8,658 research outputs found
A hybrid approach for predicting the distribution of vibro-acoustic energy in complex built-up structures
Finding the distribution of vibro-acoustic energy in complex built-up
structures in the mid-to-high frequency regime is a difficult task. In
particular, structures with large variation of local wavelengths and/or
characteristic scales pose a challenge referred to as the mid-frequency
problem. Standard numerical methods such as the finite element method (FEM)
scale with the local wavelength and quickly become too large even for modern
computer architectures. High frequency techniques, such as statistical energy
analysis (SEA), often miss important information such as dominant resonance
behaviour due to stiff or small scale parts of the structure. Hybrid methods
circumvent this problem by coupling FEM/BEM and SEA models in a given built-up
structure. In the approach adopted here, the whole system is split into a
number of subsystems which are treated by either FEM or SEA depending on the
local wavelength. Subsystems with relative long wavelengths are modelled using
FEM. Making a diffuse field assumption for the wave fields in the short wave
length components, the coupling between subsystems can be reduced to a weighted
random field correlation function. The approach presented results in an
SEA-like set of linear equations which can be solved for the mean energies in
the short wavelength subsystems
An efficient BEM for numerical solution of the biharmonic boundary value problem
This paper presents an efficient BEM for solving biharmonic equations. All boundary values including geometries are approximated by the universal high order radial basis function networks (RBFNs) rather than the usual low order interpolations. Numerical results show that the proposed BEM is considerably superior to the linear/quadratic-BEM in terms of both accuracy and convergence rate
Solving high-order partial differential equations with indirect radial basis function networks
This paper reports a new numerical method based on radial basis function networks (RBFNs) for solving high-order partial differential equations (PDEs). The variables and their derivatives in the governing equations are represented by integrated RBFNs. The use of integration in constructing neural networks allows the straightforward implementation of multiple boundary conditions and the accurate approximation of high-order derivatives. The proposed RBFN method is verified successfully through the solution of thin-plate bending and viscous flow problems which are governed by biharmonic equations. For thermally driven cavity flows, the solutions are obtained up to a high Rayleigh number
Mask breathing system for the Apollo command module
Breathing mask for Apollo command module spacecre
Regular quantum graphs
We introduce the concept of regular quantum graphs and construct connected
quantum graphs with discrete symmetries. The method is based on a decomposition
of the quantum propagator in terms of permutation matrices which control the
way incoming and outgoing channels at vertex scattering processes are
connected. Symmetry properties of the quantum graph as well as its spectral
statistics depend on the particular choice of permutation matrices, also called
connectivity matrices, and can now be easily controlled. The method may find
applications in the study of quantum random walks networks and may also prove
to be useful in analysing universality in spectral statistics.Comment: 12 pages, 3 figure
Ribozymes: the characteristics and properties of catalytic RNAs
Ribozymes, or catalytic RNAs, were discovered a little more than 15 years ago. They are found in the organelles of plants and lower eukaryotes, in amphibians, in prokaryotes, in bacteriophages, and in viroids and satellite viruses that infect plants. An example is also known of a ribozyme in hepatitis delta virus, a serious human pathogen. Additional ribozymes are bound to be found in the future, and it is tempting to regard the RNA component(s) of various ribonucleoprotein complexes as the catalytic engine, while the proteins serve as mere scaffolding - an unheard-of notion 15 years ago! In nature, ribozymes are involved in the processing of RNA precursors. However, all the characterized ribozymes have been converted, with some clever engineering, into RNA enzymes that can cleave or modify targeted RNAs (or even DNAs) without becoming altered themselves. While their success in vitro is unquestioned, ribozymes are increasingly used in vivo as valuable tools for studying and regulating gene expression. This review is intended as a brief introduction to the characteristics of the different identified ribozymes and their propertie
Design and Prototyping of a Shape-changing Rigid-body Human Foot in Gait
Traditional ankle-foot prostheses often replicate the physiological change in shape of the foot during gait via compliant mechanisms. In comparison, rigid-body feet tend to be simplistic and largely incapable of accurately representing the geometry of the human foot. Multi-segment rigid-body devices offer certain advantages over compliant mechanisms which may be desirable in the design of ankle-foot devices, including the ability to withstand greater loading, the ability to achieve more drastic shape-change, and the ability to be synthesized from their kinematics, allowing for realistic functionality without prior accounting of the complex internal kinetics of the foot. This work focuses on applying methodology of shape-changing kinematic synthesis to design and prototype a multi-segment rigid-body foot device capable of matching the dynamic change in shape of a human foot in gait. Included are discussions of an actuation strategy, mechanical design considerations, limitations, and potential prosthetic design implications of such a foot
A NEW COLD WAR? A RESURGENT RUSSIA AND THE IMPLICATIONS FOR REGIONAL AND GLOBAL SECURITY
Recent events in Eastern Europe and the Middle East have prompted world leaders to opine that the world is entering a new Cold War. These concerns are based on the recent invasions of Crimea and Ukraine, action in Syria, Russian rhetoric, and military posturing by both sides. Russian history, strategy, and strategic culture provide context for the current state of affairs. These do not, however, guarantee that the present implementation of strategy will mirror the past and that the goals are to return to a Soviet-style, Cold War-era, bipolar world order. The issue is more complex then our own cognitive biases have allowed us to comprehend. Russia is resurgent and does pose a threat to stability, but its goal is neither a Cold War nor a hot war. Rather, it seeks to be treated as an equal and to reassert a greater level of control and influence over its former lands
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