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Aerospace assembly demands high drilling position accuracy for fastener holes. Hole position error correction is a key issue to meet the required hole position accuracy. This paper aims to propose a combined hole position error correction method to achieve high positioning accuracy.

The bilinear interpolation surface function based on the shape of the aerospace structure is capable of dealing with position error of non-gravity deformation. A gravity deformation model is developed based on mechanics theory to efficiently correct deformation error caused by gravity. Moreover, three solution strategies of the average, least-squares and genetic optimization algorithms are used to solve the coefficients in the gravity deformation model to further improve position accuracy and efficiency.

Experimental validation shows that the combined position error correction method proposed in this paper significantly reduces the position errors of fastener holes from 1.106 to 0.123 mm. The total position error is reduced by 43.49% compared with the traditional mechanics theory method.

The position error correlation method could reach an accuracy of millimeter or submillimeter scale, which may not satisfy higher precision.

The proposed position error correction method has been integrated into the automatic drilling machine to ensure the drilling position accuracy.

The proposed position error method could promote the wide application of automatic drilling and riveting machining system in aerospace industry.

A combined position error correction method and the complete roadmap for error compensation are proposed. The position accuracy of fastener holes is reduced stably below 0.2 mm, which can fulfill the requirements of aero-structural assembly.

To demonstrate, through numerical models, that it is possible to simulated low‐gravity phase change (melting), of an electrically conducting material (gallium), in terrestrial conditions via the application of electromagnetic fields.

A complete three‐dimensional mathematical formulation governing a phase change process in the presence of an electromagnetic field has been developed. In addition a comprehensive parametric study has been completed to study the various effects of gravity, Stefan number, Hartmann number and electromagnetic pressure number upon the phase change process.

The results show that the application of an electromagnetic filed can be used to simulate key melting characteristics found for actual low‐gravity. However, the resulting three‐dimensional flow field in the melted region differs from actual low‐gravity. The application of an electromagnetic field creates a flow phenomenon not found in actual low‐gravity or previously seen in two‐dimensional problems.

Future work may include the use of oscillating electromagnetic fields to enhance convection in energy storage systems in a low‐gravity environment.

The ability to suppress unwanted convective flows in a phase change process without the high magnetic fields necessary in magnetic field only suppression systems.

This work fills a void in the literature related to conducting fluids and the effects of magnetic and electromagnetic fields.

Economists have had problems interpreting the gravity model, particularly as applied to migration behaviour. Its physical analogy is unappealing and theories of economic behaviour do not generally give rise to gravity laws. Some years ago Niedercorn and Bechdolt (1969) published a paper in which they provide a derivation of the gravity model using utility theory. Their approach is, however, much more applicable to other flows of human interaction across space, such as travel or communication flows, than it is to migration flows. The major purpose of this paper is to indicate a set of assumptions which might underlie a gravity model of migration and to show that these assumptions are quite severe. One important implication is that we should fully expect any real world migration pattern to deviate from the predictions of a gravity model. Nevertheless, the gravity specification may be a useful starting point in the analysis of migration behaviour, as it imposes restrictions on the formulation of the estimating equations. Thus we may fruitfully study how and why the real world migration patterns diverge from gravity flows.

The gravity theory of trade explains the potential for trade between nations, but its application to trade in halal food has been questioned by previous studies. This study aims to investigate this issue and the role of trading partners’ economic strength and their distance from one another to identify Malaysia’s potential to export food to key halal markets.

The gravity theory of trade was used to examine Malaysia’s top 10 food exports to key halal markets from 2000–2017. The gravity panels were estimated using the Hausman-Taylor modelling technique to control for endogeneity within the model.

The application of the gravity theory of trade to a halal market context provides mixed results. Although the high economic strength (gross domestic product) of the trading partners enhances halal trade, the distance between the partners does not affect the volume of halal food exports. Moreover, the study identifies Malaysia’s potential to export only a few food commodities to key halal markets.

This study challenges the applicability of the gravity theory of trade to the halal food market. The study extends the model with additional controls for behavioural aspects and applies it to commodity-specific segregated trade in halal food. The findings underscore the need to extend the theory beyond its current focus when explaining trade opportunities in halal markets.

The purpose of this paper is to present a full fourth‐order model of the gravity gradient torque of spacecraft around asteroids by taking into consideration of the inertia integrals of the spacecraft up to the fourth order, which is an improvement of the previous fourth‐order model of the gravity gradient torque.

The fourth‐order gravitational potential of the spacecraft is derived based on Taylor expansion. Then the expression of the gravity gradient torque in terms of gravitational potential derivatives is derived. By using the formulation of the gravitational potential, explicit formulations of the full fourth‐order gravity gradient torque are obtained. Then a numerical simulation is carried out to verify the model.

