Small Relative Distances Research Articles

Astrometry via close approach events: applications to main-belt asteroid (702) Alauda

ABSTRACT The release of Gaia catalogue is revolutionary to the astronomy of Solar system objects. After some effects such as atmospheric refraction and CCD geometric distortion have been taken into account, the astrometric precision for ground-based telescopes can reach the level of tens of milli-arcseconds (mas). If an object approaches a reference star in a small relative angular distance (less than 100 arcsec), which is called close approach event in this work, the relative positional precision between the object and reference star will be further improved since the systematic effects of atmospheric turbulence and local telescope optics can be reduced. To obtain the precise position of a main-belt asteroid in an close approach event, a second-order angular velocity model with time is supposed in the sky plane. By fitting the relationship between the relative angular distance and observed time, we can derive the time of maximum approximation and calculate the corresponding position of the asteroid. In practice, five nights’ CCD observations including 15 close approach events of main-belt asteroid (702) Alauda are taken for testing by the 1 m telescope at Yunnan Observatory, China. Compared with conventional solutions, our results show that the positional precision significantly improves, which reaches better than 4 mas, and 1 mas in the best case when referenced for the Jet Propulsion Laboratory (JPL) ephemeris in both right ascension and declination.

Simulation Study on Axial Location Identification of Damage in Layered Pipeline Structures Based on Damage Index

This study investigates the feasibility of identifying the axial position of circumferential defects in laminated pipeline structures based on damage indices. Wavelet packet decomposition is combined with damage indices, and the effects of dual defects with the same circumferential position but different axial positions, as well as dual defects with different circumferential and axial positions, on damage indices are separately studied. Our aim was to determine the potential to use damage indices to identify the axial position of circumferential defects in laminated pipeline structures. ABAQUS finite element analysis software was used to establish models of laminated pipeline structures with single defects and dual defects (with the same circumferential position but different axial positions, and with different circumferential and axial positions). The laminated pipeline structure was composed of a steel pipe (structural layer), a rigid polyurethane foam (insulation layer), and a high-density polyethylene (anticorrosion layer). The received sensing signals were averaged, and subjected to 5-level wavelet packet decomposition, to calculate the damage index values, which were then organized into a damage index matrix. Based on the trend of changes in the damage index matrix, the effects of variations in the number and circumferential position of the defects on the identification of the axial position of the damage were analyzed. The results indicate that the trend in damage index changes is influenced by the number of defects, and the increase in the circumferential distance between the second and the piezoelectric element sensor. This study found that when 1.7λ≤PD≤3.4λ, Idouble defect 90°<Isingle defect<Idouble defect 0°; when 3.7λ≤PD≤4λ, Idouble defect 90°<0.3<Idouble defect 0°<Isingle defect. This article demonstrates that the identification of the axial position of damage in laminated pipeline structures can be achieved using the damage index values in the damage index matrix. Additionally, this damage identification method overcomes the limitation of the wavelet packet’s inability to identify dual defects with relatively small relative axial distances. This provides new ideas and methods for finite element analysis in identifying the axial position of damage in laminated pipeline structures.

Generalized section–section interaction potentials in the geometrically exact beam theory: Modeling of intermolecular forces, asymptotic limit as strain-energy function, and formulation of rotational constraints

The present contribution proposes a universal framework to formulate generalized section–section interaction potentials (SSIP) within the geometrically exact beam theory. By exploiting the fundamental kinematic assumption of undeformable cross-sections, an objective (i.e., frame-invariant) description of SSIPs via a minimal set of six (translational and rotational) relative coordinates, either in spatial or in material form, is proposed. Based on work-pairing, work-conjugated section–section interaction forces and moments, either in spatial or in material form, are identified that can be consistently derived from a variational principle. Interestingly, it is shown that hyperelastic stored-energy functions relating the deformation measures and stress-resultants of the well-known geometrically exact Simo–Reissner beam theory can also be identified as SSIPs when considering the asymptotic limit of small relative distances and rotations between the interacting cross-sections. Moreover, the proposed variational problem formulation is demonstrated to be of a very general nature, thus allowing for the formulation of translational and rotational constraints between arbitrarily oriented cross-sections based on either a penalty or a Lagrange multiplier potential. Possible applications include fiber-based structures and materials in technical and biological systems, where the proposed approach allows to model short- or long-ranged inter-molecular (e.g., electrostatic, van der Waals or repulsive steric) interactions between fibers in geometrically complex arrangements and to formulate translational and rotational coupling constraints between different fibers (e.g., cross-linked polymer chains) or between fibers and a matrix phase (e.g., fiber-reinforced composites).

