An Efficient Algorithm for Calculating Aircraft RCS Based on the Geometrical Characteristics

Taking into account the influences of scatterer geometrical shapes on induced currents, an algorithm, termed the sparse-matrix method （SMM）, is proposed to calculate radar cross section （RCS） of aircraft configuration. Based on the geometrical characteristics and the method of moment （MOM）, the SMM points out that the strong current coupling zone could be predefined according to the shape of scatterers. Two geometrical parameters, the surface curvature and the electrical space between the field position and source position, are deducted to distinguish the dominant current coupling. Then the strong current coupling is computed to construct an impedance matrix having sparse nature, which is solved to compute RCS. The efficiency and feasibility of the SMM are demonstrated by computing electromagnetic scattering of some kinds of shapes such as a cone-sphere with a gap, a bi-arc column and a stealth aircraft configuration. The numerical results show that： （1） the accuracy of SMM is satisfied, as compared with MOM, and the computational time it spends is only about 8% of the MOM; （2） with the electrical space considered, making another allowance for the surface curvature can reduce the computation time by 9.5%.

Effects of geometrical characteristics on defect distributions in alloy components produced by selective laser melting

Selective laser melting(SLM)has been applied to manufacture various alloy components with excellent properties,but its further application is restricted by the intrinsic defects.In this work,the internal defect distributions in samples of three alloys(316L stainless steel,AlSi10Mg and Inconel 718)were investigated respectively,considering the effects of geometrical characteristics of the samples.The defects in the 316L stainless steel sample tend to be formed densely in the central part with large wall thickness,indicating a strong sensitivity to heat accumulation.Contrarily,the Inconel 718 sample shows a higher relative density with homogeneous defect distribution,indicating better formability for the SLM process.For the AlSi10Mg sample,the defect density keeps increasing as the deposition goes on.Typically,the defect density located at sample edges shows an abnormally high level comparing with the inner part,especially in the top sections of AlSi10Mg and Inconel 718 samples.The results are helpful for the geometrical design,the adjustment of building orientation and the further optimization of process parameters in the SLM process.

Modeling of rotational supercavitating evaporator and the geometrical characteristics of supercavity within

In this paper,a rotational supercavitating evaporator(RSCE)was at first modeled by means of theoretical analysis approach.The geometrical characteristics of supercavity in the modeled RSCE were then studied through numerical simulations.The current research objectives consist in determination of shape of the supercavitator(which in the plane of rotation generates supercavity occupying the most volume between blades),and location of the area suitable for steam extraction by revealing the inner structure of supercavity.Analytical analysis was performed by solving empirical equations for the shape of RSCE,through which an evaluation of two-dimensional relative position of supercavity trailing edge for different shapes of the supercavitator has been realized.Numerical simulation was then carried out,by numerically solving the unsteady Navier-Stokes equations in their conservation form coupled with the Rayleigh-Plesset cavitation and Shear-Stress Transport turbulence models,for verification of the results obtained from empirical equations.Despite unreliable assumption of applicability of empirical equations we have confirmed similarity of the supercavity shapes obtained by both methods for the same RSCE.Therefore,the shape of supercavitator calculated by using empirical equations is acceptable,which provides a simple but reliable approach for design of RSCE.The inner structure of supercavity obtained by numerical simulation has indicated position and parameters for steam extraction openings for further numerical and experimental studies on the performance of RSCE.Practical application of steam or gas extraction is suggested for solving of some problems associated with cavitating pumping of cryogenic liquid.

Geometrical and Geotechnical Characteristics of Landslides in Korea under Various Geological Conditions

