The Interactions Between Vaporizing Liquid Droplets and a Turbulent Flow: Fully Resolved DNS and Experiment

本文提出的数值/实验研究的目的是加强对湍流中液滴汽化和混合过程的理解。数值研究采用直接数值模拟（DNS）研究了自由运动汽化液滴与各向同性湍流之间的双向相互作用。液滴将在三维空间和时间中被完全分解，即不被视为点粒子，并且湍流运动的所有尺度都被分解到最小的相关长度和时间尺度。重点将放在汽化液滴和周围湍流气体之间的质量、动量和能量的双向交换上。实验采用产生湍流的有源网格和光学可及的风洞截面来测量各向同性湍流中自由飞行的单个液滴（即单向耦合）的蒸发速率，其性质与DNS中的相同。畸变光学成像和干涉条纹技术被用来提供一个大的轴向视野，具有高分辨率的液滴尺寸能力。提出的DNS研究将是第一个完全解决湍流中大量自由运动汽化液滴内部和外部流动的研究。基于液滴和湍流之间双向耦合的DNS详细结果可用于建立和验证大涡模拟（LES）亚网格尺度的数学模型以及Reynolds平均模型。应该强调的是，从拟议的研究中获得的详细DNS数据在任何已发表的实验或数值研究中都是不可用的（Birouk & Gokalp, 2006）。提出的实验研究将检验湍流对各向同性湍流中自由运动的单个液滴汽化速率的期望。除了加强对湍流和汽化之间相互作用的物理理解之外，这些测量结果将用于验证我们的DNS方法，因为没有其他可比较的实验数据存在于各向同性湍流中自由飞行的液滴蒸发，湍流的Kolmogorov长度尺度小于初始液滴尺寸。因此，耦合实验和计算将确定蒸发和湍流之间的关键相互作用。更广泛的影响拟议研究的结果将对能源的有效利用产生重大影响。这种影响源于这样一个事实，即汽化速率是燃料液滴燃烧的主要控制机制，液体燃料是所有运输方式最重要的能源来源，并且在可预见的未来仍将如此。了解湍流中汽化和混合过程的物理细节是理解化学反应过程和最终控制/优化能量转换过程的必要前提。pi在研究领域和研究与教学相结合方面取得了成就，包括向本科生和科学领域代表性不足的学生伸出援助之手。例如，三名女性博士毕业生现在是大学机械工程学院（加州大学圣地亚哥分校，丹佛大学，加州州立大学洛杉矶分校）。此外，联合pi是加州少数民族参与联盟（CAMP）的区域主任，这是一项由美国国家科学基金会资助的活动，旨在提高STEM领域中代表性不足的学生的数量和成功。本项目通过CAMP为这类学生提供研究动力。这次调查的实验也将作为COSMOS的示范，COSMOS是一项面向对科学和工程感兴趣的有才华的高中生的暑期住宿计划，该计划由共同pi参与。我们的DNS数字化数据已经传输到日本和巴西的大学以及欧洲科学基金会，作为欧洲研究人员的参考。由这些数据制作的视频显示了纵向涡结构在空间发展的湍流边界层中的运动，由G.M. Homsy教授（UCSB）选择，包括在多媒体流体力学- ii CD中，为学生和专业人士提供了一个跨平台，互动的视觉资源。拟议的研究结果将以同样方式传播

0933085ElghobashiThe objective of the proposed numerical/experimental study is to enhance the understanding of liquid droplet vaporization and mixing processes in a turbulent °ow. The numerical study employs direct numerical simulations (DNS) to examine the two way interactions between freely moving vaporizing droplets and isotropic turbulence. The droplets will be fully resolved in 3D space and time, i.e. not treated as point particles, and all the scales of the turbulent motion are resolved down to the smallest relevant length and time scales. The emphasis will be on the two way exchange of mass, momentum and energy between the vaporizing droplets and the surrounding turbulent gas. The experiment employs a turbulence generating active grid and optically accessible wind tunnel section to measure the evaporation rate of free flying single droplets (i.e. one way coupling) in isotropic turbulence with properties identical to those in the DNS. Anamorphic optics for imaging and an interferometric fringe technique are used to provide a large axial field of view with high resolution droplet-sizing capability.Intellectual meritThe proposed DNS study will be the first that fully resolves the flow inside and outside a large number of freely-moving vaporizing droplets in a turbulent flow. The detailed results of the proposed DNS, with two way coupling between the droplets and turbulence, can be used to develop and verify the mathematical models for the subgrid scales of large eddy simulations (LES) as well as Reynolds averaged models. It should be emphasized that the detailed DNS data which will be obtained from the proposed research are not available in any published experimental or numerical study (Birouk & Gokalp, 2006). The proposed experimental study will examine the exects of turbulence on the vaporization rate of a single droplet moving freely in isotropic turbulence. In addition to enhancing the understanding of the physics of interaction between turbulence and vaporization, the measurements will be used to validate our DNS methodology since no other comparable experimental data exist for free flying droplet evaporation in isotropic turbulence where the Kolmogorov length scale of turbulence is smaller than the initial droplet size. Hence, the coupled experiment and computations will determine the key interactions between evaporation and turbulence.Broader impactThe results of the proposed study will have a significant impact on the efficient utilization of energy. This impact stems from the fact that the vaporization rate is the main controlling mechanism of fuel droplet combustion and that liquid fuels are the most important source of energy for all modes of transportation and will remain as such for the foreseeable future. Understanding the physical details of the vaporization and mixing processes in a turbulent flow is an essential prerequisite to understanding the chemical reaction process and the eventual control/optimization of the energy conversion process. The PIs have a record of accomplishment in the field of study and in combining research and teaching, including outreach to undergraduate students and underrepresented students in the sciences. For example, three women Ph.D. graduates of the investigators are now University Mechanical Engineering faculty (Univ. of California San Diego, Univ. of Denver, Calif. State Univ., Los Angeles). In addition, the co-PI is Regional Director for the California Alliance for Minority Participation (CAMP), an NSF-funded activity to enhance the number and success of underrepresented students in STEM fields. Through CAMP, this project can provide research motivation for such students. The experiment in this investigation will also be used as a demonstration in COSMOS, a summer residential program for talented high school students interested in the sciences and engineering in which the co-PI participates. Our DNS digitized data have been transmitted to universities in Japan and Brazil as well as the European Science Foundation to be used as a reference for researchers in Europe. A video made of these data showing the motion of the longitudinal vortical structures in a spatially developing turbulent boundary layer has been selected by Prof. G.M. Homsy (UCSB) to include in the Multimedia Fluid Mechanics-II CD which provides a cross-platform, interactive, visual resource, for students and professional alike. The results of the proposed research will be disseminated similarly