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Caltech Authors (144.724 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 1.561

  1. 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...

  2. 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...

  3. 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...

  4. 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...

  5. 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.

  6. 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...

  7. 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...

  8. 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.

  9. 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...

  10. Herding a Flock of Birds Approaching an Airport Using an Unmanned Aerial Vehicle

    Gade, Shripad; Paranjape, Aditya A.; Chung, Soon-Jo
    The problem of herding a flock of birds is posed in a graph theoretic framework. A novel algorithm, called the n-wavefront algorithm, Is developed for enabling a single unmanned aerial vehicle to herd a flock of birds to a desired point. The technique is applied to the problem of diverting a flock of birds approaching an airport away from a protected zone around the airport. The n-wavefront algorithm is demonstrated in simulation and compared with existing strategies using graph-based metrics.

  11. Phase relations in a forced turbulent boundary layer: implications for modelling of high Reynolds number wall turbulence

    Duvvuri, Subrahmanyam; McKeon, Beverley
    Phase relations between specific scales in a turbulent boundary layer are studied here by highlighting the associated nonlinear scale interactions in the flow. This is achieved through an experimental technique that allows for targeted forcing of the flow through the use of a dynamic wall perturbation. Two distinct large-scale modes with well-defined spatial and temporal wavenumbers were simultaneously forced in the boundary layer, and the resulting nonlinear response from their direct interactions was isolated from the turbulence signal for the study. This approach advances the traditional studies of large- and small-scale interactions in wall turbulence by focusing on the direct...

  12. Scaling and interaction of self-similar modes in models of high Reynolds number wall turbulence

    Sharma, A. S.; Moarref, R.; McKeon, B. J.
    Previous work has established the usefulness of the resolvent operator that maps the terms nonlinear in the turbulent fluctuations to the fluctuations themselves. Further work has described the self-similarity of the resolvent arising from that of the mean velocity profile. The orthogonal modes provided by the resolvent analysis describe the wall-normal coherence of the motions and inherit that self-similarity. In this contribution, we present the implications of this similarity for the nonlinear interaction between modes with different scales and wall-normal locations. By considering the nonlinear interactions between modes, it is shown that much of the turbulence scaling behaviour in the...

  13. Magnetohydrodynamic implosion symmetry and suppression of Richtmyer-Meshkov instability in an octahedrally symmetric field

    Mostert, W.; Pullin, D. I.; Wheatley, V.; Samtaney, R.
    We present numerical simulations of ideal magnetohydrodynamics showing suppression of the Richtmyer-Meshkov instability in spherical implosions in the presence of an octahedrally symmetric magnetic field. This field configuration is of interest owing to its high degree of spherical symmetry in comparison with previously considered dihedrally symmetric fields. The simulations indicate that the octahedral field suppresses the instability comparably to the other previously considered candidate fields for light-heavy interface accelerations while retaining a highly symmetric underlying flow even at high field strengths. With this field, there is a reduction in the root-mean-square perturbation amplitude of up to approximately 50% at representative...

  14. A linearized porous brittle damage material model with distributed frictional-cohesive faults

    De Bellis, M. L.; Della Vecchia, G.; Ortiz, M.; Pandolfi, A.
    We present a simplified model of damaging porous material, obtained through consistent linearization from a recursive-faulting material model described in (Pandolfi et al. 2016). The brittle damage material model is characterized by special planar micro-structures, consisting of nested families of equi-spaced frictional-cohesive faults in an otherwise elastic matrix material. The linear kinematics model preserves the main microstructural features of the finite kinematics one but offers a far better computational performance. Unlike models commonly employed in geo-mechanical applications, the proposed model contains a small number of parameters, to wit, two elastic moduli, three frictional-cohesive parameters, and three hydraulic response parameters, all...