The goal of this study is to determine the geometrical and geotechnical characteristics of landslides under various geological conditions using detailed field surveys, laboratory soil tests and precipitation records. Three study areas are selected to consider different rocks, including gneiss in Jangheung, granite in Sangju and sedimentary rocks in Pohang, South Korea. Many landslides have occurred in these three areas during the rainy season.Precipitation records indicate that landslides occurring in the gneiss area of Jangheung and granite area of Sangju may be influenced by the hourly rainfall intensity rather than cumulative rainfall.However, landslides occurring in the sedimentary rock area of Pohang may be influenced by hourly rainfall intensity and cumulative rainfall. To investigate the factors that influence these types of landslides, a detailed landslide survey was performed and a series of laboratory soil tests were conducted.According to the detailed field survey, most landslides occurred on the flanks of mountain slopes, and the slope inclination where they occurred mostly ranged from 26 to 30 degrees, regardless of the geological conditions. The landslide in the gneiss area of Jangheung is larger than the landslides in the granite area of Sangju and sedimentary rock area of Pohang.Particularly, the landslide in the sedimentary rock area is shorter and shallower than the landslides in the gneiss and granite areas. Thus, the shape and size of the landslide are clearly related to the geological conditions. According to the integrated soil property and landslide occurrence analyses results, the average dry unit weight of the soils from the landslide sites is smaller than that of the soils obtained from the nonlandslide site. The average coefficient of permeability of soils obtained from the landslide sites is greater than that of soils obtained from the non-landslide sites with the same geology. These results indicate that the soils from the landslide sites are more poorly graded or looser than the soils from the non-landslide sites.

Experimental Study on 3D Geometrical and Fractal Characteristics of Fracture Surface of Rock-Like Materials Based on GIS Technique

In order to research the characteristic and mechanism of fracture of rock-like materials,the morphology of rock fracture surface under the breakages of uniaxial compression and triaxial compression was observed and measured by means of a new-type 3D laser scanning system.Based on geographic information system(GIS)technique,the fracture surfaces were 3D visualized and reestablished.According to GIS 3D statistics,the geometrical characteristics of fracture surfaces under different breakage conditions were analyzed,and then based on fractal theory,the change laws of fractal dimension of fracture surfaces were discussed under the conditions of different cell pressures and initial water contents of rock.Furthermore,the relationships between characteristics of fracture surface and mechanical properties of rock were discussed.The results indicate that cell pressure,initial water content,and mechanical parameters of rock are the important factors to influence on the geometrical characteristics of fracture surface.The research provides a new experimental method for quantitative study on the fracture characteristics of various materials under different breakage conditions.

PREDICTION OF PUMP CHARACTERISTICS ACCORDING TO GEOMETRICAL PARAMETERS OF IMPELLER AND VOLUTE CASING

The geometrical parameters of impeller or volute casing (including guide vane ofmultistage pump) have a great effect on pump characteristics, but ultimately. the pump characteris-tics are determined by the geometrical parameters of impeller and volute casing cooperatively. Inthis essay the effect of impeller and volute casing on pump characteristics will be studiedquantitatvely from the angle cf optimal matching of them.

Geometrical characteristic investigation of the Baihetan irregular columnar jointed basalt and corresponding numerical reconstruction method

Columnar jointed rock mass with unique geometric and geological properties is one spectacular example of geometrical order in nature.Columnar joints are generally accepted to be formed by spatially uniform volume contraction during cooling.In this paper,substantial field work was performed to study the geological characteristics of irregular columnar jointed basalt on the left bank dam foundation in the Baihetan Hydropower Station,where the columnar jointed rock mass is extensively exposed due to excavation.The quantitative measurements of the sizing of polygonal crack pattern of columnar joints and assessment of their degree of irregularity were summarized.Considering the irregularity of polygonal crack pattern,a modified Voronoi polygon(MVP)method was developed to model the special polygonal crack pattern of columnar joints.The new polygonal pattern obtained by the MVP method consists of a large number of irregular polygons,of which the degree of irregularity is consistent with the field measurement results.This method can reproduce the rapid evolution from an initial ideal regular hexagonal pattern to a final actual irregular polygonal pattern as the degree of irregularity increases.The compression tests of columnar jointed rock mass with different irregularity show that the geometric irregularity has a great influence on its mechanical properties.This numerical construction method provides a reliable way to reconstruct columnar joint structure with specific polygonal crack pattern,which is consistent with onsite columnar jointed basalt.

