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环境中都会受到湍流的影响。例如，通过浮游生物和海洋雪粒的沉降，营养物质的垂直通量，河流、湖泊和海床上沉积的海洋废弃物，如微塑料，以及废水处理设施中的沉淀池。准确地了解此类应用中的沉降速度将提高效率并将对环境的影响降至最低，但要获得这种了解是具有挑战性的，因为此类颗粒往往太大，无法建模为小点。因此，该项目的主要目的是了解导致湍流中较大颗粒沉降率变化的基本物理原因。该项目还将包括设计一个台式实验设施，为威斯康星大学麦迪逊分校土木工程与环境工程系的本科生提供研究、教育和拓展机会。有限尺寸颗粒在湍流中的沉降具有以下特征：(1)当雷诺数大于1时，颗粒与湍流发生相互作用；(2)当颗粒扭曲其周围的局部流动时，颗粒与湍流之间复杂且多尺度的相互作用。以前在这一领域的工作已经发现，相对于静止流体中的终端速度，湍流中的粒子沉降率会降低，但目前还不清楚应该如何从潜在的物理机制来理解这一点。这个项目试图通过对有限大小颗粒的沉降状态进行彻底的实验研究，探索一系列颗粒大小、密度和体积分数来填补这一认识上的空白。这些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。重点将放在确定新的标度参数上，这些参数捕捉到了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对改变粒子沉降率的机制的识别，该机制基于使一系列参数的数据崩溃的标度定律。反过来，这种理解将导致一种新的预测模型，可以为水处理和生态系统工程的未来设计提供信息。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.