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141.
Class IV Flextensional Transducers (FTs), which are the most popular among the various FTs used for underwater sound projection, are inherently omni-directional and therefore their possible application areas are limited. In this work, we investigate the possibility of getting directional performance from a 3 kHz Class IV FT, using a reflector. Finite element modeling (FEM), with the help of a commercially available package ATILA, has been made use of. Results indicate that a parabolic reflector with a air–water interface as the reflecting surface, would give an excellent directivity of 60° (3 dB) with a front-to-back ratio better than 30 dB. Experimental results, making use of a sound-reflecting coating as the reflector has shown a directivity of 82°, with a front-to-back ratio of 21 dB. It is also observed that the reflector causes the FTs to vibrate in asymmetrical modes, the long-term implications of which needs to be studied to ensure the reliability of the device. 相似文献
142.
We present an experimental study of the motion of a viscoelastic micellar fluid around a moving cylinder, which ranges from fluidlike flow to solidlike tearing and fracture, depending on the cylinder radius and velocity. The observation of crack propagation driven by the cylinder indicates an extremely low tear strength, approximately equal to the steady state surface tension of the fluid. At the highest speeds a driven crack is observed in front of the cylinder, propagating with a fluctuating speed equal on average to the cylinder speed, here as low as 5% of the elastic wave speed. 相似文献
143.
Abolore A. Musari Daniel P. Joubert Joseph A. Olowofela Adio T. Akinwale Gboyega A. Adebayo 《The European Physical Journal B - Condensed Matter and Complex Systems》2017,90(12):254
Pyrites (FeS2) are solid minerals that are found abundantly in Nigeria and are easy to prepare in laboratories. In this work, FeS2 is studied extensively in its pure state as well as when iron is substitutionally doped with zinc and calcium at concentrations of 0, 0.25, 0.5, 0.75 and 1. Using density functional theory, the eectronic, dynamic and thermodynamic properties were calculated. The results revealed that the lattice parameters and bulk modulus increases with increasing concentration and the obtained values are in agreement with available experimental and theoretical values. Though pyrite, when doped with zinc, obeys Vegard’s law, doping with calcium revealed pronounced deviation from this law. The calculated band structures showed that FeS2 has an indirect band gap whose size decreases after introducing zinc while doping with calcium increases the band gap. The phonon dispersion of the end members FeS2 and ZnS2 indicate that the systems are dynamically stable while CaS2 is dynamically unstate. Also, the thermodynamic properties of the pure and doped pyrites were calculated and the ranges of temperature at which the lattice and electronic degrees of freedom contribute to the specific heat capacity are presented. 相似文献
144.
Millions of miles of pipes are being used for the transportation, distribution, and local use of petroleum products, gas, water, and chemicals. Most of the pipes are buried in soil, leading to the significance of the study on the subject of guided wave propagation in pipes with soil influence. Previous investigations of ultrasonic guided wave propagation in an elastic hollow cylinder and in an elastic hollow cylinder coated with a viscoelastic material have led to the development of inspection techniques for bare and coated pipes. However, the lack of investigation on guided wave propagation in hollow cylinders embedded in infinite media like soil has hindered the development of pipe inspection methods. Therefore the influence of infinite media on wave propagation is explored in this paper. Dispersion curves and wave structures of both axisymmetric and nonaxisymmetric wave modes are developed. Due to the importance of the convergence of numerical calculations, the requirements of thickness and element number of the finite soil layer between hollow cylinder and infinite element layer are discussed, and an optimal combination is obtained in this paper. Wave structures are used for the mode identification in the non-monotonic region caused by the viscoelastic properties of coating and infinite media. 相似文献
145.
Joseph P. Sattler George J. Simonis 《International Journal of Infrared and Millimeter Waves》1984,5(4):465-473
We report the refractive indices and absorption coefficients for four beryllia (BeO) ceramic samples. These dielectric properties have been measured over the range from 4 to 18 cm–1 by use of a Michelson interferometer. The index n, follows the linear relationship n=0.6517 cm3/g×+0.7130 with density, , over the range 2.8<<3.0 g/cm3. 相似文献
146.
