A direct numerical simulation has been performed to study instantaneous behavior in lead-bismuth eutectic flows past a vertical, backward-facing step. A turbulent forced convection case and two cases of mixed convection, the first buoyancy-aided flow at a Richardson number Ri of 0.1 and the second buoyancy-opposed flow at $$Ri=0.02$$ , are simulated and discussed. The Reynolds number based on the bulk velocity and step height is 4805. A uniform heat flux is imposed on the expansion wall behind the step. In the forced convection case, the numerical results reveal two characteristic unsteady flow phenomena. The first is vortex-shedding motion along the separating shear layer, while the second is wall-normal flapping of the shear layer. These unsteady motions have significant influences on the thermal field. The vortex-shedding motion induces some streak-like low-temperature structures on the heated wall, while the flapping motion induces oscillation of the maximum temperature on the wall. In the mixed convection cases, buoyancy alters the flow field substantially. The two unsteady flow phenomena noted above constitute motions inherent to backward-facing step flow. Buoyancy plays a material role in vortex development, affecting vortex ranges and time-scales. While the vortex shedding frequency is insensitive to buoyancy, the frequency of the flapping motion increases with the buoyancy. These results contribute to an improved understanding of separating and reattaching flows, especially in association with buoyancy and temperature fluctuations. The data serve to aid future development and validation of improved heat-flux modeling of low-Prandtl-number fluids.

中文翻译：

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液态金属湍流强制和混合对流中瞬时行为的 DNS

已经进行了直接数值模拟来研究铅铋共晶流通过垂直的、面向后的台阶的瞬时行为。模拟和讨论了一种湍流强制对流情况和两种混合对流情况，Richardson 数 Ri 为 0.1 的第一个浮力辅助流和 $$Ri=0.02$$ 的第二个浮力反对流。基于体积速度和台阶高度的雷诺数为 4805。均匀的热通量施加在台阶后面的膨胀壁上。在强制对流情况下，数值结果揭示了两种特征性的非定常流动现象。第一个是沿着分离剪切层的涡旋脱落运动，而第二个是剪切层的壁法向拍动。这些非定常运动对热场有显着影响。涡旋脱落运动在受热壁上引起一些条纹状低温结构，而拍打运动引起壁上最高温度的振荡。在混合对流情况下，浮力会显着改变流场。上面提到的两种不稳定流动现象构成了向后阶梯流动所固有的运动。浮力在涡流发展中起着重要作用，影响涡流范围和时间尺度。虽然涡旋脱落频率对浮力不敏感，但扑动的频率随着浮力的增加而增加。这些结果有助于更好地理解分离和重新连接流动，特别是与浮力和温度波动相关的流动。