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At least 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Chemical nonequilibrium effects on the flow in the windward plane of symmetry of a blunted delta orbiter

The extent, and some of the consequences, of nonequilibrium effects in the inviscid orbiter flow field are examined. Calculations have been made for the reacting flow along selected streamlines in the windward plane of symmetry of a blunted delta shape representative of a shuttle orbiter. An approximate model of the pressure field in the windward plane of symmetry of a blunted delta wing was constructed from wind tunnel measurements of the body surface pressure distribution and approximate calculations of the shock shape and streamline locations. Numerical solutions for the flow along streamlines were obtained for this pressure distribution, and solutions were obtained for both equilibrium and finite-reaction rate, nonequilibrium chemical reactions. Since the streamtube solutions are much easier to compute than fully three-dimensional, reacting flows, results were obtained for a wide range of conditions. The validity of this approach is based on the assumptions that the pressure field is insensitive to nonequilibrium effects and that the flow along streamlines is quasi-one dimensional. Numerical solutions have been used to verify this approach for other blunted slender shapes such as spherically blunted cones.

Lordi, J. A.↗
NASA NTRS1972

Heat transfer distribution on a discontinuously convective cooled surface

An active cooling scheme in which discrete cooling passages are used was investigated. In this scheme, the coolant passages are placed perpendicular to the main stream direction. Streamwise temperature oscillations result from the highly cooled regions, near the coolant channels, being next to regions not directly cooled between the channels. The accuracy of the heat transfer rate predictions by these approximations was evaluated by means of both experimental and numerical investigations. Experiments were conducted at free stream Mach number of 6 with several cooling passages. Heat transfer measurements were made and corresponding theoretical analysis were performed for comparison in evaluating both local and overall deviations in the heat transfer rate.

Piva, R.↗
NASA NTRS1972

Investigation of Some Characteristics Related to PCM Thermal Capacitors

In order to provide greater flexibility, shorter computer time, and a parametric study, attention was devoted to an implicit numerical scheme. The experimental work involved outlining and monitoring approximately fourteen tests of an invertable capacitor. Surface and PCM temperatures were measured at several locations and photographs were made of the PCM during melting and solidification. Some typical experimental results are presented and discussed.

Griggs, E. I.↗
NASA NTRS1972

Thermal instability of the hydrogen-burning shell in nondegenerate stars.

An investigation is made of thermal instability in the hydrogen-burning shell of stars of moderate to high mass evolving from the end of core hydrogen burning to the early stages of core helium burning, with the help of an approximate analytic criterion for thermal instability and full nonlinear numerical calculations of stellar evolution. Two different assumptions about the chemical evolution in the stars of highest mass are adopted. Thermal pulses are found to develop only in very massive stars having convective intermediate zones, when the hydrogen profile in the shell is sufficiently steep and the shell is still moderately thick.

Stothers, R.↗
NASA NTRS1974

Analytical and experimental study of mean flow and turbulence characteristics inside the passages of an axial flow inducer

The effort conducted to gather additional understanding of the complex inviscid and viscid effects existing within the passages of a three-bladed axial flow inducer operating at a flow coefficient of 0.065 is summarized. The experimental investigations included determination of the blade static pressure and blade limiting streamline angle distributions, and measurement of the three components of mean velocity, turbulence intensities and turbulence stresses at locations inside the inducer blade passage utilizing a rotating three-sensor hotwire probe. Applicable equations were derived for the hotwire data reduction analysis and solved numerically to obtain the appropriate flow parameters. Analytical investigations were conducted to predict the three-dimensional inviscid flow in the inducer by numerically solving the exact equations of motion, and to approximately predict the three-dimensional viscid flow by incorporating the dominant viscous terms into the exact equations. The analytical results are compared with the experimental measurements and design values where appropriate.

Gorton, C. A.↗
NASA NTRS1975

Non-linear resonances in the forced responses of plates. I - Symmetric responses of circular plates

The dynamic analogue of the von Karman equations is used to study the symmetric response of a circular plate to a harmonic excitation when the frequency of the excitation is near one of the natural frequencies. It is shown that, in general, when there is no internal resonance (i.e., the natural frequencies are not commensurable), only the mode having a frequency near that of the excitation is strongly excited (i.e., is needed to represent the response in the first approximation). A clamped, circular plate is used as a numerical example to show that, when there is an internal resonance, more than one of the modes involved in this resonance can be strongly excited; moreover, when more than one mode is strongly excited, the lower modes can dominate the response, even when the frequency of the excitation is near that of the highest mode. This possibility was not revealed by any of the earlier studies which were based on the same governing equations.

