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At least 199 records · Page 11

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NASA NTRS1993

A comparison of two incompressible Navier-Stokes algorithms for unsteady internal flow

A comparative study of two different incompressible Navier-Stokes algorithms for solving an unsteady, incompressible, internal flow problem is performed. The first algorithm uses an artificial compressibility method coupled with upwind differencing and a line relaxation scheme. The second algorithm uses a fractional step method with a staggered grid, finite volume approach. Unsteady, viscous, incompressible, internal flow through a channel with a constriction is computed using the first algorithm. A grid resolution study and parameter studies on the artificial compressibility coefficient and the maximum allowable residual of the continuity equation are performed. The periodicity of the solution is examined and several periodic data sets are generated using the first algorithm. These computational results are compared with previously published results computed using the second algorithm and experimental data.

Wiltberger, N. Lyn↗

NASA NTRS1994

Quantitative 3-D imaging topogrammetry for telemedicine applications

The technology to reliably transmit high-resolution visual imagery over short to medium distances in real time has led to the serious considerations of the use of telemedicine, telepresence, and telerobotics in the delivery of health care. These concepts may involve, and evolve toward: consultation from remote expert teaching centers; diagnosis; triage; real-time remote advice to the surgeon; and real-time remote surgical instrument manipulation (telerobotics with virtual reality). Further extrapolation leads to teledesign and telereplication of spare surgical parts through quantitative teleimaging of 3-D surfaces tied to CAD/CAM devices and an artificially intelligent archival data base of 'normal' shapes. The ability to generate 'topogrames' or 3-D surface numerical tables of coordinate values capable of creating computer-generated virtual holographic-like displays, machine part replication, and statistical diagnostic shape assessment is critical to the progression of telemedicine. Any virtual reality simulation will remain in 'video-game' realm until realistic dimensional and spatial relational inputs from real measurements in vivo during surgeries are added to an ever-growing statistical data archive. The challenges of managing and interpreting this 3-D data base, which would include radiographic and surface quantitative data, are considerable. As technology drives toward dynamic and continuous 3-D surface measurements, presenting millions of X, Y, Z data points per second of flexing, stretching, moving human organs, the knowledge base and interpretive capabilities of 'brilliant robots' to work as a surgeon's tireless assistants becomes imaginable. The brilliant robot would 'see' what the surgeon sees--and more, for the robot could quantify its 3-D sensing and would 'see' in a wider spectral range than humans, and could zoom its 'eyes' from the macro world to long-distance microscopy. Unerring robot hands could rapidly perform machine-aided suturing with precision micro-sewing machines, splice neural connections with laser welds, micro-bore through constricted vessels, and computer combine ultrasound, microradiography, and 3-D mini-borescopes to quickly assess and trace vascular problems in situ. The spatial relationships between organs, robotic arms, and end-effector diagnostic, manipulative, and surgical instruments would be constantly monitored by the robot 'brain' using inputs from its multiple 3-D quantitative 'eyes' remote sensing, as well as by contact and proximity force measuring devices. Methods to create accurate and quantitative 3-D topograms at continuous video data rates are described.

Altschuler, Bruce R.↗

NASA NTRS1994

Plasma arc welding weld imaging

A welding torch for plasma arc welding apparatus has a transparent shield cup disposed about the constricting nozzle, the cup including a small outwardly extending polished lip. A guide tube extends externally of the torch and has a free end adjacent to the lip. First and second optical fiber bundle assemblies are supported within the guide tube. Light from a strobe light is transmitted along one of the assemblies to the free end and through the lip onto the weld site. A lens is positioned in the guide tube adjacent to the second assembly and focuses images of the weld site onto the end of the fiber bundle of the second assembly and these images are transmitted along the second assembly to a video camera so that the weld site may be viewed continuously for monitoring the welding process.

Rybicki, Daniel J.↗

NASA NTRS1994

Effect of ground and ceiling planes on shape of energized wakes

Results are presented from an investigation of the structure of energized wakes in the presence of ground or ceiling planes when powered-lift devices are tested at zero and low wind tunnel velocities. Solutions are presented to indicate how the presence of a nearby ground plane causes the energized wake to constrict less than when in free space. In contrast, another set of solutions indicate that the presence of a ceiling plane enhances wake construction so that the wake width is even smaller than when in free space. Hence, a ground plane tends to reduce the chance for interaction between the wake and the energizing elements, while a nearby ceiling plane tends to increase the likelihood for interference.

