scholarly journals Optical phonons polarized along the c axis of YBa2Cu3O6+x, for x = 0.5 → 0.95

1995 ◽  
Vol 73 (11-12) ◽  
pp. 663-675 ◽  
Author(s):  
C. C. Homes ◽  
T. Timusk ◽  
D. A. Bonn ◽  
R. Liang ◽  
W. N. Hardy
Keyword(s):  
High Frequency ◽  
Low Temperatures ◽  
Electronic Conductivity ◽  
Low Frequency ◽  
Phonon Spectra ◽  
Major Transformation ◽  
Low Frequency Mode ◽  
Phonon Structure ◽  
Oxygen Mode ◽  
Doping Range

The c-axis polarized phonon spectra of single crystals of YBa2Cu3O6+x, were measured for the doping range x = 0.5 → 0.95, between 10 and 300 K. The low background electronic conductivity, determined by Kramers–Kronig analysis of the reflectance, leads to a rich phonon structure. With decreased doping the five normally-active B1u modes broaden and the high-frequency apical-oxygen mode splits into two components. We associate the higher of these with the two-fold coordinated copper "sticks". The 155 cm−1 low-frequency mode, which involves the apical and the chain oxygens, splits into at least three components with decreasing doping. Some phonon anomalies that occur near Tc in the highly doped material occur well above Tc in the oxygen-reduced systems. An unusual broad phonon band develops in the normal state at ≈ 400 cm−1, which becomes more intense at low doping and low temperatures, borrowing oscillator strength from apical- and plane-oxygen modes resulting in a major transformation of the phonon spectrum below ≈150 K.

Oscillations in a Collapsed-Tube Analog of the Brachial Artery Under a Sphygmomanometer Cuff

1989 ◽  
Vol 111 (3) ◽  
pp. 185-191 ◽  
Author(s):  
C. D. Bertram ◽  
C. J. Raymond ◽  
K. S. A. Butcher
Keyword(s):  
High Frequency ◽  
Flow Characteristics ◽  
Low Frequency ◽  
Collapsible Tube ◽  
Tube Size ◽  
Low Frequency Oscillations ◽  
Central Segment ◽  
Experimental Parameters ◽  
Aqueous Flow ◽  
Low Frequency Mode

To determine whether self-excited oscillations in a Starling resistor are relevant to physiological situations, a collapsible tube conveying an aqueous flow was externally pressurized along only a central segment of its unsupported length. This was achieved by passing the tube through a shorter and wider collapsible sleeve which was mounted in Starling resistor fashion in a pressure chamber. The tube size and material, and all other experimental parameters, were as used in our previous Starling resistor studies. Both low- and high-frequency self-excited oscillations were observed, but the low-frequency oscillations were sensitive to the sleeve type and length relative to unsupported distance. Pressure-flow characteristics showed multiple oscillatory modes, which differed quantitatively from those observed in comparable Starling resistors. Slow variation of driving pressure gave differing behavior according to whether the pressure was rising or falling, in accord with the hysteresis noted on the characteristics and in the tube law. The results are discussed in terms of the various possible mechanisms of collapsible tube instability, and reasons are presented for the absence of the low-frequency mode under most physiological circumstances.

scholarly journals Investigation of Flow Around a Slender Body at High Angles of Attack

2019 ◽  
Vol 30 (1) ◽  
pp. 51-61
Author(s):  
Ibraheem AlQadi Ibraheem AlQadi
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
The Body ◽  
Slender Body ◽  
Frequency Mode ◽  
Dynamic Mode ◽  
High Frequency Mode ◽  
Wide Range ◽  
Low Frequency Mode ◽  
High Angles Of Attack

A numerical investigation of flow around a slender body at high angles of attack is presented. Large eddy simulation of the flow around an ogive-cylinder body at high angles of attack is carried out. Asymmetric vortex flow was observed at angles of attack of α = 55◦ and 65◦ . The results showed that the phenomenon is present in the absence of artificial geometrical or flow perturbation. Contrary to the accepted notion that flow asymmetry is due to a convective instability, the development of vortex asymmetry in the absence of perturbations indicates the existence of absolute instability. An investigation of the unsteady flow field was carried out using dynamic mode decomposition. The analysis identified two distinct unsteady modes; high-frequency mode and low-frequency mode. At angle of attack 45◦ the high-frequency mode is dominant in the frontal part of the body and the low-frequency mode is dominant at the rear part. At α = 55◦ , the highfrequency mode is dominant downstream of vortex breakdown. At α = 65◦ , the spectrum shows a wide range of modes. Reconstruction of the dynamical modes shows that the low-frequency mode is associated with the unsteady wake and the high-frequency mode is associated with unsteady shear layer.

