2,543 research outputs found
Jet exhaust and support interference effects on the transonic aerodynamic characteristics of a fighter model with two widely spaced engines
Jet exhaust, nozzle installation, and model support interference effects on the longitudinal aerodynamic characteristics of a twin-engine fighter model were determined. Realistic jet exhaust nozzle configurations and a reference configuration with a simulated vertical-tail support were tested. Free-stream Mach number was varied from 0.6 to 1.2, and model angle of attack from 0 deg to 9 deg. The jet exhaust affected drag more than it affected lift and pitching moment. The largest effects occurred at a Mach number of 0.9 and for the afterburning mode of exhaust nozzle operation. The combined differences between the aerodynamic characteristics of the realistic and reference configurations (which were due to afterbody and nozzle contours, jet operation, and simulated reference support interference) were considerably different from those for the jet interference alone
An Experimental Study of Jet Exhaust Simulation
An investigation was conducted to determine the effect of varying the jet exhaust ratio of specific heats, gas constants, and temperatures on jet interference on afterbody drag. Jet exhaust simulation parameters were also evaluated. In addition to air, three other exhaust gases, each with a different value of each of the gas parameters, were tested. The range of the ratios of specific heats, gas constants, and total temperatures of the four gases are identified. Tests were made using a single nacelle model with afterbodies having boattail angles of 10 degrees and 20 degrees with sonic and Mach two jet exits. Wind tunnel tests were conducted through a Mach number range of 0.60 to 1.20 and a Reynolds number per meter from 10.06 million to 14.05 million
A Bose-Einstein Condensate in a Uniform Light-induced Vector Potential
We use a two-photon dressing field to create an effective vector gauge
potential for Bose-condensed Rb atoms in the F=1 hyperfine ground state. The
dressed states in this Raman field are spin and momentum superpositions, and we
adiabatically load the atoms into the lowest energy dressed state. The
effective Hamiltonian of these neutral atoms is like that of charged particles
in a uniform magnetic vector potential, whose magnitude is set by the strength
and detuning of Raman coupling. The spin and momentum decomposition of the
dressed states reveals the strength of the effective vector potential, and our
measurements agree quantitatively with a simple single-particle model. While
the uniform effective vector potential described here corresponds to zero
magnetic field, our technique can be extended to non-uniform vector potentials,
giving non-zero effective magnetic fields.Comment: 5 pages, submitted to Physical Review Letter
Literacy practices of primary education children in Andalusia (Spain): a family-based perspective
Primary school children develop literacy practices in various domains and situations in everyday life.
This study focused on the analysis of literacy practices of children aged 8–12 years from the perspec-
tive of their families. 1,843 families participated in the non-experimental explanatory study. The
children in these families speak Spanish as a first language and are schooled in this language. The
instrument used was a self-report questionnaire about children’s home-literacy practices. The data
obtained were analysed using categorical principal components analysis (CATPCA) and analysis of
variance (ANOVA). The results show the complex relationship between literacy practices developed
by children in the domains of home and school and the limited development of a literacy-promoting
‘third space’. In conclusion, the families in our study had limited awareness of their role as literacy-
promoting agents and thought of literacy learning as restricted to formal or academic spaces
Rapid production of Rb BECs in a combined magnetic and optical potential
We describe an apparatus for quickly and simply producing \Rb87
Bose-Einstein condensates. It is based on a magnetic quadrupole trap and a red
detuned optical dipole trap. We collect atoms in a magneto-optical trap (MOT)
and then capture the atom in a magnetic quadrupole trap and force rf
evaporation. We then transfer the resulting cold, dense cloud into a spatially
mode-matched optical dipole trap by lowering the quadrupole field gradient to
below gravity. This technique combines the efficient capture of atoms from a
MOT into a magnetic trap with the rapid evaporation of optical dipole traps;
the approach is insensitive to the peak quadrupole gradient and the precise
trapping beam waist. Our system reliably produces a condensate with
