Initial Orientation Angle Research Articles

Stabilized motion of a freely falling bristled disk

Several tiny insects have peculiar porous wings composed of many bristles and perform an interesting passive flight known as parachuting. Despite numerous studies on the freefall of objects such as disks, the aerodynamic principles of the effects of a bristled configuration on the parachuting motion under external disturbances remain unexplored. Here, we experimentally investigate freely falling bristled disks over a wide range of Reynolds numbers by changing the number of bristles and the initial orientation angle and compare their kinematics with those of a full circular disk with no bristles. Given the same diameter and moment of inertia, bristled disks with a smaller area have a steady-state flow field similar to that of a circular disk by virtue of the presence of a fully formed virtual fluid barrier at low-Reynolds numbers. However, in the initial transient phase after release, the bristled disks show different damped oscillatory motions from a circular disk. Regardless of their initial orientation angle, the lateral and angular deviations of the bristled disks are smaller than those of the circular disk, producing a more stable freefall. This trend is also observed even for higher Reynolds numbers, where the bristled wings are known to be ineffective from the perspective of aerodynamic performance. By considering the vorticity fields around the disk, we suggest two vortex-related mechanisms that account for the stable falling of the bristled disk, namely, the formation of more symmetric vortex structures and the location of vortex cores closer to the disk center.

Lamb wave propagation in anisotropic multilayered piezoelectric laminates made of PVDF-θ° with initial stresses

In this paper, the influence of an initial stresses on the Lamb waves propagation in an anisotropic multilayered piezoelectric laminates is investigated. Formulations are given for only open circuit surface; while the Legendre polynomial method is employed for calculate the wave propagating characteristics. To validate our numerical strategy, this polynomial approach was applied to simulate lamina composite structures. Relatively good results were obtained compared to reliable data available from the literature ones, with relatively significant computation effort. Convergence of the present method is discussed. Influence of initial stresses and fiber orientation angle on the behaviors of Lamb waves is analyzed. Results confirm that the behavior of Lamb modes depends not only on the initial stress but also on fiber orientation. Next, the dispersion curves of fundamental Lamb modes in sandwich plates with 0°/θ°/0 configuration are calculated. Finally, the optimal angles in PVDF-θ° with initial stresses for which the Lamb modes exist are also analyzed. These results for the composite structures may render as a useful reference for the design of ultrasonic transducers in industry for non-destructive testing especially that encounters initial stresses.

A molecular dynamics study of lubricating mechanism of graphene nanoflakes embedded in Cu-based nanocomposite

Metal matrix composites containing graphene show excellent lubricating performance, while the detailed atomic scale understanding about the origin of this superior lubrication is still absent. In this study, the self-lubricating behaviors of Cu-based nanocomposites embedded with graphene nanoflakes (GNFs) were investigated by large-scale molecular dynamics simulations. The simulation results indicate that the friction reduction was achieved via the reorientation of GNFs in polycrystalline Cu matrix. We found that the friction coefficient is closely related to the coverage ratio of GNFs at the sliding interface, as the formation of van der Waals gaps in GNFs and between GNF and Cu matrix will reduce the sliding resistance. Especially, a minimum friction coefficient could be obtained when GNFs spread over the whole sliding interface. In addition, the number of layers, flake size and initial orientation angle of GNFs together with multiple GNFs in Cu matrix have also been considered. The simulated results indeed confirm the formation of van der Waals gap behaves as an effective mechanism to reduce the sliding friction.

Experiment and simulation study of erosion mechanism in float glass due to rhomboid particle impacts

A coupled FEM-SPH numerical model is established to investigate the erosion behaviors of float glass subjected to angular particle impacts. The accuracy of the model is validated by comparing with measured data onto float glass erosion experiments. The model is then used to simulate single rhomboid particle impact on float glass surfaces under various conditions including impact velocity (20~110 m/s), impact angle (60~90 deg) and initial orientation angle (−10~50 deg). The results show: (1) similar to ductile material, the critical orientation angle marking the maximum kinetic energy loss is also observed for brittle material like float glass. Under the critical impact condition, the kinetic energy loss of rhomboid particle is up to 98.69% and 94.82% during vertical and incline impact respectively; (2) The formation and propagation of radial cracks and lateral cracks are determined by the combined effects of impact angle and orientation angle, and the brittle crater formed by the backward impact is wider and shallower than that of forward impact; (3) The behaviors of brittle splashing (i.e., fragments and chips) are successfully reproduced by the model, and the particle rotation behaviors after the impact significantly influences the direction of splashing stream which is less influenced by the impact angle; (4) The impact velocity determines the extent of material damages, and has little influence on the rotation behaviors of particle. Under the condition of constant kinetic energy, the size of particle (1.0~5.0 mm) has little effect on the erosion mechanisms in terms of crater depth, crater profile, splashing flow pattern and crack propagation.

