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Results for “POTENTIAL GRADIENT”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

On the relation between 'mixing length' and 'direct interaction approximation' theories of turbulence

The capabilities of direct interaction approximations (DIA) and weak-coupling approximations (WCA) in modeling turbulence in plasma physics studies are examined. DIA and WCA formalisms are reviewed and a DIA expression for homogeneous incompressible hydrodynamics is simplified to derive an energy spectrum in an arbitrary wavenumber space. A solution is defined for the expression when applied to long wavelength phenomena generated by an instability. The equation is further generalized to yield a transport equation for modeling the spectrum generated by homogeneous turbulence in a linearly unstable system undergoing nonlinear interaction. Finally, the technique is applied to two-dimensional turbulence in a low pressure, weakly ionized plasma embedded in a homogeneous magnetic field and experiencing instabilities caused by electron density and electric potential gradients, such as found in the ionospheric E region.

Sudan, R. N.↗
NASA NTRS1987

Electric potential distributions at the interface between plasmasheet clouds

At the interface between two plasma clouds with different densities, temperatures, and/or bulk velocities, there are large charge separation electric fields which can be modeled in the framework of a collisionless theory for tangential discontinuities. Two different classes of layers were identified: the first one corresponds to (stable) ion layers which are thicker than one ion Lamor radius; the second one corresponds to (unstable) electron layers which are only a few electron Larmor radii thick. It is suggested that these thin electron layers with large electric potential gradients (up to 400 mV/m) are the regions where large-amplitude electrostatic waves are spontaneously generated. These waves scatter the pitch angles of the ambient plasmasheet electron into the atmospheric loss cone. The unstable electron layers can therefore be considered as the seat of strong pitch angle scattering for the primary auroral electrons.

Evans, D. S.↗
NASA NTRS1987

Theoretical approach to oxygen atom degradation of silver

Based on available Rutherford backscattering spectrometry (RBS), proton induced X-ray emission (PIXE) and ellipsometry data obtained on silver specimens subjected to atomic oxygen attack in low Earth orbit STS flight 41-G, a theory was developed to model the oxygen atom degradation of silver. The diffusion of atomic oxygen in a microscopically nonuniform medium is an essential constituent of the theory. The driving force for diffusion is the macroscopic electrochemical potential gradient developed between the specimen surface exposed to the ambient and the bulk of the silver specimen. The longitudinal electric effect developed parallel to the gradient is modified by space charge of the diffusing charged species. Lateral electric fields and concentration differences also exist due to the nonuniform nature of the medium. The lateral concentration differences are found to be more important than the lateral electric fields in modifying the diffusion rate. The model was evaluated numerically. Qualitative agreement exists between the kinetics predicted by the theory and kinetic data taken in ground-based experiments utilizing a plasma asher.

Fromhold, Albert T., Jr.↗
NASA NTRS1986

Experimental and theoretical dynamics of isoelectric focusing - Elucidation of a general separation mechanism

The transient states in isoelectric focusing were monitored using a potential gradient array detector. Electric field profiles are presented which show the formation of transient moving boundaries, as well as the approach of the steady-state distribution, during the focusing of two- and three-component systems. The results are completely consistent with corresponding computer-simulation data. The focusing process is comprised of two sequential phases, a relatively rapid separation phase and a much slower stabilizing phase. It was found that the second phase can last as much as 20 times longer than the separation of the constituents.

Thormann, W.↗
NASA NTRS1985

Characterization of synthetic carrier ampholytes by repetitive scanning of the electric field during focusing

This paper reports the utilization of a potential gradient array detector for monitoring the dynamics of the electric field during isoelectric focusing. Transient and steady state electric field profiles are presented for synthetic carrier ampholyte mixtures with a wide (about 3-10) pH range. Two available commercial products (Ampholine and Pharmalyte) and a laboratory synthesized mixture (PEHA ampholytes) are compared. The formation of conductivity gaps and their migration toward the cathode in extended experiments (cathodic drift) can be visualized with this system.

