a b s t r a c tA new simulation method for solving fluid-structure coupling problems has been developed. All the basic equations are numerically solved on a fixed Cartesian grid using a finite difference scheme. A volume-of-fluid formulation [Hirt, Nichols, J. Comput. Phys. 39 (1981) 201], which has been widely used for multiphase flow simulations, is applied to describing the multi-component geometry. The temporal change in the solid deformation is described in the Eulerian frame by updating a left Cauchy-Green deformation tensor, which is used to express constitutive equations for nonlinear Mooney-Rivlin materials. In this paper, various verifications and validations of the present full Eulerian method, which solves the fluid and solid motions on a fixed grid, are demonstrated, and the numerical accuracy involved in the fluid-structure coupling problems is examined.

No abstract

Small amounts of surfactant can drastically change bubble behavior. For example, a bubble in aqueous surfactant solution rises much slower than one in purified water. This phenomenon is explained by the so-called Marangoni effect caused by a nonuniform concentration distribution of surfactant along the bubble surface. In other words, a tangential shear stress appears on the bubble surface due to the surface tension variation caused by the surface concentration distribution, which results in the reduction of the rising velocity of the bubble. More interestingly, this Marangoni effect influences not only the rising velocity, but also the lateral migration in the presence of mean shear. Furthermore, these phenomena influence the multiscale nature of bubbly flows and cause a drastic change in the bubbly flow structure. In this article, we review the recent studies related to these interesting behaviors of bubbles caused by the surfactant adsorption/desorption on the bubble surface.

In order to find out whether high intensity focused ultrasound (HIFU) might be useful against hepatocellular carcinoma, we analyzed the effect of a microbubble agent (Levovist) on the temperature rise and tissue necrosis induced by HIFU. Rabbits were given 7 ml Levovist (300 mg/ml) or saline intravenously. Up to six areas per rabbit liver were exposed to HIFU for 60 s (2.18 MHz, I(SPTA)=400 W/cm(2)). The volume of the tissue coagulated by HIFU was measured 10 min after the start of HIFU. HIFU-induced lesions were larger in the animals given Levovist: (mm(3), Levovist versus saline) 371+/-104 versus 166+/-71 (P<0.001). Temperatures in the animals given Levovist were also higher 60 s after the start of exposure: ( degrees C, Levovist versus saline) 20.3+/-3.5 versus 13.2+/-3.8 (P<0.001). The amount of damage differed greatly, but the pathological changes caused by HIFU with Levovist were the same as those caused by HIFU with saline. Hemorrhagic areas and implosion cysts were seen, and many cells had been disrupted or destroyed. Microbubble agents developed for diagnostic uses could also be used in anticancer therapy.

No abstract

No abstract

The two components of the force acting on a clean almost spherical bubble rising near a plane vertical wall in a quiescent liquid are determined experimentally. This is achieved by using an apparatus in which a CCD camera and a microscope follow the rising bubble. This apparatus allows us to measure accurately the bubble radius, rise speed and distance between the bubble and the wall. Thereby the drag and lift components of the hydrodynamic force are determined for Reynolds numbers Re (based on bubble diameter, rise velocity U, and kinematic viscosity ν) less than 40. The results show the existence of two different regimes, according to the value of the dimensionless separation L* defined as the ratio between the distance from the bubble centre to the wall and the viscous length scale ν/U. When L* is O(1) or more, experimental results corresponding to Reynolds numbers up to unity are found to be in good agreement with an analytical solution obtained in the Oseen approximation by adapting the calculation of Vasseur & Cox (1977) to the case of an inviscid bubble. When L* is o(1), higher-order effects not taken into account in previous analytical investigations become important and measurements show that the deformation of the bubble is significant when the viscosity of the surrounding liquid is large enough. In this regime, experimental results for the drag force and shape of the bubble are found to agree well with recent theoretical predictions obtained by Magnaudet, Takagi & Legendre (2002) but the measured lift force tends to exceed the prediction as the separation decreases.

In the medical ultrasound field, microbubbles have recently been the subject of much interest. Controlling actively the effect of the microbubbles, a novel therapeutic method has been investigated. In this paper, our works on high intensity focused ultrasound (HIFU) lithotripsy with cavitating microbubbles are reviewed and the cavitation detection method to optimize the HIFU intensity is investigated. In the HIFU lithotripsy, collapse of the cloud cavitation is used to fragment kidney stones. Cloud cavitation is potentially the most destructive form of cavitation. When the cloud cavitation is acoustically forced into a collapse, it has the potential to concentrate a very high pressure. For the control of the cloud cavitation collapse, a novel two-frequency wave (cavitation control [C-C] waveform) is designed; a high-frequency ultrasound pulse (1-4 MHz) to create the cloud cavitation and a low-frequency trailing pulse (500 kHz) following the high-frequency pulse to force the cloud into collapse. High-speed photography showed the cavitation collapse on the stone and the shock-wave emission from the cloud. In vitro erosion tests of model and natural stones were also conducted. In the case of model stones, the erosion rate of the C-C waveform showed a distinct advantage with the combined high- and low-frequency waves over either wave alone. For the optimization of the high-frequency ultrasound intensity, the subharmonic acoustic pressure was examined. The results showed relationship between the subharmonic pressure from cavitating bubbles induced by the high-frequency ultrasound and eroded volume of the model stones. Natural stones were eroded and most of the resulting fragments were less than 1 mm in diameter. The method has the potential to provide a novel lithotripsy system with small fragments and localized cavitating bubbles on a stone.