在室内可见光通信中符号间干扰和噪声会严重影响系统性能,K均值(K-means)均衡方法可以抑制光无线信道的影响,但其复杂度较高,且在聚类边界处易出现误判。提出了改进聚类中心点的K-means(Improved Center K-means,IC-Kmeans)算法,通过随...在室内可见光通信中符号间干扰和噪声会严重影响系统性能,K均值(K-means)均衡方法可以抑制光无线信道的影响,但其复杂度较高,且在聚类边界处易出现误判。提出了改进聚类中心点的K-means(Improved Center K-means,IC-Kmeans)算法,通过随机生成足够长的训练序列,然后将训练序列每一簇的均值作为K-means聚类中心,避免了传统K-means反复迭代寻找聚类中心。进一步,提出了基于神经网络的IC-Kmeans(Neural Network Based IC-Kmeans,NNIC-Kmeans)算法,使用反向传播神经网络将接收端二维数据映射至三维空间,以增加不同簇之间混合数据的距离,提高了分类准确性。蒙特卡罗误码率仿真表明,IC-Kmeans均衡和传统K-means算法的误码率性能相当,但可以显著降低复杂度,特别是在信噪比较小时。同时,在室内多径信道模型下,与IC-Kmeans和传统Kmeans均衡相比,NNIC-Kmeans均衡的光正交频分复用系统误码率性能最好。展开更多
How to effectively evaluate the firing precision of weapon equipment at low cost is one of the core contents of improving the test level of weapon system.A new method to evaluate the firing precision of the MLRS consi...How to effectively evaluate the firing precision of weapon equipment at low cost is one of the core contents of improving the test level of weapon system.A new method to evaluate the firing precision of the MLRS considering the credibility of simulation system based on Bayesian theory is proposed in this paper.First of all,a comprehensive index system for the credibility of the simulation system of the firing precision of the MLRS is constructed combined with the group analytic hierarchy process.A modified method for determining the comprehensive weight of the index is established to improve the rationality of the index weight coefficients.The Bayesian posterior estimation formula of firing precision considering prior information is derived in the form of mixed prior distribution,and the rationality of prior information used in estimation model is discussed quantitatively.With the simulation tests,the different evaluation methods are compared to validate the effectiveness of the proposed method.Finally,the experimental results show that the effectiveness of estimation method for firing precision is improved by more than 25%.展开更多
Our study identifies a subtle deviation from Newton’s third law in the derivation of the ideal rocket equation, also known as the Tsiolkovsky Rocket Equation (TRE). TRE can be derived using a 1D elastic collision mod...Our study identifies a subtle deviation from Newton’s third law in the derivation of the ideal rocket equation, also known as the Tsiolkovsky Rocket Equation (TRE). TRE can be derived using a 1D elastic collision model of the momentum exchange between the differential propellant mass element (dm) and the rocket final mass (m1), in which dm initially travels forward to collide with m1 and rebounds to exit through the exhaust nozzle with a velocity that is known as the effective exhaust velocity ve. We observe that such a model does not explain how dm was able to acquire its initial forward velocity without the support of a reactive mass traveling in the opposite direction. We show instead that the initial kinetic energy of dm is generated from dm itself by a process of self-combustion and expansion. In our ideal rocket with a single particle dm confined inside a hollow tube with one closed end, we show that the process of self-combustion and expansion of dm will result in a pair of differential particles each with a mass dm/2, and each traveling away from one another along the tube axis, from the center of combustion. These two identical particles represent the active and reactive sub-components of dm, co-generated in compliance with Newton’s third law of equal action and reaction. Building on this model, we derive a linear momentum ODE of the system, the solution of which yields what we call the Revised Tsiolkovsky Rocket Equation (RTRE). We show that RTRE has a mathematical form that is similar to TRE, with the exception of the effective exhaust velocity (ve) term. The ve term in TRE is replaced in RTRE by the average of two distinct exhaust velocities that we refer to as fast-jet, vx<sub>1</sub>, and slow-jet, vx<sub>2</sub>. These two velocities correspond, respectively, to the velocities of the detonation