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At least 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Effects of Shrouded Stator Cavity Flows on Multistage Axial Compressor Aerodynamic Performance

Experiments were performed on a low-speed multistage axial-flow compressor to assess the effects of shrouded stator cavity flows on aerodynamic performance. Five configurations, which involved changes in seal-tooth leakage rates and/or elimination of the shrouded stator cavities, were tested. Data collected enabled differences in overall individual stage and the third stage blade element performance parameters to be compared. The results show conclusively that seal-tooth leakage ran have a large impact on compressor aerodynamic performance while the presence of the shrouded stator cavities alone seemed to have little influence. Overall performance data revealed that for every 1% increase in the seal-tooth clearance to blade-height ratio the pressure rise dropped up to 3% while efficiency was reduced by 1 to 1.5 points. These observed efficiency penalty slopes are comparable to those commonly reported for rotor and cantilevered stator tip clearance variations. Therefore, it appears that in order to correctly predict overall performance it is equally important to account for the effects of seal-tooth leakage as it is to include the influence of tip clearance flows. Third stage blade element performance data suggested that the performance degradation observed when leakage was increased was brought about in two distinct ways. First, increasing seal-tooth leakage directly spoiled the near hub performance of the stator row in which leakage occurred. Second, the altered stator exit now conditions caused by increased leakage impaired the performance of the next downstream stage by decreasing the work input of the downstream rotor and increasing total pressure loss of the downstream stator. These trends caused downstream stages to progressively perform worse. Other measurements were acquired to determine spatial and temporal flow field variations within the up-and-downstream shrouded stator cavities. Flow within the cavities involved low momentum fluid traveling primarily in the circumferential direction at about 40% of the hub wheel speed. Measurements indicated that the flow within both cavities was much more complex than first envisioned. A vortical flow structure in the meridional plane, similar to a driven cavity, existed within the upstream cavity Furthermore, other spatial and temporal variations in Row properties existed. the most prominent being caused by the upstream potential influence of the downstream blade. This influence caused the fluid within cavities near the leading edges of either stator blades in space or rotor blades in time to be driven radially inward relative to fluid near blade mid-pitch. This influence also produced large unsteady velocity fluctuations in the downstream cavity because of the passing of the downstream rotor blade.

Wellborn, Steven R.↗
NASA NTRS1998

High Pressure Water Stripping Using Multi-Orifice Nozzles

The use of multi-orifice rotary nozzles not only increases the speed and stripping effectiveness of high pressure water blasting systems, but also greatly increases the complexity of selecting and optimizing the operating parameters. The rotational speed of the nozzle must be coupled with the transverse velocity of the nozzle as it passes across the surface of the substrate being stripped. The radial and angular positions of each orifice must be included in the analysis of the nozzle configuration. Since orifices at the outer edge of the nozzle head move at a faster rate than the orifice located near the center, the energy impact force of the water stream from the outer orifice is spread over a larger area than the water streams from the inner orifice. Utilizing a larger diameter orifice in the outer radial positions increases the energy impact to compensate for its wider force distribution. The total flow rate from the combination of orifices must be monitored and kept below the pump capacity while choosing an orifice to insert in each position. The energy distribution from the orifice pattern is further complicated since the rotary path of all orifices in the nozzle head pass through the center section, contributing to the stripping in this area while only the outer most orifice contributes to the stripping in the shell area at the extreme outside edge of the nozzle. From t he outer most shell to the center section, more orifices contribute to the stripping in each progressively reduced diameter shell. With all these parameters to configure and each parameter change affecting the others, a computer model was developed to track and coordinate these parameters. The computer simulation responds by graphically indicating the cumulative affect from each parameter selected. The results from the proper choices in parameters is a well designed, highly efficient stripping system. A poorly chosen set of parameters will cause the nozzle to strip aggressively in some areas while leaving the coating untouched in adjacent sections. The high pressure water stripping system can be set to extremely aggressive conditions allowing stripping of hard to remove adhesives, paint systems, cladding and chromate conversion coatings. The energy force can be reduced to strip coatings from thin aluminum substrates without causing damage or deterioration to the substrate's surface. High pressure water stripping of aerospace components have thus proven to be an efficient and cost effective method for cleaning and removing coatings.

