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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

Thrust Stand Measurements of a Conical Inductive Pulsed Plasma Thruster

Inductive Pulsed Plasma Thrusters (iPPT) spacecraft propulsion devices in which electrical energy is capacitively stored and then discharged through an inductive coil. The thruster is electrodeless, with a time-varying current in the coil interacting with a plasma covering the face of the coil to induce a plasma current Propellant is accelerated and expelled at a high exhaust velocity (O(10 -- 100 km/s)) by the Lorentz body force arising from the interaction of the magnetic field and the induced plasma current. While this class of thruster mitigates the life-limiting issues associated with electrode erosion, inductive pulsed plasma thrusters can suffer from both high pulse energy requirements imposed by the voltage demands of inductive propellant ionization, and low propellant utilization efficiencies. While this class of thruster mitigates the life-limiting issues associated with electrode erosion, inductive pulsed plasma thrusters can suffer from both high pulse energy requirements imposed by the voltage demands of inductive propellant ionization, and low propellant utilization efficiencies. A conical coil geometry may offer higher propellant utilization efficiency over that of a at inductive coil, however an increase in propellant utilization may be met with a decrease in axial electromagnetic acceleration, and in turn, a decrease in the total axially-directed kinetic energy imparted to the propellant.

Hallock, Ashley K.↗
NASA NTRS1976

Theoretical investigations of plasma processes

System analyses are presented for electrically sustained, collision dominated plasma centrifuges, in which the plasma rotates under the influence of the Lorentz forces resulting from the interaction of the current density fields with an external magnetic field. It is shown that gas discharge centrifuges are technically feasible in which the plasma rotates at speeds up to 1 million cm/sec. The associated centrifugal forces produce a significant spatial isotope separation, which is somewhat perturbed in the viscous boundary layers at the centrifuge walls. The isotope separation effect is the more pronounced. The induced magnetic fields have negligible influence on the plasma rotation if the Hall coefficient is small. In the technical realization of collision dominated plasma centrifuges, a trade-off has to be made between power density and speeds of rotation. The diffusion of sputtered atoms to system surfaces of ion propulsion systems and the deposition of the atoms are treated theoretically by means of a simple model which permits an analytical solution. The problem leads to an inhomogeneous integral equation.

Wilhelm, H. E.↗
NASA NTRS2004

Modeling the interaction Between Ethylene Diamine and Water Films on the Surface of a Carbon Nanotube

It has been observed that a carbon nanotube (CNT) AFM tip coated with ethylene diamine (EDA) penetrates the liquid water-air interface more easily than an uncoated nanotube tip. The EDA coating remains intact through repeated cycles of dipping and removal. In order to understand the physical basis for this observation, we use ab initio quantum chemistry calculations to study the EDA-CNT-water interaction and to parameterize a force field describing this system. Molecular dynamics (MD) simulations are carried out for EDA-water mixtures and an EDA-coated carbon nanotube immmed in water. These simulations are similar to our earlier MD study that characterized the CNT-water interface. The attractive CNT-EDA and CNT-water interactions arise primarily from van der Waals forces, and the EDA-EDA, EDA-water and water-water interactions are mainly due to hydrogen bond formation. The binding energ of single EDA molecule to the nanotube is nearly three times larger than the corresponding value found for water (4.3 versus 1.5 kcal mol, respectively). The EDA molecules readily stick to and diffuse along the CNT surface. As a resulf mixing of the EDA and water films does not occur on the timescale of the MD simulations. The EDA film reduces the hydrophobicity of the nanotube surface and acts like a prototypical surfactant in stabilizing the suspension of carbon nanotubes in water. For this presentation, we use the MD simulations to determine how the presence of the carbon nanotube surface perturbs the properties of EDA-water mixtures.

