A challenging topic in nonlinear dynamics concerns the study of fractional-order systems without equilibrium points.In particular, no paper has been published to date regarding the presence of hyperchaos in these syst...A challenging topic in nonlinear dynamics concerns the study of fractional-order systems without equilibrium points.In particular, no paper has been published to date regarding the presence of hyperchaos in these systems. This paper aims to bridge the gap by introducing a new example of fractional-order hyperchaotic system without equilibrium points. The conducted analysis shows that hyperchaos exists in the proposed system when its order is as low as 3.84. Moreover, an interesting application of hyperchaotic synchronization to the considered fractional-order system is provided.展开更多
IntroductionEarthquake location is one of the inverse problems in seismology. The accurate earthquake source location is the basis of studies of earthquake images, the relation analysis between seismicity and tectonic...IntroductionEarthquake location is one of the inverse problems in seismology. The accurate earthquake source location is the basis of studies of earthquake images, the relation analysis between seismicity and tectonics, and the important parameters of earthquakes. It is also the basic data used to study the space distribution of aftershocks after a strong earthquake. The earthquake location accuracy mainly depends on reliability of the velocity models for the earth¢s crust, the reasonable design of the seismograph network, the accuracy of arrival time, and the location method used (ZHAO, 1983; ZHU, ZHAO, 1997; LI, et al, 1999). The reliability of the velocity model directly influences loca-tion results. The reasonable seismograph network design can improve the location accuracy; otherwise it will in-troduce ill-conditioned equations and large location error. If arrival time is not accurate, for example, if the seismic phase is wrongly identified or the time service is inaccurate, large arrival time error will be produced, and the location results and real hypocenters will be discordant. Different location algorithms adopted will produce differ-ent location errors. The residual criterion chosen has influence on location results. Besides, whether the event is correctly associated also influences the location quality. In practice, it can often be seen that large location devia-tion introduced by wrong event association.展开更多
Orthogonal frequency-division multiplexing (OFDM) is a multi carrier modulation scheme mainly used for digital communications. The performance of OFDM system heavily depends on the synchronization scheme used. In most...Orthogonal frequency-division multiplexing (OFDM) is a multi carrier modulation scheme mainly used for digital communications. The performance of OFDM system heavily depends on the synchronization scheme used. In most cases, the accuracy level of synchronization will be worsened by the error caused in fixed point arithmetic involved. In this paper, we analyze the impact of the fixed point arithmetic on the performance of the coarse timing and frequency synchronization. Here with an analytical approach through numerical simulations bit length of IEEE 754 standard single precision format is optimized according to the required degree of accuracy for low complexity. Also, a complete precision level requirement for FFT computations with all possible modulation types is obtained. The proposed precision model is compared with IEEE standard single precision model and its efficiency in OFDM synchronization process is proved through MATLAB simulations. Finally, the complexity reduction of proposed precision model in both addition and subtraction is proved against single precision format using hardware synthesis. Here we proved that more than 50% complexity reduction is achieved as compared to standard precision models without compromising quality. The quality retention of proposed model is proved in both timing and frequency synchronization process.展开更多
In this paper we deduce the analytic solutions of the first- and second-order vertical derivative zero points for gravity anomalies in simple regular models with single, double, and multiple edges and analyze their sp...In this paper we deduce the analytic solutions of the first- and second-order vertical derivative zero points for gravity anomalies in simple regular models with single, double, and multiple edges and analyze their spatial variation. For another simple regular models where it is difficult to obtain the analytic expression of the zero point, we try to use the profile zero points to analyze the spatial variation. The test results show that the spatial variation laws of both first- and second-order vertical derivative zero points are almost the same but the second-order derivative zero point position is closer to the top surface edge of the geological bodies than the first-order vertical derivative and has a relatively high resolution. Moreover, with an increase in buried depth, for a single boundary model, the vertical derivative zero point location tends to move from the top surface edge to the outside of the buried body but finally converges to a fixed value. For a double boundary model, the vertical derivative zero point location tends to migrate from the top surface edge to the outside of the buried body. For multiple boundary models, the vertical derivative zero point location converges from the top surface edge to the outside of the buried body where some zero points coincide and finally vanish. Finally, the effectiveness and reliability of the proposed method is verified using real field data.展开更多