Parametric study on the performance of an air curtain based on CFD simulations - New proposal for automatic operation

This paper presents a parametric study on the performance of an air curtain installed over an access door of a refrigerated room. The aim of this work is to quantify, in an integrated way, the influence of the following geometrical and dynamic parameters: door height; temperature difference between rooms, nozzle thickness; initial orientation angle; jet discharge velocity. A numerical model was used to simulate the turbulent non-isothermal 3D airflow generated in the transient period after the door is opened, with the air curtain device turned on or off. Turbulence effects were taken into account with the k-ω SST model. Results show that the optimum discharge velocity of the air curtain increases with the door height, as well as with the temperature difference between both sides of the air curtain. It was also possible to find an optimum jet nozzle width corresponding to maximum sealing efficiency and lower jet airflow rate. A proposal is made for an empirical correlation to predict the optimum jet discharge velocity as function of the variables used in the parametric study. Additionally, to ensure that the air curtain operates near the optimum conditions, the appropriate air curtain settings are discussed and a new strategy is suggested.

Distributed Formation Control: Asymptotic Stabilization Results Under Local Noisy Information.

This paper studies the relative-position-based reference-frame-free formation control in the plane for discrete-time multiagent systems under both measurement and communication noises. To achieve the desired formation, a robust distributed orientation estimate algorithm and a robust distributed formation control law are designed. The main idea of our method is to use the historical measurement information to get unbiased estimates and use the stochastic approximation method to inhibit the noise. Under mild assumptions on the measurement/communication noises and agents' initial orientation angles, we show that a common reference frame can be asymptotically shared by all agents and the desired formation can be asymptotically stabilized almost surely under the designed distributed control law, if and only if the magnitude of the bearing measurement noise is less than ( π / 4) , the communication and distance sensing topology is rooted and the corresponding bearing sensing topology is connected. Simulations are conducted to verify the correctness and effectiveness of our orientation estimate algorithm and formation control law.

Hydrodynamic properties of squirmer swimming in power-law fluid near a wall

Hydrodynamic properties of squirmer swimming in power-law fluid near a wall considering the interaction between squirmer and wall are numerically studied with an immersed boundary-lattice Boltzmann method. The power-law index, Reynolds number, initial orientation angle of squirmer, and initial distance of squirmer from the wall are all taken into account to investigate the swimming characteristics for pusher (β 0) (three kinds of swimmer types) near the no-slip boundary. Four new kinds of swimming modes are found. Results show that, for the pushers and pullers, the wall displays an increasing attraction with increasing power-law index n, which differs from the neutral squirmer who always departs from the wall after the first collision with the wall. Both the initial orientation angle and initial distance from the wall only affect the moving situations rather than the moving modes of the squirmers. However, the squirmers depart from the wall as the Reynolds number increases and chaotic orbits appear for some squirmers at Re = 5. Several typical flow fields are analyzed and the power consumption and torque for different kinds of flows are also studied. It is found that, as the absolute value of β increases, the power consumption generally increases in shear-thinning (n = 0.4), Newtonian (n = 1), and shear-thickening (n = 1.6) fluids. Moreover, the pushers (β 0) expend almost the same power if the absolute value of β remains the same. In addition, the power consumption of the squirmers is highly dependent on the power-law index n.

A new model for the acoustic wake effect in aerosol acoustic agglomeration processes

• A new model of acoustic wake effect in aerosol acoustic agglomeration is established. • The new model has been verified to be much more accurate than the existing model. • Particle agglomeration under the acoustic wake effect is calculated. The acoustic wake effect has been considered the main agglomeration mechanism in the aerosol acoustic agglomeration processes. However, the existing theoretical model for the acoustic wake effect, which was proposed half century ago, is not accurate enough and overestimates the perturbation velocity in the vicinity of particles by as high as approximately 50%. In this paper, a new model for the acoustic wake effect is established, in which an approximate expression is constructed to describe the perturbation velocity. The newly developed model has been verified to be more accurate than the existing model by both theoretical analysis of boundary conditions and computational fluid dynamics simulation . The trajectories of two particles in a horizontal sound field are calculated based on the new model and the existing model. The results show that the new model corrects the range of initial orientation angle for two particles to successfully agglomerate, which was overestimated in the existing model. Moreover, the collision efficiency of two different-sized particles under the acoustic wake effect is found to be larger in the simulation by the new model than that by the existing model. The new model can be used as a reasonably accurate tool not only for calculating the agglomeration of two particles, but also for the numerical simulation of acoustic agglomeration of an aerosol containing a large number of particles due to its low computational cost.

