2,406 research outputs found
Stability Chart of Generalized Bessel Equation
Stability analysis, due to its importance in dynamic analysis of appliedsystems, becomes very widely topic in real-world problems associated with significant effects in all applications of engineering and appliedsciences. Several contributions have been done by various authors onstability analysis of autonomous and non-autonomous systems to obtain interesting and attractive results. In these aforementioned works, and sofar, the Lyapunov methods (first and second) become the main nerve togive suitable criteria that guaranteed the stability of the dynamicalsystems,In this article, it is aimed to look for a semi-analytical relation between the eigenvalue and the order of generalized Bessel differential equation using the asymptotic iteration and perturbation methods for the periodic domain. Based on the analytical results, the stability chart between the eigenvalue and the order of theequation is constructed. Accordingly, through the path of verification and investigation by using the numerical results with Fourier spectral method, a strong fit with the analytical results is observed
Taylor University Catalog 2023-2024
The 2023-2024 academic catalog of Taylor University in Upland, Indiana.https://pillars.taylor.edu/catalogs/1128/thumbnail.jp
The dynamics and control of large space structures with distributed actuation
Future large space structures are likely to be constructed at much greater length-scales, and lower areal mass densities than has been achieved to-date. This could be enabled by ongoing developments in on-orbit manufacturing, whereby large structures are 3D-printed in space from raw feedstock materials. This thesis proposes and analyses a number of attitude control strategies which could be adopted for this next generation of ultra-lightweight, large space structures. Each of the strategies proposed makes use of distributed actuation, which is demonstrated early in the thesis to reduce structural deformations during attitude manoeuvres. All of the proposed strategies are considered to be particularly suitable for structures which are 3d-printed on-orbit, due to the relative simplicity of the actuators and ease with which the actuator placement or construction could be integrated with the on-orbit fabrication of the structure itself.
The first strategy proposed is the use of distributed arrays of magnetorquer rods. First, distributed torques are shown to effectively rotate highly flexible structures. This is compared with torques applied to the centre-of-mass of the structure, which cause large surface deformations and can fail to enact a rotation. This is demonstrated using a spring-mass model of a planar structure with embedded actuators. A torque distribution algorithm is then developed to control an individually addressable array of actuators. Attitude control simulations are performed, using the array to control a large space structure, again modelled as a spring-mass system. The attitude control system is demonstrated to effectively detumble a representative 75Ă—75m flexible structure, and perform slew manoeuvres, in the presence of both gravity-gradient torques and a realistic magnetic field model.
The development of a Distributed Magnetorquer Demonstration Platform is then presented, a laboratory-scale implementation of the distributed magnetorquer array concept. The platform consists of 48 addressable magnetorquers, arranged with two perpendicular torquers at the nodes of a 5Ă—5 grid. The control algorithms proposed previously in the thesis are implemented and tested on this hardware, demonstrating the practical feasibility of the concept. Results of experiments using a spherical air bearing and Helmholtz cage are presented, demonstrating rest-to-rest slew manoeuvres and detumbling around a single axis using the developed algorithms.
The next attitude control strategy presented is the use of embedded current loops, conductive pathways which can be integrated with a spacecraft support structure and used to generate control torques through interaction with the Earth’s magnetic field. Length-scaling laws are derived by determining what fraction of a planar spacecraft’s mass would need to be allocated to the conductive current loops in order to produce a torque at least as large as the gravity gradient torque. Simulations are then performed of a flexible truss structure, modelled as a spring-mass system, for a range of structural flexibilities and a variety of current loop geometries. Simulations demonstrate rotation of the structure via the electromagnetic force on the current carrying elements, and are also used to characterise the structural deformations caused by the various current loop geometries. An attitude control simulation is then performed, demonstrating a 90◦ slew manoeuvre of a 250×250 m flexible structure through the use of three orthogonal sets of current loops embedded within the spacecraft.