Jaffe, Richard L.↗
NASA NTRS1982

A spectral energetics analysis of atmospheric blocking

The spectral energetics of two blocking case studies from the winter of 1978-79 are calculated. One case occurred over the North Atlantic and the other over the North Pacific. The temporal evolution and geographical distribution of the energetics of the planetary-scale (zonal harmonic wavenumbers 1-4) and the intermediate-scale (wavenumbers 5-10) wave ensembles are examined to identify the physical mechanisms important in the blocking process. The Atlantic block was forced by the nonlinear interaction of intense baroclinic cyclone-scale waves with barotropic, ultralong waves. The Pacific blocking resulted from the baroclinic amplification of planetary-scale waves. In addition, at least two necessary conditions are required for the development of blocking. First, one of the forcing mechanisms must be in operation, and second, the phase of the antecedent planetary-scale waves must be such that the forcing mechanism can amplify them into a blocking pattern. In each of the cases studied, the blocking developed downstream of intense intermediate- or large-scale cyclogenesis.

Hansen, A. R.↗
NASA NTRS2011

An Investigation of Interaction of Saharan Dust and Atlantic ITCZ Using Cloudsat-Calipso and A-Train Data

In this study, we investigate the radiative forcing of Saharan dust, its interactions with the Atlantic Intertropical Convergence Zone (ITCZ), through African easterly waves (AEW), African easterly jets (AEJ), and its impacts in short term numerical forecasts of tropical cyclogenesis using the GOCART-GEOS5 forecast system. Our approach is to develop and use an A-Train satellite simulator (ATSS) to constrain the observed aerosol index of refraction and particle size distribution by finding the values that simultaneously minimize the difference between observed CALIOP, CloudSat, OMI, and MODIS radiances and simulated radiances inverted from atmospheric model output using procedures and physical principles consistent with those used in corresponding retrieval algorithms. We use observations from the A-train and TRMM to determine relationships among the Saharan dust layer, transport by the AEW, and possible responses to dust radiative forcing in developing tropical cyclones in the A-ITCZ. Preliminary model results showing physical processes associated with the generation and transport of the Saharan dust layer, their interactions with the incipient moisture, clouds and rainfall in developing tropical cyclones will be presented. Also presented will be results of a case study of possible radiative impacts on AEW and AEJ during the NAMMA field campaign.

Lau, W.↗
NASA NTRS1982

Preliminary results on the abradability of porous, sintered seal material

Preliminary results are presented for the case of titanium blade specimens, with bare tips (or covered with wear resistant, plasma sprayed materials) rubbing at 100 m/s against specimens of abradable nickel-chromium seal material moving toward the rotating blades at 0.0125 mm/s or at 0.025 mm/s. Using a two component dynamometer, the normal force of the rub interaction was measured and the shear component estimated. The elastoplastic properties of the seal material have been determined and those parameters as well as the rigidity of the rub tester system are considered in conjunction with those affecting the accuracy of the measurement of the forces arising at the blade-seal interface. The average and the 'local instantaneous' temperatures of the seal specimen and the temperature of the blade tip surface during rubbing are presented as functions of time. A seal densification factor is defined and its functional relationships with contact force components, temperature, wear ratio and blade tip abrading capability are indicated.

Wolak, J.↗
NASA NTRS2018

The Signature of Ozone Depletion in Recent Antarctic Precipitation Change: A Study with the Community Earth System Model

Although precipitation is a primary control on Antarctic ice sheet (AIS) mass balance, long-term historical AIS precipitation trends and their underlying external climate drivers remain inconclusive. In this study, we use a novel pair of climate model ensembles to identify a simulated spatial signature of ozone depletion-forced AIS precipitation change. Distinct areas of little change or precipitation decrease, arising from interaction between ozone depletion-forced atmospheric circulation changes and ice sheet topography, are outweighed by large-scale precipitation increases. This signature bears notable similarities to a new ice core-based reconstruction of AIS accumulation change and yields a significant increase in annual integrated precipitation (38 ± 10 Gt/year over the 1986–2005 period or 51 ± 11 Gt/year over the 1991–2005 period). Remarkably, this simulated ozone depletion-forced precipitation change is of a similar absolute magnitude to recent observed AIS mass loss trends and as a consequence, it may play a role in dampening recent AIS sea level rise contributions.

