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At least 469 records · Page 26

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NASA NTRS1982

A numerical solution of the supersonic flow over a rearward facing step with transverse non-reacting hydrogen injection

The present investigation represents an application of computational fluid dynamics to a problem associated with the flow in the combustor region of a supersonic combustion ramjet engine (scramjet). The governing equations are considered, taking into account the Navier-Stokes equations, a molecular viscosity calculation, the molecular thermal conductivity, molecular diffusion, and a turbulence model. The employed numerical solution is patterned after the explicit, time-dependent, unsplit, predictor-corrector, finite-difference method given by MacCormack (1969). The calculation is concerned with the supersonic flow over a rearward-facing step with transverse H2 injection at conditions germane to the combustor region of a scramjet engine. The H2 jet acts as an effective body which essentially shields the primary flow from the rearward-facing step, thus substantially changing the wave pattern in the primary flow.

Berman, H. A.↗

NASA NTRS1982

Asymptotic freedom in solids - A theorem

A cusp theorem is proved that relates the zero-separation value and slope of two-particle position correlation functions in quantum many-body systems with Coulombic interactions. The theorem is independent of the particle type and symmetry of the wave function. Its proof uses only the integral form of the Schroedinger equation and the continuity and exponential decay of the wave function. It is used to derive a sum rule for the electron-gas structure factor and an exact statement about the screening of point charges. Applications to atomic-orbital-based calculations for H2 and metallic H are described.

Carlsson, A. E.↗

NASA NTRS1982

Upper mantle anisotropy - Evidence from free oscillations

There is a growing body of evidence that much of the upper mantle may be anisotropic to the propagation of seismic waves. The present investigation shows that a small amount of anisotropy completely changes the nature of the surface wave and normal mode problem. In particular, the apparent lack of sensitivity of many of the spheroidal modes to the compressional velocity structure is due to the degeneracy in the isotropic case. The normal mode data set appears to be adequate to resolve the five elastic constants of a transversely isotropic upper mantle. Dziewonski and Anderson (1981) have shown that these data can be fitted with anisotropy restricted to the upper 200 km of the mantle.

Anderson, D. L.↗

NASA NTRS1985

MHD modelling of the earth's magnetosphere

Attention is given to the shock capturing technique, nonuniform grid system, and multiple time scale problem aspects of a global MHD model of the earth's magnetosphere that uses numerical methods. Because of the fast Alfven waves near the earth, the model is more difficult to solve than problems of hydrodynamic flow past bodies. The Rusanov (1962) scheme, which is a variant of the Lax (1954) method that maintains viscosity at a minimum value except where a large one is required, is used. The magnetosphere shape obtained by these means is noted.

Wu, C. C.↗

NASA NTRS1990

The varieties of symmetric stellar rings and radial caustics in galaxy disks

Numerical, restricted three-body and analytic calculations are used to study the formation and propagation of cylindrically symmetric stellar ring waves in galaxy disks. It is shown that such waves can evolve in a variety of ways, depending on the amplitude of the perturbation and the potential of the target galaxy. Rings can thicken as they propagate outward, remain at a nearly constant width, or be pinched off at large radii. Multiple, closely spaced rings can result from a low-amplitude collision, while an outer ring can appear well-separated from overlapping inner rings or an apparent lens structure in halo-dominated potentials. All the single-encounter rings consist of paired fold caustics. The simple, impulsive, kinematic oscillation equations appear to provide a remarkably accurate model of the numerical simulations. Simple analytic approximations to these equations permit very good estimates of oscillation periods and amplitudes, the evolution of ring widths, and ring birth and propagation characteristics.

Struck-Marcell, Curtis↗

NASA NTRS1992

Applications of a direct/iterative design method to complex transonic configurations

The current study explores the use of an automated direct/iterative design method for the reduction of drag in transport configurations, including configurations with engine nacelles. The method requires the user to choose a proper target-pressure distribution and then develops a corresponding airfoil section. The method can be applied to two-dimensional airfoil sections or to three-dimensional wings. The three cases that are presented show successful application of the method for reducing drag from various sources. The first two cases demonstrate the use of the method to reduce induced drag by designing to an elliptic span-load distribution and to reduce wave drag by decreasing the shock strength for a given lift. In the second case, a body-mounted nacelle is added and the method is successfully used to eliminate increases in wing drag associated with the nacelle addition by designing to an arbitrary pressure distribution as a result of the redesigning of a wing in combination with a given underwing nacelle to clean-wing, target-pressure distributions. These cases illustrate several possible uses of the method for reducing different types of drag. The magnitude of the obtainable drag reduction varies with the constraints of the problem and the configuration to be modified.

