Abstract Boundary layer transition over an isolated surface roughness element is investigated by means of numerical simulation. Large Eddy Simulation (LES) flow-modeling approach is employed to study flow characteristics and transition phenomenon past a roughness element immersed within an incoming developing boundary layer, at a height-based Reynolds number of 1170. LES numerical results are compared to experimental data from literature showing the time-averaged velocity distribution, the velocity fluctuation statistics and the instantaneous flow topology. Despite slight difference in the intensity of streamwise velocity fluctuations, the present LES results and experimental data show very good agreement. The mean flow visualization shows streamwise counter-rotating vortices pairs formation downstream of the obstacle. The primary pair induces an upwash motion and a momentum deficit that creates a Kelvin-Helmholtz type flow instability. The instantaneous flow topology reveals the formation of coherent K-H vortices downstream that produce turbulent fluctuations in the wake of the roughness element. These vortices are streched and lifted up when moving downstream. The velocity fluctuations results show that the onset of the turbulence is dominated by the energy transfer of large-scale vortices.

中文翻译：

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孤立斜坡型微粗糙度单元边界层过渡的大涡模拟

摘要 通过数值模拟研究了孤立表面粗糙度单元上的边界层过渡。大涡模拟 (LES) 流动建模方法用于研究流过浸入进入的发展边界层中的粗糙度元素的流动特性和过渡现象，基于高度的雷诺数为 1170。 LES 数值结果与实验数据进行比较显示时间平均速度分布、速度波动统计和瞬时流动拓扑的文献。尽管流向速度波动的强度略有不同，但目前的 LES 结果和实验数据显示出非常好的一致性。平均流可视化显示了障碍物下游的流向反向旋转涡旋对的形成。初级对引起上流运动和动量赤字，从而产生开尔文-亥姆霍兹型流动不稳定性。瞬时流动拓扑揭示了下游相干 KH 涡流的形成，这些涡流在粗糙度元素的尾流中产生湍流波动。当向下游移动时，这些涡流被拉伸并被提升。速度波动结果表明，湍流的开始主要是由大尺度涡流的能量传递决定的。