Array Of Obstacles Research Articles

A study of the occurrence of DDT in an array of obstacles: Combined effects of longitudinal and transverse obstacle spacings

Combustion wave propagation in a channel containing a stoichiometric hydrogen-air mixture and an array of obstacles was investigated, aiming to explore the effects of longitudinal (ls) and transverse (ts) spacings of the obstacles on the occurrence of deflagration-to-detonation transition (DDT). A high-order numerical algorithm and adaptive mesh refinement were adopted to solve reactive Navier-Stokes equations. The spacings, ls and ts, are varied from 1.2 to 6 times the obstacle diameter (do = 12.7 mm). The results show that the occurrence of DDT is largely dependent on ts. The DDT is likely to occur when the ratio of transverse obstacle gap to detonation cell size d/λ ≥ 0.64. The DDT run-up distance and time are closely related to ls and ts. For large ts, the DDT occurrence time increases with ls. However, the DDT distance may decrease significantly as ls increases when ls is relatively small, due to frequent local explosions in the isolated pockets of unreacted material. A requirement for the generation of these isolated explosions can be quantified as (ls/ts)2(12–4br) < 1, where br is the blockage ratio of the channel. For small ts, the increase of DDT distance with ls/ts slows down notably when ls/ts exceeds a threshold, because there is a mechanism characterized by large fluctuations in flame surface area that promotes the occurrence of DDT in the case with a large enough ls/ts.

Colloidal transport phenomena in dynamic, pulsating porous materials

AbstractWe study the transport phenomena of colloidal particles embedded within a moving array of obstacles that mimics a dynamic, time‐varying porous material. While colloidal transport in an array of stationary obstacles (“passive” porous media) has been well studied, we lack the fundamental understanding of colloidal diffusion in a nonequilibrium porous environment. We combine Taylor dispersion theory, Brownian dynamics simulations, and optical tweezer experiments to study the transport of tracer colloidal particles in an oscillating lattice of obstacles. We discover that the dispersion of tracer particles is a nonmonotonic function of oscillation frequency and exhibits a maximum that exceeds the Stokes–Einstein–Sutherland diffusivity in the absence of obstacles. By solving the Smoluchowski equation using a generalized dispersion framework, we demonstrate that the enhanced transport of the tracers depends critically on both the direct interparticle interactions with the obstacles and the fluid‐mediated, hydrodynamic interactions generated by the moving obstacles.

A free-swimming tadpole model based on immersed boundary-lattice Boltzmann method and its application

A two-dimensional (2D) free-swimming tadpole model is built in this study using the immersed boundary-lattice Boltzmann method. The tadpole is developed by connecting a passive elliptical head with a beating tail. This developed tadpole is capable of controlling the tail swing amplitude to change the swimming speed and achieve the desired swimming direction by attaching an angle offset on the tail axis. The hydrodynamics of the proposed tadpole model in swimming is investigated by regulating the width of the confined space. To be specific, three points are summarized below. First, a lower swimming speed will be produced in a narrower channel under the identical swimming pattern. Second, under the effect of a slight swing strength, a small-scale disturbance is triggered to the surrounding fluid, and a small swimming speed will be generated. Third, a relatively small or excessive swimming speed adversely affects the stability of its swimming. Moreover, a perception-response strategy for the tadpole is further formulated to achieve its autonomous locomotion control. A virtual perceptive field is proposed as the visual range, which is conducive to implementing tadpole motion control based on a set of mechanical response rules. With the above-mentioned improvements, the tadpole can effectively achieve obstacle avoidance in sophisticated obstacle array environments and tracking sine curve routines. Accordingly, this study can provide a valuable reference for the theoretical design of underwater bionic tadpole-like robots.

Reverse time migration for imaging periodic obstacles with electromagnetic plane wave

Abstract We propose novel reverse time migration (RTM) methods for the imaging of periodic obstacles using only measurements from the lower or upper side of the obstacle arrays at a fixed frequency. We analyze the resolution of the lower side and upper side RTM methods in terms of propagating modes of the Rayleigh expansion, Helmholtz–Kirchhoff equation and the distance of the measurement surface to the obstacle arrays, where the periodic structure leads to novel analysis. We give some numerical experiments to justify the competitive efficiency of our imaging functionals and the robustness against noises. Further, numerical experiments show sharp images especially for the vertical part of the periodic obstacle in the lower-RTM case, which is not shared by results for imaging bounded compactly supported obstacles.

