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At least 469 records · Page 26

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NASA NTRS1981

A new transition radiation detector for cosmic ray nuclei

Test measurements on materials for transition radiation detectors at a low Lorentz factor are reported. The materials will be based on board Spacelab-2 for determining the composition and energy spectra of nuclear cosmic rays in the 1 TeV/nucleon range. The transition radiation detectors consist of a sandwich of radiator-photon detector combinations. The radiators emit X-rays and are composed of polyolefin fibers used with Xe filled multiwired proportional chamber (MWPC) detectors capable of detecting particle Lorentz factors of several hundred. The sizing of the detectors is outlined, noting the requirement of a thickness which provides a maximum ratio of transition radiation to total signal in the chambers. The fiber radiator-MWPC responses were tested at Fermilab and in an electron cyclotron. An increase in transition radiation detection was found as a square power law of Z, and the use of six radiator-MWPC on board the Spacelab-2 is outlined.

Lheureux, J.↗

NASA NTRS1983

First detection of the ground state JK = 1 sub 0 going to 0 sub 0 submillimeter transition of interstellar ammonia

The JK = 1 sub 0 approaching O sub 0 transition of ammonia at 572.5 GHz was detected in OMC-1 from NASA's Kuiper Airborne Observatory. The central velocity of the line (VLSR approximately = 9 km/s) indicates that it originates in the molecular cloud material, not the hot core. The derived filling factor of approximately 0.09 in a 2' beam implies a source diameter of approximately 35" if it is a single clump. This clump area is much larger than that derived from observations of the sub 1 inversion transition. The larger optical depth in the 1 sub 0 approaching 0 sub 0 transition (75-350) can account for the increased source area and linewidth as compared with those seen in the 1 sub 0 inversion transition.

Keene, J.↗

NASA NTRS1983

Demonstration and evaluation of gas turbine transit buses

The Gas Turbine Transit Bus Demonstration Program was designed to demonstrate and evaluate the operation of gas turbine engines in transit coaches in revenue service compared with diesel powered coaches. The main objective of the program was to accelerate development and commercialization of automotive gas turbines. The benefits from the installation of this engine in a transit coach were expected to be reduced weight, cleaner exhaust emissions, lower noise levels, reduced engine vibration and maintenance requirements, improved reliability and vehicle performance, greater engine braking capability, and superior cold weather starting. Four RTS-II advanced design transit coaches were converted to gas turbine power using engines and transmissions. Development, acceptance, performance and systems tests were performed on the coaches prior to the revenue service demonstration.

Source record↗

NASA NTRS1983

Free stream noise and transition measurements in a Mach 3.5 pilot quiet tunnel

Free stream fluctuating pressures are determined from hot-wire measurements in a Mach 3.5 pilot quiet nozzle over a unit Reynolds number range from 10-60 million per meter. Further, the transition Reynolds numbers on a sharp tip 5 degree half-angle cone at zero angle of attack are obtained over the same range of unit Reynolds numbers from equilibrium temperature measurements on the cone. Results show that the nozzle provides noise levels in the upstream regions of the test rhombus that are substantially lower than in conventional nozzles. The normalized rms levels of the fluctuating static pressures are found to vary from extremely low values of less than 0.03% up to about 0.8% depending on the unit Reynolds number, the axial location in the test rhombus, and the bleed slot flow. By opening the boundary-layer bleed flow, the wall boundary layers over upstream regions of the nozzle become laminar and the absence of high frequency radiated noise then results in cone transition Reynolds numbers that are in the range of free-flight data. As the unit Reynolds numbers are increased, the nozzle wall boundary layers become transitional and turbulent, and the noise increases to peak levels of about 0.5% with significant energy up to 150 KHz. The cone transition Reynolds numbers then decrease to values that are in the range of those measured in conventional wind tunnels.

Beckwith, I. E.↗

NASA NTRS1983

First detection of the ground-state J(K) = 1(0)-0(0) submillimeter transition of interstellar ammonia

The JK = 1 sub 0 approaching 0 sub 0 transition of ammonia at 572.5 GHz was detected in OMC-1 from NASA's Kuiper Airborne Observatory. The central velocity of the line (VLSR approximately = 9 km/s) indicates that it originates in the molecular cloud material, not the hot core. The derived filling factor of approximately 0.09 in a 2' beam implies a source diameter of approximately 35' if it is a single clump. This clump area is much larger than that derived from observations of the sub 1 inversion transition. The larger optical depth in the 1 sub 0 approaching 0 sub 0 transition (75-350) can account for the increased source area and linewidth as compared with those seen in the 1 sub 0 inversion transition. Previously announced in STAR as N83-25630

Keene, J.↗

NASA NTRS1983

Measurement of the J = 0-1 rotational transitions of three isotopes of ArD(+)

The rotational transitions of all three isotopic species of ArD(+) in samples containing the Ar isotopes in their natural abundances have been measured by means of millimeter and submillimeter techniques that employ a magnetically enhanced abnormal glow discharge. All three transition frequency measurements were made from digitally averaged signals detected through a lock-in amplifier with a 10-msec time constant. The Ar-4OD(+) transition was easily visible in real time on an oscilloscope with SNR of about 15. It is noted that the observed transition of Ar-38D(+) is more than five orders of magnitude weaker than that due to HCO(+).

