SEARCH · Engineering Papers

Results for “forcing fields”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,117 records · Page 62

All sources NASA NTRS DOE OSTI

NASA NTRS1979

Electric generators in the magnetosphere-ionosphere system

The nature and cause of electric fields in the coupled magnetosphere-ionosphere system and their effects in the middle atmosphere are discussed. Electric fields induced by the solar wind are reported. The equivalence between forces and electric currents and also between current closure and stress balance is described. The motions of the magnetospheric and ionospheric fluids and the electric fields of interest were determined by the balance of these forces, or equivalently by the closure of the currents. The physical nature of stresses and currents was examined.

Akinson, G.↗

NASA NTRS1996

A Geophysical Flow Experiment in a Compressible Critical Fluid

The first objective of this experiment is to build an experimental system in which, in analogy to a geophysical system, a compressible fluid in a spherical annulus becomes radially stratified in density through an A.C. electric field. When this density gradient is demonstrated, the system will be augmented so that the fluid can be driven by heating and rotation and tested in preparation for a microgravity experiment. This apparatus consists of a spherical capacitor filled with critical fluid in a temperature controlled environment. To make the fluid critical, the apparatus will be operated near the critical pressure, critical density, and critical temperature of the fluid. This will result in a highly compressible fluid because of the properties of the fluid near its critical point. A high voltage A.C. source applied across the capacitor will create a spherically symmetric central force because of the dielectric properties of the fluid in an electric field gradient. This central force will induce a spherically symmetric density gradient that is analogous to a geophysical fluid system. To generate such a density gradient the system must be small (approx. 1 inch diameter). This small cell will also be capable of driving the critical fluid by heating and rotation. Since a spherically symmetric density gradient can only be made in microgravity, another small cell, of the same geometry, will be built that uses incompressible fluid. The driving of the fluid by rotation and heating in these small cells will be developed. The resulting instabilities from the driving in these two systems will then be studied. The second objective is to study the pattern forming instabilities (bifurcations) resulting from the well controlled experimental conditions in the critical fluid cell. This experiment will come close to producing conditions that are geophysically similar and will be studied as the driving parameters are changed.

Hegseth, John↗

NASA NTRS2016

Magnetospheric Multiscale Mission Observations and Non-Force Free Modeling of a Flux Transfer Event Immersed in a Super-Alfvenic Flow

We analyze plasma, magnetic field, and electric field data for a flux transfer event (FTE) to highlight improvements in our understanding of these transient reconnection signatures resulting from high-resolution data. The approximate 20 s long, reverse FTE, which occurred south of the geomagnetic equator near dusk, was immersed in super-Alfvnic flow. The field line twist is illustrated by the behavior of flows parallel perpendicular to the magnetic field. Four-spacecraft timing and energetic particle pitch angle anisotropies indicate a flux rope (FR) connected to the Northern Hemisphere and moving southeast. The flow forces evidently overcame the magnetic tension. The high-speed flows inside the FR were different from those outside. The external flows were perpendicular to the field as expected for draping of the external field around the FR. Modeling the FR analytically, we adopt a non-force free approach since the current perpendicular to the field is nonzero. It reproduces many features of the observations.

Farrugia, C. J.↗

NASA NTRS1976

Hydraulic concentration of fields in the solar photosphere. IV - Evolution of fields near equipartition

It has previously been inferred that small isolated flux tubes appearing in supergranule boundaries are compressed to 1500 gauss or more. This paper considers whether some dynamic condition within a flux tube exists which provides both stability and a 'mechanical advantage' so that a small force over a small period of time can accomplish the enormous compression from the weak-field to the strong-field state. It is found that the equipartition solutions to the hydromagnetic equations apparently may have the desired property of permitting an infinitesimal external pressure to convert a gentle flow of gas along a weak field into a very intense field through a succession of equipartition states. An illustrative example is presented, and field compression by convective forces is analyzed.

Parker, E. N.↗

NASA NTRS1989

An approximate analytic solution of a set of nonlinear model alpha-omega-dynamo equations for marginally unstable systems

An approximate analytic solution of a set of nonlinear model alpha-omega-dynamo equations is obtained. The reaction of the Lorentz force on the velocity shear which stretches and, hence, amplifies the magnetic field is incorporated into the model. To single out the effect of the Lorentz force on the omega-effect, the effect of the Lorentz force on the alpha-effect is neglected in this study. The solution represents a nonlinear oscillation with the amplitude and period determined by the dynamo number N. The amplitude is proportional to N - 1, while the period is almost exactly the same as the dissipation time of the unstable mode (proportional to N).

