Direct Numerical Simulations and Characterizations of Multiphase Flows with Complex Topology Changes in a Turbulent Channel*

The interface between different phases in multiphase flows often undergoes repeated topology changes due to the breakup and coalescence of fluid masses. With the increase of the volume fraction of one phase, the flow structures change from dispersed bubbles/drops to more complex irregular shapes, and the fluids are often highly interconnected. We place bubbles in a three-dimensional turbulent channel flow, with the average void fraction varies from 5% to 25%, and use a front tracking/finite volume numerical method for direct numerical simulations of the flows till they reach statistically steady states. The evolutions of various integral quantities, such as the average flow rate, wall-shear, and interface area are monitored and compared for these different cases.  The structures of the flows, at statistically steady state, are characterized and quantified with the distributions of some statistical descriptors, such as bubble size, interfacial area density, interface shape distribution and interface normal distribution. 3D two-point correlation functions and primary component analysis (PCA) are also employed for the quantification and reduced-order representation of the structures.