The inter-blade vortex in a Francis turbine becomes one of the main hydraulic factors that are likely to cause blade erosion at deep part load operating con- ditions. However, the causes and the mechanism of inter- bl...The inter-blade vortex in a Francis turbine becomes one of the main hydraulic factors that are likely to cause blade erosion at deep part load operating con- ditions. However, the causes and the mechanism of inter- blade vortex are still under investigation according to present researches. Thus the causes of inter-blade vortex and the effect of different hydraulic parameters on the inter-blade vortex are investigated experimentally. The whole life cycle of the inter-blade vortex is observed by a high speed camera. The test results illustrate the whole life cycle of the inter-blade vortex from generation to separation and even to fading. It is observed that the inter- blade vortex becomes stronger with the decreasing of flow and head, which leads to pressure fluctuation. Meanwhile, the pressure fluctuations in the vane-less area and the draft tube section become stronger when inter-blade vortices exist in the blade channel. The turbine will be damaged if operating in the inter-blade vortex zone, so its operating range must be far away from that zone. This paper reveals the main cause of the inter-blade vortex which is the larger incidence angle between the inflow angle and theblade angle on the leading edge of the runner at deep part load operating conditions.展开更多
Using 2 high-speed cameras, we have recorded 14 negative cloud-to-ground （CG） lightning flashes, half of which are natural and the others are artificially triggered. The two-dimensional （2D） propagation speed of d...Using 2 high-speed cameras, we have recorded 14 negative cloud-to-ground （CG） lightning flashes, half of which are natural and the others are artificially triggered. The two-dimensional （2D） propagation speed of different type leaders and the luminosity of lightning channel are analyzed in detail. Bidirectional leader processes are observed during the initial processes of two altitude triggered negative lightning （ATNL） flashes. The analysis shows： the propagation speed of the upward positive leader （UPL） before the initiation of the downward negative leader （DNL） is at the order of 10^4-10^5 m s-1; the UPL can be intensified by the initiation and development of the DNL in the way that the luminosity is enhanced and the speed is sped up; after initiation, the DNL in one ATNL flash propagates downward three times intermittently with interval of about 1 ms, while that in the other ATNL flash propagates downward continuously with a speed at the order of 10^5 m s^-1. In the five classical triggered negative lightning （CTNL） flashes, the propagation speeds of the UPLs vary between 0.35×10^5 and 7.71×10^5 m s-1, and the variations of their luminosities and speeds are quite complex during the development processes. Among the four observed natural negative lightning flashes occurred on the land, three have only one return stoke （RS） each and all of their DNLs have many branches with an average speed at the order of 10^5 m s-l; while the another one has 13 RSs. In the CG flash with 13 RSs, the DNL before the first RS has no obvious branch below 1.4 km above the ground, and its speed ranges from 2.2×10^5 to 2.3×10^6 m s-1 between the heights of 0.7 and 1.4 km and exceeds 3.9×10^6 m s-1 below 0.7 km; preceding the 4th RS, an attempted leader is observed with a speed ranging from 1.1× 10^5 to 1.1×10^6 m s-1 between 0.8 and 1.5 km. As for the three observed natural negative lightning flashes occurred on the sea, each has only one RS, and each DNL preceding the RS has a few branches, two of展开更多
The mechanism of the increasing of A-TIG welding penetration is studied by using the activating flux we developed for stainless steel, The effect of fluxon the flow and temperature fields of weld pool is simulated by ...The mechanism of the increasing of A-TIG welding penetration is studied by using the activating flux we developed for stainless steel, The effect of fluxon the flow and temperature fields of weld pool is simulated by the PHOENICS software, It shows that without flux, the fluid flow will be outward along the surface of the weld pool and then down, resulting in a flatter weld pool shape. With the flux, the oxygen, which changes the temperature dependence of surface tehsion grads froma negative value toa positive value, can cause significant changes onthe Weld penetration. Fluid flow will be inward along the surface of the weld pool toward the center and then down. This fluid flow pattern efficiently transfers heat to the weld root and produces a relatively deep and narrow weld.This change is the main cause of penetration increase. Moreover, arc construction can cause the weld width to become narrower and the penetration to become deeper, but this is not the main cause of penetration increase. The effects Of flux on fluid flow of the weld pool surface and arc profiles were observed in conventiOnai TIG welding and in A-TIG welding by using high-speed video camera; The fluid flow behavior was visualized in realtime scale by micro focused X-ray transmission video observation system. The result indicated that stronger inward fluid flow patterns leading to weld beads with narrower width and deeper penetration could be apparently identified in the case of A-TIG welding.The flux couldchange the direction of fluid flow i-n welding pool. It has a good agreement with the simulation result.展开更多