Hinata, S.↗

NASA NTRS2002

Transient Mixing Driven by Buoyancy Flows

Mixing driven by buoyancy-induced flows is of particular interest to microgravity processes, as the body force that governs the intensity of flow fields can be directly controlled. We consider a model experimental system to explore the dynamics of mixing which employs two miscible liquids inside a cavity separated initially by a divider. The two liquids are oriented vertically inside a rectangular cavity with constant width and height, and varying depths to span the range of a Hele-Shaw cell to a 3-[) configuration. The two miscible liquids can be sufficiently diluted and died, for example water and deuterium oxide, such that a distinct interface exists across the divider. The transient mixing characteristic of the two fluids is addressed by following the Lagrangian history of the interface for various aspect ratios in the z-plane (depth variation) as well as a range of pulling velocities of the divider. The mixing characteristics of the two fluids are quantified from measurement of the length stretch of the interface and its flow field using respectively image processing techniques and Particle Imaging Velocimetry. Scaling analysis shows that the length stretch depends on four governing parameters, namely the Grashof number (Gr), Schmidt number (Sc), aspect ratio (Ar), and Reynolds number (Re). Variation of the Schmidt number is taken into account through thermophysical property variation. Thus our problem reduces to a codimension three bifurcation in parametric space for Gr, Ar, and Re. Our experimental results show that for Gr on the order of 106 and a nominal cavity aspect ratio Ar = 0.2, the net effect of removal of the divider and the overwhelming buoyancy force causes an overturning motion which stretches and folds the interface to produce ml internal breakwave. The structure of the breakwave is similar to the ubiquitous Rayleigh-Taylor instability morphology. The breakwave is dissipated either through internal or wall collision depending on the impulsive velocity of the divider as prescribed by the Reynolds number. The decay of the collision event occurs through sloshing oscillations over a short time scale. The two fluids then become stably stratified with a diffusive band at the interface indicating local mass transport. The local bifurcation of the internal breakwave is investigated as a function of aspect ratio. Results show that for narrow cavities on the order of 2mm (Ar = 0.04) folding does not occur, the interface only stretches. As the cavity size increases folding occurs through a supercritical bifurcation. Insight into the mechanism of folding is obtained from measurement of the flow field which shows that in the neighborhood of the folding event, there exists hyperbolic points caused by multiple vortex interactions. The global length stretch of the interface as a function of time is nearly Gaussian; calculations of finite-time Liapunov exponents as well as construction of horseshoe maps indicate the likelihood of a chaotic transient.

W.M.B. Duval↗

NASA NTRS1991

Vortical flow control on a slender body at high angles of attack

Attention is given to the possibility of using tangential blowing to control vortical flow on a missile-like slender body at high angles of attack. Emphasis is placed on smoke- and surface oil-flow visualization, force measurements, and velocity and pressure measurements using a 5-hole probe. It is shown that the tangential blowing significantly alters the flow field downstream of the slot and creates large side forces and yawing moments to very high angles of attack. Effectiveness of the tangential blowing is discussed in terms of jet momentum, blowing location, slot area, and Reynolds number. The concept proved to be efficient at small blowing rates and at high angles of attack.

Celik, Zeki Z.↗

NASA NTRS1984

Forced convection heat transfer to air/water vapor mixtures

Heat transfer coefficients were measured using both dry and humid air in the same forced convection cooling scheme and were compared using appropriate nondimensional parameters (Nusselt, Prandtl and Reynolds numbers). A forced convection scheme with a complex flow field, two dimensional arrays of circular jets with crossflow, was utilized with humidity ratios (mass ratio of water vapor to air) up to 0.23. The dynamic viscosity, thermal conductivity and specific heat of air, steam and air/steam mixtures are examined. Methods for determining gaseous mixture properties from the properties of their pure components are reviewed as well as methods for determining these properties with good confidence. The need for more experimentally determined property data for humid air is discussed. It is concluded that dimensionless forms of forced convection heat transfer data and empirical correlations based on measurements with dry air may be applied to conditions involving humid air with the same confidence as for the dry air case itself, provided that the thermophysical properties of the humid air mixtures are known with the same confidence as their dry air counterparts.