A New Kind of Energy Transfer From the High-Frequency to Low-Frequency Mode in a Composite Laminated Plate

2010 ◽  
Author(s):  
Wei Zhang ◽  
Xiang-Ying Guo ◽  
Qian Wang ◽  
Cui-Cui Liu ◽  
Yun-cheng He
Keyword(s):  
Numerical Simulation ◽  
Natural Frequency ◽  
Thin Plate ◽  
High Frequency ◽  
Low Frequency ◽  
Frequency Mode ◽  
Rectangular Thin Plate ◽  
Chaotic Motions ◽  
Simply Supported ◽  
Low Frequency Mode

This paper focuses on the analysis on a new kind of nonlinear resonant motion with the low-frequency large-amplitude, which can be induced by the high-frequency small-amplitude mode through the mechanism of modulation of amplitude and phase. The system investigated is a simply supported symmetric cross-ply composite laminated rectangular thin plate subjected to parametric excitations. Experimental research has been carried out for the first time. The test plate was excited near the first natural frequency with parametric forces and the above mentioned high-to-low frequency mode has been observed, whose frequency is extremely lower than the first natural frequency. Theoretical job goes to analysis the above phenomenon accordingly. Based on the Reddy’s third-order shear deformation plate theory and the von Karman type equation, the nonlinear governing equations of the simply supported symmetric cross-ply composite laminated rectangular thin plate subjected to parametric excitations are formulated. The Galerkin method is utilized to discretize the governing partial differential equations into a two-degree-of-freedom nonlinear system. Numerical simulation is conducted to investigate this non-autonomous system subsequently. The results of numerical simulation demonstrate that there is a qualitative agreement between the experimental observation and the theoretical result. Besides, the multi-pulse chaotic motions are also reported in numerical simulations.

INFLUENCES OF SPRING CONSTANTS AND GENERATIONS ON THE PHONON SPECTRA OF FIBONACCI CHAIN

2008 ◽  
Vol 22 (32) ◽  
pp. 3227-3239
Author(s):  
XUBO HU ◽  
XIANGBO YANG
Keyword(s):  
High Frequency ◽  
Nearest Neighbor ◽  
Low Frequency ◽  
Critical Value ◽  
Spring Model ◽  
Phonon Spectra ◽  
Frame Work ◽  
A Chain ◽  
Spring Constants ◽  
Periodic Chain

In the frame work of the nearest-neighbor harmonic spring model, we calculate the phonon spectra by means of the transfer matrix method and investigate the influences of the spring constants, the periodicity, the degree of defect for a periodic chain and the length of a chain on the phonon spectra of Fibonacci chain. It is found that for a certain generation chain the range of the phonon spectra enlarges from the low- to high-frequency zone with the increment of the parameter λ, and when the structure of a chain is certain, the spectra range decreases and more and more phonon states with higher frequencies disappear with the increment of generation. Consequently, when the chain tends to be infinite, there only exist very few phonon states with low-frequency. On the other hand, for a certain Fibonacci chain, phonon states with high-frequency can be formed only when λ is larger than the corresponding critical value.

Some Rationale for the Unusual Behavior of the Dielectric Constant of Cu2O

1973 ◽  
Vol 51 (6) ◽  
pp. 680-685 ◽  
Author(s):  
F. L. Weichman
Keyword(s):  
Dielectric Constant ◽  
High Frequency ◽  
Low Temperatures ◽  
Low Frequency ◽  
Depletion Layer ◽  
Temperature Dependent ◽  
Unusual Behavior ◽  
Temperature Variations ◽  
Very Low Frequency ◽  
Semiconducting Material

The very low frequency dielectric constant in Cu2O varies over wide limits and is highly temperature dependent. At sufficiently low temperatures the dielectric constant for all samples reduces to the high frequency value of about 7.5. Small variations in the high frequency value are easily explained as being due to the known presence of copper crystals embedded in the semiconducting material. The low frequency and temperature variations will be related to the variation in the depletion layer formed around each copper inclusion. Calculations are based on highly oversimplified size, shape, and distribution of the copper inclusions.

