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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

SSME Seal Test Program: Test results for sawtooth pattern damper seal

Direct and transverse force coefficients for 11, sawtooth-pattern, and damper-seal configurations were examined. The designation damper seal uses a deliberately roughened stator and smooth rotor to increase the net damping force developed by a seal. The designation sawtooth-pattern refers to a stator roughness pattern. The sawtooth pattern yields axial grooves in the stator which are interrupted by spacer elements which act as flow constrictions or dams. All seals use the same smooth rotor and have the same, constant, minimum clearance. The stators examined the consequences of changes in the following design parameters: (1) axial-groove depth; (2) number of teeth: (3) number of sawtooth sections; (4) number of spacer elements; (5) dam width; (6) axially aligned sawtooth sections versus axially-staggered sawtooth sections; and (7) groove geometry. It is found that none of the sawtooth-pattern seal performs as well as the best round-hole-pattern seal. Maximum damping configurations for the sawtooth and round-hole-pattern stators have comparable stiffness performance. Several of the sawtooth pattern stators outperformed the best round-hole pattern seal.

Childs, D. W.↗
NASA NTRS2016

Magnetospheric State of Sawtooth Events

Magnetospheric sawtooth events, first identified in the early 1990s, are named for their characteristic appearance of multiple quasiperiodic intervals of slow decrease followed by sharp increase of proton differential energy fluxes in the geosynchronous region. The successive proton flux oscillations have been interpreted as recurrences of stretching and dipolarization of the nightside geomagnetic field. Due to their often extended intervals with 210 cycles, sawteeth occurrences are sometimes referred to as a magnetospheric mode. While studies of sawtooth events over the past two decades have yielded a wealth of information about such events, the magnetospheric state conditions for the occurrence of sawtooth events and how sawtooth oscillations may depend on the magnetospheric state conditions remain unclear. In this study, we investigate the characteristic magnetospheric state conditions (specified by Psw interplanetary magnetic field (IMF) Btot, IMF Bz Vsw, AE, Kp and Dst, all time shifted with respect to one another) associated with the intervals before, during, and after sawteeth occurrences. Applying a previously developed statistical technique, we have determined the most probable magnetospheric states propitious for the development and occurrence of sawtooth events, respectively. The statistically determined sawtooth magnetospheric state has also been validated by using out-of-sample events, confirming the notion that sawtooth intervals might represent a particular global state of the magnetosphere. We propose that the sawtooth state of the magnetosphere may be a state of marginal stability in which a slight enhancement in the loading rate of an otherwise continuous loading process can send the magnetosphere into the marginally unstable regime, causing it to shed limited amount of energy quickly and return to the marginally stable regime with the loading process continuing. Sawtooth oscillations result as the magnetosphere switches between the marginally stable (loading) and unstable (unloading) phases.

Fung, Shing F.↗
NASA NTRS1987

An experimental and theoretical comparison of rotordynamic coefficients for sawtooth-pattern damper seals

A brief review of the annular seal theory as related to rotordynamics for liquid seals is presented. Also included is an overview of Childs and Kim's current theory for calculating empirical turbulence coefficients and predicting stiffness and damping coefficients for surface roughened damper seals. The designation sawtooth-pattern refers to a seal stator with a roughness pattern whose cross section normal to the seal axis resembles a sawtooth with the teeth directed against the flow. The net stiffness and damping coefficients for the eleven seals are compared to each other, a smooth seal, and the optimum-configuration damper seal previously tested. The experimental force coefficients, the net damping, and the net stiffness coefficients for four of the sawtooth seals are compared to the predictions of Childs and Kim's analysis. The sawtooth-pattern seal had less net damping than the hole-pattern seal but more than the smooth seal. The stiffness was comparable to the hole-pattern. Both the sawtooth and hole-pattern seals leaked less than the smooth seal, while the sawtooth-pattern seal with maximum damping leaked more than the hole-pattern seal. The theoretical predictions compared relatively poorly to the experimental results obtained for the rotordynamic coefficients of the seals investigation.

Nolan, Steven Anthony↗
NASA NTRS1986

Test results for sawtooth-pattern damper seals - Leakage and rotordynamic coefficients

Test results consisting of direct and transverse force coefficients are presented for eleven sawtooth-pattern damper-seal configurations. The designation 'damper' seal refers to a seal which uses a deliberately roughened stator and smooth rotor to increase the net seal damping force. The designation 'sawtooth-pattern' refers to a stator-roughness pattern whose cross section normal to the axis of the seal resembles saw teeth with the teeth direction opposing fluid motion in the direction of shaft rotation. The sawtooth pattern yields axial grooves in the stator which are interrupted by spacer elements which act as flow constrictions or 'dams'. Sawtooth-pattern seals had more damping than smooth seals but less than the round-hole-pattern seals tested previously. If damping is sacrificed, sawtooth-pattern seals can be designed to leak less than round-hole-pattern seals.