pressure wave that is vectored directly towards the exhaust nozzle, and the retonation wave that is initially vectored in the direction of rocket propagation, but subsequently becomes reflected from the thrust surface of the combustion chamber to exit through the exhaust nozzle with a time lag behind the detonation wave. The detonation-retonation phenomenon is supported by experimental evidence in the published literature. Finally, we use a convolution model to simulate the composite exhaust pressure wave, highlighting the frequency spectrum of the pressure perturbations that are generated by the mutual interference between the fast-jet and slow-jet components. Our analysis offers insights into the origin of combustion oscillations in rocket engines, with possible extensions beyond rocket engineering into other fields of combustion engineering.展开更多
目的基于“紧急输血由急诊科授权医生启动,输血科15 min内发放不超过4 U O型未交叉配血悬浮红细胞”的单中心临床实践,了解该实践背景下紧急输O型未交叉配血悬浮红细胞策略的实施现状,为血液管理提供参考。方法回顾性分析2019年8月—202...目的基于“紧急输血由急诊科授权医生启动,输血科15 min内发放不超过4 U O型未交叉配血悬浮红细胞”的单中心临床实践,了解该实践背景下紧急输O型未交叉配血悬浮红细胞策略的实施现状,为血液管理提供参考。方法回顾性分析2019年8月—2024年4月四川大学华西医院紧急输O型血患者病例信息,包括申请紧急输血原因、输血申请送达输血科所需时间、输血科血液发放所需时间、患者输血前后总胆红素、间接胆红素、乳酸脱氢酶、患者血型、病情转归等。结果2019年8月—2024年4月本院申请紧急输O型血的患者有39例,共输注未经交叉配血的O型悬浮红细胞90 U。患者均为Rh(D)阳性,其中A型14例;B型6例;O型16例;AB型2例;暂无法定型1例,抗体筛查阳性2例,紧急输O型血的患者中车祸伤是主要原因,占46%(18/39),死亡率51.28%(20/39),死亡原因均为原发伤,无溶血性输血反应的临床指征:输血前后总胆红素(TBTL)、间接胆红素(IBIL)、乳酸脱氢酶(LDH)无统计学差异,P值均>0.05。患者从入院到输血科接收输血申请单和输血科从收到申请单到血液发放的中位时间分别为30.20 min和5.30 min。结论基于“紧急输血由急诊科授权医生启动,输血科15 min内发放不超过4 U O型未交叉配血悬浮红细胞”的单中心临床实践具有安全性,但申请紧急输O型血流程中在输血申请单送达输血科之前有优化空间。展开更多
The solid rocket motor driven system is one of the common ways for submarines to launch underwater missiles. It has significant advantages in improving the missile’s water exit speed, anti-interference capability, an...The solid rocket motor driven system is one of the common ways for submarines to launch underwater missiles. It has significant advantages in improving the missile’s water exit speed, anti-interference capability, and enemy striking power. The prediction of the underwater loading is a preliminary factor for the power system design of the underwater vehicle. This paper presents a rapid prediction method and validated by the experimental study for the underwater thrust of the solid rocket motor. Based on the potential flow assumption of the water field, a model of the bubble and a one-dimensional quasi-steady model of the nozzle are established to directly solve the flow status of the nozzle. The aerodynamic thrust and hydrodynamic thrust have been calculated and analyzed. The calculation results are within 5% error of the experimental results. Moreover, a design platform to predict the underwater thrust of the solid rocket motor has been developed based on Python and the PyQt library, which shows excellent system adaptability and computational efficiency.展开更多
At 11:21 on May 7,2024,the Long March 6C carrier rocket was successfully launched from China’s Taiyuan Satellite Launch Center,sending 4 satellites including Neptune 01 into their planned orbits.The launch mission wa...At 11:21 on May 7,2024,the Long March 6C carrier rocket was successfully launched from China’s Taiyuan Satellite Launch Center,sending 4 satellites including Neptune 01 into their planned orbits.The launch mission was a complete success.This mission marks the maiden flight of the Long March 6C carrier rocket,signifying the addition of a new member to the Long March rocket family.It further improves China’s new generation of Long March series launch vehicles and accelerates the upgrading of China’s in-service carrier rockets.The Long March 6C carrier rocket and Neptune 01 were developed by the Shanghai Academy of Spaceflight Technology.展开更多