In this paper,some remarks for more efficient analysis of two-dimensional elastostatic problems using the method of fundamental solutions are made.First,the effects of the distance between pseudo and main boundaries o...In this paper,some remarks for more efficient analysis of two-dimensional elastostatic problems using the method of fundamental solutions are made.First,the effects of the distance between pseudo and main boundaries on the solution are investigated and by a numerical study a lower bound for the distance of each source point to the main boundary is suggested.In some cases,the resulting system of equations becomes ill-conditioned for which,the truncated singular value decomposition with a criterion based on the accuracy of the imposition of boundary conditions is used.Moreover,a procedure for normalizing the shear modulus is presented that significantly reduces the condition number of the system of equations.By solving two example problems with stress concentration,the effectiveness of the proposed methods is demonstrated.展开更多
BACKGROUND Patients with right sided colorectal cancer are known to have a poorer prognosis than patients with left sided colorectal cancer, whatever the cancer stage. To this day, primary tumor resection(PTR) is stil...BACKGROUND Patients with right sided colorectal cancer are known to have a poorer prognosis than patients with left sided colorectal cancer, whatever the cancer stage. To this day, primary tumor resection(PTR) is still controversial in a metastatic, non resectable setting.AIM To explore the survival impact of PTR in patients with metastatic colorectal cancer(mCRC) depending on PTL.METHODS We retrospectively collected data from all consecutive patients treated for mCRC at the Centre Georges Francois Leclerc Hospital. Univariate and multivariate Cox proportional hazard regression models were used to assess the influence of PTR on survival. We then evaluated the association between PTL and overall survival among patients who previously underwent or did not undergo PTR. A propensity score was performed to match cohorts.RESULTS Four hundred and sixty-six patients were included. A total of 153(32.8%) patients had unresected synchronous mCRC and 313(67.2%) patients had resected synchronous mCRC. The number of patients with right colic cancer, left colic cancer and rectal cancer was respectively 174(37.3%), 203(43.6%) and 89(19.1%). In the multivariate analysis only PTL, PTR, resection of hepatic and or pulmonary metastases and the use of oxaliplatin, EGFR inhibitors or bevacizumab throughout treatment were associated to higher overall survival rates. Survival evaluation depending on PTR and PTL found that PTR improved the prognosis of both left and right sided mCRC. Results were confirmed by using a weighted propensity score.CONCLUSION In mCRC, PTR seems to confer a higher survival rate to patients whatever the PTL.展开更多
A complex network consisting of chaotic systems is considered and the existence of the HSlder continuous gen- eralized synchronization in the network is studied. First, we divide nodes of the network into two parts ac...A complex network consisting of chaotic systems is considered and the existence of the HSlder continuous gen- eralized synchronization in the network is studied. First, we divide nodes of the network into two parts according to their dynamical behaviour. Then, based on the Schauder fixed point theorem, sufficient conditions for the existence of the generalized synchronization between them are derived. Moreover, the results are theoretically proved. Numerical simulations validate the theory.展开更多
Grounding Points (GPs) are installed in electrical power system to drive protective devices and accomplish the person nel safety. The general grounding problem is to find the optimal locations of these points so that ...Grounding Points (GPs) are installed in electrical power system to drive protective devices and accomplish the person nel safety. The general grounding problem is to find the optimal locations of these points so that the security and reli ability of power system can be improved. This paper presents a practical approach to find the optimal location of GPs based on the ratios of zero sequence reactance with positive sequence reactance (X0/X1), zero sequence resistance with positive sequence reactance (R0/X1) and Ground Fault Factor (GFF). The optimal values of these indicators were deter-mined by considering several scenarios of fault disturbances such as single line to ground on a selected area of the Iraqi National Grid (132 KV) taking into account the statue of GPs for transformers in the other substations. From the presented results in this paper, it is noted that GFF calculated for some substations could be used to measure the effectiveness of GPs. However, the operated time of relay can be taken as a criterion of this measurement for selecting the best location of GPs.展开更多
