The computational fluid dynamics （CFD） method is used to investigate the aerodynamic characteristics of the seat/occupant with windblast protection devices. The upwind Osher scheme is used for the spatial discretisa...The computational fluid dynamics （CFD） method is used to investigate the aerodynamic characteristics of the seat/occupant with windblast protection devices. The upwind Osher scheme is used for the spatial discretisation. The detached-eddy simulation （DES） based on the Spalart-Allmaras one-equation turbulence model is ap- plied to the detached viscous flow simulation behind the seat/occupant, with Mach numbers 0.6 and 1.2 at attack angles between --10 and 30°, and at two sideslip angles of 0 and 15°, respectively. The aerodynamic characteristics of seat/occupants with and without windblast protection devices are calculated in cases of the freestream Mach numbers 0. 8 and 1.6, attack angles from 5 to 30°, and three sideslip angles of 0, --20 and --50°, respectively. Results show that simulation results agree well with experimental data. And the occupant is efficiently protected by windblast protection devices.展开更多
To achieve the secondary production in multistage fracturing wells of tight oil,milling tools are usually used to remove the multistage fracturing ball seats to achieve production with a large diameter in later.In thi...To achieve the secondary production in multistage fracturing wells of tight oil,milling tools are usually used to remove the multistage fracturing ball seats to achieve production with a large diameter in later.In this paper,first of all,the working mechanism of milling tools for multistage fracturing ball seats was studied and a mechanical analysis model of single abrasive grain was established.Then,an experimental system for milling tools was developed,and the experimental tests of the flat,the blade,and the slope milling tool were conducted in order.Besides,the morphology of chips and the surface morphology of the workpiece after the experiment were analyzed.Also,the working performance of milling tools was evaluated from the perspectives of working safety,working efficiency,and wear resistance of the milling tool.The results show that the torque of the milling tool increases nonlinearly with the increase in the cutting depth of the abrasive grain and increases linearly with the increase in the cutting width.Also,the chips are irregular particles and the size is mainly from 10 to 50μm.So,the chips should be pumped up with a small pump pressure and a large displacement.Besides this,the cutting depths of the abrasive grains are from 216.20 to 635.47μm and the bottom surface of the milling tool should be eccentric to avoid the zero point of cutting speed.Furthermore,the torque of the slope milling tool is 23.8%larger than that of the flat milling tool,which is also 30.4%smaller than that of the blade milling tool.Compared with the flat milling tool,the working efficiency of the blade milling tool improves by 79.9%and the slope milling tool improves by 111.1%.Also,the wear resistance of the blade milling tool decreases by 102.7%,while the slope milling tool declines by 32.6%when compared with the flat milling tool.Therefore,the slope milling tool has the characteristics of moderate torque,stable working conditions,the highest working efficiency,and fine wear resistance,which is preferably used to mill multistage fracturing ball seats.This study provides a theoretical basis and guidance for milling multistage fracturing ball seats on-site and realizing production with a large diameter in later stages of multistage fracturing wells.展开更多
Currently, there are two rituals coexist in the world:"taking the left for the upper"and"taking the right for the up?per". The former is usually taken in domestic affairs such as meetings, dinners,...Currently, there are two rituals coexist in the world:"taking the left for the upper"and"taking the right for the up?per". The former is usually taken in domestic affairs such as meetings, dinners, etc. while the latter enjoys an overwhelmingly fa?vor in the west and so that it becomes the international practice in every conceivable social activities. The paper intends to offer a deep explanation of the differences of two rituals with adequate evidence and examples in some specific situations, say different kinds of seat arrangement in various large and small meetings, respectively in China and the west. Then it follows to provide the underlying reasons for the differences from the perspective of cultural difference. It finds that different denotation of"left"and"right"in English and Chinese may shed some light on the understanding of the two different rituals.展开更多
