Direct numerical simulation of turbulence-interface interactions

In this thesis, the interactions between a deformable interface and turbu- lence have been investigated using Direct Numerical Simulations (DNS). The interface and the surfactant concentration are tracked using a Phase Field Method (PFM). The turbulence-interface interactions have been anal- ysed in two different flow configurations, a dispersed and a stratified flow. First, a dispersed flow is considered, a swarm of large deformable drops is re- leased in a turbulent channel flow. The coalescence and breakup rates have been characterised for different values of the surface tension and viscosity ratios. Results show that the drop size, determined by the equilibrium be- tween coalescence and breakup, is influenced either by the surface tension, either by the internal viscosity. In particular, for small values of the surface tension values, the internal viscosity enhances the stability of the interface and prevent drop breakup. Second, a viscosity stratified configuration is considered. This setup mimics a core annular flow; a low viscosity fluid is interposed between the core and the walls to decrease the pressure drop. Results show that the interface is able to damp the near-wall turbulence, an increase of the core flow rate is observed. For the range of viscosity ratios analysed, the turbulence-interface interactions play a key role for obtaining Drag Reduction (DR). The DR performance is slighty affected by the viscosity ratio.

In this thesis, the interactions between a deformable interface and turbu- lence have been investigated using Direct Numerical Simulations (DNS). The interface and the surfactant concentration are tracked using a Phase Field Method (PFM). The turbulence-interface interactions have been anal- ysed in two different flow configurations, a dispersed and a stratified flow. First, a dispersed flow is considered, a swarm of large deformable drops is re- leased in a turbulent channel flow. The coalescence and breakup rates have been characterised for different values of the surface tension and viscosity ratios. Results show that the drop size, determined by the equilibrium be- tween coalescence and breakup, is influenced either by the surface tension, either by the internal viscosity. In particular, for small values of the surface tension values, the internal viscosity enhances the stability of the interface and prevent drop breakup. Second, a viscosity stratified configuration is considered. This setup mimics a core annular flow; a low viscosity fluid is interposed between the core and the walls to decrease the pressure drop. Results show that the interface is able to damp the near-wall turbulence, an increase of the core flow rate is observed. For the range of viscosity ratios analysed, the turbulence-interface interactions play a key role for obtaining Drag Reduction (DR). The DR performance is slighty affected by the viscosity ratio.