Array Of Cubes Research Articles

Reversible shape transformations through continuous deformations in soft responsive self-repairing solids using voxel techniques

Voxel modeling is a technique in which an object is represented by an array of cubes. One advantage in design-for-manufacturing is that selected voxels may be assigned desired responsive properties through additive manufacturing. It allows for achieving reversible shape transformation in soft responsive self-repairing solids within a strategic sequence. The first step in the sequence involves selecting an equivalent geometric shape. The second step involves mapping shapes to preserve properties during continuous deformation. The third step involves transforming the shape from one form to another. The fourth step involves defining changes in dimensions. The final step involves correlating the flow of stress with the stretch of the solid. This research also considers self-repair or self-healing as part of the transformation process because the splitting or joining of solids involves continuous deformation. The newly transformed shape can also be considered as the intermediate shape, or first shape for next transformation. This leads to a chain of shapes that are topologically equivalent, and interchangeable. The iterative steps involved in shape transformation finds applications in topology optimization, generative design, metastructures, among others. One application of shape transformation in these domains is that it serves as an alternative to material reduction techniques.

Impact of urban canopy characteristics on turbulence dynamics: Insights from large eddy simulation

In this study, a series of large-eddy simulations are conducted to explore the dynamics of turbulence within the urban canopy. Two idealized urban-like geometries (staggered and aligned arrays of cubes) and one real urban region (located on the Southern University of Science and Technology campus in China) are simulated under neutral atmospheric conditions. The results show a significant difference in the dispersive momentum fluxes between OpenFOAM and PALM within the ultra-dense arrays. Furthermore, it is found that the spatial layout does not exert a significant effect on the budget of turbulent kinetic energy. Instead, it mainly depends on the ratio of the building surface area to the total surface area, i.e., packing density. In addition, a multi-scale coupled inflow boundary condition is introduced in this study to simulate the flow in a real urban area. This boundary condition yields a better match to the LiDAR measurements than the Weather Research and Forecasting model data. Finally, this study shows that the mean velocity obtained from the campus of SUSTech is in satisfactory agreement with the data from low-density idealized cube arrays and the corresponding urban canopy model. This observation suggests that mean wind speeds within partially occupied urban areas are relatively independent of wind direction, thus greatly improving the application of urban canopy models to such scenarios.

Investigation of randomly populated cylindrical, spherical, and cubical arrays for application in space, aerial and underwater collaborative beamforming

Investigation of randomly populated cylindrical, spherical, and cubical arrays for application in space, aerial and underwater collaborative beamforming

Large‐eddy simulation of a channel flow over an irregular porous matrix

AbstractFor turbulent flows in porous media, it is often assumed that the irregularity of the matrix would not affect the macroscopic, double‐averaged (in time and space), parameters of turbulence. Hence, to improve currently used modelling techniques, the ongoing efforts of the community are focused on performing high‐fidelity, scale‐resolving simulations of flows in regular, periodically repeating porous structures. This approach allows for minimizing still large computational cost, however, it assumes that results obtained from those geometries are generalizable to more disordered configurations. We investigate this assumption numerically using large‐eddy simulation and analyse the influence of the irregularity of the porous structure on turbulent characteristics in a fully turbulent flow in a channel half‐filled with porous medium. The flow statistics are examined and compared between four simulations, a reference channel with regular three‐dimensional porous matrix consisting of an array of cubes and three similar geometries created from randomly perturbing the positions of the cubes, with increasing mean value of cube displacement. We study double‐averaged flow properties, the distributions of double‐averaged Reynolds stress tensor, including the anisotropy of macroscopic turbulence and the drag force induced by the flow in the porous region of the channel. The results confirm the idea that the averaged characteristics, including the anisotropy of the stress tensor and the drag force, are not greatly influenced by the moderate perturbation to the geometry of the porous matrix. Additionally, the macroscopic turbulence seems to exhibit a high degree of anisotropy in the porous region and the adjacent boundary layer, suggesting that second‐order closure could be an adequate modelling choice for such flows.

