Surface Normal Direction Research Articles

A fast manhattan frame estimation method based on normal vectors

AbstractIn most human made scenes, such as high‐rise urban city or indoor environment, the surface normal vectors or direction vectors are concentrated in three orthogonal principal directions. The scene of such a pattern is called Manhattan World (MW), and the coordinate frame formed by the three principal directions is called Manhattan Frame (MF). MF estimation methods have been applied to many different fields, such as scene reconstruction, Visual based Simultaneous Localization And Mapping (V‐SLAM) and camera calibration. In this paper, we propose a novel MF estimation method based on a set of normal vectors. A cost function of normal vectors and MF axes is introduced based on the trigonometric function. For computational purpose, the cost function is significantly simplified by making use of vector dot and cross products, and the reduced cost function only involves 14 scalar parameters that need to be computed with O(n) complexity. The experimental results show that the proposed MF estimation method has excellent real‐time performance and gives high accuracy on both the virtual and real‐world benchmark datasets of different sizes.

Breaking Transmission Symmetry Without Breaking Reciprocity in Linear All-Dielectric Polarization-Preserving Metagratings

Transmission asymmetry in reciprocal systems offers an appealing alternative to bulkier nonreciprocal implementations for certain applications. Common reciprocal routes to transmission asymmetry of linearly polarized light involve a rotation of its polarization. Here, we explore a different route with a linear all-dielectric metagrating that preserves polarization, while lacking inversion symmetry along the surface-normal direction. Our all-angle transmission calculations reveal an abrupt transition from a symmetric to an asymmetric transmission response that traces the Bragg critical wavelength of higher-order beam emergence as a function of the incident angle. By adopting an analogy between scattering from a multiport network and the metagrating paradigm we establish why the only necessary condition for transmission symmetry breaking in this class of systems is the emergence of any higher-order Bragg diffracted beam. We further show how such a transmission symmetry breaking is consistent with reciprocity and also demonstrate the underpinning symmetry-breaking mechanism with a first-principles numerical experiment. Finally, we shed light on some previous misconceptions regarding transmission symmetry breaking related to the role of the substrate or need for change of diffraction order number at each interface. Our proposed metagrating can exhibit a strong transmission asymmetry, with contrast that can be as high as approximately 75%, thus underlining its potential as a blueprint for passive asymmetric or nonlinear self-biasing nonreciprocal metasurfaces relevant to integrated and active photonics.

Heliostat field aiming strategy optimization with post-installation calibration

Heliostat field aiming strategy significantly affects the safety and operational efficiency of the Solar Power Tower (SPT) system. Heliostats installed in fields are likely to show deviations of optical properties from design specification to some extent. Such deviations generally reflect in the surface curvatures and normal directions of heliostat facets. These differences can give rise to the inevitable degradation of optical performance, impacting the execution effect of the pre-designed aiming strategy. Focusing spot distortion with an inappropriate aiming strategy possibly leads to overheated local areas or component damages. To cope with this error and to improve the light concentration performance, an aiming strategy optimization approach with post-installation calibration was proposed in this paper. An equivalent parameter model was developed based on heliostat parameters, producing a more accurate description of optical performance. The adopted heliostat parameters were obtained by the parameter identification method. The parameter identification was formulated by photographed solar flux images and simulated flux distributions. The aiming strategy optimization model of heliostat field was built with obtained identification parameters, and the optimization goal considers utilized solar power absorbed by the heat transfer fluid and flux distribution uniformity. The simulation results showed substantial improvement from the proposed aiming strategy. • A novel approach to heliostat field aiming strategy optimization is developed. • Effective heliostat optical parameters are obtained by parameter identification. • Identified parameters calibrate heliostat deviations in aiming strategy optimization. • Flux distribution uniformity and heat loss are considered in the optimization goal.

