Effect of Perturbations on Eddy Organization in Turbulent Boundary Layers

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。最近的研究已经证明了湍流边界层中大规模组织的普遍性和重要性，这些组织被认为是由单个涡流包引起的。考虑到数据包似乎具有特定的跨向间隔，PI假设障碍物可以改变跨向结构。例如，障碍物可以间隔一段时间，以增强相邻包的稳定性，或者干扰并因此改变在光滑壁上流动时观察到的典型的展向（可能是流向）缩放。在当前的研究中，涡旋包结构将使用延伸到边界层对数区域的障碍物进行扰动。利用立体PIV和层析PIV分别测量了大型水道设施边界层的平面速度场和体积速度场。由此产生的速度场将用特征识别算法进行探测，以确定扰动对涡流和包的相互作用、演化和缩放的影响。瞬时剪应力和剪应力梯度将同时量化，以确定扰动对壁法向动量传递的潜在影响。障碍物的高度和间隔将被改变以修改数据包的组织。此外，流动速度将根据雷诺数的大小变化，并观察它们如何在实际应用中推断出更高雷诺数下的行为。这项研究的结果将有助于回答边界层中的包的形成是由在壁面附近启动的“自下而上”机制引起的，还是由远离壁面的扰动或不稳定引起的。此外，实验将展示如何通过智能控制方法改变尺度，组织和伴随特征。准确的模型将导致更有效的能源生产和转换过程，更有效的运输车辆和材料，更好地了解污染物，微生物和营养物质在大气和水生环境中的运输。将培训一名研究生，本科生将通过REU补充和UM UROP计划参与研究。此外，PI和学生将与一所有大量美洲原住民人口的学校的小学教师和学生合作，开发基于火箭、纸飞机和环境流的互动演示和流可视化活动。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Longmire0933341 Recent research has demonstrated the prevalence and importance of a large scale organization in turbulent boundary layers thought to be caused by packets of individual eddies. Given that packets appear to have a specific spanwise spacing, the PI hypothesizes that obstacles can alter the spanwise structure. For example, obstacles can be spaced at intervals to either enhance stability of neighboring packets or alternatively interfere with and hence alter the typical spanwise (and probably streamwise) scaling observed in flow over smooth walls. In the current study, the vortex packet structure will be perturbed using obstacles extending into the logarithmic region of the boundary layer. Planar and volumetric velocity fields of boundary layers in a large water channel facility will be measured by stereo and tomographic PIV, respectively. The resulting velocity fields will be probed with feature identification algorithms to determine the effect of perturbations on eddy and packet interaction, evolution, and scaling. Instantaneous shear stresses and shear stress gradients will be quantified simultaneously to determine potential effects of perturbations on wall-normal momentum transfer. Obstacle height and spanwise spacing will be varied to modify the packet organization. Further, the flow speed will be varied to scale the results with Reynolds number and to see how they might extrapolate to behavior at the much higher Reynolds numbers in practical applications. The results of this study will help answer whether packet formation in boundary layers results from a 'bottom up' mechanism initiated near the wall or it results from perturbations or instabilities initiated away from the wall. Further, the experiments will demonstrate how scaling, organization, and accompanying characteristics can be altered by intelligent control methods. Accurate models will lead to more efficient processes in energy production and conversion, more efficient transport of vehicles and materials, better understanding of pollutant, microorganism and nutrient transport in the atmosphere and aquatic environments. One graduate student will be trained, and undergraduate students will participate in the research through REU supplements and the UM UROP program. Also, the PI and students will work with elementary school teachers and students at a school with significant Native American population to develop interactive demonstrations and flow visualization activities based on rockets, paper airplanes, and environmental flows.