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NASA NTRS1992

Streamwise vortices in heated boundary layers

The nonlinear instability of the boundary layer on a heated flat plate placed in an oncoming flow is investigated. Such flows are unstable to stationary vortex instabilities and inviscid traveling wave disturbances governed by the Taylor-Goldstein equation. For small temperature differences the Taylor-Goldstein equation reduces to Rayleigh's equation. When the temperature difference between the wall and free stream is small the preferred mode of instability is a streamwise vortex. It is shown in this case that the vortex, assumed to be of small wavelength, restructures the underlying mean flow to produce a profile which can be massively unstable to inviscid traveling waves. The mean state is shown to be destabilized or stabilized to inviscid waves depending on whether the Prandtl number is less or greater than unity.

Hall, Philip↗

NASA NTRS1990

The influence of Lewis number and nonhomogeneous mixture on premixed turbulent flame structure

The structure of a premixed flame front in two-dimensional turbulence is investigated using full numerical simulation including heat release, variable fluid properties, and one-step Arrhenius chemistry. Non-unity Lewis number (Le) effects are described by comparing the local instantaneous turbulent flame structure to the steady one dimensional laminar flame structure for the same thermochemical parameters. Flame surface area, mean reactant consumption rate per unit area of flame, turbulent flame speed, and statistical descriptions of the flame geometry are also reported. Principal findings are that the local flame structure correlates strongly with the local flame curvature, while global properties depend both on strain and on curvature. Preliminary results for cases with nonhomogeneous reactant mixture strength are reported. Here, the emphasis is on the ability of a propagating flame to recover after encountering a fuel-lean pocket in an otherwise homogeneous mixture. Both one dimensional and two dimensional cases are described.

Haworth, D. C.↗

NASA NTRS1992

Diversity imaging techniques in lidar

Diversity imaging techniques have been successfully employed in conventional microwave range-Doppler imaging radars to obtain high resolution images of both natural and man-made targets. These techniques allow microwave radars to achieve image resolution which would otherwise require excessively large antennas. Recent advances in coherent laser radar techniques and signal processing have led to the development of range-Doppler imaging laser radars. While much of the theory and signal processing techniques used in microwave radars can be brought to bear on laser radars, the significant difference in wavelength results in issues peculiar to laser radar systems. Both the fundamental concepts and specific applications of diversity imaging techniques applied to laser radar imaging systems will be discussed. Angle, frequency, and bistatic angle degrees of freedom can be employed in a coherent laser radar imaging system to achieve image resolution which exceeds the traditional Rayleigh criterion associated with the receive aperture. In diversity imaging, angle and frequency degrees of freedom can be used to synthesize an effective aperture providing range and Doppler target information. The ability to vary the bistatic angle provides an additional means of synthesizing an effective aperture. Both simulated and experimentally obtained laser radar images of spinning and/or tumbling objects utilizing both angular and frequency diversity will be presented. In coherent laser radar systems, image quality can be dominated by laser speckle effects. In particular, the signal-to-noise ratio (SNR) of a coherent laser radar image is at most unity in the presence of fully developed speckle. Diversity techniques can be utilized to improve the image SNR; simple incoherent averaging of images utilizing temporal and polarization degrees of freedom can significantly improve image SNR. Both the SNR and image resolution (as defined by the synthetic aperture) contribute to image quality. The relationship between the available degrees of freedom and image quality are discussed.

Schultz, K. I.↗

NASA NTRS1992

On the spatial evolution of long-wavelength Goertler vortices governed by a viscous-inviscid interaction

The generation of long-wavelength, viscous-inviscid interactive Goertler vortices is studied in the linear regime by numerically solving the time-dependent governing equations. It is found that time-dependent surface deformations, which assume a fixed nonzero shape at large times, generate steady Goertler vortices that amplify in the downstream direction. Thus, the Goertler instability in this regime is shown to be convective in nature, contrary to the earlier findings of Ruban and Savenkov. The disturbance pattern created by steady and streamwise-elongated surface obstacles on a concave surface is examined in detail, and also contrasted with the flow pattern due to roughness elements with aspect ratio of order unity on flat surfaces. Finally, the applicability of the Briggs-Bers criterion to unstable physical systems of this type is questioned by providing a counterexample in the form of the inviscid limit of interactive Goertler vortices.

