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More About the Tetrahedral Unstructured Software System
Friday, 01 September 2006
TetrUSS is a comprehensive suite of computational fluid dynamics (CFD) programs that won the Software of the Year award in 1996 and has found increasing use in government, academia, and industry for solving realistic flow problems (especially in aerodynamics and aeroelastics of aircraft having complex shapes). TetrUSS includes not only programs for solving basic equations of flow but also programs that afford capabilities for efficient generation and utilization of computational grids and for graphical representation of computed flows (see figure). The 2004 version of the Tetrahedral Unstructured Software System (TetrUSS), which is one of two software systems reported in “NASA’s 2004 Software of the Year,” NASA Tech Briefs, Vol. 28, No. 10 (October 2004), page 18, has been improved greatly since 1996. These improvements include (1) capabilities to simulate viscous flow by solving the NavierStokes equations on unstructured grids, (2) portability to personal computers from diverse manufacturers, (3) advanced models of turbulence, (4) a parallelprocessing version of one of the unstructuredgrid NavierStokesequationsolving programs, and (5) advanced programs for generating unstructured grids.
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Posted in: Briefs, TSP, Software
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Computing Flows Using Chimera and Unstructured Grids
Friday, 01 September 2006
DRAGONFLOW is a computer program that solves the NavierStokes equations of flows in complexly shaped three dimensional regions discretized by use of a direct replacement of arbitrary grid overlapping by nonstructured (DRAGON) grid. A DRAGON grid (see figure) is a combination of a chimera grid (a composite of structured subgrids) and a collection of unstructured subgrids. DRAGONFLOW incorporates modified versions of two prior NavierStokesequationsolving programs: OVERFLOW, which is designed to solve on chimera grids; and USM3D, which is used to solve on unstructured grids. A master module controls the invocation of individual modules in the libraries. At each time step of a simulated flow, DRAGONFLOW is invoked on the chimera portion of the DRAGON grid in alternation with USM3D, which is invoked on the unstructured subgrids of the DRAGON grid. The USM3D and OVERFLOW modules then immediately exchange their solutions and other data. As a result, USM3D and OVERFLOW are coupled seamlessly.
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Posted in: Briefs, TSP, Software
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Avoiding Obstructions in Aiming a HighGain Antenna
Friday, 01 September 2006
The High Gain Antenna Pointing and Obstruction Avoidance software performs computations for pointing a Mars Rover highgain antenna for communication with Earth while (1) avoiding lineofsight obstructions (the Martian terrain and other parts of the Rover) that would block communication and (2) taking account of limits in ranges of motion of antenna gimbals and of kinematic singularities in gimbal mechanisms. The software uses simplified geometric models of obstructions and of the trajectory of the Earth in the Martian sky(see figure). It treats all obstructions according to a generalized approach, computing and continually updating the time remaining before interception of each obstruction. In cases in which the gimbalmechanism design allows two aiming solutions, the algorithm chooses the solution that provides the longest obstructionfree Earthtracking time. If the communication session continues until an obstruction is encountered in the current pointing solution and the other solution is now unobstructed, then the algorithm automatically switches to the other position. This software also notifies communicationmanaging software to cease transmission during the switch to the unobstructed position, resuming it when the switch is complete.
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Posted in: Briefs, TSP, Software
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Progress in Computational Simulation of Earthquakes
Friday, 01 September 2006
GeoFEST(P) is a computer program written for use in the QuakeSim project, which is devoted to development and improvement of means of computational simulation of earthquakes. GeoFEST(P) models interacting earthquake fault systems from the faultnucleation to the tectonic scale. The development of GeoFEST(P) has involved coupling of two programs: GeoFEST and the Pyramid Adaptive Mesh Refinement Library. GeoFEST is a messagepassinginterface parallel code that utilizes a finiteelement technique to simulate evolution of stress, fault slip, and plastic/ elastic deformation in realistic materials like those of faulted regions of the crust of the Earth. The products of such simulations are synthetic observable timedependent surface deformations on time scales from days to decades. Pyramid Adaptive Mesh Refinement Library is a software library that facilitates the generation of computational meshes for solving physical problems. In an application of GeoFEST(P), a computational grid can be dynamically adapted as stress grows on a fault. Simulations on workstations using a few tens of thousands of stress and displacement finite elements can now be expanded to multiple millions of elements with greater than 98percent scaled efficiency on over many hundreds of parallel processors (see figure).
