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At least 469 records · Page 26

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NASA NTRS1974

Use of cross-correlation measurements to investigate noise generating regions of a real jet engine and a model jet

Cross-correlations are reported of the jet static pressure fluctuations (as measured with a B and K microphone fitted with a nose cone), with the far-field radiated sound pressure. These measurements were made for various probe positions and a large number of far-field positions (at various angles). In addition, the tests were run for a number of different jet exit velocities. The measured, normalized cross-correlation functions vary between 0.004 and 0.155. These values depend upon the angular position of the far-field microphone, the jet exit Mach number, and the position of the probe. In addition, the cross-correlation technique was employed to study the symmetry of the far-field radiated sound about the jet axis. Third-octave analyses of both the probe signal and the far-field radiated sound were made. This is the first time correlation measurements have been made on a jet engine. In addition, a report is given on an extensive noise survey of a model jet. The correlations are related to sound source functions and jet source regions are discussed.

Meecham, W. C.↗

NASA NTRS2003

Characteristics of Gaseous Diffusion Flames with High Temperature Combustion Air in Microgravity

The characteristics of gaseous diffusion flames have been obtained using high temperature combustion air under microgravity conditions. The time resolved flame images under free fall microgravity conditions were obtained from the video images obtained. The tests results reported here were conducted using propane as the fuel and about 1000 C combustion air. The burner included a 0.686 mm diameter central fuel jet injected into the surrounding high temperature combustion air. The fuel jet exit Reynolds number was 63. Several measurements were taken at different air preheats and fuel jet exit Reynolds number. The resulting hybrid color flame was found to be blue at the base of the flame followed by a yellow color flame. The length and width of flame during the entire free fall conditions has been examined. Also the relative flame length and width for blue and yellow portion of the flame has been examined under microgravity conditions. The results show that the flame length decreases and width increases with high air preheats in microgravity condition. In microgravity conditions the flame length is larger with normal temperature combustion air than high temperature air.

Ghaderi, M.↗

NASA NTRS2018

Acceleration Modes and Transitions in Pulsed Plasma Accelerators

Pulsed plasma accelerators typically operate by storing energy in a capacitor bank and then discharging this energy through a gas, ionizing and accelerating it through the Lorentz body force. Two plasma accelerator types employing this general scheme have typically been studied: the gas-fed pulsed plasma thruster and the quasi-steady magnetoplasmadynamic (MPD) accelerator. The gas-fed pulsed plasma accelerator is generally represented as a completely transient device discharging in approximately 1-10 microseconds. When the capacitor bank is discharged through the gas, a current sheet forms at the breech of the thruster and propagates forward under a j (current density) by B (magnetic field) body force, entraining propellant it encounters. This process is sometimes referred to as detonation-mode acceleration because the current sheet representation approximates that of a strong shock propagating through the gas. Acceleration of the initial current sheet ceases when either the current sheet reaches the end of the device and is ejected or when the current in the circuit reverses, striking a new current sheet at the breech and depriving the initial sheet of additional acceleration. In the quasi-steady MPD accelerator, the pulse is lengthened to approximately 1 millisecond or longer and maintained at an approximately constant level during discharge. The time over which the transient phenomena experienced during startup typically occur is short relative to the overall discharge time, which is now long enough for the plasma to assume a relatively steady-state configuration. The ionized gas flows through a stationary current channel in a manner that is sometimes referred to as the deflagration-mode of operation. The plasma experiences electromagnetic acceleration as it flows through the current channel towards the exit of the device. A device that had a short pulse length but appeared to operate in a plasma acceleration regime different from the gas-fed pulsed plasma accelerators was developed by Cheng, et al. The Coaxial High ENerGy (CHENG) thruster operated on the 10-microseconds timescales of pulsed plasma thrusters, but claimed high thrust density, high efficiency and low electrode erosion rates, which are more consistent with the deflagration mode of acceleration. Separate work on gas-fed pulsed plasma thrusters (PPTs) by Ziemer, et al. identified two separate regimes of performance. The regime at higher mass bits (termed Mode I in that work) possessed relatively constant thrust efficiency (ratio of jet kinetic energy to input electrical energy) as a function of mass bit. In the second regime at very low mass bits (termed Mode II), the efficiency increased with decreasing mass bit. Work by Poehlmann et al. and by Sitaraman and Raja sought to understand the performance of the CHENG thruster and the Mode I / Mode II performance in PPTs by modeling the acceleration using the Hugoniot Relation, with the detonation and deflagration modes representing two distinct sets of solutions to the relevant conservation laws. These works studied the proposal that, depending upon the values of the various controllable parameters, the accelerator would operate in either the detonation or deflagration mode. In the present work, we propose a variation on the explanation for the differences in performance between the various pulsed plasma accelerators. Instead of treating the accelerator as if it were only operating in one mode or the other during a pulse, we model the initial stage of the discharge in all cases as an accelerating current sheet (detonation mode). If the current sheet reaches the exit of the accelerator before the discharge is completed, the acceleration mode transitions to the deflagration mode type found in the quasi-steady MPD thrusters. This modeling method is used to demonstrate that standard gas-fed pulsed plasma accelerators, the CHENG thruster, and the quasi-steady MPD accelerator are variations of the same device, with the overall acceleration of the plasma depending upon the behavior of the plasma discharge during initial transient phase and the relative lengths of the detonation and deflagration modes of operation.

