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

颗粒沉降在各种环境和工业设置中受到湍流的影响。例子包括浮游生物和海洋雪颗粒沉降造成的营养物垂直通量，微塑料等海洋废弃物在河流、湖泊和海床上的沉积，以及废水处理设施中的沉淀池。准确了解这些应用中的沉降速度将提高效率并最大限度地减少环境影响，但获得这种理解具有挑战性，因为这些颗粒通常太大而无法建模为小点。因此，本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基本物理学。该项目还将包括一个台式实验设施的设计，将提供研究，教育和推广机会，本科生在土木与环境工程系在威斯康星大学麦迪逊分校。湍流中有限尺寸颗粒的沉降特征在于：（1）颗粒与湍流的相互作用发生在雷诺数大于1的情况下;（2）复杂和多尺度的颗粒-湍流相互作用，因为颗粒扭曲了周围的局部流动。以前的工作在这方面已经发现，颗粒在湍流中的沉降速率相对于静止流体中的终端速度降低，但目前还不清楚这应该如何理解的基本物理机制。该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，探索一系列颗粒尺寸，密度和体积分数，来填补这一认识上的空白。实验将使用新建造的八角形湍流室和三维高速摄像机测量流动和颗粒运动。重点将是确定新的缩放参数，捕捉湍流对颗粒沉降速率的影响，包括颗粒聚集的影响。预计这种方法将导致识别的机制，改变颗粒沉降速率的基础上，崩溃的数据在一系列参数的标度律。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NSF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.