Pressure fluctuations in the plasma sheath from spacecraft reentry affect radiofrequency(RF) wave propagation.The influence of these fluctuations on wave propagation and wave properties is studied using methods deri...Pressure fluctuations in the plasma sheath from spacecraft reentry affect radiofrequency(RF) wave propagation.The influence of these fluctuations on wave propagation and wave properties is studied using methods derived by synthesizing the compressible turbulent flow theory,plasma theory,and electromagnetic wave theory.We study these influences on wave propagation at GPS and Ka frequencies during typical reentry by adopting stratified modeling.We analyzed the variations in reflection and transmission properties induced by pressure fluctuations.Our results show that,at the GPS frequency,if the waves are not totally reflected then the pressure fluctuations can remarkably affect reflection,transmission,and absorption properties.In extreme situations,the fluctuations can even cause blackout.At the Ka frequency,the influences are obvious when the waves are not totally transmitted.The influences are more pronounced at the GPS frequency than at the Ka frequency.This suggests that the latter can mitigate blackout by reducing both the reflection and the absorption of waves,as well as the influences of plasma fluctuations on wave propagation.Given that communication links with the reentry vehicles are susceptible to plasma pressure fluctuations,the influences on link budgets should be taken into consideration.展开更多
The randomness of turbulent reentry plasma sheaths can affect the propagation and scattering properties of electromagnetic waves.This paper developed algorithms to estimate the influences.With the algorithms and typic...The randomness of turbulent reentry plasma sheaths can affect the propagation and scattering properties of electromagnetic waves.This paper developed algorithms to estimate the influences.With the algorithms and typical reentry data,influences of GPS frequency and Ka frequency are studied respectively.Results show that,in terms of wave scattering,the scattering loss caused by the randomness of the turbulent plasma sheath increases with the increase of the ensemble average electron density,ensemble average collision frequency,electron density fluctuation and turbulence integral scale respectively.Also the scattering loss is much smaller than the dielectric loss.The scattering loss of Ka frequency is much less than that of the GPS frequency.In terms of wave propagation,the randomness arouses the fluctuations of amplitude and phase of waves.The fluctuations change with altitudes that when the altitude is below 30 km,fluctuations increase with altitude increasing,and when the altitude is above 30 km,fluctuations decrease with altitude increasing.The fluctuations of GPS frequency are strong enough to affect the tracking,telemetry,and command at appropriate conditions,while the fluctuations of Ka frequency are much more feeble.This suggests that the Ka frequency suffers less influences of the randomness of a turbulent plasma sheath.展开更多
A variational method is introduced to analyze the transmissivity of an electromagnetic wave propagating in the magnetized plasma sheath. The plasma density is modeled by two parabolic inhomogeneous regions separated b...A variational method is introduced to analyze the transmissivity of an electromagnetic wave propagating in the magnetized plasma sheath. The plasma density is modeled by two parabolic inhomogeneous regions separated by one homogeneous region. The Lagrangian density of the system is constructed based on the fluid energy density and the electromagnetic energy density.The total variation of the Lagrangian density is derived. The fluid and electromagnetic fields are numerically solved by expansion in piecewise polynomial function space. We investigate the effect of an external magnetic field on the transmissivity of the electromagnetic wave. It is found that the transmissivity is increased when an external magnetic field is applied. The dependence of transmissivity on the collision frequency between the electrons and the neutral particles has also been studied. We also show that the external magnetic field causes a shift in the critical frequency of the plasma sheath.展开更多
