Measuring and Modeling Interactions of the Turbulent Atmospheric Boundary Layer with Multiscale Ground Topology

大气中的流动和输送过程受到地形的强烈影响，地形会产生力量并造成复杂的流动扭曲。在较小的尺度上，大气表层也受到植被覆盖的很大影响。以往的工作大多集中在单一长度尺度的丘陵和植被地形的影响上，例如特定大小的单一丘陵，或者由植物组成的树冠，通常使用规定的叶面积密度分布来建模。然而，众所周知，山脉和树冠等水流障碍物的特点是长度范围很大。然而，目前还不知道如何将这种多尺度物体对低层大气动力学的影响参数化。这项研究通过一个综合的实验室实验和计算程序来解决这个问题，重点是通过分形形状的大气边界层流动。分形学在一定的尺度范围内对山脉和植被几何的内在多尺度特征提供了方便的理想化。这些实验包括对“光学折射率匹配”装置中的流动结构和阻力的实验室模型研究，在该装置中，可以在整个复杂区域内进行无障碍的、详细的流动和力测量。多平面粒子图像测速仪将提供应力和速度梯度张量的所有分量。实验工作的一个关键动机是需要验证和支持一种新的预测工具-重整化数值模拟(RNS)的进一步开发和改进。该技术使用在计算网格上显式解析的大尺度询问确定的阻力系数来模拟来自地面拓扑的未解析特征的力，然后进行动态重新缩放。RNS将被用来对穿过分形树和山脉的流动进行建模，并将预测力和流动特征与测量结果进行比较。有了详细的流动数据，将找出差异的原因并用于改进。该项目的主题--改进陆地-大气相互作用的次网格参数化的科学基础的研究，对大气和气候科学的基础设施具有广泛的影响。在大气流动现象的背景下，使用新的、光学折射率匹配的方法进行测量，提供了在可以绘制和理解流动的细节水平上进行量子步骤的可能性。此外，应用于具有分形边界的流动的适当验证的RNS的发展可能会在多尺度地面拓扑上的湍流边界层以外的区域产生更广泛的影响。在许多学科中，如生物学(分支血液网络、肺结构、珊瑚)、天体物理学(宇宙的大尺度结构、行星际磁场的间歇性)和其他地球科学方面(海岸线、云层)，分形图被用作描述工具。RNS将分形学的几何概念扩展到流体力学。这项工作的教育影响将集中在研究生教育/培训上，强调物理实验和模拟之间的相互作用。作为教育推广工作的一部分，将继续并加强与巴尔的摩市学校系统的互动。具体地说，巴尔的摩理工学院的高中生将在我们的实验室参与为期一年的研究体验，作为他们要求的研究实践的一部分。参与研究分形边界上的大气流动将有助于激励有才华的高中生考虑未来在这一领域的职业。

Flow and transport processes in the atmosphere are strongly influenced by orography that generates forces and causes complex flow distortions. At smaller scales, the atmospheric surface layer is also affected substantially by vegetation canopies. Most previous work has focused on effects of hills and vegetated terrain characterized by a single length scale, e.g. a single hill of a particular size, or canopies consisting of plants, often modeled using a prescribed leaf-area density distribution. It is well known, however, that flow obstructions such as mountain ranges and canopies are characterized by a wide range of length scales. Yet, it is not known how to parameterize the effects of such multi-scale objects on the lower atmospheric dynamics. This research addresses this issue with an integrated laboratory experimental and computational program, focusing on atmospheric boundary layer flow over fractal shapes. Fractals provide convenient idealizations of the inherently multi-scale character of mountain range and vegetation geometries, within certain ranges of scales. The experiments consist of laboratory model studies of flow structure and drag forces in an "optically index-matched" facility, where unobstructed, detailed flow and force measurements can be performed within the entire complex domain. Multi-plane particle image velocimetry measurements will provide all components of the stress and velocity gradient tensors. A key motivation for the experimental work is the need to validate and support further development and improvements of a new prediction tool, Renormalized Numerical Simulation (RNS). This technique models forces from unresolved features of the ground topology using drag coefficients determined from interrogation of the large scales that are explicitly resolved on the computational mesh, followed by dynamic rescaling. The RNS will be used to model the flow across fractal trees and mountain ranges, and compare the predicted forces and flow features with the measurements. With the detailed flow data available, causes for discrepancies will be identified and used for improvements.Research on improving scientific foundations of sub-grid parameterizations of land-atmosphere interactions, the subject of the project, has a broad impact on the infrastructure of atmospheric and climate sciences. Measurements using novel, optically index-matched, methods in the context of atmospheric flow phenomena provide the possibility of a quantum step in the level of detail with which flows can be mapped and understood. Moreover, the development of properly validated RNS applied to flow with fractal boundaries may yield broader impact in areas other than turbulent boundary layers over multi-scale ground topology. Fractals have been used as a descriptive tool in many disciplines, such as biology (branching blood network, pulmonary structures, corals), astrophysics (large-scale structure of the universe, intermittency of interplanetary magnetic fields), and other geosciences aspects (fractal coastlines, clouds). RNS extends the geometric idea of fractals to fluid dynamics. Educational impact of the work will focus on graduate education/training that stresses the interplay between physical experimentation and simulation. As part of the educational outreach effort, interactions with the Baltimore City School system will continue and strengthen. Specifically, senior high-school students from the Baltimore Polytechnic Institute will be involved in yearlong research experiences in our laboratory, as part of their required Research Practicum. Involvement in research on atmospheric flows over fractal boundaries will help motivate talented senior high-school students to consider future careers in this field.