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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

VLSI binary updown counter

A pipeline binary updown counter is comprised of simple stages that may be readily replicated. Each stage is defined by the Boolean logic equation: A(sub n)(t) = A(sub n)(t - 1) exclusive OR (U AND P(sub n)) inclusive OR (D AND Q(sub n)), where A(sub n)(t) denotes the value of the nth bit at time t. The input to the counter has three values represented by two binary signals U and D such that if both are zero, the input is zero, if U = 0 and D = 1, the input is -1 and if U = 1 and D = 0, the input is +1. P(sub n) represents a product of A(sub k)'s for 1 is less than or equal to k is less than or equal to -1, while Q(sub n) represents the product of bar A's for 1 is less than or equal to K is less than or equal to n - 1, where bar A(sub k) is the complement of A(sub k) and P(sub n) and Q(sub n) are expressed as the following two equations: P(sub n) = A(sub n - 1) A(sub n - 2)...A(sub 1) and Q(sub n) = bar A(sub n - 1) bar A(sub n - 2)...bar A(sub 1), which can be written in recursive form as P(sub n) = P(sub n - 1) AND bar A(sub n - 1) and Q(sub n) = Q(sub n - 1) AND bar A(sub n - 1) with the initial values P(sub 1) = 1 and Q(sub 1) = 1.

Truong, Trieu-Kie↗
NASA NTRS1990

Laboratory studies of photodissociation processes relevant to the formation of cometary radicals

The strength of the C2(d 3 Pi g yields a 3 Pi u) Swan band emission in the spectra of cometary comae identifies this species as a prominent constituent of the coma gas. It was previously suggested that the formation of cometary C2 proceeds via the secondary photolysis of the C2H radical. The detection of C2H in the interstellar medium and the recent analysis of the radial variation in C2(delta V=O) surface brightness of Comet Halley support the postulate that C2 is a third-generation molecule. Measurement of the C2 and C2H translational energy distributions produced from the multiphoton dissociation (MPD) of acetylene at 193 nm are identified . Time-resolved FTIR emission studies of the nascent C2H radical formed in the C2H2 yields C2H + H reaction verify that this species is produced both vibrationally and electronically excited. A survey of the internal energy distributions of the C2 fragments produced from the MPD of acetylene using a high intensity ArF laser is currently in progress in the laboratory. Recent experiments have focused on the measurement of rotational energy distribution for the C2(A 1 Pi u, a 3 Pi u) fragments. The C2(a 3 Pi u) detection capability is currently being improved by performing this experiment in a molecular beam, thus allowing for discrimination between initial emission and laser-induced fluorescence (LIF). Although the experiments performed to date provide considerable evidence in support of C2H yields C2 + H reaction, there is an important distinction to be made when comparing the laboratory conditions to those typically found in comets. The C2H radicals generated in the laboratory experiments are formed vibrationally and/or electronically excited. Any rotationally/vibrationally excited C2H present in cometary comae will quickly undergo radiative relaxation in the infrared to their lowest rotational and vibrational state. Experiments are currently under way to confirm the cometary formation of C2 via the VUV dissociation of cold C2H.

Urdahl, R. S.↗
NASA NTRS1989

On the bonding in Be2(2+)

The ground 1Sigma-g(+) state and excited 3Pi-u and 1P-u states of Be2(2+) have been studied. The ground state has 13 vibrational levels with an appreciable lifetime with respect to unimolecular decay. The 3Pi-u state also has an inner well with several vibrational levels with long lifetimes. The 1Pi-u state is repulsive, but the chemical bonding causes an inflection to appear in the potential curve.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1991

Counterflow diffusion flames of hydrogen, and hydrogen plus methane, ethylene, propane, and silane vs. air - Strain rates at extinction

Five coaxial tubular opposed jet burners (OJBs) with tube diameter D(T) of 1.8-10 mm and 5 mm conical nozzles were used to form dish-shaped counterflow diffusion flames centered by opposing laminar jets of nitrogen and hydrocarbon-diluted H2 versus air in an argon-purged chamber at 1 atm. Area-averaged air jet velocities at blowoff of the central flame, U(air), characterized extinction of the airside flame as functions of input H2 concentration on the fuelside. A master plot of extensive U(air) data at blowoff versus D(T) shows that U(air) varies linearly with D(T). This and other data sets are used to find that nozzle OJB results for U(air)/diameter average 4.24 + or - 0.28 times larger than tubular OJB results for the same fuel compositions. Critical radial velocity gradients consistent with one-dimensional stagnation point boundary theory and with plug flow inputs are estimated. The results compare favorably with published numerical results based only on potential flow.

