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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Acceleration of a trailing positron bunch in a plasma wakefield accelerator

High gradients of energy gain and high energy efficiency are necessary parameters for compact, cost-efficient and high-energy particle colliders. Plasma Wakefield Accelerators (PWFA) offer both, making them attractive candidates for next-generation colliders. Here in these devices, a charge-density plasma wave is excited by an ultra-relativistic bunch of charged particles (the drive bunch). The energy in the wave can be extracted by a second bunch (the trailing bunch), as this bunch propagates in the wake of the drive bunch. While a trailing electron bunch was accelerated in a plasma with more than a gigaelectronvolt of energy gain, accelerating a trailing positron bunch in a plasma is much more challenging as the plasma response can be asymmetric for positrons and electrons. We report the demonstration of the energy gain by a distinct trailing positron bunch in a plasma wakefield accelerator, spanning nonlinear to quasi-linear regimes, and unveil the beam loading process underlying the accelerator energy efficiency. A positron bunch is used to drive the plasma wake in the experiment, though the quasi-linear wake structure could as easily be formed by an electron bunch or a laser driver. Finally, the results thus mark the first acceleration of a distinct positron bunch in plasma-based particle accelerators.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Searching for Subsecond Stellar Variability with Wide-field Star Trails and Deep Learning

We present here a method that enables wide-field ground-based telescopes to scan the sky for subsecond stellar variability. The method has operational and image processing components. The operational component takes star trail images. Each trail serves as a light curve for its corresponding source and facilitates subexposure photometry. We train a deep neural network to identify stellar variability in wide-field star trail images. We use the Large Synoptic Survey Telescope Photon Simulator to generate simulated star trail images and include transient bursts as a proxy for variability. The network identifies transient bursts on timescales down to 10 ms. We argue that there are multiple fields of astrophysics that can be advanced by the unique combination of time resolution and observing throughput that our method offers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

SMTN-003: Trailing Losses for Moving Objects

Trailing losses occur for Solar System objects when their motion during an image results in their detection covering more than the equivalent stellar PSF for that image. This results in a decreased SNR for the moving object compared to a stationary point source. There are two distinct sources of this decreased SNR: if the moving object measurement is done with a trailed PSF that matches the moving object PSF, SNR decreases due to the additional background sky pixels (larger n_effective) in the trailed PSF; if the moving object measurement is done with a point-source PSF that does not match the moving object's PSF, SNR decreases due to the flux missed outside the point-source PSF. This technote evaluates trailing losses from both sources and provides a summary formula to calculate each from the moving object's velocity and the image's FWHM and exposure time.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1971

NASA aircraft trailing vortex research

A brief description is given of NASA's comprehensive program to study the aircraft trailing vortex problem. Wind tunnel experiments are used to develop the detailed processes of wing tip vortex formation and explore different means to either prevent trailing vortices from forming or induce early break-up. Flight tests provide information on trailing vortex system behavior behind large transport aircraft, both near the ground, as in the vicinity of the airport, and at cruise/holding pattern altitudes. Results from some flight tests are used to show how pilots might avoid the dangerous areas when flying in the vicinity of large transport aircraft. Other flight tests will be made to verify and evaluate trailing vortex elimination schemes developed in the model tests. Laser Doppler velocimeters being developed for use in the research program and to locate and measure vortex winds in the airport area are discussed. Field tests have shown that the laser Doppler velocimeter measurements compare well with those from cup anemometers.

Mcgowan, W. A.↗
NASA NTRS1973

Photography of a lithium vapor trail during the daytime.

Barium and lithium vapors were released from sounding rockets in the thermosphere and observed from aboard a jet aircraft at an altitude of 40,000 ft. The purpose of the releases was to demonstrate the feasibility of an all-weather technique for observing chemical releases and to evaluate methods of observing daytime releases. The selected flight plan of the aircraft allowed a series of observations of the trail from two different straight line paths. Data were recorded photographically. The reduction in sky brightness at the 40,000-ft altitude as compared to the ground allows the use of a filter with a 10-A bandwidth for trail photography in the daytime. These photographs verified the calculation of the usable angular field of the narrow-band filters. Photographs of a 45-min-old trail of lithium vapor were obtained up to 20 min after sunrise at the aircraft. It is concluded that now vapor trail observations may be made during the daytime without regard to weather and logistic restrictions.

