By analyzing cardiac beat-to-beat intervals and interbeat increments, we find that—unlike adults—the difference in the pattern of interbeat increments in healthy and sick newborn infants is more due to a change in t...
By analyzing cardiac beat-to-beat intervals and interbeat increments, we find that—unlike adults—the difference in the pattern of interbeat increments in healthy and sick newborn infants is more due to a change in the amplitude and much less to a change in the ordering of the interbeat increments. This suggests that very low-frequency elements of neonatal and adult heart rate variability rise from fundamentally different mechanisms.
A finite element algorithm to simulate two dimensional flows of viscous and inviscid compressible fluids for a wide range of Mach numbers is presented in this work. This model is coupled to immersed deformable structu...
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A finite element algorithm to simulate two dimensional flows of viscous and inviscid compressible fluids for a wide range of Mach numbers is presented in this work. This model is coupled to immersed deformable structures through equilibrium and compatibility conditions in order to analyze its dynamic behavior. For the fluid, time integration is performed by a two-step Taylor-Galerkin explicit scheme and Newmark’s method is used to obtain the dynamic response of the structure. An arbitrary mixed Euler-Lagrange description is used to re-define a new finite element mesh in the presence of the immersed structure displacements. Finally, several examples are included showing the model behavior and possibilities for future expansions.
A network thermodynamic model has been devised to describe the coupled movement of water and glycerol molecules within the kidney during perfusion with solutions of time-varying glycerol and impermeant comcentrations....
A network thermodynamic model has been devised to describe the coupled movement of water and glycerol molecules within the kidney during perfusion with solutions of time-varying glycerol and impermeant comcentrations. Both flows are governed by a combination of the individual osmotic potential differences between compartments, the viscoelastic behavior of the tissue, and the momentum transferred between the two flows. The system is modelled using the bond graph technique and simulated with the ENPORT7 package (Rosencode Associates, 1993).
The problem of sequencing units on a mixed-model assembly line can be viewed with several objectives in mind. Past research has focused mainly on two separate performance measures: (1) minimizing the length of the lin...
The problem of sequencing units on a mixed-model assembly line can be viewed with several objectives in mind. Past research has focused mainly on two separate performance measures: (1) minimizing the length of the line (which is equivalent to minimizing the risk of stopping the conveyor when system variability is present and the station lengths are fixed);or (2) maintaining a rate of assembly equal to the demand rate for each model type in the production schedule. The latter is the more appropriate in a just-in-time environment. We present a bicriteria formulation of the problem that can be used to examine the tradeoffs between line length and parts usage. The resultant model takes the form of a mixed integer nonlinear program and is solved with a combination of heuristics and branch and bound. Results are reported for a wide range of problem sizes, as defined by the number of stations on the line, the number of different model types, and the total number of units to be assembled. In almost all cases, at least one of the heuristics found either the optimum or the best available solution. Computation times were quite reasonable for the heuristics, but grew exponentially for branch and bound. In general, it was only possible to verify optimality on problems with less-than-or-equal-to 20 units.
A major contributor to the expense and length of time to design, build, and test new systems has been the need to build and test hardware prototypes to determine their effectiveness in meeting operational requirements...
A major contributor to the expense and length of time to design, build, and test new systems has been the need to build and test hardware prototypes to determine their effectiveness in meeting operational requirements. Recent and dramatic advances in computer simulation technologies hold forth the promise of revolutionizing design and acquisition strategies by providing the means to validate end users' requirements prior to hardware construction. By designing and operationally testing virtual prototypes in a virtual environment, these technologies will soon offer naval architects the ability to build and launch ships in computer-based cyberspace in lieu of the shipbuilder's ways. The authors of this paper provide the background for these developments, explore the significance and ramifications of these technologies to the current process of ship and system design, outline challenges lying ahead, and present their vision and recommendations for future development.
The circular cylindrical shell is described by equations that govern wave propagation in a two-dimensional homogeneous, but anisotropic and dispersive, medium. The idealization of an unbounded medium is applicable if ...
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The circular cylindrical shell is described by equations that govern wave propagation in a two-dimensional homogeneous, but anisotropic and dispersive, medium. The idealization of an unbounded medium is applicable if the source is replaced by a periodic array of forces, the repetition distance being the cylinder circumference. Analytical expressions and calculations are presented for wavefront patterns, amplitude distributions, polarizations, and phase velocities for waves on the cylinder surface. The analysis includes all three possible directions of the exciting force and gives fundamental results that can be superimposed to predict vibration fields resulting from arbitrary excitations.
Determination of the surface acoustic pressure given the surface velocity of a vibrating body can be formulated in various ways. However, for some such formulations such as the surface Helmholtz integral equation, sol...
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Determination of the surface acoustic pressure given the surface velocity of a vibrating body can be formulated in various ways. However, for some such formulations such as the surface Helmholtz integral equation, solutions are not unique at certain discrete frequencies. Such uniqueness problems can also be present for variational formulations of the problem, but the variational formulation based on the normal derivative of the Kirchhoff integral theorem has unique solutions for vibrating disks and plate-like bodies. For bodies of finite volume, but for which each surface point is vibrating in phase, the total radiated acoustic power is always unique, even though the pressure may not be. The latter conclusion is supported by numerical calculations based on the Rayleigh-Ritz technique for the case of a finite cylinder vibrating as a rigid body in the axial direction.
Characteristics of both thermoplastic and thermoset composite materials as they pertain to marine vehicle applications are discussed. Comparison of various material selection factors such as strength, damage and moist...
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Characteristics of both thermoplastic and thermoset composite materials as they pertain to marine vehicle applications are discussed. Comparison of various material selection factors such as strength, damage and moisture resistance, and flammability and toxicity as well as cost and availability of thermoset and thermoplastic composite materials are presented. Methods for testing and reducing the flammability and toxicity are discussed. Many commercially available composite systems are reported to provide favorable characteristics for marine applications. Although there seems to be a need for improved production technology for thermoplastics, they present potential advantages in physical properties over thermoset composites.
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