Bayesian estimation is applied to the analysis of backflow vortex instabilities in typical three-and four bladed liquid propellant rocket(LPR)engine inducers.The flow in the impeller eye is modeled as a set of equally...Bayesian estimation is applied to the analysis of backflow vortex instabilities in typical three-and four bladed liquid propellant rocket(LPR)engine inducers.The flow in the impeller eye is modeled as a set of equally intense and evenly spaced 2D axial vortices,located at the same radial distance from the axis and rotating at a fraction of the impeller speed.The circle theorem and the Bernoulli’s equation are used to predict the flow pressure in terms of the vortex number,intensity,rotational speed,and radial position.The theoretical spectra so obtained are frequency broadened to mimic the dispersion of the experimental data and parametrically fitted to the measured pressure spectra by maximum likelihood estimation with equal and independent Gaussian errors.The method is applied to three inducers,tested in water at room temperature and different loads and cavitation conditions.It successfully characterizes backflow instabilities using the signals of a single pressure transducer flush-mounted on the casing of the impeller eye,effectively by-passing the aliasing and data acquisition/reduction complexities of traditional multiple-sensor cross correlation methods.The identification returns the estimates of the model parameters and their standard errors,providing the information necessary for assessing the accuracy and statistical significance of the results.The flowrate is found to be the major factor affecting the backflow vortex instability,which,on the other hand,is rather insensitive to the occurrence of cavitation.The results are consistent with the data reported in the literature,as well as with those generated by the auxiliary models specifically developed for initializing the maximum likelihood searches and supporting the identification procedure.展开更多
On May 3,a Long March 5 rocket carrying the Chang’e 6 probe blasted off from its launchpad at the Wenchang Space Launch Site at 17:27.The probe separated from the rocket and entered its planned transfer orbit shortly...On May 3,a Long March 5 rocket carrying the Chang’e 6 probe blasted off from its launchpad at the Wenchang Space Launch Site at 17:27.The probe separated from the rocket and entered its planned transfer orbit shortly thereafter,demonstrating a completely successful launch.With the world’s attention,the Chang’e 6 probe will retrieve samples from the far side of the moon in a historic first.展开更多
文摘在室内可见光通信中符号间干扰和噪声会严重影响系统性能,K均值(K-means)均衡方法可以抑制光无线信道的影响,但其复杂度较高,且在聚类边界处易出现误判。提出了改进聚类中心点的K-means(Improved Center K-means,IC-Kmeans)算法,通过随机生成足够长的训练序列,然后将训练序列每一簇的均值作为K-means聚类中心,避免了传统K-means反复迭代寻找聚类中心。进一步,提出了基于神经网络的IC-Kmeans(Neural Network Based IC-Kmeans,NNIC-Kmeans)算法,使用反向传播神经网络将接收端二维数据映射至三维空间,以增加不同簇之间混合数据的距离,提高了分类准确性。蒙特卡罗误码率仿真表明,IC-Kmeans均衡和传统K-means算法的误码率性能相当,但可以显著降低复杂度,特别是在信噪比较小时。同时,在室内多径信道模型下,与IC-Kmeans和传统Kmeans均衡相比,NNIC-Kmeans均衡的光正交频分复用系统误码率性能最好。
基金National Natural Science Foundation of China(Grant Nos.11972193 and 92266201)。
文摘How to effectively evaluate the firing precision of weapon equipment at low cost is one of the core contents of improving the test level of weapon system.A new method to evaluate the firing precision of the MLRS considering the credibility of simulation system based on Bayesian theory is proposed in this paper.First of all,a comprehensive index system for the credibility of the simulation system of the firing precision of the MLRS is constructed combined with the group analytic hierarchy process.A modified method for determining the comprehensive weight of the index is established to improve the rationality of the index weight coefficients.The Bayesian posterior estimation formula of firing precision considering prior information is derived in the form of mixed prior distribution,and the rationality of prior information used in estimation model is discussed quantitatively.With the simulation tests,the different evaluation methods are compared to validate the effectiveness of the proposed method.Finally,the experimental results show that the effectiveness of estimation method for firing precision is improved by more than 25%.
