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-气相变的的多相流体动力学模拟中起着重要作用。这类流动以低韦伯数和压力、速度、温度的尖锐跳跃为特征。在出口边界处对这些跳跃处理不当会产生有害的流体扰动，这些扰动向上游传播并导致计算域内失稳。为减轻这些扰动，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，以强制数值解保持稳定性。该强迫项作为阻尼机制控制多相流中由液滴/气泡产生的涡旋，其设计为通用形式，可与固定压力出流边界条件耦合，模拟多种多相流问题。我们展示了该方法在池沸腾和流动沸腾问题模拟中的适用性——这类问题中蒸发和冷凝过程的气泡诱导涡旋对出流边界构成挑战。验证和确认案例被选用于量化该方法相较于既有基准和参考解的精度与稳定性，同时针对领导级超级计算平台上的三维模拟进行了详细的性能分析。计算实验采用Flash-X进行，这是一个专为异构架构上的多尺度流体动力学模拟设计的可组合开源软件工具。