Pectoral fins fascinate researchers for their important role in fish maneuvers. By possessing a complicated flexible structure with several fin rays made by a thin film, the fin exhibits a three-dimensional （3D） mot...Pectoral fins fascinate researchers for their important role in fish maneuvers. By possessing a complicated flexible structure with several fin rays made by a thin film, the fin exhibits a three-dimensional （3D） motion. The complex 3D fin kinematics makes it challenging to study the performance of pectoral fin. Nevertheless, a detailed study on the 3D motion pattern of pectoral fins is necessary to the design and control ofa bio-inspired fin rays. Therefore, a highspeed photography system is introduced in this paper to study the 3D motion ofa Koi Carp by analyzing the two views of its pectoral fin simultaneously. The key motions of the pectoral fins are first captured in both hovering and retreating. Next, the 3D configuration of the pectoral fins is recon- structed by digital image processing, in which the movement of fin rays during fish retreating and hovering is obtained. Fur- thermore, the method of Singular Value Decomposition （SVD） is adopted to extract the basic motion patterns of pectoral fins from extensive image sequences, i.e. expansion, bending, cupping, and undulation. It is believed that the movement of the fin rays and the basic patterns of the pectoral fins obtained in the present work can provide a good foundation for the development and control of bionic flexible pectoral fins for underwater propeller.展开更多
Research on dynamics and stability of machin-ing operations has attracted considerable attention. Cur-rently, most studies focus on the forward solution ofdynamics and stability in which material properties and thefre...Research on dynamics and stability of machin-ing operations has attracted considerable attention. Cur-rently, most studies focus on the forward solution ofdynamics and stability in which material properties and thefrequency response function at the tool tip are known topredict stable cutting conditions. However, the forwardsolution may fail to perform accurately in cases whereinthe aforementioned information is partially known or var-ies based on the process conditions, or could involve sev-eral uncertainties in the dynamics. Under thesecircumstances, inverse stability solutions are immenselyuseful to identify the amount of variation in the effectivedamping or stiffness acting on the machining system. Inthis paper, the inverse stability solutions and their use forsuch purposes are discussed through relevant examples andcase studies. Specific areas include identification of processdamping at low cutting speeds and variations in spindledynamics at high rotational speeds.展开更多
Laser Chemical Machining (LCM) is a non-conventional processing method, which enables very accurate and precise ablation of metallic surfaces. Material ablation results from laser-induced thermal activation of heterog...Laser Chemical Machining (LCM) is a non-conventional processing method, which enables very accurate and precise ablation of metallic surfaces. Material ablation results from laser-induced thermal activation of heterogeneous chemical reactions between electrolytes and a metallic surface. However, when processing metallic surfaces with LCM, large fluctuations in ablation quality can occur due to rising bubbles. The for-mation of bubbles during laser chemical machining and their influence on the ablation quality has not been investigated. For a more detailed investigation of the bubbles, ablation experiments on Titanium and Ce-ramic under different thermal process conditions were performed. The experiments were recorded by a high-speed camera. The evaluation of the video sequences was performed using Matlab. The resulting bubbles were analyzed regarding their size and frequency. The results show that boil-ing bubbles formed on both materials during processing. Titanium also produces smaller bubbles, which can be identified as process bubbles ac-cording to their size. Furthermore, it was found that undisturbed laser chemical ablation can be achieved in the presence of a boiling process, since both boiling bubbles and process bubbles were detected during machining within the process window.展开更多
基金Supported by National Natural Science Foundation of China(Grant No.51279172)Open Research Subject of Key Laboratory of Fluid Machinery of Ministry of Education,Xihua University,China(Grant No.szjj2015-022)Key Laboratory of Natural Science Fund of Education Department of Sichuan Province,China(Grant No.080704)