Assessment of thoracic spinal curvatures in static postures using spatially tracked 3D ultrasound volumes: a proof-of-concept study

The assessment of spinal posture is a difficult endeavour given the lack of identifiable bony landmarks for placement of skin markers. Moreover, potentially significant soft tissue artefacts along the spine further affect the accuracy of marker-based approaches. The objective of this proof-of-concept study was to develop an experimental framework to assess spinal postures by using three-dimensional (3D) ultrasound (US) imaging. A phantom spine model immersed in water was scanned using 3D US in a neutral and two curved postures mimicking a forward flexion in the sagittal plane while the US probe was localised by three electromagnetic tracking sensors attached to the probe head. The obtained anatomical ‘coarse’ registrations were further refined using an automatic registration algorithm and validated by an experienced sonographer. Spinal landmarks were selected in the US images and validated against magnetic resonance imaging data of the same phantom through image registration. Their position was then related to the location of the tracking sensors identified in the acquired US volumes, enabling the localisation of landmarks in the global coordinate system of the tracking device. Results of this study show that localised 3D US enables US-based anatomical reconstructions comparable to clinical standards and the identification of spinal landmarks in different postures of the spine. The accuracy in sensor identification was 0.49 mm on average while the intra- and inter-observer reliability in sensor identification was strongly correlated with a maximum deviation of 0.8 mm. Mapping of landmarks had a small relative distance error of 0.21 mm (SD = ± 0.16) on average. This study implies that localised 3D US holds the potential for the assessment of full spinal posture by accurately and non-invasively localising vertebrae in space.

Effect of relative longitudinal spacing on the dynamic behavior of two interacting ships in head waves

The influence of relative longitudinal position on the frequency characteristics of two interacting ships floating stationary in close proximity in head waves and shallow water is investigated in this paper. A CFD approach has been adopted to simulate the dynamic behavior of the interacting ships. The numerical simulation for “Aleksey Kosygin” and “Novgorod” ships floating on head waves at a small relative transverse distance was carried out using two scaled models. The effect of longitudinal separation on the frequency characteristics of both ships was studied. Heave, roll, and pitch RAOs for various cases were analyzed, and recommendations for relative longitudinal positions were made on the basis of the present analysis.

Research on the Influence of Vehicle Speed on Safety Warning Algorithm: A Lane Change Warning System Case Study.

Speed has an important impact on driving safety, however, this factor is not included in existing safety warning algorithms. This study uses lane change systems to study the influence of vehicle speed on safety warning algorithms, aiming to determine lane change warning rules for different speeds (DS-LCW). Thirty-five drivers are recruited to carry out an extreme trial and naturalistic driving experiment. The vehicle speed, relative speed, relative distance, and minimum safety deceleration (MSD) related to lane change characteristics are then analyzed and calculated as warning rule characterization parameters. Lane change warning rules for a rear vehicle in the target lane under four-speed levels of 60 ≤ v < 70 km/h, 70 ≤ v < 80 km/h, 80 ≤ v < 90 km/h, and v ≥ 90 km/h are established. The accuracy of lane change warning rules not considering speed level (NDS-LCW) and ISO 17387 are found to be 87.5% and 79.8%, respectively. Comparatively, the accuracy rate of DS-LCW under four-speed levels is 94.6%, 93.8%, 90.0%, and 92.6%, respectively, which is significantly superior. The algorithm proposed in this paper provides warning in the lane change process with a smaller relative distance, and the accuracy rate of DS-LCW is significantly superior to NDS-LCW and ISO 17387.

