Turbulent boundary layer flow over regularly and irregularly arranged truncated cone surfaces
Turbulent boundary layer flow over regularly and irregularly arranged truncated cone surfaces
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DOI：
10.1017/jfm.2021.946
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发表时间：
2022-01-04
影响因子：
3.7
通讯作者：
Schultz, Michael P.
中科院分区：
文献类型：
--
作者：
Womack, Kristofer M.;Volino, Ralph J.;Schultz, Michael P.
为了研究不同布置粗糙度单元上的粗糙壁湍流边界层结构，对规则粗糙度和随机粗糙度两种情况下的流动进行了实验研究。研究了正方形交错图形中截断锥面粗糙度单元的平面密度变化。在随机排列中也研究了相同的平面密度。速度统计采用双分量激光多普勒测速仪和立体粒子图像测速仪进行测量。由空间平均流动统计确定的摩擦速度、厚度、粗糙长度和零面位移，在相同密度下的规则排列和随机排列之间只有很小的差别。根据实验确定的粗糙度长度，对最近的先验形态测量和统计阻力预测方法进行了评估。观测到的规则和随机表面流参数的差异是由于二次流的存在，二次流在流向速度中表现为高动量路径和低动量路径。与预期相反，这些二次流在随机表面上存在，而在规则表面上看不到。以前发现的沿水流方向相干的跨向粗糙度不均匀似乎不存在，这表明这种粗糙度不均匀对于维持二次流是不必要的。证据表明，观测到的二次流开始于粗糙度的前缘，并持续到不规则粗糙度以上。由于二次流的存在，局部湍流边界层廓线并不随局部壁面剪应力的变化而变化，而是随粗糙度冠层上方的局部湍流剪切应力的变化而变化。此外，象限分析显示，弹射和扫射事件的数量发生了明显的变化。
Aiming to study the rough-wall turbulent boundary layer structure over differently arranged roughness elements, an experimental study was conducted on flows with regular and random roughness. Varying planform densities of truncated cone roughness elements in a square staggered pattern were investigated. The same planform densities were also investigated in random arrangements. Velocity statistics were measured via two-component laser Doppler velocimetry and stereoscopic particle image velocimetry. Friction velocity, thickness, roughness length and zero-plane displacement, determined from spatially averaged flow statistics, showed only minor differences between the regular and random arrangements at the same density. Recent a priori morphometric and statistical drag prediction methods were evaluated against experimentally determined roughness length. Observed differences between regular and random surface flow parameters were due to the presence of secondary flows which manifest as high-momentum pathways and low-momentum pathways in the streamwise velocity. Contrary to expectation, these secondary flows were present over the random surfaces and not discernible over the regular surfaces. Previously identified streamwise-coherent spanwise roughness heterogeneity does not seem to be present, suggesting that such roughness heterogeneity is not necessary to sustain secondary flows. Evidence suggests that the observed secondary flows were initiated at the front edge of the roughness and sustained over irregular roughness. Due to the secondary flows, local turbulent boundary layer profiles do not scale with local wall shear stress but appear to scale with local turbulent shear stress above the roughness canopy. Additionally, quadrant analysis shows distinct changes in the populations of ejection and sweep events.