The test and analysis on the chair system of homemade J80 skidder was carried out. on the basis of the study result, the important parameters, the spring rigidity and the damping coefficient of J80 skidder chair, were...The test and analysis on the chair system of homemade J80 skidder was carried out. on the basis of the study result, the important parameters, the spring rigidity and the damping coefficient of J80 skidder chair, were optimized and some suggestions were put forward for improving the riding comfort of the skidder seat.展开更多
The sleeve sealing ball seat is one of the important components in the multistage fracturing process of horizontal wells.The erosion and wear of the surface will decrease the sealing performance of the fracturing ball...The sleeve sealing ball seat is one of the important components in the multistage fracturing process of horizontal wells.The erosion and wear of the surface will decrease the sealing performance of the fracturing ball and the ball seat.This leads to pressure leakage during the fracturing process and fracturing failure.In this paper,combined with the actual ball seat materials and working conditions during the fracturing process,the erosion tests of ductile iron and tungsten carbide materials under different erosion speeds,angles,and mortar concentrations are carried out.Then the erosion test results were analyzed by mathematical fitting,and a set of erosion models suitable for sliding sleeve setting ball seat materials were innovatively established.For the first time,this paper combines the erosion model obtained from the experiment and the computational fluid dynamics(CFD)with Fluent software to simulate the erosion of the ball seat.Based on the simulation results,the morphology of the sliding sleeve seat ball after erosion is predicted.Through analysis of the test and simulation results,it is showed that the erosion rate of tungsten carbide material is lower and the wear resistance is better under the condition of small angle erosion.This research can offer a strong basis for fracturing site selection,surface treatment methods,and prediction of failure time of ball seats.展开更多
BACKGROUND: Motor vehicle crash(MVC) related injury has been identified as a major public health concern. Child restrain(CR) seat belts can minimize the mortality and morbidity from MVC. The use for seat belts is subs...BACKGROUND: Motor vehicle crash(MVC) related injury has been identified as a major public health concern. Child restrain(CR) seat belts can minimize the mortality and morbidity from MVC. The use for seat belts is substantially low in developing countries like Pakistan even though its use has been shown to decrease morbidity and mortality by a significant extent.METHODS: This was a case control study with cases from the general population(GP) and controls from the Aga Khan University(AKU) employees in a 3:1 ratio. The study questionnaire was based on parameters like gender, education level, awareness and presence of CR and also assessed the frequency of usage, reasons for not using CR and the source of knowledge regarding CR use.RESULTS: Out of 848 respondents, 212 were from AKU and 636 were from the GP. 96.7% from AKU had at least a bachelor's degree while less than half(42.6%) of those from the GP were graduate or above(P<0.001). A statistically signif icant difference was found between the two groups with drivers from AKU being generally more aware about CR and its use. 81.1% of the group from AKU compared to 59.7% from the general population were found to be aware of child restraint use(P<0.001). Media(40.6%) was found to be the most common source of information amongst the AKU employees.CONCLUSION: Most motor vehicle related injuries in children can be prevented or their severity may be reduced by the use of appropriate child restraint seat belts.展开更多