Polzin, Kurt A.↗

NASA NTRS2015

Solid Rocket Motor Combustion Instability Modeling in COMSOL Multiphysics

Combustion instability modeling of Solid Rocket Motors (SRM) remains a topic of active research. Many rockets display violent fluctuations in pressure, velocity, and temperature originating from the complex interactions between the combustion process, acoustics, and steady-state gas dynamics. Recent advances in defining the energy transport of disturbances within steady flow-fields have been applied by combustion stability modelers to improve the analysis framework. Employing this more accurate global energy balance requires a higher fidelity model of the SRM flow-field and acoustic mode shapes. The current industry standard analysis tool utilizes a one dimensional analysis of the time dependent fluid dynamics along with a quasi-three dimensional propellant grain regression model to determine the SRM ballistics. The code then couples with another application that calculates the eigenvalues of the one dimensional homogenous wave equation. The mean flow parameters and acoustic normal modes are coupled to evaluate the stability theory developed and popularized by Culick. The assumption of a linear, non-dissipative wave in a quiescent fluid remains valid while acoustic amplitudes are small and local gas velocities stay below Mach 0.2. The current study employs the COMSOL Multiphysics finite element framework to model the steady flow-field parameters and acoustic normal modes of a generic SRM. This work builds upon previous efforts to verify the use of the acoustic velocity potential equation (AVPE) laid out by Campos. The acoustic velocity potential (psi) describing the acoustic wave motion in the presence of an inhomogeneous steady high-speed flow is defined by, del squared psi - (lambda/c) squared psi - M x [M x del((del)(psi))] - 2((lambda)(M)/c + M x del(M) x (del)(psi) - 2(lambda)(psi)[M x del(1/c)] = 0. with M as the Mach vector, c as the speed of sound, and λ as the complex eigenvalue. The study requires one way coupling of the CFD High Mach Number Flow (HMNF) and mathematics module. The HMNF module evaluates the gas flow inside of a SRM using St. Robert's law to model the solid propellant burn rate, slip boundary conditions, and the supersonic outflow condition. Results from the HMNF model are verified by comparing the pertinent ballistics parameters with the industry standard code outputs (i.e. pressure drop, axial velocity, exit velocity). These results are then used by the coefficient form of the mathematics module to determine the complex eigenvalues of the AVPE. The mathematics model is truncated at the nozzle sonic line, where a zero flux boundary condition is self-satisfying. The remaining boundaries are modeled with a zero flux boundary condition, assuming zero acoustic absorption on all surfaces. The one way coupled analysis is perform four times utilizing geometries determined through traditional SRM modeling procedures. The results of the steady-state CFD and AVPE analyses are used to calculate the linear acoustic growth rate as is defined by Flandro and Jacob. In order to verify the process implemented within COMSOL we first employ the Culick theory and compare the results with the industry standard. After the process is verified, the Flandro/Jacob energy balance theory is employed and results displayed.