Limitations of Surface Current Model of Magnetic Field and a Remedy

It is well known that any realistic static magnetic field in a source-free volume can be generated by electric currents on a surface enclosing the volume. The magnetic field generated by surface currents is equivalent to that from magnetic dipoles distributed on the surface and along the surface normal directions. We demonstrate the limitations of the surface current model for instrumentation, and show vertical current loops corresponding to magnetic dipoles along the surface tangential directions may be employed to mitigate these limitations. We consider loop coil elements distributed on a cylindrical surface. A closed-surface array has loops on the entire surface, and an open-surface array does not have loops on the surface of both ends. A coil array may contain only flat elements (each loop is inside the cylindrical surface) or composite elements consisting of three orthogonal loops (one flat loop and two vertical loops) instead of just one flat loop at the location of each element. With computer simulations, the array coils were tested for generating the spatial field pattern of a vertical loop or a linear field gradient. The simulations showed that (1) an open-surface composite loop array produces less errors than a similar flat loop array in generating linear field patterns; the percentage root mean square error (RMSE) of the composite coil loops was 8.46% compared to 21.5% for the similar flat loops; (2) A composite loop array consumes less power than a corresponding flat array. For the open-surface array with a similar number of loops, the power consumption of the composite loop array was 73.9 versus 184 (arbitrary power units) for the flat loop array.

Demonstration of light reflection concepts for rendering realistic 3D tree images

With advancements in computer graphics, creating natural images has always been the main purpose, image rendering is all based on principles of physics. So, understanding the physics of image rendering will enable us to create the most realistic images. A ray of light hit a surface with different orientation and reflects as per the rules of physics. It is difficult to calculate the light reflection of complex foliage, such as trees, so, the reflection of this natural complexity needs to be adapted to rendering situations. In this research, the researchers provide demonstrations to enable students to understand the light reflection in nature, light calculation in computer graphics and methods to apply them to render realistic tree images. The researchers assign students to render 3D realistic tree images to assess the students’ understanding by applying the diffuse reflection value, specular reflection value and surface normal direction to render realistic tree images. The researchers find that most students understand of diffuse reflection, specular reflection, and surface normal direction causes the rendering results to be most realistic.

Rough-surface effect on sputtering of Cr bombarded by low-energy He plasma

• Microscopic cones grow on a Cr surface during He plasma exposure. • Angular distribution of sputtered atoms becomes more over-cosine-like (narrower). • Sputtering yield is significantly reduced. • GITR coupled with SDTrimSP simulations reproduce the experimental findings. The formation of microscopic cone-like structures on a Cr surface is observed under He plasma exposure at a low incident ion energy of ∼106 eV in the PISCES-A linear plasma device. 2D emission profiles of the Cr I line intensity, measured with a hyperspectral imaging camera, are compared with those modeled using the GITR Monte Carlo impurity transport code coupled with the SDTrimSP binary collision code. With the growth of cones, the experimental 2D emission profile becomes axially elongated and the emission intensity significantly drops, and these observations are successfully reproduced in the simulation. The simulation reveals that the deposition of sputtered Cr atoms onto the neighboring cones results in a more over-cosine-like angular distribution, i.e., a narrower distribution with more particles ejected in the surface normal direction, of sputtered Cr atoms with a significant reduction of the sputtering yield.

An enhanced semi-implicit particle method for simulating the flow of droplets with free surfaces

The accurate modeling of the surface-tension on free surfaces remains a challenging problem in meshfree particle methods. Recently, Matsunaga et al. (2020) have made an important step forward toward addressing this issue by introducing a moving surface mesh to represent the free-surface boundary in 2D. However, the surface mesh makes it difficult to extend this methodology to 3D and to treat topological changes in free surfaces. This study aims to extend this methodology to 3D and restore the capability of the model to simulate topological changes by carefully detecting the free-surface particles. In this situation, the problems regarding fluctuated free-surface boundaries and the dynamic intersection of a free surface with a wall boundary arise. To resolve these problems, a new particle shifting method in the surface normal direction is proposed for the free-surface particles to reduce fluctuations. A new contact angle model is proposed to compute the intersection point of a free surface and a wall boundary. In this manner, an enhanced particle method suitable for simulating the dynamics of liquid droplets with possible topological changes in 3D is developed. Eight numerical examples verified the developed method and demonstrated the enhancement of the new particle shifting technique.