  15. Probabilistic swarm guidance using optimal transport

    Bandyopadhyay, Saptarshi; Chung, Soon-Jo; Hadaegh, Fred Y.
    Probabilistic swarm guidance enables autonomous agents to generate their individual trajectories independently so that the entire swarm converges to the desired distribution shape. In contrast with previous homogeneous or inhomogeneous Markov chain based approaches [1], this paper presents an optimal transport based approach which guarantees faster convergence, minimizes a given cost function, and reduces the number of transitions for achieving the desired formation. Each agent first estimates the current swarm distribution by communicating with neighboring agents and using a consensus algorithm and then solves the optimal transport problem, which is recast as a linear program, to determine its transition probabilities....

  16. Stability of slender inverted flags and rods in uniform steady flow

    Sader, John E.; Huertas-Cerdeira, Cecilia; Gharib, Morteza
    Cantilevered elastic sheets and rods immersed in a steady uniform flow are known to undergo instabilities that give rise to complex dynamics, including limit cycle behaviour and chaotic motion. Recent work has examined their stability in an inverted configuration where the flow impinges on the free end of the cantilever with its clamped edge downstream: this is commonly referred to as an ‘inverted flag’. Theory has thus far accurately captured the stability of wide inverted flags only, i.e. where the dimension of the clamped edge exceeds the cantilever length; the latter is aligned in the flow direction. Here, we theoretically...

  17. Modeling of near-continuum laminar boundary layer shocks using DSMC

    Tumuklu, Ozgur; Levin, Deborah A.; Gimelshein, Sergey F.; Austin, Joanna M.
    The hypersonic flow of nitrogen gas over a double wedge was simulated by the DSMC method using two-dimensional and three-dimensional geometries. The numerical results were compared with experiments conducted in the HET facility for a high-enthalpy pure nitrogen flow corresponding to a free stream Mach number of approximately seven. Since the conditions for the double wedge case are near-continuum and surface heat flux and size of the separation are sensitive to DSMC numerical parameters, special attention was paid to the convergence of these parameters for both geometries. At the beginning of the simulation, the separation zone was predicted to be...

  18. Factors influencing flow steadiness in laminar boundary layer shock interactions

    Tumuklu, Ozgur; Levin, Deborah A.; Gimelshein, Sergey F.; Austin, Joanna M.
    The Direct Simulation Monte Carlo method has been used to model laminar shock wave boundary interactions of hypersonic flow over a 30/55-deg double-wedge and “tick-shaped” model configurations studied in the Hypervelocity Expansion Tube facility and T-ADFA free-piston shock tunnel, respectively. The impact of thermochemical effects on these interactions by changing the chemical composition from nitrogen to air as well as argon for a stagnation enthalpy of 8.0 MJ/kg flow are investigated using the 2-D wedge model. The simulations are found to reproduce many of the classic features related to Edney Type V strong shock interactions that include the attached, oblique...

  19. Compact, lensless digital holographic microscope for remote microbiology

    Serabyn, Eugene; Liewer, Kurt; Lindensmith, Chris; Wallace, Kent; Nadeau, Jay
    In situ investigation of microbial life in extreme environments can be carried out with microscopes capable of imaging 3-dimensional volumes and tracking particle motion. Here we present a lensless digital holographic microscope approach that provides roughly 1.5 micron resolution in a compact, robust package suitable for remote deployment. High resolution is achieved by generating high numerical-aperture input beams with radial gradient-index rod lenses. The ability to detect and track prokaryotes was explored using bacterial strains of two different sizes. In the larger strain, a variety of motions were seen, while the smaller strain was used to demonstrate a detection capability...

  20. Experimental Demonstration of Perching by an Articulated Wing MAV

    Paranjape, Aditya; Kim, Joseph; Gandhi, Nihar; Chung, Soon-Jo
    This paper presents an experimental demonstration of perching by a micro aerial vehicle (MAV) equipped with articulated wings. A novel feature of the MAV considered in this paper is that wing dihedral, controlled independently on both wings, is used for yaw stability and control as well as for maintaining the flight path angle. Yaw stability and control are essential for perching in tightly constrained places. The experiments described in this paper were conducted indoors and flight parameters are measured using the VICON motion capture system.

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