The flight of Alsomitra macrocarpa

Alsomitra macrocarpa is a gliding diaspore which grows on tall trees in tropical Asian forests. It does not rely on gusts nor even on a slight breeze to fly up to hundreds of meters. It is a low aspect ratio wing, which defies the conventional aerodynamic argument that efficient gliders need to be slender. Compared to pappose seeds like the dandelion, Alsomitra macrocarpa has a higher wing loading, yet it reaches a comparable terminal velocity. It also achieves a stable flight in the absence of both a vertical stabiliser and active control. What enables these remarkable flight abilities, which inspired the design of the first manned gliders, are yet to be understood. The investigation herein utilised image analysis, three-dimensional scans and wind tunnel tests. The morphological study highlighted Alsomitra macrocarpa’s uniqueness in terms of dimensions and shape. Through the use of depth cameras, the gliding path of 15 seeds was recorded. While existing literature treats Alsomitra macrocarpa as a flat, two-dimensional shape, drop tests evidenced a preferential flight orientation for every seed sample. Hence, and contrary to previous belief, the membrane wing has an intrados and an extrados. The majority of the seeds presented a helical path, while some moved in a straight oscillatory path that has not been previously reported, with oscillations on the vertical and horizontal plane. This gliding trajectory assumed to be two-dimensional, could be described by a simplified dynamics model. The phugoid style flight coupled the horizontal motion of a tumbling wing with the oscillation of a fluttering wing. Wind tunnel tests revealed how the membrane wing undergoes spanwise deformation under the loads experienced during a glide. This deformation displaces the aerodynamic centre from the plane of the membrane wing. Low-order dynamical models, which included a non-uniform mass distribution, were employed to recreate these oscillations in the vertical plane and showed good qualitative agreement with the experiments. Overall, this work provides new insights into the remarkably stable and efficient flight of Alsomitra macrocarpa. The aerodynamic conditions under which these seeds fly are in the range experienced by Micro Aerial Vehicles (MAVs). Thus, the outcomes of this thesis could aid the design of more efficient MAVs, just as early aviation pioneers were inspired when they saw Alsomitra macrocarpa glide down through the forest canopy

A separated vortex ring underlies the flight of the dandelion

Wind-dispersed plants have evolved ingenious ways to lift their seeds1,2. The common dandelion uses a bundle of drag-enhancing bristles (the pappus) that helps to keep their seeds aloft. This passive flight mechanism is highly effective, enabling seed dispersal over formidable distances3,4; however, the physics underpinning pappus-mediated flight remains unresolved. Here we visualized the flow around dandelion seeds, uncovering an extraordinary type of vortex. This vortex is a ring of recirculating fluid, which is detached owing to the flow passing through the pappus. We hypothesized that the circular disk-like geometry and the porosity of the pappus are the key design features that enable the formation of the separated vortex ring. The porosity gradient was surveyed using microfabricated disks, and a disk with a similar porosity was found to be able to recapitulate the flow behaviour of the pappus. The porosity of the dandelion pappus appears to be tuned precisely to stabilize the vortex, while maximizing aerodynamic loading and minimizing material requirements. The discovery of the separated vortex ring provides evidence of the existence of a new class of fluid behaviour around fluid-immersed bodies that may underlie locomotion, weight reduction and particle retention in biological and manmade structures

The aerodynamics of the plumed seeds of Taraxacum officinale

Anyone who has ever blown a dandelion clock, will know that its fruit (the feathery, seed-bearing structure) fly off with the slightest breeze. Each fruit is made of a group of stringy hollow tubes called lamellae, which together form the parachute, the pappus. Contrary to a popular belief, the physics of a conventional parachute cannot be applied, because of the parachute's high porosity. This structure gives outstanding flight capability to the fruit while, at the same time, saving weight. In steady descent, the flight of the fruit is characterized by two dimensionless parameters: the Reynolds number and the Darcy number. We measured the Reynolds number for the dandelion fruit by performing terminal velocity measurements for a variety of fruit weights. We tested previous theories for the flight of the dandelion by artificially weighting the dandelion, and determining the variation in the terminal velocity as a function of the parachute loading. Our experiments reveal that the parachute behaves more like a porous disk than individual independent lamellae. We performed direct numerical simulations of a porous disk of a certain Darcy number (as measured by us), and compared our results to flow visualizations realized in a specially designed vertical wind tunnel

The effects of exposure and climate on the weathering of late Pleistocene and Holocene Alpine soils

The main aim of this study was to examine the influence of exposure and consequently climate, on the chemical weathering of soils which had developed after the ice retreat of the last glaciation in Northern Italy. This was done by comparing soils developing at north- and south-facing sites on siliceous parent material. There is very little data available on weathering rates and organic matter (OM) as a function of climate and exposure in such environments. Weathering rates (elemental leaching) over the whole lifetime of the soils are higher on north-facing sites. Total organic C and N contents, organic matter stocks and organic matter fractions were analysed to decipher the causes of this difference in weathering behaviour. For the organic matter fractions, we compared the easily oxidisable and stable (resistant to H2O2 treatment) organic matter fractions, water-soluble phenolic materials and alkaline-extractable fractions of the various sites. The abundance of soil organic carbon (SOC) tends to have a non-linear climate dependency. The highest amounts of SOC were measured near the timberline. In addition, compared to south-facing sites, soils on north-facing slopes have a higher organic matter content and a significantly lower degree of humification. Undecomposed or weakly degraded organic matter accumulated on north-facing sites due to less favourable thermal conditions and a higher acidity. With northern exposure, fulvic acids were more easily transported within the soil profile than humic acids and predominately gave rise to the migration (eluviation) of Fe and Al compounds due to their –COOH and –OH functional groups. Furthermore, water-soluble phenolic materials, which are more abundant on north-facing sites, have accelerated the leaching of Al. Accumulation of weakly degraded OM and the subsequently higher production of organic ligands have enhanced the eluviation of Fe and Al. Patterns of weathering processes in Alpine environments are strongly linked to biological and (micro)climatic factors which give rise to distinct differences between north- and south-facing sites