Asymmetric secondary flows above geometrically symmetric surface roughness

The mean flow behaviour of a turbulent boundary layer over rough walls is expected to exhibit symmetries that govern the flow dynamics. In particular, when roughness elements are arranged in a spanwise symmetric manner, the mean flow above them should also exhibit spanwise symmetry. This symmetrical consideration has garnered substantial empirical support. We conduct direct numerical simulations (DNS) of flow over aligned cube arrays to test such symmetry considerations further. We vary the surface coverage density from 0.25 % to 6.25 %, and employ an averaging time of about 100 large-eddy turnover times, which is longer than the typical averaging time in prior DNS studies of rough-wall boundary layers. The results suggest the presence of spanwise asymmetry in the mean flow. Specifically, we observe the development of a prominent secondary vortex on one side of the cubical roughness, accompanied by a relatively smaller secondary vortex on the other side. This asymmetry becomes most pronounced when the surface coverage density is approximately 0.59 %, and diminishes as the coverage density approaches either a low or a high value. We also establish that this mean flow asymmetry is robust across variations in the domain size, the initial condition, and the placement of the cubes in the spanwise direction.

Effect of cube spacings on the three-dimensional flow structure over an array of wall-mounted cube

The turbulent flow over an array of cubes mounted on one of the walls of a channel has been investigated using direct numerical simulation for cube spacing that ranges between 2.0 and 4.0. The Reynolds number based on the cube size and the average streamwise velocity is chosen to be 4000. The Navier–Stokes equations have been discretized using second-order spatial and temporal discretization schemes. The present investigation focuses on the flow structures and comprehensive characterization of the separated zones surrounding the cubes, as well as the associated wall-shear stress. A vortex shedding has been observed for the cube spacings of 3.0 and 4.0 without any evidence of vortex shedding for the lowest pitch of 2.0. For the two cases having pitches of 3.0 and 4.0, the presence of the unsteady separation bubbles at the cube's top and side surfaces results in a decrease in wall-shear stress. The quadrant analysis for the region close to the top surface of the cube is performed with the help of the joint probability density function, which reveals dual peaks within the recirculation bubble at the top surface of the cube for higher cube spacings. By conducting an invariant analysis of the Reynolds stress tensor for different cube spacings, we have explored the characteristic of Reynolds stress anisotropy due to the total fluctuations. The production of negative turbulent kinetic energy (TKE) is observed in different regions within the flow domain, among which the horseshoe vortex region for each cube spacing reveals its dominant presence. The physical mechanism responsible for the production of the negative TKE has also been attempted by decomposing the production term into two parts, namely, normal and shear components.

Large-eddy simulation of the urban boundary layer using drag-porosity modeling

This work details the assessment of the performance of the drag-porosity model implemented in ARPS (Advanced Regional Prediction System) atmospheric Large-Eddy Simulation (LES) solver for simulating the atmospheric boundary layer developing over the urban canopy with comparison with literature. The flow within and above an idealized urban canopy consisting of a staggered array of cubes with various packing densities modeled with the drag-porosity approach immersed into a neutral, Coriolis-free atmospheric boundary layer at high Reynolds is investigated. Besides one-points statistics, particular interest was given to the ability of the model to reproduce the turbulent coherent structures and their characteristic scales. A detailed analysis of one-point statistics, one- and two-dimensional spectra and two-point correlation functions revealed the presence of typical structures and features found in wall-bounded turbulent flows (two-scale behavior in the roughness sublayer, ejections, sweeps, self-similar wall-attached large scale streaky motions, canopy-independent very large scale motions). Further investigation to identify the interaction mechanisms between large and small scales based on spectral filtering highlighted an interaction mechanism that resembles an amplitude modulation process, as observed in literature on wall-bounded flows. These findings therefore show that the proposed approach is able to reproduce all the key features of the flow developing over urban terrain.

Differential constant-beamwidth beamforming with cube arrays

In this paper, we present an azimuth and elevation constant-beamwidth (CB) differential beamforming approach for cube arrays. We decompose a global cube beamformer into a Kronecker-product (KP) of three sub-beamformers: two constant-beamwidth beamformers along the y and z axes, and a tunable super-directive (SD) beamformer along the x-axis. We propose two design methods to derive cube beamformers whose either white noise gain (WNG) or directivity factor (DF) may be set by design. We show that the CB threshold frequency with respect to the azimuth and elevation angles, and the WNG and DF performance, can be controlled by the number of microphones along each axis. In addition, we focus on the particular case of merely a single microphone along the x-axis, which yields a rectangular azimuth and elevation CB beamformer. We demonstrate that its CB threshold frequencies are controlled by the number of microphones along the y and z axes, and show that the WNG and DF performance measures are maximized when the two axes are equal in size. Finally, we analyze the performance of the proposed beamformers through simulations of speech signals in various reverberant scenarios, including deviations in the desired speech signal’s direction of arrival (DOA). We show that the proposed beamformers outperform previously-presented beamformers and the traditional SD and delay-and-sum (DS) beamformers, particularly in terms of the intelligibility of their corresponding time-domain enhanced signals.

