Different wake structures behind a flapping airfoil and its influence on propulsion performance.

The existence of a reverse von Karman street behind a flapping airfoil has been identified previously. The present paper numerically investigates the path through which the reverse von Karman street is established. The investigations are performed over a custom-designed airfoil performing combined motion of pitch and heave. The wake transformation path is established by increasing the flapping frequency and amplitude while visualizing the structure of the wake. The wake structure behind the flapping airfoil is identified using contours of vorticity, and the structures are arranged categorically. The force coefficients at different flapping frequencies and amplitudes are computed, and their influence on propulsion performance is assessed. The variation in wake structure behind the flapping airfoil appears to influence the force experienced by the airfoil significantly. The transition of the wake from periodic dynamics to aperiodic dynamics reduced the propulsion efficiency of the airfoil. [ABSTRACT FROM AUTHOR]