Small Variations In Geometry Research Articles

Registration of Multi-Sensor Bathymetric Point Clouds in Rural Areas Using Point-to-Grid Distances

This article proposes a method for registration of two different point clouds with different point densities and noise recorded by airborne sensors in rural areas. In particular, multi-sensor point clouds with different point densities are considered. The proposed method is marker-less and uses segmented ground areas for registration.Therefore, the proposed approach offers the possibility to fuse point clouds of different sensors in rural areas within an accuracy of fine registration. In general, such registration is solved with extensive use of control points. The source point cloud is used to calculate a DEM of the ground which is further used to calculate point to raster distances of all points of the target point cloud. Furthermore, each cell of the raster DEM gets a height variance, further addressed as reconstruction accuracy, by calculating the grid. An outlier removal based on a dynamic threshold of distances is used to gain more robustness against noise and small geometry variations. The transformation parameters are calculated with an iterative least-squares optimization of the distances weighted with respect to the reconstruction accuracies of the grid. Evaluations consider two flight campaigns of the Mangfall area inBavaria, Germany, taken with different airborne LiDAR sensors with different point density. The accuracy of the proposed approach is evaluated on the whole flight strip of approximately eight square kilometers as well as on selected scenes in a closer look. For all scenes, it obtained an accuracy of rotation parameters below one tenth degrees and accuracy of translation parameters below the point spacing and chosen cell size of the raster. Furthermore, the possibility of registration of airborne LiDAR and photogrammetric point clouds from UAV taken images is shown with a similar result. The evaluation also shows the robustness of the approach in scenes where a classical iterative closest point (ICP) fails.

Crack arrest testing at the micro-scale

Crack arrest testing of micro-sized cantilever beams (≈8 × 4 × 6 μm, length, width and height, respectively) was conducted in order to evaluate the suitability of a new method to quantify local crack arrest properties. Chevron notched cantilevers were milled to match the (1 0 0)[0 1¯ 1] crack system in α-iron, where earlier attempts to obtain brittle or rapidly propagating fracture proved difficult. Brittle crack initiation and propagation was achieved by means of the deposition of a layer of SiOX on the surface, acting as a brittle starter. All tests were performed at −75 °C, using an in-house designed cooling system. The cracks arrested after propagation into the iron cantilever. A finite element model was developed to determine the appropriate dimensionless shape factor and provide a rigorous computer analysis of these complexly shaped cantilevers. KQC and KQa, at initiation and arrest respectively, were determined and evaluated. The cantilevers were later displaced further at 40 K to allow evaluation of crack jump lengths and to obtain a more complete analysis of the fracture surfaces. The average fracture toughness was determined to be 3.89 ± 1.00 MPam, and the average arrest toughness to be 2.6 ± 0.86 MPam. The finite element model highlights the effect of small variations in geometry which was larger than anticipated and strongly affects the shape factor, up to a 25% difference in f(a/W). As small variations in geometry are inevitable when milling with FIB, the need for individual models tailored to every cantilever is discussed.

A fuzzy α-cut optimization analysis for vibration control of laminated composite smart structures under uncertainties

As a high performance and specific mechanically tailored material, composite structures present high structural behavior sensitivity to small variations in geometry or material properties. Some of these uncertainties are intrinsically related to the manufacturing process but others are usual uncertainty in material properties. A fuzzy interval analysis methodology is proposed in this paper in order to evaluate the structural behavior of laminated composite smart structures under vibration control and uncertain parameters. A Particle Swarm Optimization algorithm is used to search for input parameters in order to minimize and maximize system outputs by an anti-optimization and thus build their envelopes. Each input parameter has interval values defined by α-cut levels and interpolated by the Fuzzy reasoning. This methodology is applied to a smart structure of laminated composite material with attached piezoelectric patches and controlled by an optimal control regulator. System's interval outputs like natural frequency, mechanical vibration, and electric control input energy are investigated taking into account uncertainties in the composite and piezoelectric material properties, ply angles and layer thickness. Finally, it is concluded that these uncertainties may affect the control performance since it was found significant modifications in the dynamic behavior and therefore this should be accounted in the design stages.