BACKGROUND Synchronous colorectal carcinomas(SCRC)are two or more primary colorectal carcinomas identified simultaneously or within 6 mo of the initial presentation in a single patient.Their incidence is low and the n...BACKGROUND Synchronous colorectal carcinomas(SCRC)are two or more primary colorectal carcinomas identified simultaneously or within 6 mo of the initial presentation in a single patient.Their incidence is low and the number of pathological types of SCRC is usually no more than two.It is very unusual that the pathological findings of a patient with SCRC show more than two different pathological subtypes.Here,we report a rare case of SCRC with three pathological subtypes.CASE SUMMARY A 75-year-old woman who had no previous medical history or family history was admitted to the hospital because of intermittent hematochezia for more than a month.Colonoscopy displayed an irregularly shaped neoplasm of the rectum,a tumor-like lesion causing intestinal stenosis in the descending colon,and a polypoidal neoplasm in the ileocecum.Subsequently,she underwent total colectomy,abdominoperineal resection for rectal cancer,and ileostomy.After operation,the pathological report showed three pathological subtypes including well-differentiated adenocarcinoma of the ascending colon,moderately differen-tiated adenocarcinoma of the descending colon,and mucinous adenocarcinoma of the rectum.She is now recovering well and continues to be closely monitored during follow-up.CONCLUSION Preoperative colonoscopy examination,imaging examination,and extensive intraoperative exploration play important roles in reducing the number of missed lesions.展开更多
The method of fundamental solutions(MFS)is a boundary-type and truly meshfree method,which is recognized as an efficient numerical tool for solving boundary value problems.The geometrical shape,boundary conditions,and...The method of fundamental solutions(MFS)is a boundary-type and truly meshfree method,which is recognized as an efficient numerical tool for solving boundary value problems.The geometrical shape,boundary conditions,and applied loads can be easily modeled in the MFS.This capability makes the MFS particularly suitable for shape optimization,moving load,and inverse problems.However,it is observed that the standard MFS lead to inaccurate solutions for some elastostatic problems with stress concentration and/or highly anisotropic materials.In thiswork,by a numerical study,the important parameters,which have significant influence on the accuracy of the MFS for the analysis of two-dimensional anisotropic elastostatic problems,are investigated.The studied parameters are the degree of anisotropy of the problem,the ratio of the number of collocation points to the number of source points,and the distance between main and pseudo boundaries.It is observed that as the anisotropy of the material increases,there will be more errors in the results.It is also observed that for simple problems,increasing the distance between main and pseudo boundaries enhances the accuracy of the results;however,it is not the case for complicated problems.Moreover,it is concluded that more collocation points than source points can significantly improve the accuracy of the results.展开更多
The low frequency oscillation is a serious threat to security and stability of a power grid.How to locate the disturbance source accurately is an important issue to low frequency oscillation disposal.Existing methods ...The low frequency oscillation is a serious threat to security and stability of a power grid.How to locate the disturbance source accurately is an important issue to low frequency oscillation disposal.Existing methods have poor adaptability to the low frequency oscillation with time-varying steady-state points because of the limitations in the location criterion derivation.A disturbance source location method on a low frequency oscillation with good generality is presented in the paper.Firstly,the reasons why the steady-state points are time-varying on a low frequency oscillation are analyzed.Then,based on the energy function construction form,the branch transmission energy is decomposed into state energy,reciprocating energy and dissipation energy by mathematical derivation.The flow direction of the dissipation energy shows the source and destination of the disturbance energy,and the specific location of a disturbance source can be identified according to its flow direction.Meanwhile,to meet the needs of energy calculation,a recognition method on the electrical quantities steady-state points is also presented by using the cubic spline interpolation.Simulation results show the correctness of the derivation and analysis on energy structure in the paper,and the disturbance source can be located accurately according to the dissipation energy.展开更多