TRT-EXPERIMENTAL↗

NASA NTRS1994

Microvascular responses to body tilt in cutaneous maximus muscle of conscious rats

We investigated microvascular responses to head-up tilt (HUT) and head-down tilt (HDT) in striated muscle of conscious male rats. To observe the microcirculation in the cutaneous maximus muscle, a transparent polycarbonate chamber was implanted aseptically into a skin fold created between the shoulders. Rats were trained to sit quietly during HUT and HDT while positioned on a horizontal microscope that rotated in the sagittal plane. At 4-5 days after surgery, arteriole and venule diameters were recorded using videomicroscopy while the rat experienced 10 min each (in random order) of HUT or HDT at 20 deg or 40 deg separated by 2-h rest periods. HUT had no affect on microvessel diameter; 20 deg HDT had little affect. In response to 40 deg HDT, 'large' arterioles constricted by 18 +/- 2% and 'small' arterioles dilated by 21 +/- 3%; this difference suggested variation in mechanisms controlling arteriolar responses. Venules exhibited a larger fluctuation in diameter during 40 deg HDT compared with other body positions, suggesting that venomotor activity may be induced with sufficient fluid shift or change in central venous pressure. These observations illustrate a viable model for studying microvascular responses to gravitational stress in conscious rats.

Puri, Rohit K.↗

NASA NTRS1995

Getting granite dikes out of the source region

Whether a dike can propagate far from a magma reservoir depends upon the competition between the rate at which propagation widens the dike and the rate at which freezing constricts the aperture available for magma flow. Various formulations are developed for a viscous fluid at temperature T(sub m) intruding a growing crack in an elastic solid. The initial solid temperature equals T(sub m) at the source and decreases linearly with distance from the source. If T(sub m) is the unique freezing temperature of the fluid, dike growth is initially self-similar and an essentially exact solution is obtained; if T(sub m) is above the solidus temperature, the solution is approximate but is designed to overestimate the distance the dike may propagate. The ability of a dike to survive thermally depends primarily upon a single parameter that is a measure of the ratio of the dike frozen margin thickness to elastic thickness. Perhaps more intuitively, one may define a minimum distance from the essentially solid reservoir wall to the point at which the host rock temperature drops below the solidus, necessary for dikes to propagate far into subsolidus rock. It is concluded that for reasonable material properties and source conditions, most basalt dikes will have little difficulty leaving the source region, but most rhyolite dikes will be halted by freezing soon after the magma encounters rock at temperatures below the magma solidus. While these results can explain why granitic dikes are common near granitic plutons but rare elsewhere, the potentially large variation in magmatic systems makes it premature to rule out the possibility that most granites are transported through the crust in dikes. Nonetheless, these results highlight difficulties with such proposals and suggest that it may also be premature to rule out the possibility that most granite plutons ascend as more equidimensional bodies.

Rubin, Allan M.↗

NASA NTRS1995

Low-state disks and low-beta disks

Stellar black hole candidates (BHCs) exhibit bimodal spectral states. We calculate nonthermal disk spectra, demonstrating that a large photon index (alpha (sub x) approximately 2-3) observed in the soft (high) state is due to a copious soft photon supply, whereas soft photon starvation leads to a smaller index (alpha (sub x) approximately 1.5-2) in the hard (low) state. Thus, the absence of the soft component flux in the low state cannot be due to obscuration. A possible disk configuration during the low state is discussed. We proposed that a low-state disk may be a low-beta disk in which magnetic pressure may exceed gas pressure becuase of the suppression of field escape by a strong shear. As a result, disk material will take the form of blobs constricted by mainly toroidal magnetic fields. Fields are dissipated mainly by occasional reconnection events with a huge energy release. This will account for large-amplitude, aperiodic X-ray variations (flickering) and high-energy radiation with small alpha(sub x) from hard state BHCs and possibly from active galactic nuclei. Further, we propose a hysteretic relation between the mass-flow rate and plasma-beta, a ratio of gas pressure to magnetic pressure, for the spectral evolution of transient BHCs. The disk is in the low-beta state in quiescence and early rise. The low-beta disk is optically thin and affected by advection. A hard-to-soft transition occurs before the peak luminosity, since there is no advection-dominated branch at higher luminosities. An optically thick, high-beta disk appears at small radii. In the decay phase of the light curve, the standard-type disk becomes effectively optically thin, when a soft-hard transition is triggered. High-beta plasmas in the main body shrink to form minute blobs, and low-beta coronal plasma fills interblob space.

