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NASA NTRS1988

Mass spectrometric studies of trimethylindium pyrolysis

The kinetics of the pyrolysis of trimethylindium (TMIn) in He, D2, and H2 carriers was investigated using the atmospheric pressure flow-tube apparatus described by Larsen et al. (1987) and a time-of-flight mass spectrometer. The rate constant for the pyrolysis of TMIn in He was found to be comparable to that found by Jacko and Price (1964) for TMIn in toluene carrier (a radical scavenger), indicating that TMIn decomposes in He not by radical attack of methyl groups, but by homolytic fission. The decomposition of TMIn is enhanced in D2 and H2 carriers, where the principal products are CH3D and C2H6, and CH4 and C2H6, respectively, indicating that the reaction pathway in these carriers is different from those in He and toluene. The pyrolysis in H2 and D2 is attributed to a radical attack by H or D on TMIn. A reaction mechanism involving a short-lived hypervalent DTMIn species was proposed and was tested using numerical modeling techniques.

Buchan, N. I.↗

NASA NTRS1989

On the active control of wall-bounded turbulent flows

The direct numerical simulation technique is used to explore concepts for manipulation of turbulent boundary layers. Significant drag reduction was achieved when at each instant the normal component of velocity at the wall was prescribed to be 180 deg out of phase with the normal velocity slightly above the wall. The drag reduction is accompanied with significant reduction in the intensity of the wall-layer structures and reductions in the magnitude of Reynolds stresses throughout the flow. Suitability of wall pressure and shear stress fluctuations for detection of flow structures above the wall are examined.

Moin, P.↗

NASA NTRS1988

Bipolar coordinates for computation of transition duct flows

Numerical simulation techniques for flows in jet-engine transition ducts with changing cross sections are developed and demonstrated. The boundary-conforming grids required are generated using bipolar coordinates, and the singularities arising at the no-slip boundaries are treated as described by de Vahl Davis (1979) and Tsai and Levy (1987). Grids for several typical configurations are employed in computations with the three-dimensional laminar/turbulent viscous-flow solver of Levy et al. (1983), and the results are presented graphically. The scheme used to deal with singularities is shown to be robust, suggesting that the bipolar grids may be applicable to Navier-Stokes computations.

Madabhushi, R. K.↗

NASA NTRS1988

Multiple directed graph large-class multi-spectral processor

Numerical analysis techniques for the interpretation of high-resolution imaging-spectrometer data are described and demonstrated. The method proposed involves the use of (1) a hierarchical classifier with a tree structure generated automatically by a Fisher linear-discriminant-function algorithm and (2) a novel multiple-directed-graph scheme which reduces the local maxima and the number of perturbations required. Results for a 500-class test problem involving simulated imaging-spectrometer data are presented in tables and graphs; 100-percent-correct classification is achieved with an improvement factor of 5.

Casasent, David↗

NASA NTRS1989

Computational fluid dynamics - A personal view

This paper provides a personal view of computational fluid dynamics. The main theme is divided into two categories - one dealing with algorithms and engineering applications and the other with scientific investigations. The former category may be termed computational aerodynamics, with the objective of providing reliable aerodynamic or engineering predictions. The latter category is essentially basic research, where the algorithmic tools are used to unravel and elucidate fluid-dynamic phenomena hard to obtain in a laboratory. A critique of the numerical solution techniques for both compressible and incompressible flows is included. The discussion on scientific investigations deals in particular with transition and turbulence.

Hussaini, M. Y.↗

NASA NTRS1989

Efficient computer algebra algorithms for polynomial matrices in control design

The theory of polynomial matrices plays a key role in the design and analysis of multi-input multi-output control and communications systems using frequency domain methods. Examples include coprime factorizations of transfer functions, cannonical realizations from matrix fraction descriptions, and the transfer function design of feedback compensators. Typically, such problems abstract in a natural way to the need to solve systems of Diophantine equations or systems of linear equations over polynomials. These and other problems involving polynomial matrices can in turn be reduced to polynomial matrix triangularization procedures, a result which is not surprising given the importance of matrix triangularization techniques in numerical linear algebra. Matrices with entries from a field and Gaussian elimination play a fundamental role in understanding the triangularization process. In the case of polynomial matrices, matrices with entries from a ring for which Gaussian elimination is not defined and triangularization is accomplished by what is quite properly called Euclidean elimination. Unfortunately, the numerical stability and sensitivity issues which accompany floating point approaches to Euclidean elimination are not very well understood. New algorithms are presented which circumvent entirely such numerical issues through the use of exact, symbolic methods in computer algebra. The use of such error-free algorithms guarantees that the results are accurate to within the precision of the model data--the best that can be hoped for. Care must be taken in the design of such algorithms due to the phenomenon of intermediate expressions swell.