In this study, we investigate the optimal location of access points (APs) to connect end nodes with a service provider through power-line communication in smartgrid communication networks. APs are the gateways of po...In this study, we investigate the optimal location of access points (APs) to connect end nodes with a service provider through power-line communication in smartgrid communication networks. APs are the gateways of power-distribution communication networks, connecting users to control centers. Hence, they are vital for the reliable, safe, and economical operation of a power system. This paper proposes a planning method for AP allocation that takes into consideration economics, reliability, network delay, and (n-l) resilience. First, an optimization model for the AP location is established, which minimizes the cost of installing APs, while satisfying the reliability, network delay, and (n-1) resilience constraints. Then, an improved genetic algorithm is proposed to solve the optimization problem. The simulation results indicate that the proposed planning method can deal with diverse network conditions satisfactorily. Furthermore, it can be applied effectively with high flexibility and scalability.展开更多
In the early development of the wind energy, the majority of the wind turbines have been operated at constant speed. Subsequently, the number of variable-speed wind turbines installed in wind farms has increased. In t...In the early development of the wind energy, the majority of the wind turbines have been operated at constant speed. Subsequently, the number of variable-speed wind turbines installed in wind farms has increased. In this paper, a comparative performance of fixed and variable speed wind generators with Pitch angle control has been presented. The first is based on a squirrel cage Induction Generator (IG) of 315 kW rated power, connected directly to the grid. The second incorporated a Permanent Magnet Synchronous Generator (PMSG) of 750 kW rated power. The performances of each studied wind generator are evaluated by simulation works and variable speed operation is highlighted as preferred mode of operation.展开更多
Over recent years, there has been a clear increase in the frequency of reported flooding events around the world. Gabion structures offer one means of flood mitigation in dam spillways. These types of structures provi...Over recent years, there has been a clear increase in the frequency of reported flooding events around the world. Gabion structures offer one means of flood mitigation in dam spillways. These types of structures provide an additional challenge to the computational modeller in that flow through the porous gabions must be simulated. We have used a computational model to investigate the flow over gabion stepped spillways. The model was first validated against published experimental results. Then, gabion stepped spillways with four different step geometries were tested under the same conditions in order to facilitate inter-comparisons and to choose the best option in terms of energy dissipation. The results show that normal gabion steps can dissipate more energy than overlap, inclined, and pooled steps. An intensive set of tests with varying slope, stone size, and porosity were undertaken. The location of the inception point and the water depth at this point obtained from this study were compared with those from existing formulae. Two new empirical equations have been derived, on the basis of a regression analysis, to provide improved results for gabion stepped spillways.展开更多
Analyzed the relation between time delay difference and time delay estimation errors, based on the principles of three-point locating, a reformed threshold method for time delay difference estimation of impulse signal...Analyzed the relation between time delay difference and time delay estimation errors, based on the principles of three-point locating, a reformed threshold method for time delay difference estimation of impulse signals, called as amendment estimation for short, is developed by introducing channel equalization technique to its conventional version, named as direct estimation in this paper, to improve the estimation stability. After inherent relationship between time delay and phase shift of signals is analyzed, an integer period error compensation method utilized the diversities of both contribution share and contribution mode of concerned estimates is proposed under the condition of high precision phase lag estimation. Finally, a cooperative multi-threshold estimation method composed of amendment and direct estimations to process impulse signals with three thresholds is established. In sea trials data tests of passive locating, this method improves the estimation precision of time delay difference efficiently. The experiments verify the theoretical predictions.展开更多
