2,919 research outputs found
Modeling and analysis of water-hammer in coaxial pipes
The fluid-structure interaction is studied for a system composed of two
coaxial pipes in an annular geometry, for both homogeneous isotropic metal
pipes and fiber-reinforced (anisotropic) pipes. Multiple waves, traveling at
different speeds and amplitudes, result when a projectile impacts on the water
filling the annular space between the pipes. In the case of carbon
fiber-reinforced plastic thin pipes we compute the wavespeeds, the fluid
pressure and mechanical strains as functions of the fiber winding angle. This
generalizes the single-pipe analysis of J. H. You, and K. Inaba,
Fluid-structure interaction in water-filled pipes of anisotropic composite
materials, J. Fl. Str. 36 (2013). Comparison with a set of experimental
measurements seems to validate our models and predictions
Effective behavior of nematic elastomer membranes
We derive the effective energy density of thin membranes of liquid crystal
elastomers as the Gamma-limit of a widely used bulk model. These membranes can
display fine-scale features both due to wrinkling that one expects in thin
elastic membranes and due to oscillations in the nematic director that one
expects in liquid crystal elastomers. We provide an explicit characterization
of the effective energy density of membranes and the effective state of stress
as a function of the planar deformation gradient. We also provide a
characterization of the fine-scale features. We show the existence of four
regimes: one where wrinkling and microstructure reduces the effective membrane
energy and stress to zero, a second where wrinkling leads to uniaxial tension,
a third where nematic oscillations lead to equi-biaxial tension and a fourth
with no fine scale features and biaxial tension. Importantly, we find a region
where one has shear strain but no shear stress and all the fine-scale features
are in-plane with no wrinkling
A formal proof of the optimal frame setting for Dynamic-Frame Aloha with known population size
In Dynamic-Frame Aloha subsequent frame lengths must be optimally chosen to
maximize throughput. When the initial population size is known,
numerical evaluations show that the maximum efficiency is achieved by setting
the frame length equal to the backlog size at each subsequent frame; however,
at best of our knowledge, a formal proof of this result is still missing, and
is provided here. As byproduct, we also prove that the asymptotical efficiency
in the optimal case is , provide upper and lower bounds for the length
of the entire transmission period and show that its asymptotical behaviour is
, with .Comment: 22 pages, submitted to IEEE Trans. on Information Theor
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