Characterization of Vortex Dynamics in the Near Wake of an Oscillating Flexible Foil

Abstract

An experimental study was conducted to explore the effect of surface flexibility at the leading and trailing edges on the near-wake flow dynamics of a sinusoidal heaving foil. Mid-span particle image velocimetry measurements were taken in a closed loop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies (k = fc/U) from 0.09–0.20. Time resolved and phase locked measurements were used to describe the mean flow characteristics and phase averaged vortex structures and their evolution throughout the oscillation cycle. Large eddy scale decomposition and swirl strength analysis were used to quantify the effect of flexibility on the vortical structures. The results demonstrate that flexibility at the trailing edge has a minimal influence on the mean flow characteristics when compared to the purely rigid foil. The mean velocity deficit for the flexible trailing edge and rigid foils is shown to remain constant for all reduced frequencies tested. However, the trailing edge flexibility increases the swirl strength of the small scale structures, which results in enhanced cross stream dispersion of the mean velocity profile. Flexibility at the leading edge is shown to generate a large scale leading edge vortex for k ≥ 0.18. This results in a reduction in the swirl strength due to the complex vortex interactions when compared to the flexible trailing edge and rigid foils. Furthermore, it is shown that the large scale leading edge vortex is responsible for extracting a significant portion of the energy from the mean flow, resulting in a substantial reduction of mean flow momentum in the wake. The kinetic energy loss in the wake is shown to scale well with the energy content of the leading edge vortex.

Numerical Investigation of the Effects of Nonsinusoidal Motion Trajectory on the Propulsion Mechanisms of a Flapping Airfoil

“…Since the first works of thrust generation from flapping airfoil, several experimental [8][9][10][11][12][13][14][15] and numerical [16][17][18][19][20][21][22][23] studies were carried out to understand the mechanism of thrust generation and to improve their propulsive performances. A large number of these works focused on the effect of kinematic parameters such as the maximum amplitudes of heaving and pitching motions, the phase angle between heaving and pitching motions, the flapping frequency, and the effective angle of attack on the propulsive performance of the flapping airfoil.…”

Effect of wake interaction on the response of two tandem oscillating hydrofoils

“…7,8 Furthermore, modified motions 9,10 and flexible foils [11][12][13] are employed to improve the hydrodynamic and energy harvesting performance of an oscillating foil. Subsequently, the effects of kinematic parameters and flow evolution of the single foil experiencing a prescribed oscillation motion are thoroughly revealed by researchers using both computational and experimental methods.…”