A guaranteed-comfort and safe adaptive cruise control by considering driver’s acceptance level

One of the most important parameters which directly effects the performance of Adaptive Cruise Control systems is the reference signal. In order to design this signal based on the inter-distance policy between vehicles, the system’s conflicting constraints i.e. safety, comfort, and driver’s acceptance level must be simultaneously considered. When the front vehicle is far from the host vehicle, drivers usually tend to reduce their relative distance which may put the safety of passengers at risk. To resolve this conflict, we introduced a non-linear reference model with novel Sine and Gaussian functions. In order to meet driver’s acceptance level and meet the safety and comfort criteria of the passengers, the analytically-derived reference signals are based on functions which provide higher velocities in high relative distances and adopt smoother variations with guaranteed safety in small relative distances. Moreover, to apply the reference values on the vehicle’s dynamic parts, in this paper, a customized fuzzy controller is developed. The simulation results prove the efficiency of the proposed method so that while in high relative distances, more driving acceptance level can be achieved, in small relative distances, it satisfies the safety and comfort of passengers.

Hydrodynamic Performance Analysis of the Vertical Axis Twin-Rotor Tidal Current Turbine

The goal of this manuscript is to investigate the influence of relative distance between the twin rotors on the hydrodynamic performance of the vertical axis twin-rotor tidal current turbine. Computational fluid dynamics (CFD) simulations based on commercial software ANSYS-CFX have been performed to enhance the understanding of interactions between the twin-rotors. The interactions between the twin rotors are known to have increased the power output efficiency as a whole, and it is, therefore, of great significance to undertake deeper research. The simulation results are found to be consistent with similar research results in the literature in some aspects. The simulation results of stand-alone turbine and twin rotors are compared from three different aspects, including blade forces, power output efficiency and wake flow field. The results showed that the cyclic variations tendency of blade force coefficients of twin rotors is close to that of the stand-alone turbine. The average power output efficiency of the twin-rotors system is higher than that of the stand-alone turbine. The interactions between the turbines increase the power output of the twin turbine system as whole in a wide relative distance range. However, smaller relative distance between the twin rotors does not mean a bigger power output efficiency of such a system. The power out efficiency of such a system would decrease when the relative distance between the twin rotors exceeds the critical point. The power output of the twin rotors reaches the peak value when the ratio between the two main axis distance and diameter of the turbine is around 9/4. This research can provide a reference for the design and development of larger tidal power stations.

Bounds on the Size and Asymptotic Rate of Subblock-Constrained Codes

The study of subblock-constrained codes has recently gained attention due to their application in diverse fields. We present bounds on the size and asymptotic rate for two classes of subblock-constrained codes. The first class is binary constant subblock-composition codes (CSCCs), where each codeword is partitioned into equal sized subblocks, and every subblock has the same fixed weight. The second class is binary subblock energy-constrained codes (SECCs), where the weight of every subblock exceeds a given threshold. We present novel upper and lower bounds on the code sizes and asymptotic rates for the binary CSCCs and SECCs. For a fixed subblock length and small relative distance, we show that the asymptotic rate for CSCCs (respectively SECCs) is strictly lower than the corresponding rate for constant weight codes (CWCs) [respectively heavy weight codes (HWCs)]. Furthermore, for codes with high weight and low relative distance, we show that the asymptotic rate for CSCCs is strictly lower than that of SECCs, which contrasts with the fact that the asymptotic rate for the CWCs is equal to that of the HWCs. We also provide a correction to an earlier result by Chee et al . (2014) on the asymptotic CSCC rate. In addition, we present several numerical examples comparing the rates for the CSCCs and SECCs with those for the CWCs and HWCs.

