The aim of the present study is to develop the method of determining the kinetic boundary condition (KBC) at a vapor-liquid interface in net evaporation/condensation. We proposed a novel method for determining the KBC by combining the numerical simulations of the mean field kinetic theory and the molecular gas dynamics. The method was evaluated on steady vapor flow between two liquid slabs at different temperatures. A uniform net mass flux in the vapor phase induced by net evaporation and condensation is obtained from the numerical simulation of the mean field kinetic theory for both vapor and liquid phases. The KBC was specified by using the uniform net mass flux, and the numerical simulation of the molecular gas dynamics was conducted for the vapor phase. Comparing the macroscopic variables in the vapor phase obtained from both numerical simulations, we can validate the KBC whether the appropriate solutions are obtained. Moreover, the evaporation and condensation coefficients were estimated uniquely. The results showed that the condensation and evaporation coefficients were identical and constant in net evaporation. On the other hand, in net condensation, the condensation coefficient increased with the collision molecular mass flux. We also presented the applicable limit of the KBC which is assumed to be the isotropic Gaussian distribution at the liquid temperature. From these results, the KBCs in net evaporation and condensation, which enable the exact macroscopic variables to be determined, were proposed. C 2014 AIP Publishing LLC. [http://dx

Abstract. In the case of extracorporeal shock wave lithotripsy (ESWL), a shock wave-bubble interaction inevitably occurs near the focusing point of stones, resulting in stone fragmentation and subsequent tissue damage. Because shock wave-bubble interactions are high-speed phenomena occurring in the tissue consisting of various media with different acoustic impedance values, numerical analysis is an effective method for elucidating the mechanism of these interactions. However, the mechanism has not been examined in detail because, at present, numerical simulations capable of incorporating the acoustic impedance of various tissues do not exist. Here, we show that the improved ghost fluid method (IGFM) can treat the shock wave-bubble interactions in various media. Nonspherical bubble collapse near a rigid or soft tissue boundary (stone, liver, gelatin, and fat) was analyzed. The reflection wave of an incident shock wave at a tissue boundary was the primary cause for the acceleration or deceleration of bubble collapse. The impulse that was obtained from the temporal evolution of pressure created by the bubble collapse increased the downward velocity of the boundary and caused subsequent boundary deformation. Results of this study showed that IGFM is a useful method for analyzing the shock wave-bubble interaction near various tissues with different acoustic impedance.Shock wave-bubble interaction near soft and rigid boundaries during lithotripsy 2

Using molecular dynamics simulations, the present study investigated the precise characteristics of the binary mixture of condensable gas (vapor) and non-condensable gas (NC gas) molecules creating kinetic boundary conditions (KBCs) at a gas-liquid interface in equilibrium. We counted the molecules utilizing the improved two-boundary method proposed in previous studies by Kobayashi et al.

Using molecular dynamics simulations, the present study investigates the precise characteristics of evaporating and reflecting monatomic molecules (argon) composing a kinetic boundary condition (KBC) in a vapor-liquid equilibria. We counted the evaporating and reflecting molecules utilizing two boundaries (vapor and liquid boundaries) proposed by the previous studies (Meland et al., in Phys Fluids 16:223-243, 2004, and Gu et al., in Fluid Phase Equilibria 297:77-89, 2010). In the present study, we improved the method using the two boundaries incorporating the concept of the spontaneously evaporating molecular mass flux. The present method allows us to count the evaporating and reflecting molecules easily, to investigate the detail motion of the evaporating and reflecting molecules, and also to evaluate the velocity distribution function of the KBC at the vapor-liquid interface, appropriately. From the results, we confirm that the evaporating and reflecting molecules in the normal direction to the interface have slightly faster and significantly slower average velocities than that of the Maxwell distribution at the liquid temperature, respectively. Also, the stall time of the reflecting molecules at the interphase that is the region in the vicinity of the vapor-liquid interface is much shorter than those of the evaporating molecules. Furthermore, we discuss our method for constructing the KBC that incorporates condensation and evaporation coefficients. Based on these results, we suggest that the proposed method

For the accurate description of heat and mass transfer through a vapor-liquid interface, the appropriate modeling of the interface during nonequilibrium phase change (net evaporation/condensation) is a crucial issue. The aim of this study is to propose a microscopic interfacial model which should be imposed at the interface as the kinetic boundary condition for the Boltzmann equation. In this study, we constructed the kinetic boundary condition for monoatomic molecules over a wide range of liquid temperature based on mean field kinetic theory, and we validated the accuracy of the constructed kinetic boundary condition by solving the boundary value problem of the Boltzmann equation. These results showed that we can impose the kinetic boundary condition at the interface by simply specifying liquid temperature and simulate the complex vapor-liquid two-phase flow induced by net evaporation/condensation. Furthermore, we applied the constructed kinetic boundary condition to the boundary condition for the fluid-dynamic-type equations. This application enables us to deal with a large spatio-temporal scale of the interfacial dynamics in the vapor-liquid two-phase system with net evaporation/condensation.

We investigated the evaporation and condensation coefficients of vapor, which represent evaporation and condensation rates of vapor molecules, under high gas pressure (high gas density) conditions in a system of a vapor/gas-liquid equilibrium state. The mixture gas is composed of condensable gas (vapor) and non-condensable gas (NC gas) molecules. We performed numerical simulations of vapor/gas–liquid equilibrium systems with the Enskog–Vlasov direct simulation Monte Carlo (EVDSMC) method. As a result of the simulations, we found that the evaporation and condensation fluxes decrease with increasing NC gas pressure, which leads to a decrease in the evaporation and condensation coefficients of vapor molecules. Especially, under extremely high gas pressure conditions, the values of these coefficients are close to zero, which means the vapor molecules cannot evaporate and condensate at the interface. Moreover, we found that the vapor molecules behave as NC gas molecules under high gas pressure conditions. We also discussed the reason why NC gas molecules interfere with evaporation and condensation of vapor molecules at the vapor/gas–liquid interface.

Diffuse optical tomography using near-infrared light in a wavelength range from 700 to 1000 nm has the potential to enable non-invasive diagnoses of thyroid cancers; some of which are difficult to detect by conventional methods such as ultrasound tomography. Diffuse optical tomography needs to be based on a physically accurate model of light propagation in the neck, because it reconstructs tomographic images of the optical properties in the human neck by inverse analysis. Our objective here was to investigate the effects of three factors on light propagation in the neck using the 2D time-dependent radiative transfer equation: (1) the presence of the trachea, (2) the refractive-index mismatch at the trachea-tissue interface, and (3) the effect of neck organs other than the trachea (spine, spinal cord, and blood vessels). There was a significant influence of reflection and refraction at the trachea-tissue interface on the light intensities in the region between the trachea and the front of the neck surface. Organs other than the trachea showed little effect on the light intensities measured at the front of the neck surface although these organs affected the light intensities locally. These results indicated the necessity of modeling the refractive-index mismatch at the trachea-tissue interface and the possibility of modeling other neck organs simply as a homogeneous medium when the source and detectors were far from large blood vessels.

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