A Note on Friction Velocity and Viscous Effect for Idealized Urban Canopy Flows

Friction velocity is one of the important scaling parameters in atmospheric boundary layer studies. However, several definitions of friction velocity exist in the literature: e.g. estimated from the total drag force or used only pressure drag, etc. In this study, a series of large-eddy simulation (LES) calculations were carried out to evaluate the impact of various definitions on the friction velocity for idealized urban geometries, i.e. staggered array of cubes with different packing densities and wind directions. We first compared the normalized velocity fields with the literature data for the case with a packing density of $$25\%$$ . The results show that the LES data normalized by the friction velocity derived from the Reynolds stress using the extrinsic spatial average is more consistent with the direct numerical simulation data. Furthermore, when varying the wind direction, the distribution of Reynolds stress and pressure drag show significant change in streamwise and spanwise directions. We further found that for packing density of $$44.44\%$$ , the frictional drag accounts for more than a quarter of the total drag, and even higher than the pressure drag in parallel wind direction. This leads to the deviation of friction velocity estimated from the pressure drag and that calculated from the total drag force up to 33%. Such characteristic of viscous effect challenges the assumption widely used in wind tunnel experiments and urban canopy parameterizations that the contribution of viscous force is negligible, especially for ultra-dense arrays.

Results from the HEPD-01 detector on board the China Seismo-Electromagnetic Satellite (CSES)

CSES (China Seismo-Electromagnetic Satellite) is a sophisticated multi-channel space observatory to study the Earth and its surrounding layers. It was launched on February 2nd, 2018, on a Sun-Synchronous orbit at an altitude of 500 km. The High Energy Particle Detector (HEPD-01) is one of the main contributions of the CSES-Limadou collaboration to the mission with the goal of measuring 3-100 MeV electrons and 30-300 MeV protons, also extending to heavier nuclei. The instrument is quite compact (40.36 cm x 53.00 cm x 38.15 cm) and it is composed of a tracking system, a trigger made by a segmented layer of plastic scintillator, a calorimeter made by a tower of plastic scintillators and an array of LYSO cubes and a veto system. With its large field of view (±60°) it is capable to collect sufficient statistics to provide new and competitive measurements concerning a quite rich scientific program: the study of the radiation present in the ionospheric-magnetospheric environment, searching for transient phenomena correlated to seismic events, monitoring the solar activity, the flux of the low energy cosmic rays and studying the South Atlantic Anomaly (SAA). In this paper we provide a synthetic description of the detector and of the main scientific results obtained on the measurement of the properties of low energy charged particles.

A Load-Independent Fission-Type Inductive Power Transfer System for 3D Reconfigurable IoT Array

In recent years, the number of electronic devices and appliances that support wireless charging has rapidly increased, owing to the research and development breakthroughs in inductive power transfer (IPT) technology. The existing wireless charging protocols, such as Qi and AirFuel, support only a few devices to be charged simultaneously. They cannot be directly utilized to construct a complex IPT network. This article introduces a reconfigurable three-dimensional (3D) IPT system based on series-parallel (SP) compensation for wirelessly powering multiple adjacent cubic Internet of Things (IoT) devices. These cubes can be configured into different 3D array combinations according to the application demands. After the 3D cube array is formed, a self-detection mechanism runs to turn off the uncoupled coils, such that to maximize the power transmission efficiency of the entire system. These cubes serve as not only IoT loads but also wireless power relays. In this way, the number of loads is not limited by the size of a single transmission coil. In addition, it maintains a constant drive voltage to each load device, regardless of the load variations in local or adjacent cubes. The constant voltage design eases local power management. A prototyped IPT cube array is manufactured and tested to validate the proposed idea. This fission-type IPT system makes a valuable exploration of the 3D IPT technology toward future ubiquitous and reconfigurable IoT arrays.

Single nanoparticle sensing and waveguide applications using photonic nanojet from an array of shaped microparticles

Photonic nanojet (PNJ) is an intense and narrow beam that emerges from the shadow side of a dielectric microparticle. In this work, we have studied the dependence of the PNJ length on the size, shape, refractive index and the number of particles. Four shapes of the particle, namely, elliptical, cubical, circular and triangular have been investigated. Arrays of cubical and elliptical particles have shown the formation of PNJ for array lengths more than 30. Multiple PNJs have been observed for cubical and triangular arrays. The width of the obtained jet can be tuned from μm to nm dimension. The PNJ length increases by a factor of ∼29 when kept in a biofluid. The change in the width of the PNJ observed in presence of a single nanoparticle (NP) can be used as a sensing parameter. The PNJ width of 0.2λ has been obtained in the presence of the NP. The PNJ from an array of microparticles has been found to be useful in the waveguides due to the propagation distances ∼30 times the wavelength.