Smith, Leigh Ann↗

NASA NTRSDate not supplied

New Astrophysical Discovery with Ground and Space-Based Observatories

Astronomy is robust with discovery thanks to a wide range of international observing facilities in space and on the ground. For example, space telescopes such as Webb and Hubble are now detecting the most distant galaxies in the universe, as well as the cosmological effects of dark matter and dark energy. Ground based radio telescope arrays are peering into nurseries of star-formation, finding regions of infant stars and also their circumstellar disks in which planetary systems are forming. Even black holes have now been imaged. This overview talk will highlight recent advances in observational astrophysics, especially the scientifically powerful examples where multiple facilities are used in complement. Examples include the study of solar system bodies, the characterization of exoplanet atmospheres, and detections of electromagnetic radiation associated with cosmic sources of gravitational waves.

Jennifer J Wiseman↗

NASA NTRS1989

Numerical simulation of rotorcraft

The objective of the research is to develop and validate accurate, user-oriented viscous CFD codes (with inviscid options) for three-dimensional, unsteady aerodynamic flows about arbitrary rotorcraft configurations. Unsteady, three-dimensional Euler and Navier-Stokes codes are developed, adapted, and extended to rotor-body combinations. Flow solvers are coupled with zonal grid topologies, including rotating and nonrotating blocks. Special grid clustering and wave-fitting techniques were developed to capture low-level radiating acoustic waves. Significant progress was made in computing the propagation of acoustic waves due to the interaction of a concentrated vortex and a helicopter airfoil. The need for higher-order schemes was firmly established in relatively inexpensive two-dimensional calculations. In three dimensions, the number of grid points required to capture the low-level acoustic waves becomes very large, so that large supercomputer memory becomes essential. Good agreement was obtained between the numerical results obtained with a thin-layer Navier-Stokes code and experimental data from a model rotor. In addition, several nonrotating configurations that are sometimes proposed to simulate rotor blade tips in conventional wind tunnels were examined, and the complex flow around the radical tip shape of the world's fastest helicopter is under investigation. These studies demonstrate the flexibility and power of CFD to gain physical insight, study novel ideas, and examine various possibilities that might be difficult or impossible to set up in physical experiments. As a prelude to studies of rotor-body aerodynamic interactions, a preliminary grid topology and moving-interface strategy were developed. A new Euler/Navier-Stokes code using these techniques computes the vortical wake directly, rather than modeling it, as in most previous rotorcraft studies. Several hover cases were run for conventional and advanced-geometry blades. Numerical schemes using multi-zones and/or adaptive grids appear to be necessary to simulate the complex vortical flows in rotor wakes.

Mccroskey, William J.↗

NASA NTRS1995

Euler/Navier-Stokes Flow Computations on Flexible Configurations for Stability Analysis

Abstract Longitudinal dynamic stability derivatives required for design of aircraft are computed by using the state-of-the-art numerical methods for wing-body configurations. The flow is modeled using the Euler/Navier-Stokes equations with turbulence models and solved using an efficient finite-difference scheme suitable for patched structured grids. Computations are made at a flow regime that is beyond the limits of the current linear methods mostly used for computing stability derivatives. Flow conditions include shock-waves and viscous dominated vortical flows. Effect of Mach number and angle-of-attack on stability derivatives are demonstrated for a typical wing-body configuration. For the same configuration the effects of wing flexibility on the magnitude and phase angles of stability derivatives are also demonstrated.

