Objects Of Arbitrary Shape Research Articles

A directional ghost-cell immersed boundary method for low Mach number reacting flows with interphase heat and mass transfer

This paper presents a directional ghost-cell immersed boundary method for low Mach number reacting flows with general boundary conditions, extending the approach described in Chi et al. (2020) [17]. The method employs locally directional schemes for ghost value reconstruction and utilization along each discretization direction. In this manner, the boundary condition can be naturally imposed on the boundary intersection point along each coordinate direction, allowing an easy and straightforward implementation of complex boundary conditions. Using Taylor series approximation of fluid points near the immersed boundary, the boundary variable and its gradient governed by arbitrary boundary condition can be implicitly involved, leading to a reliable polynomial extrapolation for the ghost values. In this way, Dirichlet, Neumann and Robin boundary conditions are implemented for general variables with formally second-order accuracy. For reacting gas-solid flow with surface reactions, the reaction-related coefficients in Robin boundary condition lead to a complex implementation in conventional IBM techniques, while the present directional framework leads to a straightforward algorithm while preserving accuracy. The proposed method has been checked by a series of test cases with different boundary conditions, including basic flow, heat and species transport, Stefan problem, and finally two practical applications involving heat and mass transfer. The local accuracy of all boundary conditions exhibits nearly second-order convergence, as expected. While only a single solid object is considered in this first work, the same method can be simply extended to multiple objects of arbitrary shape, leading to fully resolved simulation of reactive particle-laden flows.

Automated balancing method of vector Illustration and its software implementation

The article proposes to develop a method for determining the conformity of vector illustration to the laws of composition and design, which will help to engage in artistic work for people without special skills and abilities. The concept of the centre of the composition is analysed. The main criterion of equilibrium is determined - the distance between the optical and composite centre. As the first stage of the developed method, it is offered to define the area of an element and to bring an arbitrary figure to a polygon. An algorithm for determining the area of an object of arbitrary shape has been developed. The process of colour density determination is analysed, and its algorithm is developed. An algorithm for determining the equilibrium of the composition is proposed. A script has been created that implements the method of determining the compliance of vector illustration with the laws of composition and design. A script based on a specific one has been tested during a number of tests. According to the test results, the conclusions about the correct operation of the script were confirmed. A prototype of an automation system for creating vector illustrations in Adobe Illustrator has been created.

A Nonstandard Path Integral Model for Curved Surface Analysis

The nonstandard finite-difference time-domain (NS-FDTD) method is implemented in the differential form on orthogonal grids, hence the benefit of opting for very fine resolutions in order to accurately treat curved surfaces in real-world applications, which indisputably increases the overall computational burden. In particular, these issues can hinder the electromagnetic design of structures with electrically-large size, such as aircrafts. To alleviate this shortcoming, a nonstandard path integral (PI) model for the NS-FDTD method is proposed in this paper, based on the fact that the PI form of Maxwell’s equations is fairly more suitable to treat objects with smooth surfaces than the differential form. The proposed concept uses a pair of basic and complementary path integrals for H-node calculations. Moreover, to attain the desired accuracy level, compared to the NS-FDTD method on square grids, the two path integrals are combined via a set of optimization parameters, determined from the dispersion equation of the PI formula. Through the latter, numerical simulations verify that the new PI model has almost the same modeling precision as the NS-FDTD technique. The featured methodology is applied to several realistic curved structures, which promptly substantiates that the combined use of the featured PI scheme greatly improves the NS-FDTD competences in the case of arbitrarily-shaped objects, modeled by means of coarse orthogonal grids.

A numerical method for suspensions of articulated bodies in viscous flows

An articulated body is defined as a finite number of rigid bodies connected by a set of arbitrary constraints that limit the relative motion between pairs of bodies. Such a general definition encompasses a wide variety of situations in the microscopic world, from bacteria to synthetic micro-swimmers, but it is also encountered when discretizing inextensible bodies, such as filaments or membranes. In this work we consider hybrid articulated bodies, i.e. constituted of both linear chains, such as filaments, and closed-loop chains, such as membranes. Simulating suspensions of such articulated bodies requires to solve the hydrodynamic interactions between large collections of objects of arbitrary shape while satisfying the multiple constraints that connect them. Two main challenges arise in this task: limiting the cost of the hydrodynamic solves, and enforcing the constraints within machine precision at each time-step. To address these challenges we propose a formalism that combines the body mobility problem in Stokes flow with a velocity formulation of the constraints, resulting in a mixed mobility-resistance problem. While resistance problems are known to scale poorly with the particle number, our preconditioned iterative solver is not sensitive to the system size, therefore allowing to study large suspensions with quasilinear computational cost. Additionally, constraint violations, e.g. due to discrete time-integration errors, are prevented by correcting the particles' positions and orientations at the end of each time-step. Our correction procedure, based on a nonlinear minimisation algorithm, has negligible computational cost and preserves the accuracy of the time-integration scheme. The versatility of our method allows to study a plethora of articulated systems within a unified framework. We showcase its robustness and scalability by exploring the locomotion modes of a model microswimmer inspired by the diatom colony Bacillaria Paxillifer, and by simulating large suspensions of bacteria interacting near a no-slip boundary. Finally, we provide a Python implementation of our framework in a collaborative publicly available code, where the user can prescribe a set of constraints through a single input file to study a wide spectrum of applications involving suspensions of articulated bodies.