A simple hydrodynamic model based on two-fluid theory, taking into account the effect of discrete particles on both the gas- and solid-phase momentum equations, was used to numerically investigate the pressure fluctua...A simple hydrodynamic model based on two-fluid theory, taking into account the effect of discrete particles on both the gas- and solid-phase momentum equations, was used to numerically investigate the pressure fluctuation characteristics in a gas-solid fluidized bed with the aid of CFX 4.4, a commercial CFD software package, by adding user-defined Fortran subroutines. Numerical simulations together with typical experimental measurements show that pressure fluctuations originate above the distributor when a gas pulse is injected into the fluidized bed. The pressure above the bubble gradually increases due to the presence of a rising bubble. When the bubble passes through the bed surface, the pressure near the bed surface gradually decreases to a lower value. Moreover, the pressure signals in the bubbling fluidized beds show obviously periodic characteristics. The major frequency of pressure fluctuations at the same vertical position is affected slightly by the operating gas velocity, and the amplitude of pressure fluctuations is related to both the operating gas velocity and the vertical height. In this study, the influence of the operating gas velocity on the pressure wave propagation velocity can be ignored, and only two peak frequencies in the power spectrum of the pressure fluctuations are observed which are associated with the bubble formation above the distributor and its eruption at the bed surface.展开更多
The attenuation characteristics of obliquely incident electromagnetic(EM) wave in L-Ka frequency band in weakly ionized dusty plasma are analyzed based on the modified Bhatnagar-Gross-Krook(BGK) collision model.Accord...The attenuation characteristics of obliquely incident electromagnetic(EM) wave in L-Ka frequency band in weakly ionized dusty plasma are analyzed based on the modified Bhatnagar-Gross-Krook(BGK) collision model.According to the kinetic equation and the charging theory,the total complex dielectric constant of the weakly ionized dusty plasma is derived by considering that the minimum velocity of the electron accessible to the dust particle surface is non-zero and the second potential part of the collision cross-section contributes to the charging.The attenuation characteristics within the modified model are compared with those within the traditional model.The influence of the dusty plasma parameters and the incident angle of EM waves on the attenuation in weakly ionized dusty plasma is further analyzed.Finally,the influence of different reentry heights on the attenuation characteristics of the obliquely incident EM wave is discussed.The results show that the effect of the minimum electron velocity and the second term of the collision cross-section on the attenuation characteristics of EM waves cannot be ignored.When the dust density and dust radius are changed,the trends of the attenuation of obliquely incident EM waves are consistent,but the influence of dust density is weaker than that of dust radius due to the constraint of orbit-limited motion(OLM) theory.The plasma thickness,electron density,and incident angle are proportional to the attenuation amplitude of EM waves.The effect of different reentry heights on the attenuation obliquely incident EM waves is related to the electron density and plasma thickness.展开更多
Research on terahertz communication under different vehicles has important guiding significance for the design of future hypersonic vehicle and Radio Frequency(RF)blackout.In this paper,a joint simulation model of pla...Research on terahertz communication under different vehicles has important guiding significance for the design of future hypersonic vehicle and Radio Frequency(RF)blackout.In this paper,a joint simulation model of plasma flow under thermochemical nonequilibrium state and terahertz transmission is developed to investigate the differences in terahertz wave transmission characteristics under different vehicle shapes and the related mechanisms.By comparing the plasma sheath characteristics and terahertz transmission among HIFIRE-5b(Hypersonic International Flight Research Experimentation-5b),RAM