Geometric Characteristics of Tropical Cyclone Eyes before Landfall in South China Based on Ground-Based Radar Observations

The geometric characteristics of tropical cyclone（TC） eyes before landfall in South China are examined using groundbased radar reflectivity. It is found that the median and mean eye area decrease with TC intensity, except for the severe typhoon category, and the eye size increases with height. The increasing rate of eye size is relatively greater in upper layers.Moreover, the ratio of eye size change in the vertical direction does not correlate with TC intensity. No relationship is presented between the ratio of eye size change in the vertical direction and the vertical wind shear. No relationship between the vertical change in eye size and the eye size at a certain level is found, inconsistent with other studies. No relationship exists between the vertical change in eye size and the intensity tendency. The eye roundness values range mainly from 0.5 to 0.7, and more intense TCs generally have eyes that are more circular.

Theory and Method of Power System Integrated Security Region Irrelevant to Operation States:An Introduction

How to comprehensively consider the power flow constraints and various stability constraints in a series of power system optimization problems without affecting the calculation speed is always a problem.The computational burden of probabilistic security assessment is even more unimaginable.In order to solve such problems,a security region(SR)methodology is proposed,which is a brand-new methodology developed on the basis of the classical point-wise method.Tianjin University has been studying the SR methodology since the 1980s,and has achieved a series of original breakthroughs that are described in this paper.The integrated SR introduced in this paper is mainly defined in the power injection space,and includes SRs to ensure steady-state security,transient stability,static voltage stability,and smalldisturbance stability.These SRs are uniquely determined for a given network topology(as well as location and clearing process for transient faults)and given system component parameters,and are irrelevant to operation states.This paper presents 11 facts and related remarks to introduce the basic concepts,composition,dynamics nature,and topological and geometric characteristics of SRs.It also provides a practical mathematical description of SR boundaries and fast calculation methods to determine them in a concise and systematic way.Thus,this article provides support for the systematic understanding,future research,and applications of SRs.The most critical finding on the topological and geometric characteristics of SRs is that,within the scope of engineering concern,the practical boundaries of SRs in the power injection space can be approximated by one or a few hyperplanes.Based on this finding,the calculation time for power system probabilistic security assessment(i.e.,risk analysis)and power system optimization with security constraints can be decreased by orders of magnitude.

Polygonal faults in the Sanzhao sag of the Songliao basin:their significance in hydrocarbon accumulation

Polygonal faults,generally distributed in fine-grained sediments,are layer-bound faults and are important in hydrocarbon accumulation.Using 3D seismic data,we analyzed the plane and profile features of faults developed in the Qingshankou formation of the Sanzhao sag.We identified these faults as having typical features of polygonal faults:1) layer-bound;2) normal faults;3) slight fault displacements and steep in dip angles;4) multi-directional in strike and 5) a single fault has a short horizontal extension.In addition,these faults intersect each other and form polygons.These polygonal faults are the result from the combined action of compaction,volume contraction and episodic hydraulic fracturing,conditions favorable for oil/gas accumulation.They are the dominant channels for migration of fluids in the Qingshankou mudstone,forming a large number of fault-lithologic oil traps.Polygonal faults improve reservoirs.

Prediction of salinity intrusion in the sheltered estuary of Terengganu River in Malaysia using 1-D empirical intrusion model

Generally one dimensional （l-D） empirical salinity intrusion model is limited to natural alluvial estuary. However, this study attempts to investigate its ability to model a sheltered alluvial estuary of the Terengganu River in Malaysia. The constructed breakwater at the mouth of the river shelters the estuary from direct influence of the open sea. The salinity density along the estuary was collected during the wet and dry seasons for scenarios before and after the constructed breakwater. Moreover, the freshwater discharges, tidal elevations and bathymetry data were also measured as model inputs. A good fit was demonstrated between simulated and observed variables, namely salinity distribution and intrusion length for both scenarios. Thus, the results show that 1-D empirical salinity model can be utilized for sheltered estuarine condition at the Terengganu Estuary, but with an appropriate determination of an initial point. Furthermore, it was observed that the salinity intrusion in the study area is largely dependent on the freshwater discharge rather than tidal elevation fluctuations. The scale of the salinity intrusion length in the study area is proportional to the river discharge of the -1/2 power. It was appeared that the two lines of the 1-D empirical salinity model and discharge power based equation fitted well to each other, with the average predicted minimum freshwater discharge of 150 m^3/s is going to be required to maintain acceptable salinity levels during high water slack （HWS） near the water intake station, which is located at 10.63 km from river mouth.

