M.Sc Thesis


M.Sc StudentAlexander Itenberg
SubjectNumerical Investigation of Flapping Wings
DepartmentDepartment of Aerospace Engineering
Supervisor Dr. Levy Yuval


Abstract

A numerical investigation was conducted to study the aerodynamic characteristics of a wing in the heave motion. The results of the current work investigate the heave motion of two and three dimensional wings in inviscid and viscous flows for low Reynolds numbers. The low Reynolds numbers investigation was chosen because of an interest in small size, low speed UAVs.

The results of inviscid two-dimensional flow show a very good agreement with the linear theory under the assumption of small angles of attack. The increase in the maximum angle of attack decreases the thrust force and the motion efficiency comparing to the linear theory predictions. The inviscid three dimensional flow investigation examined the influence of the aspect and taper ratios. The decrease in the aspect ratio and the taper ratio decreases the thrust force. The decrease in the aspect ratio decreases the motion efficiency and presents a very good compliance to the two-dimensional results at the infinity approximation of the aspect ratio. As opposed to that, the decrease in the taper ratio increases the motion efficiency.

The viscous flow results show a strong Reynolds number effect. In order to investigate the heave motion with the Reynolds numbers in range of 104 to 5ยด104 the low limit of reduced frequency was set to 0.3. The results under the low limit of the reduced frequency show mainly the "von Karman vortex street" unsteady flow phenomenon. The unsteady phenomenon has a significant influence at low angles of attack (low reduced frequencies and motion amplitudes) and its influence reduces at high angles of attack. The motion efficiency is negative at low angles of attack, meaning that the drag force is greater than the thrust force. The propulsion efficiency (the propulsion efficiency calculation is based on the pure thrust force) is always positive as it is predicted by the linear theory, but has a different behavior. The propulsion efficiency increases up to the reduced frequency, where the unsteady phenomenon loses its influence, and from that point the propulsion efficiency decreases as the linear theory. The propulsion efficiency, in the region of small unsteady influence, is in range of 35% - 55%, depending on the reduced frequency and motion amplitude. The increase in the motion amplitude decreases the propulsion efficiency due to the additional drag force at high angles of attack.