AI 中文总结
研究摩擦雷诺数880 - 1460的湍流边界层流过被动旋转圆盘以减阻，测试表面由交错圆盘组成，分析发现虽与圆盘部件相互作用使阻力增加，但隔离圆盘运动影响后有3%减阻，还利用恒等式及测速法揭示相关流动效应及减阻机制。
AI 中文摘要
在风洞中研究了摩擦雷诺数范围为$Re_{\tau}=880 - 1460$且流过齐平安装的被动旋转圆盘的湍流边界层，目的是减少表面摩擦阻力。测试表面由32个交错排列的旋转圆盘组成，由安装在圆柱形腔体内的轴承支撑。由于圆盘半被薄刚性板覆盖，壁面湍流在圆盘暴露半部施加的壁面剪应力不对称分布使圆盘稳定旋转。直接力测量表明，由于与圆盘外壳和盖板的流动相互作用，阻力相对于平板情况增加。隔离了圆盘运动的影响，相对于固定圆盘上的流动测量到3%的阻力降低。首次利用\cite{Elnahhas_Johnson_2022}扩展到包括圆盘流动效应的表面摩擦恒等式分析实验数据。通过粒子图像测速法获得的测量结果表明，由于圆盘周围的间隙，相邻圆盘之间出现粗糙度平均流效应，并且在盖板附近存在下洗二次流，类似于流过流向拉长的矩形粗糙度元件的流动。这种下洗速度由于圆盘的展向运动而在流向受到调制，并改变了Elnahhas - Johnson恒等式中的壁面法向输运项，从而局部降低了阻力。
英文摘要
Turbulent boundary layers characterised by friction Reynolds numbers in the range $Re_τ = 880 - 1460$ and flowing over flush-mounted passively rotating discs are investigated in a wind tunnel with the purpose of reducing the skin-friction drag. The test surface is composed of thirty-two rotating discs arranged in a staggered configuration and supported by bearings mounted in cylindrical cavities. As the discs are half covered by thin rigid plates, a steady rotation of the discs is sustained via the asymmetric distribution of the wall-shear stress exerted by the wall turbulence on the exposed halves of the discs. Direct force measurements reveal that the drag increases with respect to a flat-plate case because of the flow interaction with the disc housings and the covering plates. The effect of the disc motion is isolated and a 3\% drag reduction is measured with respect to the flow over stationary discs. The skin-friction identity by \cite{Elnahhas_Johnson_2022} (\emph{J. Fluid Mech.}, vol. 940, 2022), extended herein to include the disc-flow effects, is utilised for the first time to analyse experimental data. This direct slip effect, quantified by using the measured disc angular velocities in the Elnahhas-Johnson identity, is negligible. Measurements obtained by particle image velocimetry disclose that a roughness mean-flow effect occurs between adjacent discs because of the clearance gaps around the discs and that a downwash secondary flow exists near the covering plates, analogous to flows over streamwise-elongated rectangular roughness elements. This downwash velocity is streamwise modulated because of the spanwise disc motion and alters the wall-normal transport term in the Elnahhas-Johnson identity, thus reducing the drag locally.