Fischbach, S. R.↗

NASA NTRS1982

NASA Lewis Research Center combustion MHD experiment

The MHD power generation experiments were conducted in a high field strength cryomagnet which was adapted from an existing facility. In its original construction, it consisted of 12 high purity aluminum coils pool cooled in a bath of liquid neon. In this configuration, a peak field of 15 tesla was produced. For the present experiments, the center four coils were removed and a 23 cm diameter transverse warm bore tube was inserted to allow the placement of the MHD experiment between the remaining eight coils. In this configuration, a peak field of 6 tesla should be obtainable. The time duration of the experiment is limited by the neon supply which allows on the order of 1 minute of total operating time followed by an 18-hour reliquefaction period. As a result, the experiments are run in a pulsed mode. The run duration for the data presented here was 5 sec. The magnetic field profile along the MHD duct is shown. Since the working fluid is in essence superheated steam, it is easily water quenched at the exit of the diffuser and the components are designed vacuum tight so that the exhaust pipe and demister an be pumped down to simulate the vacuum of outer space.

Smith, J. M.↗

NASA NTRS1985

The solar vector magnetograph of the Okayama Astrophysical Observatory

The vector magnetograph of the Okayama Astrophysical Observatory is fed to the 65 cm solar coude telescope with a 10 m Littrow spectrograph. The polarimeter put at the telescope focus analyzes the incident polarization. Photomultipliers (PMT) at the exit of the spectrograph pick up the modulated light signals and send them to the electronic controller. The controller analyzes frequency and phase of the signal. The analyzer of the polarimeter is a combination of a single wave plate rotating at 40 Hz and a Wallaston prism. Incident linear and circular polarizations are modified at four times and twice the rotation frequency, respectively. Two compensators minimize the instrumental polarization, mainly caused by the two tilt mirrors in the optical path of the telescope. The four photomultipliers placed on the wings of the FeI 5250A line give maps of intensity, longitudinal field and transverse field. The main outputs, maps of intensity, and net linear and circular polarizations in the neighboring continuum are obtained by the other two monitor PMTs.

Makita, M.↗

NASA NTRS1991

Test equipment data package for the KC-135 fiber pulling apparatus

The Fiber Pulling Apparatus (FPA) is a device designed to produce continuous glass fibers from simulated lunar soil, and to determine the effects of reduced gravity, specifically 1/6 g on fiber formation and resultant properties. Briefly, pre-melt simulated lunar soil will be placed in a pint crucible and heated to 1200 C or higher, up to a maximum temperature of 1400 C. At a given temperature, a quartz fiber will be immersed into the melt and then pulled through a chill block and wound onto a cylindrical bobbin using a servo motor control. A high resolution video camera will record the fiber as it is being pulled. This assembly wil be enclosed in Plexiglas. Before fiber pulling commences, the apparatus will be backfilled with dry nitrogen. A separate data acquisition system will support this apparatus. This system will contain a personal computer, video recorder, and monitor. Temperature, acceleration, winding speed, and video images will be controlled and recorded using the data acquisition system. Thus, the FPA will consist of two hardware packages, the fiber production assembly and the data acquisition rack. The primary objective of this test is to determine the effects of 1/6 g on the formation of continuous glass fiber made from simulated lunar soil. Baseline studies using the FPA on the ground will provide a reference for the 1/6 g studies. Of particular interest will be the effect of 1/6 g on the free fluid zone where the fiber exits the crucible. In the fiber spinning parlance this zone is known as the upper jet region, where the boundary slope is greater than one tenth. The properties of the resulting glass fiber will depend on the jet shape as well as distributions of velocity, temperature and tension within the jet. It is unknown at this time how 1/6 g will effect these parameters.

Kosten, Sue↗

NASA NTRS1956

An analog study of a shock-position diffuser control on a supersonic turbojet engine

Shock position was assumed to be controlled by air bleed through a diffuser-exit bypass door operated by a second-order servomotor. Pressure disturbance resulting from changes in bypass-door position had whereas a disturbance in engine speed had a large effect on diffuser pressure and could impose severe requirements on diffuser control response. The controlled diffuser response improved with faster servomotor response and smaller diffuser dead time and lag. The effect of diffuser dead time on diffuser response was greater than the effect of diffuser lag.

Novik, David↗

NASA NTRS1999

Soot Aerosol Properties in Laminar Soot-Emitting Microgravity Nonpremixed Flames

The 0-g flame soot measurements reported in previous studies are extended by adding new 0-g data for different fuel flow rates and burner diameters. The new flame conditions allow more conclusive comparisons regarding the effect of characteristic flow residence times on soot field structure, the influence of fuel preheat on fuel pyrolysis rates near the flame centerline, and the premature cessation of soot growth along the soot annulus in 0-g when the fuel is preheated. The paper also reports on the implementation of thermophoretic soot sampling in a specific 0-g flame featuring burner exit velocities typical of buoyant flames and presents quantitative data on the radial variation of soot microstructure at a fixed height above the burner mouth.

