Proceedings of the Canadian Society for Mechanical Engineering International Congress 2021 CSME Congress 2021 June 27-30, 2021, Charlottetown, PE, Canada Flow Control in Low-Reynolds Flow Using Flexible Gurney Flap B. Afra 1 *, A. Tarokh 1,2 1,* Faculty of Science and Environmental Studies, Lakehead University, Thunder Bay, Canada 2 Department of Mechanical Engineering, Lakehead University, Thunder Bay, Canada * Email: bafra@lakeheadu.ca atarokh@lakeheadu.ca Abstract- The structure of the wake behind immersed bodies can be manipulated via either passive or active flow control methods. In this study, the effects of a flexible Gurney flap on the wake structure are investigated. The Gurney flap as a passive flow control does not require an extra source of energy and it proved its capability of modification of the wake structure in low Reynolds number. We analyze the flow characteristics for the case in which a flexible gurney flap has been attached to a circular cylinder and is under affected of incoming uniform flow at different Reynolds numbers. We implemented a new robust numerical method including Immersed Boundary lattice Boltzmann Methods to find the flow filed and boundary forces which is combined with Lattice Spring Model for simulation of flexible Gurney flap. In order to verify the numerical method, fluctuation of a filament attached to the cylinder in downstream has been compared in terms of the fluctuation’s amplitude. The gurney flap has been found to enhance lift experienced over the immersed body due to high pressure region generated at downstream of the flap. Also, lift to drag ratio is studied for different bending stiffness and length of the flap. The results show that flexibility changes the projected area in the normal direction, altering drag force and wake structure at downstream of the whole structure. Keywords-component; formatting; style; styling; insert (key words) I. INTRODUCTION The field of low-Reynolds number fluid structure interaction has extensively received consideration over the past decade due to the high range of applications, such as imitation of fishing motions for fabricating of soft robotics [1], heat transfer advancement using vortex generator flapping fins [2] and flow control of aerial vehicles suffers from stalls in high angle of attacks [3]. For these applications, in particular computational flow control of immersed body, understanding the effect of the lift force on the boundary edges of deformable becomes vital. On the other hand, drag forces play a paramount role in such problems as drags reduce kinetic energy of fluid flows in the vicinity of the structure. Flow control study of immersed body undergoes a low- Reynolds number incoming fluid flow has been widely performed by different research groups, either passively [4, 5] and actively [6, 7]. Active control approaches require energy and usually are efficient in high Reynolds numbers to delay and remove stalls on aircrafts’ wings. They are less-effective in very low Reynolds numbers (e.g., in order of 2 10 to 3 10 ) since passive flow control methods are as successful as active ones [8]. Active flow control methods operate when they are needed, and require control devices and actuators [8]. Thus, the problems in which lift and drag forces can be controlled passively are not reasonable to be combined by active flow control methods. In this study, hydrodynamic forces on the circular cylinder are studied to figure out how a simple, effective passive method, namely Gurney flap attachment, can optimize the flow structure at downstream of the flap. The Gurney flap attached to the structure’s boundaries are extensively seen in popular researches in the field of immersed bodies’ flow control [9, 10]. Gurney flap consists of a thin plate vertically attached to the pressure-side of immersed bodies (particularly airfoils), resulting in an increment in lift coefficient. While drag coefficients will also increase by an extra device perpendicular to the stream-wise. In the previous studies [11, 12] showed that ratio of lift to drag coefficients ( LD D ) may be larger than that of clean cylinder and differs by altering location of the flaps. There are extensive researches on Gurney flaps in the literature in high Reynolds number to examine effects of the flap’s height and its location on generating higher lift-to-drag ratio [13-15]. However, the information about the effects of the flexible flap on the lift coefficient and the structure of wakes behind the immersed body is very limited. In this study, we simulate a circular cylinder equipped by a flexible Gurney flap and determined the effects of its flexibility, size and configuration on the lift-to- drag ratio and flow behavior behind the cylinder.