Separated from the women’s prison by only a wall is a big courtyard. It is a world of green. We were told that growing in the three hectare courtyard are more than 3,000 of tall trees and bushes in more than 100 spec...Separated from the women’s prison by only a wall is a big courtyard. It is a world of green. We were told that growing in the three hectare courtyard are more than 3,000 of tall trees and bushes in more than 100 species, 3,000 pots of plants, 4,000 square meters of vegetative cover and nearly 10,000 square meters of flower beds, and lawns. There is also a rockery and a fall and more than 700 square meters of roof garden. No one could associate the place with the word "prison." But it is a prison-a prison or reform school for educating and correcting juvenile delinquents.展开更多
近年来,超宽带(Ultra-wideband,UWB)通信技术因其具有数据传输速率高、功耗低、保密性高、成本低等诸多优点,成为室内高速数据传输的一种有效解决方案。由于高速通信下码间串扰极大地影响了UWB系统的接收性能,需要在解调前采用Rake接收...近年来,超宽带(Ultra-wideband,UWB)通信技术因其具有数据传输速率高、功耗低、保密性高、成本低等诸多优点,成为室内高速数据传输的一种有效解决方案。由于高速通信下码间串扰极大地影响了UWB系统的接收性能,需要在解调前采用Rake接收机和均衡等手段消除码间串扰。针对自适应最小误码率(Adaptive Minimum Bit Error Rate,AMBER)均衡算法与Rake接收机结合进行研究,提出了一种变步长自适应最小误码率(Variable Step-Adaptive Minimum Bit Error Rate,VS-AMBER)均衡算法,在较低信噪比和较高信噪比情况下分别使用不同的变步长方案,较高信噪比下在收敛速度和收敛精度上有所提升,较低信噪比下降低了误码率和功耗。展开更多
The marine propeller typically functions within thefilowfiield generated by a water vehicle.Investigations into the geometric parameters of the propeller are commonly conducted under open‑water conditions as simultane...The marine propeller typically functions within thefilowfiield generated by a water vehicle.Investigations into the geometric parameters of the propeller are commonly conducted under open‑water conditions as simultaneously simulating both vehicle and propeller holds several computational challenges.While during operation,this propellant device must face several forces like gravity,hydrodynamic load,and centrifugal force,which cause different problems like cavitation and structural failure,etc.Since these issues affect performance,it necessitates comprehensive analysis.In this study,hydrodynamic analysis is performed by using commercial software STAR CCM+.In hydrodynamic analysis,the effect of the rake angles–5°,5°,10°and 15°on hydrodynamic coeffiicients and effiiciency of the DTMB 4119 in the open water is analyzed using Computational Fluid Dynamics(CFD)and the control volume approach.The Shear Stress Transport(SST)k‑ωturbulence model is used in Computational Fluid Dynamics(CFD)simulation.Hydrodynamic analysis reveals that the rake angles 5°and 10°cause the open water effiiciency of David Taylor Model Basin(DTMB)4119 to improve by 0.4 to 1.32%with exception of the rake angles–5°and 15°,which possess different effects on effiiciency.The angle–5°causes a decrease in propeller effiiciency under heavy loading situations(low advance coeffiicient)apart from a minorfiluctuation at light loading conditions(high advance coeffiicient),while the angle 15°produces a drop in effiiciency by higher advance ratios but little variation at lower advance ratios.展开更多
Based on the finite-discrete element method,a three-dimensional numerical model for axial impact rock breaking was established and validated.A computational method for energy conversion during impact rock breaking was...Based on the finite-discrete element method,a three-dimensional numerical model for axial impact rock breaking was established and validated.A computational method for energy conversion during impact rock breaking was proposed,and the effects of conical tooth forward rake angle,rock temperature,and impact velocity on rock breaking characteristics and energy transfer laws were analyzed.The results show that during single impact rock breaking with conical tooth bits,merely 7.52%to 12.51%of the energy is utilized for rock breaking,while a significant 57.26%to 78.10%is dissipated as frictional loss.An insufficient forward rake angle increases tooth penetration depth and frictional loss,whereas an excessive forward rake angle reduces penetration capability,causing bit rebound and greater energy absorption by the drill rod.Thus,an optimal forward rake angle exists.Regarding environmental factors,high temperatures significantly enhance impact-induced rock breaking.Thermal damage from high temperatures reduces rock strength and inhibits its energy absorption.Finally,higher impact velocities intensify rock damage,yet excessively high velocities increase frictional loss and reduce the proportion of energy absorbed by the rock,thereby failing to substantially improve rock breaking efficiency.An optimal impact velocity exists.展开更多
在水声通信中,扩频通信具有可靠性高、抗多径能力强等特点,适合于远程或低信噪比情况下的通信;但另一方面,由于其带宽利用率低加之水声信道带宽有限,通信系统的数据率将受到很大的限制。为了提高数据率,提出了一种正交M-ary/DS扩频方法...在水声通信中,扩频通信具有可靠性高、抗多径能力强等特点,适合于远程或低信噪比情况下的通信;但另一方面,由于其带宽利用率低加之水声信道带宽有限,通信系统的数据率将受到很大的限制。为了提高数据率,提出了一种正交M-ary/DS扩频方法,该方法将正交直接序列(DS)扩频与并行M-ary扩频结合在一起,能够显著提高水声通信系统的数据率与带宽利用率。经过湖上实验验证,该方法在25 km距离约0 dB的接收信噪比下,采用63、127长度G o ld序列并通过RAKE接收,分别实现了对480个符号的无误码传输,数据率为381.0 b its/s与220.5 b its/s,其带宽利用率与使用相同序列的直接序列扩频相比提高了约一个数量级。展开更多
文摘Separated from the women’s prison by only a wall is a big courtyard. It is a world of green. We were told that growing in the three hectare courtyard are more than 3,000 of tall trees and bushes in more than 100 species, 3,000 pots of plants, 4,000 square meters of vegetative cover and nearly 10,000 square meters of flower beds, and lawns. There is also a rockery and a fall and more than 700 square meters of roof garden. No one could associate the place with the word "prison." But it is a prison-a prison or reform school for educating and correcting juvenile delinquents.