Toward pore-scale simulation of combustion in porous media using a low-Mach hybrid lattice Boltzmann/finite-difference solver

A hybrid numerical model previously developed for combustion simulations is extended in this article to describe flame propagation and stabilization in porous media. The model, with a special focus on flame/wall interaction processes, is validated via corresponding benchmarks involving flame propagation in channels with both adiabatic and constant-temperature walls. Simulations with different channel widths show that the model can correctly capture the changes in flame shape and propagation speed as well as the dead zone and quenching limit, as found in channels with cold walls. The model is further assessed considering a pseudo two-dimensional porous burner involving an array of cylindrical obstacles at constant temperature, investigated in a companion experimental study. Furthermore, the model is used to simulate pore-scale flame dynamics in a randomly generated three-dimensional porous media. Results are promising, opening the door for future simulations of flame propagation in realistic porous media.

Pattern formation and transport for externally driven active matter on periodic substrates (a)

We investigate the transport of interacting active run-and-tumble particles moving under an external drift force through a periodic array of obstacles for increasing drive amplitudes. For high activity where the system forms a motility-induced phase-separated state, there are several distinct dynamic phases including a low drive pinned cluster phase, an intermediate uniform fluid, and a higher drive stripe crystal state. The transitions between the phases are correlated with signatures in the transport curves, differential mobility, and power spectra of the velocity fluctuations. In contrast, in the low activity regime the transport curves and power spectra undergo little change as a function of drive. We argue that in the high activity limit, the behavior is similar to that of driven solids on periodic substrates, while in the low activity limit the system behaves like a driven fluid.

Accelerating discrete dislocation dynamics simulations with graph neural networks

Discrete dislocation dynamics (DDD) is a widely employed computational method to study plasticity at the mesoscale that connects the motion of dislocation lines to the macroscopic response of crystalline materials. However, the computational cost of DDD simulations remains a bottleneck that limits its range of applicability. Here, we introduce a new DDD-GNN framework in which the expensive time-integration of dislocation motion is entirely substituted by a graph neural network (GNN) model trained on DDD trajectories. As a first application, we demonstrate the feasibility and potential of our method on a simple yet relevant model of a dislocation line gliding through an array of obstacles. We show that the DDD-GNN model is stable and reproduces very well unseen ground-truth DDD simulation responses for a range of straining rates and obstacle densities, without the need to explicitly compute nodal forces or dislocation mobilities during time-integration. Our approach opens new promising avenues to accelerate DDD simulations and to incorporate more complex dislocation motion behaviors.

Achieving Complete Band Gaps of Water Waves with Shallow-Draft Cylinder Arrays

Conventionally, band gaps of water waves can be obtained with periodic arrays of bottom-mounted obstacles or vertical cylinders. However, such structures need to possess a large ratio of volume in water, and may not be easy to build and move in practical ocean engineering. Here, we theoretically propose and experimentally demonstrate that when the water surface is pierced by a fixed, rigid cylinder array, a complete band gap of water waves can be achieved, in which the propagation of water waves is forbidden in all the directions. Such a band gap can exist even when the cylinders have a draft much smaller than the water depth ($d\ensuremath{\ll}h$). Based on the complete band gap, a semicircular shape of the fixed-surface disk array is further constructed, which can block omnidirectional water waves generated by a point source.

Study of interscale interactions for turbulence over the obstacle arrays from a machine learning perspective

Investigating interactions between large- and small-scale motions is essential for understanding turbulence over rough boundaries. The present work applies XGBoost models to predict the spatial distribution of ejections and sweeps and quantify their statistical dependence on scale-decomposed velocity fields. Based on large eddy simulation, the models are trained and validated at 20 horizontal planes in turbulence over two types of obstacle arrays. At each height, a default XGBoost model X0 and four comparison models ( X u L , w L , X u L , w S , X u S , w L, and X u S , w S) are trained. The model X0 is trained by the set with four scale-decomposed velocity fields { u L , u S , w L , w S }, where u and w are the streamwise and vertical velocity fluctuations and subscripts L and S refer to above-canyon and sub-canyon scales, while the comparison models are trained by subsets of the scale-decomposed velocity fields. The results indicate that the model X0 predicts the spatial distributions of both ejection and sweep events well, with the structure underestimation being less than 8% within the canopy layer and 3% above it. Along the vertical direction, the relative importance of scale-decomposed velocity fields on the prediction of ejections and sweeps is quantified by the feature importance and prediction errors. The feature importance profiles reveal that both sweeps and ejections are most strongly related to w S within the canopy, but ejections have a stronger dependence on u L well above the canopy. For the comparison models, those trained with w S (namely, X u L , w S and X u S , w S) give better predictions within the canopy layer, whereas those trained with u L (namely, X u L , w S and X u L , w L) perform better above the canopy. This study shows that a machine-learning-based approach can be designed to quantify the relative importance of different scale-decomposed velocity fields on predicting ejections and sweeps and to detect vertical changes of such relative importance.

