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At least 271 records · Page 15

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NASA NTRS1991

OPAD 1991

The Optical Plume Anomaly Detection Program, an experimental study in the attempt to create a rocket engine health monitor based on detection and possible quantification of anomalous atomic and molecular species in the exhaust plume, has been in existence for several years. The instruments developed to monitor the exhaust plumes are presented. Two optical instruments are employed: the polychromator and the spectrometer. The polychromator is a 16 channel spectroradiometer having independent channels individually adjustable for center wavelength and pass bandwidth. Those values may be mechanically reset, with the aid of certain lab equipment. The spectrometer is a multichannel spectral analyzer having two 2048 element linear photodiode arrays at the exit plane of the dispersing instrument, which, like the 16 channel device, is a 1/2 meter grating spectrograph. These instruments are equipped with ultraviolet grade multifiber optical input cables, allowing the instruments to be placed in a benign environment. Telescopes mounted on the test stand observe the plume keeping the shock structure in view. The data acquisition and control system consist of four 80386-33 MHz computers: two at the test stand for instrument control and data preconditioning, and two in the test support building providing data archiving, display, and system control. Further descriptions of the instrumentation are provided.

Powers, W. T.↗

NASA NTRS1993

Generation and exploration of aggregation abstractions for scheduling and resource allocation

This paper presents research on the abstraction of computational theories for scheduling and resource allocation. The paper describes both theory and methods for the automated generation of aggregation abstractions and approximations in which detailed resource allocation constraints are replaced by constraints between aggregate demand and capacity. The interaction of aggregation abstraction generation with the more thoroughly investigated abstractions of weakening operator preconditions is briefly discussed. The purpose of generating abstract theories for aggregated demand and resources includes: answering queries about aggregate properties, such as gross feasibility; reducing computational costs by using the solution of aggregate problems to guide the solution of detailed problems; facilitating reformulating theories to approximate problems for which there are efficient problem-solving methods; and reducing computational costs of scheduling by providing more opportunities for variable and value-ordering heuristics to be effective. Experiments are being developed to characterize the properties of aggregations that make them cost effective. Both abstract and concrete theories are represented in a variant of first-order predicate calculus, which is a parameterized multi-sorted logic that facilitates specification of large problems. A particular problem is conceptually represented as a set of ground sentences that is consistent with a quantified theory.

Lowry, Michael R.↗

NASA NTRS1993

Process control and recovery in the Link Monitor and Control Operator Assistant

This paper describes our approach to providing process control and recovery functions in the Link Monitor and Control Operator Assistant (LMCOA). The focus of the LMCOA is to provide semi-automated monitor and control to support station operations in the Deep Space Network. The LMCOA will be demonstrated with precalibration operations for Very Long Baseline Interferometry on a 70-meter antenna. Precalibration, the task of setting up the equipment to support a communications link with a spacecraft, is a manual, time consuming and error-prone process. One problem with the current system is that it does not provide explicit feedback about the effects of control actions. The LMCOA uses a Temporal Dependency Network (TDN) to represent an end-to-end sequence of operational procedures and a Situation Manager (SM) module to provide process control, diagnosis, and recovery functions. The TDN is a directed network representing precedence, parallelism, precondition, and postcondition constraints. The SM maintains an internal model of the expected and actual states of the subsystems in order to determine if each control action executed successfully and to provide feedback to the user. The LMCOA is implemented on a NeXT workstation using Objective C, Interface Builder and the C Language Integrated Production System.

Lee, Lorrine↗

NASA NTRS1992

Sub-domain decomposition methods and computational controls for multibody dynamical systems

This paper presents a concurrent methodology to simulate the dynamics of flexible multibody systems with a large number of degrees of freedom. A general class of open-loop structures is treated and a redundant coordinate formulation is adopted. A range space method is used in which the constraint forces are calculated using a preconditioned conjugate gradient method. By using a preconditioner motivated by the regular ordering of the directed graph of the structures, it is shown that the method is order N in the total number of coordinates of the system. The overall formulation has the advantage that it permits fine parallelization and does not rely on system topology to induce concurrency. It can be efficiently implemented on the present generation of parallel computers with a large number of processors. Validation of the method is presented via numerical simulations of space structures incorporating large number of flexible degrees of freedom.

Menon, R. G.↗

NASA NTRS1992

The neutral circulation in the vicinity of a stable auroral arc

The effects of asymmetric potential distributions, arc-neutral feedback, and cross-arc winds on the neutral circulation near auroral arc were examined using a sophisticated two-dimensional high-resolution model of the neutral atmosphere which was preconditioned by allowing weak diffuse aurora to exist prior to the formation of an arc. Simulations were performed with asymmetric potential structures and with an initial state which was warmed and accelerated due to a preexisting diffuse aurora, as well as with arc-zonal wind feedback and with large-scale cross-arc flow. It is shown that, with a preexisting zonal flow and asymmetrical arc structures, strong zonal winds are generated within the arc. Simulations with the arc-zonal feedback indicate that the effect of feedback is not large because of the limited duration of an arc. Simulations with a strong large-scale cross-arc wind indicate that the zonal wind is not strongly sensitive to the large-scale wind.

