High idler conversion with an intracavity optical parametric oscillator and difference frequency generation

Tunable mid IR generation is analyzed with an optical parametric oscillator (OSO) and difference frequency generation (DFG) based on two periodically poled gratings. To obtain chirped-pulse amplification techniques, in the difference frequency generation the group-velocity must be matched. If grating periods are selected, appropriately, the wider tunable range of output pulse can be obtained. Because the group velocity is matched, the parametric bandwidth and the maximum effective length will be enhanced. We theoretically demonstrate high idler conversion in a femtosecond optical parametric oscillators with two different and cascaded periodically poled gratings. In the first grating the signal and the idler 1 are generated, while in the second one, the midwave infrared idler 2 is generated. The dependence of the lengths of the two cavities on the conversion efficiency is discussed.     

Sound speed in bubble film of ship wakes

As is distinct from general gas-liquid two-phase flow, a large number of bubbles with different diameters belong to ship wakes. Feasibility of Laplace equation, used to calculate wake sound speed (WSS), is confirmed based on differential postulation. Defect for calculating the adiabatic sound speed of gases in references is showed, and a concept of WSS is proposed clearly. A minimum WSS of 24.5 m/s is got when bubble ratio reads 0.5 according to the calculation when bubble dimension is less than that of resonance. Also a weak dependence of WSS on pressures is predicted. WSS from calculation corresponds with the experimental data of references well in high frequency domain, when the actual scale of bubbles is greater than the resonant scale. Supported by the National Natural Science Foundation of China (Grant No.10274046) and Pre-study Fund of Military Equipment (Grant No. 51448030101ZK1801)     

Analytic solutions of self-similar pulse based on Ginzburg-Landau equation with constant coefficients

Based on the constant coefficients of Ginzburg-Landau equation that considers the influence of the doped fiber retarded time on the evolution of self-similar pulse, the parabolic asymptotic self-similar solutions were obtained by the symmetry reduction algorithm. The parabolic asymptotic amplitude function, phase function, strict linear chirp function and the effective temporal pulse width of self-similar pulse are given in this paper. And these theoretical results are consistent with the numerical simulations. Supported by the Natural Science Foundation of Guangdong Province of China (Grant No. 04010397)     

Optimization of constrained composite absorbers using simulated annealing

Sound reverberation is an important problem in some industrial environments. As indicated by the Occupational Safety and Health Act of 1970, noise is responsible for the psychological and physiological ills of workers. Therefore reduction of reverberation becomes essential. For maintenance and other reasons, the thickness of sound absorbers used for reverberation control may be constrained. Consequently there is interest in minimizing noise using sound absorbers with constrained thickness. Optimization of a composite absorber using a simulated annealing algorithm is presented. Simulated annealing is a stochastic relaxation technique based on analogy with the physical process of annealing metal. The algorithm requires a mathematical model for the acoustical properties of the absorber. Before optimization, the accuracy of the mathematical model was checked against experimental data. A program for optimizing in respect of broad band noise at a specified receiver has been created and run. Results prove that SA optimization provides a quick and efficient approach in designing constrained thickness composite sound absorbers.     

EDGE REYNOLDS STRESS STRUCTURE OF LOWER HYBRID WAVE INJECTION IN THE HL—1M PLASMA

The radial profiles of electrostatic Reynolds stress,plasma poloidal rotations,radial and poloidal electric fields have been measured in the plasma boundary region on the HL-1M tokamak using a multi-array of Mach/Langmuir probes.During experiments of lower hybrid wave current drive,the variations in LHW drive power xill cause changes in the edge electric field,poloidal rotation veloity and Reynolds stress.The results indicate that sheared poloidal flow can be generated in the edge plasma due to radially varied Reynolds stress.     

Effects of uneven moisture distribution on the strength of and wave velocity in concrete

Earlier findings showed that the effects of moisture (liquid or free water) in hardened concrete on its behavior, especially the lesser known effects ofuneven moisture distribution, can (a) be significant, and (b) vary from property to property. This distribution, for instance whether or not the surface layer is drier than the overall average moisture content, can be characterized by the difference between the velocity of the longitudinal wave (pulse velocity) measured in the standard through-thickness manner, and the velocity of the longitudinal wave propagating on the concrete surface. The summary of earlier findings on the effects of moisture distribution is followed by a recent investigation on pulse velocity in the special case, occurring frequently in practice, when the distribution is uneven because the liquid is concentrated in cracks in the concrete.     

A new approach for the estimation of the axial velocity using ultrasound

The most used estimation method for calculating the blood velocity in commercial scanners is the autocorrelation approach. The calculation of the mean velocity used in this method depends on the center frequency of the interacting ultrasound pulse which downshifts as a function of depth, introducing a bias. A new velocity estimator for the mean axial velocity is presented. The estimation principle is based on the 2D Fourier transform and the Radon transform. The input data are a sequence of RF data forming a 2D data input, one column for each pulse emission. A 2D segment is selected for a specific depth. This data segment is first transformed by a 2D Fourier transform, and the result is then transformed by a Radon transform. The center of gravity for the angles of the lines intersecting the origin of the R-theta coordinate system in the Radon domain gives the mean axial velocity for the data segment. The benefit of this method is an estimate of the mean axial velocity which is independent of the center frequency of the propagating ultrasound pulse. The estimate will only depend on f(s) and f(pr f). Results of the estimation method is presented based on both simple generated RF harmonic data for different signal/noise ratios and simulated acoustic RF responses from a 3D measurement situation with an array transducer and a tube with plug flow. The new method shows improvement with a factor of 1.5-4 on the standard deviation on the estimated mean velocity for the simulated case.     

Extended frequency range measurements for determining the Kneser-type acoustic relaxation time

In the present paper, the authors discuss studies carried out for many years dealing particularly with two compounds: benzene and carbon disulphide and compare them with the results obtained by numerous acoustics researchers. These compounds are typical liquids in which acoustic Kneser-type relaxation occurs, caused by an irreversible vibrational and translational (VT) transition. Since magnitudes describing the relaxation process were diverse in many papers, we have undertaken an attempt to clarify these differences and to indicate how to avoid errors resulting from instrumental imperfections and the disregard of the considerable measurement errors when investigating velocity dispersion in the hypersonic (GHz) range. The results of these researches changed the interpretation of previous papers.     

Resonance behaviour of viscoelastic fluid in Poiseuille flow in the presence of a transversal magnetic field

The oscillatory flow of a viscoelastic fluid in a circular pipe under the influence of a transversal magnetic field is studied. Exact solutions for the axial velocity and flow rate are presented. The velocity enhancement and the resonance behaviour are analysed both numerically and asymptotically in the case of small pipe radii. Approximations for the resonance frequencies and the achievable velocity enhancements are derived. Copyright © 2005 John Wiley & Sons, Ltd.