Cross-stream stereoscopic particle image velocimetry of a modified turbulent boundary layer over directional surface pattern
Cross-stream stereoscopic particle image velocimetry of a modified turbulent boundary layer over directional surface pattern
复制标题
DOI：
10.1017/jfm.2016.879
复制
发表时间：
2017-01
影响因子：
3.7
通讯作者：
K. Kevin;J. Monty;H. Bai;G. Pathikonda;B. Nugroho;J. Barros;K. Christensen;N. Hutchins
中科院分区：
文献类型：
--
作者：
K. Kevin;J. Monty;H. Bai;G. Pathikonda;B. Nugroho;J. Barros;K. Christensen;N. Hutchins
本文利用体视粒子图像测速技术在雷诺数Re_{\unicode[STIX]{x1 D 70 F}}\approximately 3900$的横流平面上研究了人字形沟槽表面上的湍流边界层。从这个实验中得到的三个速度分量在所有单点速度统计中显示出明显的展向周期性。与以前的热线研究类似类型的沟槽一致，我们观察到一个弱的时间平均二次流的形式$\unicode[STIX]{x1 D 6 FF}$填充流向涡。所观察到的表面和二次流特性的差异，相比其他异质粗糙度的研究，可能表明，不同的机制是负责在这种情况下的流量修改。瞬时速度场的观测揭示了修改和重新排列的湍流结构。瞬时快照还表明，时间平均二次流可能是由表面纹理增强的更强的瞬时湍流事件叠加而产生的伪影。此外，所观察到的瞬时二次运动似乎促进了外层的自由流吞没行为，这将表明湍流/非湍流混合的增加。它总体上表明，过渡表面粗糙度的大规模方向性的存在下，可以导致整个边界层的修改，即使当粗糙度的高度是0.5%的层厚度。
A turbulent boundary layer developed over a herringbone patterned riblet surface is investigated using stereoscopic particle image velocimetry in the cross-stream plane at $Re_{\unicode[STIX]{x1D70F}}\approx 3900$ . The three velocity components resulting from this experiment reveal a pronounced spanwise periodicity in all single-point velocity statistics. Consistent with previous hot-wire studies over similar-type riblets, we observe a weak time-average secondary flow in the form of $\unicode[STIX]{x1D6FF}$ -filling streamwise vortices. The observed differences in the surface and secondary flow characteristics, compared to other heterogeneous-roughness studies, may suggest that different mechanisms are responsible for the flow modifications in this case. Observations of instantaneous velocity fields reveal modified and rearranged turbulence structures. The instantaneous snapshots also suggest that the time-average secondary flow may be an artefact arising from superpositions of much stronger instantaneous turbulent events enhanced by the surface texture. In addition, the observed instantaneous secondary motions seem to have promoted a free-stream-engulfing behaviour in the outer layer, which would indicate an increase turbulent/non-turbulent flow mixing. It is overall demonstrated that the presence of large-scale directionality in transitional surface roughness can cause a modification throughout the entire boundary layer, even when the roughness height is 0.5 % of the layer thickness.