Walterscheid, R. L.↗

NASA NTRS1993

A least-squares finite element method for 3D incompressible Navier-Stokes equations

The least-squares finite element method (LSFEM) based on the velocity-pressure-vorticity formulation is applied to three-dimensional steady incompressible Navier-Stokes problems. This method can accommodate equal-order interpolations, and results in symmetric, positive definite algebraic system. An additional compatibility equation, i.e., the divergence of vorticity vector should be zero, is included to make the first-order system elliptic. The Newton's method is employed to linearize the partial differential equations, the LSFEM is used to obtain discretized equations, and the system of algebraic equations is solved using the Jacobi preconditioned conjugate gradient method which avoids formation of either element or global matrices (matrix-free) to achieve high efficiency. The flow in a half of 3D cubic cavity is calculated at Re = 100, 400, and 1,000 with 50 x 52 x 25 trilinear elements. The Taylor-Gortler-like vortices are observed at Re = 1,000.

Jiang, Bo-Nan↗

NASA NTRS1992

Atrial natriuretic peptide degradation by CPA47 cells - Evidence for a divalent cation-independent cell-surface proteolytic activity

Atrial natriuretic peptide (ANP) is rapidly cleared and degraded in vivo. Nonguanylate-cyclase receptors (C-ANPR) and a metalloproteinase, neutral endopeptidase (EC 3.4.24.11) (NEP 24.11), are thought to be responsible for its metabolism. We investigated the mechanisms of ANP degradation by an endothelial-derived cell line, CPA47. CPA47 cells degraded 88 percent of 125I-ANP after 1 h at 37 degrees C as determined by HPLC. Medium preconditioned by these cells degraded 41 percent of the 125I-ANP, and this activity was inhibited by a divalent cation chelator, EDTA. Furthermore, a cell-surface proteolytic activity degraded 125I-ANP in the presence of EDTA when receptor-mediated endocytosis was inhibited either by low temperature (4 degrees C) or by hyperosmolarity at 37 degrees C. The metalloproteinase, NEP 24.11, is unlikely to be the cell-surface peptidase because 125I-ANP is degraded by CPA47 cells at 4 degrees C in the presence of 5 mM EDTA. These data indicate that CPA47 cells can degrade ANP by a novel divalent cation-independent cell-surface proteolytic activity.

Frost, S. J.↗

NASA NTRS1993

A natural partitioning scheme for parallel simulation of multibody systems

A parallel partitioning scheme based on physical-co-ordinate variables is presented to systematically eliminate system constraint forces and yield the equations of motion of multibody dynamics systems in terms of their independent coordinates. Key features of the present scheme include an explicit determination of the independent coordinates, a parallel construction of the null space matrix of the constraint Jacobian matrix, an easy incorporation of the previously developed two-stage staggered solution procedure and a Schur complement based parallel preconditioned conjugate gradient numerical algorithm.

Chiou, J. C.↗

NASA NTRS1993

Three-dimensional unstructured grid Euler computations using a fully-implicit, upwind method

A method has been developed to solve the Euler equations on a three-dimensional unstructured grid composed of tetrahedra. The method uses an upwind flow solver with a linearized, backward-Euler time integration scheme. Each time step results in a sparse linear system of equations which is solved by an iterative, sparse matrix solver. Local-time stepping, switched evolution relaxation (SER), preconditioning and reuse of the Jacobian are employed to accelerate the convergence rate. Implicit boundary conditions were found to be extremely important for fast convergence. Numerical experiments have shown that convergence rates comparable to that of a multigrid, central-difference scheme are achievable on the same mesh. Results are presented for several grids about an ONERA M6 wing.

Whitaker, David L.↗

NASA NTRS1993

On solving the compressible Navier-Stokes equations for unsteady flows at very low Mach numbers

The properties of a preconditioned, coupled, strongly implicit finite-difference scheme for solving the compressible Navier-Stokes equations in primitive variables are investigated for two unsteady flows at low speeds, namely the impulsively started driven cavity and the startup of pipe flow. For the shear-driven cavity flow, the computational effort was observed to be nearly independent of Mach number, especially at the low end of the range considered. This Mach number independence was also observed for steady pipe flow calculations; however, rather different conclusions were drawn for the unsteady calculations. In the pressure-driven pipe startup problem, the compressibility of the fluid began to significantly influence the physics of the flow development at quite low Mach numbers. The present scheme was observed to produce the expected characteristics of completely incompressible flow when the Mach number was set at very low values. Good agreement with incompressible results available in the literature was observed.

