  Outflow boundaries play an important role in multiphase fluid dynamics
simulations that involve transition between liquid and vapor phases. These
flows are dominated by low Weber numbers and a sharp jump in pressure,
velocity, and temperature. Inadequate treatment of these jumps at the outlet
generates undesirable fluid disturbances that propagate upstream and lead to
instabilities within the computational domain. To mitigate these disturbances,
we introduce a forcing term that can be applied to incompressible Navier-Stokes
equations to enforce stability in the numerical solution. The forcing term acts
as a damping mechanism to control vortices that are generated by
droplet/bubbles in multiphase flows, and is designed to be a general
formulation that can be coupled with a fixed pressure outflow boundary
condition to simulate a variety of multiphase flow problems. We demonstrate its
applicability to simulate pool and flow boiling problems, where bubble-induced
vortices during evaporation and condensation present a challenge at the
outflow. Validation and verification cases are chosen to quantify accuracy and
stability of the proposed method in comparison to established benchmarks and
reference solutions, along with detailed performance analysis for
three-dimensional simulations on leadership supercomputing platforms.
Computational experiments are performed using Flash-X, which is a composable
open-source software instrument designed for multiscale fluid dynamics
simulations on heterogeneous architectures.