Hoppe, David T.↗
NASA NTRS1999

Lean Premixed Combustion Stabilized by Low Swirl a Promising Concept for Practical Applications

Since its inception, the low-swirl burner (LSB) has shown to be a useful laboratory apparatus for fundamental studies of premixed turbulent flames. The LSB operates under wide ranges of equivalence ratios, flow rates, and turbulence intensities. Its flame is lifted and detached from the burner and allows easy access for laser diagnostics. The flame brush is axisymmetric and propagates normal to the incident reactants. Therefore, the LSB is well suited for investigating detailed flame structures and empirical coefficients such as flame speed, turbulence transport, and flame generated turbulence. Due to its capability to stabilize ultra-lean premixed turbulent flames (phi approx. = 0.55), the LSB has generated interest from the gas appliance industry for use as an economical low-NO(x) burner. Lean premixed combustion emits low levels of NO(x), due primarily to the low flame temperature. Therefore, it is a very effective NO(x) prevention method without involving selective catalytic reduction (SCR), fuel-air staging, or flue gas recirculation (FGR). En the gas turbine industry, substantial research efforts have already been undertaken and engines with lean premixed combustors are already in use. For commercial and residential applications, premixed pulsed combustors and premixed ceramic matrix burners are commercially available. These lean premixed combustion technologies, however, tend to be elaborate but have relatively limited operational flexibility, and higher capital, operating and maintenance costs. Consequently, these industries are continuing the development of lean premixed combustion technologies as well as exploring new concepts. This paper summarizes the research effects we have undertaken in the past few years to demonstrate the feasibility of applying the low-swirl flame stabilization method for a wide range of heating and power generation systems. The principle of flame stabilization by low-swirl is counter to the conventional high-swirl methods that rely on a recirculation zone to anchor the flame. In LSBS, flow recirculation is not promoted to allow the premixed turbulent flames to propagate freely. A LSB with an air-jet swirler is essentially an open tube with the swirler at its mid section. The small air-jets generate swirling motion only in the annular region and leaving the central core of the flow undisturbed, When this flow exits the burner tube, the angular momentum generates radial mean pressure gradient to diverge the non-swirling reactants stream. Consequently, the mean flow velocity decreases linearly. Propagating against this decelerating flow, the flame self-sustains at the position where the local flow velocity equals the flame speed, S(sub f). The LSB operates with a swirl number, S, between 0.02 to 0.1. This is much lower than the minimum S of 0.6 required for the high-swirl burners. We found that the swirl number needed for flame stabilization varies only slightly with fuel type, flow velocity, turbulent conditions and burner dimensions (i.e. throat diameter and swirl injection angle).

Cheng, R. K.↗
NASA NTRS2004

Turbomachine Sealing and Secondary Flows: Review of Rotordynamics Issues in Inherently Unsteady Flow Systems With Small Clearances - Part 2

Today's computational methods enable the determination of forces in complex systems, but without field validation data, or feedback, there is a high risk of failure when the design envelope is challenged. The data of Childs and Bently and field data reported in NASA Conference Proceedings serve as sources of design information for the development of these computational codes. Over time all turbomachines degrade and instabilities often develop, requiring responsible, accurate, turbomachine diagnostics with proper decisions to prevent failures. Tam et al. (numerical) and Bently and Muszynska (analytical) models corroborate and implicate that destabilizing factors are related through increases in the fluid-force average circumferential velocity. The stability threshold can be controlled by external swirl and swirl brakes and increases in radial fluid film stiffness (e.g., hydrostatic and ambient pressures) to enhance rotor stability. Also cited are drum rotor self-excited oscillations, where the classic fix is to add a split or severed damper ring or cylindrical damper drum, and the Benkert-Wachter work that engendered swirl brake concepts. For a smooth-operating, reliable, long-lived machine, designers must pay very close attention to sealing dynamics and diagnostic methods. Correcting the seals enabled the space shuttle main engine high-pressure fuel turbopump (SSME HPFTP) to operate successfully.