Childs, D.↗
NASA NTRS1990

The magnetic pumping of plasmas with sawtooth waveforms

The pumping of plasmas by sawtooth-waveform magnetic induction variations is studied theoretically and by means of computer simulations. A sawtooth is a cycle waveform that is characterized by a slow increase in the magnetic induction followed by a rapid drop in the induction. Two types of sawtooth pumping are analyzed, and the types classified as to whether or not the first adiabatic invariants of the plasma particles are conserved during the rapid drops in the magnetic induction. When the invariants are conserved, the sawtooth waveforms are found to be less efficient than square waves for pumping plasmas. When the adiabatic invariations are not conserved, the pumping efficiency is found to be a slight improvement over square waves. Both types of pumping are applied to a hypothetical tokamak plasma and it is concluded that neither type of sawtooth pumping is practical for heating magnetically confined fusion plasmas.

Borovsky, Joseph E.↗
NASA NTRS1990

Tunneling calculations for GaAs-Al(x)Ga(1-x)As graded band-gap sawtooth superlattices

The transmission resonance spectra and tunneling current-voltage characteristics for direct conduction band electrons in sawtooth GaAs-Al(x)Ga(1-x)As superlattices are computed. Only direct-gap interfaces are considered. It is found that sawtooth superlattices exhibit resonant tunneling similar to that in step superlattices, manifested by correlation of peaks and regions of negative differential resistance in the current-voltage curves with transmission resonances. The Stark shift of the resonances of step-barrier superlattices is a linear function of the field, whereas in sawtooth superlattices under strong fields the shift is not a simple function of the field. This follows from the different ways in which the two structures deform under uniform electric fields: the sawtooth deforms into a staircase, at which field strength all barriers to tunneling are eradicated. The step-barrier superlattice always presents some barrier to tunneling, no matter how high the electric field strength.

Forrest, Kathrine↗
NASA NTRS1977

The sawtooth cover slide

A novel cover slide is reported which increases solar cell output by reducing the reflection of light from the cover slide surface and by redirecting incident light so that none falls on the collection grids of the cell. The new cover slide is fabricated with a sawtooth surface having a periodicity equal to that of the solar cell grids. This configuration refracts the light so that it is directed onto the semiconductor surface between the grid lines. Conventional grid patterns obstruct 7-10 percent of the light incident on the cell; at least half of this loss has been recovered by using the sawtooth cover slide. In addition, surface reflection from the conventional coated cover slide is suppressed by presenting a second surface to any light reflected at the first plane of contact. This double reflection results in a greater reduction of the reflection loss from the cover slide than does an antireflection coating on a flat surface.

Meulenberg, A., Jr.↗
NASA NTRS1991

Tunneling calculations for GaAs-Al(x)Ga(1-x) as graded band-gap sawtooth superlattices

Quantum mechanical tunneling calculations for sawtooth (linearly graded band-gap) and step-barrier AlGaAs superlattices were performed by means of a transfer matrix method, within the effective mass approximation. The transmission coefficient and tunneling current versus applied voltage were computed for several representative structures. Particular consideration was given to effective mass variations. The tunneling properties of step and sawtooth superlattices show some qualitative similarities. Both structures exhibit resonant tunneling, however, because they deform differently under applied fields, the J-V curves differ.

Forrest, Kathrine A.↗
NASA NTRS1985

Sawtooth segmentation and deformation processes on the southern San Andreas fault, California

Five contiguous 12-13 km fault segments form a sawtooth geometry on the southernmost San Andreas fault. The kinematic and morphologic properties of each segment depend on fault strike, despite differences of strike between segments of as little as 3 degrees. Oblique slip (transpression) of fault segments within the Indio Hills, Mecca Hills and Durmid Hill results from an inferred 8:1 ratio of dextral slip to convergence across the fault zone. Triggered slip and creep are confined almost entirely to transpressive segments of the fault. Durmid Hill has been formed in the last 28 + or - 6 ka by uplift at an average rate of 3 + or - 1 mm/a.

Bilham, R.↗
NASA NTRS1986

Photoelectrochemical fabrication of sawtooth gratings in n-GaAs

A photoelectrochemical approach is employed to fabricate exceptionally smooth low-pitch grating structures consisting of polished single crystals of (100) oriented n-GaAs in a sawtooth profile. The depth of the edged features is controlled by monitoring the total coulombs passed, and the rate of etching is precisely controlled by the light intensity, minimizing diffusional gradients in the liquid which may impair the uniformity. Symmetrical grooves spaced 10 microns apart will have a depth of 7.14 microns, and typically, 10-20 percent less total charge is required.

Carrabba, Michael M.↗
NASA NTRS1989

Sawtooth oscillations about helical current channels

An existing pseudospectral code for solving the three-dimensional equations of reduced magnetohydrodynamics is extended by adding a temperature equation. Resistivities and thermal conductivities are given their (isotropic) Braginskii temperature dependences, and are advanced self-consistently. Realistic-looking sawtooth oscillations are observed at modest Lundquist and Reynolds numbers. However, the oscillations are excited upon, and relax back to, a helical (rather than an axisymmetric) current channel.