Thormann, W.↗
NASA NTRS1988

Upper-level baroclinic instability

Effects on baroclinic instability of 'partial equilibration' and an Ekman lower boundary condition were examined by considering the linear stability of various zonal wind profiles having, as in the Charney (1947) problem, constant vertical shear and positive meridional gradient of potential vorticity aloft, but with reduced shear and zero or negative potential vorticity gradient at low levels. It is shown that, compared to the Charney problem, the partially equilibrated basic state zonal wind profiles obtained here have instabilities whose growth rates are less sensitive to the presence of an Ekman lower boundary condition.

Snyder, Christopher M.↗
NASA NTRS1991

Temporal study of wake formation behind a conducting body

The temporal evolution of the wake of a conducting body is studied experimentally in a pulsed plasma device. Three-dimensional measurements of the plasma potential, density, particle energy distribution, and ion currents are measured throughout the near- and mid-wake regions during the wake formation. It is found that the potential behind the conducting body is initially negative. This negative potential is caused by the higher mobility of the electrons, allowing them to flow into the ion free wake region. The negative potential in the wake region induces an electric field that pulls ions into the region behind the conducting body. However, the dominant factor in determining the length of the near wake is the thermal energy spread of the ions. At later times, as the sheath forms around the conducting body, ions are deflected by the potential gradient in the sheath region. This deflection, in addition to the thermal energy spread of the ions, determines the length of the near wake.

Meassick, S.↗
NASA NTRS1990

Cell wall and enzyme changes during the graviresponse of the leaf-sheath pulvinus of oat (Avena sativa)

The graviresponse of the leaf-sheath pulvinus of oat (Avena sativa) involves an asymmetric growth response and asymmetric processes involving degradation of starch and cell wall synthesis. Cellular and biochemical events were studied by investigation of the activities of related enzymes and changes in cell walls and their constituents. It is suggested that an osmotic potential gradient acts as the driving factor for growth, while wall extensibility is a limiting factor in pulvinus growth.

Gibeaut, David M.↗
NASA NTRS1924

Balloon flight and atmospheric electricity

The air is known to be charged with electricity (chiefly positive) with reference to the earth, so that its potential increases with the altitude and the difference in potential between two points in the same vertical line, divided by the distance between them, gives a value called the "potential gradient," which may vary greatly with the altitude, the nature of the ground and the atmospheric conditions.

BALLOON FLIGHTS↗
NASA NTRS1995

Far-infrared studies of the Galactic Center Arc using the Kuiper Airborne Observatory

Two molecular clouds are clearly interacting with the Galactic Center Radio Arc. The HII regions located at both sites of interaction - the 'Arched Filaments' and G0.18-0.04 - are powerful sources of far-infrared radiation, each having a total luminosity of approx. 10(exp 7) solar luminosity. Using the University of Texas far-IR Photometer at 50 and 90 microns, we found that the emission is substantially more extended than the filamentary HII regions, and that, while the intensity peaks at the locations of the filaments, the dust temperature shows only weak variations, implying that the heating is relatively uniform. The Arched Filaments were also observed with the University of Chicago far-IR Array Polarimeter, STOKES, at 100 microns. The greater than 100 independently sampled positions cover most of the 5 ft x 7 ft area of the Arched Filaments. The polarization is relatively strong (up to 9.4%) almost everywhere in this region. Also, the slow and smooth variations of the polarization angles across the region are consistent with the large magnetic field strengths and the highly-ordered geometry deduced from radio measurements. However, it appears that the magnetic field direction inferred from far-IR polarization (parallel to the Arched Filaments and following their curvature) is very different from that implied by the nonthermal radio filaments. The magnetic field, gas motions, and gravitational potential gradient in the arched filament region are all consistent with gravitational shearing of the molecular cloud underlying the Arched Filaments.

