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Kwok K.,Graduate Aerospace Laboratories | Pellegrino S.,California Institute of Technology
AIAA Journal | Year: 2013

This paper presents an experimental and numerical study of the folding, stowage, and deployment behavior of viscoelastic tape springs. Experiments show that during folding the relationship between load and displacement is nonlinear and varies with rate and temperature. In particular, the limit and propagation loads increase with the folding rate but decrease with temperature. During stowage, relaxation behavior leads to a reduction in internal forces that significantly impacts the subsequent deployment dynamics. The deployment behavior starts with a short, dynamic transient that is followed by a steady deployment and ends with a slow creep recovery. Unlike elastic tape springs, localized folds in viscoelastic tape springs do not move during deployment. Finite-element simulations based on a linear viscoelastic constitutive model with an experimentally determined relaxation modulus are shown to accurately reproduce the experimentally observed behavior, and to capture the effects of geometric nonlinearity, time and temperature dependence. Copyright © 2013 by the American Institute of Aeronautics and Astronautics, Inc. Source

Tan L.T.,University College London | Tan L.T.,Structural Engineering Group | Pellegrino S.,California Institute of Technology | Pellegrino S.,Graduate Aerospace Laboratories
AIAA Journal | Year: 2012

This paper presents an experimental and computational study of four deployable reflectors with collapsible edge stiffeners, to verify the differences in behavior that had been predicted in a previous theoretical study. The experimental models have different geometric configurations and are made of two different plastics. Both folding experiments and vibration tests in the fully deployed configuration are carried out on each model, and it is shown that good correlation with finite element simulations can be achieved if detailed effects such as material nonlinearity, geometric imperfections, air, and gravity effects are included in the computer models. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Source

Mallikarachchi H.M.Y.C.,University of Cambridge | Pellegrino S.,California Institute of Technology | Pellegrino S.,Graduate Aerospace Laboratories
Journal of Spacecraft and Rockets | Year: 2011

Deployable structures made from ultrathin composite materials can be folded elastically and are able to selfdeploy by releasing the stored strain energy. This paper presents a detailed study of the folding and deployment of a tape-spring hinge made from atwo-ply plain-weave laminate of carbon-fiber reinforced plastic. Aparticular version of this hinge was constructed, and its moment-rotation profile during quasi-static deployment was measured. The present study is the first to incorporate in the simulation an experimentally validated elastic micromechanical model and to provide quantitative comparisons between the simulations and the measured behavior of an actual hinge. Folding and deployment simulations of the tape-spring hinge were carried out with the commercial finite element package Abaqus/Explicit, starting from the as-built unstrained structure. The folding simulation includes the effects of pinching the hinge in the middle to reduce the peak moment required to fold it. The deployment simulation fully captures both the steady-state moment part of the deployment and the final snap back tothe deployed configuration. An alternative simulation without pinching the hinge provides an estimate of the maximum moment that could be carried by the hinge during operation. This is about double the snapback moment. Source

Chen C.,University of Arizona | Seele R.,California Institute of Technology | Seele R.,Graduate Aerospace Laboratories | Wygnanski I.,University of Arizona
AIAA Journal | Year: 2012

The application of blowing over a thick elliptical airfoil is considered for the purpose of identifying the hierarchy of additional parameters affecting the flow, such as slot width, its location, and orientation, in addition to the mass and momentum flow that were considered as being significant in the past. When a single jet emanates from a narrow slot at a prescribed location, the incremental lift coefficient scales mostly with the momentum coefficient (C μ). For wider slots, the slot width has to be considered as an independent parameter even when the jet velocity is much higher than the freestream. The lift increment depends strongly on the slot location when the latter is near the natural separation location of the flow, but it is independent of Reynolds number. Deleterious effects of steady blowing on C L were observed for low momentum inputs. For wider slots located upstream of the natural separation location, the drag is sensitive to Reynolds number and it is not solely affected by C μ. Flowfield measurements are used to explain some of the observations made. © 2012 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION. Source

Oberleithner K.,TU Berlin | Paschereit C.O.,TU Berlin | Seele R.,California Institute of Technology | Seele R.,Graduate Aerospace Laboratories | Wygnanski I.,University of Arizona
AIAA Journal | Year: 2012

This study provides quantitative insight into the formation of vortex breakdown and the onset of global instability in a turbulent swirling jet. A water jet is guided through a rotating honeycomb that imparts the rotational motion, passed through a contraction, and discharged into a large water tank. The flow states evolving at increasing swirl are mapped out via time-resolved particle image velocimetry. The experimental results scale properly with the swirl number based on the axial momentum flux when the commonly used boundary-layer approximations are omitted. The instantaneous velocity field reveals that vortex breakdown occurs intermittently at a wide range of swirl numbers before it appears in the mean flow. At this intermittent state, the evolving breakdown bubble oscillates heavily between two streamwise locations where the vortex core is subcritical. Upon further increasing the swirl, the breakdown oscillations decay and a region of reversed flow appears in the mean flowfield. The formation of this socalled axisymmetric breakdown state is accompanied by a supercritical-to-subcritical transition of the inflowing vortex core. The reversed flow region is found to grow linearly with increasing swirl until the flow undergoes a supercritical Hopf bifurcation to a global single-helical mode, and vortex breakdown adopts a spiral shape. The global mode shape is extracted from the particle image velocimetry snapshots by means of proper orthogonal decomposition and Fourier analysis. The present experiment reveals that, at gradually increasing swirl, the jet first transitions to an axisymmetric breakdown state that remains globally stable until a critical swirl number is exceeded. This sequence of flow states agrees well with the transient formation of vortex breakdown observed in laminar flows. Copyright © 2012 by Kilian Oberleithner. Source

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