Bedinger, J. F.↗
NASA NTRS1974

Cold-air study of the effect on turbine stator blade aerodynamic performance of coolant ejection from various trailing-edge slot geometries. 1: Experimental results

Trailing-edge slot configurations were investigated in a two-dimensional cascade of turbine stator blades. The trailing-edge slots were incorporated into blades with round trailing edges. The five blade configurations investigated included blades with two different trailing-edge thicknesses and four different slot widths. The results of the investigation showed that there was, in general, a significant increase in primary-air efficiency due to the coolant flow, the increase varying with slot configuration. For the five configurations tested, the average percent change in primary-air efficiency per percent coolant flow varied almost linearly from zero to about 1.4 percent over a range of coolant- to primary-air exit-velocity ratios between 0 and 1.2. However, for different configurations there was considerable deviation from the average values in the lower range of exit velocity ratios.

Prust, H. W., Jr.↗
NASA NTRS1975

Cold air study of the effect on turbine stator blade aerodynamic performance of coolant ejection from various trailing edge slot geometries. 2: Comparison of experimental and analytical results

Experimentally determined efficiencies of turbine stator blades having trailing-edge coolant ejection are compared with efficiencies predicted from two previously published approximate analytical methods. The experimental results were obtained from two-dimensional data with the temperature of the primary and coolant flows both being nearly ambient. Data from five stator blade configurations having different slotted trailing-edge geometries were included in the comparison. The two analytical methods gave results which agreed reasonably well with experimental results. An average of the absolute values of differences between experimental and predicted efficiencies for all five blade configurations showed that one method gave average efficiency differences which were about 1.3 percent different than experimental efficiencies, while the other method gave average efficiency differences that were about 0.7 percent different than experimental. However, in some instances, maximum differences of as much as 4 percent occurred. A comparison between experimental and analytical results indicated that the ratio of trailing-edge slot width to trailing-edge thickness influences the measured efficiencies to a greater extent than is accounted for by either analytical model.

Prust, H. W., Jr.↗
NASA NTRS1976

Low-speed wind-tunnel investigation of various segments of flight spoilers as trailing-vortex-alleviation devices on a transport aircraft model

An investigation was made in the Langley V/STOL tunnel to determine, by the trailing wing sensor technique, the effectiveness of various segments of the existing flight spoilers on a transport aircraft model when they were deflected as trailing-vortex-alleviation devices. The four combinations of flight-spoiler segments investigated were effective in reducing the induced rolling moment on the trailing wing model by as much as 40 to 50 percent at distances behind the transport model of from about 7 to 27 transport wing spans, 27 spans being the downstream limit of distances used in this investigation. Results obtained at about 7 wing spans downstream for two of the spoiler configurations indicate that essentially all the reduction in induced rolling moment on the trailing wing model was realized at a spoiler deflection of about 30 deg.

Croom, D. R.↗
NASA NTRS1975

Flow visualization of vortices locked by spanwise blowing over wings featuring a unique leading and trailing-edge flap system

Flow visualization studies were conducted to qualitatively determine the effects of active generation and augmentation of vortex flow over wings by blowing a discrete jet in a spanwise direction in the channel formed by extension of upper surface leading- and trailing-edge flaps. Spanwise blowing from a reflection plane over a rectangular wing was found to generate and lock a dual corotating vortex system within the channel and, at sufficient blowing rates, cause the separated flow off the upper end of the leading-edge flap to reattach to the trailing-edge flap. Test parameters included wing angle of attack, jet momentum coefficient, leading- and trailing-edge flap deflection angle, and jet location above the wing surface. Effects due to removal of the leading- and trailing-edge flap were also investigated.