Morris, Mark↗
NASA NTRS1999

Evolution of Local Microstructures (ELMS): Spatial Instabilities of Coarsening

This work examines the diffusional growth of discrete phase particles dispersed within a matrix. Engineering materials are microstructurally heterogeneous, and the details of the microstructure determine how well that material performs in a given application. Critical to the development of designing multiphase microstructures with long-term stability is the process of Ostwald ripening. Ripening, or phase coarsening, is a diffusion-limited process which arises in polydisperse multiphase materials. Growth and dissolution occur because fluxes of solute, driven by chemical potential gradients at the interfaces of the dispersed phase material, depend on particle size. The kinetics of these processes are "competitive," dictating that larger particles grow at the expense of smaller ones, overall leading to an increase of the average particle size. The classical treatment of phase coarsening was done by Todes, Lifshitz, and Slyozov, (TLS) in the limit of zero volume fraction, V(sub v), of the dispersed phase. Since the publication of TLS theory there have been numerous investigations, many of which sought to describe the kinetic scaling behavior over a range of volume fractions. Some studies in the literature report that the relative increase in coarsening rate at low (but not zero) volume fractions compared to that / 2 1/ 3 predicted by TLS is proportional to V(sub v)(exp 1/2), whereas others suggest V(sub v)(exp 1/3). This issue has been resolved recently by simulation studies at low volume fractions in three dimensions by members of the Rensselaer/MSFC team.

Glicksman, Martin E.↗
NASA NTRS1999

Evolution of Local Microstructures: Spatial Instabilities of Coarsening Clusters

This work examines the diffusional growth of discrete phase particles dispersed within a matrix. Engineering materials are microstructurally heterogeneous, and the details of the microstructure determine how well that material performs in a given application. Critical to the development of designing multiphase microstructures with long-term stability is the process of Ostwald ripening. Ripening, or phase coarsening, is a diffusion-limited process which arises in polydisperse multiphase materials. Growth and dissolution occur because fluxes of solute, driven by chemical potential gradients at the interfaces of the dispersed phase material, depend on particle size. The kinetics of these processes are "competitive," dictating that larger particles grow at the expense of smaller ones, overall leading to an increase of the average particle size. The classical treatment of phase coarsening was done by Todes, Lifshitz, and Slyozov, (TLS) in the limit of zero volume fraction, V(sub v), of the dispersed phase. Since the publication of TLS theory there have been numerous investigations, many of which sought to describe the kinetic scaling behavior over a 0 range of volume fractions. Some studies in the literature report that the relative increase in coarsening rate at low (but not zero) volume fractions compared to that predicted by TLS is proportional to v(sub v)(exp 1/2), whereas others suggcest V(sub v)(exp 1/3). This issue has been resolved recently by simulation studies at low volume fractions in three dimensions by members of the Rensselaer/MSFC team. Our studies of ripening behavior using large-scale numerical simulations suggest that although there are different circumstances which can lead to either scaling law, the most important length scale at low volume fractions is the diffusional analog of the Debye screening length. The numerical simulations we employed exploit the use of a recently developed "snapshot" technique, and identifies the nature of the coarsening dynamics at various volume fractions. Preliminary results of numerical and experimental investigations, focused on the growth of finite particle clusters, provide important insight into the nature of the transition between the two scaling regimes. The companion microgravity experiment centers on the growth within finite particle clusters, and follows the temporal dynamics driving microstructural evolution, using holography.

Frazier, Donald O.↗
NASA NTRS2001

Evolution of Local Microstructures: Spatial Instabilities of Coarsening Clusters

This work examines the diffusional growth of discrete phase particles dispersed within a matrix. Engineering materials are often microstructurally heterogeneous, and the details of the microstructure determine how well that material performs in a given application. Critical to the development of designing multiphase microstructures with long-term stability is the process of Ostwald ripening. Ripening, or phase coarsening, is diffusion-limited and arises in polydisperse multiphase materials. Growth and dissolution occur because fluxes of solute, driven by chemical potential gradients at the interfaces of the dispersed phase material, depend on particle size. Competitive kinetics of these processes dictates that larger particles grow at the expense of smaller ones, overall leading to an increase of the average particle size. The classical treatment of phase coarsening was done by Todes, Lifshitz, and Slyozov, (TLS) in the limit of zero volume fraction, V(sub V)=0 of the dispersed phase. Since the publication of TLS theory, there have been numerous investigations, many of which sought to describe the kinetic scaling behavior over a range of volume fractions. Some studies in the literature report that the relative increase in coarsening rate at low (but not zero) volume fractions compared to that predicted by TLS is proportional to V(exp 1/2)(sub v) whereas others suggest V(exp 1/3)(sub v).This issue has been addressed recently by simulation studies at low volume fractions in three dimensions by members of the Rensselaer/MSFC team.