Bowman, W. C.↗

NASA NTRS1983

Behavior of transition-region lines during impulsive solar flares

The emissions observed in lines originating in the transition-region plasma, particularly during impulsive flares, are investigated using data collected by the Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission. Data concerning lines from ions such as Si II, C IV, Si IV, O V, and Fe XXI are analyzed. Topics examined include the transition region away from sunspots, oscillatory motions above sunspots, the shape of the transition region, the transition region during flares, and O V and Fe XXI during flare flash phases.

Tandberg-Hanssen, E.↗

NASA NTRS1983

Transition to turbulence in plane channel flow

A numerical simulation of the final stages of transition to turbulence in plane channel flow is reported. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time-evolution of two and three dimensional finite amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32x51x32 grid. Results are presented for no-slip boundary conditions at the solid walls as well as for periodic suction-blowing to simulate active control of transition by mass transfer. Solutions indicate that the method is capable of simulating the complex character of vorticity dynamics during the various stages of transition and final breakdown. In particular, evidence points to the formation of a lambda-shape vortex and the subsequent system of horseshoe vortices inclined to the main flow direction as the main elements of transition. Calculations involving suction-blowing indicate that interference with a wave of suitable phase and amplitude reduces the disturbance growth rates.

Biringen, S.↗

NASA NTRS1984

Correlations of supersonic boundary-layer transition on cones including effects of large axial variations in wind-tunnel noise

Transition data on sharp tip cones in two pilot low disturbance wind tunnels at Mach numbers of 3.5 and 5 were correlated in terms of noise parameters with data from several conventional wind tunnels and with data from supersonic flight tests on a transition cone. The noise parameters were developed to account for the large axial variations of the free stream noise and the very high frequency noise spectra that occurred in the low disturbance tunnels for some test conditions. The noise could be varied in these tunnels from high levels, approaching those in conventional tunnels, to extremely low levels. The correlations indicated that transition in the low disturbance tunnels was dominated by the local stream noise that was incident on the cone boundary layer unstream of the neutral stability point. The correlation results also suggested that high frequency components of the low disturbance tunnel noise spectra had significant effects on transition when the noise was incident on the boundary layer both upstream and downstream of the neutral stability point.

Chen, F. J.↗

NASA NTRS1984

Transition to turbulence in plane channel flows

Results obtained from a numerical simulation of the final stages of transition to turbulence in plane channel flow are described. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time evolution of two and three dimensional finite amplitude disturbances. Computations are performed on CYBER-203 vector processor for a 32x51x32 grid. Results are presented for no-slip boundary conditions at the solid walls as well as for periodic suction blowing to simulate active control of transition by mass transfer. Solutions indicate that the method is capable of simulating the complex character of vorticity dynamics during the various stages of transition and final breakdown. In particular, evidence points to the formation of a lambda-shape vortex and the subsequent system of horseshoe vortices inclined to the main flow direction as the main elements of transition. Calculations involving periodic suction-blowing indicate that interference with a wave of suitable phase and amplitude reduces the disturbance growth rates.

Biringen, S.↗

NASA NTRS1984

Effects of streamwise variations in noise levels and spectra on supersonic boundary-layer transition

Transition data for sharp cones in two quiet wind tunnels at Mach numbers 3.5 and 5.0 have been correlated in terms of noise parameters with data from several conventional wind tunnels and from the flight data for the AEDC transition cone. The noise parameters were developed to account for the large axial variations of the rms stream noise and the high frequency noise spectra that occurred in the quiet tunnels for some test conditions. The correlation results indicated transition in the quiet tunnels was dominated by the local stream noise that was incident on the cone boundary layer upstream of the neutral stability point. The correlation results also suggested that the energy in high frequency components of the quiet tunnel noise spectra had significant adverse effects on transition when the noise was incident on the boundary layer both upstream and downstream of the neutral stability point.