High-Order Photonic Cavity Modes Enabled 3D Structural Colors.

It remains a challenge to directly print arbitrary three-dimensional shapes that exhibit structural colors at the micrometer scale. Woodpile photonic crystals (WPCs) fabricated via two-photon lithography (TPL) are elementary building blocks to produce 3D geometries that generate structural colors due to their ability to exhibit either omnidirectional or anisotropic photonic stop bands. However, existing approaches produce structural colors on WPCs when illuminating from the top, requiring print resolutions beyond the limit of commercial TPL, which necessitates postprocessing techniques. Here, we devised a strategy to support high-order photonic cavity modes upon side illumination on WPCs that surprisingly generate prominent reflectance peaks in the visible spectrum. Based on that, we demonstrate one-step printing of 3D photonic structural colors without requiring postprocessing or subwavelength features. Vivid colors with reflectance peaks exhibiting a full width at half-maximum of ∼25 nm, a maximum reflectance of 50%, a gamut of ∼85% of sRGB, and large viewing angles were achieved. In addition, we also demonstrated voxel-level manipulation and control of colors in arbitrary-shaped 3D objects constituted with WPCs as unit cells, which has potential for applications in dynamic color displays, colorimetric sensing, anti-counterfeiting, and light-matter interaction platforms.

Slippery Antifouling Polymer Coatings Fabricated Entirely from Biodegradable and Biocompatible Components.

We report the design of slippery liquid-infused porous surfaces (SLIPS) fabricated from building blocks that are biodegradable, edible, or generally regarded to be biocompatible. Our approach involves infusion of lubricating oils, including food oils, into nanofiber-based mats fabricated by electrospinning or blow spinning of poly(ε-caprolactone), a hydrophobic biodegradable polymer used widely in medical implants and drug delivery devices. This approach leads to durable and biodegradable SLIPS that prevent fouling by liquids and other materials, including microbial pathogens, on objects of arbitrary shape, size, and topography. This degradable polymer approach also provides practical means to design "controlled-release" SLIPS that release molecular cargo at rates that can be manipulated by the properties of the infused oils (e.g., viscosity or chemical structure). Together, our results provide new designs and introduce useful properties and behaviors to antifouling SLIPS, address important issues related to biocompatibility and environmental persistence, and thus advance new potential applications, including the use of slippery materials for food packaging, industrial and marine coatings, and biomedical implants.

Cross-Polarization Carpet Cloaking Based on Polarization Conversion Metasurfaces

The principle of phase compensation on the metasurfaces is applied to realize the cloaking function, which provides a new idea for the further development of invisibility technology. However, the uppermost layer of the metasurface carpet cloak previously studied is mostly a metal structure with ohmic loss and low coupling efficiency. In practical applications, these ordinary invisibility cloaks are easy to be corroded, and does not endure high temperature, and only has the function of invisibility under the action of the same polarization. Here, we propose to use the unit structure of high-refractive-index dielectric silicon resonators to construct the invisibility cloak, which can better reduce loss, improve reflection efficiency, and can be better applied to practical applications. A unique cross polarization stealth device with the broadband cloaking effect is designed to effectively resist cross polarization detectors. Objects of arbitrary shape can be hidden under the triangular carpet stealth device designed to achieve the purpose that the detector cannot detect, so as to achieve stealth function. The working wavelength of the cloak proposed is at 1550nm, achieving the effect of cross polarization invisibility in the communication band.