文摘The inter-blade vortex in a Francis turbine becomes one of the main hydraulic factors that are likely to cause blade erosion at deep part load operating con- ditions. However, the causes and the mechanism of inter- blade vortex are still under investigation according to present researches. Thus the causes of inter-blade vortex and the effect of different hydraulic parameters on the inter-blade vortex are investigated experimentally. The whole life cycle of the inter-blade vortex is observed by a high speed camera. The test results illustrate the whole life cycle of the inter-blade vortex from generation to separation and even to fading. It is observed that the inter- blade vortex becomes stronger with the decreasing of flow and head, which leads to pressure fluctuation. Meanwhile, the pressure fluctuations in the vane-less area and the draft tube section become stronger when inter-blade vortices exist in the blade channel. The turbine will be damaged if operating in the inter-blade vortex zone, so its operating range must be far away from that zone. This paper reveals the main cause of the inter-blade vortex which is the larger incidence angle between the inflow angle and theblade angle on the leading edge of the runner at deep part load operating conditions.
基金the National Natural Science Foundation of China under Grant No.40605004the Ministry of Science and Tech.nology of China under Grant Nos.2004DEA71070 and GYHY2007622.
文摘Using 2 high-speed cameras, we have recorded 14 negative cloud-to-ground （CG） lightning flashes, half of which are natural and the others are artificially triggered. The two-dimensional （2D） propagation speed of different type leaders and the luminosity of lightning channel are analyzed in detail. Bidirectional leader processes are observed during the initial processes of two altitude triggered negative lightning （ATNL） flashes. The analysis shows： the propagation speed of the upward positive leader （UPL） before the initiation of the downward negative leader （DNL） is at the order of 10^4-10^5 m s-1; the UPL can be intensified by the initiation and development of the DNL in the way that the luminosity is enhanced and the speed is sped up; after initiation, the DNL in one ATNL flash propagates downward three times intermittently with interval of about 1 ms, while that in the other ATNL flash propagates downward continuously with a speed at the order of 10^5 m s^-1. In the five classical triggered negative lightning （CTNL） flashes, the propagation speeds of the UPLs vary between 0.35×10^5 and 7.71×10^5 m s-1, and the variations of their luminosities and speeds are quite complex during the development processes. Among the four observed natural negative lightning flashes occurred on the land, three have only one return stoke （RS） each and all of their DNLs have many branches with an average speed at the order of 10^5 m s-l; while the another one has 13 RSs. In the CG flash with 13 RSs, the DNL before the first RS has no obvious branch below 1.4 km above the ground, and its speed ranges from 2.2×10^5 to 2.3×10^6 m s-1 between the heights of 0.7 and 1.4 km and exceeds 3.9×10^6 m s-1 below 0.7 km; preceding the 4th RS, an attempted leader is observed with a speed ranging from 1.1× 10^5 to 1.1×10^6 m s-1 between 0.8 and 1.5 km. As for the three observed natural negative lightning flashes occurred on the sea, each has only one RS, and each DNL preceding the RS has a few branches, two of
文摘The mechanism of the increasing of A-TIG welding penetration is studied by using the activating flux we developed for stainless steel, The effect of fluxon the flow and temperature fields of weld pool is simulated by the PHOENICS software, It shows that without flux, the fluid flow will be outward along the surface of the weld pool and then down, resulting in a flatter weld pool shape. With the flux, the oxygen, which changes the temperature dependence of surface tehsion grads froma negative value toa positive value, can cause significant changes onthe Weld penetration. Fluid flow will be inward along the surface of the weld pool toward the center and then down. This fluid flow pattern efficiently transfers heat to the weld root and produces a relatively deep and narrow weld.This change is the main cause of penetration increase. Moreover, arc construction can cause the weld width to become narrower and the penetration to become deeper, but this is not the main cause of penetration increase. The effects Of flux on fluid flow of the weld pool surface and arc profiles were observed in conventiOnai TIG welding and in A-TIG welding by using high-speed video camera; The fluid flow behavior was visualized in realtime scale by micro focused X-ray transmission video observation system. The result indicated that stronger inward fluid flow patterns leading to weld beads with narrower width and deeper penetration could be apparently identified in the case of A-TIG welding.The flux couldchange the direction of fluid flow i-n welding pool. It has a good agreement with the simulation result.