C(Radio Attenuation Measurement C)and ARD(Atmospheric Reentry Demonstrator)vehicles,it is found that the sheath thickness and electron density of ARD vehicles is significantly larger than that of HIFIRE-5b and RAM C vehicles,resulting in greater terahertz wave attenuation.Collision absorption plays a major role in the terahertz attenuation of vehicles,and the contribution of reflection effects is only observed in the ARD vehicle due to its larger plasma sheath thickness and spatial structure variation.Based on the above comparison results,a shape design scheme of reducing the vehicle head and tail for mitigating RF blackout is proposed,and the scheme is proved by further analyzing the effects of different vehicle heads and tails on the terahertz communication.With the decrease of the head radius and tail width of hypersonic vehicle,the wave attenuation at the same terahertz frequency decreases,and the contribution of reflection effect to wave attenuation gradually disappears.Therefore,the shape design scheme of reducing the vehicle head and tail can effectively alleviate the RF blackout problem,which provides an important reference value for future hypersonic vehicle design.展开更多
高超声速飞行器等离子鞘套由于湍流、姿态调整、非均匀烧蚀等因素的影响,其等离子参数具有一定的随机特性。随机媒质的时域有限差分法(Stochastic Finite Difference Time Domain,SFDTD)可用于快速分析随机媒质中电波传播的统计特性。...高超声速飞行器等离子鞘套由于湍流、姿态调整、非均匀烧蚀等因素的影响,其等离子参数具有一定的随机特性。随机媒质的时域有限差分法(Stochastic Finite Difference Time Domain,SFDTD)可用于快速分析随机媒质中电波传播的统计特性。本文将普通媒质的S-FDTD方法扩展到等离子色散媒质,并采用该方法分析了典型鞘套电子密度分布下,电子密度随机特性对电波传播的影响。结果表明:电子密度的随机变化会引起透射电磁波的幅度与相位的抖动,抖动幅度与电子密度变化幅度具有线性相关性;等离子碰撞频率越高,相同强度的电子密度变化引起的透射系数幅度与相位变化越小;提高入射波频率,有助于有减少透射电磁波幅度与相位抖动。展开更多
Injection-induced rail pressure fluctuations are proven to cause nonuniform spray development.These fluctuations are also responsible for generating lower injection pressures,to the detriment of jet penetration length...Injection-induced rail pressure fluctuations are proven to cause nonuniform spray development.These fluctuations are also responsible for generating lower injection pressures,to the detriment of jet penetration length and break-up timing.Despite the vast literature dealing with such issues,several aspects of rail pressure fluctuations remain unclear.Additionally,the need for compliance with the emission legislation has shed light on the potential of alternative fuels,which represent a pathway for sustainable mobility.This scenario has motivated the present study dealing with the assessment of the time history of rail pressure correlated with fuel properties.Tests have been performed using a last-generation common rail injection equipment under various injection settings,employing diesel and 2-methylfuran-diesel blend.This paper describes the research activity and aims to provide new insights into the correlation of rail pressure fluctuations with fuel properties.展开更多
基金supported by the National Basic Research Program of China(No.2014CB340205)National Natural Science Foundation of China(No.61301173)
文摘Pressure fluctuations in the plasma sheath from spacecraft reentry affect radiofrequency(RF) wave propagation.The influence of these fluctuations on wave propagation and wave properties is studied using methods derived by synthesizing the compressible turbulent flow theory,plasma theory,and electromagnetic wave theory.We study these influences on wave propagation at GPS and Ka frequencies during typical reentry by adopting stratified modeling.We analyzed the variations in reflection and transmission properties induced by pressure fluctuations.Our results show that,at the GPS frequency,if the waves are not totally reflected then the pressure fluctuations can remarkably affect reflection,transmission,and absorption properties.In extreme situations,the fluctuations can even cause blackout.At the Ka frequency,the influences are obvious when the waves are not totally transmitted.The influences are more pronounced at the GPS frequency than at the Ka frequency.This suggests that the latter can mitigate blackout by reducing both the reflection and the absorption of waves,as well as the influences of plasma fluctuations on wave propagation.Given that communication links with the reentry vehicles are susceptible to plasma pressure fluctuations,the influences on link budgets should be taken into consideration.
