Firas F. Siala

THE EFFECTS OF A PASSIVELY ACTUATED TRAILING EDGE ON THE AERODYNAMICS OF AN OSCILLATING WING

This study examines the use of a passively actuated trailing edge of a thin wing during oscillation motion. The integration of a flexible trailing edge with an oscillating wing has the ability to alter the transient lift and drag characteristics, as well as the time averaged values. The results are obtained for a chord-length based Reynolds number of 0 and 40,000, and at oscillation frequencies of 0.5 and 1 Hz. The non-dimensional heaving amplitude is fixed at 0.25 and the pitching is 20°. The flexibility of the trailing edge is controlled by a torsion rod between the main wing and the trailing edge. Three conditions are evaluated: a very stiff rod (essentially non-flexible trailing edge), a moderately flexible rod and a very flexible rod. Results obtained indicate that lift and drag have a shift in the time averaged values, where the drag and lift both decrease as the trailing edge flexibility increases. These findings have application to both enhanced propulsion and energy harvesting.Copyright

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

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. Midspan particle image velocimetry (PIV) measurements were taken in a closed-loop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies (k1⁄4 fc/U) from 0.09 to 0.20. Time-resolved and phase-locked measurements are used to describe the mean flow characteristics and phase-averaged vortex structures and their evolution. Large-eddy 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 small-scale structures, resulting in enhanced cross-stream dispersion. Flexibility at the leading edge is shown to generate a large-scale 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 large-scale 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. [DOI: 10.1115/1.4033959]

Wake Dynamics and Structure of a Heaving Flexible Foil Based on PIV Measurements

The effect of surface flexibility at the leading and trailing edge on the flow dynamics in the near wake region of a heaving foil is investigated using 2D particle image velocimetry measurements. The experiments were conducted in a closed loop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies from 0.09 to 0.2. Swirling strength analysis and velocity triple decomposition are used to quantify the vortical structures and their influence on momentum entrainment, respectively. The results show at large reduced frequencies, flexibility at the leading edge generates a significantly different wake structure when compared to the flexible trailing edge and rigid foils, by allowing a large scale vortex to form and shed from the leading edge. The swirl strength is shown to decrease with increasing reduced frequency due to vortex compression from the three dimensional effects. In addition, it is observed that the dominant mechanism of momentum transfer is associated with the leading edge vortex.