Direct Numerical Simulation on the Influence of Adjacent Cylinder/Ramp and Spanwise Spacing on the Transition of Flat Plate Flow

In order to address the issue of unclarified effect of adjacent roughness elements on plate transition. direct numerical simulation was employed to investigate the transition induced by the spanwise arrangement of adjacent roughness elements, comprising cylinders and ramps. These roughness elements were resolved utilizing the embedded boundary method, with spanwise intervals set at 2.5h and 54, respectively, where & denotes the height of the roughness elements. The study elucidated the effects of transient and time-averaged flow characteristics, coherent structures, and characteristic modes. Findings indicated that ramps with significant disturbances predominantly influenced the breakdown of hairpin vortices, the onset of transition, and the energy transferred from time-averaged flow to the fluctuating flow. With the ramp neglected. a decrease in spanwise spacing effectively promoted transition. The interplay between roughness elements of varying shapes could generate a more potent nonlinear coherent structure than that observed among elements of uniform shape. The intense disturbance caused by the ramp predominantly contributed to the wall-normal and spanwise fluctuating waves of the coherent structure, rather than to the streamwise fluctuating waves. The interaction between adjacent roughness elements of diverse shapes, facilitated by an optimal reduction in spanwise spacing, promoted the evolution of the nonlinear structure at the central position. This structure could be approximated by several linear modes across the full frequency spectrum. When the spanwise spacing was well-chosen, roughness elements with different shapes might induce low-frequency resonance of nonlinear structures in a region near the downstream, substantially enhancing their evolutionary dynamics.