| The head-on collision of two equal-sized drops driven by a hyperbolic flow is investigated numerically. An axisymmetric volume of fluid (VOF) method is used to simulate the motion of a drop toward a symmetry plane where it interacts with, and possibly coalesces with, its mirror image. The van der Waals forces are included in the Navier-Stokes equations. Collisions resulting in bouncing and stable coalescence are simulated by manipulating the volume fraction' boundary conditions.; A new approach to calculate curvature using a level set function is introduced. A piecewise parabolic interface reconstruction algorithm for axisymmetric coordinates, in which curvature is evaluated conveniently in terms of the coefficients of the parabolic segments approximating the interface, is presented. These two methods, as well as a commonly used method which calculates curvature from a smoothed volume fraction field, are used in the study of a static drop problem. The results showed that the accuracy of curvature calculation significantly affects the amplitude of the spurious currents. Moreover, as the Ohnesorge number decreases, amplitude of the spurious currents increases.; Two new numerical methods have been developed to calculate the van der Waals forces. The body force method applies a force on the drops which is computed as the negative gradient of van der Waals interaction potential between the drops. The disjoining pressure method calculates the van der Waals forces in terms of a disjoining pressure in the film which depends only on the film thickness.; For coalescence collisions, the influence of van der Waals forces on the evolution and rupture of the film between the drops is examined. Comparisons of the results of the two methods showed that the van der Waals forces calculated from the two methods have qualitatively similar effects on coalescence. The disjoining pressure method is much less computationally intensive and captures the main features of the evolution of the film, but may not provide adequate accuracy under all conditions.; The dynamics of the bouncing and coalescence collisions are investigated. The effects of Reynolds number, Weber number and viscosity ratio on energy evolution, drop deformation, collision time, and film drainage are studied. |