Choudhari, Meelan↗

NASA NTRS1992

Thermal conductance of augmented pressed metallic contacts at liquid helium temperatures

The thermal conductance of uncoated oxygen-free high conductivity (OFHC) copper, 6061-T6 aluminum, free-machining brass, and 304 stainless steel sample pairs which were augmented with a gold coated 6061-T6 aluminum washer inserted between the contact surfaces was measured over the temperature range of 1.6 to 6.0 K, with applied forces from 22 to 670 N. The contact surfaces of the sample pairs were prepared with a 0.8 micron lapped finish, while the finish of the aluminum washer was 0.2 micron lapped. The contribution to the overall thermal impedance by the bulk conductance of the aluminum washer was negligible. It was found that addition of the washer offered no significant conductance improvement over an uncoated single contact pair; any benefits from using the gold plated washer were counteracted by the addition of two more contact surfaces. Additionally, the thermal conductance of a 'combination' aluminum sample pair having one gold coated and one uncoated surface was measured and compared to the washer pair. The ratio of the conductance of the washer pair to half the conductance of the 'combination' pair was found to be constant and near unity over the temperature range of the data obtained, within experimental error.

Salerno, L. J.↗

NASA NTRS1991

Linear system theory

The aim of this book is to provide a systematic and rigorous access to the main topics of linear state-space system theory in both the continuous-time case and the discrete-time case; and the I/O description of linear systems. The main thrusts of the work are the analysis of system descriptions and derivations of their properties, LQ-optimal control, state feedback and state estimation, and MIMO unity-feedback systems.

Callier, Frank M.↗

NASA NTRS1991

Unified nonlinear analysis for nonhomogeneous anisotropic beams with closed cross sections

A unified methodology for geometrically nonlinear analysis of nonhomogeneous, anisotropic beams is presented. A 2D cross-sectional analysis and a nonlinear 1D global deformation analysis are derived from the common framework of a 3D, geometrically nonlinear theory of elasticity. The only restrictions are that the strain and local rotation are small compared to unity and that warping displacements are small relative to the cross-sectional dimensions. It is concluded that the warping solutions can be affected by large deformation and that this could alter the incremental stiffnes of the section. It is shown that sectional constants derived from the published, linear analysis can be used in the present nonlinear, 1D analysis governing the global deformation of the beam, which is based on intrinsic equations for nonlinear beam behavior. Excellent correlation is obtained with published experimental results for both isotropic and anisotropic beams undergoing large deflections.

Atilgan, Ali R.↗

NASA NTRS1991

Voyager photometry of Triton - Haze and surface photometric properties

The Voyager whole-disk observations of Triton at 0.41, 0.48, and 0.56 micron filter wavelengths are analyzed using a model which combines an improved version of Hapke's photometric equation with a thin atmospheric haze layer in the appropriate spherical geometry. The model is shown to describe accurately the phase curves over a range of phase angles and to agree with disk-resolved brightness scans along the photometric equator and mirror meridian. According to the model, the photometric parameters of Triton's regolith are reasonably typical of icy satellites, except for the extremely high (close to unity) single-scattering albedo.

Hillier, J.↗

NASA NTRS1991

Dissociation of diatomic gases

The Landau-Zener theory of reactive cross sections has been applied to diatomic molecules dissociating from a ladder of rotational and vibrational states. Although the preexponential factor of the Arrhenius rate expression is shown to be a complex function of the dimensionless activation energy, the average over all states in the ladder is well represented by a single factor that varies about as T exp (-n), where the coefficient n is the order of unity. This relation agrees very well with experimental data for dissociation of O2 and N2, for example. The results validate previous empirical assignment of a single preexponential factor in the Arrhenius expression and justify the extrapolation of the expression well beyond the range of data. The theory is then used to calculate the effect of vibrational nonequilibrium on dissociation rate. For Morse oscillators the results are about the same as for harmonic oscillators, and the dissociation from a ladder of equilibrium rotational and nonequilibrium vibrational states is close to an analytic approximation provided by Hammerling, Kivel, and Teare for harmonic oscillators all dissociating from the ground rotational state.