A Raman study of aqueous DMF and DMA solutions at low temperatures

2004 ◽  
Vol 82 (10) ◽  
pp. 1468-1473 ◽  
Author(s):  
K Tomikawa ◽  
H Kanno ◽  
H Kimoto
Keyword(s):  
Aqueous Solution ◽  
High Frequency ◽  
Low Temperatures ◽  
Low Frequency ◽  
Frequency Component ◽  
Solute Concentration ◽  
Area Ratio ◽  
High Frequency Component ◽  
Frequency Components ◽  
Raman Study

Hydrogen bonding in aqueous N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) solutions was studied by Raman spectroscopy of the OH-stretching spectra of water as functions of solute concentration and temperature (from –50 to 50 °C). The Raman spectra were decomposed successfully into two components (the low- and high-frequency components). The peak area ratio of the low-frequency component to the high-frequency component increases exponentially with decreasing temperature. The frequency of the low-frequency component decreases linearly with lowering temperature down to –50 °C at all solute concentrations (solute mol fraction from 0.3 to 0.7). It is shown that the hydrogen bonds in the clathrate structures around DMA molecules are stronger than those around DMF molecules.Key words: Raman spectrum, DMF, DMA, aqueous solution, low temperature.

A new kind of energy transfer from high frequency mode to low frequency mode in a composite laminated plate

2013 ◽  
Vol 224 (12) ◽  
pp. 2937-2953 ◽  
Author(s):  
X. Y. Guo ◽  
W. Zhang ◽  
M. H. Zhao ◽  
Y. C. He
Keyword(s):  
Energy Transfer ◽  
High Frequency ◽  
Low Frequency ◽  
Frequency Mode ◽  
Laminated Plate ◽  
High Frequency Mode ◽  
Composite Laminated Plate ◽  
Low Frequency Mode

scholarly journals Self-sustained hydrodynamic oscillations in lifted jet diffusion flames: origin and control

2015 ◽  
Vol 775 ◽  
pp. 201-222 ◽  
Author(s):  
Ubaid Ali Qadri ◽  
Gary J. Chandler ◽  
Matthew P. Juniper
Keyword(s):  
Stability Analysis ◽  
High Frequency ◽  
Passive Control ◽  
Diffusion Flames ◽  
Low Frequency ◽  
Frequency Mode ◽  
High Frequency Mode ◽  
Jet Diffusion Flames ◽  
Local Stability Analysis ◽  
Low Frequency Mode

We use direct numerical simulation (DNS) of the Navier–Stokes equations in the low-Mach-number limit to investigate the hydrodynamic instability of a lifted jet diffusion flame. We obtain steady solutions for flames using a finite rate reaction chemistry, and perform a linear global stability analysis around these steady flames. We calculate the direct and adjoint global modes and use these to identify the regions of the flow that are responsible for causing oscillations in lifted jet diffusion flames, and to identify how passive control strategies might be used to control these oscillations. We also apply a local stability analysis to identify the instability mechanisms that are active. We find that two axisymmetric modes are responsible for the oscillations. The first is a high-frequency mode with wavemaker in the jet shear layer in the premixing zone. The second is a low-frequency mode with wavemaker in the outer part of the shear layer in the flame. We find that both of these modes are most sensitive to feedback involving perturbations to the density and axial momentum. Using the local stability analysis, we find that the high-frequency mode is caused by a resonant mode in the premixing region, and that the low-frequency mode is caused by a region of local absolute instability in the flame, not by the interaction between resonant modes, as proposed in Nichols et al. (Phys. Fluids, vol. 21, 2009, article 015110). Our linear analysis shows that passive control of the low-frequency mode may be feasible because regions up to three diameters away from the fuel jet are moderately sensitive to steady control forces.

