Prediction of strong shock structure using the bimodal distribution function

A modified Mott-Smith method for predicting the one-dimensional shock wave solution at very high Mach numbers is constructed by developing a system of fluid dynamic equations. The predicted shock solutions in a gas of Maxwell molecules, a hard sphere gas and in argon using the newly proposed formalism are compared with the experimental data, direct-simulation Monte Carlo (DSMC) solution and other solutions computed from some existing theories for Mach numbers M<50. In the limit of an infinitely large Mach number, the predicted shock profiles are also compared with the DSMC solution. The density, temperature and heat flux profiles calculated at different Mach numbers have been shown to have good agreement with the experimental and DSMC solutionsComment: 22 pages, 9 figures, Accepted for publication in Physical Review

The Inertial Cavitation Threshold in Soft Tissue Using a Dual-Frequency Driving Signal

High Intensity Focused Ultrasound (HIFU) is a non-invasive technology that can be applied for treatment of different diseases and ablation of tumours in different parts of the body. When high intensity ultrasound propagates through the medium bubbles can be formed, a phenomenon known as acoustic cavitation. There are two different regimes of acoustic cavitation: stable cavitation when a bubble just oscillates around an equilibrium state, and inertial cavitation which is accompanied by bubble collapse. These two different regimes can be used for different biomedical applications. However, in some cases it can also make the treatment less predictable. Therefore, fundamental understanding of these effects is very important. In the current study theoretical investigation of the bubble dynamics in viscoelastic medium is performed and inertial cavitation thresholds have been calculated. To describe the bubble dynamics, Gilmore-Akulichev-Zener model has been used, which is suitable for a large bubble oscillations and high ultrasound powers. The results showed that using the dual-frequency driving signal the threshold value of inertial cavitation can be significantly reduced compared to single-frequency signal mode. Large difference between frequencies in the dual-frequency signal leads to lower threshold values. Numerical simulations also showed the dependencies of the cavitation threshold on the bubble radius

Prediction of strong shock structure using the bimodal distribution function

A modified Mott-Smith method for predicting the one-dimensional shock wave solution at very high Mach numbers is constructed by developing a system of fluid dynamic equations. The predicted shock solutions in a gas of Maxwell molecules, a hard sphere gas and in argon using the newly proposed formalism are compared with the experimental data, direct-simulation Monte Carlo (DSMC) solution and other solutions computed from some existing theories for Mach numbers M<50. In the limit of an infinitely large Mach number, the predicted shock profiles are also compared with the DSMC solution. The density, temperature and heat flux profiles calculated at different Mach numbers have been shown to have good agreement with the experimental and DSMC solutionsComment: 22 pages, 9 figures, Accepted for publication in Physical Review

Effects of acoustic nonlinearity and blood flow cooling during HIFU treatment

International audienceThe present study is aimed at predicting liver tumor temperature during a high-intensity focused ultrasound (HIFU) thermal ablation in a patient-specific liver geometry. The model comprises the nonlinear Westervelt equation and bioheat equations in liver and blood vessel. The nonlinear hemodynamic equations are also taken into account with the convected cooling and acoustic streaming effects being taken into account. We found from this three-dimensional three-field coupling study that in large blood vessel both convective cooling and acoustic streaming can change the temperature near blood vessel. Acoustic streaming velocity magnitude can be 4-5 times larger than the blood vessel velocity. Nonlinear wave propagation effects lead to the enhanced heating in the focal area and help to ablate tumor close to the blood vessel wall

Simulation of nonlinear Westervelt equation for the investigation of acoustic streaming and nonlinear propagation effects

International audienceThis study investigates the influence of blood flow on temperature distribution during high-intensity focused ultrasound (HIFU) ablation of liver tumors. A three-dimensional acoustic-thermal-hydrodynamic coupling model is developed to compute the temperature field in the hepatic cancerous region. The model is based on the nonlinear Westervelt equation, bioheat equations for the perfused tissue and blood flow domains. The nonlinear Navier-Stokes equations are employed to describe the flow in large blood vessels. The effect of acoustic streaming is also taken into account in the present HIFU simulation study. A simulation of the Westervelt equation requires a prohibitively large amount of computer resources. Therefore a sixth-order accurate acoustic scheme in three-point stencil was developed for effectively solving the nonlinear wave equation. Results show that focused ultrasound beam with the peak intensity 2470 W/cm2 can induce acoustic streaming velocities up to 75 cm/s in the vessel with a diameter of 3 mm. The predicted temperature difference for the cases considered with and without acoustic streaming effect is 13.5 °C or 81% on the blood vessel wall for the vein. Tumor necrosis was studied in a region close to major vessels. The theoretical feasibility to safely necrotize the tumors close to major hepatic arteries and veins was shown