Route to turbulence in Strongly Stratified Slope Flows

湍流，或称高速混沌流体运动，在自然界中是常态，而不是例外。描述和预测流体湍流是复杂的，需要计算机模拟。在均匀大气流动条件下，湍流的计算机模拟已经取得了许多进展。然而，天气预报中的一个严重问题是对分层大气和复杂地形地区的湍流建模不足。在分层大气中，浮力(由于密度的变化)与重力的作用方向相同，阻碍了良好混合流动条件的形成。在山区和大冰盖和冰川的夜间，通常会观察到分层的情况。准确预测南极洲和格陵兰岛上空的风(气流)是了解气候变化影响的一个重要方面，因为风的强度和结构与冰盖的消融和相关的海平面上升直接相关。此外，山区夜间风的预测对空气质量、农业和国防作战具有重要意义。该项目有望改进计算机天气模型中分层条件下的风向预报。计划为现有的大学预科课程开展外展活动。该项目将产生与分层流体湍流相关的开源教育材料。主要技术目标是调查新发现的倾斜地形上的流体不稳定性，并揭示它们在向间歇性和片状湍流状态发展过程中的作用。由于层结相对于流动切变的倾斜度，表面倾斜改变了湍流动力学，导致进一步背离了经典的边界层剖面。该项目将采用流体动力稳定性理论的最新技术和直接的数值模拟，以得出流体不稳定性随表面倾角和新引入的无因次分层扰动参数的函数而变化的图谱。这项研究有望揭示足够高的层结扰动数时出现的新的流动特征，并提高对以猝发现象和片状湍流形式出现的流动间歇现象的理解。该项目最终将以一个定量标准来描述从层流到完全湍流条件的中间流态，从而填补目前与部分湍流分层坡流相关的知识空白。这一项目的成果有望在未来改进数值天气预报模式中稳定分层条件的次网格尺度参数化。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。

Turbulence, or high-speed chaotic fluid motion, is the norm rather than the exception in nature. Describing and predicting fluid turbulence is complex and requires computer simulations. Many advances have been achieved in computer modeling of turbulence under well-mixed atmospheric flow conditions. However, a serious concern in weather forecasting has been inadequate modeling of turbulence within a stratified atmosphere and in regions of complex terrain. In a stratified atmosphere, buoyancy force (due to density variations) acts in the same direction as gravity, hindering the formation of well-mixed flow conditions. Stratified conditions are commonly observed during nighttime in mountainous terrain and over large ice sheets and glaciers. Accurate prediction of winds (air flow) over Antarctica and Greenland is an important aspect of understanding the impact of climate change, because the strength and structure of winds are directly connected to the ablation of ice sheet and the associated sea level rise. Moreover, prediction of nocturnal winds in mountainous terrain has important implications for air quality, agriculture, and defense operations. This project is expected to lead to improved prediction of winds under stratified conditions in computer models of weather. Outreach activities are planned for existing precollege preparatory programs. The project will result in open-source educational materials related to stratified fluid turbulence.The chief technical objective is to investigate newly discovered fluid instabilities over sloping terrain and unravel their role in progression toward an intermittent and patchy turbulent state. Inclination of the surface alters the turbulence dynamics due to obliqueness of stratification relative to flow shear, leading to further departure from a classical boundary layer profile. The project will adopt contemporary techniques from hydrodynamic stability theory and direct numerical simulations to arrive at an atlas of fluid instabilities as a function of the surface inclination and the newly introduced dimensionless stratification perturbation parameter. The investigation is expected to reveal new flow features emerging at sufficiently high stratification perturbation numbers and improve the comprehension of flow intermittency in the form of bursting phenomena and patchy turbulence. The project will culminate with a quantitative criterion to characterize intermediate flow regimes that range from laminar to fully turbulent conditions, thus filling the current knowledge gap pertinent to partially turbulent stratified slope flows. The outcomes of this project are expected to lead to future improvements of subgrid-scale parameterizations of stably stratified conditions in numerical weather prediction models.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.