Lenaerts, Jan T. M.↗
NASA NTRS1999

Radiative Importance of Aerosol-Cloud Interaction

Aerosol particles are input into the troposphere by biomass burning, among other sources. These aerosol palls cover large expanses of the earth's surface. Aerosols may directly scatter solar radiation back to space, thus increasing the earth's albedo and act to cool the earth's surface and atmosphere. Aerosols also contribute to the earth's energy balance indirectly. Hygroscopic aerosol act as cloud condensation nuclei (CCN) and thus affects cloud properties. In 1977, Twomey theorized that additional available CCN would create smaller but more numerous cloud droplets in a cloud with a given amount of liquid water. This in turn would increase the cloud albedo which would scatter additional radiation back to space and create a similar cooling pattern as the direct aerosol effect. Estimates of the magnitude of the aerosol indirect effect on a global scale range from 0.0 to -4.8 W/sq m. Thus the indirect effect can be of comparable magnitude and opposite in sign to the estimates of global greenhouse gas forcing Aerosol-cloud interaction is not a one-way process. Just as aerosols have an influence on clouds through the cloud microphysics, clouds have an influence on aerosols. Cloud droplets are solutions of liquid water and CCN, now dissolved. When the cloud droplet evaporates it leaves behind an aerosol particle. This new particle does not have to have the same properties as the original CCN. In fact, studies show that aerosol particles that result from cloud processing are larger in size than the original CCN. Optical properties of aerosol particles are dependent on the size of the particles. Larger particles have a smaller backscattering fraction, and thus less incoming solar radiation will be backscattered to space if the aerosol particles are larger. Therefore, we see that aerosols and clouds modify each other to influence the radiative balance of the earth. Understanding and quantifying the spatial and seasonal patterns of the aerosol indirect forcing may have even greater consequences. Presently we know that through the use of fossil fuel and land-use changes we have increased the concentration of greenhouse gases in the atmosphere. In parallel, we have seen a modest increase of global temperature in the last century. These two observations have been linked as cause and effect by climate models, but this connection is still experimentally not verified. The spatial and seasonal distribution of aerosol forcing is different from that of greenhouse gases, thus generating a different spatial fingerprint of climate change. This fingerprint was suggested as a method to identify the response of the climate system to anthropogenic forcing of greenhouse gases and aerosol. The aerosol fingerprint may be the only way to firmly establish the presence (or absence) of human impact on climate. Aerosol-cloud interaction through the indirect effect will be an important component of establishing this fingerprint.

Tsay, Si-Chee↗
NASA NTRS1990

Stability analysis of position and force control problems for robot arms

Stability issues involving the control of a robot arm under the influence of external forces are discussed. Several different scenarios are considered: position control with the external force as an unmodeled disturbance, compliant control for a bounded external force in some subspace, and compliant control for a force due to the interaction with an environment whose dynamical behavior can be modeled. In each of these cases, a stability analysis using the Lyapunov method is presented. An explanation of instability is put forth for the case in which the environment has flexibility and the gains are inappropriately chosen. When the environment is stiff in the force control subspace, robust stability can be achieved with the integral force feedback.

Wen, John T.↗
NASA NTRS2001

Carbon Nanotubes in Water: MD Simulations of Internal and External Flow, Self Organization

We have developed computational tools, based on particle codes, for molecular dynamics (MD) simulation of carbon nanotubes (CNT) in aqueous environments. The interaction of CNTs with water is envisioned as a prototype for the design of engineering nano-devices, such as artificial sterocillia and molecular biosensors. Large scale simulations involving thousands of water molecules are possible due to our efficient parallel MD code that takes long range electrostatic interactions into account. Since CNTs can be considered as rolled up sheets of graphite, we expect the CNT-water interaction to be similar to the interaction of graphite with water. However, there are fundamental differences between considering graphite and CNTs, since the curvature of CNTs affects their chemical activity and also since capillary effects play an important role for both dynamic and static behaviour of materials inside CNTs. In recent studies Gordillo and Marti described the hydrogen bond structure as well as time dependent properties of water confined in CNTs. We are presenting results from the development of force fields describing the interaction of CNTs and water based on ab-initio quantum mechanical calculations. Furthermore, our results include both water flows external to CNTs and the behaviour of water nanodroplets inside heated CNTs. In the first case (external flows) the hydrophobic behaviour of CNTs is quantified and we analyze structural properties of water in the vicinity of CNTs with diagnostics such as hydrogen bond distribution, water dipole orientation and radial distribution functions. The presence of water leads to attractive forces between CNTs as a result of their hydrophobicity. Through extensive simulations we quantify these attractive forces in terms of the number and separation of the CNT. Results of our simulations involving arrays of CNTs indicate that these exhibit a hydrophobic behaviour that leads to self-organising structures capable of trapping water clusters. In the second case (internal flows) we study the behaviour of water droplets confined inside CNTs. Constant temperature simulations allow us to capture structural properties such as the contact angles and density profiles of the equilibrated drops. By heating and subsequently cooling of the CNT, we are able to measure the evaporation and the condensation rate of the entrapped water.