FLAME STRUCTURE↗

DOE OSTI2021

Investigation of theoretical scaling laws using large eddy simulations for airborne spreading of viral contagion from sneezing and coughing

Using a set of large eddy point-particle simulations, we explore the fluid dynamics of an ejected puff resulting from a cough/sneeze. The ejection contains over 61,000 potentially virus-laden droplets at an injection Reynolds number of about 46,000, comparable to an actual cough/ sneeze. We observe that global puff properties, such as centroid, puff volume, momentum, and buoyancy vary little across realizations. Other properties, such as maximum extent, shape, and edge velocity of the puff, may exhibit substantial variation. In many realizations, a portion of the puff splits off and advances along a random direction, while keeping airborne droplet nuclei afloat. This peeled-off portion provides a mechanism for virus-laden droplets to travel over large distances in a short amount of time. We also observe that the vast majority of droplets remain suspended within the puff after all liquid has evaporated. The main objectives of the study are to (i) evaluate assumptions of Balachandar’s et al. theory [Int. J. Multiphase Flow 132, 103439 (2020)], which include buoyancy effects, shape of the puff, and droplet evaporation rate, (ii) obtain values of closure parameters, which include location and time of the virtual origin, and puff entrainment and drag coefficients, and (iii) evaluate the accuracy of the theory in predicting the shape, size, and location of the puff, as well as droplet number density long after ejection. The theory adequately predicts global puff properties including size, velocity, and distance traveled, the largest size of droplets that exit the puff due to settling, and the droplet size distribution within the puff long after ejection.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2021

Bioeconomic benefits of managing fishing effort in a coexisting small- and large-scale fishery game

Abstract Fishing systems provide employment, income generation, poverty alleviation, and food security. The coexistence of small-scale fisheries (SSFs) and large-scale fisheries (LSFs) increases management complexity. Management actions have ecological and social implications that must be addressed carefully. We applied a bioeconomic game-theoretical model to the four-gear mullet fishery in southern Brazil—one industrial LSF (purse seine) and three artisanal SSFs (gillnet, beach seine, and drift net). All fishing gears target adult individuals during mullet's reproductive migration. First, we explored whether the current fishing efforts of all fishing gears could persist over time. Second, we investigated their interactions through a non-cooperative game. Finally, we studied the response of these interactions when fishing effort was restricted. We found that when the current fishing effort was maintained, the stock reduced to 26.4% of its capacity in 25 years. In addition, under non-cooperation, the traditional beach seine fleet exited the fishery. Interestingly, the constrained scenario had a coexistence output with increasing values for the final stock size and the per capita labour income, suggesting that limiting fishing effort can maintain all fishing gears in the fishery with social and ecological benefits.

de Azevedo, Eric Zettermann Dias (ORCID:0000000323↗

NASA NTRS1982

Inductive electric field at the magnetopause

The electric field data for two crossings of the magnetopause by ISEE-1 on November 20, 1977, have been analyzed with high time resolution. In both cases the electric field has a negative dawn-dusk component in the boundary layer, so it must reverse somewhere within the current layer to the positive value outside. If there is a component parallel to the moving magnetopause current it is small, and by no means obvious. In the case of the exit crossing from the boundary layer to the magnetosheath the data show that the electric field vector is turning for about two seconds at roughly the satellite spin rate; this changing direction suggests that the electric field has a curl. Such a curl could be caused by a travelling localized perturbation of the magnetopause surface current associated with impulsive plasma transport through the magnetopause.

Heikkila, W. J.↗

NASA NTRS1991

An information theory approach for evaluating earth radiation budget (ERB) measurements - Nonuniform sampling of diurnal longwave flux variations

Satellite measurements are subject to a wide range of uncertainties due to their temporal, spatial, and directional sampling characteristics. An information-theory approach is suggested to examine the nonuniform temporal sampling of ERB measurements. The information (i.e., its entropy or uncertainty) before and after the measurements is determined, and information gain (IG) is defined as a reduction in the uncertainties involved. A stochastic model for the diurnal outgoing flux variations that affect the ERB is developed. Using Gaussian distributions for the a priori and measured radiant exitance fields, the IG is obtained by computing the a posteriori covariance. The IG for the monthly outgoing flux measurements is examined for different orbital parameters and orbital tracks, using the Earth Observing System orbital parameters as specific examples. Variations in IG due to changes in the orbit's inclination angle and the initial ascending node local time are investigated.