Subsolidus Conceptual Design of CaO-AI_2O_3-TiO_2-SiO_2 System and Its Significance for Manufacturing Advanced Ceramics

The article deals the structure of the CaO -Al2O3 - TiO2 -SiO2 system as a basis for obtaining advanced ce- ramics with a complex of high-operational characteris- tics. As a result o[＇investigations the phase charctcteristics are represented trod the graph of the relationship of the elementaory tetrahedra has been plotted. The most techno- logical system area ,for the production of heat resistant advanced ceramics has been determined. The batches for producing heat and chemical resistant dense advanced ceramics made of natural, technical and technogenic raw materials have been developed.

Learning to Identify Footholds from Geometric Characteristics for a Six-legged Robot over Rugged Terrain

Foothold identification is a key ability for legged robots that allows generating terrain adaptive behaviors(e.g.,gait and control parameters)and thereby improving mobility in complex environment.To this end,this paper addresses the issue of foothold characterization and identification over rugged terrain,from the terrain geometry point of view.For a terrain region that might be a potential foothold of a robotic leg,the characteristic features are extracted as two first-order partial derivatives and two curvature parameters of a quadric regression surface at this location.These features are able to give an intuitive and,more importantly,accurate characterization towards the specific geometry of the ground location.On this basis,a supervised learning technique,Support Vector Machine(SVM),is employed,seeking to Ieam a foothold identification policy from human expert demonstration.As a result,an SVM classifier is leamt using the extracted features and human-demonstrated labels,which is able to identify whether or not a certain ground location is suited as a safe foot support for a robotic leg.It is shown that over 90%identification rate can be achieved with the proposed approach.Finally,preliminary experiment is implemented with a six-legged robot to demonstrate the effectiveness of the proposed approach.

Leaf shape simulation of castor bean and its application in nondestructive leaf area estimation

The leaf shape and leaf area measurement are crucial in plant growth modeling.The castor bean leaf is large,palm-shaped with multiple clefts.The leaf shape simulation and leaf area estimation were less studied.The circular model and nonrectangular hyperbolic model were developed to describe the standard leaf shape of castor bean in this study,providing a model for simulating the leaf shape and a nondestructive way for estimating the leaf area respectively.In addition,a formula was established to estimate the leaf area by the parameter of the standard leaf shape of castor bean.Based on validation results,the circular model fits the landmarks and nonrectangular hyperbolic model fits the lobe margins very well.The leaf area was accurately estimated by using the established formula.This study could provide a theoretical reference for leaf visualization,a nondestructive and easy way to estimate the leaf area for other complex leaves with multiple lobes.

Phenotyping of poplar seedling leaves based on a 3D visualization method

In plant research,there is a demand for non-destructive and non-invasive trait measurement methods for phenotyping that can be used to accurately analyze various aspects of plants,such as stem length,leaf area,and leaf inclination.In this study,a method for measuring the leaf geometric characteristics of poplar seedlings based on 3D visualization via the use of time-of-flight(ToF)and digital cameras was proposed.Firstly,the average distance density function method was applied to process outliers of leaves.Secondly,to improve the accuracy of data fitting,a specific method using the angle of adjacent two-point normal vectors was introduced to filter redundant data,kept essential sample values as the control points,and then used the control points to fit the leaf surface based on non-uniform rational B-splines(NURBS).At the same time,NURBS was used to fit the trunk according to its control points and an iterative method to fit the other branches.Finally,3D visualization of poplar seedlings was achieved,and leaf traits,including leaf width,leaf length,leaf area,and leaf inclination angle,were calculated.To obtain accurate results,multiple experiments were conducted including assessments of poplar seedlings exhibiting normal growth and those grown under water shortage.The results of the proposed method were compared with the real values of the leaves.The RMSE for leaf width,leaf length,leaf area,and leaf angle were 0.18 cm,0.21 cm,1.14 cm^(2),and 1.97°,respectively.The results proved that this approach could be used to accurately measure the leaf characteristics of poplar seedlings via visualization.