Size of a 2D ring polymer topologically unentangled with a planar array of obstacles

We readdress the statistical mechanical problem of the size of a 2D ring polymer, topologically unentangled with a planar lattice array of regularly spaced obstacles. It is commonly assumed in the literature that such a polymer adopts a randomly branched type of configuration, in order to ostensibly maximise chain entropy, while minimising obstacle entanglement. Via an innovative analytic approach, valid in the condensed polymer region, we are able to provide a greater theoretical understanding, and justification, for this presumed polymer behaviour. Our theoretically derived results could also potentially have important implications for the structure of interphase chromosomes, as well as electrophoretic ring polymer dynamics.

Optimal design of an array of porous obstacles in a partially heated channel using lattice Boltzmann method for the heat transfer enhancement

The present work investigates the heat transfer enhancement in a channel includes a number of porous obstacles. A partially heated channel is considered, and an array of porous blocks installed on the channel wall is used as an effective heat transfer enhancement mechanism. The flow and temperature fields are resolved using a well-tuned lattice Boltzmann method. An optimal design is obtained for the porous obstacles considering the blocks' height and pitch together with the permeability of the porous medium as the design parameters. In addition, the impact of Da on the thermal performance of the porous block is studied. The PEC number (Performance Evaluation Criteria), which simultaneously accounts for the pressure drop penalty and the heat transfer rate augmentation, is chosen as a proper thermal performance evaluation criteria. The obtained results show an optimum value for both blocks' height and pitch, which depends on the Da. Moreover, the impact of obstacle geometry and the distribution of porous material inside the channel is investigated on the thermal performance of the heat-removing system.

Applications of Polyaniline-Based Molybdenum Disulfide Nanoparticles against Brain-Eating Amoebae.

Primary amoebic meningoencephalitis and granulomatous amoebic encephalitis are distressing infections of the central nervous system caused by brain-eating amoebae, namely, Naegleria fowleri and Acanthamoeba spp., respectively, and present mortality rates of over 90%. No single drug has been approved for use against these infections, and current therapy is met with an array of obstacles including high toxicity and limited specificity. Thus, the development of alternative effective chemotherapeutic agents for the management of infections due to brain-eating amoebae is a crucial requirement to avert future mortalities. In this paper, we synthesized a conducting polymer-based nanocomposite entailing polyaniline (PANI) and molybdenum disulfide (MoS2) and explored its anti-trophozoite and anti-cyst potentials against Acanthamoeba castellanii and Naegleria fowleri. The intracellular generation of reactive oxygen species (ROS) and ultrastructural appearances of amoeba were also evaluated with treatment. Throughout, treatment with the 1:2 and 1:5 ratios of PANI/MoS2 at 100 μg/mL demonstrated significant anti-amoebic effects toward A. castellanii as well as N. fowleri, appraised to be ROS mediated and effectuate physical alterations to amoeba morphology. Further, cytocompatibility toward human keratinocyte skin cells (HaCaT) and primary human corneal epithelial cells (pHCEC) was noted. For the first time, polymer-based nanocomposites such as PANI/MoS2 are reported in this study as appealing options in the drug discovery for brain-eating amoebae infections.

Short and long-range cyclic patterns in flows of DNA solutions in microfluidic obstacle arrays.

We observe regular patterns emerging across multiple length scales with high-concentration DNA solutions in microfluidic pillar arrays at low Reynolds numbers and high Deborah numbers. Interacting vortices between pillars lead to long-range order in the form of large travelling waves consisting of DNA at high concentration and extension. Waves are formed in quadratic arrays of pillars, while randomizing the position of the pillar in each unit cell of a quadratic array leads to suppression of the long-range patterns. We find that concentrations exceeding the overlap concentration of the DNA enables the waves, and exploring the behavior of the waves as a function of flow rate, buffer composition, concentration and molecular length, we identify elastic effects as central to the origin of the waves. Our work may not only help increase the low throughput that often limits sample processing in microfluidics, it may also provide a platform for further studies of the underlying viscoelastic mechanisms.

