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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Structural Stability and gamma-Transitions

In this work, we review some new discoveries and physical interpretations that concern the secular and dynamical instabilities and the associated structural changes in rotating, self-gravitating, incompressible fluids. For such fluids, we have been able to examine in depth and to understand physically a variety of classic results. Here we discuss the structural stability and the lambda-transitions of astrophysical fluids. We conclude with a summary of all the different types of lambda-transitions found in the course of our investigation and with a discussion of the implications of our results for superfluids, catastrophes, thermodynamical phase transitions, and the breaking of symmetry and topology.

Christodoulou, Dimitris M.↗
NASA NTRS1975

Closed-loop structural stability for linear-quadratic optimal systems

An explicit parametrization of a subclass of linear constant gain feedback maps that never destabilize an originally open-loop stable system is presented. These results are used to obtain several new structural stability results for multiinput linear-quadratic feedback optimal designs.

Wong, P. K.↗
NASA NTRS1975

Closed-loop structural stability for linear-quadratic optimal systems

This paper contains an explicit parameterization of a subclass of linear constant gain feedback maps that never destabilize an originally open-loop stable system. These results can then be used to obtain several new structural stability results for multi-input linear-quadratic feedback optimal designs.

Wong, P. K.↗
NASA NTRS1976

Closed-loop structural stability for linear-quadratic optimal system

This paper contains an explicit parameterization of a subclass of linear constant gain feedback maps that will not destabilize an originally open-loop stable system. These results can then be used to obtain several new structural stability results for multi-input linear-quadratic feedback optimal designs.

Wong, P. K.↗
NASA NTRS1977

Closed-loop structural stability for linear-quadratic optimal systems

This paper contains an explicit parametrization of a subclass of linear constant gain feedback maps that will not destabilize an originally open-loop stable system. These results can then be used to obtain several new structural stability results for multiinput linear-quadratic feedback optimal designs.

Wong, P. K.↗
NASA NTRS1976

Closed-loop structural stability for linear-quadratic optimal systems

This paper contains an explicit parametrization of a subclass of linear constant gain feedback maps that will not destabilize an originally open-loop stable system. These results can then be used to obtain several new structural stability results for multi-input linear-quadratic feedback optimal designs.

Wong, P. K.↗
NASA NTRS1989

Structural stability augmentation system design using BODEDIRECT: A quick and accurate approach

A methodology is presented for a modal suppression control law design using flight test data instead of mathematical models to obtain the required gain and phase information about the flexible airplane. This approach is referred to as BODEDIRECT. The purpose of the BODEDIRECT program is to provide a method of analyzing the modal phase relationships measured directly from the airplane. These measurements can be achieved with a frequency sweep at the control surface input while measuring the outputs of interest. The measured Bode-models can be used directly for analysis in the frequency domain, and for control law design. Besides providing a more accurate representation for the system inputs and outputs of interest, this method is quick and relatively inexpensive. To date, the BODEDIRECT program has been tested and verified for computational integrity. Its capabilities include calculation of series, parallel and loop closure connections between Bode-model representations. System PSD, together with gain and phase margins of stability may be calculated for successive loop closures of multi-input/multi-output systems. Current plans include extensive flight testing to obtain a Bode-model representation of a commercial aircraft for design of a structural stability augmentation system.

Goslin, T. J.↗
NASA NTRS1993

Partially Grooved Domain Stabilization Structures for Vertical Bloch Line Memory

Bias field stability ranges were measured and numerically simulated for magnetic domains ingarnets stabilized by partially grooved rectangular and ring grooves. Simulation results agreefavorably with experimental results when finite slope effects of the groove walls are included. Asbias fields increase, rectangular and ring domains both destabilize through stripe head recession. Asbias fields decrease, destabilization in rectangular domains occurs by runout, while destabilization inring domains occurs by midstripe domain buckling. While ring domains are stable at lower biasfields than rectangular domains, bias field stability ranges are approximately equal. Hence, for thesame partial grooving depth, rectangular domains are preferred because they offer higher storagedensity potential in Vertical Block Line (VBL) storage arrays as long as bit propagation margins atstripe ends are sufficient.

Katti, R. R.↗
NASA NTRS1989

Towards real-time simulation of large space structures: Stabilization of fluid/thermal/structure interactions and implementation on high performance supercomputers

Within the Center for Space Construction, the SIMSTRUC project's objectives center around the development of simulation tools for the realistic analysis of large space structures. The word 'tools' is the broad sense; it designates mathematical models, finite element/finite difference formulations, computational algorithms, implementations on advanced computer architectures, and visualization capabilities. The results of our activities during the first year within the SIMSTRUC project are reported. On the modeling side, an alternative approach to fluid/thermal/structure interaction analysis that is a departure from the 'loosely coupled' and 'unified' approaches that are being currently practiced are described. The advantages of our approach both in terms of accuracy and computational efficiency were demonstrated. On the computational side, a software architecture for parallel/vector and massively parallel supercomputers that speeds up finite element and finite difference computations by several orders of magnitude is presented. As an example, the simulation of the deployment of a space structure that used to require over six hours of a workstation using a conventional finite element software, now runs on a multiprocessor using a parallel computation strategy in less than three seconds. In order to promote the physical understanding of the simulation behavior, a real-time visualization capability on the Connection Machine, which allows the analyst to watch the graphical animation of the results at the same time these are generated, was also developed. It is believed that by combining efficient analytical formulations with the state-of-the-art high performance computer implementations and superfast visualization capabilities, SIMSTRUC is moving fast towards the real-time simulation of large space structures. The designers as well as the researchers will certainly benefit from this technology.

Farhat, C.↗