Keith Mertens

Reconfiguration and the reduction of vortex-induced vibrations in broad leaves.

SUMMARY Flexible plants, fungi and sessile animals reconfigure in wind and water to reduce the drag acting upon them. In strong winds and flood waters, for example, leaves roll up into cone shapes that reduce drag compared with rigid objects of similar surface area. Less understood is how a leaf attached to a flexible leaf stalk will roll up stably in an unsteady flow. Previous mathematical and physical models have only considered the case of a flexible sheet attached to a rigid tether in steady flow. In this paper, the dynamics of the flow around the leaf of the wild ginger Hexastylis arifolia and the wild violet Viola papilionacea are described using particle image velocimetry. The flows around the leaves are compared with those of simplified physical and numerical models of flexible sheets attached to both rigid and flexible beams. In the actual leaf, a stable recirculation zone is formed within the wake of the reconfigured cone. In the physical model, a similar recirculation zone is observed within sheets constructed to roll up into cones with both rigid and flexible tethers. Numerical simulations and experiments show that flexible rectangular sheets that reconfigure into U-shapes, however, are less stable when attached to flexible tethers. In these cases, larger forces and oscillations due to strong vortex shedding are measured. These results suggest that the three-dimensional cone structure in addition to flexibility is significant to both the reduction of vortex-induced vibrations and the forces experienced by the leaf.

Subsurface Trapping of Oil Plumes in Stratification: Laboratory Investigations

Laboratory experiments demonstrating how the addition of surfactants creates the possibility of trapping buoyant immiscible fluids are presented. In particular, these experiments demonstrate that buoyant immiscible plumes like those which occurred during the Deepwater Horizon Gulf oil spill can be trapped as they rise through an ambient, stratified fluid. The addition of surfactants is an important mechanism by which trapping can occur. In this paper, we describe experiments and theory on trapping/escape of plumes containing an oil/water/surfactant mixture released into nonlinear stratification. We also present results on the timescale for trapping and for destabilization and release of trapped subsurface plumes. This timescale is shown to be a function of the oil to surfactant ratio.

Numerical simulations and experimental measurements of dense-core vortex rings in a sharply stratified environment

We present three-dimensional direct numerical simulations of a vortex ring settling in sharply stratified miscible ambient fluids for near two-layer configurations, and comparisons of these simulations with the results from laboratory experiments. The core fluid of the vortex rings has density higher than both the top and the bottom layers of the ambient fluid, and is fully miscible in both layers. This setup ensures a rich parameter space that we partially explore in this study. In particular, a critical (bifurcation) phenomenon is identified that distinguishes the long-time behavior of the settling vortex ring as either being fully trapped at the ambient density layer or continuing through the layer in its downward motion. This critical behavior is determined by the initial conditions (e.g. the size and speed of the vortex ring, the initial distance to the layer, etc). The numerical simulations are able to provide evidence for this in qualitative agreement with an experimental phase diagram. Our setup isolates essential elements of mixing, trapping and escape through stratified fluids in a variety of situations, such as the mixing and dispersion of pollutants and plankton in the ocean.

Comment on “The role of wetting heterogeneities in the meandering instability of a partial wetting rivulet” by Couvreur S. and Daerr A.

Rivulets and their meandering on a partially wetting surface present an interesting problem, as complex behavior arises from a deceptively simple setup. Recently Couvreur and Daerr suggested that meandering is caused by an instability developing as the flow rate

DYNAMIC, FREE-SPACE USER INTERACTIONS FOR MACHINE CONTROL

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Free-space user interface and control using virtual constructs

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NON-LINEAR MOTION CAPTURE USING FRENET-SERRET FRAMES

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Optimal mixing of buoyant jets and plumes in stratified fluids: theory and experiments

, with stationary (pinned) meandering being the final state of the flow. We tried to verify this assertion experimentally, but instead produced results contradicting the claim of Couvreur and Daerr. The likely reason behind the discrepancy is the persistence of flow-rate perturbations. Moreover, the theory presented in this paper cannot reproduce the states as considered and disagrees with other theories.