The goal of the present study is to provide a better understanding of the flapping wing propulsion of a robotic bird for varying kinematics of the motion. For that purpose the flow over 2D pitching and plunging airfoil sections of the wing of the robotic bird has been numerically simulated. The airfoil motion is implemented using a dynamic mesh, which moves the airfoil within a structured C-grid in a re-meshing unstructured grid. The results of the numerical simulation of the flow over the mid-wing airfoil section of the robotic bird identify the influence of different kinematics. It shows that for an increasing Strouhal number, the maximum effective angle of attack increases. This results in a stronger Leading Edge Vortex (LEV) and therewith a stronger reverse von Karman vortex street in the wake. In the thrust-producing wake, the air is accelerated in between the vortices, which yields time-averaged a pronounced streamwise jet behind the airfoil. It has also been found that higher amplitudes of the pitching angles yield lower effective angles of attack. For that case the formed LEV's are milder and higher efficiencies are obtained, however, resulting in a lower cruise velocity. The efficiency for different pitching angles peaks for 0.1＜St＜0.3, which is close to the optimum Strouhal number regime found in nature. The maximum efficiency is always found for a similar maximum effective angle of attack of around 11 degrees. The jet-like velocity profiles obtained for a range of Strouhal numbers are compared with results of wind-tunnel experiments for the flapping robotic bird. Streamwise jets have been measured with strength of order of magnitude comparable to the ones found in the numerical simulations for the flow around the mid-wing airfoil section.
展开▼