Mineshige, Shin↗

NASA NTRS1995

Flexible Borescope For Inspecting Ducts

Borescope and associated equipment developed specifically for use in optical inspection of inside of flexible joints in rocket-engine fuel duct. Apparatus includes assembly, characterized as "mouse/canoe," at sensing end of 10-ft borescope cable. Borescope tip rides in mouse/canoe. The mouse/canoe made laterally compressible, to facilitate movement past constrictions and provides fixed distance from inspection point permitting accurate description in duct inspected.

Shinozaki, Keith↗

NASA NTRS1996

Megavolt, Multigigawatt Pulsed Plasma Switch

Plasma switch proposed for use in high-voltage, high-current pulse power system. Designed not only to out-perform conventional spark-gap switch but also relatively compact and lightweight. Features inverse-pinch configuration to prevent constriction of current sheets into filaments, plus multiple-ring-electrode structure to resist high-voltage breakdown.

Lee, Ja H.↗

NASA NTRS1995

Mechanisms of Microgravity Effect on Vascular Function

The overall goal of the project is to characterize the effects of simulated microgravity on vascular function. Microgravity is simulated using the hindlimb unweighted (HU) rat, and the following vessels are removed from HU and paired control rats for in vitro analysis: abdominal aorta, carotid and femoral arteries, jugular and femoral veins. These vessels are cut into 3 mm long rings and mounted in tissue baths for the measurement of either isometric contraction, or relaxation of pre- contracted vessels. The isolated mesenteric vascular bed is perfused for the measurement of changes in perfusion pressure as an index of arteriolar constriction or dilation. This report presents, in addition to the statement of the overall goal of the project, a summary list of the specific hypotheses to be tested. These are followed by sections on results, conclusions, significance and plans for the next year.

Purdy, Ralph E.↗

NASA NTRS1996

Advanced liquid Oxygen (LO2) propellant conditioning concept testing

Extensive testing was performed on the promising L02 propellant conditioning concept of passive recirculation (no-bleed). Data from the project is being used to further anchor models in L02 conditioning behavior and broaden the data base of no-bleed and low-bleed conditioning. Data base expansion includes results from testing the limits of no-bleed and low-bleed conditioning with various configuration changes to the test facility and designed test article. Configuration changes include low velocity effects in the recirculation loop above the test article, test article internal constriction impacts, test article out-of-plane effects, impact from an actual Titan L02 pump attachment, feed duct slope effects, and up-leg booster effects. LN2 was used as the test fluid. The testing was conducted between July 1994 and January 1995 at the west test area of Marshall Space Flight Center. Data have shown that in most cases passive recirculation was demonstrated when the aforementioned limits were applied.

Hasting, J. H.↗

NASA NTRS1997

West-Antarctic Ice Streams: Analog to Ice Flow in Channels on Mars

Sounding of the sea floor in front of the Ross Ice Shelf in Antarctica recently revealed large persistent patterns of longitudinal megaflutes and drumlinoid forms, which are interpreted to have formed at the base of ice streams during the list glacial advance. The flutes bear remarkable resemblance to longitudinal grooves and highly elongated streamlined islands found on the floors of some large martian channels, called outflow channels. ln addition, other similarities exist between Antarctic ice streams and outflow channels. Ice streams are 30 to 80 km wide and hundreds of kilometers long, as are the martian channels. Ice stream beds are below sea level. Floors of many martian outflow channels lie below martian datum, which may have been close to or below past martian sea levels. The Antarctic ice stream bed gradient is flat and locally may go uphill, and surface slopes are exceptionally low. So are gradients of martian channels. The depth to the bed in ice streams is 1 to 1.5 km. At bankful stage, the depth of the fluid in outflow channels would have been 1 to 2 km. These similarities suggest that the martian outflow channels, whose origin is commonly attributed to gigantic catastrophic floods, were locally filled by ice that left a conspicuous morphologic imprint. Unlike the West-Antarctic-ice streams, which discharge ice from an ice sheet, ice in the martian channels came from water erupting from the ground. In the cold martian environment, this water, if of moderate volume, would eventually freeze. Thus it may have formed icings on springs, ice dams and jams on constrictions in the channel path, or frozen pools. Given sufficient thickness and downhill surface gradient, these ice masses would have moved; and given the right conditions, they could have moved like Antarctic ice streams.

