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Caltech Authors (167.915 recursos)

Repository of works by Caltech published authors.

Group = GALCIT

Mostrando recursos 1 - 20 de 410

  1. Charge carrier transport across grain boundaries in graphene

    Mendez, J. P.; Arca, F.; Ramos, J.; Ortiz, M.; Ariza, M. P.
    We evaluate the charge carrier transmission across asymmetric grain boundaries (GB) in a graphene lattice within the Landauer-Büttiker formalism. We employ a tight-binding model for C-based materials that accounts for lattice strain introduced by topological defects, such as grain boundaries. In particular, we investigate electronic transmission across grain boundaries found to be stable up to high temperatures. Our calculations suggest that the introduction of GBs generally preserves the zero-transport gap property of pristine graphene. However, only some specific asymmetric GBs open a moderate transport gap, which can be as high as ≈ 1.15 eV. We find that the GBs that...

  2. Non-normality and classification of amplification mechanisms in stability and resolvent analysis

    Symon, Sean; Rosenberg, Kevin; Dawson, Scott T. M.; McKeon, Beverley J.
    Eigenspectra and pseudospectra of the mean-linearized Navier-Stokes operator are used to characterize amplification mechanisms in laminar and turbulent flows in which linear mechanisms are important. Success of mean flow (linear) stability analysis for a particular frequency is shown to depend on whether two scalar measures of non-normality agree: (1) the product between the resolvent norm and the distance from the imaginary axis to the closest eigenvalue and (2) the inverse of the inner product between the most amplified resolvent forcing and response modes. If they agree, the resolvent operator can be rewritten in its dyadic representation to reveal that the...

  3. Three-dimensional Floquet stability analysis of the wake of a circular cylinder

    Barkley, Dwight; Henderson, Ronald D.
    Results are reported from a highly accurate, global numerical stability analysis of the periodic wake of a circular cylinder for Reynolds numbers between 140 and 300. The analysis shows that the two-dimensional wake becomes (absolutely) linearly unstable to three-dimensional perturbations at a critical Reynolds number of 188.5±1.0. The critical spanwise wavelength is 3.96 ± 0.02 diameters and the critical Floquet mode corresponds to a ‘Mode A’ instability. At Reynolds number 259 the two-dimensional wake becomes linearly unstable to a second branch of modes with wavelength 0.822 diameters at onset. Stability spectra and corresponding neutral stability curves are presented for Reynolds...

  4. Mixing, scalar boundedness, and numerical dissipation in large-eddy simulations

    Sharan, Nek; Matheou, Georgios; Dimotakis, Paul E.
    Numerical schemes for scalar transport and mixing in turbulent flows must be high-order accurate, and observe conservation and boundedness constraints. Discretization accuracy can be evaluated from the truncation error, and assessed by its dispersion and dissipation properties. Dispersion errors can cause violation of physical scalar bounds, whereas numerical dissipation is key to mitigating those violations. Numerical dissipation primarily alters the energy at small scales that are critical to turbulent mixing. Influence of additional dissipation on scalar mixing in large-eddy simulations (LES) of incompressible temporally evolving shear flow is examined in terms of the resolved passive-scalar field, z. Scalar fields in...

  5. Relation between a singly-periodic roughness geometry and spatio-temporal turbulence characteristics

    Morgan, J.; McKeon, B. J.
    The structure of a turbulent boundary layer over a singly-periodic roughness of large wavelength is shown to give insight into the physics of rough-wall boundary layers. To this end, a roughness consisting of a single spanwise-varying mode and a single streamwise-varying mode was 3D printed with wavelengths on the order of the boundary layer thickness. The large length scale introduced by such a roughness creates spatial inhomogeneity of the mean velocity field throughout the entire boundary layer. A hot-wire probe was used to take time series of streamwise velocity at a grid of points in the x,y, and z directions,...

  6. Nonlinear vibration of transversely-loaded spinning membranes

    Delapierre, Mélanie; Lohaus, Stefan Haegeli; Pellegrino, Sergio
    The paper studies the transverse nonlinear vibration of an isotropic and homogeneous annular membrane spinning at constant angular velocity, under the action of a uniform transverse load. The equilibrium configuration of the membrane, clamped along the inner edge and free along the outer edge, and the natural frequencies of vibration of the membrane are calculated. A Galerkin procedure is used to determine a reduced order model describing the weakly nonlinear vibration of the membrane, and it is shown that near-resonance vibration can be modeled as a single degree of freedom Helmholtz-Duffing oscillator. A detailed study at vibration frequencies close to...

  7. Stress fluctuation, crack renucleation and toughening in layered materials

    Hsueh, C.-J.; Avellar, L.; Bourdin, B.; Ravichandran, G.; Bhattacharya, K.
    It has been established that contrast in the elastic properties can lead to enhancement of fracture toughness in heterogeneous materials. Focussing on layered materials as a model system, we show that this enhancement is a result of two distinct phenomena – first, fluctuations in stress leading to regions where the stress intensity at the crack is considerably smaller than that of the macroscopically applied value; and second, the lack of stress intensity when a crack is at a compliant to stiff interface thereby requiring renucleation. Using theoretical, computational and experimental methods, we study two geometries – a layered material and...

