Characterization of parallel flow through rod bundles is of key importance in assessing the performance and safety of several engineering systems, including a majority of nuclear reactor concepts. Inhomogeneities in the bundle cross-section can present complex flow phenomena, including varying local conditions of turbulence. With the ever-increasing capabilities of high-performance computing, Direct Numerical Simulation (DNS) of turbulent flows is becoming more feasible. Through resolving all scales of turbulence, DNS can serve as a “numerical experiment,” and can provide substantial insight into flow physics, but at considerable computational cost. Thus to date, the DNS in open literature for rod bundle flows is relatively scarce, and largely limited to unit-cell domains. Since wall effects are important in rod bundle flows, a multiple-pin DNS study can expand understanding of rod bundle flows while providing valuable reference data for evaluating reduced-resolution techniques. In this work, DNS of a 5x5 square bare rod bundle representative of typical light water reactor fuel dimensions was performed using the spectral element code Nek5000. Turbulent microscales based on an advanced Reynolds-Averaged Navier–Stokes model were used to establish the required DNS resolution. Velocity and Reynolds stress fields are analyzed in detail, and invariant analysis is used for further investigation into flow physics. The results show stark changes in the structure of turbulence in the edge gaps, suggesting the presence of gap vortices in these regions. In addition, turbulent kinetic energy budgets are presented to more fully illustrate the various turbulent processes. These data can prove useful for rigorous evaluation of lower-fidelity turbulence modeling approaches.

通过棒束的平行流的表征对于评估包括大多数核反应堆概念在内的多个工程系统的性能和安全性至关重要。束横截面的不均匀性会呈现复杂的流动现象，包括湍流的不同局部条件。随着高性能计算能力的不断增强，湍流的直接数值模拟 (DNS) 变得越来越可行。通过解析所有尺度的湍流，DNS 可以用作“数值实验”，并且可以提供对流动物理的深入了解，但需要相当大的计算成本。因此，迄今为止，关于杆束流的公开文献中的 DNS 相对稀缺，并且主要限于单元域。由于壁效应在棒束流中很重要，多针 DNS 研究可以扩展对杆束流的理解，同时为评估降低分辨率技术提供有价值的参考数据。在这项工作中，代表典型轻水反应堆燃料尺寸的 5x5 方形裸棒束的 DNS 使用光谱元素代码 Nek5000 进行。基于先进的雷诺平均纳维-斯托克斯模型的湍流微尺度用于建立所需的 DNS 解析。详细分析了速度和雷诺应力场，并使用不变分析来进一步研究流动物理。结果显示边缘间隙中的湍流结构发生了明显变化，表明这些区域存在间隙涡流。此外，还提供了湍流动能预算以更全面地说明各种湍流过程。