有人把“他坐下”译为 He seated down。这就把seat与sit混淆了。sit是不及物动词。而seat是及物动词,有“使…就座”之意,可以说 He was seated.或 He seated himself. 象seat这样的动词在英语里有不少,我们常见的还有:astonish(使惊奇...有人把“他坐下”译为 He seated down。这就把seat与sit混淆了。sit是不及物动词。而seat是及物动词,有“使…就座”之意,可以说 He was seated.或 He seated himself. 象seat这样的动词在英语里有不少,我们常见的还有:astonish(使惊奇),cheer(使高兴),excite(使激动)。展开更多
动词 seat 与 sit 词义相似,但用法有同异,以前大部分的讲解者只谈其用法的不同点却忽略了其相同之处,故使得同学们在学习、掌握和使用该两词时产生了模糊的概念。现将其归纳整理如下,以期对读者有所帮助。要搞明白这两词的用法,必须抓...动词 seat 与 sit 词义相似,但用法有同异,以前大部分的讲解者只谈其用法的不同点却忽略了其相同之处,故使得同学们在学习、掌握和使用该两词时产生了模糊的概念。现将其归纳整理如下,以期对读者有所帮助。要搞明白这两词的用法,必须抓住以下两点:(一)看其是及物动词还是不及物动词;(二)看其是表动作还是表状态。一、seat(seated,seated,seating),是及物动词,后接宾语(常接反身代词)展开更多
基金Supported by the Aeronautical Science Foundation of China(2008ZC52039)~~
文摘The computational fluid dynamics （CFD） method is used to investigate the aerodynamic characteristics of the seat/occupant with windblast protection devices. The upwind Osher scheme is used for the spatial discretisation. The detached-eddy simulation （DES） based on the Spalart-Allmaras one-equation turbulence model is ap- plied to the detached viscous flow simulation behind the seat/occupant, with Mach numbers 0.6 and 1.2 at attack angles between --10 and 30°, and at two sideslip angles of 0 and 15°, respectively. The aerodynamic characteristics of seat/occupants with and without windblast protection devices are calculated in cases of the freestream Mach numbers 0. 8 and 1.6, attack angles from 5 to 30°, and three sideslip angles of 0, --20 and --50°, respectively. Results show that simulation results agree well with experimental data. And the occupant is efficiently protected by windblast protection devices.
基金supported by the National Science and Technology Major Project under Grant Nos.2016ZX05042004 and 2017ZX05072the Joint Funds of the National Natural Science Foundation of China under Grant No.U1762104+2 种基金the Postgraduate Innovation Project Foundation under Grant No.YCX2019054the Fundamental Research Funds for the Central Universities under Grant No.20CX02306Athe Opening Fund of National Engineering Laboratory of Offshore Geophysical and Exploration Equipment。
文摘To achieve the secondary production in multistage fracturing wells of tight oil,milling tools are usually used to remove the multistage fracturing ball seats to achieve production with a large diameter in later.In this paper,first of all,the working mechanism of milling tools for multistage fracturing ball seats was studied and a mechanical analysis model of single abrasive grain was established.Then,an experimental system for milling tools was developed,and the experimental tests of the flat,the blade,and the slope milling tool were conducted in order.Besides,the morphology of chips and the surface morphology of the workpiece after the experiment were analyzed.Also,the working performance of milling tools was evaluated from the perspectives of working safety,working efficiency,and wear resistance of the milling tool.The results show that the torque of the milling tool increases nonlinearly with the increase in the cutting depth of the abrasive grain and increases linearly with the increase in the cutting width.Also,the chips are irregular particles and the size is mainly from 10 to 50μm.So,the chips should be pumped up with a small pump pressure and a large displacement.Besides this,the cutting depths of the abrasive grains are from 216.20 to 635.47μm and the bottom surface of the milling tool should be eccentric to avoid the zero point of cutting speed.Furthermore,the torque of the slope milling tool is 23.8%larger than that of the flat milling tool,which is also 30.4%smaller than that of the blade milling tool.Compared with the flat milling tool,the working efficiency of the blade milling tool improves by 79.9%and the slope milling tool improves by 111.1%.Also,the wear resistance of the blade milling tool decreases by 102.7%,while the slope milling tool declines by 32.6%when compared with the flat milling tool.Therefore,the slope milling tool has the characteristics of moderate torque,stable working conditions,the highest working efficiency,and fine wear resistance,which is preferably used to mill multistage fracturing ball seats.This study provides a theoretical basis and guidance for milling multistage fracturing ball seats on-site and realizing production with a large diameter in later stages of multistage fracturing wells.
