Stringent Boundary Conditions Research Articles

A High-Efficiency Spectral Method for Two-Dimensional Ocean Acoustic Propagation Calculations.

The accuracy and efficiency of sound field calculations highly concern issues of hydroacoustics. Recently, one-dimensional spectral methods have shown high-precision characteristics when solving the sound field but can solve only simplified models of underwater acoustic propagation, thus their application range is small. Therefore, it is necessary to directly calculate the two-dimensional Helmholtz equation of ocean acoustic propagation. Here, we use the Chebyshev–Galerkin and Chebyshev collocation methods to solve the two-dimensional Helmholtz model equation. Then, the Chebyshev collocation method is used to model ocean acoustic propagation because, unlike the Galerkin method, the collocation method does not need stringent boundary conditions. Compared with the mature Kraken program, the Chebyshev collocation method exhibits a higher numerical accuracy. However, the shortcoming of the collocation method is that the computational efficiency cannot satisfy the requirements of real-time applications due to the large number of calculations. Then, we implemented the parallel code of the collocation method, which could effectively improve calculation effectiveness.

A Solution Strategy to Include the Opening of the Opercular Slits in Moving-Mesh CFD Models of Suction Feeding.

The gill cover of fish and pre-metamorphic salamanders has a key role in suction feeding by acting as a one-way valve. It initially closes and avoids an inflow of water through the gill slits, after which it opens to allow outflow of the water that was sucked through the mouth into the expanded buccopharyngeal cavity. However, due to the inability of analytical models (relying on the continuity principle) to calculate the flow of fluid through a cavity with two openings and that was changing in shape and size, stringent boundary conditions had to be used in previously developed mathematical models after the moment of the valve's opening. By solving additionally for the conservation of momentum, computational fluid dynamics (CFD) has the capacity to dynamically simulate these flows, but this technique also faces complications in modeling a transition from closed to open valves. Here, I present a relatively simple solution strategy to incorporate the opening of the valves, exemplified in an axisymmetrical model of a suction-feeding sunfish in ANSYS Fluent software. By controlling viscosity of a separately defined fluid entity in the region of the opercular cavity, early inflow can be blocked (high viscosity assigned) and later outflow can be allowed (changing viscosity to that of water). Finally, by analyzing the CFD solution obtained for the sunfish model, a few new insights into the biomechanics of suction feeding are gained.

Trajectory Optimization of Reentry Capsule

Abstract Reentry vehicles are required to meet stringent boundary conditions and path constraints, while minimizing certain cost functional. Parameter optimal control is easier and faster to solve than indirect methods for complex problems, such as the problem of re-entry trajectory optimization subject to multiple path constraints. In this paper, a direct transcription method is applied for finding optimal trajectory of 2 problems. Firstly, Newtonian method is used for finding aerodynamic coefficients of blunt nose conical re-entry capsule in hypersonic regime. Using these coefficients, trajectory satisfying path constraints corresponding to heat influx, G-load and control angles is generated while minimizing the vehicle manoeuvres. In the second problem Apollo capsule's multiphase optimization problem is solved based on the aerodynamic coefficients obtained from wind tunnel test data.

Fabrication of X-ray mirrors for synchrotron applications

Application of conic section geometries play an important role for soft X-ray synchrotron beams. Mirror design for collimation or focusing under stringent boundary conditions may end up with very aspheric geometries. In particular, the aspheric departure of these surfaces from “base geometries” in combination with high-quality requirements for slope and roughness is challenging. Mirror manufacturing of very aspheric mirrors at Carl Zeiss Laser Optics will be illustrated by recent examples. Fabrication issues of ellipsoids and 2D-parabolas are discussed. In addition, the close interaction of metrology and polishing will be highlighted.

Rethinking the commons

Common property has been theoretically linked to environmental degradation through the metaphor of “the tragedy of the commons,” which discounts local solutions to commons dilemmas and typically posits the need for strong states or privatization. Though neither solution is theoretically or empirically adequate—because of the nature of states and nature in the real world—local arrangements for averting the tragedy suffer certain lacunaeas well, including stringent boundary conditions and overlapping/overarching commons situations that necessitate larger scale cooperation than is possible in the face-to-face communities that are conducive to cooperation. Second-order or meta-commons issues expand the scope of inquiry necessarily beyond conservationto preservation.The Sundarbans illustrates the contradictory implications of the Leviathan solution to comons dilemmas, as well as the centrality of alternative perceptual framings of natural systems.

Abundance Clues to Early Galactic Chemical Evolution

High S/N spectroscopic studies of the abundance patterns characterizing extremely metal-deficient halo field stars and globular cluster stars have served to provide significant clues to and increasingly stringent boundary conditions upon the chemical evolution of the halo population of our galaxy. Guided by our current knowledge of nucleosynthesis as a function of stellar mass occurring in stars and supernovae, we identify some interesting constraints that these combined observational and theoretical considerations impose upon theories of the early history of our galaxy.

The effect of stringent boundary conditions on the solution of a transient heat conduction problem

A notation system is proposed for the writing of the finite difference equations related to a conduction heat transfer problem in a composite medium with variable thermal properties. An example is worked out with Euler explicit, Euler implicit and Crank-Nicolson methods. Special care is taken of the conductivity expression and an analysis shows the importance of using mean conductivity values around the nodes especially when stringent Dirichlet boundary conditions are imposed.

Recuperators for the HHT-Demonstration Plant

Recuperators of closed cycle gas turbine plants are characterized by large heat transfer areas accommodated in rather small volumes. Clean gas conditions, moderate operating temperatures and modest requirements with respect to tightness have resulted in generally good operational records. In the case of recuperators for direct cycle high temperature reactors, more stringent boundary conditions are being imposed than for conventional plants. In this paper studies and experiments performed for the recuperator of the HHT demonstration plant (670 MWe) are described. Five different design concepts are discussed and mutually compared. All designs use straight tubes. The main concept is a proposal with longitudinal flow on the shell side, but also the proven cross-counterflow concept has its merits. Special subjects are sensitivity to temperature streaks and optimum subdivision into modules.