Lucchitta, B. K.↗

NASA NTRS1998

Transient Simulation of Pressure Oscillations in the Fuel Feedline of the Fastrac Engine Thrust Chamber

During mainstage testing of the 60,000 lbf thrust Fastrac thrust chamber at MSFC's Test Stand 116 (TS 116), sustained, large amplitude oscillations near 530 Hz were observed in the pressure data. These oscillations were detected both in the RP-1 feedline, downstream of the cavitating venturi, and in the combustion chamber. The driver of the instability is believed to be feedline excitation driven by either periodic cavity collapse at the exit of the cavitating venturi or combustion instability. In covitating venturi, static pressure drops as the flow passes through a constriction resembling a converging-diverging nozzle until the vapor pressure is reached. At the venturi throat, the flow is essentially choked, which is why these devices are typically used for mass flow rate control and disturbance isolation. Typically, a total pressure drop of 15% or more across the venturi is required for cavitation. For much larger pressure differentials, unstable cavities can form and subsequently collapse downstream of the throat. Although the disturbances generated by cavitating venturis is generally considered to be broad-band, this type of phenomena could generate periodic behavior capable of exciting the feedline. An excitation brought about by combustion instability would result from the coupling of a combustion chamber acoustic mode and a feedline resonance frequency. This type of coupling is referred to as "buzz" and is not uncommon for engines in this thrust range.

Bullard, Brad↗

NASA NTRS1996

Sensor-Based Collision Avoidance: Theory and Experiments

A new on-line control strategy for sensor-based collision avoidance of manipulators and supporting experimental results are presented in this article. This control strategy is based on nullification of virtual forces applied to the end-effector by a hypothetical spring-plus-damper attached to the object's surface. In the proposed approach, the real-time arm control software continuously monitors the object distance measured by the arm-mounted proximity sensors. When this distance is less than a preset threshold, the collision avoidance control action is initiated to inhibit motion toward the object and thus prevent collision. This is accomplished by employing an outer feedback loop to perturb the end-effector nominal motion trajectory in real-time based on the sensory data. The perturbation is generated by a proportional-plus-integral (PI) collision avoidance controller acting on the difference between the sensed distance and the preset threshold. This approach is computationally very fast, requires minimal modification to the existing manipulator positioning system, and provides the manipulator with an on-line collision avoidance capability to react autonomously and intelligently. A dexterous RRC robotic arm is instrumented with infrared proximity sensors and is operated under the proposed collision avoidance strategy. Experimental results are presented to demonstrate end-effector collision avoidance both with an approaching object and while reaching inside a constricted opening.

Seraji, Homayoun↗

NASA NTRS1987

Further Discussion: Parametric Study of Wind Generated Supermicron Particle Effects in Large Fires

In their reply (Porch et al., 1987) to our comments (Turco et al., 1987) on their smoke-scavenging-by-dust paper, Porch et al. attempt to justify a number of parameter assumptions in their original article, again revealing the extreme nature of those assumptions, particularly in the situation where all are taken simultaneously. In critiquing Porch et al.'s calculations, have not applied "opinion", but rather physical reality and common sense expressed through basic experimental results and logical physical bounds. A few examples of the unrealistic conditions required by the Porch et al. scavenging scheme, as described in their paper and comments, should suffice here. ) Porch et al. have fabricated a "fetch" region for dust particles in large fire plumes that logically must extend over an area up to 50 times greater than the fire area itself. Alternatively, they have invoked significant "necking down' of the fire plume, so that its cross-sectional area is at most a few percent of the fire area. Such severe constriction is seen only in very small fires with strong, organized vorticity, and then only over a limited plume rise region. No "fetch" has ever been noted in any large-scale fires we have observed, or for which accounts are available. Indeed, as we deduced in our original comments, complete dust scavenging even within the fire zone would probably occur less than 10% of the time for large urban fires.