Pellett, G. L.↗
NASA NTRS1991

Solution of steady-state, two-dimensional conservation laws by mathematical programming

A truly two-dimensional algorithm is created for solving the steady-state two-dimensional conservation-law problem. An overdetermined system of algebraic equations is obtained through discretization by finite-volume formulas. These equations are perturbed nonsingularly and are solved by an efficient geometrically oriented l(1) procedure. The basic algorithm and the theory for the linear case f(u) = u are presented, and computational results for the nonlinear case f(u) = sq u are also analyzed. It is noted that the l(1) procedure captures boundary shocks as well as oblige and zigzag interior shocks in bands that are one cell wide, and the solution values are accurate up to the edge of the shock.

Lavery, John E.↗
NASA NTRS1991

The shock wave equation of state of brucite Mg(OH)2

New shock equation of state (EOS) data on magnesium hydroxide between 12 and 60 GPa is presented. It is found that the brucite EOS data between 12 and 97 GPa can be fit with a single linear U(s) - u(p) relationship: U(s) = 4.76(0.11) + 1.35(0.05)u(p). The third order Birch-Murnaghan equation parameters are K(os) = 51 + or - 4 GPa and K(os)prime = 5.0 + or - 0.4. The first partial release states measured for brucite Mg(OH)2 are reported. Calculated phase boundaries using the EOS data are found to be consistent with the experimental data and indicate that brucite is unlikely to be stable under lower mantle conditions. At high pressure, bulk sound velocities calculated for MgO and Mg(OH)2 are very similar, indicating that the presence of hydrous assemblages in the lower mantle may not produce anomalous bulk seismic velocities. A comparison of densities in brucite and other high-pressure phases under mantle conditions shows that the water content of the lower mantle is between 0 and 3 wt pct.

Duffy, Thomas S.↗
NASA NTRS1991

Optimal least-squares finite element method for elliptic problems

An optimal least squares finite element method is proposed for two dimensional and three dimensional elliptic problems and its advantages are discussed over the mixed Galerkin method and the usual least squares finite element method. In the usual least squares finite element method, the second order equation (-Delta x (Delta u) + u = f) is recast as a first order system (-Delta x p + u = f, Delta u - p = 0). The error analysis and numerical experiment show that, in this usual least squares finite element method, the rate of convergence for flux p is one order lower than optimal. In order to get an optimal least squares method, the irrotationality Delta x p = 0 should be included in the first order system.

Jiang, Bo-Nan↗
NASA NTRS1992

The structure of premixed particle-cloud flames

The structure of premixed flames propagating in combustible systems, containing uniformly distributed volatile fuel particles, in an oxidizing gas mixture, is analyzed. It is presumed that the fuel particles vaporize first to yield a gaseous fuel of known chemical structure, which is subsequently oxidized in the gas phase. The analysis is performed in the asymptotic limit, where the value of the characteristic Zeldovich number, based on the gas-phase oxidation of the gaseous fuel is large, and for values of phi(u) greater than or equal to 1.0, where phi(u) is the equivalence ratio based on the fuel available in the fuel particles. The structure of the flame is presumed to consist of a preheat vaporization zone where the rate of the gas-phase chemical reaction is small, a reaction zone where convection and the rate of vaporization of the fuel particles are small and a convection zone where diffusive terms in the conservation equations are small. For given values phi(u) the analysis yields results for the burning velocity and phi(g) where phi(g) is the effective equivalence ratio in the reaction zone. The analysis shows that even though phi(u) greater than or equal to 1.0, for certain cases the calculated value of phi(g) is less than unity. This prediction is in agreement with experimental observations.