Sound absorption and compression properties of perpendicular-laid nonwovens

This study presents an investigation into the sound absorption behavior and compression properties of perpendicular-laid nonwovens. Seven types of perpendicular-laid nonwovens produced by vibrating and rotating perpendicular lappers were selected. Nonwovens with varying thickness and areal density were prepared by the heat-pressing method to investigate the effect of structural parameters such as thickness and areal density on sound absorption ability. Measurements of sound absorption properties were carried out with a Brüel and Kjær measuring instrument. The effect of manufacturing techniques on sound absorption performance and compression properties was investigated. The effect of porosity on sound absorption ability was studied. The influence of density and fiber orientation angle on compression properties was analyzed. The results show that samples prepared by vibrating perpendicular lapper exhibit better compression properties, whereas there is no significant influence of two manufacturing techniques on sound absorption performance. The increase of areal density results in improvement in the sound absorption ability. The increase of thickness can improve the sound absorption coefficient in the low-frequency range, but decrease of the coefficient occurred in the high-frequency range. A quadratic relationship between porosity and sound absorption ability has been found. The results also show that compressional resistance has a strong relation with density – the correlation coefficient is 0.95, indicating that the compressional resistance is directly proportional to the density of perpendicular-laid nonwovens. The results indicate that the perpendicular-laid nonwovens with higher initial fiber orientation angle have better compression properties.

Polarimetric decomposition for the selection of persistent scatterers

ABSTRACTIn this paper, the issue of employing polarimetric processing methods for persistent scatterers (PS) selection is addressed. The scope of the paper regards the processing methods for PS generation by means of coherent target decomposition (CTD). The key is to exploit scatterer physical characteristics, besides scatterer coherence and stability, so as to classify them as PS utilizing as less data sets as possible. Cameron’s CTD is proposed for PS classification based on scatterers physical properties as an alternative to polarization phase difference (PPD) for it can eliminate the scatterer initial orientation angle. Additionally, provides a complete set of elemental scattering mechanisms in order to describe the physical properties of the scatterer, and thus create a more reliable PS net. These assets make Cameron’s CTD outperform PPD in exploiting scatterer physical properties for PS selection. The study also focuses on minimizing the number of used data sets for PS net generation, which led to experimenting with only two fully polarimetric SAR data sets. The results are promising enough to establish the proposed method as an auxiliary tool to be utilized prior to any classic interferometric techniques for early scene evaluation in order to decide whether the scene would be fruitful for applying permanent scatterer interferometry or not.

Nanovoid growth in BCC α-Fe: influences of initial void geometry

The growth of voids has a great impact on the mechanical properties of ductile materials by altering their microstructures. Exploring the process of void growth at the nanoscale helps in understanding the dynamic fracture of metals. While some very recent studies looked into the effects of the initial geometry of an elliptic void on the plastic deformation of face-centered cubic metals, a systematic study of the initial void ellipticity and orientation angle in body-centered cubic (BCC) metals is still lacking. In this paper, large scale molecular dynamics simulations with millions of atoms are conducted, investigating the void growth process during tensile loading of metallic thin films in BCC α-Fe. Our simulations elucidate the intertwined influences on void growth of the initial ellipticity and initial orientation angle of the void. It is shown that these two geometric parameters play an important role in the stress–strain response, the nucleation and evolution of defects, as well as the void size/outline evolution in α-Fe thin films. Results suggest that, together with void size, different initial void geometries should be taken into account if a continuum model is to be applied to nanoscale damage progression.

Numerical Study of Impact Behaviors of Angular Particles on Metallic Surface Using Smoothed Particle Hydrodynamics

ABSTRACTModeling and studying the impact behaviors of angular particles is critical in understanding the mechanisms of erosive wear on solid surfaces. This article focuses on effective mesh-free model based on the smoothed particle hydrodynamics (SPH) method to simulate impacts of angular particles on metallic surfaces. The predicted results are compared with the available experimental data, and good agreement has been achieved. Our simulations under different incident conditions successfully reproduce the general impact behaviors of angular particles, including rotating behavior and rebound behavior, which enables detailed examinations of erosion mechanisms. We find that the rotating behaviors are mainly determined by initial orientation and impact angle, whereas impact velocity has little effect. For backward impact involving a prying-off action, there generally exsits a critical impact velocity below which the cutting process would never be finished, which may result in a rebound angle greater than 90°. Further, multiple and overlapping impacts are simulated to reveal the effect of a pre-created crater on the subsequent impact. The results demonstrate the ability of the present model to handle the extremely deformed surface by overlapping impacts. The proposed SPH model and the present study could be useful in the study of erosive wear on the surface of metal devices that carry granular substances.

