Jaehoon Han

Secondary breakup of axisymmetric liquid drops. I. Acceleration by a constant body force

The secondary breakup of liquid drops, accelerated by a constant body force, is examined for small density differences between the drops and the surrounding fluid. Two cases are examined in detail: a density ratio close to unity (ρd/ρo=1.15, where the Boussinesq approximation is valid) and a density ratio of ten. A finite difference/front tracking numerical technique is used to solve the unsteady Navier–Stokes equations for both the drops and the surrounding fluid. The breakup is controlled by the Eotvos number (Eo), the Ohnesorge number (Oh), and the viscosity and density ratios. If viscous effects are small (small Oh), the Eotvos number is the main controlling parameter. In the Boussinesq limit, as Eo increases the drops break up in a backward facing bag, transient breakup, and a forward facing bag mode. At a density ratio of ten, similar breakup modes are observed, with the exception that the forward facing bag mode is replaced by a shear breakup mode. Similar breakup modes have been seen experimentall...

Secondary breakup of axisymmetric liquid drops. II. Impulsive acceleration

The secondary breakup of impulsively accelerated liquid drops is examined for small density differences between the drops and the ambient fluid. Two cases are examined in detail: a density ratio close to unity and a density ratio of 10. A finite difference/front tracking numerical technique is used to solve the unsteady axisymmetric Navier–Stokes equations for both the drops and the ambient fluid. The breakup is governed by the Weber number, the Reynolds number, the viscosity ratio, and the density ratio. The results show that Weber number effects are dominant. In the higher density ratio case, ρd/ρo=10, different breakup modes—oscillatory deformation, backward-facing bag mode, and forward-facing bag mode—are seen as the Weber number increases. The forward-facing bag mode observed at high Weber numbers is an essentially inviscid phenomenon, as confirmed by comparisons with inviscid flow simulations. At the lower density ratio, ρd/ρo=1.15, the backward-facing bag mode is absent. The deformation rate also b...

Direct numerical simulations of fluid flow, heat transfer and phase changes

Direct numerical simulations of fluid flow, heat transfer, and phase changes are presented. The simulations are made possible by a recently developed finite difference/front tracking method based on the “one-field” formulation of the governing equations where a single set of conservation equations is written for all the phases involved. The conservation equations are solved on a fixed rectangular grid, but the phase boundaries are kept sharp by tracking them explicitly by a moving grid of lower dimension. The method is discussed and applications to boiling heat transfer and the solidification of drops colliding with a wall are shown.

Direct numerical simulations in material processing

A unified approach for direct simulations of fluid flow, heat transfer, and phase changes is discussed. The method is based on writing one set of conservation equation for all phases involved, allowing arbitrary changes in material properties and adding singular terms at phase boundary to ensure that the correct boundary conditions are incorporated. This approach allows the conservation equations to ge solved on a fixed grid in a very efficient way. By explicit tracking of the phase boundary by a lower dimensional moving grid, the method is capable of producing accurate solutions for complex phase boundaries. Examples of simulations of the solidification of pure materials, binary alloys, and drops impinging on a solid surface are shown.