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Caltech Authors (147.820 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 1.570

  1. Experiments on imperfection insensitive axially loaded cylindrical shells

    Ning, Xin; Pellegrino, Sergio
    This paper presents an experimental study of imperfection insensitive composite wavy cylindrical shells subject to axial compression. A fabrication technique for making cylindrical shells with intricate shape of cross-sections has been developed. A photogrammetry technique to measure the geometric imperfections has also been developed. The behavior of the wavy shells under axial compression was predicted through simulations and measured through compression tests. Both the analyses and experiments have confirmed that the wavy shells are imperfection insensitive. Comparisons between the wavy shells and circular shells have also confirmed that introducing optimal symmetry-breaking wavy cross-sections can significantly reduce the imperfection sensitivity and...

  2. Flow-induced vibration of a circular cylinder surrounded by two, four and eight wake-control cylinders

    Silva-Ortega, M.; Assi, G. R. S.
    The present work investigates the use of a polar array of 2, 4 and 8 wake-control cylinders as a means to suppress the vortex-induced vibration (VIV) of a larger circular cylinder. The diameter of the control cylinders and the gap between their walls have been varied in 27 different configurations. Experiments have been performed in water at Reynolds numbers between 5000 and 50,000. Cross-flow amplitude of displacement, frequency of vibration, mean drag and fluctuating lift coefficients are presented. While some configurations of control cylinders suppressed VIV, others produced a galloping-like response. The best VIV suppressor was composed of 8 control...

  3. Reducing Versatile Bat Wing Conformations to a 1-DoF Machine

    Hoff, Jonathan; Ramezani, Alireza; Chung, Soon-Jo; Hutchinson, Seth
    Recent works have shown success in mimicking the flapping flight of bats on the robotic platform Bat Bot (B2). This robot has only five actuators but retains the ability to flap and fold-unfold its wings in flight. However, this bat-like robot has been unable to perform folding-unfolding of its wings within the period of a wingbeat cycle, about 100 ms. The DC motors operating the spindle mechanisms cannot attain this folding speed. Biological bats rely on this periodic folding of their wings during the upstroke of the wingbeat cycle. It reduces the moment of inertia of the wings and limits...

  4. From Rousettus aegyptiacus (bat) Landing to Robotic Landing: Regulation of CG-CP Distance Using a Nonlinear Closed-Loop Feedback

    Syed, Usman A.; Ramezani, Alireza; Chung, Soon-Jo; Hutchinson, Seth
    Bats are unique in that they can achieve unrivaled agile maneuvers due to their functionally versatile wing conformations. Among these maneuvers, roosting (landing) has captured attentions because bats perform this acrobatic maneuver with a great composure. This work attempts to reconstruct bat landing maneuvers with a Micro Aerial Vehicle (MAV) called Allice. Allice is capable of adjusting the position of its Center of Gravity (CG) with respect to the Center of Pressure (CP) using a nonlinear closed-loop feedback. This nonlinear control law, which is based on the method of input-output feedback linearization, enables attitude regulations through variations in CG-CP distance....

  5. Describing Robotic Bat Flight with Stable Periodic Orbits

    Ramezani, Alireza; Ahmed, Syed Usman; Hoff, Jonathan; Chung, Soon-Jo; Hutchinson, Seth
    From a dynamic system point of view, bat locomotion stands out among other forms of flight. During a large part of bat wingbeat cycle the moving body is not in a static equilibrium. This is in sharp contrast to what we observe in other simpler forms of flight such as insects, which stay at their static equilibrium. Encouraged by biological examinations that have revealed bats exhibit periodic and stable limit cycles, this work demonstrates that one effective approach to stabilize articulated flying robots with bat morphology is locating feasible limit cycles for these robots; then, designing controllers that retain the closed-loop...

  6. Co-Design of Strain-Actuated Solar Arrays for Spacecraft Precision Pointing and Jitter Reduction

    Chilan, Christian M.; Herber, Daniel R.; Nakka, Yashwanth Kumar; Chung, Soon-Jo; Allison, James T.; Aldrich, Jack B.; Alvarez-Salazar, Oscar S.
    This work presents a novel spacecraft attitude control architecture using strain-actuated solar arrays (SASA) that does not require the use of conventional attitude control hardware. SASA enables attitude slewing maneuvers and precision-pointing (image acquisition) stares, while simultaneously suppressing structural vibrations. Distributed piezoelectric actuators help achieve higher precision, higher bandwidth, and quieter operation than reaction wheels. To understand the design tradeoffs for this architecture, a framework for the integrated design of distributed structural geometry and distributed control is presented. The physical properties of the array are modeled and designed with respect to a piecewise linear distributed thickness profile. The distributed control...