Jaffe, Richard L.↗
NASA NTRS1992

The effect of space flight on spatial orientation

Both during and following early space missions, little neurosensory change in the astronauts was noted as a result of their exposure to microgravity. It is believed that this lack of in-flight adaptation in the spatial orientation and perceptual-motor system resulted from short exposure times and limited interaction with the new environment. Parker and Parker (1990) have suggested that while spatial orientation and motion information can be detected by a passive observer, adaptation to stimulus rearrangement is greatly enhanced when the observer moves through or acts on the environment. Experience with the actual consequences of action can be compared with those consequences expected on the basis of prior experience. Space flight today is of longer duration, and space craft volume has increased. These changes have forced the astronauts to interact with the new environment of microgravity, and as a result substantial changes occur in the perceptual and sensory-motor repsonses reflecting adaptation to the stimulus rearrangement of space flight. We are currently evaluating spatial orientation and the perceptual-motor systems' adaptation to microgravity by examining responses of postural control, head and gaze stability during locomotion, goal oriented vestibulo-ocular reflex (VOR), and structured quantitative perceptual reports. Evidence suggests that humans can successfully replace the gravitational reference available on Earth with cues available within the spacecraft or within themselves, but that adaptation to microgravity is not appropriate for a return to Earth. Countermeasures for optimal performance on-orbit and a successful return to earth will require development of preflight and in-flight training to help the astronauts acquire and maintain a dual adaptive state. An understanding of spatial orientation and motion perception, postural control, locomotion, and the VOR will aid in this process.

Reschke, Millard F.↗
NASA NTRS1998

Compressibility and Turbulence Effects Due to Airfoil Clocking in Axial Compressors

Axial compressors have inherently unsteady flow fields because of relative motion between rotor and stator airfoils. This relative motion leads to viscous and inviscid (potential) interactions between blade rows. As the number of stages increases in a turbomachine, the buildup of convected wakes can lead to progressively more complex wake/wake and wake/airfoil interactions. Variations in the relative circumferential positions of stators or rotors can change these interactions, leading to different unsteady forcing functions on airfoils and different compressor efficiencies. In addition, as the Mach number increases the interaction between blade rows is intensified due to potential effects. The current study uses an unsteady, two-dimensional Navier-Stokes approach to investigate the unsteady aerodynamics of stator clocking in a 1-1/2 stage compressor, typical of high-pressure compressors used in advanced commercial jet engines. The effects of turbulence are modeled with both algebraic and two-equation models. Results include surface pressures, efficiencies, boundary layer quantities and turbulence quantities. In addition, the growth of turbulence and the effects of compressibility on airfoil are discussed.

Dorney, Daniel J.↗
NASA NTRS1989

Optimal force distribution for payload positioning using a planar dual-arm robot

This paper presents a parameter optimization technique for deciding the force distribution on a payload being transported along a predetermined trajectory using two planar manipulator arms. The methodology begins by transforming the singular dynamics of two-arm transport to an ordinary set of differential equations and then proceeds to obtain a relation between the torques exerted by each arm. This relation is then used in a quadratic torque cost which is subsequently minimized to yield an optimal torque distribution. Significant savings in energy were found to occur when the arms were allowed to interact by transmission of forces through the payload. Even more significant are the savings found over one-arm transport of payloads where the arm torques are fixed by the prescribed trajectory.