Guruswamy, Guru P.↗

NASA NTRS2017

A New Formulation of Time Domain Boundary Integral Equation for Acoustic Wave Scattering in the Presence of a Uniform Mean Flow

It has been well-known that under the assumption of a constant uniform mean flow, the acoustic wave propagation equation can be formulated as a boundary integral equation, in both the time domain and the frequency domain. Compared with solving partial differential equations, numerical methods based on the boundary integral equation have the advantage of a reduced spatial dimension and, hence, requiring only a surface mesh. However, the constant uniform mean flow assumption, while convenient for formulating the integral equation, does not satisfy the solid wall boundary condition wherever the body surface is not aligned with the uniform mean flow. In this paper, we argue that the proper boundary condition for the acoustic wave should not have its normal velocity be zero everywhere on the solid surfaces, as has been applied in the literature. A careful study of the acoustic energy conservation equation is presented that shows such a boundary condition in fact leads to erroneous source or sink points on solid surfaces not aligned with the mean flow. A new solid wall boundary condition is proposed that conserves the acoustic energy and a new time domain boundary integral equation is derived. In addition to conserving the acoustic energy, another significant advantage of the new equation is that it is considerably simpler than previous formulations. In particular, tangential derivatives of the solution on the solid surfaces are no longer needed in the new formulation, which greatly simplifies numerical implementation. Furthermore, stabilization of the new integral equation by Burton-Miller type reformulation is presented. The stability of the new formulation is studied theoretically as well as numerically by an eigenvalue analysis. Numerical solutions are also presented that demonstrate the stability of the new formulation.

Hu, Fang↗

NASA NTRS2002

Pickup Ions at Dione and Enceladus

Voyager images of the icy satellites of Saturn, Dione and Enceladus, suggest they have been geologically active and are not only composed of ice. Recent observations by HST have shown the presence of ozone at both Dione and Rhea which also implies the presence of molecular oxygen at these bodies. The Cassini Plasma Spectrometer (CAPS) will provide the capability to determine the global composition of these bodies by measuring the pickup ions produced by the ionization of their sputter produced atmospheres. We will present a model of these atmospheres and associated pickup ions and demonstrate CAPS ability to distinguish the freshly produced picked up ions from the ambient plasma. Such ions are expected to form a ring distribution that will have a uniquely different energy-angle dependence than the ambient plasma ions. In the case of Dione we expect the potential for a moderate strength interaction for which both Voyager 1 and Pioneer 11 spacecraft measured ion cyclotron waves centered on the Dione L shell and near the equatorial plane. Since Enceladus may be the source of the E-ring, some surprises may be encountered during its close encounter with the Cassini spacecraft. In the case of Dione we will show that a wake pass at 500 km altitude is more than an order of magnitude better than an upstream pass at 500 km altitude. Pickup ion detection for minor ion species such as NH3+ is possible for 500 km altitude wake pass but not for a 500 km altitude upstream pass at closest approach. For navigation reasons a 100 km pass is not allowed and therefore it is essential to have a wake pass to maximize the science return for a targeted flyby with Dione. The CAPS observations when combined with magnetometer, plasma wave and energetic particle observations will allow us to estimate the source of ions into Saturn's magnetosphere due to these two bodies and to characterize the nature of the interaction with Saturn's magnetosphere.

Sittler, E.↗

NASA NTRS1993

Analysis of unsteady wave processes in a rotating channel

The impact of passage rotation on the gas dynamic wave processes is analyzed through a numerical simulation of ideal shock-tube flow in a closed rotating-channel. Initial conditions are prescribed by assuming homentropic solid-body rotation. Relevant parameters of the problem such as wheel Mach number, hub-to-tip radius ratio, length-to-tip radius ratio, diaphragm temperature ratio, and diaphragm pressure ratio are varied. The results suggest possible criteria for assessing the consequences of passage rotation on the wave processes, and they may therefore be applicable to pressure-exchange wave rotors. It is shown that for a fixed geometry and initial conditions, the contact interface acquires a distorted three-dimensional time-dependent orientation at non-zero wheel Mach numbers. At a fixed wheel Mach number, the level of distortion depends primarily on the density ratio across the interface as well as the hub-to-tip radius ratio. Rarefaction fronts, shocks, and contact interfaces are observed to propagate faster with increasing wheel Mach number.

