Study of presence of bubble cloud in liquids has been of interest to researchers because of its positive or negative effects in many industrial processes. This paper aims to understand the behavior of a homogeneous mixture of bubbles and liquid. Due to usual simplifications, the equations governing bubble cloud behavior do not offer a complete analysis of this phenomenon. In this research, the works of the other scientists are reviewed especially the studies carried out by Wang and Brennen. In this study, first, the code devised according to numerical algorithm of this research is validated by results of Wang and Brennen. Then, by taking into account the compressibility property at the bubbles boundary for homogeneous mixture of bubbles in liquid, the governing equations are derived. Next, these equations under different applied sound pressure levels to the cloud, and a variety of gas percentage in liquid are numerically solved by the devised code. The severe reduction in maximum growth of bubbles with compressibility considerations, and the occurrence of collapses under some conditions are of the obtained results. Also, when bubble presence ratio is low the behavior of bubble cloud does not relate to compressibility. Some other outcomes of this research are manners of bubbles collapse in outer and inner layers of bubble cloud, serious compressibility effects on intensity and time of collapses, and maximum bubbles growth.

In this paper, bubble formation process has been studied numerically by entering air into a submerged orifice placed in a cylinder. The SOLA-VOF method was used to simulate the bubble formation. In This code, complete form of Navier-stocks equations was predicted two dimension and using finite difference method. Also in this study, the effect of air flow rate on the bubble size and its formation time at very low rates was studied.

Large-eddy Simulation of a laminar separation bubble on a flat plate has been performed and compared with the data in the literature. Suitability of different subgrid-scale models has been examined for Simulation of transition. Comparison of various parameters and three-dimensional visualization of instantaneous flow fields indicate that standard Smagorinsky model, being too dissipative, is not suitable for this kind of problem and fails to properly resolve transition. With the application of low Reynolds number correction and a reduced model constant, a good agreement with the dynamic model is obtained at a lower computational cost. Of the three SGS models investigated, dynamic model gives the most physically accurate description of transition. The Simulations illustrate that the appearance of Λ-vortices, vortex stretching and break down of longitudinal streaks characterize the transition process. Low values of reverse flow make it clear that a convective instability is involved. It is concluded that the initial amplification of disturbances is due to Tollmien-Schlichting mechanism while the roll-up of the shear layer takes place due to Kelvin-Helmholtz instability. It is observed that the universal log-law profile is not reached by the velocity profiles even far downstream.

Three-dimensional Simulations are presented on the motion of a deformable bubble in a combined coquette and poiseuille flow at finite Reynolds number. The full Navier-Stokes equation are solved by a finite volume method on a regular, fixed and staggered grid. Interface is tracked by marker points on an irregular, triangle and moving grid. The effect of surface tension is included. The effects of capillary number and Reynolds number on the lateral migration of a bubble are studied. It is found that a bubble migration to an equilibrium position about halfway between the wall and the centerline regardless of its original position. Simulations also show that the bubble shape is strongly dependent on the capillary number. As the Reynolds number increases, the effect of viscosity decreases and the equilibrium position moves closer to the wall. The effect of the radius of a bubble is also examined. With increasing the radius of the bubble, the centroid of the bubble moves to the centerline. The bubbles are more deformed with negative pressure gradient and migrate to the centerline.

In this work, rising of a single bubble in a quiescent liquid under microgravity condition was simulated. The related unsteady incompressible full Navier-Stokes equations were solved using a conventional finite difference method with a structured staggered grid. The interface was tracked explicitly by connected marker points via hybrid front capturing and tracking method. One field approximation was used, while one set of governing equations was only solved in the entire domain and different phases treated as one fluid with variable physical properties. The interfacial effects are accounted for by adding appropriate source terms to the governing equations. The results show that the bubble moves in a straight path under microgravity condition, compared to the zigzag motion of bubbles in the presence of gravity. Also, in the absence of gravity and temperature gradients, the hydrodynamic effect can still cause the upward motion of the bubble. This phenomenon was explicitly shown in our results.

Due to the increasing applications, micro droplet generators have become an important research area. Researchers presented different methods to produce controlled microdroplets. In this article, spark bubble-induced droplet generation has been studied through an axisymmetric chamber. The process includes the growth and collapse of the bubble and the formation resultant droplet until the pinch-off time. Pressure contours and velocity vectors in the fluid surrounding the bubble are obtained by using a combination of boundary element and finite difference method, and the effect strength parameter, chamber depth and inclination angle of feeder canal on the behavior of bubble and droplet is assessed. The results showed that by increasing the strength parameter, the length of droplet and its volume increase and droplet pinch-off happens earlier. By increasing inclination angle of feeder canal the droplet length and its volume will increase. The maximum pinch-off time belongs to-15 degree. By increasing the chamber depth, the droplet length will increase. In addition, for-15 degree the pinch-off time increases and the droplet volume decreases, while for 0 and 15 degrees the pinch-off time will decrease and the droplet volume will increase. Besides, by increasing bubble-free surface distance the droplet size decreases.

In the current study, the non-Newtonian fluid over a stationary circular cylinder has been simulated using the lattice Boltzmann method. The regime of 2D and unsteady flow (Re=100) for different non-newotonian power-law indices (0. 4≤ n≤ 1. 8) and four different aspect ratios of 2, 4, 6 and 8 has been investigated. The power-law model was used for describing the non-Newtonian behavior of the fluid. The results have been compared with previous experimental and numerical results. The effects of the power– law index and aspect ratio on the vorticity contour, average drag coefficient, fluctuations of lift coefficient and time averaged of pressure coefficient have been studied in detail. The results show that the steady or unsteady fluid behavior depend on power-law index and aspect ratio. Generally, the average of drag coefficient deacreases by increasing aspect ratio at fixed power-law index. For all considered aspect ratio, except B=2, the average of drag coefficient increases when the fluid behavior changes from shear thining to shear-thickening.