Signed algebraic level sets on NURBS surfaces and implicit Boolean compositions for isogeometric CAD–CAE integration

Boundary-representation (B-rep) geometrical models, often mathematically represented using Non-Uniform Rational B-Spline (NURBS) surfaces, are the starting point for complex downstream product life-cycle evaluations including Computer-Aided Engineering (CAE). Boolean operations during B-rep model generation require surface intersection computations to describe the composed entity. However, for parametric NURBS surfaces, intersection operations are non-trivial and typically carried out numerically. The numerical intersection computations introduce challenges relating to the accuracy of the resulting representation, efficiency with which the computation is carried out, and robustness of the result to small variations in geometry. Often, for downstream CAE evaluations, an implicit, procedural knowledge of the Boolean operations between the composed objects that can resolve point containment queries (exact to the original NURBS bounding surfaces) maybe sufficient during quadrature. However, common point containment queries on B-rep models are numerical, iterative and relatively expensive. Thus, the first goal of the present paper is to describe a purely algebraic, and therefore non-iterative, approach to carrying out point containment queries on complex B-rep models built using low-degree NURBS surfaces. For CAE operations, the boundary representation of B-rep solids is, in general, not convenient and as a result, the B-rep model is converted to a meshed volumetric approximation. The major challenges to such a conversion include capturing the geometric features accurately when constructing the secondary (meshed) representation, apart from the efficiency of carrying out such a mesh generation step repeatedly as the geometric shape evolves. Thus, an ideal analysis procedure would operate directly on B-rep CAD models, without needing a secondary mesh, and would procedurally unify the geometric operations during CAD as well as CAE stages. Therefore, the second and broader goal of the present paper is to demonstrate CAD–CAE integration using signed algebraic level set operations directly on B-rep models by embedding or immersing the bounding surfaces within a discretized domain while preserving the geometric accuracy of the surfaces exact to the original NURBS representation during analysis.

Passive separation control by acoustic resonance

At transitional Reynolds numbers, the laminar boundary layer separation and possible reattachment on a smooth airfoil, or wing section, are notoriously sensitive to small variations in geometry or in the fluid environment. We report here on the results of a pilot study that adds to this list of sensitivities. The presence of small holes in the suction surface of an Eppler 387 wing has a transformative effect upon the aerodynamics, by changing the mean chordwise separation line location. These changes are not simply a consequence of the presence of the small cavities, which by themselves have no effect. Acoustic resonance in the backing cavities generates tones that interact with intrinsic flow instabilities. Possible consequences for pas- sive flow control strategies are discussed together with potential problems in measurements through pressure taps in such flow regimes.

Optical Properties of Chiral Plasmonic Tetramers: Circular Dichroism and Multipole Effects

Chiral metal nanoparticle assemblies exhibit plasmonic circular dichroism (CD) in the visible spectral interval. It was found previously that the circular dichroism signals can be induced by dipolar interactions between nanoparticles in a chiral assembly. In order to enhance plasmonic circular dichroism response, one can take advantage of multipole effects and anisotropy of nanostructures. We calculate the plasmonic circular dichroism of several nanoparticle (NP) assemblies using the interacting point-dipole approach and the purely numerical method based on the discrete dipole approximation (DDA). We found that the multipole effects revealed by the DDA calculations are crucial to describe and understand CD responses of tightly packed assemblies. The chiral equilateral tetrahedral 4-NP complexes are especially interesting because they do not have the dipolar contribution to the CD signal. Therefore, CD signals of equilateral tetrahedral 4-NP complexes originate solely from the multipole interactions. The strength of CD signals rapidly decreases with the particle–particle distance as 1/R9.7 for the helices and as 1/R18.1 for the equilateral tetrahedral 4-NP complexes, where R is a particle–particle distance. We show that the CD spectra are much more sensitive to the geometry of a plasmonic complex compared to the extinction spectra. Small variations in geometry can result in large changes in CD responses. This study can be used to design nanostructures with strong CD for optical and sensor applications.