Ralph Weissleder David D. Stark Guillermo Elizondo Peter F. Hahn Carolyn Compton Sanjay Saini Jack Wittenberg Joseph T. Ferrucci 《Magnetic resonance imaging》1988,6(6):675-681
Thirteen patients with biopsy proven hepatic lymphoma (2 Hodgkin, 11 Non-Hodgkin) and a control group of 15 patients with hepatic metastases were analyzed quantitatively and qualitatively by MRI. Focal hepatic lymphoma was most reliably detected (eight of eight patients) and appeared hypointense relative to liver on T1 weighted (CNR − 7.4 ± 2.3) and hyperintense on T2 weighted (CNR + 8.4 ± 2.9) images. The mean T1 and T2 relaxation times of focal hepatic lymphoma (T1 = 832 ± 234 msec, T2 = 84 ± 16 ms) differed significantly from adjacent non-tumorous liver (T1 = 420 ± 121 ms, T2 = 51 ± 9 ms; p < 0.05), however CNR values and relaxation times were similar to those of hepatic metastases. Diffuse hepatic lymphoma (microscopic periportal infiltration) was undetectable by MRI in three patients by either morphologic features or quantitative criteria. A mixed pattern of hepatic lymphoma (focal lesions and diffuse infiltration) showed focal areas of slightly decreased signal intensity on T1 weighted images (CNR = −1.7 ± 0.4) while T2 weighted images revealed multiple regions of focal hyperintensity (CNR = +13.3 ± 8.4) superimposed on a diffusely hyperintense liver. Our experience demonstrates that either T1 or T2 weighted techniques are useful in detecting focal and that T2 weighted techniques are useful in detecting mixed hepatic lymphoma. Conventional image derived relaxation time measurements and quantitative parameters were of no additional diagnostic value. 相似文献
147.
Joseph Zaks 《Geometriae Dedicata》1997,67(2):223-232
Extending earlier results in the plane, we prove that every n-polygon in sufficiently general position in d-dimensional projective space, n d + 2, gives rise to a derived n-polygon, which preserves a few functions; these functions are the cyclial product of (actually affine) ratios of various points, obtained by proper projections on suitable lines. 相似文献
148.
Richard Bellman Joseph Bentsman Semyon M. Meerkov 《Journal of Mathematical Analysis and Applications》1983,91(1):152-191
A nonlinear system with S-shape steady state characteristic is referred to as a system with Arrhenius dynamics. The negative slope part of the S-shape curve represents a set of unstable steady states. Using two examples of Arrhenius systems (catalytic reactor and continuous stirred tank reactor), it is shown that introduction of sufficiently fast oscillations in the parameters of the system generates a new Arrhenius system, the steady state characteristic of which has a smaller negative slope part. Results of analytical investigation as well as numerical simulation are presented. It is shown that vibrational stabilization of Arrhenius systems gives an increase in productivity of the plants. 相似文献
149.
Joseph Akeyo Omolo 《Pramana》2008,71(6):1311-1320
The quantum dynamics of a two-mode non-resonant parametric down-conversion process is studied by recasting the time evolution
equations for the basic operators in an equivalent spin equation form with simpler exact solutions for a pump field with harmonic
time dependence. Expectation values of suitable operators for studying important features such as squeezing and quantum revivals
are presented in simple forms.
相似文献
150.
Froncisz W Camenisch TG Ratke JJ Anderson JR Subczynski WK Strangeway RA Sidabras JW Hyde JS 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2008,193(2):297-304
A reference arm W-band (94 GHz) microwave bridge with two sample-irradiation arms for saturation recovery (SR) EPR and ELDOR experiments is described. Frequencies in each arm are derived from 2 GHz synthesizers that have a common time-base and are translated to 94 GHz in steps of 33 and 59 GHz. Intended applications are to nitroxide radical spin labels and spin probes in the liquid phase. An enabling technology is the use of a W-band loop-gap resonator (LGR) [J.W. Sidabras, R.R. Mett, W. Froncisz, T.G. Camenisch, J.R. Anderson, J.S. Hyde, Multipurpose EPR loop-gap resonator and cylindrical TE011 cavity for aqueous samples at 94 GHz, Rev. Sci. Instrum. 78 (2007) 034701]. The high efficiency parameter (8.2 GW−1/2 with sample) permits the saturating pump pulse level to be just 5 mW or less. Applications of SR EPR and ELDOR to the hydrophilic spin labels 3-carbamoyl-2,2,5,5-tetra-methyl-3-pyrroline-1-yloxyl (CTPO) and 2,2,6,6,-tetramethyl-4-piperidone-1-oxyl (TEMPONE) are described in detail. In the SR ELDOR experiment, nitrogen nuclear relaxation as well as Heisenberg exchange transfer saturation from pumped to observed hyperfine transitions. SR ELDOR was found to be an essential method for measurements of saturation transfer rates for small molecules such as TEMPONE. Free induction decay (FID) signals for small nitroxides at W-band are also reported. Results are compared with multifrequency measurements of T1e previously reported for these molecules in the range of 2–35 GHz [J.S. Hyde, J.-J. Yin, W.K. Subczynski, T.G. Camenisch, J.J. Ratke, W. Froncisz, Spin label EPR T1 values using saturation recovery from 2 to 35 GHz. J. Phys. Chem. B 108 (2004) 9524–9529]. The values of T1e decrease at 94 GHz relative to values at 35 GHz. 相似文献