Roeland de Kat

Aerodynamic Experiments on DelFly II: Unsteady Lift Enhancement

Particle image velocimetry measurements and simultaneous force measurements have been performed on the DelFly II flapping-wing MAV, to investigate the flow-field behavior and the aerodynamic forces generated. For flapping wing motion it is expected that both the clap and peel mechanism and the occurrence of a leading edge vortex during the translational phase play an important role in unsteady lift generation. Furthermore, the flexibility of the wing foil is also considered of primary relevance. The PIV analysis shows a strong influx between the wings during the peel but no downward expelling jet during the clap. The force measurements reveal that the peel, oppositely to the clap, contributes significantly to the lift. The PIV visualization suggests the occurrence of a leading edge vortex during the first half of the in- and outstroke, which is supported by a simultaneous augmentation in lift. The early generation of a leading edge vortex during the flex cannot be assessed from the PIV images due to optical obstruction, but is likely to appear since the wing flexing is accompanied with a large increase in lift.

Flow visualization and force measurements on a hovering flapping-wing MAV 'DelFly II'

Particle image velocimetry measurements and simultaneous force measurements have been performed on the DelFly II flapping-wing MAV, to investigate the flow-field behavior and the aerodynamic forces generated. For flapping wing motion it is expected that both the clap and peel mechanism and the occurrence of a leading edge vortex during the translational phase play an important role in unsteady lift generation. Furthermore, the flexibility of the wing foil is also considered of primary relevance. The PIV analysis shows a strong influx between the wings during the peel but no downward expelling jet during the clap. The force measurements reveal that the peel, oppositely to the clap, contributes significantly to the lift. The PIV visualization suggests the occurrence of a leading edge vortex during the first half of the in- and outstroke, which is supported by a simultaneous augmentation in lift. The early generation of a leading edge vortex during the flex cannot be assessed from the PIV images due to optical obstruction, but is likely to appear since the wing flexing is accompanied with a large increase in lift.

Feather roughness reduces flow separation during low Reynolds number glides of swifts.

ABSTRACT Swifts are aerodynamically sophisticated birds with a small arm and large hand wing that provides them with exquisite control over their glide performance. However, their hand wings have a seemingly unsophisticated surface roughness that is poised to disturb flow. This roughness of about 2% chord length is formed by the valleys and ridges of overlapping primary feathers with thick protruding rachides, which make the wing stiffer. An earlier flow study of laminar–turbulent boundary layer transition over prepared swift wings suggested that swifts can attain laminar flow at a low angle of attack. In contrast, aerodynamic design theory suggests that airfoils must be extremely smooth to attain such laminar flow. In hummingbirds, which have similarly rough wings, flow measurements on a 3D printed model suggest that the flow separates at the leading edge and becomes turbulent well above the rachis bumps in a detached shear layer. The aerodynamic function of wing roughness in small birds is, therefore, not fully understood. Here, we performed particle image velocimetry and force measurements to compare smooth versus rough 3D-printed models of the swift hand wing. The high-resolution boundary layer measurements show that the flow over rough wings is indeed laminar at a low angle of attack and a low Reynolds number, but becomes turbulent at higher values. In contrast, the boundary layer over the smooth wing forms open laminar separation bubbles that extend beyond the trailing edge. The boundary layer dynamics of the smooth surface varies non-linearly as a function of angle of attack and Reynolds number, whereas the rough surface boasts more consistent turbulent boundary layer dynamics. Comparison of the corresponding drag values, lift values and glide ratios suggests, however, that glide performance is equivalent. The increased structural performance, boundary layer robustness and equivalent aerodynamic performance of rough wings might have provided small (proto) birds with an evolutionary window to high glide performance. Summary: Swift feather roughness enhances boundary layer mixing, which reduces flow separation during low Reynolds number glides, enabling swifts to attain high glide performance with rough wings.

Gliding Swifts Attain Laminar Flow over Rough Wings

Swifts are among the most aerodynamically refined gliding birds. However, the overlapping vanes and protruding shafts of their primary feathers make swift wings remarkably rough for their size. Wing roughness height is 1–2% of chord length on the upper surface—10,000 times rougher than sailplane wings. Sailplanes depend on extreme wing smoothness to increase the area of laminar flow on the wing surface and minimize drag for extended glides. To understand why the swift does not rely on smooth wings, we used a stethoscope to map laminar flow over preserved wings in a low-turbulence wind tunnel. By combining laminar area, lift, and drag measurements, we show that average area of laminar flow on swift wings is 69% (n = 3; std 13%) of their total area during glides that maximize flight distance and duration—similar to high-performance sailplanes. Our aerodynamic analysis indicates that swifts attain laminar flow over their rough wings because their wing size is comparable to the distance the air travels (after a roughness-induced perturbation) before it transitions from laminar to turbulent. To interpret the function of swift wing roughness, we simulated its effect on smooth model wings using physical models. This manipulation shows that laminar flow is reduced and drag increased at high speeds. At the speeds at which swifts cruise, however, swift-like roughness prolongs laminar flow and reduces drag. This feature gives small birds with rudimentary wings an edge during the evolution of glide performance.

Instantaneous Pressure Field Determination around a Square-Section Cylinder using Time-Resolved Stereo-PIV

This paper describes the determination of instantaneous planar pressure fields from time-resolved particle image velocimetry around a stationary square-section cylinder, with the face normal to the flow, for ReD = 9,500, where D is the section dimension. The mean flow and instantaneous flow around the cylinder are described. To validate the pressure evaluation approach two signals are extracted from the series of instantaneous planar pressure fields and compared to microphone and orifice measurements at the side surface and at the base. The results for the side surface of the model show good agreement in time. Mean and fluctuating pressure values are consistent with previous studies. The results for the base show agreement for the mean pressure and a fair agreement in time, which give a good prospect for instantaneous planar pressure determination in three-dimensional flows.

Aero-electro-mechanical Coupling of Electro-Active Membrane Wings

This study focuses on providing and validating an analytical model of the aero-electromechanical coupling for an acrylic electro-active membrane wing manufactured using 3M V HB 4905 and tested at 0% pre-strain. Load measurements and photogrammetry measurements of a perimeter reinforced membrane of AR=1 were conducted in a low speed wind tunnel at 10 m/s and a Reynolds number of 66,000. By formulating an analytical solution using the Weber number, a whole range of stiffness values and aerodynamic loading conditions are determined. The analytical solution shows good agreement with the photogrammetry data in the pre-stall region. Furthermore, from an aerodynamic perspective, voltage activation of the membrane is shown to behave as an excess length problem (at least in the pre-stall region).