Inviscid Modes within the Boundary-Layer Flow of a Rotating Disk with Wall Suction and in an External Free-Stream

The purpose of this paper is to investigate the linear stability analysis for the laminar-turbulent transition region of the high-Reynolds-number instabilities for the boundary layer flow on a rotating disk. This investigation considers axial flow along the surface-normal direction, by studying analytical expressions for the steady solution, laminar, incompressible and inviscid fluid of the boundary layer flow due to a rotating disk in the presence of a uniform injection and suction. Essentially, the physical problem represents flow entrainment into the boundary layer from the axial flow, which is transferred by the spinning disk surface into flow in the azimuthal and radial directions. In addition, through the formation of spiral vortices, the boundary layer instability is visualised which develops along the surface in spiral nature. To this end, this study illustrates that combining axial flow and suction together may act to stabilize the boundary layer flow for inviscid modes.

Solar cell BRDF measurement and modeling with out-of-plane data.

In this work, a CCD-augmented complete angle scatter instrument (CASI) with a visible red laser source was used to measure the BRDF of a commercially available solar cell designed for small satellites, simultaneously capturing both in-plane and out-of-plane data with high angular resolution surrounding the specular direction. The measurements exhibited three distinct scatter features: a central specular peak, an offset specular peak, and a diffraction pattern. The two peaks were caused by different material surfaces with slightly different normal directions, and the diffraction pattern arose from periodically-spaced metal conducting bars running in one direction across the solar cell surface. The diffraction pattern measurements were verified in-plane with an original single-pixel CASI detector and then used to inform the creation of a single closed-form BRDF model capable of describing the out-of-plane features. Both specular peaks were modeled using a traditional microfacet formulation, but the offset peak model implemented a rotation of the incident and scatter directions to account for the difference in surface normal direction. The diffraction pattern-which is not typically described with microfacet models-was described based on Fraunhofer diffraction through two rectangular stripes, adjusted in terms of microfacet coordinates. Parameters for the model were chosen manually, based largely on physical material properties when possible, rather than using optimized fitting algorithms. Model results were compared to the measurements by using the same CCD pixel scatter coordinates. Qualitatively, the model successfully replicated the observed features, and quantitatively, the modeled peak values agree with the measurements within an order of magnitude.

Direct treatment of interaction between laser-field and electrons for simulating laser processing of metals

Laser ablation is often simulated by the two-temperature model in which electrons are assumed to be thermalized by laser irradiation, while an explicit representation of interaction between laser-field and electrons is challenging but beneficial as being free from any adjustable parameters. Here, an ab initio method based on the time-dependent density functional theory (TDDFT) in which electron-ion dynamics under a laser field are numerically simulated is examined as a tool for simulating femtosecond laser processing of metals. Laser-induced volume expansion in surface normal directions of Cu(111) and Ni(111) surfaces are simulated by using repeating slab models. The amount of simulated volume expansion is compared between Cu(111) and Ni(111) slabs for the same laser pulse conditions, and the Ni slab is found to expand more than the Cu slab despite the smaller thermal expansion coefficient of Ni compared with Cu. The analyzed electronic excitation and lattice motion were compared to those in the two-temperature model. The threshold fluence to release surface Cu atom deduced from current TDDFT approach is found to be comparable to those of Cu ablation reported experimentally.

Model Study of Penetration of Cl− Ions from Solution into Organic Self-Assembled-Monolayer on Metal Substrate: Trends and Modeling Aspects

Organic molecules that form self-assembled-monolayers on metal substrates may provide efficient corrosion protection. Herein we study how such self-assembled-monolayers hinder the penetration of ions from aqueous solution toward the metal substrate. We first elucidate some aspects that are relevant for modeling charged ions near surfaces with slab models that utilize periodic-boundary-conditions, in particular: (i) solvation effects, (ii) inter-ion electrostatics, (iii) depolarization effects, and (iv) effects of periodic-boundary-conditions along lateral directions and, for multi-slab models, also along the surface normal direction. The last two effects are artifacts hence they can be avoided or at least minimized by proper modeling. We further present a simple scheme that describes the activation barrier for penetration into self-assembled-monolayer as a function of the electrode potential and show that the activation barrier decreases as the electrode potential increases, as would be intuitively expected, however, for thick self-assembled-monolayers the barrier remains sizable even at rather positive potentials, which may be one of the reasons why dense and sufficiently thick self-assembled-monolayers can efficiently inhibit corrosion. By utilizing a simple model where metal substrate, organic layer, and aqueous solvent are described implicitly by dielectric continuum slabs, we analyze two important effects by which self-assembled-monolayers hinder the penetration of ions toward the metal substrate. The first effect is due to inferior solvation of ions in organic layer compared to that in aqueous solvent and the estimated difference is larger than 1 eV. This effect is independent of the thickness of the organic layer, provided that the layer is sufficiently thick (≳10 Å). The second effect is due to electric field at the electrochemical interface and the extent by which it affects the penetration of depends on the electrode potential and on the thickness of the organic layer. Other effects, such as local deformation of organic layer during penetration, cannot be described by current simple models and will be considered in our next publication. Finally, calculations indicate that due to stronger solvation of Na+ counter-ions their penetration into organic layer is inferior to that of . Energetically the most favorable way for Na+ to penetrate is in the form of Na+/ ion-pairs, but it is inferior to penetration of alone.