文摘Our study identifies a subtle deviation from Newton’s third law in the derivation of the ideal rocket equation, also known as the Tsiolkovsky Rocket Equation (TRE). TRE can be derived using a 1D elastic collision model of the momentum exchange between the differential propellant mass element (dm) and the rocket final mass (m1), in which dm initially travels forward to collide with m1 and rebounds to exit through the exhaust nozzle with a velocity that is known as the effective exhaust velocity ve. We observe that such a model does not explain how dm was able to acquire its initial forward velocity without the support of a reactive mass traveling in the opposite direction. We show instead that the initial kinetic energy of dm is generated from dm itself by a process of self-combustion and expansion. In our ideal rocket with a single particle dm confined inside a hollow tube with one closed end, we show that the process of self-combustion and expansion of dm will result in a pair of differential particles each with a mass dm/2, and each traveling away from one another along the tube axis, from the center of combustion. These two identical particles represent the active and reactive sub-components of dm, co-generated in compliance with Newton’s third law of equal action and reaction. Building on this model, we derive a linear momentum ODE of the system, the solution of which yields what we call the Revised Tsiolkovsky Rocket Equation (RTRE). We show that RTRE has a mathematical form that is similar to TRE, with the exception of the effective exhaust velocity (ve) term. The ve term in TRE is replaced in RTRE by the average of two distinct exhaust velocities that we refer to as fast-jet, vx<sub>1</sub>, and slow-jet, vx<sub>2</sub>. These two velocities correspond, respectively, to the velocities of the detonation pressure wave that is vectored directly towards the exhaust nozzle, and the retonation wave that is initially vectored in the direction of rocket propagation, but subsequently becomes reflected from the thrust surface of the combustion chamber to exit through the exhaust nozzle with a time lag behind the detonation wave. The detonation-retonation phenomenon is supported by experimental evidence in the published literature. Finally, we use a convolution model to simulate the composite exhaust pressure wave, highlighting the frequency spectrum of the pressure perturbations that are generated by the mutual interference between the fast-jet and slow-jet components. Our analysis offers insights into the origin of combustion oscillations in rocket engines, with possible extensions beyond rocket engineering into other fields of combustion engineering.
