The PIs plan novel laboratory tests of a new and fundamentally different theoretical view of wall turbulence. Organized flows (coherent structures) near smooth walls play a central role in turbulence production over a wide range of Reynolds numbers. Recent theoretical work with large scale numerical computation has revealed a class of unstable, exact Navier-Stokes solutions termed "exact coherent structures" (ECS), which capture essential features of classic coherent structures. In particular, theory suggests ECS can be used construct simplified descriptions of wall turbulence. While theory/numerics have made a compelling case for an ECS-based description in highly idealized turbulent flows with periodic boundary conditions, very little is known about whether this viewpoint can describe turbulence in the lab. The PIs plan experimental studies of ECS in circular Couette flow where turbulence is initialized by precise, optically-imposed disturbances and measured by 3D velocimetry. The PI's unique method of optical distributed flow actuation has already been tested in other flows; thus, the experiments will focus directly on flow physics instead of actuator development. Laboratory testing of an ECS-based description will begin first with the adaptation of theoretical state space visualization techniques to experiments. The experiments will then identify important ECS. In particular, the PIs will focus on the Lower Branch ECS, which appears to play a key "gatekeeper" role in the transition between laminar and turbulent flow. Characterization of Lower Branch ECS will set the stage for novel approaches to turbulence flow control. Turbulence is a major consideration in the design of transport vehicles on land, at sea or in the air. In practical applications; even modest improvements in turbulence control could have enormous economic impact. The tools of dynamical systems are becoming ever more important in solving engineering problems. Thus, our planned work will implement a laboratory module on state space visualization of pendulum dynamics that will be included in a novel introductory calculus-based engineering physics curriculum at Georgia Tech and at Spelman College, the country's leading Historically Black College/University for women.