TRANSFERRING LEGAL INFORMATION TO THE ELDERLY: ADDRESSING ACCESSIBILITY TO THEIR RIGHTS AND THE RESTRICTIONS THEY ENCOUNTER

The paper addresses areas such as language and legal knowledge in connection with the elders’ awareness of their rights based on proper legal communication. The elder population encounter a wide array of legal obstacles, many of which prevent them from obtaining up-to-date legal information, thereby unwittingly imposing restrictions on them. The first part of the paper explains the terms related to the barriers the older people face with particular emphasis on ageism. What is more, it provides data which emphasize the lack of adequate information, leading to the purpose of this paper: improving the methods in which older generations obtain legal advice. The second part of the paper recognizes the most common, in our opinion, problems the elderly find difficult to deal with. Primarily, we try to concentrate on the reasons lying at the heart of that daunting situation which respectively would be access to the Internet and know-how to use electronic devices in order to get different pieces of information, stressing the point why state-of-the-art methods are not the best educational choice for the older generation. Subsequently, we put forward the way of sorting the issue, highlighting the significance of well-performed face-to-face communication which according to our research works best for people over the age of 60. The final part provides conclusions drawn from the results obtained from the literature review, as well as the dire need of implementing some innovative solutions that may help the elderly in everyday life.

Experimental investigation of blast mitigation of pre-fractal obstacles

Obstacles arranged into a pre-fractal shape (Sierpinski carpet) were tested for their blast attenuation abilities using 250 g PE4 at three different scaled distances ( Z = 1.87, 2.24, 2.99 m/kg1/3). Three pre-fractal iterations were tested, as well as free-field tests for comparative purposes. Reductions in peak overpressure up to 26% and peak specific impulse up to 19% were observed, attributed to a mechanism known as ‘trapping’. This mechanism is characterised by a reduction in the ability of a blast wave to advect downstream, with corresponding increases in pressure observed within the bounds of the pre-fractal obstacle. Attenuation magnitudes and areas of reduced pressure and impulse were found to be drastically different with each pre-fractal iteration, with a transition from shadowing to wave trapping as the obstacles more closely resembled true fractals. A linear dependence on a newly-defined obstruction factor ( OF) was found for arrival time, overpressure and impulse at the sensor locations, suggesting that the attenuation of a pre-fractal obstacle is inherently determinable. The results indicate that the mechanism of blast mitigation of pre-fractal obstacles is fundamentally different from singular or arrays of regular obstacles, and could be exploited further to develop novel protective structures with enhanced blast attenuation.

Non-symmetric pinning of topological defects in living liquid crystals

Topological defects, such as vortices and disclinations, play a crucial role in spatiotemporal organization of equilibrium and non-equilibrium systems. The defect immobilization or pinning is a formidable challenge in the context of the out-of-equilibrium system, like a living liquid crystal, a suspension of swimming bacteria in lyotropic liquid crystal. Here we control the emerged topological defects in a living liquid crystal by arrays of 3D-printed microscopic obstacles (pillars). Our studies show that while −1/2 defects may be easily immobilized by the pillars, +1/2 defects remain motile. Due to attraction between oppositely charged defects, positive defects remain in the vicinity of pinned negative defects, and the diffusivity of positive defects is significantly reduced. Experimental findings are rationalized by computational modeling of living liquid crystals. Our results provide insight into the engineering of active systems via targeted immobilization of topological defects.

Windowed Green function method for wave scattering by periodic arrays of 2D obstacles

AbstractThis paper introduces a novel boundary integral equation (BIE) method for the numerical solution of problems of planewave scattering by periodic line arrays of two‐dimensional penetrable obstacles. Our approach is built upon a direct BIE formulation that leverages the simplicity of the free‐space Green function but in turn entails evaluation of integrals over the unit‐cell boundaries. Such integrals are here treated via the window Green function method. The windowing approximation together with a finite‐rank operator correction—used to properly impose the Rayleigh radiation condition—yield a robust second‐kind BIE that produces superalgebraically convergent solutions throughout the spectrum, including at the challenging Rayleigh–Wood anomalies. The corrected windowed BIE can be discretized by means of off‐the‐shelf Nyström and boundary element methods, and it leads to linear systems suitable for iterative linear algebra solvers as well as standard fast matrix–vector product algorithms. A variety of numerical examples demonstrate the accuracy and robustness of the proposed methodology.