The SuperFGD prototype PID beam tests results

The near detector ND280 of the T2K experiment will be upgraded in 2022 with the aim of measuring precisely CP violation in neutrinos. The ND280 upgrade consists of the installation of 3 new sub-detector types including SuperFGD, a novel neutrino active target concept. SuperFGD (Super-Fine-Grained-Detector) will have 2 million 1 × 1 × 1cm3 plastic scintillator cubes forming a cube array of 184 × 56 × 192 cm3. Each of the cubes will be intersected by 3 orthogonal WLS fibers with an MPPC on one end. Thanks to its super-fine segmentation, high light yield, and excellent time resolution, great particle identification (PID) capabilities are expected. Since 2018, a set of prototypes have been exposed to particle beams (charged tracks and neutrons) to test this concept. Here the results concerning the particle identification (PID) capabilities using the last prototype are presented.

Fast O(N log N) Algorithm for Generating Rank-Minimized H2-Representation of Electrically Large Volume Integral Equations

In this article, we propose a fast algorithm to generate a rank-minimized <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {H}^{2}$ </tex-math></inline-formula> -representation for solving electrically large volume integral equations (VIEs). Unlike existing methods whose complexity is as high as quadratic, the proposed algorithm has linearithmic complexity, and thus, it can handle problems with large electrical sizes. Furthermore, the algorithm is purely algebraic and kernel independent. Numerical experiments on electrically large 3-D arrays of dielectric cubes having over 33 million unknowns demonstrate the efficiency and accuracy of the proposed algorithm.

Light Interaction with Cluster Chiral Nanostructures by High-Order Bessel Beam

Interactions between cluster chiral nanoparticles and a high-order Bessel beam (HOBB) with arbitrary illuminations are investigated. The generalized Lorenz–Mie theory (GLMT) is applied to derive the expansions of HOBB. Based on the additional theorem, multiple scattering results of cluster chiral nanoparticles are obtained by taking into account the tangential continuous boundary conditions. The present theory and codes proved to be effective when confronted with the simulations obtained from the Computer Simulation Technology (CST) software. Numerical results concerning the effects of beam order, beam conical angle, incident angles, beam polarization state, the chirality, and the material loss on the scattering of various types of aggregated chiral particles are displayed in detail, including the linearly chiral sphere chain, the chiral cube array, and the complex models composed of aggregated chiral spheres. This study may provide critical support to analytically understand the optical scattering characteristics with aggregated chiral particles of complex shapes, and may find important applications in manipulating collective chiral particles.

Evaluation of the SPARTACUS-Urban Radiation Model for Vertically Resolved Shortwave Radiation in Urban Areas

The heterogenous structure of urban environments impacts interactions with radiation, and the intensity of urban–atmosphere exchanges. Numerical weather prediction (NWP) often characterizes the urban structure with an infinite street canyon, which does not capture the three-dimensional urban morphology realistically. Here, the SPARTACUS (Speedy Algorithm for Radiative Transfer through Cloud Sides) approach to urban radiation (SPARTACUS-Urban), a multi-layer radiative transfer model designed to capture three-dimensional urban geometry for NWP, is evaluated with respect to the explicit Discrete Anisotropic Radiative Transfer (DART) model. Vertical profiles of shortwave fluxes and absorptions are evaluated across domains spanning regular arrays of cubes, to real cities (London and Indianapolis). The SPARTACUS-Urban model agrees well with the DART model (normalized bias and mean absolute errors < 5.5%) when its building distribution assumptions are fulfilled (i.e., buildings randomly distributed in the horizontal). For realistic geometry, including real-world building distributions and pitched roofs, SPARTACUS-Urban underestimates the effective albedo (< 6%) and ground absorption (< 16%), and overestimates wall-plus-roof absorption (< 15%), with errors increasing with solar zenith angle. Replacing the single-exponential fit of the distribution of building separations with a two-exponential function improves flux predictions for real-world geometry by up to half. Overall, SPARTACUS-Urban predicts shortwave fluxes accurately for a range of geometries (cf. DART). Comparison with the commonly used single-layer infinite street canyon approach finds SPARTACUS-Urban has an improved performance for randomly distributed and real-world geometries. This suggests using SPARTACUS-Urban would benefit weather and climate models with multi-layer urban energy balance models, as it allows more realistic urban form and vertically resolved absorption rates, without large increases in computational cost or data inputs.