It is found that the model is more sound and precise than the previous fourth‐order model due to the consideration of higher‐order inertia integrals of the spacecraft. Numerical simulation results show that the motion of the previous fourth‐order model is quite different from the exact motion, while the full fourth‐order model fits the exact motion very well. The full fourth‐order model is precise enough for high‐precision attitude dynamics and control around asteroids.

This high‐precision model is of importance for the future asteroids missions for scientific explorations and near‐Earth objects (NEOs) mitigation.

In comparison with previous models, a gravity gradient torque model around asteroids that is more sound and precise is established. This model is valuable for high‐precision attitude dynamics and control around asteroids.

This study aims to estimate the determinants of international tourist flows between destinations by using the panel gravity model.

The multi-dimensional panel gravity model was used to analyse tourism originating from 30 different countries to the 14 most-visited countries in the world between 2008 and 2016. Income (i.e. per capita gross domestic product for both the origin and destination countries), distance between countries, various economic indicators and six dummy variables were added to the gravity model as control variables.

The results indicated that tourist arrivals depended mainly on economic factors, i.e. income and trade variables were significant determinants of tourist arrivals. The results also suggested that estimated international tourist flows are a negative function of distance, as is postulated in economic theory.

In recent years, gravity models have been used frequently to analyse international tourism demand and have demonstrated their ability to evaluate the effects of various determinants of international tourism for many countries. The literature includes studies that used a two-dimensional panel gravity model to analyse the determinants of tourism demand to a single country from many different countries. This study differs in terms of specificity; in that, it relied on a three-dimensional panel gravity model that allowed for modelling of multiple destination countries. As a result, more comprehensive and general results relative to the determinants of tourism demand were obtained. In addition, the application of a non-nested three-dimensional panel data model, which has limited use, contributes a new perspective to the econometric literature.

本研究旨在通过采用面板引力模型来估计目的地之间国际旅游流量的决定因素。

多维面板引力模型用于分析2008年至2016年间来自30个不同国家的游客赴世界上前14位旅游目的地国家的旅游情况。收入（即客源国和目的地国家的人均国内生产总值）, 国家之间的距离, 各种经济指标和六个虚拟变量作为控制变量加入到引力模型中。

结果表明, 游客到访主要取决于经济因素, 即收入和贸易变量是游客到访的重要决定因素。结果还表明, 估计的国际旅游流量是距离的负函数。

近年来, 引力模型经常被用来分析国际旅游需求, 并证明了它们能够评估许多国家的各种国际旅游决定因素的影响。文献包括使用二维面板引力模型来分析来自许多不同国家的单个国家的旅游需求的决定因素的研究。该研究的不同之处在于, 它依赖于非嵌套集合模型三维面板引力模型, 允许对多个目的地国家进行建模。因此, 此研究获得了与旅游需求决定因素相关的更全面和广泛的结果。此外, 使用不常运用的非嵌套三维面板数据模型的应用为计量经济学文献提供了新的视角。

国际旅游需求, 重力模型, 多维面板数据模型

Este estudio tiene como objetivo estimar los determinantes de los flujos turísticos internacionales entre destinos mediante el uso del modelo de panel de gravedad.

El modelo de gravedad de panel multidimensional se utilizó para analizar el turismo que se originó en 30 países diferentes a los 14 países más visitados del mundo entre 2008 y 2016.

Los resultados indicaron que las llegadas de turistas dependían principalmente de factores económicos, es decir, las variables de ingresos y comercio fueron determinantes significativos de las llegadas de turistas. Los resultados también sugirieron que los flujos turísticos internacionales estimados son una función negativa de la distancia.

En los últimos años, los modelos de gravedad se han utilizado con frecuencia para analizar la demanda turística internacional y han demostrado su capacidad para evaluar los efectos de varios determinantes del turismo internacional para muchos países. La literatura incluye estudios que utilizaron un modelo de gravedad de panel bidimensional para analizar los determinantes de la demanda turística a un solo país desde muchos países diferentes. Este estudio difiere en términos de especificidad en que se basó en un modelo tridimensional de panel de gravedad que permitió el modelado de múltiples países de destino.

