Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session E25: Focus Session: Flow Past Passive Hairy Surfaces |
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Chair: Kaitlyn Hood, Massachusetts Institute of Technology Room: Georgia World Congress Center B313 |
Sunday, November 18, 2018 5:10PM - 5:23PM |
E25.00001: Abstract Withdrawn
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Sunday, November 18, 2018 5:23PM - 5:36PM |
E25.00002: Dynamic Friction and Lubrication in Soft Hydrogel Interfaces Nicholas Cuccia, Suraj Pothineni, Justin C. Burton The increasing use of biologically-inspired hydrogel material and the unusual mechanical qualities that these gels offer drive the need for a better, more intuitive understanding of the mechanical response of such continua. Using a custom low-force pin-on-disc tribometer, we have probed the single-contact frictional properties of both polyacrylamide and agarose hydrogels under a variety of environmental conditions. Under a constant, physical contact, these hydrogels exhibit a dynamic frictional transition characterized by a precipitous drop in the friction coefficient at a critical velocity. Within a range of speeds near this dynamic frictional transition, transient behavior can be observed. Upon increasing the speed, the coefficient of friction decreases exponentially, with a characteristic decay time of order 10 minutes. We show how our results can be interpreted as a competition between the relaxation time of the polymers and shear rate in the fluid (i.e. Wiessenberg number), and that this transition can be tuned by varying the liquid salt concentration, liquid viscosity, and sliding geometry. |
Sunday, November 18, 2018 5:36PM - 5:49PM |
E25.00003: A Brinkman model for elastic fibers David Stein, Michael John Shelley An important class of fluid-structure problems involve the dynamics of ordered arrays of immersed, flexible fibers. While specialized numerical methods have been developed to study fiber-fluid systems, they become infeasible when there are many, rather than a few, fibers present, and do not lend themselves to analytical calculation. In this talk I will introduce a coarse-grained continuum model, based on local-slender body theory for elastic fibers immersed in a viscous Newtonian fluid. We will explore some of the basic properties of these systems subjected to steady and oscillatory shear flows; and then show how qualitatively different phenomenon can emerge in some systems as the fiber density is varied. Finally, we will show how the model can be used to study pumping in beds of actuated cilia. |
Sunday, November 18, 2018 5:49PM - 6:02PM |
E25.00004: Elastic feathers spreading fluid-impact force Kinjal Bhar, Brian Chang, Lorian Straker, Emmanuel Virot, Romain Paris, Christophe Clanet, Sunghwan Jung Northern Gannets are seabirds that frequently employ a hunting tactic called ‘plunge-diving’ in which they dive through the water surface at high-speeds to catch underwater prey; often reaching speeds of up to 24 m/s at the moment of impact. It can result in forces as high as 500 N acting on the bird’s neck during impact, yet the bird escapes uninjured, despite making 20-100 dives per foraging trips. However, little is known about how such high impact forces affect the rest of the body. The goal of this study is to investigate the role played by the feathers in spreading the hydrodynamic forces on the skin around the impact zone. We model the feathers as elastic beams taking into account their pre-curvature and non-uniform cross-section. Results from our experiments with polycarbonate beams suggest that the interaction of feathers on the skin patch redistributes the force, thereby reducing the impact on any particular area of the skin. |
Sunday, November 18, 2018 6:02PM - 6:15PM |
E25.00005: Honey bee leg hairs grip onto pollen Marguerite E Matherne, Suraj Puvvada, Oliver Howington, Ben Guy, Wilson Poon, David L Hu Honey bees are masters at collecting pollen that varies vastly in size and shape. Its strategy is to gradually form a pollen basket, a 2-mm pellet of pollen mixed with nectar that is skewered onto its leg hairs. In this experimental study, we collect and examine pollen baskets and film bees removing the baskets from their legs. Because of the high volume fraction of the basket, the pollen is in a shear-thickened state. Based on the rheological properties of the pollen basket, we rationalize the shear rates the honey bees must apply to the basket to remove it. |
Sunday, November 18, 2018 6:15PM - 6:28PM |
E25.00006: Rake or Sieve: design principles for flow past an array of rigid hairs Kaitlyn Hood, Mani Sai Suryateja Jammalamadaka, Anette E. Hosoi Crustaceans have appendages with an array of rigid hairs covered in chemoreceptors, used to sense and track food. By changing the speed of flow past the hairy surface, and thereby manipulating the Reynolds number (Re) of the flow, crustaceans directly influence the flow behavior. Flow acts either as a rake – diverting flow around the hair array, or as a sieve – penetrating into the hair array. In our experiments, we uncover a third transitional phase: deflection – where the flow partially penetrates the hair array and is deflected laterally. We develop a design principle for constructing hair arrays that exhibit each flow phase. We find that the flow is characterized by the depth of the boundary layer on a single hair. As Re increases, the depth of the boundary layer decreases non-linearly. If the distance δ between hairs is less than the depth of the boundary layer, then we observe rake flow. Conversely, if δ is greater than twice the boundary layer depth, then we observe sieve flow. And we observe deflection flow for intermediate values of δ. This principle can be used to design steady state devices that use beds of rigid hairs to direct flow. Potential applications include chemo-sensing and filtering particles. |
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