow over a blunt trailing edge configuration. The turbulent fluctuations in the boundary layers at the inflow region of the LES domain are generated by a synthesized turbulence method. The hybrid RANS-LES model showed considerable improvement in prediction accuracy even at a moderate grid resolution. The aerodynamic comparison with experimental data shows like results for the pressure distributions surrounding a flatback airfoil. To predict accurately the noise radiation from the blunt trailing edge and to save computational costs, the near-field region is computed by embedded LES while the surrounding region is simultaneously computed by RANS. The Brooks, Pope, and Marcolini (BPM) semi-empirical model is used for noise comparison with the hybrid RANS-LES result and experimental data. The present hybrid RANS-LES method is found to be adequate for predicting aerodynamic noise generation by vortical flow in the vicinity of a blunt trailing edge airfoil over a range of frequencies.

Optimization of flatback airfoils for wind turbine blades

PurposeIn recent years, the airfoil sections with blunt trailing edge (called flatback airfoils) have been proposed for the inboard regions of large wind‐turbine blades because they provide several structural and aerodynamic performance advantages. The purpose of this paper is to optimize the shape of these airfoils for optimal performance using a multi‐objective genetic algorithm.Design/methodology/approachA multi‐objective genetic algorithm is employed for shape optimization of flatback airfoils to achieve two objectives, namely the generation of maximum lift as well as the maximum lift to drag ratio. The commercially available software FLUENT is employed for calculation of the flow field using the Reynolds‐Averaged Navier‐Stokes (RANS) equations in conjunction with a two‐equation Shear Stress Transport (SST) turbulence model and a three‐equation k‐kl‐ω turbulence model.FindingsIt is shown that the multi‐objective genetic algorithm based optimization can generate superior flatback airfoils compared to those obtained by using a single objective genetic algorithm.Research limitations/implicationsThe method of employing genetic algorithms for shape optimization of flatback airfoils could be considered as an excellent example for the optimization of other types of wind turbine blades such as DU FX and S series airfoils.Originality/valueThis paper is the first to employ the multi‐objective genetic algorithm for shape optimization of flatback airfoils for wind‐turbine blades to achieve superior performance.

Optimization of Flatback Airfoils for Wind-Turbine Blades Using a Genetic Algorithm

In recent years, the airfoil sections with blunt trailing edges (called flatback airfoils) have been proposed for the inboard regions of largewind-turbine blades because they provide several structural and aerodynamic performance advantages. In this paper, a genetic algorithm is used for shape optimization of flatback airfoils for generating maximum lift-to-drag ratio. The computational efficiency of a genetic algorithm can be significantly enhanced with an artificial neural network. The commercially available software FLUENT is used for calculation of the flowfield using the Reynolds-averaged Navier–Stokes equations in conjunction with a turbulence model. It is shown that the genetic algorithm optimization technique is capable of accurately and efficiently finding globally optimal flatback airfoils.

Computational Analyses on Aerodynamic Characteristics of Flatback Wind Turbine Airfoils

The flatback wind turbine airfoils have good structure and aerodynamic performance. Blunt trailing edge or flatback airfoils are studied especially for the inboard region of large wind turbine blades. The drawback of the flatback design is known as the production of large vortices downstream of the airfoil that tend to degrade the performance of the blade. The overall objective of the present research is to have a better understanding of the airfoil performance characteristics such as the loss in aerodynamic performance balanced by the structural performance gain. The performance of the S809-100 flatback airfoil is simulated and compared with the original thin-trailing edge S809 airfoil by using the computational fluid dynamics (CFD) technique. It is shown that the flatback airfoil can increase the sectional maximum lift coefficient and lift curve slope,and reduce the well-documented sensitivity of the lift characteristics of thin airfoils to surface soiling in comparison with the traditional thin trailing edge airfoil.

Experimental Analysis of Thick Blunt Trailing-Edge Wind Turbine Airfoils

An experimental investigation of blunt trailing-edge or flatback airfoils was conducted in the University of California, Davis aeronautical wind tunnel. The blunt trailing-edge airfoil is created by symmetrically adding thickness to both sides of the camber line of the FB-3500 airfoil, while maintaining the maximum thickness-to-chord ratio of 35%. Three airfoils of various trailing-edge thicknesses (0.5%, 8.75%, and 17.5% chord) are discussed in this paper. In the present study, each airfoil was tested under free and fixed boundary layer transition flow conditions at Reynolds numbers of 333,000 and 666,000. The fixed transition conditions were used to simulate surface soiling effects by placing artificial tripping devices at 2% chord on the suction surface and 5% chord on the pressure surface of each airfoil. The results of this investigation show that lift increases and the well-documented thick airfoil sensitivity to leading-edge transition reduces with increasing trailing-edge thickness. The flatback airfoils yield increased drag coefficients over the sharp trailing-edge airfoil due to an increase in base drag. The experimental results are compared against numerical predictions obtained with two different computational aerodynamics methods. Computations at bounded and unbounded conditions are used to quantify the wind tunnel wall corrections for the wind tunnel tests.