Pletcher, R. H.↗

NASA NTRS1993

Nonrecursive formulations of multibody dynamics and concurrent multiprocessing

Since the late 1980's, research in recursive formulations of multibody dynamics has flourished. Historically, much of this research can be traced to applications of low dimensionality in mechanism and vehicle dynamics. Indeed, there is little doubt that recursive order N methods are the method of choice for this class of systems. This approach has the advantage that a minimal number of coordinates are utilized, parallelism can be induced for certain system topologies, and the method is of order N computational cost for systems of N rigid bodies. Despite the fact that many authors have dismissed redundant coordinate formulations as being of order N(exp 3), and hence less attractive than recursive formulations, we present recent research that demonstrates that at least three distinct classes of redundant, nonrecursive multibody formulations consistently achieve order N computational cost for systems of rigid and/or flexible bodies. These formulations are as follows: (1) the preconditioned range space formulation; (2) penalty methods; and (3) augmented Lagrangian methods for nonlinear multibody dynamics. The first method can be traced to its foundation in equality constrained quadratic optimization, while the last two methods have been studied extensively in the context of coercive variational boundary value problems in computational mechanics. Until recently, however, they have not been investigated in the context of multibody simulation, and present theoretical questions unique to nonlinear dynamics. All of these nonrecursive methods have additional advantages with respect to recursive order N methods: (1) the formalisms retain the highly desirable order N computational cost; (2) the techniques are amenable to concurrent simulation strategies; (3) the approaches do not depend upon system topology to induce concurrency; and (4) the methods can be derived to balance the computational load automatically on concurrent multiprocessors. In addition to the presentation of the fundamental formulations, this paper presents new theoretical results regarding the rate of convergence of order N constraint stabilization schemes associated with the newly introduced class of methods.

Kurdila, Andrew J.↗

NASA NTRS1993

Removal of carbonaceous contaminants from silica aerogel

Capture of micrometeorite material from low Earth orbit or dust grains around active comets for return to terrestrial laboratories, capable of practicing the most up to date techniques of chemical isotopic and mineralogical analysis, will greatly enhance our knowledge of primitive material in the solar system. The next generation of space launched cosmic dust collectors will undoubtedly include extremely low density target materials such as silica aerogel as the decelerating and arresting medium. This material has been found to be clean from the point of view of inorganic elements and is thus acceptable for the purpose of harvesting grains to be studied by, for example PIXE, INAA, or SXRF. However, the process used in making aerogel leaves substantial carbon and hydrogen containing residues which would negate their suitability for collection and subsequent investigation of the very important CHON particles. Attempts to precondition aerogel by solvent extraction or heating at 500 C and 750 C in air for 24 hours or under a vacuum of 2(7)(exp -7) torr at 260 C were largely ineffective except that pyrolysis did reduce volatile species. In this investigation we have examined the use of supercritical fluids for the purpose of extracting organic residues. The logic of the new approach is that beyond the supercritical point a substance has the solvating properties of a liquid but the viscosity characteristics of a gas. For example carbon dioxide becomes supercritical at a pressure of 73 atmospheres and a temperature of 31 C; in consequence it can transform to a very powerful and ultraclean solvent. It can dissolve organic matter from low molecular weight up to molecules containing 90 carbon atoms. On release of pressure the fluid reverts to a gas which can easily be pumped away and removed from the substrate being extracted.

Huang, Hui-Ping↗

NASA NTRS1993

On solving the compressible Navier-Stokes equations for unsteady flows at very low Mach numbers

The properties of a preconditioned, coupled, strongly implicit finite difference scheme for solving the compressible Navier-Stokes equations in primitive variables are investigated for two unsteady flows at low speeds, namely the impulsively started driven cavity and the startup of pipe flow. For the shear-driven cavity flow, the computational effort was observed to be nearly independent of Mach number, especially at the low end of the range considered. This Mach number independence was also observed for steady pipe flow calculations; however, rather different conclusions were drawn for the unsteady calculations. In the pressure-driven pipe startup problem, the compressibility of the fluid began to significantly influence the physics of the flow development at quite low Mach numbers. The present scheme was observed to produce the expected characteristics of completely incompressible flow when the Mach number was set at very low values. Good agreement with incompressible results available in the literature was observed.

