CFD Letters 14, Issue 5 (2022) 16-23 16 CFD Letters Journal homepage: https://semarakilmu.com.my/journals/index.php/CFD_Letters/index ISSN: 2811-3969 The Drag of an Elliptical Airfoil at a Reynolds Number of 1000 Sheila Tobing 1,* 1 Faculty of Engineering, University of Indonesia, UI Depok Campus, Depok 16424, Indonesia ARTICLE INFO ABSTRACT Article history: Received 17 March 2022 Received in revised form 10 May 2022 Accepted 17 May 2022 Available online 31 May 2022 There have been many studies on the mechanisms of unsteady aerodynamics, such as leading-edge vortex (LEV) formation, wing-wake interaction, and spanwise flow. Spanwise flow can only be observed on three-dimensional wing models; however other phenomena such as LEV and wing-wake interaction can be captured using two- dimensional airfoil models. This study focuses on two-dimensional elliptical airfoil because this profile can generate counter-rotating vortices used by insects to generate aerodynamic forces. This research aims to analyze the drag production of two- dimensional elliptical airfoils flapping with bumblebee-inspired kinematics in asymmetrical normal-hovering mode at a typical Reynolds number range of Re = (10 3 ). It is found that drag is generated during the downstroke while thrust during the upstroke. It is also found that the creation and shedding of counter-rotating vortices are closely related to the generation of thrust. The results also indicate that asymmetrical strokes can be used in normal hovering to minimize drag or produce thrust. Keywords: Elliptical airfoil; bumblebee kinematics; counter-rotating vortices 1. Introduction There have been many studies on flapping wing flight on two-dimensional (2D) objects such as airfoil [1–4] and three-dimensional objects such as nature-inspired wings [5–9]. These past studies on flapping wing flight have led to the discovery of unsteady aerodynamic mechanisms such as leading-edge vortex (LEV), spanwise flow and wing-wake interaction. Leading-edge vortex is formed by the flow that separates at the wing’s leading edge and reattaches before leaving the trailing edge [5, 10]. Spanwise flow is a base-to-tip flow that limits the growth of leading-edge vorticity, and thus stabilizes LEV and delays its separation from the wing surface [5]. Wing-wake interaction causes the rapid change in aerodynamic forces after supination and pronation due to the interaction between wing and vortices shed during the previous flapping cycles [6, 11]. The unsteady aerodynamic mechanism explains why insects are able to fly, including bumblebees that are deemed unfit to fly based on the aerodynamics of stationary wings. Bumblebees can carry a heavy load with their small wings [12]. A trait that might relate to the unique vortex rings observed on the wings of bumblebees. The vortex ring on each wing of a * Corresponding author. E-mail address: sheilatobing1@gmail.com (Sheila Tobing) https://doi.org/10.37934/cfdl.14.5.1623