Flame acceleration and DDT in a channel with fence-type obstacles: Effect of obstacle shape and arrangement

Experiments and numerical simulations were conducted to study the effects of obstacle shape and arrangement on the flame acceleration and deflagration-to-detonation transition (DDT) in an obstructed channel filled with a stoichiometric hydrogen–oxygen mixture. Two different shapes and arrangements of fence-type obstacles with the same blockage area were considered: continuous triangular and discrete rectangular obstacles. In the experiments, high-speed schlieren photography was used to record the flame acceleration and DDT process with a blockage ratio of 0.5. In the calculations with the different blockage ratios, a high-order numerical algorithm was used to solve the multidimensional, fully compressible, reactive Navier–Stokes equations coupled to a calibrated single-step chemical-diffusive model. The quantitative agreement between the calculations and experiments allows a detailed analysis of the DDT mechanism and the influence of obstacle shape/arrangement using the numerical results. The results show that the differences in the flame acceleration and distance to DDT between the triangular and rectangular obstacle arrays are closely related to the blockage ratio. For a low blockage ratio (br<0.5), flame accelerates faster and DDT occurs in a shorter time in the channel with the triangular obstacles because the inclined surfaces of the triangular obstacles allow stronger flame-vortex interactions and are more favorable to detonation survival once a detonation is initiated in the gap between obstacles. Nevertheless, for a high blockage ratio (br>0.5), the lateral triangle surfaces become steeper, and the advantages of triangular obstacles for promoting DDT become less effective. In addition, the triangles have a more significant weakening effect on the leading shock, while the rectangular obstacles allow the formation of Mach stems. These effects make the rectangular obstacles with a high blockage ratio more conducive to the occurrence of DDT than their triangular counterparts.

Inverse wave scattering in the time domain for point scatterers

Let Δα,Y be the bounded from above self-adjoint realization in L2(R3) of the Laplacian with n point scatterers placed at Y={y1,…,yn}⊂R3, the parameters (α1,…αn)≡α∈Rn being related to the scattering properties of the obstacles. Let ufϵα,Y and ufϵ∅ denote the solutions of the wave equations corresponding to Δα,Y and to the free Laplacian Δ respectively, with a source term given a pulse fϵ supported in ϵ-neighborhoods of the points in XN={x1,…,xN}, XN∩Y=∅. We show that, for any fixed λ>sup⁡σ(Δα,Y)≥0, there exists N∘≥1 such that the locations of the points in Y can be determined by the knowledge of a finite-dimensional scattering data operator FλN:RN→RN, N≥N∘. Such an operator is defined in terms of the limit as ϵ↘0 of the Laplace transform of ufϵα,Y(t,xk)−ufϵ∅(t,xk), k=1,…,N. We exploit the factorized form of the resolvent difference (−Δα,Y+λ)−1−(−Δ+λ)−1 and a variation on the finite-dimensional factorization in the MUSIC algorithm. Multiple scattering effects are not neglected; our model can be interpreted as the time-domain version of a frequency-domain scattering from an array of Foldy's point-like obstacles.

Blockchain Technology to Ensure Traceability of Agriculture Supply Chain Management

Agriculture is an industry that is essential to the functioning of any economy. Our entire economic structure revolves around this one factor. Agriculture is not only an important source of food and raw materials, but it is also an important source of employment opportunities for a significant fraction of the total population. In agriculture food supply chains, also known as ASCs, customers are the ones who pay for the agricultural products that farmers produce. During this process, customers place a higher priority on the safety of agriculturally produced foods, while farmers anticipate an increase in their revenues. The effectiveness of traceability and management for agri-food products is met with a vast array of obstacles as a result of the complexities and dynamism of ASCs. However, the majority of the currently available solutions are not able to satisfactorily meet the standards of ASCs regarding traceability and management. Chen et al., proposed a blockchain-based ASC framework to provide product traceability. This framework guarantees decentralized security for the agriculture food tracing data stored in ASCs. This was done in order to address the problems that have been presented. In order to guarantee the quality of the food, it is critical to have complete visibility into the agricultural supply chain management. Along with that, it improves the overall pleasure of the customers and the productivity of the peers. The centralized storage of data makes it more difficult to guarantee the items' quality and rate, as well as their point of origin. Consequently, we are in need of a decentralized system that allows for transparency and provides everyone, from the producers to the consumers, with the satisfaction they seek. The supply chain can now have both greater traceability and transparency thanks to blockchain technology, which is a form of digital technology. According to Madumidha utilization of this technology results in an improvement to the community that exists between the various stakeholders and the farmers. Increased capacity, improved security, immutability, minting, speedier settlement, and full traceability of all recorded transaction records are fundamentally provided by blockchain's features.