The final concept investigated in this thesis is a self-reconfiguring OrigamiSat, where reconfiguration of the proposed OrigamiSat is triggered by changes in the local surface optical properties of an origami structure to harness the solar radiation pressure induced acceleration. OrigamiSats are origami spacecraft with reflective panels which, when flat, operate as a conventional solar sail. Shape reconfiguration, i.e. “folding” of the origami design, allows the OrigamiSat to change operational modes, performing different functions as per mission requirements. For example, a flat OrigamiSat could be reconfigured into the shape of a parabolic reflector, before returning to the flat configuration when required to again operate as a solar sail, providing propellant-free propulsion. Shape reconfiguration or folding of OrigamiSats through the use of surface reflectivity modulation is investigated in this thesis. First, a simplified, folding facet model is used to perform a length-scaling analysis, and then a 2d multibody dynamics simulation is used to demonstrate the principle of solar radiation presure induced folding. A 3d multibody dynamics simulation is then developed and used to demonstrate shape reconfiguration for different origami folding patterns. Here, the attitude dynamics and shape reconfiguration of OrigamiSats are found to be highly coupled, and thus present a challenge from a control perspective. The problem of integrating attitude and shape control of a Miura-fold pattern OrigamiSat through the use of variable reflectivity is then investigated, and a control algorithm developed which uses surface reflectivity modulation of the OrigamiSat facets to enact shape reconfiguration and attitude manoeuvres simultaneously
To the Last Drop: Affective Economies of Extraction and Sentimentality
The romance of extraction underlies and partly defines Western modernity and our cultural imaginaries. Combining affect studies and environmental humanities, this volume analyzes societies' devotion to extraction and fossil resources. This devotion is shaped by a nostalgic view on settler colonialism as well as by contemporary "affective economies" (Sara Ahmed). The contributors examine the links between forms of extractivism and gendered discourses of sentimentality and the ways in which cultural narratives and practices deploy the sentimental mode (in plots of attachment, sacrifice, and suffering) to promote or challenge extractivism
Neural Networks for Constitutive Modeling -- From Universal Function Approximators to Advanced Models and the Integration of Physics
Analyzing and modeling the constitutive behavior of materials is a core area
in materials sciences and a prerequisite for conducting numerical simulations
in which the material behavior plays a central role. Constitutive models have
been developed since the beginning of the 19th century and are still under
constant development. Besides physics-motivated and phenomenological models,
during the last decades, the field of constitutive modeling was enriched by the
development of machine learning-based constitutive models, especially by using
neural networks. The latter is the focus of the present review, which aims to
give an overview of neural networks-based constitutive models from a methodical
perspective. The review summarizes and compares numerous conceptually different
neural networks-based approaches for constitutive modeling including neural
networks used as universal function approximators, advanced neural network
models and neural network approaches with integrated physical knowledge. The
upcoming of these methods is in-turn closely related to advances in the area of
computer sciences, what further adds a chronological aspect to this review. We
conclude this review paper with important challenges in the field of learning
constitutive relations that need to be tackled in the near future
Improving Pseudo-Time Stepping Convergence for CFD Simulations With Neural Networks
Computational fluid dynamics (CFD) simulations of viscous fluids described by
the Navier-Stokes equations are considered. Depending on the Reynolds number of
the flow, the Navier-Stokes equations may exhibit a highly nonlinear behavior.
The system of nonlinear equations resulting from the discretization of the
Navier-Stokes equations can be solved using nonlinear iteration methods, such
as Newton's method. However, fast quadratic convergence is typically only
obtained in a local neighborhood of the solution, and for many configurations,
the classical Newton iteration does not converge at all. In such cases,
so-called globalization techniques may help to improve convergence.
In this paper, pseudo-transient continuation is employed in order to improve
nonlinear convergence. The classical algorithm is enhanced by a neural network
model that is trained to predict a local pseudo-time step. Generalization of
the novel approach is facilitated by predicting the local pseudo-time step
separately on each element using only local information on a patch of adjacent
elements as input. Numerical results for standard benchmark problems, including
flow through a backward facing step geometry and Couette flow, show the
performance of the machine learning-enhanced globalization approach; as the
software for the simulations, the CFD module of COMSOL Multiphysics is
employed
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