文摘目的基于“紧急输血由急诊科授权医生启动,输血科15 min内发放不超过4 U O型未交叉配血悬浮红细胞”的单中心临床实践,了解该实践背景下紧急输O型未交叉配血悬浮红细胞策略的实施现状,为血液管理提供参考。方法回顾性分析2019年8月—2024年4月四川大学华西医院紧急输O型血患者病例信息,包括申请紧急输血原因、输血申请送达输血科所需时间、输血科血液发放所需时间、患者输血前后总胆红素、间接胆红素、乳酸脱氢酶、患者血型、病情转归等。结果2019年8月—2024年4月本院申请紧急输O型血的患者有39例,共输注未经交叉配血的O型悬浮红细胞90 U。患者均为Rh(D)阳性,其中A型14例;B型6例;O型16例;AB型2例;暂无法定型1例,抗体筛查阳性2例,紧急输O型血的患者中车祸伤是主要原因,占46%(18/39),死亡率51.28%(20/39),死亡原因均为原发伤,无溶血性输血反应的临床指征:输血前后总胆红素(TBTL)、间接胆红素(IBIL)、乳酸脱氢酶(LDH)无统计学差异,P值均>0.05。患者从入院到输血科接收输血申请单和输血科从收到申请单到血液发放的中位时间分别为30.20 min和5.30 min。结论基于“紧急输血由急诊科授权医生启动,输血科15 min内发放不超过4 U O型未交叉配血悬浮红细胞”的单中心临床实践具有安全性,但申请紧急输O型血流程中在输血申请单送达输血科之前有优化空间。
文摘The solid rocket motor driven system is one of the common ways for submarines to launch underwater missiles. It has significant advantages in improving the missile’s water exit speed, anti-interference capability, and enemy striking power. The prediction of the underwater loading is a preliminary factor for the power system design of the underwater vehicle. This paper presents a rapid prediction method and validated by the experimental study for the underwater thrust of the solid rocket motor. Based on the potential flow assumption of the water field, a model of the bubble and a one-dimensional quasi-steady model of the nozzle are established to directly solve the flow status of the nozzle. The aerodynamic thrust and hydrodynamic thrust have been calculated and analyzed. The calculation results are within 5% error of the experimental results. Moreover, a design platform to predict the underwater thrust of the solid rocket motor has been developed based on Python and the PyQt library, which shows excellent system adaptability and computational efficiency.
文摘At 11:21 on May 7,2024,the Long March 6C carrier rocket was successfully launched from China’s Taiyuan Satellite Launch Center,sending 4 satellites including Neptune 01 into their planned orbits.The launch mission was a complete success.This mission marks the maiden flight of the Long March 6C carrier rocket,signifying the addition of a new member to the Long March rocket family.It further improves China’s new generation of Long March series launch vehicles and accelerates the upgrading of China’s in-service carrier rockets.The Long March 6C carrier rocket and Neptune 01 were developed by the Shanghai Academy of Spaceflight Technology.
文摘Bayesian estimation is applied to the analysis of backflow vortex instabilities in typical three-and four bladed liquid propellant rocket(LPR)engine inducers.The flow in the impeller eye is modeled as a set of equally intense and evenly spaced 2D axial vortices,located at the same radial distance from the axis and rotating at a fraction of the impeller speed.The circle theorem and the Bernoulli’s equation are used to predict the flow pressure in terms of the vortex number,intensity,rotational speed,and radial position.The theoretical spectra so obtained are frequency broadened to mimic the dispersion of the experimental data and parametrically fitted to the measured pressure spectra by maximum likelihood estimation with equal and independent Gaussian errors.The method is applied to three inducers,tested in water at room temperature and different loads and cavitation conditions.It successfully characterizes backflow instabilities using the signals of a single pressure transducer flush-mounted on the casing of the impeller eye,effectively by-passing the aliasing and data acquisition/reduction complexities of traditional multiple-sensor cross correlation methods.The identification returns the estimates of the model parameters and their standard errors,providing the information necessary for assessing the accuracy and statistical significance of the results.The flowrate is found to be the major factor affecting the backflow vortex instability,which,on the other hand,is rather insensitive to the occurrence of cavitation.The results are consistent with the data reported in the literature,as well as with those generated by the auxiliary models specifically developed for initializing the maximum likelihood searches and supporting the identification procedure.
文摘On May 3,a Long March 5 rocket carrying the Chang’e 6 probe blasted off from its launchpad at the Wenchang Space Launch Site at 17:27.The probe separated from the rocket and entered its planned transfer orbit shortly thereafter,demonstrating a completely successful launch.With the world’s attention,the Chang’e 6 probe will retrieve samples from the far side of the moon in a historic first.