On the role of initial void geometry in plastic deformation of metallic thin films: A molecular dynamics study

Void growth is usually considered one of the most critical phases leading to dynamic fracture of ductile materials. Investigating the detailed process of void growth at the nanoscale aids in understanding the damage mechanism of metals. While most atomistic simulations by far assume circular or spherical voids for simplicity, recent studies highlight the significance of the initial void ellipticity in mechanical response of voided metals. In this work, we perform large scale molecular dynamics simulations with millions of atoms to investigate the void growth in plastic deformation of thin films in face-centered cubic Cu. It is found that the initial ellipticity and the initial orientation angle of the void have substantial impacts on the dislocation nucleation, the void evolution, and the stress-strain response. In particular, the initial dislocation emission sites and the sequence of slip plane activation vary with the initial void geometry. For the void size evolution, three regimes are identified: (I) the porosity increases relatively slowly in the absence of dislocations, (II) the porosity grows much more rapidly after dislocations start to glide on different slip planes, and (III) the rate of porosity variation becomes much more slowly when dislocations are saturated in the model, and the void surface becomes irregular, non-smooth. In terms of the stress-strain response, the effects of the initial orientation angle are more pronounced when the initial void ellipticity is large; the influence of the initial void ellipticity is different for different initial orientation angles. The effects of the temperature, the strain rate, the loading direction, and the initial porosity in the void growth are also explored. Our results reveal the underlying mechanisms of initial void geometry-dependent plastic deformation of metallic thin films and shed light on informing more accurate theoretical models.

RELATIVISTIC DOPPLER BEAMING AND MISALIGNMENTS IN AGN JETS

ABSTRACT Radio maps of active galactic nuclei often show linear features, called jets, on both parsec and kiloparsec scales. These jets supposedly possess relativistic motion and are oriented close to the line of sight of the observer, and accordingly the relativistic Doppler beaming makes them look much brighter than they really are in their respective rest frames. The flux boosting due to the relativistic beaming is a very sensitive function of the jet orientation angle, as seen by the observer. Sometimes, large bends are seen in these jets, with misalignments being 90° or more, which might imply a change in the orientation angle that should cause a large change in the relativistic beaming factor. Hence, if relativistic beaming does play an important role in these jets such large bends should usually show high contrast in the brightness of the jets before and after the bend. It needs to be kept in mind that sometimes a small intrinsic change in the jet angle might appear as a much larger misalignment due to the effects of geometrical projection, especially when seen close to the line of sight. What really matters are the initial and final orientation angles of the jet with respect to the observer’s line of sight. Taking the geometrical projection effects properly into account, we calculate the consequences of the presumed relativistic beaming and demonstrate that there ought to be large brightness ratios in jets before and after the observed misalignments.

Modeling and simulation of microcrack propagation behavior under shear stress using phase-field-crystal

Phase-field-crystal (PFC) method is applied to simulate the propagation behavior of microcrack under shear stress in single crystal, which is seldom touched before. The microscopic mechanism is explored through the center precast microcrack on the (111) crystal plane in face-centered cubic (FCC) crystal. The influence of different factors on the crack propagation behavior is analyzed. Results show that the crack shape has influence on the occurrence of propagation. Propagation occurs earlier if the crack contains sharper inner angles, otherwise occurs later. Moreover, crack shape also affects the direction of propagation. The rate of applied shear stress can produce effect on the propagation speed, without altering the specific process of crack propagation. Improving shear rate can accelerate propagation. The initial orientation angle of solid phase has a great influence on the propagation mode and morphology of crack. Curves of change in free energy obtained in this work all monotonically ascend. We can divide the curves into five stages, corresponding to different shear strain stages. When simulation area is slightly small or shear rate is relatively large, the curve contains only three stages. Through this research, the PFC simulation is proved to be an effective means to study microcrack propagation behavior under applied shear stress.