Richards, D. R.↗

NASA NTRS1999

Comparison of Radiative Forcing Calculations Due to Mineral Dust from a Transport Model, Satellite Measurements and an Assimilation System

This study uses information on mineral aerosol from a transport model to calculate global radiative forcing values. The transport model is driven by assimilated meteorology and outputs three-dimensional dust spatial information for various size ranges. The dust fields are input to an off-line radiative transfer calculation to obtain the direct radiative forcing due to the dust fields. During June, July and August of 1988 presence of dust 1) reduces the global net incoming radiation at the top of atmosphere (TOA) by 0.3 to 0.7 W/sq m and 2) reduces net incoming radiation at the earth's surface by 1.3 to 2.0 W/sq m. Over Africa our estimates of the reduction of radiation at the top of atmosphere compare well with TOA reductions derived from ERBE and TOMS satellite data. However, our heating rates are not consistent with analysis temperature increments produced by the assimilation system over regions of high aerosol loading. These increments are based on differences between temperature observations and temperatures from the assimilation general circulation model. One explanation is that the lower tropospheric temperatures retrieved by TOVS are being contaminated by mineral aerosol.

Weaver, Clark J.↗

NASA NTRS2004

Electric-field-driven Phenomena for Manipulating Particles in Micro-Devices

Compared to other available methods, ac dielectrophoresis is particularly well-suited for the manipulation of minute particles in micro- and nano-fluidics. The essential advantage of this technique is that an ac field at a sufficiently high frequency suppresses unwanted electric effects in a liquid. To date very little has been achieved towards understanding the micro-scale field-and shear driven behavior of a suspension in that, the concepts currently favored for the design and operation of dielectrophoretic micro-devices adopt the approach used for macro-scale electric filters. This strategy considers the trend of the field-induced particle motions by computing the spatial distribution of the field strength over a channel as if it were filled only with a liquid and then evaluating the direction of the dielectrophoretic force, exerted on a single particle placed in the liquid. However, the exposure of suspended particles to a field generates not only the dielectrophoretic force acting on each of these particles, but also the dipolar interactions of the particles due to their polarization. Furthermore, the field-driven motion of the particles is accompanied by their hydrodynamic interactions. We present the results of our experimental and theoretical studies which indicate that, under certain conditions, these long-range electrical and hydrodynamic interparticle interactions drastically affect the suspension behavior in a micro-channel due to its small dimensions.

Khusid, Boris↗

NASA NTRS1974

Magneto-fluid-mechanics free convection turbulent flow

The present work is an experimental study of the influence of a uniform magnetic field on the structure of a free convection turbulent boundary layer in a conducting fluid. The boundary layer was formed along the heated vertical wall of a cell. The applied magnetic field was normal to the wall. The measured mean temperature profiles, temperature turbulent intensity distributions, and temperature spectra along the wall, indicated that transition from turbulent to laminar flow occurs at a constant value of the ratio (Rayleigh number)/(Hartmann number). The study of the recorded spectra indicated that the presence of the magnetic field enhances the mechanism of turbulent suppression due to the buoyancy forces. Finally, a possible mechanism by which turbulence is suppressed by the presence of a magnetic field is discussed.

Papailiou, D. D.↗

NASA NTRS2016

Aerosol Complexity and Implications for Predictability and Short-Term Forecasting

There are clear NWP and climate impacts from including aerosol radiative and cloud interactions. Changes in dynamics and cloud fields affect aerosol lifecycle, plume height, long-range transport, overall forcing of the climate system, etc. Inclusion of aerosols in NWP systems has benefit to surface field biases (e.g., T2m, U10m). Including aerosol affects has impact on analysis increments and can have statistically significant impacts on, e.g., tropical cyclogenesis. Above points are made especially with respect to aerosol radiative interactions, but aerosol-cloud interaction is a bigger signal on the global system. Many of these impacts are realized even in models with relatively simple (bulk) aerosol schemes (approx.10 -20 tracers). Simple schemes though imply simple representation of aerosol absorption and importantly for aerosol-cloud interaction particle-size distribution. Even so, more complex schemes exhibit a lot of diversity between different models, with issues such as size selection both for emitted particles and for modes. Prospects for complex sectional schemes to tune modal (and even bulk) schemes toward better selection of size representation. I think this is a ripe topic for more research -Systematic documentation of benefits of no vs. climatological vs. interactive (direct and then direct+indirect) aerosols. Document aerosol impact on analysis increments, inclusion in NWP data assimilation operator -Further refinement of baseline assumptions in model design (e.g., absorption, particle size distribution). Did not get into model resolution and interplay of other physical processes with aerosols (e.g., moist physics, obviously important), chemistry

