3,738 research outputs found

    Impact and flexural properties of hybrid jute/HTPET fibre reinforced epoxy composites

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    In this study, hybridizing jute with high tenacity polyethylene terephthalate (HTPET) fibres has been proposed. Different hybrid jute/HTPET fibre reinforced epoxy composite samples have been fabricated with different weight ratios of HTPET, and subjected to charpy impact and three-point bending test. The results show that increasing the content of HTPET fibres in the composites will considerably improve their energy absorption capabilities and structural integrity after impact and bending test. However, hybrid composites with HTPET show lower flexural stress and bending modulus than pure jute composites

    Shear flow induced isotropic to nematic transition in a suspension of active filaments

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    We study the effects of externally applied shear flow on a model of suspensions of motors and filaments, via the equations of active hydrodynamics [PRL {\bf 89} (2002) 058101; {\bf 92} (2004) 118101]. In the absence of shear, the orientationally ordered phase of {\it both} polar and apolar active particles is always unstable at zero-wavenumber. An imposed steady shear large enough to overcome the active stresses stabilises both apolar and moving polar phases. Our work is relevant to {\it in vitro} studies of active filaments, the reorientation of endothelial cells subject to shear flow and shear-induced motility of attached cells.Comment: 8 pages, 4 figures submitted to Europhysics Letter

    Introducing a new formulation for the warehouse inventory management systems : with two stochastic demand patterns

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    This paper presents a new formulation for warehouse inventory management in a stochastic situation. The primary source of this formulation is derived from FP model, which has been proposed by Fletcher and Ponnambalam for reservoir management. The new proposed mathematical model is based on the first and the second moments of storage as a stochastic variable. Using this model, the expected value of storage, the variance of storage, and the optimal ordering policies are determined. Moreover, the probability of within containment, surplus, and shortage are computable without adding any new variables. To validate the optimization model, a Monte Carlo simulation is used. Furthermore, to evaluate the performance of the optimal FP policy, It is compared to (s*,S*) policy, as a very popular policy used in the literature, in terms of the expected total annual cost and the service level. It is also demonstrated that the FP policy has a superior performances than (s*,S*) policy

    Nematic and Polar order in Active Filament Solutions

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    Using a microscopic model of interacting polar biofilaments and motor proteins, we characterize the phase diagram of both homogeneous and inhomogeneous states in terms of experimental parameters. The polarity of motor clusters is key in determining the organization of the filaments in homogeneous isotropic, polarized and nematic states, while motor-induced bundling yields spatially inhomogeneous structures.Comment: 4 pages. 3 figure

    A Complete Characterization of the Gap between Convexity and SOS-Convexity

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    Our first contribution in this paper is to prove that three natural sum of squares (sos) based sufficient conditions for convexity of polynomials, via the definition of convexity, its first order characterization, and its second order characterization, are equivalent. These three equivalent algebraic conditions, henceforth referred to as sos-convexity, can be checked by semidefinite programming whereas deciding convexity is NP-hard. If we denote the set of convex and sos-convex polynomials in nn variables of degree dd with C~n,d\tilde{C}_{n,d} and ΣC~n,d\tilde{\Sigma C}_{n,d} respectively, then our main contribution is to prove that C~n,d=ΣC~n,d\tilde{C}_{n,d}=\tilde{\Sigma C}_{n,d} if and only if n=1n=1 or d=2d=2 or (n,d)=(2,4)(n,d)=(2,4). We also present a complete characterization for forms (homogeneous polynomials) except for the case (n,d)=(3,4)(n,d)=(3,4) which is joint work with G. Blekherman and is to be published elsewhere. Our result states that the set Cn,dC_{n,d} of convex forms in nn variables of degree dd equals the set ΣCn,d\Sigma C_{n,d} of sos-convex forms if and only if n=2n=2 or d=2d=2 or (n,d)=(3,4)(n,d)=(3,4). To prove these results, we present in particular explicit examples of polynomials in C~2,6ΣC~2,6\tilde{C}_{2,6}\setminus\tilde{\Sigma C}_{2,6} and C~3,4ΣC~3,4\tilde{C}_{3,4}\setminus\tilde{\Sigma C}_{3,4} and forms in C3,6ΣC3,6C_{3,6}\setminus\Sigma C_{3,6} and C4,4ΣC4,4C_{4,4}\setminus\Sigma C_{4,4}, and a general procedure for constructing forms in Cn,d+2ΣCn,d+2C_{n,d+2}\setminus\Sigma C_{n,d+2} from nonnegative but not sos forms in nn variables and degree dd. Although for disparate reasons, the remarkable outcome is that convex polynomials (resp. forms) are sos-convex exactly in cases where nonnegative polynomials (resp. forms) are sums of squares, as characterized by Hilbert.Comment: 25 pages; minor editorial revisions made; formal certificates for computer assisted proofs of the paper added to arXi

    p-species integrable reaction-diffusion processes

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    We consider a process in which there are p-species of particles, i.e. A_1,A_2,...,A_p, on an infinite one-dimensional lattice. Each particle AiA_i can diffuse to its right neighboring site with rate DiD_i, if this site is not already occupied. Also they have the exchange interaction A_j+A_i --> A_i+A_j with rate rij.r_{ij}. We study the range of parameters (interactions) for which the model is integrable. The wavefunctions of this multi--parameter family of integrable models are found. We also extend the 2--species model to the case in which the particles are able to diffuse to their right or left neighboring sites.Comment: 16 pages, LaTe

    Massive spinor fields in flat spacetimes with non-trivial topology

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    The vacuum expectation value of the stress-energy tensor is calculated for spin 121\over 2 massive fields in several multiply connected flat spacetimes. We examine the physical effects of topology on manifolds such as R3×S1R^3 \times S^1, R2×T2R^2\times T^2, R1×T3R^1 \times T^3, the Mobius strip and the Klein bottle. We find that the spinor vacuum stress tensor has the opposite sign to, and twice the magnitude of, the scalar tensor in orientable manifolds. Extending the above considerations to the case of Misner spacetime, we calculate the vacuum expectation value of spinor stress-energy tensor in this space and discuss its implications for the chronology protection conjecture.Comment: 18 pages, Some of the equations in section VI as well as typographical errors corrected, 5 figures, Revtex

    New Dependencies of Hierarchies in Polynomial Optimization

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    We compare four key hierarchies for solving Constrained Polynomial Optimization Problems (CPOP): Sum of Squares (SOS), Sum of Diagonally Dominant Polynomials (SDSOS), Sum of Nonnegative Circuits (SONC), and the Sherali Adams (SA) hierarchies. We prove a collection of dependencies among these hierarchies both for general CPOPs and for optimization problems on the Boolean hypercube. Key results include for the general case that the SONC and SOS hierarchy are polynomially incomparable, while SDSOS is contained in SONC. A direct consequence is the non-existence of a Putinar-like Positivstellensatz for SDSOS. On the Boolean hypercube, we show as a main result that Schm\"udgen-like versions of the hierarchies SDSOS*, SONC*, and SA* are polynomially equivalent. Moreover, we show that SA* is contained in any Schm\"udgen-like hierarchy that provides a O(n) degree bound.Comment: 26 pages, 4 figure
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