Sensing of keyhole shape and size is important for controlling of plasma arc welding process. In this study, a synchronous vision system is developed by using two CCD cameras to observe the keyhole entrance and exit s...Sensing of keyhole shape and size is important for controlling of plasma arc welding process. In this study, a synchronous vision system is developed by using two CCD cameras to observe the keyhole entrance and exit simultaneously. Experiments are conducted on stainless steel workpiece, Whole keyhole exit image and part of keyhole entrance image are captured synchronously under different welding conditions. Image processing and calibration algorithms are proposed. The boundary and front point of the keyhole entrance and exit are extracted respectively. The effects of welding current and welding speed on ,front point of keyhole entrance and exit are analyzed. Compared to the front point of the keyhole entrance, the position of the front point of the keyhole exit is better to reflect the variation of the welding parameters.展开更多
China has a vast territory with a great demand for electricity. However, the resources are in reverse distribution in the country. Therefore, high voltage direct current transmission has great practical significance a...China has a vast territory with a great demand for electricity. However, the resources are in reverse distribution in the country. Therefore, high voltage direct current transmission has great practical significance and been widely used. However, traditional fault location methods have a lot of problems in engineering application for the length of transmission line and the complexity of the terrain. This paper proposes a comprehensive evaluation algorithm based on the travelling wave method and time domain method. It also proposes a concept of fault point reliability. This algorithm analyzes the fault point reliability in the whole transmission line to determine the specific location of the fault point. This paper proves that the algorithm has high reliability by PSCAD simulation software.展开更多
文摘A challenging topic in nonlinear dynamics concerns the study of fractional-order systems without equilibrium points.In particular, no paper has been published to date regarding the presence of hyperchaos in these systems. This paper aims to bridge the gap by introducing a new example of fractional-order hyperchaotic system without equilibrium points. The conducted analysis shows that hyperchaos exists in the proposed system when its order is as low as 3.84. Moreover, an interesting application of hyperchaotic synchronization to the considered fractional-order system is provided.
文摘IntroductionEarthquake location is one of the inverse problems in seismology. The accurate earthquake source location is the basis of studies of earthquake images, the relation analysis between seismicity and tectonics, and the important parameters of earthquakes. It is also the basic data used to study the space distribution of aftershocks after a strong earthquake. The earthquake location accuracy mainly depends on reliability of the velocity models for the earth¢s crust, the reasonable design of the seismograph network, the accuracy of arrival time, and the location method used (ZHAO, 1983; ZHU, ZHAO, 1997; LI, et al, 1999). The reliability of the velocity model directly influences loca-tion results. The reasonable seismograph network design can improve the location accuracy; otherwise it will in-troduce ill-conditioned equations and large location error. If arrival time is not accurate, for example, if the seismic phase is wrongly identified or the time service is inaccurate, large arrival time error will be produced, and the location results and real hypocenters will be discordant. Different location algorithms adopted will produce differ-ent location errors. The residual criterion chosen has influence on location results. Besides, whether the event is correctly associated also influences the location quality. In practice, it can often be seen that large location devia-tion introduced by wrong event association.
文摘Orthogonal frequency-division multiplexing (OFDM) is a multi carrier modulation scheme mainly used for digital communications. The performance of OFDM system heavily depends on the synchronization scheme used. In most cases, the accuracy level of synchronization will be worsened by the error caused in fixed point arithmetic involved. In this paper, we analyze the impact of the fixed point arithmetic on the performance of the coarse timing and frequency synchronization. Here with an analytical approach through numerical simulations bit length of IEEE 754 standard single precision format is optimized according to the required degree of accuracy for low complexity. Also, a complete precision level requirement for FFT computations with all possible modulation types is obtained. The proposed precision model is compared with IEEE standard single precision model and its efficiency in OFDM synchronization process is proved through MATLAB simulations. Finally, the complexity reduction of proposed precision model in both addition and subtraction is proved against single precision format using hardware synthesis. Here we proved that more than 50% complexity reduction is achieved as compared to standard precision models without compromising quality. The quality retention of proposed model is proved in both timing and frequency synchronization process.