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Posted in: Briefs, TSP, Software
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Stochastic Evolutionary Algorithms for Planning Robot Paths
Friday, 01 September 2006
A computer program implements stochastic evolutionary algorithms for planning and optimizing collisionfree paths for robots and their jointed limbs. Stochastic evolutionary algorithms can be made to produce acceptably close approximations to exact, optimal solutions for pathplanning problems while often demanding much less computation than do exhaustivesearch and deterministic inversekinematics algorithms that have been used previously for this purpose. Hence, the present software is better suited for application aboard robots having limited computing capabilities (see figure). The stochastic aspect lies in the use of simulated annealing to (1) prevent trapping of an optimization algorithm in local minima of an energylike error measure by which the fitness of a trial solution is evaluated while (2) ensuring that the entire multidimensional configuration and parameter space of the pathplanning problem is sampled efficiently with respect to both robot joint angles and computation time. Simulated annealing is an established technique for avoiding local minima in multidimensional optimization problems, but has not, until now, been applied to planning collisionfree robot paths by use of lowpower computers.
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Posted in: Briefs, TSP, Software
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Tracking the Martian CO_{2} Polar Ice Caps in Infrared Images
Friday, 01 September 2006
Researchers at NASA’s Jet Propulsion Laboratory have developed a method for automatically tracking the polar caps on Mars as they advance and recede each year (see figure). The seasonal Mars polar caps are composed mainly of CO2 ice and are therefore cold enough to stand out clearly in infrared data collected by the Thermal Emission Imaging System (THEMIS) onboard the Mars Odyssey spacecraft. The Bimodal Image Temperature (BIT) histogram analysis algorithm analyzes raw, uncalibrated data to identify images that contain both “cold” (“polar cap”) and “warm” (“not polar cap”) pixels. The algorithm dynamically identifies the temperature that separates these two regions. This flexibility is critical, because in the absence of any calibration, the threshold temperature can vary significantly from image to image. Using the identified threshold, the algorithm classifies each pixel in the image as “polar cap” or “not polar cap,” then identifies the image row that contains the spatial transition from “polar cap” to “not polar cap.” While this method is useful for analyzing data that has already been returned by THEMIS, it has even more significance with respect to data that has not yet been collected. Instead of seeking the polar cap only in specific, targeted images, the simplicity and efficiency of this method makes it feasible for direct, onboard use. That is, THEMIS could continuously monitor its observations for any detections of the polarcap edge, producing detections over a wide range of spatial and temporal conditions. This effort can greatly contribute to our understanding of longterm climatic change on Mars.
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Posted in: Briefs, TSP, Software
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Rapid Aeroelastic Analysis of Blade Flutter in Turbomachines
Friday, 01 September 2006
The LINFLUXAE computer code predicts flutter and forced responses of blades and vanes in turbomachines under subsonic, transonic, and supersonic flow conditions. The code solves the Euler equations of unsteady flow in a blade passage under the assumption that the blades vibrate harmonically at small amplitudes. The steadystate nonlinear Euler equations are solved by a separate program, then equations for unsteady flow components are obtained through linearization around the steadystate solution. A structuraldynamics analysis (see figure) is performed to determine the frequencies and mode shapes of blade vibrations, a preprocessor interpolates mode shapes from the structuraldynamics mesh onto the LINFLUX computational fluiddynamics mesh, and an interface code is used to convert the steadystate flow solution to a form required by LINFLUX. Then LINFLUX solves the linearized equations in the frequency domain to calculate the unsteady aerodynamic pressure distribution for a given vibration mode, frequency, and interblade phase angle. A postprocessor uses the unsteady pressures to calculate generalized aerodynamic forces, response amplitudes, and eigenvalues (which determine the flutter frequency and damping). In comparison with the TURBOAE aeroelasticanalysis code, which solves the equations in the time domain, LINFLUXAE is 6 to 7 times faster.
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Posted in: Briefs, TSP, Software
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