3,814 research outputs found
Dynamics of poroelastic filaments
We investigate the stability and geometrically non-linear dynamics of slender
rods made of a linear isotropic poroelastic material. Dimensional reduction
leads to the evolution equation for the shape of the poroelastica where, in
addition to the usual terms for the bending of an elastic rod, we find a term
that arises from fluid-solid interaction. Using the poroelastica equation as a
starting point, we consider the load controlled and displacement controlled
planar buckling of a slender rod, as well as the closely related instabilities
of a rod subject to twisting moments and compression when embedded in an
elastic medium. This work has applications to the active and passive mechanics
of thin filaments and sheets made from gels, plant organs such as stems, roots
and leaves, sponges, cartilage layers and bones.Comment: 34 pages, 13 figures, to appear in the Proceeding of the Royal
Societ
Monami as an oscillatory hydrodynamic instability in a submerged sea grass bed
The onset of monami ~-- the synchronous waving of sea grass beds driven by a
steady flow -- is modeled as a linear instability of the flow. Unlike previous
works, our model considers the drag exerted by the grass in establishing the
steady flow profile, and in damping out perturbations to it. We find two
distinct modes of instability, which we label Mode 1 and Mode 2. Mode 1 is
closely related to Kelvin-Helmholtz instability modified by vegetation drag,
whereas Mode 2 is unrelated to Kelvin-Helmholtz and arises from an interaction
between the flow in the vegetated and unvegetated layers. The vegetation
damping, according to our model, leads to a finite threshold flow for both
these modes. Experimental observations for the onset and frequency of waving
compare well with model predictions for the instability onset criteria and the
imaginary part of the complex growth rate respectively, but experiments lie in
a parameter regime where the two modes can not be distinguished. % The
inclusion of vegetation drag differentiates our mechanism from the previous
linear stability analyses of monami.Comment: 4 figures, 13 page
Origin of transition metal clustering tendencies in GaAs based dilute magnetic semiconductors
While isovalent doping of GaAs (e.g. by In) leads to a repulsion between the
solute atoms, two Cr, Mn, or Fe atoms in GaAs are found to have lower energy
than the well-separated pair, and hence attract each other. The strong bonding
interaction between levels with t2 symmetry on the transition metal (TM) atoms
results in these atoms exhibiting a strong tendency to cluster. Using
first-principles calculations, we show that this attraction is maximal for Cr,
Mn and Fe while it is minimal for V. The difference is attributed to the
symmetry of the highest occupied levels. While the intention is to find
possible choices of spintronic materials that show a reduced tendency to
cluster, one finds that the conditions that minimize clustering tendencies also
minimize the stabilization of the magnetic state.Comment: To appear in Appl. Phys. Let
Periodic Review, Push Inventory Policies for Remanufacturing
Sustainability has become a major issue in most economies, causing many leading companies to focus on product recovery and reverse logistics. This research is focused on product recovery, and in particular on production control and inventory management in the remanufacturing context. We study a remanufacturing facility that receives a stream of returned products according to a Poisson process. Demand is uncertain and also follows a Poisson process. The decision problems for the remanufacturing facility are when to release returned products to the remanufacturing line and how many new products to manufacture. We assume that remanufactured products are as good as new. In this paper, we employ a "push" policy that combines these two decisions. It is well known that the optimal policy parameters are difficult to find analytically; therefore, we develop several heuristics based on traditional inventory models. We also investigate the performance of the system as a function of return rates, backorder costs and manufacturing and remanufacturing lead times; and we develop approximate lower and upper bounds on the optimal solution. We illustrate and explain some counter-intuitive results and we test the performance of the heuristics on a set of sample problems. We find that the average error of the heuristics is quite low.inventory;reverse logistics;remanufacturing;environment;heuristics
Sinking flux of particulate organic matter in the oceans: Sensitivity to particle characteristics
© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Omand, M. M., Govindarajan, R., He, J., & Mahadevan, A. Sinking flux of particulate organic matter in the oceans: Sensitivity to particle characteristics. Scientific Reports, 10(1), (2020): 5582, doi:10.1038/s41598-020-60424-5.The sinking of organic particles produced in the upper sunlit layers of the ocean forms an important limb of the oceanic biological pump, which impacts the sequestration of carbon and resupply of nutrients in the mesopelagic ocean. Particles raining out from the upper ocean undergo remineralization by bacteria colonized on their surface and interior, leading to an attenuation in the sinking flux of organic matter with depth. Here, we formulate a mechanistic model for the depth-dependent, sinking, particulate mass flux constituted by a range of sinking, remineralizing particles. Like previous studies, we find that the model does not achieve the characteristic ‘Martin curve’ flux profile with a single type of particle, but instead requires a distribution of particle sizes and/or properties. We consider various functional forms of remineralization appropriate for solid/compact particles, and aggregates with an anoxic or oxic interior. We explore the sensitivity of the shape of the flux vs. depth profile to the choice of remineralization function, relative particle density, particle size distribution, and water column density stratification, and find that neither a power-law nor exponential function provides a definitively superior fit to the modeled profiles. The profiles are also sensitive to the time history of the particle source. Varying surface particle size distribution (via the slope of the particle number spectrum) over 3 days to represent a transient phytoplankton bloom results in transient subsurface maxima or pulses in the sinking mass flux. This work contributes to a growing body of mechanistic export flux models that offer scope to incorporate underlying dynamical and biological processes into global carbon cycle models.We thank NSF (OCE 1260080), NASA (NNX16AR48G), and the Ministry of Earth Sciences, Government of India (Monsoon Mission Project on the Bay of Bengal) for support. This work was largely done in 2012 while MMO was a postdoctoral associate at WHOI, during a visit by RG supported by The Mary Sears visiting scholar program to the Woods Hole Oceanographic Institution. Thanks also to Benjamin Hodges for many thoughtful contributions
Transitions to Nematic states in homogeneous suspensions of high aspect ratio magnetic rods
Isotropic-Nematic and Nematic-Nematic transitions from a homogeneous base
state of a suspension of high aspect ratio, rod-like magnetic particles are
studied for both Maier-Saupe and the Onsager excluded volume potentials. A
combination of classical linear stability and asymptotic analyses provides
insight into possible nematic states emanating from both the isotropic and
nematic non-polarized equilibrium states. Local analytical results close to
critical points in conjunction with global numerical results (Bhandar, 2002)
yields a unified picture of the bifurcation diagram and provides a convenient
base state to study effects of external orienting fields.Comment: 3 Figure
Peeling from a patterned thin elastic film
Inspired by the observation that many naturally occurring adhesives arise as
textured thin films, we consider the displacement controlled peeling of a
flexible plate from an incision-patterned thin adhesive elastic layer. We find
that crack initiation from an incision on the film occurs at a load much higher
than that required to propagate it on a smooth adhesive surface; multiple
incisions thus cause the crack to propagate intermittently. Microscopically,
this mode of crack initiation and propagation in geometrically confined thin
adhesive films is related to the nucleation of cavitation bubbles behind the
incision which must grow and coalesce before a viable crack propagates. Our
theoretical analysis allows us to rationalize these experimental observations
qualitatively and quantitatively and suggests a simple design criterion for
increasing the interfacial fracture toughness of adhesive films.Comment: 8 pages, To appear in Proceedings of Royal Society London, Ser.
- …