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NASA NTRS1988

Boundary shape identification problems in two-dimensional domains related to thermal testing of materials

The identification of the geometrical structure of the system boundary for a two-dimensional diffusion system is reported. The domain identification problem treated here is converted into an optimization problem based on a fit-to-data criterion and theoretical convergence results for approximate identification techniques are discussed. Results of numerical experiments to demonstrate the efficacy of the theoretical ideas are reported.

Banks, H. T.↗

NASA NTRS1987

Effects of intensity modulations on the power spectra of random processes

Intensity-modulated random processes (IMRPs), defined as the products of (1) deterministic modulating functions or processes and (2) stationary modulated processes statistically independent of (1), are investigated analytically. Instantaneous power spectra are derived for IMRPs with different classes of (1), and a spectrum series expansion with a known locally stationary approximation as its first term is obtained. Numerical results for sample IMRPs with known (deterministic), stationary, nonstationary, ergodic, onset, and bell-shaped types of (1) are presented graphically and briefly characterized.

Mark, W. D.↗

NASA NTRS1987

Approximation techniques for domain identification in two-dimensional parabolic systems under boundary observations

The identification of the geometrical structure of the system boundary for a two-dimensional diffusion system is considered. The domain identification treated is converted into an optimization problem based on a fit-to-data criterion and theoretical convergence results for approximate identification techniques are discussed. Results of numerical experiments to demonstrate the efficiency of the theoretical ideas are reported.

Banks, H. T.↗

NASA NTRS1988

Numerical prediction of flow in slender vortices

The slender vortex approximation was investigated using the Navier-Stokes equations written in cylindrical coordinates. It is shown that, for free vortices without external pressure gradient, the breakdown length is proportional to the Reynolds number. For free vortices with adverse pressure gradients, the breakdown length is inversely proportional to the value of its gradient. For low Reynolds numbers, the predictions of the simplified system agreed well with the ones obtained from solutions of the full Navier-Stokes equations, whereas for high Reynolds numbers, the flow became quite sensitive to pressure fluctuations; it was found that the failure of the slender vortex equations corresponded to the critical condition as identified by Benjamin (1962) for inviscid flows. The predictions obtained from the approximating system were compared with available experimental results. For low swirl, a good agreement was obtained; for high swirl, on the other hand, upstream effects on the pressure gradient produced by the breakdown bubble caused poor agreement.

Reyna, Luis G.↗

NASA NTRS1988

A 20-DOF hybrid stress general shell element

A hybrid-stress general shell element is developed based on the Hellinger-Reissner principle modified for relaxed element compatibility conditions. The element is based on a thin-shell theory with Love-Kirchhoff hypothesis. It is of quadrilateral shape with only four corner nodes and five degrees of freedom per node. The geometry of the element is approximated through a cubic polynomial surface patch. Numerical examples consisting of torsion-loaded slit cylinder and pinched cylinders with open ends and rigid diaphragmed ends demonstrate excellent performance of the present element.

Kang, David S.↗

NASA NTRS1989

Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes

Production of polycyclic aromatic hydrocarbons in carbon-rich circumstellar envelopes was investigated using a kinetic approach. A detailed chemical reaction mechanism of gas-phase PAH formation and growth, containing approximately 100 reactions of 40 species, was numerically solved under the physical conditions expected in cool stellar winds. The chemistry is based on studies of soot production in hydrocarbon pyrolysis and combustion. Several first-ring and second-ring cyclization processes were considered. A linear lumping algorithm was used to describe PAH growth beyond the second aromatic ring. PAH production using this mechanism was examined with respect to a grid of idealized constant velocity stellar winds as well as several published astrophysical models. The basic result is that the onset of PAH production in the interstellar envelopes is predicted to occur within the temperature interval of 1100 to 900 K. The absolute amounts of the PAHs formed, however, are very sensitive to a number of parameters, both chemical and astrophysical, whose values are not accurately known. Astrophysically meaningful quantities of PAHs require particularly dense and slow stellar winds and high initial acetylene abundance. It is suggested that most of the PAHs may be produced in a relatively small fraction of carbon-rich red giants.

Frenklach, Michael↗

NASA NTRS1989

Flow of rarefied gases over two-dimensional bodies

A kinetic-theory analysis is made of the flow of rarefied gases over two-dimensional bodies of arbitrary curvature. The Boltzmann equation simplified by a model collision integral is written in an arbitrary orthogonal curvilinear coordinate system, and solved by means of finite-difference approximation with the discrete ordinate method. A numerical code is developed which can be applied to any two-dimensional submerged body of arbitrary curvature for the flow regimes from free-molecular to slip at transonic Mach numbers. Predictions are made for the case of a right circular cylinder.