文摘Currently, there are two rituals coexist in the world:"taking the left for the upper"and"taking the right for the up?per". The former is usually taken in domestic affairs such as meetings, dinners, etc. while the latter enjoys an overwhelmingly fa?vor in the west and so that it becomes the international practice in every conceivable social activities. The paper intends to offer a deep explanation of the differences of two rituals with adequate evidence and examples in some specific situations, say different kinds of seat arrangement in various large and small meetings, respectively in China and the west. Then it follows to provide the underlying reasons for the differences from the perspective of cultural difference. It finds that different denotation of"left"and"right"in English and Chinese may shed some light on the understanding of the two different rituals.
文摘The test and analysis on the chair system of homemade J80 skidder was carried out. on the basis of the study result, the important parameters, the spring rigidity and the damping coefficient of J80 skidder chair, were optimized and some suggestions were put forward for improving the riding comfort of the skidder seat.
基金This research was funded by the National Natural Science Foundation of China(grant number 51675534).
文摘The sleeve sealing ball seat is one of the important components in the multistage fracturing process of horizontal wells.The erosion and wear of the surface will decrease the sealing performance of the fracturing ball and the ball seat.This leads to pressure leakage during the fracturing process and fracturing failure.In this paper,combined with the actual ball seat materials and working conditions during the fracturing process,the erosion tests of ductile iron and tungsten carbide materials under different erosion speeds,angles,and mortar concentrations are carried out.Then the erosion test results were analyzed by mathematical fitting,and a set of erosion models suitable for sliding sleeve setting ball seat materials were innovatively established.For the first time,this paper combines the erosion model obtained from the experiment and the computational fluid dynamics(CFD)with Fluent software to simulate the erosion of the ball seat.Based on the simulation results,the morphology of the sliding sleeve seat ball after erosion is predicted.Through analysis of the test and simulation results,it is showed that the erosion rate of tungsten carbide material is lower and the wear resistance is better under the condition of small angle erosion.This research can offer a strong basis for fracturing site selection,surface treatment methods,and prediction of failure time of ball seats.
文摘BACKGROUND: Motor vehicle crash(MVC) related injury has been identified as a major public health concern. Child restrain(CR) seat belts can minimize the mortality and morbidity from MVC. The use for seat belts is substantially low in developing countries like Pakistan even though its use has been shown to decrease morbidity and mortality by a significant extent.METHODS: This was a case control study with cases from the general population(GP) and controls from the Aga Khan University(AKU) employees in a 3:1 ratio. The study questionnaire was based on parameters like gender, education level, awareness and presence of CR and also assessed the frequency of usage, reasons for not using CR and the source of knowledge regarding CR use.RESULTS: Out of 848 respondents, 212 were from AKU and 636 were from the GP. 96.7% from AKU had at least a bachelor's degree while less than half(42.6%) of those from the GP were graduate or above(P<0.001). A statistically signif icant difference was found between the two groups with drivers from AKU being generally more aware about CR and its use. 81.1% of the group from AKU compared to 59.7% from the general population were found to be aware of child restraint use(P<0.001). Media(40.6%) was found to be the most common source of information amongst the AKU employees.CONCLUSION: Most motor vehicle related injuries in children can be prevented or their severity may be reduced by the use of appropriate child restraint seat belts.
文摘有人把“他坐下”译为 He seated down。这就把seat与sit混淆了。sit是不及物动词。而seat是及物动词,有“使…就座”之意,可以说 He was seated.或 He seated himself. 象seat这样的动词在英语里有不少,我们常见的还有:astonish(使惊奇),cheer(使高兴),excite(使激动)。
文摘动词 seat 与 sit 词义相似,但用法有同异,以前大部分的讲解者只谈其用法的不同点却忽略了其相同之处,故使得同学们在学习、掌握和使用该两词时产生了模糊的概念。现将其归纳整理如下,以期对读者有所帮助。要搞明白这两词的用法,必须抓住以下两点:(一)看其是及物动词还是不及物动词;(二)看其是表动作还是表状态。一、seat(seated,seated,seating),是及物动词,后接宾语(常接反身代词)