Sridhar, S.↗
NASA NTRS1982

Stability of Goertler vortices in boundary layers

A formal analysis of Goertler-type instability is presented. The boundary-layer and disturbance equations are formulated in a general, orthogonal, curvilinear system of coordinates constructed from the inviscid flow over a curved surface. Effects of curvature on the boundary-layer flow are analyzed. The basic approximation for the disturbance equations is presented and solved numerically. Previous analyses are discussed and compared with our analysis. It is shown that the general system of coordinates developed in this analysis and the correct order-of-magnitude analysis of the disturbance velocities with two velocity scales leads to a rational foundation for future work in Goertler vortices.

Floryan, J. M.↗
NASA NTRS1982

Detonation propulsion experiments and theory

Test data are presented for the use of a single detonation of explosives in long-cone, short-cone, straight, and firing-plug nozzles to provide propulsion in a simulated Jupiter atmosphere, as well as the ambient gases N, CO2 and He. The long-cone nozzle yielded a progressive increase with ambient pressure for the higher molecular weight gases CO2 and N, while the lower molecular weight He and simulated Jupiter atmosphere showed a specific pulse decrease with increasing ambient pressure. The short-plug nozzle yielded a small specific impulse reduction with increasing ambient pressure, and its results were found to be nearly independent of ambient gas molecular weight. All data gathered are analyzed by using first principles, approximate blast wave theory predictions, and two-dimensional numerical calculations. Rarefaction and oscillatory wave phenomena are found to significantly influence specific impulse.

Back, L. H.↗
NASA NTRS1983

An experimental and numerical investigation of the impingement of an oblique shock wave on a body of revolution

Attention is given to the experimental study and numerical simulation of the impingement of oblique shock wave on a cylinder, in order both to document the complex three-dimensional shock wave and boundary layer interaction occurring in practical problems (such as stores carriage interference in a supersonic tactical aircraft) and to conduct a critical comparison of experimental measurements and numerical computations for such complex flows. A thin layer approximation of the Navier-Stokes equations was solved by means of a mixed explicit-implicit scheme. Experimental measurements reveal a highly complex flow field with two distinct adjacent separation zones, regions of high cross flow, and multiply reflected shocks and expansion fans.

Brosh, A.↗
NASA NTRS1982

Solution of 3-dimensional time-dependent viscous flows. Part 3: Application to turbulent and unsteady flows

A numerical scheme is developed for solving the time dependent, three dimensional compressible viscous flow equations to be used as an aid in the design of helicopter rotors. In order to further investigate the numerical procedure, the computer code developed to solve an approximate form of the three dimensional unsteady Navier-Stokes equations employing a linearized block implicit technique in conjunction with a QR operator scheme is tested. Results of calculations are presented for several two dimensional boundary layer flows including steady turbulent and unsteady laminar cases. A comparison of fourth order and second order solutions indicate that increased accuracy can be obtained without any significant increases in cost (run time). The results of the computations also indicate that the computer code can be applied to more complex flows such as those encountered on rotating airfoils. The geometry of a symmetric NACA four digit airfoil is considered and the appropriate geometrical properties are computed.

Weinberg, B. C.↗
NASA NTRS1984

Impact of computers on aerodynamics research and development

Factors motivating the development of computational aerodynamics as a discipline are traced back to the limitations of the tools available to the aerodynamicist before the development of digital computers. Governing equations in exact and approximate forms are discussed together with approaches to their numerical solution. Example results obtained from the successively refined forms of the equations are presented and discussed, both in the context of levels of computer power required and the degree of the effect that their solution has on aerodynamic research and development. Factors pacing advances in computational aerodynamics are identified, including the amount of computational power required to take the next major step in the discipline. Finally, the Numerical Aerodynamic Simulation (NAS) Program - with its 1987 target of achieving a sustained computational rate of 1 billion floating-point operations per second operating on a memory of 240 million words - is briefly discussed in terms of its projected effect on the future of computational aerodynamics.

Peterson, V. L.↗
NASA NTRS1985

Application of closed-form solutions to a mesh point field in silicon solar cells

A computer simulation method is discussed that provides for equivalent simulation accuracy, but that exhibits significantly lower CPU running time per bias point compared to other techniques. This new method is applied to a mesh point field as is customary in numerical integration (NI) techniques. The assumption of a linear approximation for the dependent variable, which is typically used in the finite difference and finite element NI methods, is not required. Instead, the set of device transport equations is applied to, and the closed-form solutions obtained for, each mesh point. The mesh point field is generated so that the coefficients in the set of transport equations exhibit small changes between adjacent mesh points. Application of this method to high-efficiency silicon solar cells is described; and the method by which Auger recombination, ambipolar considerations, built-in and induced electric fields, bandgap narrowing, carrier confinement, and carrier diffusivities are treated. Bandgap narrowing has been investigated using Fermi-Dirac statistics, and these results show that bandgap narrowing is more pronounced and that it is temperature-dependent in contrast to the results based on Boltzmann statistics.

