Numerical Simulation And Analysis Of Complex Multiphase Flows

Multiphase flow is a complex flow which involves complex interactions betweenphases such as the transient motion of interface, complex interfacial forces and unsteadycharacteristics. Among gas/vapour-liquid, cavitating flows and ventilated gas-liquid flowsare common and have been widely applied in hydraulic machinery, underwater launching,aeronautical and aerospace engineering, chemical engineering and so on.In this paper, a numerical simulation method is developed in the calculation ofcavitating flows and gas-liquid flows. Applicability of the method is investigated throughthe comparison between simulation results and experimental data.In addition, the cavitating flows around a disc and the gas-liquid flows in a bubblecolumn are simulated with this method, and the Lagrangian Coherent Structure (LCS)based on Finite-Time Lyapunov Exponent (FTLE) is utilized in the mechanism andstructure analysis. The main research achievements are as follows:1) The numerical method suitable for the calculation of cavitating flows andgas-liquid flows is developed.On the basis of RANS turbulence model, the method utilizes inhomogeneous modelwhich takes the interfacial forces and transfer between phases into account and couplesLevel Set interface tracking method. The turbulence is modeling by k-ε model and masstransfer is described in Kubota cavitation model.2) The applicability of Inhomogeneous model in cavitating flows and gas-liquidflows is investigated.The numerical method based on inhomogeneous model can be applied well to thecavitating and gas-liquid flows. For the cavitating flows, the periodical shedding of thelarge scale bubbles around the hydrofoil and the pure vapor region in the attached cavity areobserved. For the gas-liquid flows, the break and floating of the bubble above the plane arecaptured and the surface tension in the interface region is calculated accurately, which iscrucial to the prediction of unsteady characteristics.3) The cavitating flows around a disc and the gas-liquid flows in a bubble columnare analyzed, combining Finite-Time Lyapunov Exponent (FTLE) and LagrangianCoherent Structure (LCS) with traditional Eulerian methods such as Q criterion.FTLE is a scalar which measures the amount of stretching about the trajectory of neighboring points and LCS could be the boundaries between regions with differentdynamic behaviors. In the cavitating flows around a disc, the straight LCS can reflect thehindered motion of fluid caused by the disc, and compared to the Q criterion, the circularLCS can capture the vortex better. In the gas-liquid flows in a bubble column, the interfacebetween the forward and backward flows and the boundary of large scale vortex arecaptured by LCS clearly.In the cavitating flows around a disc, the vortex is intensified and the distribution isconcentrated with the development of the cavity. It is the periodic changes in the intensityof re-entrant flow that causes the periodic expansion and contraction of cavity and themotion of vortex. Three-dimensional effect is also observed around the support rod.In the gas-liquid flows in a bubble column, the process of gaseous flow injected intowater can be divided into two stages: the bubble hose formation and bubble hosefluctuation stage. In the first stage, the gas is injected into water, forming a bubble hosewhich is extending along vertical direction to the top wall. Meanwhile, the vortex structuresare formed in the liquid symmetrically. In the second stage, the bubble hose starts tooscillate periodically, and the vortexes move to the bottom of the column alternately.