Bluff Body Shedding and Stall Cells: Numerical Simulation of a Flatback Airfoil at High Angles of Attack
Creators: name: Manolesos, Marinos
name: Papadakis, George
Publisher: name: Zenodo
Year: 2021

description: The inboard part of wind turbine blades must satisfy structural, aeroelastic, transport and, finally, aerodynamic requirements. As a result, very thick airfoils are commonly used and lately their flatback versions, i.e. airfoils with blunt trailing edge (TE) (Griffith and Richards, 2014; Manolesos and Voutsinas, 2016). Such airfoils provide aerodynamic, structural and aeroelastic benefits compared to traditional airfoils of the same thickness. Computing the flow past flatback airfoils at low angles of attack is a challenge for traditional Computational Fluid Dynamics methods such as Reynolds Averaged Navier Stokes (RANS) solvers because of the unsteady and 3D character of the flow in the wake of the blunt TE. Higher fidelity options such as Detached Eddy Simulations (DES) offer improved predictions especially with regards to wake frequency predictions (Papadakis and Manolesos, 2020; Papadakis <em>et al.</em>, 2020). Considering flatback airfoils at high angles of attack increases the flow complexity decisively. Wind turbine thick airfoils experience TE stall and as such, they develop Stall Cells on their suction side (Broeren and Bragg, 2001; Manolesos and Voutsinas, 2016). Stall cells are 3D coherent structures of separated flow and their numerical simulation is a challenge on its own (Kamenetskiy <em>et al.</em>, 2014; Manolesos, Papadakis and Voutsinas, 2014). The present study is a continuation of the work presented in (Papadakis and Manolesos, 2020; Papadakis <em>et al.</em>, 2020) and focuses on the flow past a flatback airfoil at high angles of attack. The flow is simulated using RANS and DES options using the MaPFlow solver (Papadakis and Voutsinas, 2019) and the results are validated against experiments (Manolesos and Voutsinas, 2016) and are found in good agreement. The aim of the study is dual; we wish to identify a best practice when it comes to simulating the flow in question and also shed light in the flow mechanism involved in this highly 3D and unsteady flow. Preliminary analysis suggests that, even for low angles of attack, low Aspect Ratio (AR) simulations are artificially constraining the 3D character of the flow, although the dominant shedding frequency is predicted correctly. At stall, low AR simulations fail to predict the formation of Stall Cells and as a result overpredict stall angle and maximum lift. An interesting flow feature is the low drag pocket that appears for an incidence range before stall and after the lift gradient has slightly dropped from its value at α = 0°. This region is observed in the experiments, RANS and DES simulations, although the AoA range differs. Analysis of the high-fidelity simulations reveals that the wake flow is more coherent in the low drag pocket regime because the top vortex is weaker. It appears that this happens because the boundary layer on the suctions side is decelerated significantly by the adverse pressure gradient. Marinos Manolesos would like to acknowledge the contribution by Supergen ORE Hub Early Career Researcher Research Fund. The authors acknowledge Supercomputing Wales for CPU time. References Broeren, A. P. and Bragg, M. B. (2001) ‘Spanwise variation in the unsteady stalling flowfields of two-dimensional airfoil models’, <em>AIAA Journal</em>, 39(9), pp. 1641–1651. Griffith, D. T. and Richards, P. W. (2014) ‘Investigating the effects of flatback airfoils and blade slenderness on the design of large wind turbine blades’, <em>EWEA Conference</em>, pp. 1–8. Kamenetskiy, D. S. <em>et al.</em> (2014) ‘Numerical Evidence of Multiple Solutions for the Reynolds-Averaged Navier–Stokes Equations’, <em>AIAA Journal</em>. American Institute of Aeronautics and Astronautics, 52(8), pp. 1686–1698. doi: 10.2514/1.j052676. Manolesos, M., Papadakis, G. and Voutsinas, S. G. (2014) ‘Experimental and computational analysis of stall cells on rectangular wings’, <em>Wind Energy</em>, 17(6), pp. 939–955. doi: 10.1002/we.1609. Manolesos, M. and Voutsinas, S. G. (2016) ‘Experimental Study of Drag-Reduction Devices on a Flatback Airfoil’, <em>AIAA Journal</em>. American Institute of Aeronautics and Astronautics Inc., 54(11), pp. 3382–3396. doi: 10.2514/1.J054901. Papadakis, G. <em>et al.</em> (2020) ‘DES vs RANS: The flatback airfoil case’, <em>Journal of Physics: Conference Series</em>, 1618(5), p. 052062. doi: 10.1088/1742-6596/1618/5/052062. Papadakis, G. and Manolesos, M. (2020) ‘The flow past a flatback airfoil with flow control devices: Benchmarking numerical simulations against wind tunnel data’, <em>Wind Energy Science</em>, 5(3), pp. 911–927. doi: 10.5194/wes-5-911-2020. Papadakis, G. and Voutsinas, S. G. (2019) ‘A strongly coupled Eulerian Lagrangian method verified in 2D external compressible flows’, <em>Computers and Fluids</em>. Elsevier Ltd, 195, p. 104325. doi: 10.1016/j.compfluid.2019.104325.