Born approximation of acoustic radiation force and torque

The Born approximation provides a simple method for calculating acoustic radiation force and torque on compressible objects of arbitrary shape, which may be inhomogeneous. The specific acoustic impedance throughout the object must not differ significantly from that of the background fluid. Also, the incident field must not be too similar to a progressive plane wave, because the approximation is associated with radiation forces due to time-averaged energy density gradients in the incident field. Comparisons with a complete theory for radiation force and torque exerted by standing plane waves on compressible spheroids, based on expansions of the incident and scattered fields in spheroidal wave functions, indicate that the Born approximation is accurate for impedance contrasts up to at least 30% and for spheroids with sizes on the order of a wavelength. This presentation will review the derivation, validation, and applications of the Born approximation. Closed-form solutions for spheres and finite cylinders are presented and for selected distributions of material inhomogeneity. Applications include acoustofluidic separation of biological targets, and acoustic forces on nonspherical objects near interfaces. The approximation also serves as a useful benchmark for other methods of calculation. [T.S.J. was supported by an ARL:UT McKinney Fellowship in Acoustics.]

Cooperative transportation: realizing the promises of robotic networks using a tailored software/hardware architecture

Abstract With cooperative transportation, the paper looks at a demanding problem from distributed robotics. At its heart, the proposed transportation scheme uses distributed model predictive control. Yet, distributed control alone does not suffice to solve the task. Thus, also distributed organization, a custom software architecture, simulation, and custom robotic hardware are dealt with, bridging the gap between distributed control theory and practical robotics. The robots are enabled to transport arbitrarily-shaped objects, automatically adapting to changing circumstances and numbers of robots.

Universal Theory of Light Scattering of Randomly Oriented Particles: A Fluctuational-Electrodynamics Approach for Light Transport Modeling in Disordered Nanostructures.

Disordered nanostructures are commonly encountered in many nanophotonic systems, from colloid dispersions for sensing to heterostructured photocatalysts. Randomness, however, imposes severe challenges for nanophotonics modeling, often constrained by the irregular geometry of the scatterers involved or the stochastic nature of the problem itself. In this Article, we resolve this conundrum by presenting a universal theory of averaged light scattering of randomly oriented objects. Specifically, we derive expansion-basis-independent formulas of the orientation-and-polarization-averaged absorption cross section, scattering cross section, and asymmetry parameter, for single or a collection of objects of arbitrary shape. These three parameters can be directly integrated into traditional unpolarized radiative energy transfer modeling, enabling a practical tool to predict multiple scattering and light transport in disordered nanostructured materials. Notably, the formulas of average light scattering can be derived under the principles of fluctuational electrodynamics, allowing the analogous mathematical treatment to the methods used in thermal radiation, nonequilibrium electromagnetic forces, and other associated phenomena. The proposed modeling framework is validated against optical measurements of polymer composite films with metal-oxide microcrystals. Our work may contribute to a better understanding of light–matter interactions in disordered systems, such as plasmonics for sensing and photothermal therapy, photocatalysts for water splitting and CO2 dissociation, photonic glasses for artificial structural colors, and diffuse reflectors for radiative cooling, to name just a few.

A Discontinuous Galerkin Combined Field Integral Equation Formulation for Electromagnetic Modeling of Piecewise Homogeneous Objects of Arbitrary Shape

We present a novel discontinuous Galerkin (DG) surface integral equation approach, based on the electric and magnetic current combined field integral equations (JMCFIE), for the electromagnetic analysis of arbitrarily shaped piecewise homogeneous objects. In the proposed scheme, nonoverlapping boundary surfaces and interfaces between materials can be handled independently, without any continuity requirement through multimaterial junctions and tear lines between surfaces in contact. The use of nonconformal meshes provides improved flexibility for computer-aided-design (CAD) prototyping and tessellation. The proposed formulation can readily address nonconformal multimaterial junctions, where three or more material regions meet. The continuity of the electric surface current across the junction contours is enforced by the combination of the boundary conditions implicit in the JMCFIE formulation and the weakly imposed interior penalty (IP) between the contacting surfaces within each region. This completely avoids the cumbersome junction problem, which no longer requires any special treatment. Numerical experiments are included to validate the accuracy and demonstrate the great versatility of the proposed JMCFIE-DG formulation for the management and solution of complex composite objects with junctions.

Grasping of arbitrary shape objects by a two-finger hand with an RGB-D hand-eye camera

Grasping of arbitrary shape objects by a two-finger hand with an RGB-D hand-eye camera

Continuously growing multi-layered hydrogel structures with seamless interlocked interface

<h2>Summary</h2> Many natural tissues feature layered structures for multi-functionality as each layer consists of different cell types and therefore exhibits unique physicochemical properties. Creating such complex multi-layered structures in human-made soft materials is hardly achievable with common gelation methods, lacking spatiotemporal control, and thus demands a living polymerization-based controllable continuous growing process to imitate bio-tissue growth. Herein, a method, namely, ultraviolet-triggered surface catalytically initiated radical polymerization (UV-SCIRP), is demonstrated in living growth of uniform hydrogel layers at room temperature, facilitated by the <i>in situ</i> generation of Fe²⁺ ion catalysts from the surface-bound Fe³⁺ ions of the hydrogel substrate. Reiterative application of UV-SCIRP enables fabrication of multi-layered hydrogels with diverse components and layered features. The surface-catalyzed gelation makes the method effective in constructing complex hydrogel patterns, non-flat arbitrary-shaped hydrogel objects, and blood vessel-like intricate multi-layered hydrogel tubes. This work manifests a new route to engineer bioinspired structural hydrogels and complex soft architectures with on-demand functions.