Hansen, C. F.↗

NASA NTRS1990

The spectral emissivity of prairie and pasture grasses at Konza Prairie, Kansas

Field measurements of spectral radiances are used to determine precise values of the spectral emissivity of grass-thatch-soil complexes to facilitate remote temperature determinations. The emissivity variation with wavelength is very small, emissivity is close to unity, and emissivity is fairly constant in terms of emission angle, land practice, and season. The prairie surface is therefore similar to a grey body and a quasiideal emitter, although determinations of the kinetic temperature are required to confirm the results.

Palluconi, Frank↗

NASA NTRS1991

The particle size of Martian aeolian dunes

The effective particle size of unconsolidated materials on the Martian surface can be determined from thermal inertia, due to a pore size dependence of thermal conductivity at Martian atmospheric pressures. Because dunes consist of a narrow range of well-sorted, unconsolidated particles, they provide for a test of the relationship between particle size and thermal inertia calculated from midinfrared emission data for the Martian surface. Two independent approaches are used. First, thermal inertia data indicate that Martian dunes have an average particle size of about 500 +/-100 microns, or medium to coarse sand. Second, expected dune particle sizes are determined from grain trajectory calculations and the particle size transition from suspension to saltation. On earth, the transition occurs for a grain when the ratio of the terminal fall velocity to the wind friction speed, u*(t) is near unity; for grains at u*(t) this occurs at about 52 microns. Terrrestrial dune sands have a mean of 250 microns and are composed entirely of grains greater than 52 microns. The corresponding Martian transition grain size is about 210 microns, suggesting that Martian dunes should be significantly coarser than terrestrial dunes. Grain saltation path length as a function of particle size also shows that, under Martian conditions, larger grains than on earth will become suspended. Both approaches indicate that Martian dune sand should be coarser than terrestrial dune sand. These results closely match the grain sizes determined from thermal inertia models, providing the first direct test of the validity of these models for actual Martian surface materials.

Edgett, Kenneth S.↗

NASA NTRS1992

Multiple resonant scattering in the Compton upscatter model of gamma-ray bursts

Resonant Compton scattering, an increasingly popular mechanism for suppressing X-rays and producing gamma rays, must be treated as a multiple-scattering process for conditions thought characteristic of gamma-ray bursts. Photons that multiply scatter with a beamed power-law electron distribution in a uniform magnetic field produce a flat spectrum between the cyclotron frequency and an optical-depth-dependent critical energy; this critical energy ranges between several hundred keV and several MeV. Above this critical energy, the gamma-ray spectrum has a shape determined by the electron distribution and described by a single-scattering model. Only electron distributions that are nearly proportional to the electron momentum are able to simultaneously suppress X-rays and produce a single-scattering spectrum. As the Thomson optical depth approaches unity, photons that experience multiple scatterings often spawn additional photons at a rate that makes the model unphysical.

Brainerd, J. J.↗

NASA NTRS1992

The secondary outburst maximum of T Coronae Borealis - Implications for the physics of accretion disks

The hypothesis of Webbink (1976) that the accretion of a torus of matter onto a main-sequence star caused secondary maxima during the 1866 and 1946 eruptions of the recurrent nova T CrB is examined. Simple 1D calculations show that the accretion disk viscosity must increase with time to produce light curves resembling observations. It is inferred that the physical mechanism responsible for producing the viscous dissipation in the accretion disk has a fast growth rate and saturates to a viscosity parameter of order unity. The good agreement between parameters estimated from observations and those derived from a physical mechanism suggests that the growth rate instability identified by Balbus and Hawley (1991) is a promising source for accretion disk viscosity.