Baras, J. S.↗

NASA NTRS1989

Measurement of terms and parameters in turbulent models

Experimental measurements of the mean and turbulent velocity field in a water flow, turn-around-duct is documented. The small radius of curvature duct experiments were made over a range of Reynolds numbers (based on a duct height of 10 cm) from 70,000 to 500,000. For this particular channel, the flow is dominated by the inertia forces. Use of the local bulk velocity to non-dimensionalize the local velocity was found to limit Reynolds number effects to the regions very close to the wall. Only secondary effects on the flow field were observed when the inlet or exit boundary conditions were altered. The flow over the central two-thirds of the channel was two-dimensional. Mean tangetial and radial velocities, streamlines, pressure distributions, surface shear stress; tangential, radial and lateral turbulent velocities and the Reynolds turbulent shear values are tabulated in other reports. It is evident from the experimental study that a complex numerical modeling technique must be developed to predict the flow in the turn-around-duct. The model must be able to predict relaminarization along the inner-convex-wall. It must also allow for the major increase in turbulence produced by the outer-concave-wall.

Sandborn, Virgil A.↗

NASA NTRS1990

Estimation of multidimensional precipitation parameters by areal estimates of oceanic rainfall

The parameters of the multidimensional precipitation model proposed by Waymire et al. (1984) are estimated using the areal-averaged radar measurements of precipitation of the Global Atlantic Tropical Experiment (GATE) data set. The procedure followed was the fitting of the first- and second-order moments at different aggregation scales by nonlinear regression techniques. The numerical estimates of the parameters using different subsets of GATE information were reasonably stable, i.e., they were not affected by changes of the area-averaging size, temporal length of the records, and percentage of areal coverage of rainfall. This suggests that the estimation procedure is relatively robust and suitable to estimate the parameters of the multidimensional model in areas of sparse density of rain gages. The use of the space-time spectrum of rainfall to help in the determination of sampling errors due to intermittent visits of future space-borne low-altitude sensors of precipitation is also discussed.

Valdes, J. B.↗

NASA NTRS1990

Optimal placement of actuators and sensors in control augmented structural optimization

A control-augmented structural synthesis methodology is presented in which actuator and sensor placement is treated in terms of (0,1) variables. Structural member sizes and control variables are treated simultaneously as design variables. A multiobjective utopian approach is used to obtain a compromise solution for inherently conflicting objective functions such as strucutal mass control effort and number of actuators. Constraints are imposed on transient displacements, natural frequencies, actuator forces and dynamic stability as well as controllability and observability of the system. The combinatorial aspects of the mixed - (0,1) continuous variable design optimization problem are made tractable by combining approximation concepts with branch and bound techniques. Some numerical results for example problems are presented to illustrate the efficacy of the design procedure set forth.

Sepulveda, A. E.↗

NASA NTRS1990

Recurrence in truncated Boussinesq models for nonlinear waves in shallow water

The rapid spatial recurrence of weakly nonlinear and weakly dispersive progressive shallow-water waves is examined using a numerical integration technique on the discretized and truncated form of the Boussinesq equations. This study primarily examines recurrence in wave fields with Ursell number O(1) and characterizes the sensitivity of recurrence to initial spectral shape and number of allowed frequency modes. It is shown that the rapid spatial recurrence is not an inherent property of the considered Boussinesq systems for evolution distances of 10-50 wavelengths. The main result of the study is that highly truncated Boussinesq models of resonant shallow-water ocean surface gravity waves predict rapid multiple recurrence cycles, but that this is an artifact dependent on the number of allowed modes. For initial conditions consisting of essentially all energy concentrated in a single mode, damping of the recurrence cycles increases as the number of low-power background modes increases. When more than 32 modes are allowed, the recurrence behavior is relatively insensitive to the number of allowed modes.

Elgar, Steve↗

NASA NTRS1989

Computational fluid dynamics at the Lewis Research Center: An overview

Lewis is a multidisciplinary Center with strong research and development programs in aeronautical and space propulsion, power, space communications, space experiments and materials. Computational fluid dynamics (CFD) is playing an important and growing role in most of these areas. Described here is how CFD is integrated into these programs and highlights elements of the CFD activities. Examples are presented of codes developed to predict flow fields in advanced propulsion systems and several of the code validation experiments are described. The CFD effort at Lewis ranges from basic research on new and improved algorithms through code development to the application of these codes to specific engineering problems. Because of the substantial improvement in CFD's predictive capability, its use at Lewis is on a steep growth path, spreading rapidly into new areas which had not traditionally taken advantage of the techniques of numerical simulation. Multidisciplinary codes and the future direction of CFD at Lewis are discussed.

Stubbs, Robert M.↗

NASA NTRS1991

Coherent motions in the turbulent boundary layer

The role of coherent structures in the production and dissipation of turbulence in a boundary layer is characterized, summarizing the results of recent investigations. Coherent motion is defined as a three-dimensional region of flow where at least one fundamental variable exhibits significant correlation with itself or with another variable over a space or time range significantly larger than the smallest local scales of the flow. Sections are then devoted to flow-visualization experiments, statistical analyses, numerical simulation techniques, the history of coherent-structure studies, vortices and vortical structures, conceptual models, and predictive models. Diagrams and graphs are provided.