Erickson, G. E.↗
NASA NTRS1976

Low-speed wind-tunnel investigation of flight spoilers as trailing-vortex-alleviation devices on a medium range wide-body tri-jet airplane model

An investigation was made in the V/STOL tunnel to determine, by the trailing wing sensor technique, the effectiveness of various segments of the existing flight spoilers on a medium range wide body tri-jet transport airplane model when they were deflected as trailing vortex alleviation devices. The four combinations of flight spoiler segments investigated were effective in reducing the induced rolling moment on the trailing wing model by as much as 15 to 60 percent at distances behind the transport model of from 3.9 to 19.6 transport wing spans, 19.6 spans being the downstream limit of distances used. Essentially all of the reduction in induced rolling moment on the trailing wing model was realized at a spoiler deflection of about 45 deg.

Croom, D. R.↗
NASA NTRS1976

Low speed wind tunnel investigation of flight spoilers as trailing-vortex-alleviation devices on an extended-range wide body tri-jet airplane model

An investigation was made in the Langley V/STOL tunnel to determine, by the trailing wing sensor technique, the effectiveness of various segments of the existing flight spoilers on an extended-range wide-body tri-jet transport airplane model when they were deflected as trailing-vortex-alleviation devices. On the transport model with the approach flap configuration, the four combinations of flight-spoiler segments investigated were effective in reducing the induced rolling moment on the trailing wing model by as much as 25 to 45 percent at downstream distances behind the transport model of 9.2 and 18.4 transport wing spans. On the transport airplane model with the landing flap configuration, the four combinations of flight-spoiler segments investigated were effective in reducing the induced rolling moment on the trailing wing model by as much as 35 to 60 percent at distances behind the transport model of from 3.7 to 18.4 transport wing spans, 18.4 spans being the downstream limit of distances used.

Croom, D. R.↗
NASA NTRS1977

Numerical solutions of Navier-Stokes equations for the structure of a trailing vortex

The structure and decay of a trailing vortex were analyzed during the numerical solutions of the full Navier-Stokes equations. Unsteady forms of the governing equations were recast in terms of circulation, vorticity, and stream function as dependent variables, and a second upwind finite difference scheme was used to integrate them with prescribed initial and boundary conditions. The boundary conditions at the outer edge and at the outflow section of the trailing vortex were considered. Different models of the flow were postulated, and solutions were obtained describing the development of the flow as integration proceeds in time. A parametric study was undertaken with a view to understanding the various phenomena that may possibly occur in the trailing vortex. Using the Hoffman and Joubert law of circulation at the inflow section, the results of this investigation were compared with experimental data for a Convair 990 wind model and a rectangular wing. With an exponentially decaying law of circulation at the inflow section and an adverse pressure gradient at the outer edge of the trailing vortex, solutions depict vortex bursting through the sudden expansion of the core and/or through the stagnation and consequent reversal of the flow on the axis. It was found that this bursting takes place at lower values of the swirl ratio as the Reynolds number increases.

Jain, A. C.↗
NASA NTRS1977

Wind-tunnel and flight evaluation of spoilers as trailing-vortex hazard alleviation devices

This paper presents the results of ground-based and flight investigations that have been performed at NASA for the purpose of development of spoilers as trailing-vortex hazard alleviation devices. Based on the results obtained in these investigations, it was found that the induced rolling moment on a trailing model can be reduced by spoilers located near the mid-semispan of a vortex-generating wing. Substantial reductions in induced rolling moment occur when the spoiler vortex attenuator is located well forward on both unswept and swept wing models. In addition, it was found by ground-based model tests and verified by full-scale flight tests that the existing flight spoilers on the B-747 aircraft are effective as trailing vortex attenuators. Based on the results of wind-tunnel investigations of the DC-10-30 and L-1011 aircraft models, the existing flight spoilers on both the DC-10-30 and L-1011 aircraft may also be effective trailing vortex attenuators.

Croom, D. R.↗
NASA NTRS1977

Experimental studies of trailing-edge conditions on an oscillating airfoil at frequency parameters of up to one

This paper describes an investigation to find the range of reduced frequencies over which the classical Kutta-Joukowsky condition is valid and the nature of deviations beyond this range. An NACA 64A010 airfoil with a 15-cm chord is supported between the side walls of a 25-cm x 35-cm low-speed wind tunnel and oscillated over reduced frequencies ranging from 0.05 to 1.2. The experimentally determined loading near the trailing edge is measured and compared with unsteady incompressible small-disturbance theory. Experimental data are obtained both with and without a boundary-layer trip. It is observed that application of the Kutta-Joukowsky condition in the theoretical analysis is valid below a reduced frequency of about 0.6 in predicting the loading in the trailing-edge region. At reduced frequencies beyond 0.8, the predicted results underestimate the measured loading near the trailing edge, and the deviations increase further with reduced frequency. Similarly, the experimental phase angle of the loading in the trailing-edge region agrees reasonably with the linear theory up to k = 0.8 but lags the predicted value beyond k = 0.8.