Theobald, M. L.↗
NASA NTRS1992

Rotordynamic Behavior Of Sawtooth-Pattern Damping Seals

Report describes comparative experimental and theoretical study of rotordynamics of several different annular liquid pump seals. Introduces damping seals of new type in which inner surfaces of stators have saw-tooth-cross-section axial grooves interrupted at regular axial intervals by circumferential dams. Teeth of sawtooth patterns directed against rotations in attempt to reduce further asymptotic circumferential velocities of fluids.

Nolan, Steven A.↗
NASA NTRS1995

A Parametric Investigation of Nozzle Planform and Internal/External Geometry at Transonic Speeds

An experimental investigation of multidisciplinary (scarfed trailing edge) nozzle divergent flap geometry was conducted at transonic speeds in the NASA Langley 16-Foot Transonic Tunnel. The geometric parameters investigated include nozzle planform, nozzle contouring location (internal and/or external), and nozzle area ratio (area ratio 1.2 and 2.0). Data were acquired over a range of Mach Numbers from 0.6 to 1.2, angle-of-attack from 0.0 degrees to 9.6 degrees and nozzle pressure ratios from 1.0 to 20.0. Results showed that increasing the rate of change internal divergence angle across the width of the nozzle or increasing internal contouring will decrease static, aeropropulsive and thrust removed drag performance regardless of the speed regime. Also, increasing the rate of change in boattail angle across the width of the nozzle or increasing external contouring will provide the lowest thrust removed drag. Scarfing of the nozzle trailing edges reduces the aeropropulsive performance for the most part and adversely affects the nozzle plume shape at higher nozzle pressure ratios thus increasing the thrust removed drag. The effects of contouring were primary in nature and the effects of planform were secondary in nature. Larger losses occur supersonically than subsonically when scarfing of nozzle trailing edges occurs. The single sawtooth nozzle almost always provided lower thrust removed drag than the double sawtooth nozzles regardless the speed regime. If internal contouring is required, the double sawtooth nozzle planform provides better static and aeropropulsive performance than the single sawtooth nozzle and if no internal contouring is required the single sawtooth provides the highest static and aeropropulsive performance.

Cler, Daniel L.↗
NASA NTRS1973

Temperature control system with a pulse width modulated bridge

A temperature control system, which includes a modified wheatstone bridge with a resistive-capacitive (RC) circuit in one leg of the bridge, is disclosed. The RC circuit includes a resistor which provides an effective resistance as a function of its absolute resistance and the on-time to off-time ratio of pulses supplied to a switch connected across it. A sawtooth voltage is produced across the RC circuit. The voltage is compared with the voltage across a temperature sensor with heat being applied during each pulse period portion when the sawtooth voltage exceeds the voltage across the temperature sensor.

Heyser, R. C.↗
NASA NTRS1975

The effect of intermediate-scale motions on line formation

The problem of LTE and non-LTE line formation in the presence of nonthermal velocity fields with geometric scales between the microscopic and macroscopic limits is investigated in the cases of periodic sinusoidal and sawtooth waves. For a fixed source function (the LTE case), it is shown that time-averaged line profiles progress smoothly from the microscopic to the macroscopic limits as the geometric scale of the motions increases, that the sinusoidal motions produce symmetric time-averaged profiles, and that the sawtooth motions cause a redshift. In several idealized non-LTE cases, it is found that intermediate-scale velocity fields can significantly increase the surface source functions and line-core intensities. Calculations are made for a two-level atom in an isothermal atmosphere for a range of velocity scales and non-LTE coupling parameters and also for a two-level atom and a four-level representation of Na I line formation in the Harvard-Smithsonian Reference Atmosphere (1971) solar model. It is found that intermediate-scale velocity fields in the solar atmosphere could explain the central intensities of the Na I D lines and other strong absorption lines without invoking previously suggested high electron densities.

Shine, R. A.↗
NASA NTRS1977

Triaxial digital fluxgate magnetometer for NASA applications explorer mission: Results of tests of critical elements

Tests performed to prove the critical elements of the triaxial digital fluxgate magnetometer design were described. A method for improving the linearity of the analog to digital converter portion of the instrument was studied in detail. A sawtooth waveform was added to the signal being measured before the A/D conversion, and averaging the digital readings over one cycle of the sawtooth. It was intended to reduce bit error nonlinearities present in the A/D converter which could be expected to be as much as 16 gamma if not reduced. No such nonlinearities were detected in the output of the instrument which included the feature designed to reduce these nonlinearities. However, a small scale nonlinearity of plus or minus 2 gamma with a 64 gamma repetition rate was observed in the unit tested. A design improvement intended to eliminate this small scale nonlinearity was examined.

Mcleod, M. G.↗