Seshadri, K.↗
NASA NTRS1952

Flame Speeds of Methane-Air, Propane-Air, and Ethylene-Air Mixtures at Low Initial Temperatures

Flame speeds were determined for methane-air, propane-air, and ethylene-air mixtures at -73 C and for methane-air mixtures at -132 C. The data extend the curves of maximum flame speed against initial mixture temperature previously established for the range from room temperature to 344 C. Empirical equations for maximum flame speed u(cm/ sec) as a function of initial mixture temperature T(sub O) were determined to be as follows: for methane, for T(sub O) from 141 to 615 K, u = 8 + 0.000160 T(sub O)(exp 2.11); for propane, for T(sub O) from 200 to 616 K, u = 10 + 0.000342 T(sub O)(exp 2.00); for ethylene, for T(sub O) from 200 to 617 K, u = 10 + 0.00259 T(sub O)(exp 1.74). Relative flame speeds at low initial temperatures were predicted within approximately 20 percent by either the thermal theory as presented by Semenov or by the diffusion theory of Tanford and Pease. The same order was found previously for high initial temperatures. The low-temperature data were also found to extend the linear correlations between maximum flame speed and calculated equilibrium active-radical concentrations, which were established by the previously reported high-temperature data.

Dugger, Gordon L↗
NASA NTRS1994

LQG optimal compensator transfer function for the NASA LaRC CSI Evolutionary Model

Following the general form for LQG optimal compensators for flexible structures with collocated rate sensors we develop an explicit compensator transfer function for the NASA LaRC CSI Evolutionary model in the form: psi(i omega) = g i omega B(sub u)(sup *)(-M(sub b)omega(exp 2) + T(i omega) + i gamma omega B(sub u)Bu(sub u)(sup *))(exp -1)B(sub u) where T(i omega) is a 48 x 48 positive definite matrix whose derivation is the main result of this report. The undamped mode frequencies can be expressed in terms of T(i omega) as the zeros of Det (-omega(exp 2)M(sub b) + T(i omega)) while 'clamped-clamped' modes of the structure (with all nodes clamped) are the poles.

Balakrishnan, A. V.↗
NASA NTRS1995

Acoustic field in unsteady moving media

In the interaction of an acoustic field with a moving airframe the authors encounter a canonical initial value problem for an acoustic field induced by an unsteady source distribution, q(t,x) with q equivalent to 0 for t less than or equal to 0, in a medium moving with a uniform unsteady velocity U(t)i in the coordinate system x fixed on the airframe. Signals issued from a source point S in the domain of dependence D of an observation point P at time t will arrive at point P more than once corresponding to different retarded times, Tau in the interval (0, t). The number of arrivals is called the multiplicity of the point S. The multiplicity equals 1 if the velocity U remains subsonic and can be greater when U becomes supersonic. For an unsteady uniform flow U(t)i, rules are formulated for defining the smallest number of I subdomains V(sub i) of D with the union of V(sub i) equal to D. Each subdomain has multiplicity 1 and a formula for the corresponding retarded time. The number of subdomains V(sub i) with nonempty intersection is the multiplicity m of the intersection. The multiplicity is at most I. Examples demonstrating these rules are presented for media at accelerating and/or decelerating supersonic speed.

Bauer, F.↗
NASA NTRS1994

The 630 nm MIG and the vertical neutral wind in the low latitude nighttime thermosphere

It is shown that large negative divergences (gradients) in the horizontal neutral wind in the equatorial thermosphere can support downward neutral winds in excess of 20 m/s. With attention to the meridional and vertical winds only, the pressure tendency equation is used to derive the expression U(sub z0) approximately equals (Partial derivative U(sub y)/Partial derivative y)H for the vertical wind U(sub z0) at the reference altitude for the pressure tendency equation; H is the atmospheric density scale height, and (Partial derivative U(sub y)/Partial derivative y) is the meridional wind gradient. The velocity gradient associated with the Meridional Intensity Gradient (MIG) of the O((sup 1)D) emission (630 nm) at low latitudes is used to estimate the vertical neutral wind in the MIG region. Velocity gradients derived from MIG data are about 0.5 (m/s)/km) or more, indicating that the MIG region may contain downward neutral winds in excess of 20 m/s. Though direct measurements of the vertical wind are scarce, Fabry-Perot interferometer data of the equatorial F-region above Natal, Brazil, showed downward winds of 30 m/s occurring during a strong meridional wind convergence in 1982. In-situ measurements with the WATS instrument on the DE-2 satellite also show large vertical neutral winds in the equatorial region.