Lamorte, M. F.↗
NASA NTRS1986

Stability analysis of spectral methods for hyperbolic initial-boundary value systems

A constant coefficient hyperbolic system in one space variable, with zero initial data is discussed. Dissipative boundary conditions are imposed at the two points x = + or - 1. This problem is discretized by a spectral approximation in space. Sufficient conditions under which the spectral numerical solution is stable are demonstrated - moreover, these conditions have to be checked only for scalar equations. The stability theorems take the form of explicit bounds for the norm of the solution in terms of the boundary data. The dependence of these bounds on N, the number of points in the domain (or equivalently the degree of the polynomials involved), is investigated for a class of standard spectral methods, including Chebyshev and Legendre collocations.

Gottlieb, D.↗
NASA NTRS1986

Fast-shock formation in line-tied magnetic reconnection models of solar flares

In a previous study by the author, an approximately stationary fast shock was tentatively identified in a numerical experiment designed to study line-tied magnetic reconnection. Here the evidence for the occurrence of a stationary fast shock is reexamined, and the previous identification is confirmed. In the numerical experiment, line-tied reconnection is modeled by a configuration which produces two supermagnetosonic outflow jets - one directed upward, away from the photosphere, and one directed downward, toward an arcade of closed magnetic loops tied to the photosphere. The fast shock occurs when the downward-directed jet encounters the obstacle formed by the closed loops. Although the existence of a stationary, or nearly stationary, fast shock is confirmed, the transition from the supermagnetosonic flow region upstream of the shock to the nearly static region downstream of the shock is more complicated than was previously thought. Immediately downstream of the shock, there exists a deflection sheath in which the submagnetosonic flow coming out of the shock is diverted around the region of static closed loops. The MHD jump conditions are used to investigate the characteristics of the fast shock and to show that a stationary shock cannot exist unless accompanied by a deflection sheath. Analysis of the shock's location and dimensions suggests that such fast shocks may contribute to particle acceleration and to thermal condensation in flares.

Forbes, T. G.↗
NASA NTRS1986

Modeling turbulent, reacting flow

Several of the approximations or models involved in the development of a numerical combustor flow code are examined. In the first section, the importance of numerical accuracy is illustrated, and the impact that improved-accuracy schemes have on slowing convergence is demonstrated. Solution algorithms that can speed convergence are discussed and some performance features of these algorithms are illustrated. A sample calculation displaying the importance of boundary conditions on a three-dimensional numerical prediction is presented. The inaccuracy of a current turbulence model in highly turbulent (nonequilibrium) regions is described. Finally, the surprisingly good performance of a six-flux model in describing radiation heat transfer is displayed. In all the areas examined, continued research is still needed, but valuable engineering tools are available today.

Claus, Russell W.↗
NASA NTRS1987

A versatile and low order hybrid stress element for general shell geometry

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 consistent first order thin shell theory with Love Kirchhoff hypotheses. 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 bi-cubic polynomial surface patch. Numerical examples consist of torsion-loaded slit cylinder and pinched cylinder with open ends and rigid diaphragmed ends. Also, the representation of the rigid body motion is studied by series of parametric eigenvalue analysis of the stiffness matrix.

Kang, David S.↗
NASA NTRS1988

Domain decomposition methods for the parallel computation of reacting flows

Domain decomposition is a natural route to parallel computing for partial differential equation solvers. Subdomains of which the original domain of definition is comprised are assigned to independent processors at the price of periodic coordination between processors to compute global parameters and maintain the requisite degree of continuity of the solution at the subdomain interfaces. In the domain-decomposed solution of steady multidimensional systems of PDEs by finite difference methods using a pseudo-transient version of Newton iteration, the only portion of the computation which generally stands in the way of efficient parallelization is the solution of the large, sparse linear systems arising at each Newton step. For some Jacobian matrices drawn from an actual two-dimensional reacting flow problem, comparisons are made between relaxation-based linear solvers and also preconditioned iterative methods of Conjugate Gradient and Chebyshev type, focusing attention on both iteration count and global inner product count. The generalized minimum residual method with block-ILU preconditioning is judged the best serial method among those considered, and parallel numerical experiments on the Encore Multimax demonstrate for it approximately 10-fold speedup on 16 processors.

Keyes, David E.↗
NASA NTRS1989

Navier-Stokes solutions for vortical flows over a tangent-ogive cylinder

Reynolds number (Re) effects on low-speed vortical flows over a 3.5 caliber tangent-ogive cylinders at two angles of attack (alpha = 20 and alpha = 30 degrees) are computationally assessed for Re(D) = 0.2 -3.0 million (D: maximum diameter). The flow field results are steady-state solutions to the three-dimensional, incompressible Navier-Stokes equations in their thin-layer approximation. Using a properly modified algebraic turbulence model, the numerical results are in good to excellent agreement with experiments.

Hartwich, PETER-M.↗