The [formula omitted] asymmetric nuclear matter two-body density distributions versus those of [formula omitted

The theoretical computations of the electron–nucleus scattering can be improved, by employing the asymmetric nuclear matter (ASM) two-body density distributions (TBDD). But, due to the sophistications of the calculations, the TBDD with arbitrary isospin asymmetry have not yet been computed in the Fermi Hypernetted Chain (FHNC) or the Monte Carlo (MC) approaches. So, in the present work, we intend to find the ASMTBDD, in the states with isospinT, spin S and spin projection Sz, in the Lowest Order Constrained Variational (LOCV) method. It is demonstrated that, at small relative distances, independent of the proton to neutron ratio β, the state-dependent TBDD have a universal shape. Expectedly, it is observed that, at low (high) β values, the nucleons prefer to make a pair in the T=1(0) states. In addition, the strength of the tensor-dependent correlations is investigated, using the ratio of the TBDD in the TSSz=010 state with θ=π∕2 and that of θ=0. The mentioned ratios peak at r∼0.9fm, considering different β values. It is hoped that, the present results could help a better reproduction of the experimental data of the electron–nucleus scattering.

Robust Hybrid Supervisory Control for a 3-DOF Spacecraft in Close-Proximity Operations

In this paper we propose a hybrid control strategy to solve the problem of rendezvous, proximity operations, and docking of an autonomous spacecraft in 3D. Due to the different constraints and tasks to perform, a hybrid systems approach is implemented to solve the problem in three phases: 1) rendezvous; 2) rendezvous with smaller relative distance; 3) docking phase; and 4) docked phase; with range and angle measurements. In this approach, we implement a supervisor that robustly coordinates the individual controllers to accomplish the whole mission. We also present the designs of these individual controllers that solve the appropriate control problems for the individual phases. Numerical results for both the nominal and perturbed case validate the hybrid control strategy for the spacecraft close-proximity mission.

Effect of parametric variation of strut layout and position on the performance of a typical two-strut based scramjet combustor

The two equation Shear stress transport k-ω turbulence model and the 2D compressible Reynolds-Averaged Navier–Stokes (RANS) equations along with single-step chemical reaction mechanism have been applied to analyze the effect of strut lip height, staggered strut and the strut position relative to the combustor entrance on the combustion enhancement of a two-strut scramjet combustor. Here all the computational simulations have been carried out using ANSYS 14-FLUENT code. Moreover, validation has been performed for the single strut scramjet engine by comparing its outcome with the open experimental data. The computational results show acceptable similarity with the experimental results available in the open literature. The presence of two struts improves the mixing and combustion efficiency of the combustor appreciably compared to single strut. The current results represent the presence of an optimal lip height of the strut for which the performance of the two-strut combustor notably increases and strong combustion phenomena take place within the combustion region. Again mixing phenomena are not enhanced with the use of staggered strut due to the formation of separation region on both the wall two times during front as well as aft fuel injection. Further combustion phenomena are totally affected by the small relative distance between the entrance of the combustor and strut base. The combustion efficiency is found to be maximum only when overlapping occurs between the intersectional point and the divergence point inside the two-strut combustor.

Distance Verification for Classical and Quantum LDPC Codes

The techniques of distance verification known for general linear codes are first applied to the quantum stabilizer codes. Then, these techniques are considered for classical and quantum (stabilizer) low-density-parity-check (LDPC) codes. New complexity bounds for distance verification with provable performance are derived using the average weight spectra of the ensembles of LDPC codes. These bounds are expressed in terms of the erasure-correcting capacity of the corresponding ensemble. We also present a new irreducible-cluster technique that can be applied to any LDPC code and takes advantage of parity-checks’ sparsity for both the classical and quantum LDPC codes. This technique reduces complexity exponents of all existing deterministic techniques designed for generic stabilizer codes with small relative distances, which also include all known families of the quantum stabilizer LDPC codes.