Predictability↗

NASA NTRS2000

Electrostatics of Granular Material (EGM): Space Station Experiment

Aggregates were observed to form very suddenly in a lab-contained dust cloud, transforming (within seconds) an opaque monodispersed cloud into a clear volume containing rapidly-settling, long hair-like aggregates. The implications of such a "phase change" led to a series of experiments progressing from the lab, to KC-135, followed by micro-g flights on USML-1 and USML-2, and now EGM slated for Space Station. We attribute the sudden "collapse" of a cloud to the effect of dipoles. This has significant ramifications for all types of cloud systems, and additionally implicates dipoles in the processes of cohesion and adhesion of granular matter. Notably, there is the inference that like-charged grains need not necessarily repel if they are close enough together: attraction or repulsion depends on intergranular distance (the dipole being more powerful at short range), and the D/M ratio for each grain, where D is the dipole moment and M is the net charge. We discovered that these ideas about dipoles, the likely pervasiveness of them in granular material, the significance of the D/M ratio, and the idea of mixed charges on individual grains resulting from tribological processes --are not universally recognized in electrostatics, granular material studies, and aerosol science, despite some early seminal work in the literature, and despite commercial applications of dipoles in such modern uses as "Krazy Glue", housecleaning dust cloths, and photocopying. The overarching goal of EGM is to empirically prove that (triboelectrically) charged dielectric grains of material have dipole moments that provide an "always attractive" intergranular force as a result of both positive and negative charges residing on the surfaces of individual grains. Microgravity is required for this experiment because sand grains can be suspended as a cloud for protracted periods, the grains are free to rotate to express their electrostatic character, and Coulombic forces are unmasked. Suspended grains will be "interrogated" by applied electrical fields. In one module, grains will be immersed in an inhomogeneous electric field and allowed to be attracted towards or repelled from the central electrode of the module: part of the grain's speed will be a function of its net charge (monopole), part will be a function of the dipole. Observed grain position vs. time will provide a curve that can be deconvolved into the dipole and monopole forces responsible, since both have distinctive radial dependencies. In a second approach, the inhomogeneous field will be alternated at low frequency (e.g., every 5-10 seconds) so that the grains are alternately attracted and repelled from the center of the field. The resulting "zigzag" grain motion will gradually drift inwards, then suddenly change to a unidirectional inward path when a critical radial distance is encountered (a sort of "Coulombic event horizon") at which the dipole strength supersedes the monopole strength --thus proving the presence of a dipole, while also quantifying the D/M ratio. In a second module, an homogeneous electric field eliminates dipole effects (both Coulombic and induced) to provide calibration of the monopole and to more readily evaluate net charge statistical variance. In both modules, the e-fields will be exponentially step-ramped in voltage during the experiment, so that the field "nominalizes" grain speed while spreading the response time --effectively forcing each grain to "wait its turn" to be measured. In addition to rigorously quantifying M, D, and the D/M ratio for many hundreds of grains, the experiment will also observe gross electrometric and RF discharge phenomena associated with grain activity. The parameter space will encompass grain charging levels (via intentional triboelectrification), grain size, cloud density, and material type. Results will prove or disprove the dipole hypothesis. In either case, light will be shed on the role of electrostatic forces in governing granular systems. Knowledge so gained can be applied to natural clouds such as protostellar and protoplanetary dust and debris systems, planetary rings, planetary dust palls and aerosols created by volcanic, impact, aeolian, firestorm, or nuclear winter processes. The data are also directly applicable to adhesion, cohesion, transport, dispersion, and collection of granular materials in industrial, agricultural, pharmaceutical applications, and in fields as diverse as dust contamination of space suits on Mars and crop spraying on Earth.