atoms every 16\second
Quantitative localized proton-promoted dissolution kinetics of calcite using scanning electrochemical microscopy (SECM)
Scanning electrochemical microscopy (SECM) has been used to determine quantitatively the kinetics of proton-promoted dissolution of the calcite (101̅4) cleavage surface (from natural “Iceland Spar”) at the microscopic scale. By working under conditions where the probe size is much less than the characteristic dislocation spacing (as revealed from etching), it has been possible to measure kinetics mainly in regions of the surface which are free from dislocations, for the first time. To clearly reveal the locations of measurements, studies focused on cleaved “mirror” surfaces, where one of the two faces produced by cleavage was etched freely to reveal defects intersecting the surface, while the other (mirror) face was etched locally (and quantitatively) using SECM to generate high proton fluxes with a 25 μm diameter Pt disk ultramicroelectrode (UME) positioned at a defined (known) distance from a crystal surface. The etch pits formed at various etch times were measured using white light interferometry to ascertain pit dimensions. To determine quantitative dissolution kinetics, a moving boundary finite element model was formulated in which experimental time-dependent pit expansion data formed the input for simulations, from which solution and interfacial concentrations of key chemical species, and interfacial fluxes, could then be determined and visualized. This novel analysis allowed the rate constant for proton attack on calcite, and the order of the reaction with respect to the interfacial proton concentration, to be determined unambiguously. The process was found to be first order in terms of interfacial proton concentration with a rate constant k = 6.3 (± 1.3) × 10–4 m s–1. Significantly, this value is similar to previous macroscopic rate measurements of calcite dissolution which averaged over large areas and many dislocation sites, and where such sites provided a continuous source of steps for dissolution. Since the local measurements reported herein are mainly made in regions without dislocations, this study demonstrates that dislocations and steps that arise from such sites are not needed for fast proton-promoted calcite dissolution. Other sites, such as point defects, which are naturally abundant in calcite, are likely to be key reaction sites
Differential Cross Section for γ d →ωd Using CLAS at Jefferson Lab
The cross section for coherent ω-meson photoproduction off the deuteron has been measured for the first time as a function of the momentum transfer t=(Pγ−Pω)2 and photon energy Eγ using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are measured in the energy range 1.4 γ\u3c3.4 GeV. A model based on ω−N rescattering is consistent with the data at low and intermediate momentum transfer, |t|. For 2.8 γ\u3c3.4 GeV, the total cross-section of ω−N scattering, based on fits within the framework of the Vector Meson Dominance model, is in the range of 30–40 mb
First upper limit analysis and results from LIGO science data: stochastic background
I describe analysis of correlations in the outputs of the three LIGO
interferometers from LIGO's first science run, held over 17 days in August and
September of 2002, and the resulting upper limit set on a stochastic background
of gravitational waves. By searching for cross-correlations between the LIGO
detectors in Livingston, LA and Hanford, WA, we are able to set a 90%
confidence level upper limit of h_{100}^2 Omega_0 < 23 +/- 4.6.Comment: 7 pages; 1 eps figures; proceeding from 2003 Edoardo Amaldi Meeting
on Gravitational Wave
White noise reduction for wideband linear array signal processing
The performance of wideband array signal processing algorithms is dependent on the noise level in the system. A method is
proposed for reducing the level of white noise in wideband linear arrays via a judiciously designed spatial transformation followed
by a bank of highpass filters. A detailed analysis of the method and its effect on the spectrum of the signal and noise is presented.
The reduced noise level leads to a higher signal to noise ratio (SNR) for the system, which can have a significant beneficial
effect on the performance of various beamforming methods and other array signal processing applications such as direction of
arrival (DOA) estimation. Here we focus on the beamforming problem and study the improved performance of two well-known
beamformers, namely the reference signal based (RSB) and the linearly constrained minimum variance (LCMV) beamformers.
Both theoretical analysis and simulation results are provided
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