Fusion hindrance in heavy systems with time-dependent Hartree-Fock

We analyze fusion hindrance in heavy systems, where the fusion probability around the Coulomb barrier is hindered compared with that in light and medium-mass systems. We perform simulations of central collisions around the Coulomb barrier in heavy systems with time-dependent Hartree‒Fock (TDHF) and find that the fusion hindrance is realized in TDHF simulations. We extract nucleus‒nucleus potential and energy dissipation in heavy systems by a method combining a microscopic TDHF evolution to a macroscopic collective equation of motion. We find that the extracted potentials exhibit a dynamical increase at small relative distances, while the extracted friction coefficients show rather a behavior similar to that in light and medium-mass systems. We find from our analysis that the dynamical increase in potential is a main contribution to this fusion hindrance.

Exploration and Mapping Technique Suited for Visual-features Based Localization of MAVs

An approach for long term localization, stabilization, and navigation of micro-aerial vehicles (MAVs) in unknown environment is presented in this paper. The proposed method relies strictly on onboard sensors of employed MAVs and does not require any external positioning system. The core of the method consists in extraction of information from pictures consequently captured using a camera carried by the particular MAV. Visual features are obtained from images of the surface under the MAV, and stored into a map that is represented by these features. The position of the MAV is then obtained through matching with previously stored features. An important part of the proposed system is a novel approach for exploration and mapping of the workspace of robots. This method enables efficient exploring of the unknown environment, while keeping the iteratively built map of features consistent. The proposed algorithm is suitable for mapping of surfaces, both outdoor and indoor, with various density of the image features. The sufficient precision and long term persistence of the method allows its utilization for stabilization of large MAV groups that work in formations with small relative distances between particular vehicles. Numerous experiments with quadrotor helicopters and various numerical simulations have been realized for verification of the entire system and its components.

Microscopic analysis of fusion hindrance in heavy nuclear systems

Background: Heavy-ion fusion reactions involving heavy nuclei at energies around the Coulomb barrier exhibit fusion hindrance, where the probability of compound nucleus formation is strongly hindered compared with that in light- and medium-mass systems. The origin of this fusion hindrance has not been well understood from a microscopic point of view. Purpose: Analyze the fusion dynamics in heavy systems by a microscopic reaction model and understand the origin of the fusion hindrance. Method: We employ the time-dependent Hartree-Fock (TDHF) theory. We extract nucleus--nucleus potential and energy dissipation by the method combining TDHF dynamics of the entrance channel of fusion reactions with one-dimensional Newton equation including a dissipation term. Then, we analyze the origin of the fusion hindrance using the properties of the extracted potential and energy dissipation. Results: Extracted potentials show monotonic increase as the relative distance of two nuclei decreases, which induces the disappearance of an ordinary barrier structure of the potential. This is different from those in light- and medium-mass systems and from density-constraint TDHF calculations. Extracted friction coefficients show sizable energy dependence and universal value of their magnitude, which are rather similar to those in light- and medium-mass systems. Using these properties, we analyze the origin of the fusion hindrance and find that contribution of the increase in potential to the extra-push energy is larger than that of the accumulated dissipation energy in most systems studied in this article. Conclusions: By the analysis of the origin of the fusion hindrance, we conclude that, as the system becomes heavier, the dynamical increase in potential at small relative distances plays a more important role than the dissipation during the fusion reaction for understanding the origin of the fusion hindrance.

The LOCV averaged two-nucleon interactions versus the density-dependent M3Y potential for the heavy-ion collisions