  8. Scintillation Yield Estimates of Colloidal Cerium-Doped LaF_3 Nanoparticles and Potential for “Deep PDT”

    Kudinov, Konstantin A.; Cooper, Daniel R.; Ha, Jonathan K.; Hill, Colin K.; Nadeau, Jay L.; Seuntjens, Jan P.; Bradforth, Stephen E.
    A hybrid of radiotherapy and photodynamic therapy (PDT) has been proposed in previously reported studies. This approach utilizes scintillating nanoparticles to transfer energy to attached photosensitizers, thus generating singlet oxygen for local killing of malignant cells. Its effectiveness strongly depends upon the scintillation yield of the nanoparticles. Using a liquid scintillator as a reference standard, we estimated the scintillation yield of Ce_(0.1)La_(0.9)F_3/LaF_3 core/shell nanoparticles at 28.9 mg/ml in water to be 350 photons/MeV under orthovoltage X-ray irradiation. The subsequent singlet oxygen production for a 60 Gy cumulative dose to cells was estimated to be four orders of magnitude lower than...

  9. Atomistic Simulation of Hydrogen Diffusion in Palladium Nanoparticles Using a Diffusive Molecular Dynamics Method

    Sun, Xingsheng; Ariza, Pilar; Ortiz, Michael; Wang, Kevin G.
    Understanding the transport of hydrogen within metals is crucial for the advancement of energy storage and the mitigation of hydrogen embrittlement. Using nanosized palladium particles as a model, recent experimental studies have revealed several highly nonlinear phenomena that occur over a long period of time. The time scale of these phenomena is beyond the capability of established atomistic models. In this work, we present the application of a new model, referred to as diffusive molecular dynamics (DMD), to simulating long-term diffusive mass transport at atomistic length scale. Specifically, we validate the model for the long-term dynamics of a single hydrogen...

  10. CTH shock physics simulation of non-linear material effects within an aerospace CFRP fastener assembly due to direct lightning attachment

    Kirchdoerfer, T.; Liebscher, A.; Ortiz, M.
    This study examines detailed CTH simulation results of a carbon fiber composite panel assembly where the binding titanium fastener joint is the attachment point of a direct lightning strike. A refined COMSOL Multiphysics electromagnetics calculation is performed on the undeformed installed fastener geometry to obtain current and electric field solutions in time using SAE ARP5412B lightning test waveforms. Electro-mechanical coupling is achieved through the use of Joule heating resulting from J·E, the power per unit volume dispersed throughout the structure due to the lightning energy transport. The shock physics code CTH is used to model the fluid-structure calculations which involve...

  11. Rayleigh Waves Generated by a Thermal Source: A Three-Dimensional Transient Thermoelasticity Solution

    Georgiadis, H. G.; Lykotrafitis, G.
    A three-dimensional transient thermoelastic solution is obtained for Rayleigh-type disturbances propagating on the surface of a half-space. These surface waves are generated by either a buried or surface thermal source, which has the form of a concentrated heat flux applied impulsively. In an effort to model this problem as realistically as possible, the half-space material is taken to respond according to Biot’s fully coupled thermoelasticity. The problem has relevance to situations involving heat generation due to: (i) laser action (impulsive electromagnetic radiation) on a surface target, (ii) underground nuclear activity, and (iii) friction developed during underground fault motions related to...

  12. Heterodyne transverse velocimetry for pressure-shear plate impact experiments

    Kettenbeil, Christian; Mello, Michael; Bischann, Moriah; Ravichandran, Guruswami
    Pressure-shear plate impact experiments have traditionally relied on free space beam interferometers to measure transverse and normal particle velocities at the rear surface of the target plate. Here, we present two different interferometry schemes that leverage heterodyne techniques, which enable the simultaneous measurement of normal and transverse velocities using short-time Fourier transforms. Both techniques rely on diffracted 1st order beams that are generated by a specular, metallic grating deposited on the rear surface of the target plate. The diffracted beam photonic Doppler velocimetry technique interferes each 1st order beam with a reference of slightly higher wavelength to create a constant...

  13. A Simple Model of Mixing and Chemical Reaction in a Turbulent Shear Layer

    Broadwell, J. E.; Breidenthal, R. E.

  14. Theoretical and Experimental Investigations of Axial Flow Compressors

    Bowen, John T.; Sabersky, Rolf H.; Rannie, W. Duncan; Millikan, Clark B.

  15. Water Tunnel Tests on NACA-66 (Modified) and YS-920 Foil Sections with and whitout Surface Roughness

    Baloga, Paul E.

  16. A Research Program on Solid Propellant Physical Behavior

    Blatz, P. J.; Tschoegl, N. W.; Landel, R. F.

  17. Physicomechanical Behavior of Rubberlike Materials

    Blatz, P. J.

  18. A Scaling Technique for the Design of Idealized Electromagnetic Lenses

    Baum, Carl E.

  19. Constant Pressure Laminar Mixing with Arbitrary Initial Velocity and Stagnation Enthalpy Profiles

    Reeves, Barry L.

  20. Periodic Cavity Oscillations, Shear flow Instabilities and Nonequilibrium Turbulent Wakes

    Behrens, Wilhelm; Sarohia, Virendra

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