Recent advances in the understanding of laminar-turbulent transition in rod bundles

Rod bundles are critical components in various engineering systems, including heat exchangers and nuclear reactor cores. This study employs high-resolution Direct Numerical Simulations (DNS) to investigate laminar-turbulent transition in axial flow through rod bundles, focusing on the effects of pitch-to-diameter ratio (P/D) and Reynolds number (Re) on transition onset. Using the GPU-accelerated spectral element code NekRS on the Summit supercomputer, the research examines exceptionally long computational domains to capture spatially developing transitions. Results demonstrate that laminar-turbulent transitions occur even at relatively large P/D ratios and very low Reynolds numbers, primarily driven by gap instability during the initial phases. Detailed analyses reveal the significant impact of these transitions on flow mixing, complemented by frequency analysis that further elucidates the underlying dynamics. These findings offer valuable insights into the fluid dynamics and mixing mechanisms within rod bundle geometries.