Larosiliere, L. M.↗

NASA NTRS1993

Analysis of unsteady wave processes in a rotating channel

Larosiliere, Louis M.↗

NASA NTRS1972

Theoretical prediction of interference loading on aircraft stores. Part 2: Supersonic speeds

A method is developed for theoretically predicting the loading on pylon-mounted stores in supersonic flow. Linear theory is used, without two dimensional or slender body assumptions, to predict the flow field produced by the aircraft wing, nose, inlet, and pylons. Aircraft shock wave locations are predicted, and their effect on the flow field is included through a transformation of the aircraft geometry. The interference loading is integrated over the store length by considering the local crossflow, its axial and radial derivatives, and buoyancy. Store moment calculations under an F-4 aircraft at Mach 1.2 are compared to wind tunnel data. The method is computerized, and program user information is included.

Danfernandes, F.↗

NASA NTRS1974

Experimental Investigation of Electron Cloud Containment in a Nonuniform Magnetic Field

Dense clouds of electrons were generated and studied in an axisymmetric, nonuniform magnetic field created by a short solenoid. The operation of the experiment was similar to that of a low-pressure (approximately 0.000001 Torr) magnetron discharge. Discharge current characteristics are presented as a function of pressure, magnetic field strength, voltage, and cathode end-plate location. The rotation of the electron cloud is determined from the frequency of diocotron waves. In the space charge saturated regime of operation, the cloud is found to rotate as a solid body with frequency close to V sub a/phi sub a where V sub a is the anode voltage and phi suba is the total magnetic flux. This result indicates that, in regions where electrons are present, the magnetic field lines are electrostatic equipotentials (E bar, B bar = 0). Equilibrium electron density distributions suggested by this conditions are integrated with respect to total ionizing power and are found consistent with measured discharge currents.

Eninger, J. E.↗

NASA NTRS1981

Three dimensional boundary conditions in supersonic flow

A theoretical analysis of the flow pattern at a solid surface in three dimensional supersonic flow is presented. The additional information necessary to overcome the nonuniqueness associated with the body tangency condition in three dimensions was developed. The analysis is based on the fact that three dimensional waves propagate locally exactly as they do in axisymmetric flow when viewed in the osculating plane to the streamline. The supersonic flow over an infinite swept corner is examined by both the classical solution and the three dimensional solution in the osculating plane and the results are shown to be identical. A simple numerical algorithm is proposed which accounts for the three wave surfaces that interact at a solid boundary.

Rudman, S.↗

NASA NTRS1986

Infrared imaging and photometry of Comet Giacobini-Zinner

Infrared images and photometry were obtained to determine the spatial distribution and physical characteristics (temperature, albedo, size distribution, total mass, etc.) of the grains in the coma of Comet GZ. A 10.8 m image of Comet GZ obtained on August 4 represents the first groundbased thermal-infrared image of a Comet. Among the most significant results are: (1)An estimate of the number of grains that the ICE spacecraft must have encountered, which led the plasma wave team to conclude that they could only detect impacts on the antennae and not on the whole body of the ICE spacecraft; (2)The discovery of a population of large grains (radius > 100 micrometer), not observed in most other comets, which formed a curved tail near the nucleus (within 80 arcsec or 34,000 km); and (3)the detection of structure in the spatial distribution in the coma of the particle albedo, which was tentatively attributed to the presence of very fluffy grains which are likely to have multiple internal scattering of incident sunlight. The albedo map of Comet GZ was obtained by combining the 10.8 micrometer image shown with a simultaneous image taken at 0.68 micrometer, a bandpass which isolates the scattered continuum.

Campins, H.↗

NASA NTRS1990

CFD spinoff - Computational electromagnetics for radar cross section (RCS) studies

A finite-volume discretization procedure derived from proven CFD methods is used to solve the conservation form of the time-domain Maxwell's equations, in order to compute EM scattering from layered objects. This time-domain approach handles both single-frequency/continuous wave and broadband-frequency/pulse incident excitation. Arbitrarily shaped objects are modeled by means of a body-fitted coordinate transformation; complex internal/external structures with many material layers are treated through the implementation of a multizone framework capable of handling any type of zonal boundary condition. Results are presented for various two- and three-dimensional problems.

Shankar, Vijaya↗