Session T06: Aerodynamics: Folding, Flapping, Twisting and Rotating Wings

Wing folding and twisting synergistically boost lift generation for flapping wing flight

We designed and built a three degree-of-freedom flapping wing platform to study the aerodynamic benefits of wing folding and twisting during flapping flight. We tested this physical model in a wind tunnel with a Strouhal number range of 0.2 – 0.4, typical for animal flight. Motion tracking was employed to ensure that the wing motion closely followed the prescribed motion, while aerodynamic forces and moments were measured using a six-axis load cell. The results show that the air jet produced by wing clapping may be directed by means of controlling the wing twisting, and that the rotational lift generated by rapid wing twisting can be vectored by varying wing folding angles. Finally, the negative lift during upstroke is alleviated by wing twisting. PIV measurements to visualize the directed jet, are planned and will be presented if time permits.   

Static Longitudinal Stability in a Flapping Robot

While static longitudinal stability is well understood for fixed wing aircraft, it has yet to be fully explored for flapping flight. Numerical and analytical works have suggested flapping is either inherently unstable or merely amplifies existing instabilities already present in gliding flight. Experimental work is lacking to support or refute these hypotheses. To address this, we recorded triaxial forces and moments on a 1 degree-of-freedom flapping robot mounted in the wind tunnel test section. The inertial and aerodynamic forces associated with flapping are decomposed from the complete force data to identify how each contributes to stability. Forces and moments are recorded at several different wind speeds, flapping frequencies, and with wings of varying elasticity. Static longitudinal stability is determined by evaluating the relationship between the pitching moment and angle of attack.   

Direct numerical simulation of a flapping wing in turbulent environments

Micro-aerial vehicles (MAVs) are often designed to fly at low speeds and low altitudes, potentially experiencing strong turbulence conditions where the characteristic length-scale of the flow perturbations can be comparable to their size. Being such conditions remarkably different from those of more conventional aircraft, we still lack a thorough understanding on how atmospheric turbulence affects the aerodynamic performance of MAVs and similar devices. Here, we present a high-fidelity computational methodology to investigate low-Reynolds aerodynamic problems in the presence of a turbulent free stream with well-defined and controllable properties (e.g., homogeneity, isotropy, spectral energy distribution). First, useful indications are provided by a benchmark between the flow obtained using a synthetic turbulence generator and the fully-resolved simulation of grid-induced turbulence. Hence, focusing on bio-inspired MAVs, we characterize the case of a flapping wing impinged by fluctuations of high turbulence intensity and integral length-scale comparable to the chord, highlighting the most relevant variations in the aerodynamic response with respect to the unperturbed free-stream case.   

Experimental force measurement study of Pantala flavescens in tailwind flight

Wind compensation is vital in insects migrating under the influence of tailwinds and crosswinds. An insect requires tailwind compensation to cover large distances and crosswind drift compensation to stay on course. The aerodynamic force measurements can provide insight into the mechanism involved in the migration in tailwind and crosswind configurations. We conducted a tethered flight experiment in a wind tunnel at a wind speed of 2.7 m/s on the dragonfly species Pantala flavescens, a long-distance migrant migrating between India and Africa. We positioned a dragonfly in a tailwind and three crosswind-tailwind configurations at 5°, 10°, and 15°. On average, the dragonfly generates thrust three times its weight at all orientations. Our results show that the dragonfly employs a new flapping mode for high lift generation and generates a mean lift equal to its weight. It produces side force approximately 0.5 times its weight, opposing crosswind to compensate for wind drift. Wind drift compensation becomes increasingly difficult with an increase in crosswind angles. In tailwind and crosswind flights, the strategy is to generate large amounts of thrust to propel itself and lift to sustain a level flight while compensating for wind drift by generating an opposing side force.    

Wind compensation and flight in tailwind by dragonfly species Pantala flavescens, during the transoceanic migration

The dragonfly species Pantala flavescens (PF), perform the longest known insect migration (14-16000 kms) annually between India and Africa bridging two distant ecologies and, is consequently of tremendous importance amidst rising climate change concerns. Field observations of transoceanic insect migration are unavailable due to the lack of lightweight trackers for dragonflies; therefore, we developed a blend of wind tunnel experiments and theory to study the migration route, timing, and the role of wind. We propose a novel energetics model and modified Dijkstra's path planning algorithm to identify the migration network, timing, and favorable winds. The migration from Africa to India (~2800kms) is completed in merely 45 hours non-stop with assistance from the strong Somali jet; the return journey originates in north-east India involving a complex network of stopovers in southern India, Sri Lanka, Maldives, Seychelles stretching over months; thus, highlighting the critical role of winds. Our wind tunnel experiments reveal that PF actively compensates for crosswind to remain on course. Also, thrust production dominates tailwind flight to enhance the distance covered in contrast to lift generation in headwind flight. Finally, we highlight how climate change endangers this migration.   

Characterization of vorticity transport mechanisms during the transient phase of a rotating wing

Studies have demonstrated that in the quasi-steady phase, spanwise convection of vorticity and the Coriolis tilting, among others, lead to the transport of vorticity from the leading-edge vortex (LEV), which mediates the LEV. However, understanding the vorticity transport mechanisms in the transient phase is equally important and has not yet received much attention. This study aims to explore the effect of wing acceleration on the mechanisms that regulate the development of LEV on a rotating wing. As such, the flow field over the wing was experimentally quantified using Rotating Three-Dimensional Velocimetry (R3DV) technique under varying wing acceleration and Reynolds numbers. Vorticity transport analysis in the rotating frame will be used to quantify the in-plane and out-of-plane fluxes of vorticity. Initial findings suggest that when the acceleration is low, the surface diffusive flux (due to secondary vorticity) annihilates the excess vorticity transported by the shear layer, thus mediating the growth of the LEV. At higher acceleration rates, a lag in the production of secondary vorticity is observed, leading to a higher amount of vorticity transfer from the shear layer to the LEV. In this study, along with these in-plane fluxes, the contribution of out-of-plane fluxes to the vorticity transport from the LEV will also be investigated.   

Active flow control of a transitional-open, three-dimensional cavity with lateral apertures in a channel flow configuration: Mitigation of the pressure footprint and fluctuations.

Flows over transitional-open cavities, with a shear-layer that partly enters the cavity are relevant to many branches of engineering. These flows are known to be a source of instabilities, high pressure disturbances and large recirculation regions, leading to excessive wall pressure footprint. The subject of this study is a 6.44 length-to-depth ratio and 3.29 width-to-depth ratio cavity, considering wall proximity effects. The objective is to investigate the mitigation of the wall pressure footprint and fluctuations through steady blowing upstream of the cavity leading edge.

Unsteady numerical CFD simulations and experiments have been conducted to identify the mechanisms underlying the flow dynamics of both baseline and controlled cases. Concomitant pressure, and Particle Image Velocimetry (PIV) measurements have been performed at a Reynolds number of 2.8*10⁵. Centerline wall pressure coefficient distribution Cp and cavity inner flow fields are compared between experimental and numerical data. They show good agreement.

The control strategy changes the flow drastically upstream of the cavity. The boundary layer is thickened, and the ground clearance flow rate is reduced. It transforms the dynamics of the shear layer impinging onto the cavity and impacts the inner recirculation bubble. The steady blowing is successful to reduce the pressure gradient between upstream and downstream cavity walls, with almost 20%. It has also reduced the fluctuations all along the cavity walls with an amount of 20%.