Experimental investigation into wake flapping wing robotic bird

To further the development of bird-like drones a better understanding of flapping wing flight at Re ~= 10⁵ is necessary. For this purpose the wake of a flapping wing has been investigated using a 7-hole probe with a seven pressure ports distributed over its hemispherical tip. By measuring the pressures at the seven ports it is possible to determine the direction and the magnitude of the flow velocity as well as the static and total pressure. Since the 7-hole pressure probe is to be used to measure unsteady flows as occur in the wake of a flap-ping wing, the dynamic behavior of the probe and transducer system has been analyzed. Based on this analysis a probe has been designed to function within the range of frequencies expected in the wake. A test setup has been realized in the open-jet, closed-circuit wind tunnel of the University of Twente. This setup consists of the port-side wing of the robotic bird, and a mechanism designed to produce the flapping motion of the wing. The pressure probe has been installed in a frame allowing it to be positioned along traverses at various span wise positions in the wake of the wing, within a fixed plane normal to the free stream. First measurements have been taken for the stationary wing at two angles of attack. Subsequently, for the flapping wing the repeatability of the measurement method has been checked by considering results of two sets of measurements for a number of combinations of test parameters. For the stationary wing it was found that several expected flow features, such as the wingtip vortex, could be resolved in the results. Moreover, for different angles of attack a significant difference in the distributions of the upwash and downwash was found, corresponding to a difference in the lift generated by the wing. For the flapping wing the time-averaged streamwise velocity distribution was found to exhibit a jet-like profile similar to that found in previous studies. In addition, significant down-wash was found in the wake, indicating the generation of lift. Analyzing the data collected at a single location in the wake provided the flow velocity components and static pressure as a function of time. However, it was not possible to correlate the time-dependent data at various locations, because the wake measurement could not be synchronized with the mechanism flapping the wing.