Robinson, Stephen K.↗

NASA NTRS1990

Hypersonic waverider design from given shock waves

An attempt is made to generate waverider flows from given shock wave geometries using two approaches. In the first approach, axisymmetric flows are used to construct more general flows based on the concept of osculating cones. Conical waverider design can thus be extended to yield results also for shocks forming a slope surface. The second approach involves solving the ill-posed problem of prescribing a shock wave and finding the flow field behind it. A new numerical marching technique with some features of characteristic cross marching is used to solve the Euler equation. The code selection and some test cases are discussed.

Sobieczky, H.↗

NASA NTRS1991

Non-linear dynamics of a geared rotor-bearing system with multiple clearances

The nonlinear frequency response characteristics of a geared rotor-bearing system are examined. A three-DOF dynamic model is developed which includes nonlinearities associated with radial clearances in the radial rolling element bearings and backlash between a spur gear pair; linear time-invariant gear meshing stiffness is assumed. The corresponding linear system problem is also solved, and predicted natural frequencies and modes match with finite element method results. The applicability of both analytical and numerical solution techniques to the multi-DOF nonlinear problem is investigated. Satisfactory agreement is found between the theory presented here and the available experimental data. Nonlinear modal interactions and differences between internal static transmission error excitation and external torque excitation are discussed. In addition, parametric studies are performed to understand the effect of system parameters such as bearing stiffness to gear mesh stiffness ratio, alternating to mean force ratio and radial bearing preload to mean force ratio on the nonlinear dynamic behavior.

Kahraman, A.↗

NASA NTRS1990

Active turbulence control in wallbounded flow using direct numerical simulations

An exploratory study of concepts for active control of turbulent boundary layers using the direct numerical simulation technique was performed. Significant drag reduction was achieved when the surface boundary condition was modified such that it could suppress the large-scale structures present in the wall region. This was achieved by prescribing the normal component of velocity at the wall to be 180 deg out of phase with the normal velocity slightly above the wall at each instant. The drag reduction was accompanied with significant reduction in the intensity of the wall-layer structures and reductions in the magnitude of Reynolds stresses throughout the flow. Suitability of wall-pressure and shear-stress fluctuations for detection of flow structures above the wall was examined. A preliminary result obtained by applying the present control strategy to a transitional flow is also briefly described, from which one can infer a possible linkage between the control strategy and flow stability.

Kim, J.↗

NASA NTRS1990

Two-color holography concept (T-CHI)

The Material Processing in the Space Program of NASA-MSFC was active in developing numerous optical techniques for the characterization of fluids in the vicinity of various materials during crystallization and/or solidification. Two-color holographic interferometry demonstrates that temperature and concentration separation in transparent (T-CHI) model systems is possible. The experiments were performed for particular (succinonitrile) systems. Several solutions are possible in Microgravity Sciences and Applications (MSA) experiments on future Shuttle missions. The theory of the T-CHI concept is evaluated. Although particular cases are used for explanations, the concepts developed will be universal. A breadboard system design is also presented for ultimate fabrication and testing of theoretical findings. New developments in holography involving optical fibers and diode lasers are also incorporated.

Vikram, C. S.↗

NASA NTRS1991

Design of 3-dimensional complex airplane configurations with specified pressure distribution via optimization

A subcritical panel method applied to flow analysis and aerodynamic design of complex aircraft configurations is presented. The analysis method is based on linearized, compressible, subsonic flow equations and indirect Dirichlet boundary conditions. Quadratic dipol and linear source distribution on flat panels are applied. In the case of aerodynamic design, the geometry which minimizes differences between design and actual pressure distribution is found iteratively, using numerical optimization technique. Geometry modifications are modeled by surface transpiration concept. Constraints in respect to resulting geometry can be specified. A number of complex 3-dimensional design examples are presented. The software is adopted to personal computers, and as result an unexpected low cost of computations is obtained.

Kubrynski, Krzysztof↗

NASA NTRS1991

Programmable digital modem

The design of the Programmable Digital Modem (PDM) is outlined. The PDM will be capable of operating with numerous modulation techniques including: 2-, 4-, 8- and 16-ary phase shift keying (PSK), minimum shift keying (MSK), and 16-ary quadrature amplitude modulation (QAM), with spectral occupancy from 1.2x to 2x the data symbol rate. It will also be programmable for transmission rates ranging from 2.34 to 300 Mbit/s, where the maximum symbol rate is 75 Msymbol/s. Furthermore, these parameters will be executable in independent burst, dependent burst, or continuous mode. In dependent burst mode the carrier and clock oscillator sources are common from burst to burst. To achieve as broad a set of requirements as these, it is clear that the essential signal processing must be digital. In addition, to avoid hardware changes when the operational parameters are changed, a fixed interface to an analog intermediate frequency (IF) is necessary for transmission; and, common system level architectures are necessary for the modulator and demodulator. Lastly, to minimize size and power, as much of the design as possible will be implemented with application specific integrated circuit (ASIC) chips.

Poklemba, John J.↗