Cavitation Scaling Experiments With Headforms: Bubble Acoustics

Recently Ceccio and Brennen [1][2][3] have examined the interaction between individual traveling cavitation bubbles and the structure of the boundary layer and flow field in which the bubble is growing and collapsing. They were able to show that individual bubbles are often fissioned by the fluid shear and that this process can significantly effect the acoustic signal produced by the collapse. Furthermore they were able to demonstrate a relationship between the number of cavitation events and the nuclei number distribution measured by holographic methods in the upstream flow. Kumar and Brennen [4][5] have further examined the statistical properties of the acoustical signals from individual cavitation bubbles on two different headforms in order to learn more about the bubble/flow interactions. All of these experiments were, however, conducted in the same facility with the same size of headform (5.08cm in diameter) and over a fairly narrow range of flow velocities (around 9m/s). Clearly this raises the issue of how the phenomena identified change with speed, scale and facility. The present paper will describe further results from experiments conducted in order to try to answer some of these important questions regarding the scaling of the cavitation phenomena. These experiments (see also Kuhn de Chizelle et al. [6][7]) were conducted in the Large Cavitation Channel of the David Taylor Research Center in Memphis Tennessee, on similar Schiebe headforms which are 5.08, 25.4 and 50.8cm in diameter for speeds ranging up to 15m/s and for a range of cavitation numbers

Effect of dipolar interactions on optical nonlinearity of two-dimensional nanocomposites

In this work, we calculate the contribution of dipole-dipole interactions to the optical nonlinearity of the two-dimensional random ensemble of nanoparticles that possess a set of exciton levels, for example, quantum dots. The analytical expressions for the contributions in the cases of TM and TE-polarized light waves propagating along the plane are obtained. It is shown that the optical nonlinearity, caused by the dipole-dipole interactions in the planar ensemble of the nanoparticles, is several times smaller than the similar nonlinearity of the bulk nanocomposite. This type of optical nonlinearity is expected to be observed at timescales much larger than the quantum dot exciton rise time. The proposed method may be applied to various types of the nanocomposite shapes.Comment: 8 page

TEM investigation of YBa2Cu3O7 thin films on SrTiO3 bicrystals

YBa2Cu3O7 films in c-axis orientation on bicrystalline SrTiO3 substrates are investigated by TEM. The films and the substrates are examined in cross-section and in plane view. The grain boundary of the bicrystal substrate contains (110) faceted voids, but is otherwise straight on a nanometer scale. Contrary to this, the film grain boundary is not straight grain boundary can be up to 100 nm for a 100 nm thick film. The deviation from the intended position of the YBCO grain boundary can already occur at the film/substrate interface where it can be as much as ±50 nm

Pulsed THz radiation due to phonon-polariton effect in [110] ZnTe crystal

Pulsed terahertz (THz) radiation, generated through optical rectification (OR) by exciting [110] ZnTe crystal with ultrafast optical pulses, typically consists of only a few cycles of electromagnetic field oscillations with a duration about a couple of picoseconds. However, it is possible, under appropriate conditions, to generate a long damped oscillation tail (LDOT) following the main cycles. The LDOT can last tens of picoseconds and its Fourier transform shows a higher and narrower frequency peak than that of the main pulse. We have demonstrated that the generation of the LDOT depends on both the duration of the optical pulse and its central wavelength. Furthermore, we have also performed theoretical calculations based upon the OR effect coupled with the phonon-polariton mode of ZnTe and obtained theoretical THz waveforms in good agreement with our experimental observation.Comment: 9 pages, 5 figure