Hendricks, R. C.↗
NASA NTRS1991

A nonradial pulsation model for the rapidly rotating Delta Scuti star Kappa(2) Bootis

A sectorial nonradial pulsation model is used to construct theoretical line profiles which mimic the variations for Kappa(2) Boo. Synthetic spectra generated with the appropriate Teff and log g are used as input. It is found that the data can be reproduced by the combination of a high-degree l is approximately equal to 12 mode with P(osc) aproximately equal to 0.071 d, and a low-degree mode, l is approximately equal to 0-2 with P(osc) approximately equal to 0.071-0.079 d. The projected rotational velocity (v sin i - 115 +/-5 km/s) was determined by fitting synthetic line profiles to the observed spectra. The velocity amplitude of the high-degree oscillations is estimated to be about 3.5 km/s. It is found that the ratio of the horizontal and radial pulsation amplitudes is small (about 0.02) and consistent with p-mode oscillations. Comparisons are made with models invoking starspots, and it is impossible to fit the observations of Kappa(2) Boo by a starspot model without assuming unrealistic values of radius or equatorial velocity.

Kennelly, E. J.↗
NASA NTRS1998

The [N II] Kinematics of R Aquarii

We report a kinematic study of the symbiotic star system R Aqr derived from [N H]lambda 6584 emission observations with a Fabry-Perot imaging spectrometer. The [N II] spatial structure of the R Aqr jet, first observed circa 1977, and surrounding hourglass-shaped nebulosity, due to an explosion approximately 660 years ago, are derived from 41 velocity planes spaced at approximately 12 km/s intervals. Fabry-Perot imagery shows the elliptical nebulosity comprising the waist of the hourglass shell is consistent with a circular ring expanding radially at 55 km/s as seen at an inclination angle, i approximately 70 deg. Fabry-Perot imagery shows the two-sided R Aqr jet is collimated flow in opposite directions. The intensity-velocity structure of the strong NE jet component is shown in contrast to the amorphous SW jet component. We offer a idealized schematic model for the R Aqr jet motion which results in a small-scale helical structure forming around a larger-scale helical path. The implications of such a jet model are discussed. We present a movie showing a side-by-side comparison of the spatial structure of the model and the data as a function of the 41 velocity planes.

Hollis, J. M.↗
NASA NTRS1993

The radial evolution of the amplitudes of 'dissipationless' turbulent solar wind fluctuations

We examine the evolution with heliocentric distance of the amplitude of fluctuations in the interplanetary magnetic and velocity fields assuming a model of homogeneous, steady state turbulence. Both the Alfven ratio and the normalized cross helicity are taken to be constant, compared to other variations, and the turbulence is assumed to be nearly incompressive. The fluctuation amplitudes are found by assuming the heating of the plasma by the fluctuations to be negligible; quasi-steady state turbulence with damping balanced by large-scale energy input will lead to the same conclusions for inertial range fluctuations. While the assumptions of this model contrast sharply with those for purely Alfvenic fluctuations, the radial dependence of the amplitude of the fluctuations for reasonable parameters is very nearly that found from both WKB analysis and recent turbulence modeling. The robustness of this result suggests why some predictions of WKB theory are apparently correct in solar wind conditions where the theory is not expected to be valid.

Verma, M. K.↗
NASA NTRS1994

A simulation of the intracluster medium with feedback from cluster galaxies

We detail method and report first results from a three-dimensional hydrodynamical and N-body simulation of the formation and evolution of a Coma-sized cluster of galaxies, with the intent of studying the history of the hot, X-ray emitting intracluster medium. Cluster gas, galaxies, and dark matter are included in the model. The galaxies and dark matter fell gravitational forces; the cluster gas also undergoes hydrodynamical effects such as shock heating and PdV work. For the first time in three dimensions, we include modeling of ejection of processed gas from the simulated galaxies by winds, including heating and heavy element enrichment. For comparison, we employ a `pure infall' simulation using the same initial conditions but with no galaxies or winds. We employ an extreme ejection history for galactic feedback in order to define the boundary of likely models. As expected, feedback raises the entropy of the intracluster gas, preventing it from collapsing to densities as high as those attained in the infall model. The effect is more pronounced in subclusters formed at high redshift. The cluster with feedback is always less X-ray luminous, but experiences more rapid luminosity evolution, than the pure infall cluster. Even employing an extreme ejection model, the final gas temperature is only approximately 15% larger than in the infall model. The radial temperature profile is very nearly isothermal within 1.5 Mpc. The cluster galaxies in the feedback model have a velocity dispersion approximately 15% lower than the dark matter. This results in the true ratio of specific energies in galaxies to gas being less than one, beta(sub spec) approximately 0.7. The infall model predicts beta(sub spec) approximately 1.2. Large excursions in these values occur over time, following the complex dynamical history of the cluster. The morphology of the X-ray emission is little affected by feedback. The emission profiles of both clusters are well described by the standard beta-model with beta(sub fit) approximately equal to 0.7 - 0.9. X-ray mass estimates based on the assumptions of hydrostatic equilibrium and the applicability of the beta-model are quite accurate in both cases. A strong, radial iron abundance gradient is present, which develops as a consequence of the steepening of the galaxy density profile over time. Spectroscopic observations using nonimaging detectors with wide (approximately 45 min) fields of view dramatically smear the gradient. Observations with arcminute resolution, made available with the ASCA satellite, would readily resolve the gradient.