A phase-field crack model based on a directional strain decomposition and a stress-driven Crack-Opening Indicator

We present a phase-field model for pre-cracked solids with a directional strain decomposition and a stress-driven crack-opening indicator (COI). We first introduce the COI to distinguish the opening and closure of pre-cracks and propose a novel criterion for solids based on the stress component in the direction of the crack surface normal. The stress-driven COI can correctly detect crack opening when principal strains have different signs, which cannot be achieved by most existing phase-field models. We then split the strain tensor into an effective part that induces stress and a crack-induced part that does not result in stress, and suggest a mapping tensor to transform the total strain tensor to the effective strain tensor. The mapping tensor is based on the crack surface normal, the phase variable, and the COI, thus leading to a directional decomposition of the strain. Eventually, we obtain the fourth-order material tangent mapping an arbitrary strain tensor to its stress tensor in the way that the shear stress along the crack surface vanishes and the normal stress in the crack surface normal direction is eliminated when the crack is open. Besides, owing to the directional strain decomposition, the direction where the stiffness decreases is invariant to the current strain once a diffusive crack is generated, which is physically more reasonable. More importantly, the decomposition together with the COI ensures that when the crack is open the spurious Poisson effect associated with the crack-induced strain is eliminated. Numerical examples are provided and the results have demonstrated the capability of the proposed model in simulating material behaviours in the post-fracture stage. Particularly, in a shear test of a squared plate with an initial crack, our model that simulates both the pre-crack and crack growth using the phase-field approach is able to reproduce the crack evolution pattern that has only been obtained in phase-field modelling by considering the initial crack as a geometric notch.

Anisotropic defect distribution in He+-irradiated 4H-SiC: Effect of stress on defect distribution

Irradiation-induced anisotropic swelling in hexagonal α-SiC is known to degrade the mechanical properties of SiC; however, the associated physical mechanism and microstructural process remain insufficiently understood. In this study, an anisotropic swelling condition where the surface normal direction was allowed to freely expand with constraint in the lateral direction was introduced in 4H-SiC using selected-area He+ irradiation, and the internal defect distribution was investigated using transmission electron microscopy (TEM) and advanced scanning TEM. The defect distribution was compared to that in non-selected-area He+-irradiated 4H-SiC and electron-irradiated TEM-foil 4H-SiC. An anisotropic defect distribution was observed in the selected-area He+-ion-irradiated 4H-SiC, with interstitial defects preferentially redistributed in the surface normal direction ([0004]) and negative volume defects (such as vacancies and/or carbon antisite defects) dominantly located in the lateral directions ([112¯0] and [101¯0]). This anisotropy of the defect distribution was substantially lower in the non-selected-area He+-irradiated and electron-irradiated samples. The stress condition in the three samples was also measured and analyzed. In the selected-area He+-irradiated 4H-SiC, compressive stress was introduced in the lateral directions (([101¯0] and [112¯0])), with little stress introduced in the surface normal direction ([0004]); this stress condition was introduced at the beginning of ion irradiation. The compressive stress likely inhibits the formation of interstitial defects in the lateral directions, enhancing the anisotropy of the defect distribution in SiC.