Dissipative Particle Dynamics Models of Orientation of Weakly‐Interacting Anisometric Silicate Particles in Polymer Melts under Shear Flow: Comparison with the Standard Orientation Models

Dissipative particle dynamics (DPD) models of orientation of weakly‐interacting silicate particles in a polymer matrix are presented. To examine the DPD models, the evolution of orientation under shear flow is compared with the predictions of the standard orientation models, namely, the Folgar–Tucker (FT) and the strain reduction factor (SRF) models. While the orientation patterns are the same in all models, the slow orientation kinetics observed in previous experiments is only predicted in the DPD and SRF models. Since the coefficients of the SRF model are in good agreement with the experiments, the good tally between the DPD and SRF models supports the capability of DPD to successfully simulate the orientation process. The orientation in a large cell constructed from unit cells with various averaged initial orientation angles is evaluated from evolutions in the unit cells based on the affine deformation assumption. The good agreement between such calculations and SRF model predictions supports that the affine deformation assumption in the large cell is reasonable. It is argued that the nonaffine deformation originated from the particle‐based nature of DPD models at the lower scale could be combined with the affine deformation at the upper scale to yield appropriate estimations of the orientation state. image

Numerical investigation of single and multi impacts of angular particles on ductile surfaces

Models of single and multi-particle impact(s) on metallic targets allow an understanding of fundamental erosion mechanisms. This work was focused on the creation of a three-dimensional finite element model of the single and multi-particle impacts on a ductile copper target using ANSYS Autodyn V. 14.57 tool. The finite element model was formulated in a Lagrange reference frame. Even at small initial impact velocities, the Lagrange formulation suffered from large material deformation and large element distortion. This resulted in an inefficient increase in simulation time. Therefore, element erosion approach was used to remove the highly distorted elements that were responsible for the time step problems. The copper plate was modeled with a shock equation of state in order to consider the shock propagation as realistic as possible. The Johnson–Cook strength model was used in combination with the Johnson–Cook failure model. Good agreement between simulation results and experimental data was obtained. Moreover, a parameter analysis was carried out by varying the initial input conditions. Erosion mechanisms, such as cratering by material pile-up and chip formation were observed. It was found that mainly the initial orientation angle of the particle θi, and the impact angle αi, determine the erosion mechanism. For a given constant impact angle αi, there is a specific initial orientation angle θs, in which the transition from kinetic energy into internal energy is maximized. For values of θi<θs, the particle rotated forwards after the impact with the copper plate. For initial orientation angles θi>θs, the particle rotated backwards after the impact. The modeling was performed with multi-particle impacts as well. It was found that the rising rate of penetration depth decreases gradually when the number of non-overlapping particle impacts has increased.

Evaluation of Static and Dynamic Stress Intensity Factors under Pure Mode I and Mixed Mode I/II Fracture

The focus of this work is to investigate pre-cracked plate element fracture under mixed mode I/II loading. In order to look into element with inclined initial crack, the test procedure was also performed due to pure opening fracture (I mode) and in-plane shear mode (II mode). For this purpose, static and dynamic tests were performed with original testing device, in which the specimen was fixed in so called “Arcan disc”. The results showed fracture characteristics dependence under initial crack orientation angle, i.e. due opening mode, in-plane shear mode and mixed mode I/II fracture.

Simulation of orthogonal micro-cutting of FCC materials based on rate-dependent crystal plasticity finite element model

Micro-machining of face centered cubic (FCC) metallic materials is simulated via the theory of rate-dependent crystal plasticity. This approach accounts for slip systems and crystallographic orientations in its constitutive framework in order to accurately model the evolution of localized shear band formed during severe plastic deformation of crystalline materials. Through developing a user-defined subroutine in the ABAQUS/Explicit FE platform, the constitutive model is implemented and used to study the influence of workpiece crystallographic orientation on the cutting and thrust specific energies of the process. Due to the high rate of deformation, mechanical properties of texture can be strongly affected by activation mechanism of dislocation motion. Hence, the effects of strain rate and thermal softening are considered in the investigation. Simulations have been carried out for oxygen-free high conductivity copper (OFHC), and the effects of various parameters such as initial orientation angle of the texture on the prediction of localized deformation in the form of adiabatic shear bands are examined. The results delineate the efficiency of the model. Also, this procedure can be made applicable to calibration of force model in micro-milling process.

Formation Control and Network Localization via Orientation Alignment

We propose a formation control strategy based on inter-agent displacements for single-integrator modeled agents in the plane. Since the orientations of the local reference frames of the agents are not aligned with each other due to the absence of a common sense of orientation, the proposed strategy consists of an orientation alignment law and a formation control law. Under the proposed strategy, if the interaction graph is uniformly connected and all the initial orientation angles belong to an interval with length less than π, the orientations are exponentially aligned and the formation exponentially converges to the desired formation. We also show that the proposed strategy can be utilized for network localization as a dual problem.