Leading edge vortex dynamics in airfoils: Effect of pitching motion at large amplitudes

• Published in: Journal of fluids and structures 2023-01, Vol.116, p.103796, Article 103796
• Main Authors: Seshadri, Pradeep Kumar, Aravind, Akhil, De, Ashoke
• Format: Article
• Language: English
• Subjects: Immersed boundary method; Leading-edge vortex; Pitching airfoil; Sharp interface method; Unsteady aerodynamics; Vortex dynamics; Vortex tracking
• ISSN: 0889-9746; 1095-8622
• DOI: 10.1016/j.jfluidstructs.2022.103796

Airfoils pitching in the stalled regime have been of keen interest in recent years due to their desirable aerodynamic force characteristics. This study numerically investigates the unsteady flow past a NACA (National Advisory Committee for Aeronautics) 0012 airfoil under sinusoidally pitching motion using a sharp interface immersed boundary framework. The flow is investigated in the low Reynolds number regime (Re = 3000) for reduced frequencies of 0.628 and 3.14 at three pivot locations (one-third chord, mid-chord, and two-third chord lengths from the leading edge). The airfoil is subjected to sinusoidal motion, with its incidence angle varying from 15° to 45°. Leading-edge vortices (LEVs) formed during the pitching motion dictate the transient aerodynamic characteristics. The flow field data is used to identify individual LEVs in the flow field. The spatio-temporal evolution of LEVs in terms of their strengths and vorticity weighted centroid is traced throughout the pitching cycle to obtain a quantitative estimate of their evolution. The cycle-to-cycle variation in the vorticity evolution is characterized, and its corresponding influence on the lift/drag characteristics has been explored in detail. At low frequencies, all the pitching cycles evolve in two different patterns; however, at high frequencies, the LEVs from different cycles merge and evolve in the form of a vortex cluster. The effect of pivot location and pitching frequency on the vortex evolution and aerodynamic forces is investigated. An increased pitching frequency delays LEV formation and drastically alters the vortex dynamics. The maximum values of the lift and drag coefficients increase with pitching frequency. Additionally, pivot locations have a stronger influence on the magnitude and phase of aerodynamic coefficients at higher frequencies. However, moving the pivot axis aftward at all studied frequencies delays LEV evolution and causes a phase lag on the aerodynamic forces.