The structural, electronic, and optical properties of ladder-type polyheterofluorenes: a theoretical study

The ladder-type polyheterofluorenes were investigated theoretically by using density functional theory (DFT) to reveal their optical and electronic properties for applications in organic optoelectronic devices. The incorporation of heteroatoms (B, Si, Ge, N, P, O, and S) into the ladder-type highly fused polyfluorene backbone can influence and modify the optoelectronic properties significantly. The functionalization on the heteroatoms allows for facile derivation and incorporation of substitutes to further tune the properties. Small geometry variations between the ground, anionic/cationic, the first excited singlet and triplet states were observed due to the very rigid ladder-type coplanar backbone. Ladder-type polycarbazole was predicted to have the highest HOMO and LUMO energy levels, polyphosphafluorene oxide have the lowest HOMO energy level, polyborafluorene have the lowest LUMO energy level and bandgap, and polysulfafluorene has the highest bandgap and triplet energy. The ladder-type carbazole and borafluorene show the highest hole and electron injection abilities respectively; while sulfafluorene has the highest electron transfer rate. Most ladder-type heterofluorenes show bipolar charge transport character suggested by the reorganization energy. All of them have significantly short effective conjugation length in comparison with linear conjugated polymers. Their absorption and emission spectra were also simulated and discussed. The diversified electronic and optical properties of the ladder-type polyheterofluorenes with the different incorporated heteroatom and the substituent on it indicate their broad potential applications in organoelectronics.

Application of the Dang Van Fatigue Criterion to the Rail/Wheel Contact Problem

Great interest is dedicated to the wheel/rail contact problem, in particular as a result of some accidents worldwide and also in Portugal. In the present work a three dimensional finite element analysis of the wheel/ rail contact problem was performed using the software ABAQUS. A preliminary study on simpler geometries was carried out, in order to identify the solution strategies giving more accurate solutions. The influence of mesh refinement, friction coefficient, and numerical techniques as Lagrange and penalty functions were analysed in the simpler cases of contact of two cylinders along a generatrix, and the contact of a cylinder and a rigid plane. The corresponding Hertz solutions were programmed using MATLAB in order to compare with the finite element analyses. The numerical procedures giving better results were later applied to the wheel/rail contact problem, using standard rail and wheels profiles used by the Portuguese Railways company (CP). The influence of small geometry variations on the stress analysis results was then studied, and the study of initiation of defects using the Dang Van fatigue criterion was performed.

A Study of Pipe Jointing Technology with Reference to ITER Requirements

The successful performance of remote automated welding of pipe joints depends on the precise control of many parameters. In practice, small variations in geometry, metallurgy and weld power prevent the creation of the optimum joint every time. To minimise the risks of producing welded joints of unacceptable quality, the approach at JET was to simplify the joint design. This meant specifying autogenous, single pass welds for pipe joints wherever possible. The requirements of ITER are such that this simplification of the weld process cannot necessarily be achieved. The pipe sizes and thicknesses will probably demand the use of multi-pass welds with the addition of filler metal. This paper looks back at the experience of the JET project in endeavouring to achieve this type of joint and highlights the difficulties that ITER will need to overcome. Similarly for the pipe cutting process, the JET team was able to develop tooling that was simple and reliable due largely to the thin pipe wall thickness used on the project. The ITER requirement for all processes to be fully remote will demand the development of more sophisticated welding and cutting tools with particular capabilities for recovery from failure. In response to the difficulties foreseen with applying welding and cutting principles to the pipe jointing challenges at ITER, research is currently underway with the aim of identifying an approach that is more amenable to Remote Handling (RH). This paper documents a proposal for a novel UHV pipe jointing system based on an in situ reversible brazing technique.