Subwavelength thermally controlled acoustic topological interface states in split hollow spheres

This article puts forward theory and design approaches for three thermally controlled subwavelength acoustic topological insulators. They are presented by immersing arrays of split hollow spheres (SHSs) or cubes in an air background and performing a study on the band structures and topological interface states. The periodically immersed rhombic shape unit cell comprising of two SHSs is analyzed firstly. A subwavelength local resonance bandgap and a Dirac point at the corner of the irreducible Brillouin zone are observed. After inducing a gap for the Dirac cone with unequal inner radius of spheres, we observe topological phase transition by mode shape and valley Chern-number analysis. Generally, local resonance systems are always limited to narrow and fixed bandgaps, therefore active and passive control of the structure are demonstrated. A strip supercell is constructed to induce topologically protected interface mode and to capture highly localized acoustic energies at the interface. Similar analyses are performed on the enlarged hexagonal model with six SHSs and the rhombic model with two split hollow cubes. This study provides a new degree of freedom for active control of acoustic topological modes and sound wave propagation.

Possibility for survival of macroscopic turbulence in porous media with high porosity

The direct numerical simulation (DNS) study by Jin et al. (J. Fluid Mech, vol. 766, 2015, pp. 76–103) shows that the turbulent structures are generally restricted in size to the pore scale, leading to the pore-scale prevalence hypothesis (PSPH). Although the PSPH has been validated under most conditions, it might become invalid as the porosity approaches unity. In order to investigate the valid domain of the PSPH, we have studied the turbulent flows in porous matrices which have one or two length scales using DNS and macroscopic simulation methods. The large porous elements are made of staggered arrays of square cylinders, which might stimulate strong macroscopic (large-scale) turbulence. The small porous elements are made of aligned arrays of spheres or cubes, which suppress the macroscopic turbulence. The analyses are performed for various values of the Reynolds number, Darcy number, pore-scale ratio and porosity. Turbulent two-point correlations, integral length scales and premultiplied energy spectra are calculated from the DNS and macroscopic simulation results to determine the length scale of the turbulent structures. Our numerical results show that the flow becomes turbulent when the Reynolds number is sufficiently large. However, the length scale of turbulence is not considerably affected by the Reynolds number, Darcy number and pore-scale geometry. The PSPH is valid when the porosity has small or medium values. At a sufficiently large Reynolds number, large-scale turbulence survives if the porosity is larger than a critical value. Our DNS and macroscopic simulation results show that this critical value is in the range 0.93–0.98 for porous matrices with large Darcy numbers (0.3–1.26 using the definition in this study). The dependence of the critical porosity on the pore-scale geometry still needs to be further investigated.

Modeling of Cube Array Roughness: RANS, Large Eddy Simulation, and Direct Numerical Simulation

AbstractFlow over arrays of cubes is an extensively studied model problem for rough wall turbulent boundary layers. While considerable research has been performed in computationally investigating these topologies using direct numerical simulation (DNS) and large eddy simulation (LES), the ability of sublayer-resolved Reynolds-averaged Navier–Stokes (RANS) to predict the bulk flow phenomena of these systems is relatively unexplored, especially at low and high packing densities. Here, RANS simulations are conducted on six different packing densities of cubes in aligned and staggered configurations. The packing densities investigated span from what would classically be defined as isolated, up to those in the d-type roughness regime, filling in the gap in the present literature. Three different sublayer-resolved turbulence closure models were tested for each case: a low Reynolds number k–ϵ model, the Menter k–ω SST model, and a full Reynolds stress model. Comparisons of the velocity fields, secondary flow features, and drag coefficients are made between the RANS results and existing LES and DNS results. There is a significant degree of variability in the performance of the various RANS models across all comparison metrics. However, the Reynolds stress model demonstrated the best accuracy in terms of the mean velocity profile as well as drag partition across the range of packing densities.

EFFECT OF MOMENTUM PROVISION IN COMPUTATIONAL FLUID DYNAMICS ON TURBULENT STATISTICS IN URBAN BOUNDARY LAYER

To reproduce homogeneous flow fields above urban-like turbulent boundary layers in computational fluid dynamics (CFD), numerous CFDs with a limited numerical domain employ periodic boundary conditions in both lateral directions with artificial momentum provision to maintain airflows. Although such artificial momentum sources are clearly different from the momentum provision in wind-tunnel experiments, the effects of these numerical conditions on the airflow in the boundary layer are not well studied. In this paper, therefore, we investigate how the momentum provision differentiates the vertical Reynolds stress and turbulent kinetic energy profiles using large-eddy simulations of airflows over cubical block arrays.