  7. Three-dimensional director-field predictions of viral DNA packing arrangements

    Klug, W. S.; Feldmann, M. T.; Ortiz, M.
    In this work we develop a discrete director-field model for coarse-grained description of packing arrangements of DNA within bacteriophage virus heads. This computational lattice model allows us to explore the complex energy landscape of fully three-dimensional configurations of packaged DNA. By minimizing the system’s free energy by means of the simulated annealing and the conjugate gradient methods, we make predictions about favorable packing conformations. In particular we show that the planar-wrapped inverse spool conformation is stable everywhere inside a virus except in a central core region, where the DNA tends to buckle out of the spooling plane.

  8. Visual-Inertial Curve SLAM: Creating a Sparse Structured World without Feature Points

    Meier, Kevin; Chung, Soon-Jo; Hutchinson, Seth
    We present a simultaneous localization and mapping (SLAM) algorithm that uses Bézier curves as static landmark primitives rather than feature points. Our approach allows us to estimate the full 6-DOF pose of a robot while providing a structured map which can be used to assist a robot in motion planning and control. We demonstrate how to reconstruct the 3-D location of curve landmarks from a stereo pair and how to compare the 3-D shape of curve landmarks between chronologically sequential stereo frames to solve the data association problem. We also present a method to combine curve landmarks for mapping purposes,...

  9. On the mechanics of surface and side-wall chipping from line-wedge indentation

    Chai, Herzl; Ravichandran, Guruswami
    Chipping in glass plates from line-wedge contact loading is studied as function of the wedge’s subtended angle 2β, its inclination angle ϕ, and the distance h from a corner having a subtended angle 90° − θ A brittle-fracture analysis in conjunction with the FEM technique is used to elucidate the role of geometric variables on chip morphology and chipping load. Closed-form relations are developed for the latter by invoking the principle of geometric similarity and taking into consideration the details of contact forces transmitted to the crack mouth. The fracture progresses stably until surface effects alter the crack trajectory to...

  10. Non-uniform, axisymmetric misfit strain: in thin films bonded on plate substrates/substrate systems: the relation between non-uniform film stresses and system curvatures

    Huang, Yonggang; Ngo, D.; Rosakis, A. J.
    Current methodologies used for the inference of thin film stress through curvature measurements are strictly restricted to stress and curvature states which are assumed to remain uniform over the entire film/substrate system. By considering a circular thin film/substrate system subject to non-uniform, but axisymmetric misfit strain distributions in the thin film, we derived relations between the film stresses and the misfit strain, and between the plate system’s curvatures and the misfit strain. These relations feature a "local" part which involves a direct dependence of the stress or curvature components on the misfit strain at the same point, and a "non-local"...

  11. Distances and diameters in concentration inequalities: from geometry to optimal assignment of sampling resources

    Sullivan, Tim J.; Owhadi, Houman
    This note reviews, compares and contrasts three notions of "distance" or "size" that arise often in concentration-of-measure inequalities. We review Talagrand′s convex distance and McDiarmid′s diameter, and consider in particular the normal distance on a topological vector space

  12. Detonation wave diffraction in H_2–O_2–Ar mixtures

    Gallier, S.; Le Palud, F.; Pintgen, F.; Mével, R.; Shepherd, J. E.
    In the present study, we have examined the diffraction of detonation in weakly unstable hydrogen–oxygen–argon mixtures. High accuracy and computational efficiency are obtained using a high-order WENO scheme together with adaptive mesh refinement, which enables handling realistic geometries with resolution at the micrometer level. Both detailed chemistry and spectroscopic models of laser induced fluorescence and chemiluminescence were included to enable a direct comparison with experimental data. Agreement was found between the experiments and the simulations in terms of detonation diffraction structure both for sub-critical and super-critical regimes. The predicted wall reflection distance is about 12–14 cell widths, in accordance with...

  13. Experimental and numerical studies of fuel and hydrodynamic effects on piloted turbulent premixed jet flames

    Smolke, Jennifer; Lapointe, Simon; Paxton, Laurel; Blanquart, Guillaume; Carbone, Francesco; Fincham, Adam M.; Egolfopoulos, Fokion N.
    An experimental and numerical investigation of fuel and hydrodynamic effects is performed on piloted premixed jet flames. The investigation is carried out at a constant laminar flame speed, varying heat losses, jet Reynolds number, fuel molecular weight, and fuel chemical classification. Large Eddy Simulations are performed in an attempt to reproduce the behaviors observed experimentally. Simulations are compared against well-characterized boundary conditions, well-resolved two-dimensional velocity fields from particle image velocimetry, and line-of-sight CH⁎ profiles. Experimental results indicate that small amounts of heat losses may play a significant role on the jet reactivity as the flame heights scale with the heat...