基金jointly supported by the National Major Science and Technology Program (No. 2008ZX05025)the National 973 Program (Grant No. 2009CB219400)
文摘In this paper we deduce the analytic solutions of the first- and second-order vertical derivative zero points for gravity anomalies in simple regular models with single, double, and multiple edges and analyze their spatial variation. For another simple regular models where it is difficult to obtain the analytic expression of the zero point, we try to use the profile zero points to analyze the spatial variation. The test results show that the spatial variation laws of both first- and second-order vertical derivative zero points are almost the same but the second-order derivative zero point position is closer to the top surface edge of the geological bodies than the first-order vertical derivative and has a relatively high resolution. Moreover, with an increase in buried depth, for a single boundary model, the vertical derivative zero point location tends to move from the top surface edge to the outside of the buried body but finally converges to a fixed value. For a double boundary model, the vertical derivative zero point location tends to migrate from the top surface edge to the outside of the buried body. For multiple boundary models, the vertical derivative zero point location converges from the top surface edge to the outside of the buried body where some zero points coincide and finally vanish. Finally, the effectiveness and reliability of the proposed method is verified using real field data.
文摘In this paper,some remarks for more efficient analysis of two-dimensional elastostatic problems using the method of fundamental solutions are made.First,the effects of the distance between pseudo and main boundaries on the solution are investigated and by a numerical study a lower bound for the distance of each source point to the main boundary is suggested.In some cases,the resulting system of equations becomes ill-conditioned for which,the truncated singular value decomposition with a criterion based on the accuracy of the imposition of boundary conditions is used.Moreover,a procedure for normalizing the shear modulus is presented that significantly reduces the condition number of the system of equations.By solving two example problems with stress concentration,the effectiveness of the proposed methods is demonstrated.
文摘BACKGROUND Patients with right sided colorectal cancer are known to have a poorer prognosis than patients with left sided colorectal cancer, whatever the cancer stage. To this day, primary tumor resection(PTR) is still controversial in a metastatic, non resectable setting.AIM To explore the survival impact of PTR in patients with metastatic colorectal cancer(mCRC) depending on PTL.METHODS We retrospectively collected data from all consecutive patients treated for mCRC at the Centre Georges Francois Leclerc Hospital. Univariate and multivariate Cox proportional hazard regression models were used to assess the influence of PTR on survival. We then evaluated the association between PTL and overall survival among patients who previously underwent or did not undergo PTR. A propensity score was performed to match cohorts.RESULTS Four hundred and sixty-six patients were included. A total of 153(32.8%) patients had unresected synchronous mCRC and 313(67.2%) patients had resected synchronous mCRC. The number of patients with right colic cancer, left colic cancer and rectal cancer was respectively 174(37.3%), 203(43.6%) and 89(19.1%). In the multivariate analysis only PTL, PTR, resection of hepatic and or pulmonary metastases and the use of oxaliplatin, EGFR inhibitors or bevacizumab throughout treatment were associated to higher overall survival rates. Survival evaluation depending on PTR and PTL found that PTR improved the prognosis of both left and right sided mCRC. Results were confirmed by using a weighted propensity score.CONCLUSION In mCRC, PTR seems to confer a higher survival rate to patients whatever the PTL.
基金Project supported by the National Natural Science Foundation of China (Grant Nos.11002061 and 10901073)the Fundamental Research Funds for the Central Universities,China (Grant No.JUSRP11117)
文摘A complex network consisting of chaotic systems is considered and the existence of the HSlder continuous gen- eralized synchronization in the network is studied. First, we divide nodes of the network into two parts according to their dynamical behaviour. Then, based on the Schauder fixed point theorem, sufficient conditions for the existence of the generalized synchronization between them are derived. Moreover, the results are theoretically proved. Numerical simulations validate the theory.
文摘Grounding Points (GPs) are installed in electrical power system to drive protective devices and accomplish the person nel safety. The general grounding problem is to find the optimal locations of these points so that the security and reli ability of power system can be improved. This paper presents a practical approach to find the optimal location of GPs based on the ratios of zero sequence reactance with positive sequence reactance (X0/X1), zero sequence resistance with positive sequence reactance (R0/X1) and Ground Fault Factor (GFF). The optimal values of these indicators were deter-mined by considering several scenarios of fault disturbances such as single line to ground on a selected area of the Iraqi National Grid (132 KV) taking into account the statue of GPs for transformers in the other substations. From the presented results in this paper, it is noted that GFF calculated for some substations could be used to measure the effectiveness of GPs. However, the operated time of relay can be taken as a criterion of this measurement for selecting the best location of GPs.