Jeng, Duen-Ren↗

NASA NTRS1989

Boundary shape identification problems in two-dimensional domains related to thermal testing of materials

The identification of the geometrical structure of the system boundary for a two-dimensional diffusion system is reported. The domain identification problem treated here is converted into an optimization problem based on a fit-to-data criterion and theoretical convergence results for approximate identification techniques are discussed. Ressults of numerical experiments to demonstrate the efficacy of the theoretical ideas are reported.

Banks, H. T.↗

NASA NTRS1989

The stability of a compressible stratified shear layer

The stability of a shear layer under the effect of gravity is investigated using the compressible magnetohydrodynamic (MHD) equations, including an effective gravity term to represent the curvature effects of the flow and magnetic field line geometry. A general eigenmode equation is derived for a two-dimensional MHD fluid, and an energy-principle analysis to explain the effect of compressibility on the critical Richardson number is presented. For the case of a hyperbolic tangent shear flow and exponential density profile, it was found that, in the Boussinesq approximation, the compressibility raises the critical Richardson number from 1/4 to as much as 1/2, with the exact value depending on the value of the magnetic field at infinity. Under approximation of a strong asymptotic magnetic field, without invoking the Boussinesq approximation, it is shown both analytically and numerically that the density gradient terms cause the shear instability to be dispersive. The long-wavelength stability boundary for the Richardson number J = 0 is characterized by a normalized phase velocity c =

Wang, Z.↗

NASA NTRS1990

Critical evaluation of Jet-A spray combustion using propane chemical kinetics in gas turbine combustion simulated by KIVA-2

Jet-A spray combustion has been evaluated in gas turbine combustion with the use of propane chemical kinetics as the first approximation for the chemical reactions. Here, the numerical solutions are obtained by using the KIVA-2 computer code. The KIVA-2 code is the most developed of the available multidimensional combustion computer programs for application of the in-cylinder combustion dynamics of internal combustion engines. The released version of KIVA-2 assumes that 12 chemical species are present; the code uses an Arrhenius kinetic-controlled combustion model governed by a four-step global chemical reaction and six equilibrium reactions. Researchers efforts involve the addition of Jet-A thermophysical properties and the implementation of detailed reaction mechanisms for propane oxidation. Three different detailed reaction mechanism models are considered. The first model consists of 131 reactions and 45 species. This is considered as the full mechanism which is developed through the study of chemical kinetics of propane combustion in an enclosed chamber. The full mechanism is evaluated by comparing calculated ignition delay times with available shock tube data. However, these detailed reactions occupy too much computer memory and CPU time for the computation. Therefore, it only serves as a benchmark case by which to evaluate other simplified models. Two possible simplified models were tested in the existing computer code KIVA-2 for the same conditions as used with the full mechanism. One model is obtained through a sensitivity analysis using LSENS, the general kinetics and sensitivity analysis program code of D. A. Bittker and K. Radhakrishnan. This model consists of 45 chemical reactions and 27 species. The other model is based on the work published by C. K. Westbrook and F. L. Dryer.

Nguyen, H. L.↗

NASA NTRS1989

Review of a code development and calibration program in support of the aeroassist flight experiment

The code development and calibration program for the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) is reviewed, with emphasis directed toward support of the Aeroassist Flight Experiment (AFE). The flight project is designed to obtain data which will be used in the validation of computational fluid dynamic approximation methods. Comparisons between experimental data and numerical simulation focus on perfect-gas tests over a scale model of the AFE and on flight and ground tests which challenge some aspect of the thermochemical nonequilibrium model. In the first case, the gas model is simple, but the grid-related problems of defining the real vehicle are present. In the second case, the vehicle geometries are simple, but thermochemical processes must be modeled correctly in order to compare with the experimental data. These comparisons are described as calibration runs because they test elements of the numerical simulation, but no single data set adequately simulates the full-scale AFE flight conditions. Comparisons between computation and experiment over a broad range of data sets show generally good agreement, though some aspects of the numerical model require further development.

Gnoffo, Peter A.↗

NASA NTRS1991

Effect of splitter plate on unsteady flows around a body of revolution at incidence

Numerical solutions of the thin-layer approximation of the 3D Navier-Stokes equations are presented for flows around an ogive-cylinder body with and without a splitter plate. It is suggested that the presence of the splitter plate prevents the interaction between flows on either side of the symmetry plane. It is concluded that, as a result of the enforced symmetry, the antisymmetric mode of the convective instability near the apex of the body cannot be excited and, therefore, the vortices remain symmetric, staying low and parallel to the upper body surface. The antisymmetric mode of the absolute instability mechanism cannot be initiated, which suppresses the alternate shedding of vortices from the cylindrical portion of the body. High-frequency fluctuations of the shear layer are found to remain virtually unaffected by the presence of the splitter plate.

Degani, David↗

NASA NTRS1992

Trajectory And Heating Of A Hypervelocity Projectile

Technical paper presents derivation of approximate, closed-form equation for relationship between velocity of projectile and density of atmosphere. Results of calculations based on approximate equation agree well with results from numerical integrations of exact equations of motion. Comparisons of results presented in series of graphs.