Metzler, Christopher A.↗
NASA NTRS1981

Elasto visco-plastic flow with special attention to boundary conditions

A simple but nontrivial steady-state creeping elasto visco-plastic (Maxwell fluid) radial flow problem is analyzed, with special attention given to the effects of the boundary conditions. Solutions are obtained through integration of a governing equation on stress using the Runge-Kutta method for initial value problems and finite differences for boundary value problems. A more general approach through the finite element method, an approach that solves for the velocity field rather than the stress field and that is applicable to a wide range of problems, is presented and tested using the radial flow example. It is found that steady-state flows of elasto visco-plastic materials are strongly influenced by the state of stress of material as it enters the region of interest. The importance of this boundary or initial condition in analyses involving materials coming into control volumes from unusual stress environments is emphasized.

Shimazaki, Y.↗
NASA NTRS2017

A Method to Predict Magnetopause Expansion in Radial IMF Events by MHD Simulations

This paper presents a method for taking into account changes of solar wind parameters in the foreshock using global MHD simulations. We simulate four events with very distant subsolar magnetopause crossings that occurred during quasi-radial interplanetary magnetic field (IMF) intervals lasting from one to several hours. Using previous statistical results, we suggest that the density and velocity in the foreshock cavity decrease to approx. 60% and approx. 94% of the ambient solar wind values when the IMF cone angle falls below 50 deg. This diminishes the solar wind dynamic pressure to 53% and causes a corresponding magnetospheric expansion. We change the upstream solar wind parameters in a global MHD model to take these foreshock effects into account. We demonstrate that the modified model predicts magnetopause distances during radial IMF intervals close to those observed by THEMIS. The strong total pressure decrease in the data seems to be a local, rather than a global, phenomenon. Although the simulations with decreased solar wind pressure generally reproduce the observed total pressure in the magnetosheath well, the total pressure in the magnetosphere often agrees better with results for nonmodified boundary conditions. The last result reveals a limitation of our method: we changed the boundary conditions along the whole inflow boundary, although a more correct approach would be to vary parameters only in the foreshock. A model with the suggested global modification of the boundary conditions better predicts the location of part of the magnetopause behind the foreshock but may fail in predicting the rest of the magnetopause.

Samsonov, A. A.↗
NASA NTRS2000

Monitoring Global Sea Level: Eustatic Variations, Local Variations, and Solid Earth Effects

Project BIFROST (Baseline Inferences for Fennoscandian Rebound Observations. Sea-level and Tectonics) combines networks of continuously operating CPS receivers in Sweden and Finland to measure ongoing crustal deformation due to glacial isostatic adjustment, (CIA). We present an analysis of data collected in the years 1993-1998. We compare the CPS determinations of three-dimensional crustal motion to predictions calculated using the high resolution Fennoscandian deglaciation model recently proposed by Lambeck et al. We find that the the maximum observed uplift rate (approx. 10 mm/ yr) and the maximum predicted uplift rate agree to better than 1 mm/ yr. The patterns of uplift also agree quite well, although differences are discernible. The chi(exp 2) difference between predicted and GPS-observed radial rates is reduced by a factor of 5-6 compared to that for the "null" (no uplift) model, depending on whether a mean difference is first removed. The north components of velocity agree at about the same relative level. whereas the agreement for the east components is worse, a problem possibly related to the lack of bias fixing. We have also compared the values for the observed radial deformation rates to those based on sea-level rates from Baltic tide gauges. The weighted RMS difference between CPS and tide-gauge rates (after removing a mean) is 0.6 mm/ yr, giving an indication of the combined accuracy of the CPS and tide-gauge measurement systems. Spectral analysis of the time series of position estimates yields spectral indices in the range -1 to -2. An EOF analysis indicates, however, that much of this power is correlated among the sites. The correlation appears to be regional and falls off only slightly with distance. Some of this correlated noise is associated with snow accumulation on the antennas or, for those antennas with radomes, on the radomes. This problem has caused us to modify the radomes used several times, leading to one of our more significant sources of uncertainty.

