Title: Wake patterns and mode switching at low Reynolds numbers

Authors: Nabih Naeem; Mahmoud Fouda; Mertcan Güney; Dilek Funda Kurtuluş

Addresses: German Aerospace Center (DLR), Institute of System Architectures in Aeronautics, Hamburg, Germany ' German Aerospace Center (DLR), Institute of System Architectures in Aeronautics, Hamburg, Germany ' Aerospace Engineering Department, Middle East Technical University, Ankara, Turkey ' Aerospace Engineering Department, Middle East Technical University, Ankara, Turkey

Abstract: Instantaneous wake structures behind a 2% thick NACA 0002 symmetric airfoil are numerically studied in 2D at six different angles of attack for Reynolds numbers ranging from 100 to 3,000. Classification of the flow patterns based on vortex structure is discussed. An in-depth study of the various flow modes of vortex dynamics in the wake of the airfoil is presented, considering the amplitude spectrum of the lift coefficient and Poincaré maps. A new mode in addition to a mode switching phenomenon is identified to extend the Kurtulus modes defined in the literature. For 25°, 30°, and 35° angles of attack and Reynolds number above 1,100, considering 2% thick NACA airfoil, mode switching is identified where the flow could not achieve a stable state but rather fluctuates between two modes. New links are made between the modes' transitions, mode switching, Reynolds-based bifurcations, and the sudden changes of the forces acting on the airfoil considering the change in mean aerodynamic coefficients with variations in Reynolds number. Lastly, the evolution and the stability of the wake patterns have been examined, describing their effect on the airfoil's aerodynamic performances.

Keywords: mode switching; Reynolds-based bifurcations; wake patterns; symmetric airfoil; NACA 0002; CFD; two dimensional flow; Kurtulus modes.

DOI: 10.1504/PCFD.2023.129762

Progress in Computational Fluid Dynamics, An International Journal, 2023 Vol.23 No.2, pp.87 - 110

Received: 23 Apr 2021
Accepted: 22 Jan 2022
Published online: 25 Mar 2023 *

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