scholarly journals Inertial regimes in a curved electromagnetically forced flow

2017 ◽  
Vol 813 ◽  
pp. 860-881 ◽  
Author(s):  
J. Boisson ◽  
R. Monchaux ◽  
S. Aumaître
Keyword(s):  
Magnetic Field ◽  
Secondary Flow ◽  
High Frequency ◽  
Characteristic Length ◽  
Low Frequency ◽  
Frequency Mode ◽  
Square Duct ◽  
High Frequency Mode ◽  
Duct Geometry ◽  
Low Frequency Mode

We investigated experimentally the flow driven by a Lorentz force induced by an axial magnetic field $\boldsymbol{B}$ and a radial electric current $I$ applied between two fixed concentric copper cylinders. The gap geometry corresponds to a rectangular section with an aspect ratio of $\unicode[STIX]{x1D702}=4$ and we probe the azimuthal and axial velocity profiles of the flow along the vertical axis by using ultrasonic Doppler velocimetry. We have performed several runs at moderate magnetic field strengths, corresponding to moderate Hartmann numbers $M\leqslant 300$. At these forcing parameters and because of the geometry of our experimental device, we show that the inertial terms are not negligible and an azimuthal velocity that depends on both $I$ and $B$ is induced. From measurements of the vertical velocity we focus on the characteristics of the secondary flow: the time-averaged velocity profiles are compatible with a secondary flow presenting two pairs of stable vortices, as pointed out by previous numerical studies. The flow exhibited a transition between two dynamical modes, a high- and a low-frequency one. The high-frequency mode, which emerges at low magnetic field forcing, corresponds to the propagation in the radial $r$-direction of tilted vortices. This mode is consistent with our previous experiments and with the instability described in Zhao et al. (Phys. Fluids, vol. 23 (8), 2011, 084103) taking place in an elongated duct geometry. The low-frequency mode, observed for high magnetic field forcing, consists of large excursions of the vortices. The dynamics of these modes matches the first axisymmetric instability described in Zhao & Zikanov (J. Fluid Mech., vol. 692, 2012, pp. 288–316) taking place in an square duct geometry. We demonstrated that this transition is controlled by the inertial magnetic thickness $H^{\prime }$ which is the characteristic length we introduce as a balance between the advection and the Lorentz force. The key point here is that when the inertial magnetic thickness $H^{\prime }$ is comparable to one geometric characteristic length ($H/2$ in the vertical or $\unicode[STIX]{x0394}r$ in the radial direction) the corresponding mode is favoured. Therefore, when $H^{\prime }/(H/2)\approx 1$ we observe the high-frequency mode taking place in an elongated duct geometry, and when $H^{\prime }/\unicode[STIX]{x0394}r\approx 1$ we observe the low-frequency mode taking place in square duct geometry and high magnetic field.

THE LOW-FREQUENCY DIELECTRIC PROPERTIES OF ETHYLENE OXIDE AND ETHYLENE OXIDE HYDRATE

1963 ◽  
Vol 41 (6) ◽  
pp. 1424-1434 ◽  
Author(s):  
D. W. Davidson ◽  
G. J. Wilson
Keyword(s):  
Ethylene Oxide ◽  
High Frequency ◽  
Low Temperatures ◽  
Low Frequency ◽  
Dielectric Constants ◽  
Water Molecules ◽  
Kcal Mole ◽  
Absorption Region ◽  
Experimental Values ◽  
Static Dielectric Constants

The static dielectric constant of liquid ethylene oxide has been measured between 158 and 286 °K. The hydrate of ethylene oxide exhibits a dispersion–absorption region characterized by static dielectric constants about one-third as large as those of ice and by relatively large "high-frequency" dielectric constants (ε1 = 7.5 at 0 °C). This region may be approximately described as a circular arc locus, but may be represented somewhat better by a superposition of two (or three) semicircular dispersions. In either case, the activation energy for the relaxation of water molecules, to which this region is ascribed, is ca. 6.7 kcal/mole, except at low temperatures, where it becomes smaller. Experimental values of ε1 agree roughly with those calculated for comparatively rapid orientation of ethylene oxide molecules in the cavities of the hydrate. Such orientation may account for absorption maxima observed at 11 Mc/sec and above 100 Mc/sec at 90 °K.

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