RIPPLE: A Computer Program for Incompressible Flows with Free Surfaces. We present the RIPPLE computer program* for modeling transient, two-dimensional, incompressible fluid flows with surface tension on free surfaces of general topology. Finite difference solutions to the incompressible Navier-Stokes equations are obtained on an Eulerian, rectilinear mesh in Cartesian or cylindrical geometries. Free surfaces are represented with volume-of-fluid (VOF) data on the mesh. Surface tension is modeled as a volume force derived from the continuum surface force ( CSF) model. A two-step projection method is used for the incompressible fluid flow solutions, aided by an incomplete Cholesky conjugate gradient (ICCG) solution technique for the pressure Poisson equation (PPE). Momentum advection is estimated with the weakly monotonic, second order upwind method of van Leer. Flow obstacles and curved boundaries interior to the mesh are represented with a partial cell treatment. The improvements and enhancements of RIPPLE relative to its predecessor, NASA-VOF2D, have resulted in a versatile tool capable of modeling a wide range of applications, being especially suited for low-Bond number, low-Weber number, and low-Capillary number flows in which fluid accelerations are weak and fluid restoring forces (e.g., surface tensions) are strong. After a brief summary of the primary features of RIPPLE, we describe the model equations, the numerical method, and the structure of the computer program. Example calculations then illustrate the methodâ€™s properties, with instructions given on the use of the program

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