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.

翻译：出流边界在涉及液-气相态转变的多相流体动力学模拟中扮演重要角色。此类流动以低韦伯数以及压力、速度和温度的尖锐跳跃为特征。若在出口处对这些跳跃处理不当，将产生不利的流体扰动，并在计算域内向上游传播引发不稳定性。为抑制这些扰动，我们引入一个可应用于不可压缩纳维-斯托克斯方程的强迫项，以强制数值解的稳定性。该强迫项作为阻尼机制，用于控制多相流中液滴/气泡产生的涡旋，其设计为通用公式，可与固定压力出流边界条件耦合，从而模拟各类多相流问题。我们展示了其在模拟池沸腾和流动沸腾问题中的适用性——在蒸发与冷凝过程中，气泡诱导的涡旋对出流边界构成挑战。通过选取验证与确认算例，对比所提方法与既有基准及参考解的精度与稳定性，并在前沿超算平台上进行三维模拟的详细性能分析。计算实验基于Flash-X完成，该软件是一款可组合的开源工具，专为异构架构上的多尺度流体动力学模拟而设计。