Experimental verification of ion impact angle distribution at divertor surfaces using micro-engineered targets on DiMES at DIII-D

• The first detailed experimental verification of polar ion impact angle distribution (IAD). • Specially fabricated micro-trenches were exposed to L-mode D plasmas at DIII-D DiMES. • Calculated polar and azimuthal IADs were combined with a Monte-Carlo MPR code. • MPR revealed the areal erosion yield on the trench floor due to D sputtering. • Good agreement between MPR erosion and EDS net C deposition profiles was seen. • Such agreement strongly verifies the IADs, and the modeling of the Chodura sheath. We report the first detailed experimental verification of the polar deuterium ion impact angle distribution (IAD) on the DIII-D divertor surface in L-mode plasmas using micro-engineered trenches in samples mounted on the DiMES probe. These trenches were fabricated via focused ion beam (FIB) milling of a silicon surface partially coated with aluminum. The sample surfaces were exposed to eight repeat L-mode deuterium discharges (30 s total exposure time). The samples were examined by scanning electron microscopy (SEM), which revealed changes on the trench floor due to material deposition and evidence for shadowing of the incident deuterium ions by the trench walls. The areal distribution of carbon and aluminum deposition was measured by energy-dispersive X-ray spectroscopy (EDS). One-dimensional profiles of this deposition are in agreement with net erosion profiles calculated from a Monte Carlo micro-patterning and roughness (MPR) code for ion sputtering using as input the polar and azimuthal deuterium IADs reported previously (Chrobak et al., Nucl. Fusion 58, 106019 (2018)). The deposition profiles verified the characteristic shape of the polar IADs, which have a broad maximum from 79° to 86°, over the experimental range of 68°–83°, where 0° is the surface normal direction.

Calculating the gravity-free shape of sheet metal parts

This paper presents an iterative finite element (FE)–based method to calculate the gravity-free shape of nonrigid parts from an optical measurement performed on a non-over-constrained fixture. Measuring these kinds of parts in a stress-free state is almost impossible because deflections caused by their weight occur. To solve this problem, a simulation model of the measurement is created using available methods of reverse engineering. Then, an iterative algorithm calculates the gravity-free shape. The approach does not require a CAD model of the measured part, implying the whole part can be fully scanned. The application of this method mainly addresses thin, unstable sheet metal parts, like those commonly used in the automotive or aerospace industry. To show the performance of the proposed method, validations with simulation and experimental data are presented. The shown results meet the predefined quality goal to predict shapes within a tolerance of ± 0.05 mm measured in surface normal direction.

Stability analysis of thin-walled parts end milling considering cutting depth regeneration effect

Weak rigid parts are widely used in aerospace industry. These parts are characterized by thin walls. Most of these parts have the meter-level size and mesh surface, thus have a requirement for tool accessibility. Therefore, the end milling is an applicable process to reduce the thickness of these parts. Due to the weak rigidity of these parts in the normal direction, severe chatter will occur in the machining process, which will reduce the machining efficiency and spoil the machining surface, so the processing stability of the thin-walled part needs to be studied. In this paper, the dynamic model for end milling of thin-walled parts is presented. The regeneration effect in the surface normal direction is proposed, and the dynamic equation is constructed considering the vibration of the workpiece in the cutting tool axial direction. The stability lobe diagram is obtained by numerical method. It is found that the stability of weak rigid parts end milling is related to the cutting speed, the feed rate, and the width of cut, but not to the cutting axial depth. Finally, cutting experiments are conducted. The experimental result is coincident well with the prediction, thus verifying the proposed dynamic model and the stability analysis method.

Erosion behavior of aluminum by an inclined cavitating jet

The erosion behavior of pure aluminum by an inclined cavitating jet in the acceleration period was experimentally investigated for potential applications in peening and cleaning. The mass loss and surface topography of the eroded specimens were investigated to unravel the erosion mechanism under inclined impingement with various feeding pressure. The cavitation cloud motion was numerically investigated in various erosion stages with two-phase CFD calculation. Results show that at the first optimum standoff distance of mass loss l1 close to the nozzle bottom, erosion occurs in two ring-like areas, whereas the erosion at the downstream second optimum standoff distance l2 appears as a single ring at inclined angles αi∈ [0∘, 15∘] and as separated crescent-shaped areas at αi = 30∘ and 45∘. The increasing αi induces a higher cumulative erosion ratio in the acceleration period at Pin = 15 MPa and 13 MPa, whereas the aggressive ability of inclined impingement is attenuated at Pin = 11 MPa due to the low cavitation intensity. An obvious transition from plastic deformation to erosion damage is found on the boundary of the main erosion area. Under the inclined impingement, the upper side of the main erosion area shows much more severe erosion damage than the lower side due to the blocking effect, where the upward spread of the cavitation clouds is blocked and their collapse primarily occurs in the erosion valley. The erosion develops with the a ring-shaped pattern at αi∈ [0∘, 5∘]. At higher inclined impingement angles αi∈ [15∘, 45∘], the erosion primarily develops along the surface normal direction in the part of the main erosion area closer to the nozzle exit.

