Measurement and Analysis of Unsteady Flows Using a Lagrangian Framework

漩涡——强烈旋转的区域——通常被称为“流体运动的肌肉”，因为它们负责质量和动量的传输。此外，涡量的产生、重定向和湮灭的研究在自然界和工程中广泛存在于非定常气动、水动力和血流动力学问题中。无论是小尺度的扑翼飞行，还是大尺度的风力涡轮机叶片失速，了解涡度是如何在物体附近产生并重新定向到尾迹的，对于这类系统的优化和流动控制至关重要。幸运的是，由于目前在相机、激光和处理能力方面的进步，目前正在引入时间分辨、体积成像的新方法。这些新方法被称为“摇箱法”（STB）或层析粒子跟踪测速法（Tomo-PTV），有望打破实验室和现场实验中长期存在的障碍。与现有的基于欧拉的成像技术（如层析粒子图像测速（Tomo-PIV））不同，这些新的拉格朗日方法在超高的播种密度下提供详细的拉格朗日跟踪信息（长路径），在任何给定的时间步长都可以跟踪10万个粒子。这种新方法开辟了一个全新的分析技术范围，从稳定和运动物体附近的压力场提取到尾迹中的直接拉格朗日相干结构（LCS）识别。

Vortices - regions of strong rotation - are often referred to as the 'sinews and muscle of fluid motions' for the very fact that they are responsible for the transport of mass and momentum. Furthermore, the study of vorticity production, reorientation and annihilation are ubiquitous to a broad range of unsteady aerodynamic, hydrodynamic and hemodynamic problems found both in nature and engineering. Whether at the small scales of flapping flight or at large scales of stall on wind-turbine blades, understanding how vorticity is generated near a body and reoriented into the wake is critical towards optimization and flow control in such systems. Fortunately, due to current advances in camera, laser and processing capabilities, new methodologies in time-resolved, volumetric imaging are currently being introduced. These new methods are referred to as 'Shake-The-Box' (STB) or Tomographic Particle Tracking Velocimetry (Tomo-PTV), and are slated to break down long-standing barriers in lab- and field-based experimentation. Unlike existing Eulerian-based imaging techniques such as Tomographic Particle Image Velocimetry (Tomo-PIV), these new Lagrangian approaches provide detailed Lagrangian tracking information (long pathlines) at ultra-high seeding densities, on the order of 100,000 tracked particles at any given time step. This new approach opens up a whole new gamut of analysis techniques from pressure-field extraction near steady and moving bodies to direct Lagrangian Coherent Structure (LCS) identification in wakes. Since the development of numerical modelling approaches for complex flows demands high-quality experimentation for their validation and calibration, such new Lagrangian experimental techniques are expected to provide much needed insight into these elusive problems. Examples of such challenging cases include: high Reynolds number (turbulent) flows with a large range of temporal and spatial scales; unsteady aerodynamic and hydrodynamic interactions with structures; multiphase and transitional flows such as are found in the heart; and gust characterization in the atmosphere. The proposed research program will explore the use of these new Lagrangian-based measurement techniques and their associated post-processing strategies as a means to offer insight into the underlying physics of such flows. The proposal will begin by describing how the implementation of Tomo-PTV measurements is likely to shape the direction we take when analyzing these problems. In particular, we will look at how new formulations for networking tracked particles and their acceleration content will enable extraction of flow features such as pressure. With vorticity, we will investigate how measurements near unsteady bodies will provide insight into vorticity production and its reorientation in the wake, thus identifying possible new avenues for flow control and optimization.