No abstract

Eckart-type acoustic streaming induced in confined sound beams from a piston source is examined in water theoretically and experimentally. Axisymmetric flow equations with a spatially distributed driving force in the beams are based on the continuity equation and the Navier–Stokes equation in a viscous, incompressible fluid. They are solved numerically by the stream-function vorticity method [T. Kamakura et al., J. Acoust. Soc. Am. 97, 2740–2746 (1995)]. Experiments are conducted using a 5-MHz planar transducer with a 9.5-mm radius aperture. All measurements of the streaming velocities are carried out by a laser Doppler velocimeter and are compared with the numerical computations including the enhancement of the force due to finite-amplitude sound distortion. These measurements agree well with the theoretical prediction. It is noted that diffraction of sound beams plays an important role in the generation of streaming, particularly in the early stage. Consistency between experiments and computations suggests that both acoustic and hydrodynamic nonlinearities should be taken into account in the present observation system.

Axisymmetric flow equations for a viscous incompressible fluid are transformed into the vorticity transport and Poisson’s equations. They are numerically solved via a finite difference method imposing appropriate initial and boundary conditions. A model source of 1-cm radius and 5-cm focal length with Gaussian amplitude distribution radiates 5-MHz ultrasound beams in water. Numerical examples are shown for buildup of acoustic streaming along and across the acoustic axis. Evidently, hydrodynamic nonlinearity has an essential effect on the streaming generation in comparison with a linear flow case; the nonlinearity reduces the streaming velocity in the focal and prefocal region, whereas it tends to accelerate the flow in the postfocal region.

AlN layers are homoepitaxially grown on (0001) AlN substrates. The surfaces are atomically smooth, and the X-ray diffraction rocking curves for the symmetric and asymmetric planes indicate narrow line widths in the range of 10-30 arcsec. The oxygen, silicon, and carbon concentrations are below the detection limits of secondary ion mass spectroscopy. Due to these superior structural properties and low impurity concentrations, sharp free and donor-bound excitons dominate the photoluminescence spectra at low temperatures, while free excitons dominate at elevated temperatures. #

Peculiar magnetic and electrical properties of new Heusler compounds Ru 2−x Fe x CrSi are reported. It was found that Ru 2−x Fe x CrSi for 0.3ഛ x ഛ 1.8 exhibits ferromagnetic ordering. The saturation magnetization and the Curie temperature T C both varies almost linearly with x for 0.3ഛ x ഛ 1.4, whereas, for x ജ 1.4 T C remains constant at approximately 500 K. On the other hand, the Ru-rich sample with x ഛ 0.2 exhibits an antiferromagnetic transition, below which a spin-glass-like behavior was observed. Meanwhile, the magnitude and the temperature dependence of the electrical resistivity vary systematically with x. In the temperature dependence, a metallic behavior for x Ͼ 0.7 and a semiconduncting behavior for x ഛ 0.7 were observed; i.e., a metalsemiconductor crossover was found in these compounds. These results are compared with a band structure calculation that has proposed that this series of materials show high spin polarization.