Vortices are ubiquitous in fluids and can form on scales as small as a fraction of a millimeter to as large as hundreds of kilometers. The formation of hurricanes is just one of the extreme examples of large- scale atmospheric phenomenon, which can have devastating consequences. This proposal is aimed at understanding fundamental physics that governs the formation, interaction, and evolution of vortices in a thin sheet of fluid that behave two dimensionally. It is believed that if fluid flow is predominantly two dimensional and turbulent, as in the atmosphere and in oceans, the flow can self organize into large, power structures. A mathematical theory, similar to thermodynamic theory of mixing of chemicals, has been formulated to explain this remarkable process. This proposal is an attempt to test such a theory using well-controlled laboratory experiments. A significant part of this research is to educate and train students, at different levels, who will be the new to science and engineering. The research project itself is very attractive to students because the physical phenomena are intriguing and visually pleasing.
This proposal is aimed at understanding fundamental physics that governs the formation, interaction, and evolution of vortices in thin sheets of fluid that behave two dimensionally. This latter property prevents vortex stretching and is believed to be responsible for the formation of large-scale coherent structures with remarkable stability. Unlike vortices occurring in nature, laboratory vortices can be created in a controlled manner, allowing detailed studies to be carried out. It has been postulated that stability of these large-scale coherent structures results from the system being close to a unique state governed by the principle of maximum mixing entropy. This is an important hypothesis and our experiment is designed to test whether this entropy principle works for vortices as well as for an ordinary thermodynamic system. A significant part of this research is to educate and train students who will be new to science and engineering. The research project itself is very attractive to students of all age groups because the physical phenomena are intriguing but visually pleasing.
The aim of this research is to understand fluid flow and turbulence in flowing thin films. Most of fluid dynamic studies of turbulence focus on bulk fluids that can move freely in three-dimensional spaces. However, fluid motions in low spatial dimensions, such as those restricted to move in a plane, are unique and have important properties that are shared by oceanic and atmospheric flows. One of important consequence, among several others, is the intensification of vortices in low-dimensional flows, i.e., small vortices instead of breaking up, as in three-dimensional spaces, they combine to form larger and more powerful ones. This intriguing effect, though theoretically predicted based on the physical principles of energy and angular momentum conservation, has not been systematically studied in a laboratory. The implication of the effect is very significant because the mechanism of vortex intensification underlies formation of all hurricanes and typhoons that can cause havoc to society as we have witnessed almost every year. Thanks to the NSF support we developed various methods by which fluid flow and turbulence can be created in free suspended liquid films that restrict the flow to the plane of the films and thus two dimensional. The flow field is studied using advanced optical techniques such as Doppler velocimetry and fast video imaging. The laboratory setting allows experimental parameters such as the flow rate, the size of vortices, and turbulent intensity to be well controlled, facilitating detailed comparisons with mathematical theory of turbulence in two dimensions. Our experiments demonstrate the existence of annihilation of small vortices by large ones, a physical process known as energy inverse cascade. The flow structures that are responsible for the energy transfer from small to large scales are characterized. Our experiments also demonstrate for the first time the significant difference between turbulence that is freely decaying and that is in a steady state driven continuously by an external force. This difference manifests itself in the distribution of the sizes of vortices in the fluid, which is directly measured by a fast video camera. Our turbulence study also leads to several new research directions that we believe will be beneficial to science, technology, and society. In an effort to find a way to generate individual vortices in a flowing film, we discovered that fast moving, micron-sized particles can be readily transmitted through very thin liquid films without damaging them. We show that upon transmission, the particles are coated by a thin layer of the fluid that composes of the film. This observation can be exploited to make efficient encapsulation of small particles, which is useful for many industrial processes. A notable physical effect of vortices in fluids is the enhanced dispersal and mixing of chemicals. This has an important consequence for marine microorganisms, such as bacteria and phytoplankton. How these marine microorganisms sense their chemical environments and respond accordingly? This question is not well studied at present but is an important for the health of world’s oceans because they constitute the major biomass and are a crucial part of the food web in this unique environment. For the last couple of years, we have devoted some of our attention to investigate how marine bacteria swim in water and how they response to transitory chemical stimulation. The NSF support not only advances significantly our fundamental understanding of fluid turbulence but also helps train a cadre of young physicists who are now at the forefront of research in academia, government agencies, and industry. This contribution to human resource development is invaluable, and we wish it will continue for years to come.
