Regimes of particle settling for finite-sized particles in the inertial range of turbulence

在各种环境和工业环境中，颗粒沉降受到湍流的影响。例如，通过浮游生物和海洋雪颗粒沉降的营养物的垂直通量，河流、湖泊和海床上微塑料等海洋垃圾的沉积，以及废水处理设施中的沉降池。在此类应用中，对沉降速度的准确理解将提高效率，并最大限度地减少对环境的影响，但由于此类颗粒通常太大，无法将其建模为小点，因此获得这种理解是具有挑战性的。因此，该项目的主要目的是了解导致湍流中较大颗粒沉降速率改变的潜在物理原理。该项目还将包括设计一个台式实验设施，该设施将为威斯康星大学麦迪逊分校土木与环境工程系的本科生提供研究、教育和推广机会。有限尺寸颗粒在湍流中的沉降表现为：(1)颗粒与湍流的相互作用发生在雷诺数大于1时；(2)复杂的多尺度颗粒与湍流的相互作用，因为颗粒扭曲了周围的局部流动。先前的研究已经发现，相对于静止流体中的终端速度，湍流中的粒子沉降率降低了，但尚不清楚如何从潜在的物理机制来理解这一点。本项目旨在通过对有限尺寸颗粒的沉降机制进行彻底的实验研究，探索颗粒大小、密度和体积分数的范围，来填补这一理解上的空白。实验将使用新建造的八角形湍流室和三维高速相机测量流动和粒子运动。重点将是确定新的尺度参数，以捕获湍流对颗粒沉降率的影响，包括颗粒聚集的影响。预计这种方法将导致确定改变粒子沉降率的机制，该机制基于在一系列参数范围内崩溃数据的标度定律。反过来，这种理解将导致一个新的预测模型，可以为未来的水处理和生态系统工程设计提供信息。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Particle settling is affected by turbulence in various environmental and industrial settings. Examples include vertical fluxes of nutrients via the settling of plankton and marine snow particles, deposition of marine debris such as microplastics on river, lake and sea beds, and settling tanks in wastewater treatment facilities. An accurate understanding of settling velocities in such applications would improve efficiency and minimize environmental impacts, but gaining this understanding is challenging since such particles are often too large to be modelled as small points. Thus, the principal aim of this project is to understand the underlying physics that leads to altered settling rates of larger particles in turbulent flows. The project will also include the design of a bench-top experimental facility that will provide research, education, and outreach opportunities for undergraduate students in the Civil and Environmental Engineering department at UW-Madison. Settling of finite-sized particles in turbulence are characterized by (1) particle interactions with turbulent flow that occur at Reynolds numbers above unity and (2) complex and multiscale particle-turbulence interactions as the particles distort the local flow around them. Previous work in this regime has found that particle settling rates in turbulence are reduced relative to terminal velocities in quiescent fluid, but it is unclear how this should be understood in terms of the underlying physical mechanisms. This project seeks to fill this gap in understanding by conducting a thorough experimental investigation into settling regimes of finite-sized particles, exploring a range of particle sizes, densities, and volume fractions. The experiments will use the newly constructed octagonal turbulence chamber and three-dimensional high-speed camera measurements of flow and particle motion. The focus will be on identifying new scaling parameters that capture turbulence effects on particle settling rates, including the effects of particle clustering. It is expected that this approach will lead to the identification of mechanisms that alter particle settling rates based on scaling laws that collapse data across a range of parameters. In turn, this understanding will lead to a new predictive model that could inform future designs in water treatment and ecosystem engineering.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.