Herrero, F. A.↗
NASA NTRS1994

Differential rotation and turbulent convection: A new Reynolds stress model and comparison with solar data

In most hydrodynamic cases, the existence of a turbulent flow superimposed on a mean flow is caused by a shear instability in the latter. Boussinesq suggested the first model for the turbulent Reynolds stresses bar-(u(sub i)u(sub j)) in which the mean shear S(sub ij) is the cause (or source) of turbulence represented by the stress bar-(u(sub i)u(sub j)). In the case of solar differential rotation, exactly the reverse physical process occurs: turbulence (which must pre-exist) generates a mean flow which manifests itself in the form of differential rotation. Thus, the Boussinesq model is wholly inadequate because in the solar case, cause and effect are reversed. Since the Boussinesq model is inadequate, one needs an alternative model for the Reynolds stresses. We present a new dynamical model for the Reynolds stresses, convective fluxes, turbulent kinetic energy, and temperature fluctuations. The complete model requires the solution of 11 differential equations. We then introduce a set of simplifying assumptions which reduce the full dynamical model to a set of algebraic Reynolds stress models. We explicitly solve one of these models that entails only one differential equation. The overall agreement with the data is obtained with a model that is neither phenomenological nor one that requires a full numerical simulation, since it is algebraic in nature. The new model can play an important role in understanding the complex physics underlying the interplay between solar differential rotation and convection, as many physical processes can naturally be incorporated into the model.

Canuto, V. M.↗
NASA NTRS1994

Shock waves data for minerals

Shock compression of the materials of planetary interiors yields data which upon comparison with density-pressure and density-sound velocity profiles constrain internal composition and temperature. Other important applications of shock wave data and related properties are found in the impact mechanics of terrestrial planets and solid satellites. Shock wave equation of state, shock-induced dynamic yielding and phase transitions, and shock temperature are discussed. In regions where a substantial phase change in the material does not occur, the relationship between the particle velocity, U(sub p), and the shock velocity, U(sub s), is given by U(sub s) = C(sub 0) + S U(sub p), where C(sub 0) is the shock velocity at infinitesimally small particle velocity, or the ambient pressure bulk sound velocity. Numerical values for the shock wave equation of state for minerals and related materials of the solar system are provided.

Ahrens, Thomas J.↗
NASA NTRS1996

Polymer-Induced Depletion Interaction and Its Effect on Colloidal Sedimentation in Colloid-Polymer Mixtures

In this paper we focus on the polymer-induced depletion attraction and its effect on colloidal sedimentation in colloid-polymer mixtures. We first report a small angle neutron scattering (SANS) study of the depletion effect in a mixture of hard-sphere-like colloid and non-adsorbing polymer. Then we present results of our recent sedimentation measurements in the same colloid-polymer mixture. A key parameter in controlling the sedimentation of heavy colloidal particles is the interparticle potential U(tau), which is the work required to bring two colloidal particles from infinity to a distance tau under a give solvent condition. This potential is known to affect the average settling velocity of the particles and experimentally one needs to have a way to continuously vary U(tau) in order to test the theory. The interaction potential U(tau) can be altered by adding polymer molecules into the colloidal suspension. In a mixture of colloid and non-adsorbing polymer, the potential U(tau) can develop an attractive well because of the depletion effect, in that the polymer chains are expelled from the region between two colloidal particles when their surface separation becomes smaller than the size of the polymer chains. The exclusion of polymer molecules from the space between the colloidal particles leads to an unbalanced osmotic pressure difference pushing the colloidal particles together, which results in an effective attraction between the two colloidal particles. The polymer-induced depletion attraction controls the phase stability of many colloid-polymer mixtures, which are directly of interest to industry.