文摘近年来,超宽带(Ultra-wideband,UWB)通信技术因其具有数据传输速率高、功耗低、保密性高、成本低等诸多优点,成为室内高速数据传输的一种有效解决方案。由于高速通信下码间串扰极大地影响了UWB系统的接收性能,需要在解调前采用Rake接收机和均衡等手段消除码间串扰。针对自适应最小误码率(Adaptive Minimum Bit Error Rate,AMBER)均衡算法与Rake接收机结合进行研究,提出了一种变步长自适应最小误码率(Variable Step-Adaptive Minimum Bit Error Rate,VS-AMBER)均衡算法,在较低信噪比和较高信噪比情况下分别使用不同的变步长方案,较高信噪比下在收敛速度和收敛精度上有所提升,较低信噪比下降低了误码率和功耗。
文摘The marine propeller typically functions within thefilowfiield generated by a water vehicle.Investigations into the geometric parameters of the propeller are commonly conducted under open‑water conditions as simultaneously simulating both vehicle and propeller holds several computational challenges.While during operation,this propellant device must face several forces like gravity,hydrodynamic load,and centrifugal force,which cause different problems like cavitation and structural failure,etc.Since these issues affect performance,it necessitates comprehensive analysis.In this study,hydrodynamic analysis is performed by using commercial software STAR CCM+.In hydrodynamic analysis,the effect of the rake angles–5°,5°,10°and 15°on hydrodynamic coeffiicients and effiiciency of the DTMB 4119 in the open water is analyzed using Computational Fluid Dynamics(CFD)and the control volume approach.The Shear Stress Transport(SST)k‑ωturbulence model is used in Computational Fluid Dynamics(CFD)simulation.Hydrodynamic analysis reveals that the rake angles 5°and 10°cause the open water effiiciency of David Taylor Model Basin(DTMB)4119 to improve by 0.4 to 1.32%with exception of the rake angles–5°and 15°,which possess different effects on effiiciency.The angle–5°causes a decrease in propeller effiiciency under heavy loading situations(low advance coeffiicient)apart from a minorfiluctuation at light loading conditions(high advance coeffiicient),while the angle 15°produces a drop in effiiciency by higher advance ratios but little variation at lower advance ratios.
基金Supported by Major Instrument Project of National Natural Science Foundation of China(52327803)Major Project of National Natural Science Foundation of China(52192622).
文摘Based on the finite-discrete element method,a three-dimensional numerical model for axial impact rock breaking was established and validated.A computational method for energy conversion during impact rock breaking was proposed,and the effects of conical tooth forward rake angle,rock temperature,and impact velocity on rock breaking characteristics and energy transfer laws were analyzed.The results show that during single impact rock breaking with conical tooth bits,merely 7.52%to 12.51%of the energy is utilized for rock breaking,while a significant 57.26%to 78.10%is dissipated as frictional loss.An insufficient forward rake angle increases tooth penetration depth and frictional loss,whereas an excessive forward rake angle reduces penetration capability,causing bit rebound and greater energy absorption by the drill rod.Thus,an optimal forward rake angle exists.Regarding environmental factors,high temperatures significantly enhance impact-induced rock breaking.Thermal damage from high temperatures reduces rock strength and inhibits its energy absorption.Finally,higher impact velocities intensify rock damage,yet excessively high velocities increase frictional loss and reduce the proportion of energy absorbed by the rock,thereby failing to substantially improve rock breaking efficiency.An optimal impact velocity exists.
文摘在水声通信中,扩频通信具有可靠性高、抗多径能力强等特点,适合于远程或低信噪比情况下的通信;但另一方面,由于其带宽利用率低加之水声信道带宽有限,通信系统的数据率将受到很大的限制。为了提高数据率,提出了一种正交M-ary/DS扩频方法,该方法将正交直接序列(DS)扩频与并行M-ary扩频结合在一起,能够显著提高水声通信系统的数据率与带宽利用率。经过湖上实验验证,该方法在25 km距离约0 dB的接收信噪比下,采用63、127长度G o ld序列并通过RAKE接收,分别实现了对480个符号的无误码传输,数据率为381.0 b its/s与220.5 b its/s,其带宽利用率与使用相同序列的直接序列扩频相比提高了约一个数量级。
