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Caltech Authors (142.336 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 1.551

  1. 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.

  2. 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...

  3. 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.

  4. 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...

  5. 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...

  6. 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...

  7. 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...

  8. 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....

  9. 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...

  10. 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...

  11. 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...

  12. 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...

  13. 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.

  14. Output feedback stabilization of linear PDEs with finite dimensional input-output maps and Kelvin-Voigt damping

    Paranjape, Aditya A.; Chung, Soon-Jo
    In this paper, we consider systems of partial differential equations with a finite relative degree between the input and the output. In such systems, an output feedback controller can be constructed to regulate the output with the desired convergence properties. Although the zero dynamics are infinite dimensional, we show that the controller alters the boundary conditions in such a way that it leads to a predictable expansion in the stable operating envelope of the system. Moreover, the expansion of the stable envelope depends only on the boundary conditions and the structure of the PDE, and is independent of the system...

  15. PDE Boundary Control for Flexible Articulated Aircraft Wings

    Paranjape, Aditya A.; Chung, Soon-Jo; Krstic, Miroslav
    The paper considers a boundary control formulation for PDEs with a system output given by a spatial integral of weighted functions of the state. This formulation is directly applicable to the control of an aircraft with articulated exible wings, in which case the output of interest is a net aerodynamic force or moment. Flexible wings can be controlled via actuation at the root or the tip. The problem of beam twist is analysed in detail to illustrate the formulation, and it shown that the control law ensures that the error between the desired output signal and the actual output signal decreases...

  16. Performance and Stability of an Agile Tail-less MAV with Flexible Articulated Wings

    Paranjape, Aditya; Chakravarthy, Animesh; Chung, Soon-Jo; Hilton, Harry H.
    This paper considers the problems of (a) modelling the ight mechanics of a tail-less MAV equipped with exible articulated wings, and (b) the analysis of its turning performance. The wings are assumed to have two degrees of freedom - heave and twist. They are assumed to be actuated from the root, which is the abstraction of an experimental control mechanism being developed by the authors. The dihedral and twist angles at the wing root are controlled. A novel actuator concept of axial tension to control wing stiffness has been explored in this paper. It is shown that axial tension in the wing...

  17. Probabilistic guidance of distributed systems using sequential convex programming

    Morgan, Daniel; Subramanian, Giri Prashanth; Bandyopadhyay, Saptarshi; Chung, Soon-Jo; Hadaegh, Fred Y.
    In this paper, we integrate, implement, and validate formation flying algorithms for a large number of agents using probabilistic guidance of distributed systems with inhomogeneous Markov chains and model predictive control with sequential convex programming. Using an inhomogeneous Markov chain, each agent determines its target position during each iteration in a statistically independent manner while the distributed system converges to the desired formation. Moreover, the distributed system is robust to external disturbances or damages to the formation. Once the target positions are assigned, an optimal control problem is formulated to ensure that the agents reach the target positions while avoiding...

  18. Switched systems with multiple invariant sets

    Dorothy, Michael; Chung, Soon-Jo
    This paper explores dwell time constraints on switched systems with multiple, possibly disparate invariant limit sets. We show that, under suitable conditions, trajectories globally converge to a superset of the limit sets and then remain in a second, larger superset. We show the effectiveness of the dwell-time conditions by using examples of switching limit cycles commonly found in robotic locomotion and flapping flight.

  19. Swarm Keeping Strategies for Spacecraft under J_2 and Atmospheric Drag Perturbations

    Morgan, Daniel; Chung, Soon-Jo; Blackmore, Lars; Acikmese, Behcet; Bayard, David S.; Hadaegh, Fred Y.
    This paper presents several new open-loop guidance methods for spacecraft swarms composed of hundreds to thousands of agents with each spacecraft having modest capabilities. These methods have three main goals: preventing relative drift of the swarm, preventing collisions within the swarm, and minimizing the propellant used throughout the mission. The development of these methods progresses by eliminating drift using the Hill-Clohessy-Wiltshire equations, removing drift due to nonlinearity, and minimizing the J_2 drift. In order to verify these guidance methods, a new dynamic model for the relative motion of spacecraft is developed. These dynamics include the two main disturbances for spacecraft...

  20. Spacecraft Swarm Guidance Using a Sequence of Decentralized Convex Optimizations

    Morgan, Daniel; Chung, Soon-Jo; Hadaegh, Fred
    This paper presents partially decentralized path planning algorithms for swarms of spacecraft composed of hundreds to thousands of agents with each spacecraft having limited computational capabilities. In our prior work, J2-invariant orbits have been found to provide collision free motion for hundreds of orbits. This paper develops algorithms for the swarm reconfiguration which involves transferring from one J2-invariant orbit to another avoiding collisions and minimizing fuel. To perform collision avoidance, it is assumed that the spacecraft can communicate their trajectories with each other. The algorithm uses sequential convex programming to solve a series of approximate path planning problems until the solution converges....

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