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 nearwake flow dynamics of a sinusoidal heaving foil. Midspan particle image velocimetry (PIV) measurements were taken in a closedloop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies (k = fc/U) from 0.09 to 0.20. Timeresolved and phaselocked measurements are used to describe the mean flow characteristics and phaseaveraged vortex structures and their evolution. Largeeddy scale (LES) decomposition and swirling strength analysis are used to quantify the vortical structures. The results demonstrate that trailing edge flexibility has minimal influence on the mean flow characteristics. The mean velocity deficit for the flexible trailing edge and rigid foils remains constant for all reduced frequencies tested. However, the trailing edge flexibility increases the swirling strength of the smallscale structures, resulting in enhanced crossstream dispersion. Flexibility at the leading edge is shown to generate a largescale leading edge vortex (LEV) for k ≥ 0.18. This results in a reduction in the swirling strength due to vortex interactions when compared to the flexible trailing edge and rigid foils. Furthermore, it is shown that the largescale LEV is responsible for extracting a significant portion of energy from the mean flow, reducing the mean flow momentum in the wake. The kinetic energy loss in the wake is shown to scale with the energy content of the LEV.