Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers
Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers
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DOI：
10.1017/jfm.2019.1014
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发表时间：
2020-01
影响因子：
3.7
通讯作者：
T. Medjnoun;C. Vanderwel;B. Ganapathisubramani
中科院分区：
文献类型：
--
作者：
T. Medjnoun;C. Vanderwel;B. Ganapathisubramani
实验研究了展向非均匀表面形状对湍流边界层二次流和表面摩擦的影响。表面由不同形状和宽度的光滑流线型脊组成，展向波长与边界层厚度（$S/\unicode[STIX]{x1 D 6 FF}\approx O（1）$）相当。采用横流立体粒子图像测速技术和油膜干涉技术相结合的方法，对绕流流场进行了研究，并对阻力进行了评估。结果表明，表面摩擦力的展向分布随平均流的不均匀性而变化，并且高度依赖于表面的几何形状。旋流强度图显示了不同脊形的二次流结构的显著变化。对于宽脊，拓扑结构发生变化，显示出与大尺度二级结构共存的三级涡旋的外观。有利于这些三级结构的不平衡发生在一定的宽度上，它接管了二级结构，导致低动量和高动量路径的位置交换。此外，结果表明，展向间距本身不足以消除表面不均匀性。一个新的参数（$\unicode[STIX]{x1 D 709}$），这是基于周长的比率超过和低于平均表面高度，充分捕捉表面摩擦和流向环流的二次运动的变化。三重分解允许量化的分散应力的所有这些情况下，这可以贡献高达55美元的总剪应力时，发生强烈的二次运动。
The effect of spanwise heterogeneous surface geometry on turbulent boundary layer secondary flows and on skin friction is investigated experimentally. The surfaces consist of smooth streamwise-aligned ridges of different shapes and widths with spanwise wavelengths comparable to the boundary layer thickness ($S/\unicode[STIX]{x1D6FF}\approx O(1)$). Cross-stream stereoscopic particle image velocimetry combined with oil-film interferometry is used to investigate the flow field and assess the drag. Results show that the spanwise distribution of the skin friction varies as a consequence of the mean flow heterogeneity and is highly dependent on surface geometry. The swirling strength maps revealed remarkable changes in the secondary flow structures for different ridge shapes. For wide ridges, topological changes occur showing the appearance of tertiary vortices coexisting with the large-scale secondary structures. An imbalance in favour of these tertiary structures occurs over a certain width which take over the secondary structures, causing a swap in the locations of the low- and high-momentum pathways. Furthermore, the results indicate that the spanwise spacing alone is insufficient to characterise the surface heterogeneity. A new parameter ($\unicode[STIX]{x1D709}$), which is based on the ratio of the perimeter over and below the mean surface height, is shown to adequately capture the changes in skin friction and streamwise circulation of the secondary motions. Triple decomposition allowed the quantification of the dispersive stresses for all these cases, which can contribute up to $55\,\%$ of the total shear stress when strong secondary motions occur.