Davis, James L.↗
NASA NTRS1971

Neutron production mechanism in a plasma focus.

The neutrons emitted by a plasma focus were analyzed by using a time-of-flight method. Flight paths as large as 80 m were used to obtain better than 10% energy resolution. The energy spectrum of neutrons from d-d reactions in the plasma focus shows a sharp onset with average maximum energies of 2.8 and 3.2 MeV in the radial and the axial directions, respectively. The average half-width of the energy spectrum was 270 keV with a shot-to-shot variation between 150 and 400 keV. Simultaneous measurements in the axial and radial directions showed no appreciable difference in the half-widths and thus indicated randomly oriented ion velocities in the plasma. A converging ion model is described which is found to be in agreement with the measured quantities.

Lee, J. H.↗
NASA NTRS1972

Slowly rotating relativistic stars. VI - Stability of the quasi-radial modes.

Dynamical analysis of the quasi-radial modes of slowly and rigidly rotating, relativistic stars. The quasi-radial modes are those modes which would be radial if the star were not rotating. They are the crucial modes for determining the stability of the rotating star and also the modes which store energy for the longest time against dissipation by gravitational radiation. An equation is derived that makes it possible to calculate the effects of the slow and rigid star rotation on the frequency of the quasi-radial modes of oscillation of relativistic stars. The rotation is treated to second order in the angular velocity, but no other approximations are made.

Hartle, J. B.↗
NASA NTRS1980

Formation of the terrestrial planets

Two growth mechanisms are identified for the development of the terrestrial planets: (1) gravitational instability leading to a collapse, and (2) gravitational accumulation caused by two-body collisions and coherence. The presence of a dynamically-significant gas phase would not affect either mechanism. Theoretical expressions are presented for the production of giant gaseous protoplanets by gravitational instability within a central dust layer. Gravitational accumulation is discussed with reference to the accumulation of planetesimals from a gas-free circumsolar swarm of bodies. Numerical simulations are given for the early stages of accumulation. The Safronov steady-state velocity is considered, noting that the competition between mutual collisional damping and gravitational acceleration by the members of a solar swarm yields a steady-state velocity distribution where the mean velocity is comparable to the escape velocity of the largest body. A time scale for accumulation is postulated on the basis of the radial distribution of a swarm of non-accreting bodies of equal size. The simultaneous gas-free accumulation of several terrestrial planets is noted. Attention is also given to growth mechanisms in gas-rich interplanetary media.

Wetherill, G. W.↗
NASA NTRS1980

Time correlation functions in classical dense fluids

The velocity autocorrelation functions and memory functions of dense classical fluids may be directly obtained from the static radial distribution function g(r) in an approximate way. Following the Mori projection operator formalism, the memory functions may be related to the fluctuating force correlation. At low densities, these functions may be evaluated by following the trajectories of particle pairs in the interatomic potential. At higher densities, the force correlation functions can be evaluated approximately from particle pair trajectories via the potential of the mean force. The results for argon fluids at various densities and temperatures agree satisfactorily with the molecular dynamics and the Enskog values. The decrease of the diffusion coefficient with density is partly due to the nature of g(r) which reflects the stronger clustering of atoms at higher densities.

Tsang, T.↗
NASA NTRS1992

The 3D rocket combustor acoustics model

The theory and procedures for determining the characteristics of pressure oscillations in rocket engines with prescribed burning rate oscillations are presented. Analyses including radial and hub baffles and absorbers can be performed in one, two, and three dimensions. Pressure and velocity oscillations calculated using this procedure are presented for the SSME to show the influence of baffles and absorbers on the burning rate oscillations required to achieve neutral stability. Comparisons are made between the results obtained utilizing 1-D, 2-D, and 3-D assumptions with regards to capturing the physical phenomena of interest and computational requirements.

Priem, Richard J.↗