Development of seeder for mixed planting of corn and soybeans

Corn and soybeans should be sown between 5 cm and 10 cm apart in mixed cultivation to increase protein content and improve productivity of the forage.However,existing sowers cannot plant at intervals of less than 20 cm.Consequently,mixed cultivation of corn and soybeans is currently performed by first sowing corn seeds with a tractor and then manually planting soybean seeds.This method results in irregular intervals between the seeds,it is laborious and time consuming.This study aimed at developing a seeder that can simultaneously,precisely and efficiently plant corn and soybean.The geometrical and rheological properties of corn and soybeans were initially measured.The seed conveying equipment were designed using the EDEM software.The sowing interval between seeds,depth of soil over planted seed,and sowing performance were analyzed.The EDEM simulation results indicated that a 6-mm-wide and 3-mm-deep grooved seed-delivering roller had the highest particle mobility of the designs considered,with a 2.5%misplanting rate.A performance test showed that no misplanting occurred in the sections sowed with soybean seeds at a seeding interval that averaged 32 mm(321 seeds sown in 10 m)and that misplanting occurred in one section sowed with corn at a seeding interval that averaged 247 mm(40 seeds sown over 10 m).The sowing efficiency for both corn and soybeans was found to be 0.42 h/hm2.The average depth of soil over seed was 32.7 mm for soybean and 39.7 mm for corn.These average depths are within the stipulated range for the depth of soil over seed,which is 5 to 10 times the seed size.This study developed an efficient seeding machine that can simultaneously plant soybean and corn precisely,consequently improving forage yield and saving man-hours.

Numerical analysis of a ventilated supercavity under periodic motion of the cavitator

This paper presents a numerical simulation of the geometry and the pressure distribution of a ventilated supercavity at different cavitator amplitudes and periods of motion.The numerical method is validated by comparing with the results of a semi-empirical formula under specific conditions.It is shown that the simulation can capture the boundary fluctuations of the ventilated supercavity and its internal pressure variations in a cavitator motion cycle.The simulation results show that the supercavity boundary experiences wave-like deformations when the wavelength of the disturbance caused by the cavitator motion is comparable to the supercavity length.It is also shown that the supercavity closure changes in form between a re-entrant jet and a twin vortex owing to the variations of the pressure difference between the outside and the inside of the supercavity near the closure region.The maximum diameter of the ventilated supercavity exhibits periodic changes with a double peak in each cavitator motion cycle,caused by the corresponding changes of the difference between the internal and external pressures.With the increase of the amplitude of motion of the cavitator,the supercavity boundary has enhanced wave-like undulations,with an increased maximum diameter,and with fluctuations in the cavitation number.As the period of the cavitator motion increases,the wavelength of the disturbances caused by this motion becomes greater than the supercavity length,and so the wave-like undulations of the supercavity boundary and the maximum diameter of the supercavity gradually decrease,but the variations of the cavitation number increase.Moreover,with the increase of the periods,the delay effects on the variations of the characteristics of the supercavity geometry caused by cavitator motion gradually decrease,and they practically vanish for large periods.

Dynamic Capture Method for Out-of-step Center of Power Grid Based on Measurement Information

This paper dynamically captures the out-of-step center for a bulk power system based on measurement information.First,the position characteristics of the out-of-step center are discussed from the perspective of triangular geometry under different potential and uniform impedance distributions based on a two-machine system.The correlation between the out-of-step center and the oscillation center is analyzed.Second,combined with analytical geometry,it is inferred that the distribution characteristics of the out-of-step center voltage vector ends are a circle or a straight line,and the position of the out-of-step center corresponds to the intersection between the voltage drop of the line and the distribution characteristics;then,the accurate location of the out-of-step center and the determination of the appearance moment is realized.Finally,a dynamic equivalence method for a power grid is proposed based on the electrical parameters of the transmission line,and an alternative iterative correction of equivalent parameters is carried out by trajectory sensitivity,which provides a new way for capturing the out-ofstep center dynamically in a multimachine system.The proposed method is validated using a three-machine system,the IEEE-39 system and the Northwest China Power Grid,and shows potential for online applications.