Halyo, Nesim↗

NASA NTRS1991

Supersonic quasi-axisymmetric vortex breakdown

An extensive computational study of supersonic quasi-axisymmetric vortex breakdown in a configured circular duct is presented. The unsteady, compressible, full Navier-Stokes (NS) equations are used for the present computational study. The NS equations are solved for quasi-axisymmetric flow using an implicit, upwind, flux-difference splitting, finite-volume scheme. The quasi-axisymmetric solutions are time accurate and are obtained by forcing the components of the flowfield vector to be equal on two axial planes, which are in close proximity of each other. The computational study addresses the effect of the Reynolds number, for laminar flows, on the evolution and persistence of vortex breakdown. The effect of boundary conditions at the duct exit on vortex breakdown is also studied. Finally, the effect of swirl ratio at the duct inlet is investigated.

Kandil, Osama A.↗

NASA NTRS1993

Initial results from the NASA-Lewis wave rotor experiment

Wave rotors may play a role as topping cycles for jet engines, since by their use, the combustion temperature can be raised without increasing the turbine inlet temperature. In order to design a wave rotor for this, or any other application, knowledge of the loss mechanisms is required, and also how the design parameters affect those losses. At NASA LeRC, a 3-port wave rotor experiment operating on the flow-divider cycle, has been started with the objective of determining the losses. The experimental scheme is a three factor Box-Behnken design, with passage opening time, friction factor, and leakage gap as the factors. Variation of these factors is provided by using two rotors, of different length, two different passage widths for each rotor, and adjustable leakage gap. In the experiment, pressure transducers are mounted on the rotor, and give pressure traces as a function of rotational angle at the entrance and exit of a rotor passage. In addition, pitot rakes monitor the stagnation pressures for each port, and orifice meters measure the mass flows. The results show that leakage losses are very significant in the present experiment, but can be reduced considerably by decreasing the rotor to wall clearance spacing.

Wilson, Jack↗

NASA NTRS1993

Initial results from the NASA Lewis wave rotor experiment

Wilson, Jack↗

NASA NTRS1996

UNIX-based operating systems robustness evaluation

Robust operating systems are required for reliable computing. Techniques for robustness evaluation of operating systems not only enhance the understanding of the reliability of computer systems, but also provide valuable feed- back to system designers. This thesis presents results from robustness evaluation experiments on five UNIX-based operating systems, which include Digital Equipment's OSF/l, Hewlett Packard's HP-UX, Sun Microsystems' Solaris and SunOS, and Silicon Graphics' IRIX. Three sets of experiments were performed. The methodology for evaluation tested (1) the exception handling mechanism, (2) system resource management, and (3) system capacity under high workload stress. An exception generator was used to evaluate the exception handling mechanism of the operating systems. Results included exit status of the exception generator and the system state. Resource management techniques used by individual operating systems were tested using programs designed to usurp system resources such as physical memory and process slots. Finally, the workload stress testing evaluated the effect of the workload on system performance by running a synthetic workload and recording the response time of local and remote user requests. Moderate to severe performance degradations were observed on the systems under stress.

Chang, Yu-Ming↗

NASA NTRS1994

Modulation of Cloud Optical Properties by Vertical Circulations Associated with a Jet Streak Exit Region: The November 26 FIRE Cirrus Case Study

The period from 18 UTC 26 November 1991 to roughly 23 UTC 26 November 1991 has become a focal case study of the FIRE (First International Satellite Cloud Climatology Regional Experiment) Cirrus-11 field campaign. The middle and upper tropospheric cloud data that were collected have allowed FIRE scientists to learn a great deal about the morphological structure and microphysical and radiative characteristics of the mid-latitude cirrus that occurred during that time. An important component of this effort in determining the synoptic scale forcing that existed during this time. By forcing, we mean the coupling between the background vertical air motions and the large scale moisture budget that initiated and maintained cirrus cloud in the study region. Defining the synoptic scale forcing is one of the stated scientific objectives of the FIRE program.

Mace, Gerald G.↗