Toon, O. B.↗

NASA NTRS1999

Rodent Studies of Cardiovascular Deconditioning

Changes in blood pressure can occur for two reasons: 1) A decrease in cardiac output resulting from the altered contractility of the heart or through changes in venous filling pressure via the Frank Starling mechanism or; 2) A change in systemic vascular resistance. The observed changes in cardiac output and blood pressure after long term space flight cannot be entirely explained through changes in contractility or heart rate alone. Therefore, alterations in filling pressure mediated through changes in systemic venous capacitance and arterial resistance function may be important determinants of cardiac output and blood pressure after long term space flight. Our laboratory and previous studies have shown the importance of veno-constriction mediated by the carotid sinus baroreceptor reflex system on overall circulatory homeostasis and in the regulation of cardiac output. Our proposed experiments test the overall hypothesis that alterations in venous capacitance function and arterial resistance by the carotid sinus baroreceptor reflex system are an important determinant of the cardiac output and blood pressure response seen in astronauts after returning to earth from long term exposure to microgravity. This hypothesis is important to our overall understanding of circulatory adjustments made during long term space flight. It also provides a framework for investigating counter measures to reduce the incidence of orthostatic hypotension caused by an attenuation of cardiac output. We continue to use hind limb unweighted (HLU) rat model to simulate the patho physiological effects as they relate to cardiovascular deconditioning in microgravity. We have used this model to address the hypothesis that microgravity induced cardiovascular deconditioning results in impaired vascular responses and that these impaired vascular responses result from abnormal alpha-1 AR signaling. The impaired vascular reactivity results in attenuated blood pressure and cardiac output responses to an orthostatic challenge. We have used in vitro vascular reactivity assays to explore abnormalities in vascular responses in vessels from HLU animals and, cardiac output (CO), blood pressure (BP) and heart rate (HR) measurements to characterize changes in hemodynamics following HLU.

Shoukas, Artin A.↗

NASA NTRS2000

Measurement Requirements for Improved Modeling of Arcjet Facility Flows

Current efforts to develop new reusable launch vehicles and to pursue low-cost robotic planetary missions have led to a renewed interest in understanding arc-jet flows. Part of this renewed interest is concerned with improving the understanding of arc-jet test results and the potential use of available computational-fluid- dynamic (CFD) codes to aid in this effort. These CFD codes have been extensively developed and tested for application to nonequilibrium, hypersonic flow modeling. It is envisioned, perhaps naively, that the application of these CFD codes to the simulation of arc-jet flows would serve two purposes: first. the codes would help to characterize the nonequilibrium nature of the arc-jet flows; and second. arc-jet experiments could potentially be used to validate the flow models. These two objectives are, to some extent, mutually exclusive. However, the purpose of the present discussion is to address what role CFD codes can play in the current arc-jet flow characterization effort, and whether or not the simulation of arc-jet facility tests can be used to eva1uate some of the modeling that is used to formu1ate these codes. This presentation is organized into several sections. In the introductory section, the development of large-scale, constricted-arc test facilities within NASA is reviewed, and the current state of flow diagnostics using conventional instrumentation is summarized. The motivation for using CFD to simulate arc-jet flows is addressed in the next section, and the basic requirements for CFD models that would be used for these simulations are briefly discussed. This section is followed by a more detailed description of experimental measurements that are needed to initiate credible simulations and to evaluate their fidelity in the different flow regions of an arc-jet facility. Observations from a recent combined computational and experiment.al investigation of shock-layer flows in a large-scale arc-jet facility are then used to illustrate the current state of development of diagnostic instrumentation, CFD simulations, and general knowledge in the field of arc-jet characterization. Finally, the main points are summarized and recommendations for future efforts are given.

Fletcher, Douglas G.↗

NASA NTRS2000

Nonintrusive Diagnostic Strategies for Arcjet Stream Characterization

In the previous lecture, the issues related to arcjet flow modeling were introduced, and the limitations of conventional instrumentation in addressing these issues were discussed. The general level of understanding of the arcjet flows was seen to preclude the use of arcjets as aerothermodynamic test facilities beyond the current role in aerothermal material testing, despite their long test duration capability. In this section, the focus will be on new developments in spectroscopic instrumentation and techniques that can be brought to bear on the fundamental problem of arcjet stream characterization. Although a wide selection of arcjet facilities were introduced in the previous section, the discussion of nonintrusive diagnostic instrumentation will be restricted to the large-scale, segmented, constricted-arc heater facilities that are most widely used in thermal protection material testing for aerospace applications. After a brief review of the important features of arcjet flows, the topic of nonintrusive, optical diagnostics is introduced with a discussion of some of the basic aspects of radiative transitions. The lecture is then organized into two sections covering emission measurements and laser-induced fluorescence measurements. Emission measurements are presented next for different regions of arcjet flows, while the fluorescence measurements are presented for the free stream region only. Summaries are given for each of the two main sections, and observations on arcjet characterization by optical diagnostics in general are given at the end.

Fletcher, Douglas G.↗