文摘BACKGROUND Synchronous colorectal carcinomas(SCRC)are two or more primary colorectal carcinomas identified simultaneously or within 6 mo of the initial presentation in a single patient.Their incidence is low and the number of pathological types of SCRC is usually no more than two.It is very unusual that the pathological findings of a patient with SCRC show more than two different pathological subtypes.Here,we report a rare case of SCRC with three pathological subtypes.CASE SUMMARY A 75-year-old woman who had no previous medical history or family history was admitted to the hospital because of intermittent hematochezia for more than a month.Colonoscopy displayed an irregularly shaped neoplasm of the rectum,a tumor-like lesion causing intestinal stenosis in the descending colon,and a polypoidal neoplasm in the ileocecum.Subsequently,she underwent total colectomy,abdominoperineal resection for rectal cancer,and ileostomy.After operation,the pathological report showed three pathological subtypes including well-differentiated adenocarcinoma of the ascending colon,moderately differen-tiated adenocarcinoma of the descending colon,and mucinous adenocarcinoma of the rectum.She is now recovering well and continues to be closely monitored during follow-up.CONCLUSION Preoperative colonoscopy examination,imaging examination,and extensive intraoperative exploration play important roles in reducing the number of missed lesions.
基金The first author would like to acknowledge the support received from the Vice Chancellor of Research at Shiraz University under Grant No.99GRC1M1820.
文摘The method of fundamental solutions(MFS)is a boundary-type and truly meshfree method,which is recognized as an efficient numerical tool for solving boundary value problems.The geometrical shape,boundary conditions,and applied loads can be easily modeled in the MFS.This capability makes the MFS particularly suitable for shape optimization,moving load,and inverse problems.However,it is observed that the standard MFS lead to inaccurate solutions for some elastostatic problems with stress concentration and/or highly anisotropic materials.In thiswork,by a numerical study,the important parameters,which have significant influence on the accuracy of the MFS for the analysis of two-dimensional anisotropic elastostatic problems,are investigated.The studied parameters are the degree of anisotropy of the problem,the ratio of the number of collocation points to the number of source points,and the distance between main and pseudo boundaries.It is observed that as the anisotropy of the material increases,there will be more errors in the results.It is also observed that for simple problems,increasing the distance between main and pseudo boundaries enhances the accuracy of the results;however,it is not the case for complicated problems.Moreover,it is concluded that more collocation points than source points can significantly improve the accuracy of the results.
基金This work was supported in part by National Natural key R&D Program of China(2016YFB0900100).
文摘The low frequency oscillation is a serious threat to security and stability of a power grid.How to locate the disturbance source accurately is an important issue to low frequency oscillation disposal.Existing methods have poor adaptability to the low frequency oscillation with time-varying steady-state points because of the limitations in the location criterion derivation.A disturbance source location method on a low frequency oscillation with good generality is presented in the paper.Firstly,the reasons why the steady-state points are time-varying on a low frequency oscillation are analyzed.Then,based on the energy function construction form,the branch transmission energy is decomposed into state energy,reciprocating energy and dissipation energy by mathematical derivation.The flow direction of the dissipation energy shows the source and destination of the disturbance energy,and the specific location of a disturbance source can be identified according to its flow direction.Meanwhile,to meet the needs of energy calculation,a recognition method on the electrical quantities steady-state points is also presented by using the cubic spline interpolation.Simulation results show the correctness of the derivation and analysis on energy structure in the paper,and the disturbance source can be located accurately according to the dissipation energy.