Demanda turística internacional, Modelo de gravedad, Modelo de datos de panel multidimensional

The molecular dynamics simulation of micro‐Poiseuille flow for liquid argon in nanoscale was performed in non‐dimensional unit system with the control parameters of channel size, coupling parameters between solid wall and liquid particles, and the gravity force. The molecular forces are considered not only among the liquid molecules, but also between the solid wall and liquid molecules. The simulation shows that a larger gravity force produces a larger shear rate and a higher velocity distribution. In terms of the gravity force, there are three domain regions each with distinct flow behaviors: free molecule oscillation, coupling and gravity force domain regions. Stronger fluid/wall interactions can sustain a larger coupling region, in which the flow is controlled by the balance of the intermolecular force and the gravity force. Strong surface interaction leads to small slip lengths and the slip lengths are increased slightly with increasing the shear rate. Weak surface interaction results in higher slip lengths and the slip lengths are dramatically decreased with increasing the shear rate. The viscosities are nearly kept constant (Newton flow behavior) if the non‐dimensional shear rate is below 2.0. At higher non‐dimensional shear rate larger than 2.0, the viscosities have a sharp increase with increasing the shear rate, and the non‐Newton flow appears.

The purpose of this paper is to examine the effects of gravity variables and trade costs on China's export margins.

Following the structural gravity model with firm heterogeneity, the paper measures the extensive margins and intensive margins of China's export across 46 export destinations and estimates the linkage between export margins and its potential determinants.

The empirical results confirm the gravity relationship hold for bilateral trade and export margins. Furthermore, trade costs have different influence on extensive margins and intensive margins as the structural gravity model with firm heterogeneity expected. The paper also shows the rapid growth of China's export is mainly along the intensive margins which are increasing in fixed cost for export.

The paper contributes to the measurements of China's export margins and the empirical research on effects of trade liberalization on China's foreign trade.

The purpose of this paper is to present an investigation of the effect of different gravity conditions on the penetration mechanism using the two-dimensional Distinct Element Method (DEM), which ranges from high gravity used in centrifuge model tests to low gravity incurred by serial parabolic flight, with the aim of efficiently analyzing cone penetration tests on the lunar surface.

Seven penetration tests were numerically simulated on loose granular ground under different gravity conditions, i.e. one-sixth, one-half, one, five, ten, 15 and 20 terrestrial gravities. The effect of gravity on the mechanisms is examined with aspect to the tip resistance, deformation pattern, displacement paths, stress fields, stress paths, strain and rotation paths, and velocity fields during the penetration process.

First, under both low and high gravities, the penetration leads to high gradients of the value and direction of stresses in addition to high gradients in the velocity field near the penetrometer. In addition, the soil near the penetrometer undergoes large rotations of the principal stresses. Second, high gravity leads to a larger rotation of principal stresses and more downward particle motions than low gravity. Third, the tip resistance increases with penetration depth and gravity. Both the maximum (steady) normalized cone tip resistance and the maximum normalized mean (deviatoric) stress can be uniquely expressed by a linear equation in terms of the reciprocal of gravity.

This study investigates the effect of different gravity conditions on penetration mechanisms by using DEM.

This study aims to propose a novel method based on model learning with sparsity inducing norms for estimating dynamic gravity terms of the serial manipulators. This method is realized by operating the robot, acquiring data and filtering the features in signal acquisition to adapt to the dynamic gravity parameters.

The core principle of the method is to analyze the dictionary composition of the basis function of the model based on the dynamic equation and the Jacobian matrix of an arm. According to the structure of the basis function and the sparsity of the features, combined with joint-angle and driving-torque data acquisition, the effective features of dynamic gravity parameters are screened out using L1-norm optimization and learning algorithms.

The theoretical analysis revealed that training data obtained based on joint angles and driving torques could rapidly update dynamic gravity parameters. The simulation experiment was carried out by using the publicly available robot model and compared with the previous disassembly method to evaluate the feasibility and performance. The real 7-degree of freedom (DOF) industrial manipulator was used to further discuss the effects of the feature selection. The results show that this estimation method can be fully operational and efficient in industrial applications.

This approach is applicable to most serial robots with multi-DOF and the dynamic gravity parameters of the robot are estimated through learning and optimization. The method does not require prior knowledge of the robot arm structure and only requires joint-angle and driving-torque data acquisition under low-speed motion. Furthermore, as it is a data-driven-based method, it can be applied to gravity parameters updating.

Different from previous general robot dynamic modelling methods, the sparsity of the analytical form of dynamic equations was exploited and model learning was formulated as a convex optimization problem to achieve effective gravity parameters screening. The novelty of this estimation approach is that the method does not only require any prior knowledge but also does not require a specifically designed trajectory. Thus, this method can avoid the laborious work of parameter calibration and the induced modelling errors. By using a data-driven learning approach, the new parameter updating process can be completed conveniently when the robot carries additional mass or the end-effector changes for different tasks.