Kabarcıklı Türbülanslı Akışlarda Yüzey Aktif Maddesi ve Viskoelastisitenin Etkileri

Interface-resolved direct numerical simulations are performed to examine the sole and combined effects of soluble surfactant and viscoelasticity on the structure of a bubbly turbulent channel flow. The incompressible flow equations are solved fully coupled with the FENE-P viscoelastic model and the equations governing interfacial and bulk surfactant concentrations. The latter coupling is achieved through a non-linear equation of state which relates the surface tension to the surfactant concentration at the interface. The two-fluid Navier-Stokes equations are solved using a front-tracking method. First, the effects of soluble surfactant on the lateral migration of a bubble in a pressure-driven channel flow are examined. Extensive computations are performed to investigate the bubble dynamics for a wide range of parameters. It is found that surfactant dramatically changes the bubble dynamics. In the clean case, the bubble position depends on its deformability, characterized by the Eotvos () and the capillary () numbers. The spherical bubble moves towards the wall, while the deformable one migrates away from it. On the other hand, in the presence of the surfactant, even the spherical bubble moves away from the wall. It is also found that the contaminated bubble stays away from the wall for and while it migrates towards the wall for . Also, at high , the onset of path instability is observed for both the clean and the contaminated cases. However, adding surfactant to the system triggers the path instability earlier and amplifies the oscillations afterwards. Next, simulations are performed to examine the combined effects of soluble surfactant and viscoelasticity on the structure of a bubbly turbulent channel flow. It is found that, for the surfactant-free case, bubbles move toward the wall due to inertial lift force, resulting in formation of wall layers and a significant decrease in the flow rate. Conversely, a high-enough concentration of surfactant changes the direction of lateral migration of bubbles, i.e., the contaminated bubbles move toward the core region and spread out across the channel. When viscoelasticity is considered, viscoelastic stresses counteract the Marangoni stresses, promoting formation of bubbly wall-layers and consequently strong decrease in the flow rate. The formation of bubble wall-layers for combined case depends on the interplay of the inertial and elastic, and Marangoni forces. Further, effects of different types of surfactant are examined using the physical sorption kinetics. For Newtonian turbulent bubbly flow, effects of Triton X-100 and 1-Pentanol are examined. It is observed that the sorption kinetics highly affect the dynamics of bubbly flow. Minute amount of Triton X-100 is found to be sufficient to prevent the formation of bubble clusters. On the other hand, 100 times more of 1-Pentanol surfactant is not sufficient to prevent the formation of layers. For viscoelastic turbulent flow, it is found that, polymer drag reduction is completely lost for the surfactant-free case, and the addition of small amount of surfactant (Triton X-100) restores the polymer drag reduction for the viscoelastic turbulent bubbly flows. Lastly, the polymer drag reduction of turbulent poly-dispersed bubbly flow is examined in the presence of soluble surfactant. It is found that the drag increases for the clean viscoelastic poly-dispersed bubbly flow but it is lower than the corresponding Newtonian flow due the lateral migration of large bubbles to the center of channel. In the presence of surfactant, the drag reduction for viscoelastic poly-dispersed bubbly flow is revived.