Marshall, J.↗

NASA NTRS1987

Aeroelastic control of stability and forced response of supersonic rotors by aerodynamic detuning

Aerodynamic detuning, defined as designed passage-to-passage differences in the unsteady aerodynamic flow field of a rotor blade row, is a new approach to passive flutter and forced response control. In this paper, a mathematical model for aerodynamic detuning is developed and utilized to demonstrate the aeroelastic stability enhancement due to aerodynamic detuning of supersonic blade rows. In particular, a model is developed to analyze both the torsion mode and the coupled bending-torsion mode unstalled supersonic flutter and torsion mode aerodynamically forced response characteristics of an aerodynamically detuned rotor operating in a supersonic inlet flow field with a subsonic leading edge locus. As small solidity variations do not have a dominant effect on the steady-state performance of a rotor, the aerodynamic detuning mechanism considered is nonuniform circumferential spacing of adjacent blades.

Hoyniak, Daniel↗

NASA NTRS1992

Precision GPS ephemerides and baselines

The required knowledge of the Global Positioning System (GPS) satellite position accuracy can vary depending on a particular application. Application to relative positioning of receiver locations on the ground to infer Earth's tectonic plate motion requires the most accurate knowledge of the GPS satellite orbits. Research directed towards improving and evaluating the accuracy of GPS satellite orbits was conducted at the University of Texas Center for Space Research (CSR). Understanding and modeling the forces acting on the satellites was a major focus of the research. Other aspects of orbit determination, such as the reference frame, time system, measurement modeling, and parameterization, were also investigated. Gravitational forces were modeled by truncated versions of extant gravity fields such as, Goddard Earth Model (GEM-L2), GEM-T1, TEG-2, and third body perturbations due to the Sun and Moon. Nongravitational forces considered were the solar radiation pressure, and perturbations due to thermal venting and thermal imbalance. At the GPS satellite orbit accuracy level required for crustal dynamic applications, models for the nongravitational perturbation play a critical role, since the gravitational forces are well understood and are modeled adequately for GPS satellite orbits.

Source record↗

NASA NTRS1992

Radiation force on a single atom in a cavity

We consider the radiation pressure microscopically. Two perfectly conducting plates are parallelly placed in a vacuum. As the vacuum field hits the plates they get pressure from the vacuum. The excessive outside modes of the vacuum field push the plates together, which is known as the Casimer force. We investigate the quantization of the standing wave between the plates to study the interaction between this wave and the atoms on the plates or between the plates. We show that even the vacuum field pushes the atom to place it at nodes of the standing wave.

Kim, M. S.↗

NASA NTRS1975

The escape of magnetic flux from a turbulent body of gas

Topological conditions on the surface of a turbulent body of highly conductive fluid under which escape of magnetic force lines would be impossible (Drobyshevski, Yuferev 1974) are studied with exact calculation of intermediate field configurations as a possible explanation of the decay of stellar magnetic fields via turbulent mixing. It is shown that upward escape of magnetic force lines in the regions between convective cells is possible through rapid reconnection of the force lines (studied as neutral point annihilation) such as occurs continually in rapidly evolving sunspot groups. It is concluded that there exists no topological barrier to the escape of magnetic flux from the sun or from the local galaxy.

Parker, E. N.↗

NASA NTRS1992

Evolution of a magnetic flux tube in two-dimensional penetrative convection

Highly supercritical compressible convection is simulated in a two-dimensional domain in which the upper half is unstable to convection while the lower half is stably stratified. This configuration is an idealization of the layers near the base of the solar convection zone. Once the turbulent flow is well developed, a toroidal magnetic field B sub tor is introduced to the stable layer. The field's evolution is governed by an advection-diffusion-type equation, and the Lorentz force does not significantly affect the flow. After many turnover times the field is stratified such that the absolute value of B sub tor/rho is approximately constant in the convective layer, where rho is density, while in the stable layer this ratio decreases linearly with depth. Consequently most of the magnetic flux is stored in the overshoot layer. The inclusion of rotation leads to travelling waves which transport magnetic flux latitudinally in a manner reminiscent of the migrations seen during the solar cycle.

Jennings, R. L.↗