An electrostatic micromotor is compared with a conventional electromagnetic mieromotor, and the scaling effect of driving forces is analysed according to their different operation principles. Characteristic dimension ...An electrostatic micromotor is compared with a conventional electromagnetic mieromotor, and the scaling effect of driving forces is analysed according to their different operation principles. Characteristic dimension L represents the linear scale of micromotors. Electromagnetic driving force is proportional to high-order power of characteristic dimension L whereas electrostatic driving force is not. So, in micro domain, the effect of electrostatic force is larger than that of electromagnetic force, which makes electrostatic micromotor more competitive against electromagnetic alternative in MEMS. In assessing the performance of a micromotor, the power per unit volume and efficiency are the two most important criteria. Hence, the two kinds of micromotors are further compared from these two aspects. The results indicate that electrostatic a micromotor has higher power per unit volume and efficiency, moreover, its structure is simple, it can be made sufficiently small. For those advantages of electrostatic micromotors, they can be used in optical devices, aerospace equipment and medical instruments.展开更多
In the last 50 years,the methodology of large-eddy simulation(LES)has been greatly developed,while lots of different subgridscale(SGS)models have appeared.However,the understanding of the procedure of SGS modeling is ...In the last 50 years,the methodology of large-eddy simulation(LES)has been greatly developed,while lots of different subgridscale(SGS)models have appeared.However,the understanding of the procedure of SGS modeling is still not clear.The present contribution aims at reviewing the recent SGS models and,more importantly,expressing our recent understanding on the SGS modeling of LES in physical space.Taking the Kolmogorov equation for filtered quantities(KEF)as an example,it is argued that the KEF alone is not enough to be a closure method.Three physical laws are then introduced to complete this closure procedure and are expected to inspire the future researches of SGS modeling.展开更多
Micro-indentation tests at scales on the order of sub-micron have shown that the measured hardness increases strongly with the indent depth or indent size decreasing, which is frequently referred to as the size effect...Micro-indentation tests at scales on the order of sub-micron have shown that the measured hardness increases strongly with the indent depth or indent size decreasing, which is frequently referred to as the size effect. However, the trend is at odds with the size-independence implied by conventional elastic-plastic theory. In this paper, strain gradient plasticity theory is used to model the size effect for materials undergoing the micro-indenting. Meanwhile, the micro-indentation experiments for single crystal copper and single crystal aluminum are carried out. By the comparison of the theoretical predictions with experimental measurements, the micro-scale parameter of strain gradient plasticity theory is predicted, which is fallen into the region of 0.8—1.5 micron for the conventional metals such as copper (Cu), aluminum (Al) and silver (Ag). Moreover, the phenomena of the pile-up and sink-in near micro-indent boundary are investigated and analyzed in detail.展开更多
The miniaturization of products requires the mass production of microparts.The microforming can well meet this requirement.Due to the emergence of decreasing flow stress scale effect in the micro scale,the traditional...The miniaturization of products requires the mass production of microparts.The microforming can well meet this requirement.Due to the emergence of decreasing flow stress scale effect in the micro scale,the traditional forming process and theory may fail.Based on the crystal plasticity theory,upsetting tests of micro copper cylinders with different dimensions and grain sizes were simulated,and the decreasing flow stress scale effect was studied and discussed.Results show that with the decrease of billet dimensions,the flow stress is gradually decreased,and the decreasing flow stress scale effect is emerged;with the increase of grain size,the decreasing flow stress scale effect is more remarkable.It can also be seen that the decreasing flow stress scale effect can be well simulated with the crystal plasticity theory,and the necessary relevant information is provided for deeper understanding on this scale effect,as well as the design of processes and die structures in the microforming.展开更多
文摘An electrostatic micromotor is compared with a conventional electromagnetic mieromotor, and the scaling effect of driving forces is analysed according to their different operation principles. Characteristic dimension L represents the linear scale of micromotors. Electromagnetic driving force is proportional to high-order power of characteristic dimension L whereas electrostatic driving force is not. So, in micro domain, the effect of electrostatic force is larger than that of electromagnetic force, which makes electrostatic micromotor more competitive against electromagnetic alternative in MEMS. In assessing the performance of a micromotor, the power per unit volume and efficiency are the two most important criteria. Hence, the two kinds of micromotors are further compared from these two aspects. The results indicate that electrostatic a micromotor has higher power per unit volume and efficiency, moreover, its structure is simple, it can be made sufficiently small. For those advantages of electrostatic micromotors, they can be used in optical devices, aerospace equipment and medical instruments.
基金supported by the National Natural Science Foundation of China(Grant Nos.11202013 and 51420105008)
文摘In the last 50 years,the methodology of large-eddy simulation(LES)has been greatly developed,while lots of different subgridscale(SGS)models have appeared.However,the understanding of the procedure of SGS modeling is still not clear.The present contribution aims at reviewing the recent SGS models and,more importantly,expressing our recent understanding on the SGS modeling of LES in physical space.Taking the Kolmogorov equation for filtered quantities(KEF)as an example,it is argued that the KEF alone is not enough to be a closure method.Three physical laws are then introduced to complete this closure procedure and are expected to inspire the future researches of SGS modeling.
基金This work was supported by the National Natural Science Foundation of China (Grant Nos. 19891180 and19925211) jointly supported by the Fundamental Research Project from the Chinese Academy of Sciences (Grant No. KJ95-1-201).
文摘Micro-indentation tests at scales on the order of sub-micron have shown that the measured hardness increases strongly with the indent depth or indent size decreasing, which is frequently referred to as the size effect. However, the trend is at odds with the size-independence implied by conventional elastic-plastic theory. In this paper, strain gradient plasticity theory is used to model the size effect for materials undergoing the micro-indenting. Meanwhile, the micro-indentation experiments for single crystal copper and single crystal aluminum are carried out. By the comparison of the theoretical predictions with experimental measurements, the micro-scale parameter of strain gradient plasticity theory is predicted, which is fallen into the region of 0.8—1.5 micron for the conventional metals such as copper (Cu), aluminum (Al) and silver (Ag). Moreover, the phenomena of the pile-up and sink-in near micro-indent boundary are investigated and analyzed in detail.
基金the National Natural Science Foundation of China(Nos.50835002 and 50975174)the Ph.D.Programs Foundation of Ministry of Education of China(Nos.200802480053 and 20100073110044)
文摘The miniaturization of products requires the mass production of microparts.The microforming can well meet this requirement.Due to the emergence of decreasing flow stress scale effect in the micro scale,the traditional forming process and theory may fail.Based on the crystal plasticity theory,upsetting tests of micro copper cylinders with different dimensions and grain sizes were simulated,and the decreasing flow stress scale effect was studied and discussed.Results show that with the decrease of billet dimensions,the flow stress is gradually decreased,and the decreasing flow stress scale effect is emerged;with the increase of grain size,the decreasing flow stress scale effect is more remarkable.It can also be seen that the decreasing flow stress scale effect can be well simulated with the crystal plasticity theory,and the necessary relevant information is provided for deeper understanding on this scale effect,as well as the design of processes and die structures in the microforming.