In order to have an effective two-nucleon potential based on the microscopic many-body calculations with a bare phenomenological nucleon–nucleon potential for the heavy-ion scattering calculations, the lowest order constrained variational (LOCV) method for the symmetric nuclear matter is applied to obtain the direct and the exchange parts of the averaged effective two-body interactions (AEI). The Reid68, the Reid68-Δ, and the Av18 interactions are used as the input phenomenological potentials. Then, it is tried to decompose the AEI into a radial and a density-dependent parts, such that the fitted LOCV AEI potentials give the same equation of state for the nuclear matter as the exact LOCV calculations. The results are compared with the various density-dependent M3Y potentials. It is shown that the direct and the exchange parts of the LOCV AEI with different input potentials are reasonably in agreement with each other and besides the different behaviors of their direct terms at small relative distances (<0.6 fm), they also roughly have the same magnitude as the direct parts of those of M3Y potentials. However, in contrast to the M3Y interaction, the exchange parts of the LOCV AEI have completely different behavior, i.e. their density dependence rises with increasing density. Finally, it is concluded that, since the direct and exchange parts of the LOCV AEI are constructed based on the many-body calculations with the phenomenological nucleon–nucleon potential, they are specially more trustable for the nucleus–nucleus collision calculations, in which the double-folding model is applied.

Relativistic description of BCS–BEC crossover in nuclear matter

We study theoretically the di-neutron spatial correlations and the crossover from superfluidity of neutron Cooper pairs in the S01 pairing channel to Bose–Einstein condensation (BEC) of di-neutron pairs for both symmetric and neutron matter in the microscopic relativistic pairing theory. We take the bare nucleon–nucleon interaction Bonn-B in the particle–particle channel and the effective interaction PK1 of the relativistic mean-field approach in the particle–hole channel. It is found that the spatial structure of neutron Cooper pair wave function evolves continuously from BCS-type to BEC-type as density decreases. We see a strong concentration of the probability density revealed for the neutron pairs in the fairly small relative distance around 1.5 fm and the neutron Fermi momentum kFn∈[0.6,1.0] fm−1. However, from the effective chemical potential and the quasiparticle excitation spectrum, there is no evidence for the appearance of a true BEC state of neutron pairs at any density. The most BEC-like state may appear at kFn∼0.2 fm−1 by examining the density correlation function. From the coherence length and the probability distribution of neutron Cooper pairs as well as the ratio between the neutron pairing gap and the kinetic energy at the Fermi surface, some features of the BCS–BEC crossover are seen in the density regions, 0.05 fm−1<kFn<0.7(0.75) fm−1, for the symmetric nuclear (pure neutron) matter.

Interaction of two elongated dust grains in flowing plasmas studied by numerical simulations

Interaction potentials between two insulating elongated dust grains in a supersonic plasma flow are studied by a particle-in-cell numerical method. It is found that the spatial variations of the charge distribution and the plasma potential depend on the orientation and the relative distance of the dust grains. For the case of two grains aligned perpendicular to the flow, a single wake and region of enhanced ion density are observed for small relative distances. The ion flux can be enhanced between such grains. For grains aligned parallel to the flow, the resulting wake is not a linear combination of the wakes behind a single grain, and the grain charge and interaction potentials strongly depend on their relative orientations. The simulations are carried out in two spatial dimensions, treating ions and electrons as individual particles. The case of two spherical grains, with separation vector perpendicular to the plasma flow direction, is considered for comparison.

Analytical Solution for Power-Limited Optimal Rendezvous near an Elliptic Orbit

This paper presents an analytical solution to the problem of the optimal rendezvous using power-limited propulsion for a spacecraft in an elliptic orbit in a gravitational field. The derivation of the result assumes small relative distances, but does not make any assumption on the eccentricity of the orbit and does not require numerical integration. The results are generalized to include the possibility of different weights on the control effort for each axis (radial, along-track, and out-of-plane). When the weights on the control efforts are unequal, several integrals are used whose solutions may be represented by infinite series in a small parameter dependent on the eccentricity. A methodology is introduced where the series can be extended trivially to as many terms as desired. Furthermore, for a given numerical tolerance, an upper bound on the number of terms required to represent the series is also obtained. When the weights are equal for all the three axes, the series representations are no longer necessary. The results can be used easily to design optimal feedback controls for rendezvous maneuvers, or for generating initial guesses for two-point boundary-value problems for numerical solutions to the nonlinear rendezvous problem.