基金supported by the National High Technology Research and Development Program of China(2012AA050801)
文摘In this study, we investigate the optimal location of access points (APs) to connect end nodes with a service provider through power-line communication in smartgrid communication networks. APs are the gateways of power-distribution communication networks, connecting users to control centers. Hence, they are vital for the reliable, safe, and economical operation of a power system. This paper proposes a planning method for AP allocation that takes into consideration economics, reliability, network delay, and (n-l) resilience. First, an optimization model for the AP location is established, which minimizes the cost of installing APs, while satisfying the reliability, network delay, and (n-1) resilience constraints. Then, an improved genetic algorithm is proposed to solve the optimization problem. The simulation results indicate that the proposed planning method can deal with diverse network conditions satisfactorily. Furthermore, it can be applied effectively with high flexibility and scalability.
文摘In the early development of the wind energy, the majority of the wind turbines have been operated at constant speed. Subsequently, the number of variable-speed wind turbines installed in wind farms has increased. In this paper, a comparative performance of fixed and variable speed wind generators with Pitch angle control has been presented. The first is based on a squirrel cage Induction Generator (IG) of 315 kW rated power, connected directly to the grid. The second incorporated a Permanent Magnet Synchronous Generator (PMSG) of 750 kW rated power. The performances of each studied wind generator are evaluated by simulation works and variable speed operation is highlighted as preferred mode of operation.
基金supported by the Higher Committee for Education Development(HCED)in Iraq
文摘Over recent years, there has been a clear increase in the frequency of reported flooding events around the world. Gabion structures offer one means of flood mitigation in dam spillways. These types of structures provide an additional challenge to the computational modeller in that flow through the porous gabions must be simulated. We have used a computational model to investigate the flow over gabion stepped spillways. The model was first validated against published experimental results. Then, gabion stepped spillways with four different step geometries were tested under the same conditions in order to facilitate inter-comparisons and to choose the best option in terms of energy dissipation. The results show that normal gabion steps can dissipate more energy than overlap, inclined, and pooled steps. An intensive set of tests with varying slope, stone size, and porosity were undertaken. The location of the inception point and the water depth at this point obtained from this study were compared with those from existing formulae. Two new empirical equations have been derived, on the basis of a regression analysis, to provide improved results for gabion stepped spillways.
文摘Analyzed the relation between time delay difference and time delay estimation errors, based on the principles of three-point locating, a reformed threshold method for time delay difference estimation of impulse signals, called as amendment estimation for short, is developed by introducing channel equalization technique to its conventional version, named as direct estimation in this paper, to improve the estimation stability. After inherent relationship between time delay and phase shift of signals is analyzed, an integer period error compensation method utilized the diversities of both contribution share and contribution mode of concerned estimates is proposed under the condition of high precision phase lag estimation. Finally, a cooperative multi-threshold estimation method composed of amendment and direct estimations to process impulse signals with three thresholds is established. In sea trials data tests of passive locating, this method improves the estimation precision of time delay difference efficiently. The experiments verify the theoretical predictions.
基金The authors are grateful to the financial support for this research from the Natural Science Foundation of Shandong Province under grant No.ZR2011EEM013 and Independent Innovation Foundation of Shandong University under grant No.2010TS108
文摘Sensing of keyhole shape and size is important for controlling of plasma arc welding process. In this study, a synchronous vision system is developed by using two CCD cameras to observe the keyhole entrance and exit simultaneously. Experiments are conducted on stainless steel workpiece, Whole keyhole exit image and part of keyhole entrance image are captured synchronously under different welding conditions. Image processing and calibration algorithms are proposed. The boundary and front point of the keyhole entrance and exit are extracted respectively. The effects of welding current and welding speed on ,front point of keyhole entrance and exit are analyzed. Compared to the front point of the keyhole entrance, the position of the front point of the keyhole exit is better to reflect the variation of the welding parameters.
文摘China has a vast territory with a great demand for electricity. However, the resources are in reverse distribution in the country. Therefore, high voltage direct current transmission has great practical significance and been widely used. However, traditional fault location methods have a lot of problems in engineering application for the length of transmission line and the complexity of the terrain. This paper proposes a comprehensive evaluation algorithm based on the travelling wave method and time domain method. It also proposes a concept of fault point reliability. This algorithm analyzes the fault point reliability in the whole transmission line to determine the specific location of the fault point. This paper proves that the algorithm has high reliability by PSCAD simulation software.