Carignan, C. R.↗
NASA NTRS1985

Investigation of limb-sidestick dynamic interaction with roll control

A fixed-base simulation has been carried out to investigate and quantify limb/manipulator dynamics and interactions between the neuromuscular subsystem, force sensing sidestick configuration, high-gain command augmentation, and command filtering, and also to investigate possible relationships between these interactions and the roll ratchet phenomenon. The data obtained support the suggestion that the roll ratchet phenomenon is a closed-loop pilot-vehicle system interaction in which the pilot's neuromuscular system dynamics play a central role. Ratchet tendencies are shown to be most severe on force sensing sidestick manipulators with small stick deflections.

Johnston, D. E.↗
NASA NTRS1978

Hot-wire, laser anemometer and force balance measurements of cross-sectional planes of single and interacting trailing vortices

Single and multiple trailing vortices shed from semi-span wings and a transport model in a wind tunnel were studied by means of a laser-velocimeter, hot-wire anemometer, and a trailing model incorporating a 6-component force balance. Velocity profile and turbulence data from the laser-velocimeter and hot-wire anemometer are presented and shown to compare well with the Betz inviscid circulation model. Lift and rolling moment measurements on the following model are compared with those predicted from the flow field measurements.

Iversen, J. D.↗
NASA NTRS1983

Wall-temperature effects on the aerodynamics of a hydrogen-fueled transport concept in Mach 8 blowdown and shock tunnels

Results are presented from two separate tests on the same blended wing-body hydrogen fueled transport model at a Mach number of about 8 and a range of Reynolds numbers (based on theoretical body length) of 0.597 x 10 to the 6th power to about 156.22 x 10 to the 6th power. Tests were made in conventional hypersonic blowdown tunnel and a hypersonic shock tunnel at angles of attack of -2 deg to about 8 deg, with an extensive study made at a constant angle of attack of 3 deg. The model boundary-layer flow varied from laminar at the lower Reynolds numbers to predominantly turbulent at the higher Reynolds numbers. Model wall temperatures and stream static temperatures varied widely between the two tests, particularly at the lower Reynolds numbers. These temperature differences resulted in marked variations of the axial-force coefficients between the two tests, due in part to the effects of induced pressure and viscous interaction variations. The normal-force coefficient was essentially independent of Reynolds number. Analysis of results utilized current theoretical computer programs and basic boundary-layer theory.

Penland, J. A.↗
NASA NTRS1980

Sudden stratospheric warmings forced by mountains

A global, quasigeostrophic model of planetary wave interaction with the mean zonal flow has been used to simulate sudden stratospheric warmings forced by mean tropospheric zonal winds interacting with orography. Planetary waves m = 1 and 2 are simultaneously computed and interact with each other only through the mean zonal flow. An increase in the jet stream maximum from 36 to 48 m/sec is sufficient to initiate repeated major warmings with an approximate 80 day cycle. These results are in agreement with the observed anticorrelation of the annual variation of the winter monthly averaged jet stream strength and polar night jet strength.

Schoeberl, M. R.↗
NASA NTRS1987

Origins of hydrodynamic forces on centrifugal pump impellers

Hydrodynamic interactions that occur between a centrifugal pump impeller and volute are experimentally and theoretically investigated. The theoretical analysis considers the inability of the blades to perfectly guide the flow through the impeller, and also includes a quasi-one dimensional treatment of the flow in the volute. The disturbance at the impeller discharge and the resulting forces are determined by the theoretical model. The model is then extended to obtain the hydrodynamic force perturbations that are caused by the impeller whirling eccentrically in the volute. Under many operating conditions, these force perturbations were found to be destablizing. Comparisons are made between the theoretical model and the experimental measurements of pressure distributions and radial forces on the impeller. The theoretical model yields fairly accurate predictions of the radial forces caused by the flow through the impeller. However, it was found that the pressure acting on the front shroud of the impeller has a substantial effect on the destablizing hydrodynamic forces.

Adkins, Douglas R.↗