Cannizzo, John K.↗

NASA NTRS1992

Estimating redshifts for gamma-ray bursts

A test of distances to gamma-ray bursters is described that might be done with data from the BATSE experiment on the Gamma Ray Observatory. The test is equivalent to the formulation of the Hubble diagram for galactic redshifts. If the typical redshift of gamma-ray bursters is of the order of unity, the weakest bursts should have softer spectra than the strongest. There is some evidence for such a correlation in the BATSE data. If confirmed, this would provide the first spectroscopic indication that the burst sources lie at cosmological distances. There is also evidence that the redshift effect has been detected in the duration of a subset of the BATSE bursts. If confirmed, this could be evidence that not only the photons but also the light curves are stretched by the cosmological redshift.

Paczynski, Bohdan↗

NASA NTRS1991

Biological effectiveness of high-energy protons - Target fragmentation

High-energy protons traversing tissue produce local sources of high-linear-energy-transfer ions through nuclear fragmentation. The contribution of these target fragments to the biological effectiveness of high-energy protons using the cellular track model is examined. The effects of secondary ions are treated in terms of the production collision density using energy-dependent parameters from a high-energy fragmentation model. Calculations for mammalian cell cultures show that at high dose, at which intertrack effects become important, protons deliver damage similar to that produced by gamma rays, and with fragmentation the relative biological effectiveness (RBE) of protons increases moderately from unity. At low dose, where sublethal damage is unimportant, the contribution from target fragments dominates, causing the proton effectiveness to be very different from that of gamma rays with a strongly fluence-dependent RBE. At high energies, the nuclear fragmentation cross sections become independent of energy. This leads to a plateau in the proton single-particle-action cross section, below 1 keV/micron, since the target fragments dominate.

Cucinotta, Francis A.↗

NASA NTRS1992

Fe II fluorescence and anomalous C IV doublet intensities in symbiotic novae

The variation of absolute intensities of Bowen-excited Fe II emission in the symbiotic stars RR Tel, RX Pup, and AG Peg is examined. The C IV doublet intensity ratios in RR Tel were not anomalous between 1979 and 1989, and the ratio had typical values within the optically thin range. The intensity of individual Fe II Bowen-excited lines is correlated with the C IV 1548.2 A flux, suggesting the presence of a foreground Fe II region in which fluorescent-excited material responds to flux variations of C IV 1548.2 A. In RX Pup the combined fluxes of Fe II Bowen-pumped lines can account for an appreciable fraction of the flux deficit in the C IV 1548.2 A line when the C IV doublet ratio is less than the optically thick limit of unity. The Fe II Bowen lines in RX Pup exhibit a velocity range from 0 to 80 km/s, where several strong Fe II emission lines correspond to deep absorption structure in the C IV 1548.2 A line profile. In AG Peg and C IV 1548.2 A flux deficit cannot be explained by Fe II fluorescent absorption alone when the C IV doublet ratio anomaly is at an extreme.

Michalitsianos, A. G.↗

NASA NTRS1992

Intrinsically asymmetric astrophysical jets

Previous work on the origin of self-collimated electromagnetic jets is extended to the general case where there is no reflection symmetry of the magnetic field about the equatorial plane of the disk. The axisymmetric field structure inside the disk is obtained by solving for the magnetic flux function, and the toroidal magnetic field, from the generalized thin-disk induction equation, which is derived. The asymptotic (large-z) magnetic field structure outside the disk is obtained by solving the force-free Grad-Shafranov equation semianalytically. Jet solutions are found in which the power flow is carried mainly by the Poynting flux of the electromagnetic field and the angular momentum outflow from the disk is carried by the magnetic field. The ratio of jet luminosities (top/bottom) depends directly on the degree of asymmetry of the field and can easily be much greater than unity. It is argued that the degree of field asymmetry in the disk is determined by the asymmetry of the weak galactic field fed into the disk at large distances over long periods of time.

Wang, J. C. L.↗

NASA NTRS1992

Unusually distant bow shock encounters at Venus

The location of the Venus bow shock on two days (July 6, 1981 and August 4, 1971) when the IMF was strong and the Mach number low is examined. When the solar wind Mach number approaches unity the bow shock appears to be greatly detached from the obstacle. Thus inferences from observations of shocks near planets should be treated cautiously, especially if the Mach number is small or not well known. The case of the Phobos mission to Mars is examined as an example of such a situation.

Russell, C. T.↗