Satyanarayana, B.↗
NASA NTRS1977

An experimental investigation of the trailing edge noise mechanism

An experimental investigation has been conducted to understand the physical mechanism of noise generation from a turbulent wall jet discharging over a flat plate and interacting with its sharp trailing edge. An aspect ratio 10 rectangular nozzle was used to provide the wall jet. Measurements made consist of farfield noise, surface pressure fluctuations, turbulent velocity fluctuations, and two-point space-time cross-correlations among these quantities. Results are presented which suggest strongly that the generation mechanism is the interaction of the convecting large scale quasi-orderly disturbance in the upper free shear layer of the wall jet with the trailing edge. The interaction also excites large scale strong vortical motion in the trailing edge wake. The dominant part of the sound field is highly coherent and in phase opposition across the trailing edge.

Yu, J. C.↗
NASA NTRS1979

On sound radiation from the trailing edge of an isolated airfoil in a uniform flow

An experimental investigation has been conducted to determine the physical process of the noise production from the trailing edge of an isolated two-dimensional airfoil embedded in a low turbulence uniform mean flow. The Reynolds number of the airfoil based on chord was greater than 1,000,000 and that the boundary layer was fully turbulent at the trailing edge. Smoke injection technique and spark shadowgraphy were used to study the structural features of the boundary layer. Two point joint statistical analyses were made on the surface pressure, the radiated sound and the relationship between the two quantities. Operating conditions included two free stream velocities and two angles of attack for both a naturally developed and an artificially tripped boundary layer. Flow visualizations revealed the existence of large scale coherent motions in the outer region of the boundary layer. The observed coherent motions had scales of the order of boundary layer thickness and a convection velocity near the free stream velocity. The production process of the airfoil trailing edge noise was determined to be the convection of the large scale coherent eddies over the trailing edge of the airfoil. The noise field so generated was found to be rather coherent and dipole-like.

Yu, J. C.↗
NASA NTRS1979

Trailing-edge flows at high Reynolds number

An investigation of trailing-edge flows at high Reynolds number and subsonic Mach numbers is presented. Symmetric and asymmetric trailing-edge flows are studied, each flow having pressure gradient regions upstream of the trailing edge similar to an airfoil. Measurements include model surface pressures, mean velocity, turbulent shear stress, and turbulent kinetic energy profiles in the trailing-edge and near-wake regions. Comparisons of the symmetric data with numerical solutions of boundary layer as well as Navier-Stokes equations employing two different turbulence models show increasing effects on viscous interactions as the Mach number increases. Both turbulence models yielded solutions of the mean flow of comparable quality. The experimental results of the asymmetric case are discussed.

Viswanath, P. R.↗
NASA NTRS1980

Trailing edge noise from hovering rotors

A method has been developed to predict the high frequency broadband noise due to the interaction of convecting turbulent eddies with the trailing edges of a hovering rotor. The trailing edge noise from each blade was modeled as point dipole noise with spanwise loading corrections. This point dipole approximation was checked by applying the concept to a stationary airfoil in a moving medium with excellent results. In order to estimate the strength of the point dipole, the trailing edge noise theory of Amiet was used. The method was applied specifically to blade boundary layer turbulence and compared to incident atmospheric turbulence noise. The results indicate that the relative importance of these two mechanisms is related to the magnitudes of the intensity and of the length scales of the inflow and boundary layer turbulence. The results tend to fall below some available experimental data indicating that in those experiments other broadband noise sources were stronger than boundary layer-trailing edge noise. The approach which was developed is also applicable to other blade-turbulence interaction mechanisms such as local stall and tip noise.

Kim, Y. N.↗