Lossy Point Cloud Geometry Compression via Region-Wise Processing

Point cloud geometry (PCG) is used to precisely represent arbitrary-shaped 3D objects and scenes, is of great interest to vast applications which puts forward the pressing desire of high-efficiency PCG compression for transmission and storage. Existing PCG coding mostly relies on the octree model by which point-wise processing is applied without exploring nonlocal regional geometry similarity across the entire 3D surface. This work, instead, suggests the region-wise processing to leverage the region similarity to exploit inter-region redundancy for efficient lossy point cloud geometry compression. Towards this goal, a given PCG is first segmented into numerous local regions each of which comprises a portion of point cloud surface, and can be represented by a surface vector that describes the geometry shape numerically in a projected principal space. Subsequently, these regions are grouped into several discriminative clusters, assuring that inter-cluster similarity is minimized and intra-cluster similarity is maximized simultaneously, where the similarity is calculated using the regional surface vectors. In each cluster, we set a reference region having the largest similarity score to the others, which enables the non-reference region prediction from the reference one using alignment transform. In the end, we encode the reference regions directly using the lossless mode of the Geometry-based Point Cloud Compression (G-PCC), while corresponding non-reference regions are signaled using associated transform parameters. Compared with the state-of-the-art G-PCC using octree model, our region-wise approach can offer remarkable coding efficiency improvement, e.g., 32.4% and 22.0% Bjontegaard-delta rate (BD-Rate) gains for respective point-to-point ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D1$ </tex-math></inline-formula> ) and point-to-plane ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D2$ </tex-math></inline-formula> ) distortion evaluations, across a variety of common test sequences used in standard committee.

Fast and versatile fluid-solid coupling for turbulent flow simulation

The intricate motions and complex vortical structures generated by the interaction between fluids and solids are visually fascinating. However, reproducing such a two-way coupling between thin objects and turbulent fluids numerically is notoriously challenging and computationally costly: existing approaches such as cut-cell or immersed-boundary methods have difficulty achieving physical accuracy, or even visual plausibility, of simulations involving fast-evolving flows with immersed objects of arbitrary shapes. In this paper, we propose an efficient and versatile approach for simulating two-way fluid-solid coupling within the kinetic (lattice-Boltzmann) fluid simulation framework, valid for both laminar and highly turbulent flows, and for both thick and thin objects. We introduce a novel hybrid approach to fluid-solid coupling which systematically involves a mesoscopic double-sided bounce-back scheme followed by a cut-cell velocity correction for a more robust and plausible treatment of turbulent flows near moving (thin) solids, preventing flow penetration and reducing boundary artifacts significantly. Coupled with an efficient approximation to simplify geometric computations, the whole boundary treatment method preserves the inherent massively parallel computational nature of the kinetic method. Moreover, we propose simple GPU optimizations of the core LBM algorithm which achieve an even higher computational efficiency than the state-of-the-art kinetic fluid solvers in graphics. We demonstrate the accuracy and efficacy of our two-way coupling through various challenging simulations involving a variety of rigid body solids and fluids at both high and low Reynolds numbers. Finally, comparisons to existing methods on benchmark data and real experiments further highlight the superiority of our method.

Born approximation of acoustic radiation force and torque on inhomogeneous objects.

The Born approximation developed previously to model acoustic radiation force and torque exerted on homogeneous compressible objects of arbitrary shape [Jerome et al., J. Acoust. Soc. Am. 145, 36-44 (2019)] is extended to include objects that are inhomogeneous. The same general restrictions apply to this extended model, mainly that the incident field is not too similar to a progressive plane wave, that the material properties of the object do not differ substantially from those of the surrounding fluid, and that the size of the object is not much larger than a wavelength. Two applications of the model are presented, one for objects consisting of connected homogeneous regions with different material properties, and the other for objects with continuously varying material properties. Calculations are presented for spheres, finite cylinders, and prolate spheroids.