Single molecule characterization of anomalous transport in a thin, anisotropic film

The diffusion of small, charged molecules incorporated in an anisotropic polyelectrolyte multilayer (PEM) was tracked in three dimensions by combining single-molecule fluorescence localization (to characterize lateral diffusion) with Förster resonance energy transfer (FRET) between diffusing molecules and the supporting surface (to measure diffusion in the surface-normal direction). Analysis of the surface-normal diffusion required model-based statistical analysis to account for the inherently noisy FRET signal. Combining these distinct single-molecule methods, which are inherently sensitive to different length-scales, permitted simultaneous characterization of severely anisotropic diffusion, which was more than three orders of magnitude slower in the surface-normal direction. We hypothesize that the anomalously slow surface-normal diffusion was related to the periodic distribution of charge in the PEM, which created electrostatic barriers. The motion was strongly subdiffusive, with anomalous temporal scaling exponents in lateral and normal directions, suggesting a connection to the transient, random fractal conformation of polymer chains in the film’s matrix.

Temperature dependent lattice expansions of epitaxial GaN-on-Si heterostructures characterized by in- and ex-situ X-ray diffraction

In-situ X-ray diffraction (XRD), with the sample temperature stepwise increased up to 900 °C, together with ex-situ high-resolution XRD (HRXRD), has been used to study the lattice expansion along the surface normal direction of GaN-on-Si heterostructures grown by metalorganic chemical vapor deposition (MOCVD). The thermal stabilities induced by step-graded (SG) AlxGa1−xN/AlN and multiple low-temperature (MLT) AlN buffer layers have been addressed for the heterostructures grown on 700 and 1500 µm thick Si (111) wafers, respectively. It reveals that the thermal expansion of the GaN epilayer is slightly larger than that of the Si substrate and the nitride growth tends to decrease the thermal expansion of the Si (111) wafer at lower temperatures. Onset of drop in the lattice expansions has been observed when the temperature is increased to a transition point, Ttr, and the Ttr of the MLT-AlN buffered GaN on the 1500 µm thick Si is ~500 °C, which is apparently lower than that of the SG-AlxGa1−xN/AlN buffered GaN on the 700 µm thick Si. These observations have been interpreted and attributed to convolutions among the residual lattice strains, their relaxations, and the thermal expansion coefficient mismatch induced wafer curvatures (opposite to that during MOCVD growth) at elevated temperatures.

Self-Assembly of Polymer End-Tethered Gold Nanorods into Two-Dimensional Arrays with Tunable Tilt Structures

We demonstrated a facile yet effective strategy for self-assembly of polymer end-tethered gold nanorods (GNRs) into tunable two-dimensional (2D) arrays with the assistance of supramolecules of hydrogen bonded poly(4-vinyl pyridine) (P4VP) and 3-n-pentadecylphenol (PDP). Well-ordered 2D arrays with micrometer size were obtained by rupturing the assembled supramolecular matrix with a selective solvent. The formation of long-range ordered 2D arrays during a drying process was observed via small-angle X-ray scattering. Interestingly, the packing structure of the ordered arrays strongly depends on the molecular weight (Mw) of the polymer ligands and the size of the GNRs. By increasing Mw of the polymer ligands, tilted arrays can be obtained. The average angle between GNRs and the surface normal direction of the layered 2D arrays changes from 0 to 37° with the increase in Mw of the polymer ligands. A mechanism for assembly behavior of dumbbell shapes with a soft shell structure has been proposed. The resulting GNR arrays with different orientations showed anisotropic surface-enhanced Raman scattering (SERS) performance. We showed that the vertically ordered GNR arrays exhibited ∼3 times higher SERS signals than the tilt ordered arrays. The results prove that the polymer end-tethered GNRs can be used as a building block for preparing the tilted 2D arrays with tunable physicochemical properties, which could have a wide range of potential applications in photonics, electronics, plasmonics, etc.