  14. Linking Internal Dissipation Mechanisms to the Effective Complex Viscoelastic Moduli of Ferroelectrics

    Wojnar, Charles S.; Kochmann, Dennis M.
    Microstructural mechanisms such as domain switching in ferroelectric ceramics dissipate energy, the nature, and extent of which are of significant interest for two reasons. First, dissipative internal processes lead to hysteretic behavior at the macroscale (e.g., the hysteresis of polarization versus electric field in ferroelectrics). Second, mechanisms of internal friction determine the viscoelastic behavior of the material under small-amplitude vibrations. Although experimental techniques and constitutive models exist for both phenomena, there is a strong disconnect and, in particular, no advantageous strategy to link both for improved physics-based kinetic models for multifunctional rheological materials. Here, we present a theoretical approach that...

  15. Experimental Validation of Metaconcrete Blast Mitigation Properties

    Briccola, Deborah; Ortiz, Michael; Pandolfi, Anna
    We provide experimental evidence of the mitigation properties of metaconcrete under blast loading. Mitigation is achieved through resonance of engineered aggregates consisting of a heavy and stiff core coated by a light and compliant outer layer. These engineered aggregates replace the standard gravel in conventional concrete. To assess experimentally the attenuation properties of metaconcrete, we have cast two batches of cylindrical specimens. The mortar matrix of the first batch consists of cement combined with a regular sand mix, while the mortar matrix of the second batch consists of cement combined with sand mix, fine gravel, and polymeric fibers. One of...

  16. Calculation of Optimal Bounds on the Probability of Failure of Soft Biological Tissues

    Balzani, Daniel; Schmidt, Thomas; Ortiz, Michael
    In this contribution, a methodology for the calculation of optimal bounds on the probability of failure of soft biological tissues is presented. Two potential rupture criteria are considered and an uncertainty quantification method [1] is applied to a virtual experimental data set. The results for both criteria are compared in a finite element example.

  17. Atomimetic Mechanical Structures with Nonlinear Topological Domain Evolution Kinetics

    Frazier, Michael J.; Kochmann, Dennis M.
    A mechanical metamaterial, a simple, periodic mechanical structure, is reported, which reproduces the nonlinear dynamic behavior of materials undergoing phase transitions and domain switching at the structural level. Tunable multistability is exploited to produce switching and transition phenomena whose kinetics are governed by the same Allen–Cahn law commonly used to describe material-level, structural-transition processes. The reported purely elastic mechanical system displays several key features commonly found in atomic- or mesoscale physics of solids. The rotating-mass network shows qualitatively analogous features as, e.g., ferroic ceramics or phase-transforming solids, and the discrete governing equation is shown to approach the phase field equation...

  18. Post-cure shape errors of ultra-thin symmetric CFRP laminates: Effect of ply-level imperfections

    Steeves, John; Pellegrino, Sergio
    This paper discusses the effect of misalignments in ply orientation, uniform variations in ply thickness, and through-thickness thermal gradients on the post-cure shape errors for symmetric cross-ply laminates constructed from ultra-thin composite materials. Photogrammetry-based surface measurements are performed for laminates cured at elevated temperatures. Significant out-of-plane shape errors are observed, with amplitudes ∼75 times the laminate thickness. The magnitude of each imperfection is also characterized experimentally on coupon-level samples. A non-linear finite element model is developed and demonstrates that these imperfections result in cylindrical and twisting modes of deformation. Results are compared to Classical Lamination Theory predictions which are shown...

  19. Effects of Shock-Tube Cleanliness on Hypersonic Boundary Layer Transition at High Enthalpy

    Jewell, Joseph S.; Parziale, Nicholaus J.; Leyva, Ivett A.; Shepherd, Joseph E.
    The prediction of a high-speed boundary-layer transition (BLT) location is critical to hypersonic vehicle design; this is because the increased skin friction and surface heating rate after transition result in increased weight of the thermal protection system. Experimental studies using hypervelocity wind tunnels are one component of BLT research.

  20. Micromechanics Models for Viscoelastic Plain-Weave Composite Tape Springs

    Kwok, Kawai; Pellegrino, Sergio
    The viscoelastic behavior of polymer composites decreases the deployment force and the postdeployment shape accuracy of composite deployable space structures. This paper presents a viscoelastic model for single-ply cylindrical shells (tape springs) that are deployed after being held folded for a given period of time. The model is derived from a representative unit cell of the composite material, based on the microstructure geometry. Key ingredients are the fiber volume density in the composite tows and the constitutive behavior of the fibers (assumed to be linear elastic and transversely isotropic) and of the matrix (assumed to be linear viscoelastic). Finite-element-based homogenizations...

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