Chen, F.-J.↗

NASA NTRS1984

Computer graphic visualization of orbiter lower surface boundary-layer transition

Computer graphic techniques are applied to the processing of Shuttle Orbiter flight data in order to create a visual presentation of the extent and movement of the boundary-layer transition front over the orbiter lower surface during entry. Flight-measured surface temperature-time histories define the onset and completion of the boundary-layer transition process at any measurement location. The locus of points which define the spatial position of the boundary-layer transition front on the orbiter planform is plotted at each discrete time for which flight data are available. Displaying these images sequentially in real-time results in an animated simulation of the in-flight boundary-layer transition process.

Throckmorton, D. A.↗

NASA NTRS1984

Boundary layer transition effects on flow separation around V/STOL engine inlets at high incidence

Numerical methods for calculating laminar and turbulent boundary layer development around vertical-short take off and landing engine inlets at high incidence angles are investigated. Various transition models were compared and evaluated in calculations off flow separation bound inside the inlet. Results of the transition effects on the boundary layer characteristics at onset of separation for two types of engine inlet geometries are presented. Some of the numerical results are compared with existing wind-tunnel test data for scaled inlet models to demonstrate the effects of transition models in the numerical scheme. The effects of transition modeling on the boundary layer development are illustrated for typical engine operating conditions.

Chou, D. C.↗

NASA NTRS1983

Altitude/path-angle transitions in fuel-optimal problems for transport aircraft

Altitude/path angle transitions to fuel-optimal, energy climb and descent paths are examined for subsonic transport aircraft. These transitions are termed boundary layers in singular perturbation theory, the framework used herein to simplify solutions to the state-Euler system. The energy climb and descent paths provide equilibrium or stationary points for the boundary layer system, and thus for moderate transitions, presumed valid for the class of aircraft considered, the boundary layer system may be linearized. This simplification allows derivation of an explicit solution to the two-point boundary-value, boundary layer problem and, as a consequence, yields a nearly optimal control law, in feedback form, for the transitions. Numerical simulation results using the feedback control law are presented for a Boeing 737 airframe (NASA-ATOPS Vehicle) employing twin JT8D-7-7A engines.

Gracey, C.↗

NASA NTRS1984

Analysis of airfoil transitional separation bubbles

A previously developed local inviscid-viscous interaction technique for the analysis of airfoil transitional separation bubbles, ALESEP (Airfoil Leading Edge Separation), has been modified to utilize a more accurate windward finite difference procedure in the reversed flow region, and a natural transition turbulence model has been incorporated for the prediction of transition within the separation bubble. Numerous calculations and experimental comparisons are presented to demonstrate the effects of the windward differencing scheme and the natural transition turbulence model. Grid sensitivity and convergence capabilities of this inviscid-viscous interaction technique are briefly addressed. A major conclusion of this paper is that a second, counter-rotating eddy has been found to exist in the wall layer of the primary separation bubble with the use of windward differencing.

Davis, R. L.↗

NASA NTRS1984

SMM/UVSP observations of the distribution of transition region oscillations and other properties in a sunspot

Observations of a sunspot in the C IV line at 1548 A formed in the transition region have been analyzed to obtain the time variations and/or mean values of the velocity, intensity, longitudinal magnetic field, and line width. Oscillations with periods between approximately 110 and 200 s are observed only over the umbra where the transition region magnetic field is highest and the line width is smallest. When periodic intensity variations occur at the same frequency as the velocity oscillations, the peak intensities occur slightly before the maximum upward motions. No periodic variations in the transition region magnetic field have been detected. Scatter diagrams are presented which show possible relationships between the flow velocity, emission line intensity, line width, and transition region magnetic field.

Henze, W.↗

NASA NTRS1985

Transition in Turbines

The concept of a large disturbance bypass mechanism for the initiation of transition is reviewed and studied. This mechanism, or some manifestation thereof, is suspected to be at work in the boundary layers present in a turbine flow passage. Discussion is presented on four relevant subtopics: (1) the effect of upstream disturbances and wakes on transition; (2) transition prediction models, code development, and verification; (3) transition and turbulence measurement techniques; and (4) the hydrodynamic condition of low Reynolds number boundary layers.

Source record↗

NASA NTRS1984

Predictions of nucleation theory applied to Ehrenfest thermodynamic transitions

A modified nucleation theory is used to determine a critical nucleus size and a critical activation-energy barrier for second-order Ehrenfest thermodynamic transitions as functions of the degree of undercooling, the interfacial energy, the heat-capacity difference, the specific volume of the transformed phase, and the equilibrium transition temperature. The customary approximations of nucleation theory are avoided by expanding the Gibbs free energy in a Maclaurin series and applying analytical thermodynamic expressions to evaluate the expansion coefficients. Nonlinear correction terms for first-order-transition calculations are derived, and numerical results are presented graphically for water and polystyrene as examples of first-order and quasi-second-order transitions, respectively.

Barker, R. E., Jr.↗