Role of flow reversals in transition to turbulence and relaminarization of pulsatile flows
Role of flow reversals in transition to turbulence and relaminarization of pulsatile flows
复制标题
DOI：
10.1017/jfm.2021.269
复制
发表时间：
2021-04-26
影响因子：
3.7
通讯作者：
Andreopoulos, Yiannis
中科院分区：
文献类型：
--
作者：
Gomez, Joan;Yu, Huidan;Andreopoulos, Yiannis
脉动流中的不稳定性和向湍流的转变及其演化，涉及反向流和非定常流分离，是本实验工作的主要焦点。由可编程直流伺服电机驱动的活塞用于设置水流系统并提供脉动特性。使用连续波激光器和高帧率数码相机在折射率匹配装置中获取时间分辨粒子图像测速数据。活塞的位置由激光接近传感器连续记录。进行了五次不同的实验，雷诺数范围为 535-4825，沃默斯利数范围为 11.91 至 23.82。通过结合涉及数据低通和高通滤波的趋势消除方法，并使用经验模态分解和相关的希尔伯特-黄变换来确定固有模态函数，解决了数据的非平稳性。后一种方法更适合非线性和非平稳情况，涉及经典傅立叶分解的传统分析不能直接适用。研究发现，向湍流的转变是覆盖整个近壁区域的自发事件。瞬时涡度剖面显示出大范围的环状附壁涡旋层（WVL）的发展，其具有比叠加的脉动频率更高频率的较小涡旋，这表明剪切层开尔文-亥姆霍兹（K-H）类型的不稳定性。弯曲不稳定性导致流动分离并形成叠加 K-H 涡流的主要卷起结构。这种结构在方位角方向上分解为具有涡流内容的较小湍流斑块，这似乎是每个研究的雷诺数 (Re) 下流动的主要结构内容。 Re 值较高时，涡流的强度和范围较大，并且在流动的自由流区域中出现显着的扰动，这是过渡 Re 值下管流的典型特征。湍流似乎是在所附着的 WVL 内最大或最小涡度的位置、分离的 WVL 周围的 K-H 涡流和流撞击壁的次级涡流的上游侧之间的脊中产生的。通过 K-H 涡流的强烈喷发，壁湍流在大约 Re >= 2200 处脱离到管道的中间部分。
The instability and transition to turbulence and its evolution in pulsatile flows, which involve reverse flows and unsteady flow separations, is the primary focus of this experimental work. A piston driven by a programmable DC servo motor was used to set-up a water flow system and provide the pulsation characteristics. Time-resolved particle image velocimetry data were acquired in a refractive index matching set-up by using a continuous wave laser and a high-frame-rate digital camera. The position of the piston was continuously recorded by a laser proximity sensor. Five different experiments were carried out with Reynolds numbers in the range of 535-4825 and Womersley numbers from 11.91 to 23.82. The non-stationarity of the data was addressed by incorporating trend removal methods involving low- and high-pass filtering of the data, and using empirical mode decomposition together with the relevant Hilbert-Huang transform to determine the intrinsic mode functions. This latter method is more appropriate for nonlinear and non-stationary cases, for which traditional analysis involving classical Fourier decomposition is not directly applicable. It was found that transition to turbulence is a spontaneous event covering the whole near-wall region. The instantaneous vorticity profiles show the development of a large-scale ring-like attached wall vortical layer (WVL) with smaller vortices of higher frequencies than the pulsation frequency superimposed, which point to a shear layer Kelvin-Helmholtz (K-H) type of instability. Inflectional instability leads to flow separation and the formation of a major roll-up structure with the K-H vortices superimposed. This structure breaks down in the azimuthal direction into smaller turbulence patches with vortical content, which appears to be the prevailing structural content of the flow at each investigated Reynolds number (Re). At higher Re numbers, the strength and extent of the vortices are larger and substantial disturbances appear in the free stream region of the flow, which are typical of pipe flows at transitional Re numbers. Turbulence appears to be produced at the locations of maximum or minimum vorticity within the attached WVL, in the ridges between the K-H vortices around the separated WVL and the upstream side of the secondary vortex where the flow impinges on the wall. This wall turbulence breaks away into the middle section of the pipe, at approximately Re >= 2200, by strong eruptions of the K-H vortices.