Pletcher, R. H.↗

NASA NTRS1993

Efficiency and reliability enhancements in propulsion flowfield modeling

The implementation of traditional CFD algorithms in practical propulsion related flowfields often leads to dramatic reductions in efficiency and/or robustness. The present research is directed at understanding the reasons for this deterioration and finding methods to circumvent it. Work to date has focussed on low Mach number regions, viscous dominated regions, and high grid aspect ratios. Time derivative preconditioning, improved definition of the local time stepping, and appropriate application of boundary conditions are employed to decrease the required time to obtain a solution, while maintaining accuracy. A number of cases having features typical of rocket engine flowfields are computed to demonstrate the improvement over conventional methods. These cases include laminar and turbulent high Reynolds number flat plate boundary layers, flow over a backward-facing step, a diffusion flame, and wall heat-flux calculations in a turbulent converging-diverging nozzle. Results from these cases show convergence that is virtually independent of the local Mach number and the grid aspect ratio, which translates to a convergence speed-up of up to several orders of magnitude over conventional algorithms. Current emphasis is in extending these results to three-dimensional flows with highly stretched grids.

Buelow, Philip E. O.↗

NASA NTRS1993

Computation of propulsion-related flowfields using unstructured adaptive meshes

In this paper, we describe a computational fluid dynamics (CFD) technique based on unstructured triangular/tetrahedral meshes. A finite-volume scheme is used in conjunction with a multi-stage Runge-Kutta algorithm. Convergence enhancements in the form of dual time-stepping and time-derivative preconditioning are used to overcome the limitations of conventional multi-stage schemes. The method is applied to propulsion-related flows and shown to perform satisfactorily.

Weiss, Jonathan M.↗

NASA NTRS1993

Extending substructure based iterative solvers to multiple load and repeated analyses

Direct solvers currently dominate commercial finite element structural software, but do not scale well in the fine granularity regime targeted by emerging parallel processors. Substructure based iterative solvers--often called also domain decomposition algorithms--lend themselves better to parallel processing, but must overcome several obstacles before earning their place in general purpose structural analysis programs. One such obstacle is the solution of systems with many or repeated right hand sides. Such systems arise, for example, in multiple load static analyses and in implicit linear dynamics computations. Direct solvers are well-suited for these problems because after the system matrix has been factored, the multiple or repeated solutions can be obtained through relatively inexpensive forward and backward substitutions. On the other hand, iterative solvers in general are ill-suited for these problems because they often must restart from scratch for every different right hand side. In this paper, we present a methodology for extending the range of applications of domain decomposition methods to problems with multiple or repeated right hand sides. Basically, we formulate the overall problem as a series of minimization problems over K-orthogonal and supplementary subspaces, and tailor the preconditioned conjugate gradient algorithm to solve them efficiently. The resulting solution method is scalable, whereas direct factorization schemes and forward and backward substitution algorithms are not. We illustrate the proposed methodology with the solution of static and dynamic structural problems, and highlight its potential to outperform forward and backward substitutions on parallel computers. As an example, we show that for a linear structural dynamics problem with 11640 degrees of freedom, every time-step beyond time-step 15 is solved in a single iteration and consumes 1.0 second on a 32 processor iPSC-860 system; for the same problem and the same parallel processor, a pair of forward/backward substitutions at each step consumes 15.0 seconds.

Farhat, Charbel↗

NASA NTRS1994

An implicit numerical scheme for the simulation of internal viscous flows on unstructured grids

The Navier-Stokes equations are solved numerically for two-dimensional steady viscous laminar flows. The grids are generated based on the method of Delaunay triangulation. A finite-volume approach is used to discretize the conservation law form of the compressible flow equations written in terms of primitive variables. A preconditioning matrix is added to the equations so that low Mach number flows can be solved economically. The equations are time marched using either an implicit Gauss-Seidel iterative procedure or a solver based on a conjugate gradient like method. A four color scheme is employed to vectorize the block Gauss-Seidel relaxation procedure. This increases the memory requirements minimally and decreases the computer time spent solving the resulting system of equations substantially. A factor of 7.6 speed up in the matrix solver is typical for the viscous equations. Numerical results are obtained for inviscid flow over a bump in a channel at subsonic and transonic conditions for validation with structured solvers. Viscous results are computed for developing flow in a channel, a symmetric sudden expansion, periodic tandem cylinders in a cross-flow, and a four-port valve. Comparisons are made with available results obtained by other investigators.

Jorgenson, Philip C. E.↗

NASA NTRS1993

Relaxation schemes for Chebyshev spectral multigrid methods

Two relaxation schemes for Chebyshev spectral multigrid methods are presented for elliptic equations with Dirichlet boundary conditions. The first scheme is a pointwise-preconditioned Richardson relaxation scheme and the second is a line relaxation scheme. The line relaxation scheme provides an efficient and relatively simple approach for solving two-dimensional spectral equations. Numerical examples and comparisons with other methods are given.

Kang, Yimin↗