基金Acknowledgments This research is financially supported by National Natural Science Foundation of China （50975270） and the Fundamental Research Funds for the Central Universities （WK2090090002）. The presented work is arisen from the collaboration between the two teams in USTC and NTU （Singapore） on bio-inspired robotics.
文摘Pectoral fins fascinate researchers for their important role in fish maneuvers. By possessing a complicated flexible structure with several fin rays made by a thin film, the fin exhibits a three-dimensional （3D） motion. The complex 3D fin kinematics makes it challenging to study the performance of pectoral fin. Nevertheless, a detailed study on the 3D motion pattern of pectoral fins is necessary to the design and control ofa bio-inspired fin rays. Therefore, a highspeed photography system is introduced in this paper to study the 3D motion ofa Koi Carp by analyzing the two views of its pectoral fin simultaneously. The key motions of the pectoral fins are first captured in both hovering and retreating. Next, the 3D configuration of the pectoral fins is recon- structed by digital image processing, in which the movement of fin rays during fish retreating and hovering is obtained. Fur- thermore, the method of Singular Value Decomposition （SVD） is adopted to extract the basic motion patterns of pectoral fins from extensive image sequences, i.e. expansion, bending, cupping, and undulation. It is believed that the movement of the fin rays and the basic patterns of the pectoral fins obtained in the present work can provide a good foundation for the development and control of bionic flexible pectoral fins for underwater propeller.
文摘Research on dynamics and stability of machin-ing operations has attracted considerable attention. Cur-rently, most studies focus on the forward solution ofdynamics and stability in which material properties and thefrequency response function at the tool tip are known topredict stable cutting conditions. However, the forwardsolution may fail to perform accurately in cases whereinthe aforementioned information is partially known or var-ies based on the process conditions, or could involve sev-eral uncertainties in the dynamics. Under thesecircumstances, inverse stability solutions are immenselyuseful to identify the amount of variation in the effectivedamping or stiffness acting on the machining system. Inthis paper, the inverse stability solutions and their use forsuch purposes are discussed through relevant examples andcase studies. Specific areas include identification of processdamping at low cutting speeds and variations in spindledynamics at high rotational speeds.
文摘Laser Chemical Machining (LCM) is a non-conventional processing method, which enables very accurate and precise ablation of metallic surfaces. Material ablation results from laser-induced thermal activation of heterogeneous chemical reactions between electrolytes and a metallic surface. However, when processing metallic surfaces with LCM, large fluctuations in ablation quality can occur due to rising bubbles. The for-mation of bubbles during laser chemical machining and their influence on the ablation quality has not been investigated. For a more detailed investigation of the bubbles, ablation experiments on Titanium and Ce-ramic under different thermal process conditions were performed. The experiments were recorded by a high-speed camera. The evaluation of the video sequences was performed using Matlab. The resulting bubbles were analyzed regarding their size and frequency. The results show that boil-ing bubbles formed on both materials during processing. Titanium also produces smaller bubbles, which can be identified as process bubbles ac-cording to their size. Furthermore, it was found that undisturbed laser chemical ablation can be achieved in the presence of a boiling process, since both boiling bubbles and process bubbles were detected during machining within the process window.