Prediction of noise levels and annoyance from aircraft run-ups at Vancouver International Airport

Annoyance complaints resulting from engine run-ups have been increasing at Vancouver International Airport for several years. To assist the Airport in managing run-up noise levels, a prediction tool based on a Green's function parabolic equation (GFPE) model has been consolidated, evaluated, and applied. It was extended to include more realistic atmospheric and ground input parameters. Measurements were made of the noise-radiation characteristics of a CRJ200 jet aircraft. The GFPE model was validated by comparing predictions with results in the literature. A sensitivity analysis showed that predicted levels are relatively insensitive to small variations in geometry and ground impedance, but relatively sensitive to variations in wind speed, atmosphere type, and aircraft heading and power setting. Predicted noise levels were compared with levels measured at noise monitoring terminals. For the four cases for which all input information was available, agreement was within 10 dBA. For events for which some information had to be estimated, predictions were within 20 dBA. The predicted annoyance corresponding to the run-up events considered ranged from 1.8% to 9.5% of people awoken, suggesting that noise complaints can be expected.

Nanoparticles in nematic liquid crystals: Interactions with nanochannels

A mesoscale theory for the tensor order parameter Q is used to investigate the structures that arise when spherical nanoparticles are suspended in confined nematic liquid crystals (NLCs). The NLC is "sandwiched" between a wall and a small channel. The potential of mean force is determined between particles and the bottom of the channels or between several particles. Our results suggest that strong NLC-mediated interactions between the particles and the sidewalls of the channels, on the order of hundreds of k(B)T, arise when the colloids are inside the channels. The magnitude of the channel-particle interactions is dictated by a combination of two factors, namely, the type of defect structures that develop when a nanoparticle is inside a channel, and the degree of ordering of the nematic in the region between the colloid and the nanochannel. The channel-particle interactions become stronger as the nanoparticle diameter becomes commensurate with the nanochannel width. Nanochannel geometry also affects the channel-particle interactions. Among the different geometries considered, a cylindrical channel seems to provide the strongest interactions. Our calculations suggest that small variations in geometry, such as removing the sharp edges of the channels, can lead to important reductions in channel-particle interactions. Our calculations for systems of several nanoparticles indicate that linear arrays of colloids with Saturn ring defects, which for some physical conditions are not stable in a bulk system, can be stabilized inside the nanochannels. These results suggest that nanochannels and NLCs could be used to direct the assembly of nanoparticles into ordered arrays with unusual morphologies.

Numerical Approaches to the Nonlinear Stability Analysis of Single Layer Reticulated and Grid-Shell Structures

The paper illustrates the characteristics of a procedure for evaluating the buckling and post-buckling behaviour and the influence of imperfections on reticulated and grid-shells structures. The presentation is composed by two steps: firstly, a simple way to comprehend the effects of initial imperfections by means of an “equivalent” fictitious load vector, acting on the ideal design structure, is presented. The fictitious load's characteristics are evaluated so to qualitatively reproduce the behaviour of an unknown structure, affected by an arbitrary distribution of small geometry variations with respect to the design structure. The second step allows to detect exactly the position of critical points and to trace the secondary equilibrium paths by means of a perturbation technique. Numerical examples illustrate the characteristics and performances of the approaches and suggest practical applications.

A Simple Way to Represent Geometrical Imperfections by Means of an Equivalent Load

Abstract A simple way of understanding initial geometrical imperfections in analytical models is presented. The effects of initial imperfections are introduced by an “equivalent” fictitious load vector, acting on the ideal design structure, whose characteristics are evaluated so as to qualitatively reproduce the behavioral characteristics of a different structure affected by an unknown distribution of small geometry variations with respect to the design structure. An approximate evaluation of the above-defined equivalent load is furnished and some numerical examples are presented to demonstrate the effectiveness of the method and suggest practical applications.