The broad goal of the proposed research is to investigate the structure of turbulence eddies in pipe flows for the purpose of developing passive drag reducing surfaces suitable for implementation in large-scale engineering systems such as the U. S. national network of pipelines. Despite the many drag reducing strategies that have been tried to reduce wall friction caused by turbulent flow over surfaces, it is difficult to find any clear, undisputed set of principles that explains how one should design surfaces to reduce skin friction drag. The surfaces that have been successful, such as riblets, have been devised by trial and error methods coupled with the knowledge of the simplest aspects of wall turbulence, or by bio-mimetic approaches that copy methods used by life forms such as shark skin. However, progress over the past decade in understanding the coherent structures responsible for creating friction stresses now brings the possibility of a systematic approach to drag reduction within reach. The matter of transition to turbulent flow in pipe flow is an especially vexing question that has remained open for more that a century. The answers to these questions seem likely to be essential, or at the least very valuable, to the invention of drag reducing agencies that operate by inhibiting the formation of turbulence or diminishing its strength. The specific goals are to investigate the transition to turbulence in pipe flow at Reynolds numbers well above the lower critical value; to investigate the eddy mechanisms leading to re-laminarization of accelerating turbulent flows; and to develop large eddy simulation methods capable of revealing, with high fidelity, the effects of drag reducing surfaces at high Reynolds numbers.
Intellectual Merit : The transition to turbulence in pipe flow is one of the longest standing mysteries in fluid mechanics. Understanding the origins of turbulence in pipe flow will guide efforts to design drag reducing agents. Explaining the eddy mechanisms underlying the behavior of relaminarizing flow will test current physical models of wall turbulence such as the hairpin packet paradigm, and also provide insights into a form of turbulence behavior that reduces drag. Exploring singular perturbation methods for devising models for simulating high Reynolds number flow with fully resolved near-wall physics may improve our ability to simulate and model turbulent flow over surfaces, including complex drag reducing patterns.
Broader Impacts : Tools that will contribute to the eventual development of drag reducing methods for pipe flow would enable economically valuable energy savings. Overcoming turbulent friction consumes a surprisingly large fraction of the United States' national energy budget. The system of pipelines carrying oil and natural gas throughout the U. S. requires 3% of the roughly 1 T$ annual energy expenditure to pump the fluids, corresponding about 30B$ per annum to overcome the flow resistance due to turbulence in the pipes.
