881 research outputs found
Self-synchronizing Schlieren photography and interferometry for the visualization of unsteady transonic flows
The use of self synchronizing stroboscopic Schlieren and laser interferometer systems to obtain quantitative space time measurements of distinguished flow surfaces, steakline patterns, and the density field of two dimensional flows that exhibit a periodic content was investigated. A large field single path stroboscopic Schlieren system was designed, constructed and successfully applied to visualize four periodic flows: near wake behind an oscillating airfoil; edge tone sound generation; 2-D planar wall jet; and axisymmetric pulsed sonic jet. This visualization technique provides an effective means of studying quasi-periodic flows in real time. The image on the viewing screen is a spatial signal average of the coherent periodic motion rather than a single realization, the high speed motion of a quasi-periodic flow can be reconstructed by recording photographs of the flow at different fixed time delays in one cycle. The preliminary design and construction of a self synchronizing stroboscopic laser interferometer with a modified Mach-Zehnder optical system is also reported
High-temperature phase transitions in SrBi_2Ta_2O_9 film: a study by THz spectroscopy
Time-domain THz transmission experiment was performed on a film deposited on sapphire substrate. Temperatures between 300
and 923 K were investigated and complex permittivity spectra of the film were
determined. The lowest frequency optic phonon near 28 cm reveals a slow
monotonic decrease in frequency on heating with no significant anomaly near the
phase transitions. We show that the dielectric anomaly near the ferroelectric
phase transition can be explained by slowing down of a relaxational mode,
observed in the THz spectra. A second harmonic generation signal observed in a
single crystal confirms a loss of center of symmetry in the ferroelectric phase
and a presence of polar clusters in the intermediate ferroelastic phase.Comment: subm. to J. Phys.: Condens. Matte
Experimental Investigation of Loop Caused Influences on Parallel Flow-Induced Vibration of Fuel Pins
Communication Subsystems for Emerging Wireless Technologies
The paper describes a multi-disciplinary design of modern communication systems. The design starts with the analysis of a system in order to define requirements on its individual components. The design exploits proper models of communication channels to adapt the systems to expected transmission conditions. Input filtering of signals both in the frequency domain and in the spatial domain is ensured by a properly designed antenna. Further signal processing (amplification and further filtering) is done by electronics circuits. Finally, signal processing techniques are applied to yield information about current properties of frequency spectrum and to distribute the transmission over free subcarrier channels
Associative Behavior and Affinity for Anthropogenic Habitats in Two Relocated Timber Rattlesnakes
Associative Behavior and Affinity for Anthropogenic Habitats in Two Relocated Timber Rattlesnakes
Associative Behavior and Affinity for Anthropogenic Habitats in Two Relocated Timber Rattlesnakes
Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3
Terahertz and far-infrared electric and magnetic responses of hexagonal
piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic
resonance is observed in the spectra of magnetic permeability mu_a [H(omega)
oriented within the hexagonal plane] below the Neel temperature T_N. This
excitation softens from 41 to 32 cm-1 on heating and finally disappears above
T_N. An additional weak and heavily-damped excitation is seen in the spectra of
complex dielectric permittivity epsilon_c within the same frequency range. This
excitation contributes to the dielectric spectra in both antiferromagnetic and
paramagnetic phases. Its oscillator strength significantly increases on heating
towards room temperature thus providing evidence of piezomagnetic or
higher-order couplings to polar phonons. Other heavily-damped dielectric
excitations are detected near 100 cm-1 in the paramagnetic phase in both
epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior.
These excitations appearing in the frequency range of magnon branches well
below polar phonons could remind electromagnons; however, their temperature
dependence is quite different. We have used density functional theory for
calculating phonon dispersion branches in the whole Brillouin zone. A detailed
analysis of these results and of previously published magnon dispersion
branches brought us to the conclusion that the observed absorption bands stem
from phonon-phonon and phonon- paramagnon differential absorption processes.
The latter is enabled by a strong short-range in-plane spin correlations in the
paramagnetic phase.Comment: subm. to PR
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