Tauber, Michael E.↗

NASA NTRS1993

Program Computes Flows Of Fluids And Heat

SINDA'85/FLUINT incorporates lumped-parameter-network and one-dimensional-flow mathematical models. System enables analysis of mutual influences of thermal and flow phenomena. Offers two finite-difference numerical solution techniques: forward-difference explicit approximation and Crank-Nicholson approximation. Enables simulation of nonuniform heating and facilitates mathematical modeling of thin-walled heat exchangers. Ability to model nonequilibrium behavior within two-phase volumes included. Recent changes in program improve modeling of real evaporator pumps and other capillary-assist evaporators. Written in FORTRAN 77.

Cullimore, Brent↗

NASA NTRS1993

Numerical simulations of a pulsed detonation wave augmentation device

We present here the concept of a hybrid engine for Single Stage To Orbit (SSTO) air-breathing hypersonic vehicle. This concept relies on the use of pulsed detonation waves, both for thrust generation and mixing/combustion augmentation. We describe the principles behind the engine concept, which we call the Pulsed Detonation Wave Augmentor (PDWA). We demonstrate the principles of operation for two possible configurations through numerical simulations. We also attempt a first approximation to engine design, and propose various applications.

Cambier, Jean-Luc↗

NASA NTRS1993

Emplacement of multiple flow units on very shallow slopes, east Kawelu Planitia flow field, Venus

Lobate flows with variable radar brightness occur at several locations on Venus. Here we present preliminary mapping results for a lava flow field on the eastern side of Kawelu Planitia, centered at approximately 41 deg N, 251 deg E. Numerous individual flows of varying brightness can be traced for up to 400 km at this location. Megallan image F-MIDRP.40N251;1 contains two major flow complexes as well as a distinct minor flow field associated with a volcanic dome. The southern flow field (unit F) changes from radar dark to radar bright in a convex arc from west to east. The flow path is diverted by a ridge system (unit B) and associated N-S faulted zone (unit A) evident in the Magellan altimetry data. The 130-km-long radar-bright flow field located at 41.5 deg N and 249-250 deg E is comprised of a narrow, channeled flow (unit E) overlying an intermediate brightness flow (unit D), all of which appears to emanate from a solitary dome at 249 deg E. The southern lobe of unit E disappears under a broad leveed channel (41.3 deg N, 250.1 deg E) in unit 5, only to reappear south of the unit 5 boundary. The source of this largest flow field is not evident, but could be associated with the region of small domes west of the F-MIDRP image (centered at 41 deg N, 246 deg E). Another possible source is the fractured area south of unit C. The flow complex is clearly a conglomeration of many flow episodes, not necessarily from a single source.

Helgerud, Michael B.↗

NASA NTRS1993

An application of small-gap equations in sealing devices

The study of a thin, incompressible Newtonian fluid layer trapped between two almost parallel, sliding surfaces has been actively pursued in the last decades. This subject includes lubrication applications such as slider bearings or the sealing of non-pressurized fluids with rubber rotary shaft seals. In the present work we analyze numerically the flow of lubricant fluid through a micro-gap of sealing devices. The first stage of this study is carried out assuming that a 'small-gap' parameter delta attains an extreme value in the Navier-Stokes equations. The precise meaning of small-gap is achieved by the particular limit delta = 0 which, within the bounds of the hypotheses, predicts transport of lubricant through the sealed area by centrifugal instabilities. Numerical results obtained with the penalty function approximation in the finite element method are presented. In particular, the influence of inflow and outflow boundary conditions, and their impact in the simulated flow, are discussed.

Vionnet, Carlos A.↗

NASA NTRS1995

Crustal flushing and its relationship to magnetic and hydrothermal processes on the East Pacific Rise crest

The deep-towed Argo I optical/acoustical vehicle and a geographic information system (GIS) have been used to establish the abundance, widths, and spatial distribution of fissures, as well as the relative age distribution of lavas along the narrow (less than 500 m wide) axial zone of the East Pacific Rise (EPR) from 9 deg 12 min to 9 deg 54 min N. On a second-order scale (approximately 78 km long), wider but less numerous fissures are found in the northern portion of the survey area; this changes to narrower, more abundant fissures in the south. A profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike (near 9 deg 49 min, 9 deg 35 min, and 9 deg 15 min N), where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle, filling and covering older fissures. On a fourth-order scale (5-15 km long) the mean density of fissuring on a given segment is greater where relative axial lava age is greater. Fissure density also correlates with hydrothermal vent abundance and type. Increased cracking toward segment tips is observed at the second-order scale, whereas fourth-order segments tend to be more cracked in the middle. Cracking on a fourth-order scale may be driven by the propagation of dikes, rather than by the far-field plate stresses. The above relations constrain the model of Haymon et al. (1991) in which individual fourth-order segments are in different phases of a volcanic-hydrothermal-tectonic cycle.

Wright, Dawn J.↗