2.10a_Manolesos: A Detached Eddy Simulation study of drag reduction devices for flatback airfoils
Creators: name: Papadakis, George
name: Diakakis, Konstantinos
name: Manolesos, Marinos
Publisher: name: Zenodo
Year: 2019
Subjects: Detached Eddy Simulation, Flatback Airfoil, Drag Reduction, Flow Control

description: The current trend in wind turbine design is upscaling with manufacturers producing models of ever greater nominal power and with ever larger rotor size. As the rotor diameter increases, designing sufficiently light blades with the desired aeroelastic properties becomes a significant challenge. One of the possible solutions is the use of flatback airfoils, i.e. airfoil profiles with blunt trailing edge TE. The benefits are both structural (larger internal area, which allows for larger beams closer to the TE, and increased flapwise stiffness because of the blunt TE) and aerodynamic (reduced sensitivity to tripping and higher maximum lift coefficient). It has been shown that by using flatback airfoils, a 16% decrease in blade wake can be achieved without loss in performance [1]. The main drawback of flatback airfoils is that they have significantly higher drag values, which reduce their overall efficiency. This is a result of the low base pressure at the TE, and a number of TE devices have been proposed to improve their performance [2,3]. The latest addition to these devices was a flap, located at the top edge of the blunt TE [4]. In an experimental comparative study, it was found that the flap, combined with a horizontal flat plate at the bottom edge of the TE, was the best performing device, leading to approximately 50% decrease in drag and 30% increase in Lift to Drag ratio, at the design lift value. The present study revisits the same flatback airfoil (LI30-FB10, [5]) and devices, this time numerically, using the in-house solver MaPFlow [6]. Both Reynolds Averaged Navier Stokes Simulations (RANS) and Delayed Detached Eddy Simulations (DDES – implemented according to [7]) are performed. The aim is to identify the drag reduction mechanism in more detail, which would allow an improved design. For all cases, the flow Reynolds number is 1.5 million and a number of devices are tested. The numerical approach is validated against the available experimental data from [4,8]. Preliminary results show that RANS tend to overpredict drag values for the flatback airfoil, with or without the flow control devices. DDES results match very well with the published results in terms of both force coefficient values and wake frequencies. As postulated in the experiments, the numerical results show that the TE devices lead to a delayed roll-up of the shear layer at the wake of the airfoil. Flow fluctuations are weaker and faster when the flap is used, as the flow is entrained in the wake. This talk will present the latest findings of this ongoing study as a step to deeper understanding of drag reduction mechanism for flatback airfoils. [1] Griffith, P.W. Richards, The snl100-03 blade: Design studies with flatback airfoils for the Sandia 100-meter blade, 2014. [2] Kahn, C.P. van Dam, D.E. Berg, Trailing edge modifications for flatback airfoils, Sandia National Laboratories, 2008. [3] Baker, C.P. Van Dam, B.L. Gilbert, Flatback airfoil wind tunnel experiment, Sandia National Laboratories, USA, 2008. [4] Manolesos andVoutsinas, Experimental Study of Drag-Reduction Devices on a Flatback Airfoil, AIAA J. 54 (2016) 3382–3396 [5] Boorsma, et al. New aerodynamics rotor concepts specifically for very large offshore wind turbines, 2015. [6] Papadakis, PhD Thesis, NTUA, 2014. [7] Shur et al., Int. J. Heat Fluid Flow. 29 (2008) 1638–1649 [8] Baker et al.,<em> J. Sol. Energy Eng.</em> <strong>128</strong> 422–431 (2006)