Abstract The coolant flow behavior inside fuel bundles influences the subchannel mass and momentum exchange and the cladding temperature, which should be considered when designing fuel assemblies. In this paper, three numerical methodologies of RANS (Reynolds-Averaged Navier-Stokes), URANS (Unsteady Reynolds-Averaged Navier-Stokes) and LES (Large Eddy Simulation) were adopted to investigate the turbulent flow characteristics of water inside a 3 × 3 bare bundle at atmospheric pressure. Emphasis was focused on clarifying the features related to the flow through rod bundles, such as the distribution of mean flow, Reynolds stresses and the characteristics of large-scale flow pulsation. It was found that the prediction of RANS simulation deviates relatively large from the experimental data, on the other hand, the predictions given by URANS and LES are comparable and agree well with the experimental measurements. The time-averaged velocity, Reynolds stresses and turbulent kinetic energy were plotted along various lines of interest. The quasi-periodic large-scale flow pulsation was identified by URANS and LES, but the pulsation period and oscillation magnitude deviate slightly. Finally, power spectrum density was conducted for URANS and LES to study the axial and lateral velocity variations in the frequency domain.

中文翻译：

---

3×3 棒束中水的湍流——来自 RANS、URANS 和 LES 的数值结果

摘要 燃料束内的冷却剂流动行为会影响子通道质量和动量交换以及包壳温度，这是设计燃料组件时应考虑的因素。本文采用RANS（Reynolds-Averaged Navier-Stokes）、URANS（Unsteady Reynolds-Averaged Navier-Stokes）和LES（Large Eddy Simulation）三种数值方法研究了3×3空间内水的湍流特性。大气压下的裸束。重点阐明了与杆束流相关的特征，如平均流的分布、雷诺应力和大尺度流动脉动的特征。发现RANS模拟的预测与实验数据偏差较大，另一方面，URANS 和 LES 给出的预测具有可比性，并且与实验测量结果非常吻合。时间平均速度、雷诺应力和湍流动能沿各种感兴趣的线绘制。URANS和LES识别出准周期性大尺度流动脉动，但脉动周期和振荡幅度略有偏差。最后，对 URANS 和 LES 进行功率谱密度以研究频域中的轴向和横向速度变化。