For the sake of expounding the relationship between coarse/fine aggregate volume content and properties of self-compacting concrete(SCC),a parameter of aggregate space coefficient is proposed to study systematically t...For the sake of expounding the relationship between coarse/fine aggregate volume content and properties of self-compacting concrete(SCC),a parameter of aggregate space coefficient is proposed to study systematically the effects of fine/ coarse aggregates on workability,strength and volume stability of SCC by experiments. The optimized coarse/fine aggregates volume content of SCC was discussed with regard to specified properties. Results indicate there is a close relationship between aggregate space coefficient and properties of SCC not only in fresh state but also in hardened state. With the increasing coarse/fine aggregate space coefficients,workability of fresh SCC will be improved,the compressive strength decreases slightly and shrinkage of SCC however increases. When coarse aggregate space coefficient ranges from 1.2 to 1.7 and the fine aggregate space coefficients is from 0.6 to 1.0,SCC can meet self-compactability requirements in fresh state and have a good volume stability and appropriate strength in hardened state. The results will benefits for developing a new mixing design method for SCC.展开更多
This paper proposes a new method to estimate the ballistic coefficient（BC） of low earth orbit space debris.The data sources are the historical two-line elements（TLEs）.Since the secular variation of semi-major axes...This paper proposes a new method to estimate the ballistic coefficient（BC） of low earth orbit space debris.The data sources are the historical two-line elements（TLEs）.Since the secular variation of semi-major axes is mainly caused by the drag perturbation for space objects with perigee altitude below 600 km,the ballistic coefficients are estimated based on variation of the mean semi-major axes derived from the TLEs.However,the approximate parameters used in the calculation have error,especially when the upper atmosphere densities are difficult to obtain and always estimated by empirical model.The proportional errors of the approximate parameters are cancelled out in the form of ratios,greatly mitigating the effects of model error.This method has been also been validated for space objects with perigee altitude higher than 600 km.The relative errors of estimated BC values from the new method are significantly smaller than those from the direct estimation methods used in numerical experiments.The estimated BC values are used for the prediction of the semi-major axes,and good performance is obtained.This process is also a feasible method for prediction over a long period of time without an orbital propagator model.展开更多
The double-disc opener of maize precision seeder is an important component which affects sowing quality.After the double-disc opening operation,there will be many unfavorable phenomena such as a W-shaped bottom with p...The double-disc opener of maize precision seeder is an important component which affects sowing quality.After the double-disc opening operation,there will be many unfavorable phenomena such as a W-shaped bottom with pointed ridge,returning soil to furrow,loose and rough furrow sidewall,and large soil blocks in the furrow bottom.These phenomena often cause the problems of poor sowing depth consistency and seed spacing uniformity.In order to solve the above problems,the furrow compaction device with opener was designed to compact and reshape the original seed furrow,eventually forming a smooth and flat V-shaped seed furrow.Through theoretical calculations and kinematic analysis,the main structural parameters of the device were limited to a small range:the spring stiffness coefficient k=0.96-4.19 N/mm and the angle of the furrow compaction wheelφ=30°-60°.In the soil-bin experiment,the rotary combination design was adopted to study the effects of the parameters of the furrow compaction device with opener on the seeds location variation.The regression model of two factors with respect to each indicator was established in the Design-Expert software,revealing the effects of two factors on the indicators.Finally,the optimal structural parameters obtained were:the spring stiffness coefficient k=4.0 N/mm,and the angle of furrow compaction wheelφ=42.4°.The field test was carried out to verify the effect of the furrow compaction device with opener on the performance of precision seeder.The results showed that the average values of the sowing depth variable coefficient,the lateral deviation and the seed spacing variable coefficient respectively were 5.77%,5.1 mm and 9.54%in the treatment of the furrow compaction device with opener.All indicators were superior to the traditional double-disc opener.This research can provide references for the design of furrow opening device and maize precision seeder.展开更多
文摘For the sake of expounding the relationship between coarse/fine aggregate volume content and properties of self-compacting concrete(SCC),a parameter of aggregate space coefficient is proposed to study systematically the effects of fine/ coarse aggregates on workability,strength and volume stability of SCC by experiments. The optimized coarse/fine aggregates volume content of SCC was discussed with regard to specified properties. Results indicate there is a close relationship between aggregate space coefficient and properties of SCC not only in fresh state but also in hardened state. With the increasing coarse/fine aggregate space coefficients,workability of fresh SCC will be improved,the compressive strength decreases slightly and shrinkage of SCC however increases. When coarse aggregate space coefficient ranges from 1.2 to 1.7 and the fine aggregate space coefficients is from 0.6 to 1.0,SCC can meet self-compactability requirements in fresh state and have a good volume stability and appropriate strength in hardened state. The results will benefits for developing a new mixing design method for SCC.
基金the research support from Applied Astronomy Research Group,Yunnan Observatories,Chinese Academy of Sciencesthe grant support from the National Natural Science Foundation of China(No.61372162)
文摘This paper proposes a new method to estimate the ballistic coefficient（BC） of low earth orbit space debris.The data sources are the historical two-line elements（TLEs）.Since the secular variation of semi-major axes is mainly caused by the drag perturbation for space objects with perigee altitude below 600 km,the ballistic coefficients are estimated based on variation of the mean semi-major axes derived from the TLEs.However,the approximate parameters used in the calculation have error,especially when the upper atmosphere densities are difficult to obtain and always estimated by empirical model.The proportional errors of the approximate parameters are cancelled out in the form of ratios,greatly mitigating the effects of model error.This method has been also been validated for space objects with perigee altitude higher than 600 km.The relative errors of estimated BC values from the new method are significantly smaller than those from the direct estimation methods used in numerical experiments.The estimated BC values are used for the prediction of the semi-major axes,and good performance is obtained.This process is also a feasible method for prediction over a long period of time without an orbital propagator model.
基金The financial support for this study came from three projects:the National Key Research and Development Program of China(Grant No.2017YFD0700703)the Development Program of Science and Technology of Jilin Province of China(Grant No.20170312005ZX)the National Natural Science Foundation of China(Grant No.51905318).
文摘The double-disc opener of maize precision seeder is an important component which affects sowing quality.After the double-disc opening operation,there will be many unfavorable phenomena such as a W-shaped bottom with pointed ridge,returning soil to furrow,loose and rough furrow sidewall,and large soil blocks in the furrow bottom.These phenomena often cause the problems of poor sowing depth consistency and seed spacing uniformity.In order to solve the above problems,the furrow compaction device with opener was designed to compact and reshape the original seed furrow,eventually forming a smooth and flat V-shaped seed furrow.Through theoretical calculations and kinematic analysis,the main structural parameters of the device were limited to a small range:the spring stiffness coefficient k=0.96-4.19 N/mm and the angle of the furrow compaction wheelφ=30°-60°.In the soil-bin experiment,the rotary combination design was adopted to study the effects of the parameters of the furrow compaction device with opener on the seeds location variation.The regression model of two factors with respect to each indicator was established in the Design-Expert software,revealing the effects of two factors on the indicators.Finally,the optimal structural parameters obtained were:the spring stiffness coefficient k=4.0 N/mm,and the angle of furrow compaction wheelφ=42.4°.The field test was carried out to verify the effect of the furrow compaction device with opener on the performance of precision seeder.The results showed that the average values of the sowing depth variable coefficient,the lateral deviation and the seed spacing variable coefficient respectively were 5.77%,5.1 mm and 9.54%in the treatment of the furrow compaction device with opener.All indicators were superior to the traditional double-disc opener.This research can provide references for the design of furrow opening device and maize precision seeder.