Acoustic radiation force on objects of arbitrary shape and composition

Acoustic radiation force exerted on an object of arbitrary shape and composition by an arbitrary incident sound beam can be calculated using spherical harmonic expansions of the pressure field. The coefficients in the expansions of the incident and scattered fields, when substituted into existing models, determine the radiation force on the object. Analytical expressions for the coefficients of both the incident and scattered fields are available for very few cases, such as a plane wave incident on a rigid sphere. In the present work, finite element modeling is used to calculate the coefficients, and resulting radiation force, for a variety of incident fields and object geometries. Different compositions of the objects are also considered. Validation of the approach is demonstrated by comparison with semi-analytical results available for the radiation force exerted by progressive and standing plane waves scattered by compressible spheroids in different orientations with respect to the incident field. Motivation for this approach is development of a model that can be used to compare with direct measurements of radiation force on irregularly shaped objects of different compositions. [BES is supported by the ARL:UT Chester M. McKinney Graduate Fellowship in Acoustics.]

Acoustic radiation force and torque on objects with continuous and discrete inhomogeneity

At the Fall ASA meeting in 2019, the Born approximation was extended to acoustic radiation force and torque on objects of arbitrary shape with spatially varying density and compressibility [Proc. Meet. Acoust. 39, 045007 (2020)]. This model is applied here to a standing plane wave incident on objects either with continuously varying material properties or composed of connected homogeneous regions with different material properties. The approximation for inhomogeneous objects is subject to the same restrictions on incident field structure, scatterer size, and material contrast as for homogeneous objects. Results are presented for cylinders with a variety of strongly asymmetric inhomogeneity distributions. Closed-form expressions are obtained for a sphere with material properties that vary linearly with distance from its center and for a sphere surrounded by concentric spherical layers. Results for the latter case are shown to be in close agreement with existing theory for the radiation force on a nucleated cell modeled as a multilayered compressible sphere [Wang et al., J. Appl. Phys. 122, 094902 (2017)]. The Born approximation thus proves convenient for determining radiation force and torque on soft objects with both arbitrary shape and inhomogeneity. [TSJ was supported by the ARL:UT McKinney Fellowship in Acoustics.]

Revealing Hidden Features in Multilayered Artworks by Means of an Epi-Illumination Photoacoustic Imaging System.

Photoacoustic imaging is a novel, rapidly expanding technique, which has recently found several applications in artwork diagnostics, including the uncovering of hidden layers in paintings and multilayered documents, as well as the thickness measurement of optically turbid paint layers with high accuracy. However, thus far, all the presented photoacoustic-based imaging technologies dedicated to such measurements have been strictly limited to thin objects due to the detection of signals in transmission geometry. Unavoidably, this issue restricts seriously the applicability of the imaging method, hindering investigations over a wide range of cultural heritage objects with diverse geometrical and structural features. Here, we present an epi-illumination photoacoustic apparatus for diagnosis in heritage science, which integrates laser excitation and respective signal detection on one side, aiming to provide universal information in objects of arbitrary thickness and shape. To evaluate the capabilities of the developed system, we imaged thickly painted mock-ups, in an attempt to reveal hidden graphite layers covered by various optically turbid paints, and compared the measurements with standard near-infrared (NIR) imaging. The obtained results prove that photoacoustic signals reveal underlying sketches with up to 8 times improved contrast, thus paving the way for more relevant applications in the field.

Ge2Sb2Te5-based reconfigurable metasurface for polarization-insensitive, full-azimuth, and switchable cloaking.

Electromagnetic (EM) metasurface mantles afford an alternative avenue, allowing for the possibility of rendering arbitrary-shape objects unobservable. But the available mechanisms either depend on the states of polarization or the azimuth of wave incidence, or cannot dynamically manipulate cloaking responses without altering the structures. Herein, a three-dimensional closed-ring-based metasurface carpet cloak involving Ge2Sb2Te5 that circumvents current drawbacks of metasurface structures is proposed. By judiciously designing meta-atoms on the external surface of a spherical object, the scattered wavefront, including the distributions of EM fields and polarizations, can be reconstructed, resembling what is deflected from a flat plane. Enabled by the perfectly symmetric distribution of meta-atoms, the carpet cloak is demonstrated to work well under arbitrary states of polarization and arbitrary azimuthal angles of incident light. Meanwhile, by converting Ge2Sb2Te5 from the amorphous to crystalline state, the designed scheme is empowered with the ability to switch "ON" and "OFF" of stealth states. Furthermore, the unique design achieves invisibility over ±20∘ angular span in the mid-infrared range from 8800 to 9450 nm. The validated recipe empowers robust steps forward to achieve full-polarization, full-azimuth operation, and switchable cloaking in the real-world, showing great potential applications in stealth, camouflage, and illusion fields.