This integrated research and education study investigates the physics associated with the interaction of boundary layer flows with morphing surfaces (defined here as thin skins capable of generating small, distributed or discrete, morphological or geometrical perturbations to the wall boundary condition). Fundamental questions to be addressed include how well can the characteristics of a boundary layer be manipulated via a morphing surface, what additional fundamental understanding of fluid physics can be gained from this new means of activation and how should graduate students best be prepared for this interdisciplinary research? An interdisciplinary approach will focus on: identification of the flow physics in new regimes that can now be interrogated using active materials; in situ characterization of the interaction between boundary layer flows and morphing surfaces; and investigation of open-loop actuation using surface morphing as a precursor to closed-loop control. Arrays of individually-addressable micro- or nano- actuators will be replaced with a coherent surface, or "skin" capable of either morphological changes or small adaptation of local geometry. Activation of properly designed morphing surfaces represents minimal control effort, with the potential for relatively simpler models relating the input to resultant structural changes in the flow. This research will expand our understanding of flow physics in receptive flows into a parameter range not previously accessible, or perhaps imaginable, before the advent of some newer materials. Classical hypotheses can then be revisited for canonical flows after the introduction of time-dependent wall motion and providing potential means for actuation for future closed-loop control work. Experiments on morphing surfaces also promise further insight into fundamental questions concerning a statically rough wall, mixing and vorticity flux in boundary layers. Broader impacts address environmental concerns and expanded air vehicle performance using active control or optimization of vehicular boundary layers via artificial manipulation of the boundary layer structure. This research will show how this could be achieved without large weight or structural penalties, and ultimately leading to a design feature for new vehicles. Collaborations with the active materials industry and a leading controls researcher seek to unite multiple fields to address this highly interdisciplinary topic. The educational goal is to develop a synergistic training scheme in flow control, suitable for addressing the research described here and educating graduate researchers to inhabit the middle ground between traditional disciplines associated with flow control, including fluid mechanics, control, dynamics and solid mechanics. This will culminate with a graduate level course designed to unify experimental, computational and modeling approaches to tackle advanced flow control problems. Outreach will target general public interest in fluid dynamics, with emphasis on girls of high school age and a continuing commitment to undergraduate research participation.

Project Report

" concern the demonstration of the control authority that can be exerted on a range of receptive fluid flows by very small time-varying surface perturbations. We deem these perturbations "morphing surfaces", where the morphing is on the scale of the surface roughness (see figures 1 and 2). The present work has proved the potential of morphing surfaces to control flow and gain insight into the driving flow physics for bluff body and wall-bounded turbulent flows. The success of such an interdisciplinary approach is directly attributable to the robust flow system properties that we have identified and exploited in this work. In particular, the influence of a mechanically-driven roughness element on the force vector acting on a sphere through the critical Reynolds number has been shown to permit the generation of a lateral force that can be up to an order of magnitude larger than the drag force (figure 3 and primary image). We have also made significant progress towards understanding of the basic mechanisms behind the sustenance of wall turbulence, described in terms of the mean velocity profile as the response of the Navier-Stokes operator to small noise forcing. A focus of the work has been the communication of the potential control of a range of complex physical flows of societal interest using tiny surface perturbations induced by morphing surfaces on and its origin in the robust characteristics of the underlying flow physics. This has been achieved both through the mainstream archival literature and through public outreach via national media demonstrations of the influence of surface roughness on flow over the controversial 2010 World Cup soccer ball, on-campus connections with high-schoolers and the general public via a public lecture on turbulence.

National Science Foundation (NSF)
Division of Chemical, Bioengineering, Environmental, and Transport Systems (CBET)
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Dimitrios Papavassiliou
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California Institute of Technology
United States
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