Physical and Computational Concepts for Systematic Development of Drag Reducing Surfaces in Pipes

Physical and Computational Concepts for Systematic Development of Drag Reducing Surfaces in Pipes Physical and Computational Concepts for Systematic Development of Drag Reducing Surfaces in Pipes The broad goal of the proposed research is to investigate the structure of the eddies in moderate Reynolds number pipe flow turbulence for the purpose of developing a physical models and computational models capable of systematically guiding the development of 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 devices and strategies that have been developed or 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 cut-and-try 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 wall shear stress now brings the possibility of a more systematic approach to drag reduction within reach. Current physical models offer usable, though admittedly incomplete, explanations of the sizes and geometries of the coherent structures in cases where the turbulence is fully developed or in equilibrium. But, there is still little or no understanding of how this turbulence grows out of laminar flow, or decays back to laminar flow. The matter of transition to turbulent flow in pipe flow is an especially vexing question that has remained opened 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 therefore, 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.