Abstract The turbulent/non-turbulent interface (TNTI) in an adverse pressure gradient (APG, β = 1.45) turbulent boundary layer (TBL) is explored here by using direct numerical simulation (DNS) data; β is the Clauser pressure gradient parameter. For comparison, the DNS data for a zero pressure gradient (ZPG) TBL is included. The interface is extracted with an approach based on enstrophy criteria. Depending on the enstrophy, the outer boundary layer flow can be classified into the free stream, boundary layer wake, and intermittent flow regimes. The fractal dimension of the interface is obtained by using the box-counting algorithm, and was found to be constant over a long range of box sizes. The TNTI shows a monofractal behavior. The geometric complexity of a TNTI can be determined in terms of the genus, which is defined as the number of handles in a geometric object. We examine the volume and projection area of the genus of the TNTI to analyze the entrainment process. The geometric complexity of the APG TBL interface and the local entrainment are greater than those of the ZPG TBL, as is evident in the increases in the genus near the interface. The local entrainment velocity is dominantly affected by the viscous diffusion at the interface.

中文翻译：

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逆压力梯度湍流边界层中的湍流/非湍流界面

摘要 这里通过使用直接数值模拟 (DNS) 数据探索了逆压力梯度 (APG, β = 1.45) 湍流边界层 (TBL) 中的湍流/非湍流界面 (TNTI)；β 是克劳瑟压力梯度参数。为了进行比较，包括零压力梯度 (ZPG) TBL 的 DNS 数据。使用基于熵标准的方法提取界面。根据熵增的不同，外边界层流可分为自由流、边界层尾流和间歇流状态。界面的分形维数是通过使用盒子计数算法获得的，并且在很长的盒子尺寸范围内是恒定的。TNTI 表现出单分形行为。可以根据属确定 TNTI 的几何复杂度，它被定义为几何对象中的手柄数。我们检查了 TNTI 属的体积和投影面积，以分析夹带过程。APG TBL 界面的几何复杂性和局部夹带大于 ZPG TBL 的几何复杂性，这在界面附近的属增加中很明显。局部夹带速度主要受界面处的粘性扩散影响。