Abstract Direct numerical simulation is used to investigate turbulent flow separations generated by a forward-facing step exposed to an incoming fully-developed turbulent plane channel flow at Re τ = 180 . The step height is 25% of the inlet channel height. The results are analyzed in terms of the topology of the Reynolds stresses, transport of turbulence kinetic energy (TKE) and unsteadiness of separation bubbles using a broad range of post-processing techniques, including the Lumley triangle, quadrant analysis, auto-correlation, joint probability density function, proper orthogonal decomposition and linear stochastic estimation. The incoming turbulence intensity is elevated as the step is approached and peaks along the mean separating streamline over the step. The topology of the Reynolds stresses switches from two-component to axisymmetric along the mean separating streamline over the step. The production term of TKE peaks in the separated shear layer, while the turbulent diffusion term tends to transport TKE away from the shear layer. An expanded separation bubble upstream of the step is typically associated with a contracted separation bubble over the step. This pattern is captured by the first proper orthogonal decomposition (POD) mode. In the first POD mode, the streamwise and vertical fluctuating velocities switch signs abruptly near the mean separating streamline over the step. This is characterized by two pairs of counter-rotating vortices: one pair of counter-rotating vortices leaning over the step and another pair of opposite-signed counter-rotating vortices near the top surface of the step.

中文翻译：

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由前向台阶引起的湍流分离的直接数值模拟

摘要 直接数值模拟用于研究由暴露于 Re τ = 180 处的进入的完全发展的湍流平面通道流的前向台阶产生的湍流分离。台阶高度是入口通道高度的 25%。使用广泛的后处理技术（包括 Lumley 三角、象限分析、自相关、联合分析），根据雷诺应力的拓扑结构、湍流动能 (TKE) 传递和分离气泡的不稳定性对结果进行分析。概率密度函数、适当的正交分解和线性随机估计。进入的湍流强度随着台阶的接近而升高，并沿着台阶上的平均分离流线达到峰值。雷诺应力的拓扑结构沿着台阶上的平均分离流线从两分量切换为轴对称。TKE 的产生项在分离的剪切层中达到峰值，而湍流扩散项倾向于将 TKE 运离剪切层。台阶上游的膨胀分离气泡通常与台阶上方的收缩分离气泡相关联。这种模式由第一个适当的正交分解 (POD) 模式捕获。在第一个 POD 模式下，流向和垂直波动的速度在台阶上的平均分离流线附近突然切换符号。这以两对反向旋转涡流为特征：一对反向旋转涡流靠在台阶上，另一对符号相反的反向旋转涡流靠近台阶的顶面。