Tong, Penger↗
NASA NTRS1997

O2 Herzberg State Reaction with N2: A Possible Source of Stratospheric N2O

The goal of this one-year investigation was to determine whether N2O is formed in atmospherically significant quantities by the reaction of vibrationally excited levels of the O2((A3 Sigma(sub u)(sup +)) state with nitrogen. O2(A3 Sigma(sub u)(sup +)) is made throughout the upper stratosphere in considerable amounts by solar photoabsorption, and only a very small reactive yield is necessary for this mechanism to be a major N2O source. By long-term 245-252 nm irradiation of O2/N2 mixtures on- and off-resonance with absorption lines in the O2(A3 Sigma(sub u)(sup +) - X3 Sigma(sub g)(sup -)) transition, followed by N2O analysis by frequency-modulated diode laser absorption spectroscopy, we determined an upper limit for the N2O yield of the candidate reaction. This limit, 3 x 10(exp -5), eliminates O2(A3 Sigma(sub u)(sup +)) + N2 as a significant channel for the generation of stratospheric N2O. In further measurements, we established that N2O is stable under our photolysis conditions, showing that the small amounts of ozone generated from the reaction of O2(A) and O2 do not indirectly lead to destruction of N2O.

Slanger, Tom G.↗
NASA NTRS2000

The Turbulent/Non-Turbulent Interface Bounding a Far-Wake

The velocity fields of a turbulent wake behind a flat plate obtained from the direct numerical simulations of Moser et al. are used to study the structure of the flow in the intermittent zone where there are, alternately, regions of fully turbulent flow and non-turbulent velocity fluctuations either side of a thin randomly moving interface. Comparisons are made with a wake that is 'forced' by amplifying initial velocity fluctuations. There is also a random temperature field T in the flow; T varies between constant values of 0.0 and 1.0 on the sides of the wake. The value of the Reynolds number based on the centreplane mean velocity defect and halfwidth b of the wake is Re approx. = 2000. It is found that the thickness of the continuous interface is about equal to 0.07b, whereas the amplitude of fluctuations of the instantaneous interface displacement y(sub I)(t) is an order of magnitude larger, being about 0.5b. This explains why the mean statistics of vorticity in the intermittent zone can be calculated in terms of the probability distribution of y(sub I) and the instantaneous discontinuity in vorticity across the interface. When plotted as functions of y - y(sub I), the conditional mean velocity (U) and temperature (T) profiles show sharp jumps Delta(U) and Delta(T) at the interface adjacent to a thick zone where (U) and (T) vary much more slowly. Statistics for the vorticity and velocity variances, available in such detail only from DNS data, show how streamwise and spanwise components of vorticity are generated by vortex stretching in the bulges of the interface. Flow fields around the interface, analyzed in terms of the local streamline pattern, confirm previous results that the advancement of the vortical interface into the irrotational flow is driven by large-scale eddy motion. It is argued that because this is an inviscid mechanism the entrainment process is not sensitive to the value of Re, and that small-scale nibbling only plays a subsidiary role. While mean Reynolds stresses decrease gradually in the intermittent zone, conditional stresses are found to decrease sharply towards zero at the interface. Using one-point turbulence models applied to either unconditional or conditional statistics for the turbulent region and then averaged, the entrainment rate E(sub b) would, if calculated exactly, be zero. But if computed with standard computational methods, E(sub b) would be non-zero because of numerical diffusion. It is concluded that the current practice in statistical models of approximating entrainment by a diffusion process is computationally arbitrary and physically incorrect. An analysis shows how E(sub b) is related to Delta(U) and the jump in shear stress at the interface, and correspondingly to Delta(T) and the heat flux.

Bisset, David K.↗
NASA NTRS1996

FEM R-Matrix Calculations of Electron-H2 Collisions

Electron-H2 Collisions have been studied using the finite-element R-matrix method. Our approach uses a mixed finite-element and Gaussian basis to describe the continuum electron. In the inner region, the multi-centered nature of the Gaussian basis provides an efficient representation in the regions of space near the nuclei, whereas the piecemeal and energy-independent nature of the FEM basis is particular well suited for R-matrix calculations. Fixed nuclei calculations have been carried out at 1.4 a(sub 0), the equilibrium internuclear distance of the ground state. Up to six target states are included: the X(sup 1)E+(sub g), b(sup 3)E+(sub u), a(sup 3)E+(sub g), B(sup 1)E+(sub u), c(sup 3)II(sub u), and C(sup 1)II(sub u) states, and configuration-interaction functions are used to describe the target states. The results will be compared with available theoretical and experimental data.

Huo, Winifred M.↗