Glicksman, Martin E.↗
NASA NTRS2001

Antimatter Production at a Potential Boundary

Current antiproton production techniques rely on high-energy collisions between beam particles and target nuclei to produce particle and antiparticle pairs, but inherently low production and capture efficiencies render these techniques impractical for the cost-effective production of antimatter for space propulsion and other commercial applications. Based on Dirac's theory of the vacuum field, a new antimatter production concept is proposed in which particle-antiparticle pairs are created at the boundary of a steep potential step formed by the suppression of the local vacuum fields. Current antimatter production techniques are reviewed, followed by a description of Dirac's relativistic quantum theory of the vacuum state and corresponding solutions for particle tunneling and reflection from a potential barrier. The use of the Casimir effect to suppress local vacuum fields is presented as a possible technique for generating the sharp potential gradients required for particle-antiparticle pair creation.

LaPointe, Michael R.↗
NASA NTRS1987

Wall relaxation in growing stems: comparison of four species and assessment of measurement techniques

This study was carried out to develop improved methods for measuring in-vivo stress relaxation of growing tissues and to compare relaxation in the stems of four different species. When water uptake by growing tissue is prevented, in-vivo stress relaxation occurs because continued wall loosening reduces wall stress and cell turgor pressure. With this procedure one may measure the yield threshold for growth (Y), the turgor pressure in excess of the yield threshold (P-Y), and the physiological wall extensibility (phi). Three relaxation techniques proved useful: "turgor-relaxation", "balance-pressure" and "pressure-block". In the turgor-relaxation method, water is withheld from growing tissue and the reduction in turgor is measured directly with the pressure probe. This technique gives absolute values for P and Y, but requires tissue excision. In the balance-pressure technique, the excised growing region is sealed in a pressure chamber, and the subsequent reduction in water potential is measured as the applied pressure needed to return xylem sap to the cut surface. This method is simple, but only measures (P-Y), not the individual values of P and Y. In the pressure-block technique, the growing tissue is sealed into a pressure chamber, growth is monitored continuously, and just sufficient pressure is applied to the chamber to block growth. The method gives high-resolution kinetics of relaxation and does not require tissue excision, but only measures (P-Y). The three methods gave similar results when applied to the growing stems of pea (Pisum sativum L.), cucumber (Cucumis sativus L.), soybean (Glycine max (L.) Merr.) and zucchini (Curcubita pepo L.) seedlings. Values for (P-Y) averaged between 1.4 and 2.7 bar, depending on species. Yield thresholds averaged between 1.3 and 3.0 bar. Compared with the other methods, relaxation by pressure-block was faster and exhibited dynamic changes in wall-yielding properties. The two pressure-chamber methods were also used to measure the internal water-potential gradient (between the xylem and the epidermis) which drives water uptake for growth. For the four species it was small, between 0.3 and 0.6 bar, and so did not limit growth substantially.

NASA Program Space Biology↗
NASA NTRS2006

Towards the Identification of the Keeper Erosion Cause(s): Numerical Simulations of the Plasma and Neutral Gas Using the Global Cathode Model OrCa2D-II

Numerical simulations with the time-dependent Orificed Cathode (OrCa2D-II) computer code show that classical enhancements of the plasma resistivity can not account for the elevated electron temperatures and steep plasma potential gradients measured in the plume of a 25-27.5 A discharge hollow cathode. The cathode, which employs a 0.11-in diameter orifice, was operated at 5.5 sccm without an applied magnetic field using two different anode geometries. It is found that anomalous resistivity based on electron-driven instabilities improves the comparison between theory and experiment. It is also estimated that other effects such as the Hall-effect from the self-induced magnetic field, not presently included in OrCa2D-II, may contribute to the constriction of the current density streamlines thus explaining the higher plasma densities observed along the centerline.

plasma modeling↗