Numerical Simulation on Ship Bubbly Wake

Based on a volume of fluid two-phase model imbedded in the general computational fluid dynamics code FLUENT6.3.26, the viscous flow with free surface around a model-scaled KRISO container ship （KCS） was first numerically simulated. Then with a rigid-lid-free-surface method, the underwater flow field was computed based on the mixture muitiphase model to simulate the bubbly wake around the KCS hull. The realizable k-e two-equation turbulence model and Reynolds stress model were used to analyze the effects of turbulence model on the ship bubbly wake. The air entrainment model, which is relative to the normal velocity gradient of the free surface, and the solving method were verified by the qualitatively reasonable computed results.

Numerical investigation of turbulent bubbly wakes created by the ventilated partial cavity

This paper presents a numerical study on the turbulent bubbly wakes created by the ventilated partial cavity.A semi-empirical approach is introduced to model the discrete interface of the ventilated cavity and its complex gas leakage rate induced by the local turbulent shear stress.Based on the Eulerian-Eulerian two-fluid modeling framework,a population balance approach based on MUltiple-SIze-Group (MUSIG) model is incorporated to simulate the size evolution of the sheared off microbubbles and its complex interactions with the two-phase flow structure in the wake region.Numerical predictions at various axial locations downstream of the test body were in satisfactory agreement with the experimental measurements.The captured bubbly wake structure illustrates that the bubbles may disperse as a twin-vortex tube driven by gravity effect.The predicted Sauter mean bubble diameter has confirmed the dominance of the coleascense process in the axial direction.As the bubbles develop downstream,the coleascense and breakup rate gradually reach balance,resulting in the stable bubble diameter.A close examination of the flow structures,gas void fraction distributions and the bubble size evolution provides valuable insights into the complex physical phenomenon induced by ventilated cavity.

Numerical simulation of trajectory and deformation of bubble in tip vortex

According to the behaviors of a bubble in the ship wake flow, the numericai simulation is divided into two stages, quasi-spherical motion and non-spherical motion, based on whether the bubble is captured by the vortex or not. The one-way coupled particle tracking method （PTM） and the boundary element method （BEM） are adopted to simulate these two stages, respectively. Meanwhile, the initial condition of the second stage is taken as the output of the first one, and the entire simulation is connected and completed. Based on the numerical results and the published experimental data, the cavitation inception is studied, and the wake bubble is tracked. Besides, the split of the bubble captured by the vortex and the following sub-bubbles are simulated, including motion, deformation, and collapse. The results provide some insights into the control on wake bubbles and optimization of the wake flow.

Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase（CEP） breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional（2D） particle-in-cell（PIC） simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.