基金supported by the National Basic Research Program of China(No.2014CB340205)National Natural Science Foundation ofChina(Nos.61301173 and 61473228)
文摘The randomness of turbulent reentry plasma sheaths can affect the propagation and scattering properties of electromagnetic waves.This paper developed algorithms to estimate the influences.With the algorithms and typical reentry data,influences of GPS frequency and Ka frequency are studied respectively.Results show that,in terms of wave scattering,the scattering loss caused by the randomness of the turbulent plasma sheath increases with the increase of the ensemble average electron density,ensemble average collision frequency,electron density fluctuation and turbulence integral scale respectively.Also the scattering loss is much smaller than the dielectric loss.The scattering loss of Ka frequency is much less than that of the GPS frequency.In terms of wave propagation,the randomness arouses the fluctuations of amplitude and phase of waves.The fluctuations change with altitudes that when the altitude is below 30 km,fluctuations increase with altitude increasing,and when the altitude is above 30 km,fluctuations decrease with altitude increasing.The fluctuations of GPS frequency are strong enough to affect the tracking,telemetry,and command at appropriate conditions,while the fluctuations of Ka frequency are much more feeble.This suggests that the Ka frequency suffers less influences of the randomness of a turbulent plasma sheath.
基金supported by the National Key Basic Research Program of China (No. 2014CB340203)
文摘A variational method is introduced to analyze the transmissivity of an electromagnetic wave propagating in the magnetized plasma sheath. The plasma density is modeled by two parabolic inhomogeneous regions separated by one homogeneous region. The Lagrangian density of the system is constructed based on the fluid energy density and the electromagnetic energy density.The total variation of the Lagrangian density is derived. The fluid and electromagnetic fields are numerically solved by expansion in piecewise polynomial function space. We investigate the effect of an external magnetic field on the transmissivity of the electromagnetic wave. It is found that the transmissivity is increased when an external magnetic field is applied. The dependence of transmissivity on the collision frequency between the electrons and the neutral particles has also been studied. We also show that the external magnetic field causes a shift in the critical frequency of the plasma sheath.
基金support from National Basic Research Program of China(No.2009CB219801)National Natural Science Foundation of China(No.20976191)+1 种基金International Cooperative Program of Guizhou Province([2009]700110)Program for New Century Excellent Talents in University(NCET-09-0342)
文摘A simple hydrodynamic model based on two-fluid theory, taking into account the effect of discrete particles on both the gas- and solid-phase momentum equations, was used to numerically investigate the pressure fluctuation characteristics in a gas-solid fluidized bed with the aid of CFX 4.4, a commercial CFD software package, by adding user-defined Fortran subroutines. Numerical simulations together with typical experimental measurements show that pressure fluctuations originate above the distributor when a gas pulse is injected into the fluidized bed. The pressure above the bubble gradually increases due to the presence of a rising bubble. When the bubble passes through the bed surface, the pressure near the bed surface gradually decreases to a lower value. Moreover, the pressure signals in the bubbling fluidized beds show obviously periodic characteristics. The major frequency of pressure fluctuations at the same vertical position is affected slightly by the operating gas velocity, and the amplitude of pressure fluctuations is related to both the operating gas velocity and the vertical height. In this study, the influence of the operating gas velocity on the pressure wave propagation velocity can be ignored, and only two peak frequencies in the power spectrum of the pressure fluctuations are observed which are associated with the bubble formation above the distributor and its eruption at the bed surface.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.U20B2059 and 61627901)the Foundation for Innovative Research Groups of the National Natural Science Foundation of China(Grant No.61621005)+4 种基金the Natural Science Foundation of Shaanxi Province of China(Grant No.2019JM-206)the National Key Laboratory Foundation of China(Grant No.HTKJ2019KL504014)the Fundamental Research Funds for the Central Universities,Chinathe Innovation Fund of Xidian University and the 111 Project(Grant No.B17035)。
文摘The attenuation characteristics of obliquely incident electromagnetic(EM) wave in L-Ka frequency band in weakly ionized dusty plasma are analyzed based on the modified Bhatnagar-Gross-Krook(BGK) collision model.According to the kinetic equation and the charging theory,the total complex dielectric constant of the weakly ionized dusty plasma is derived by considering that the minimum velocity of the electron accessible to the dust particle surface is non-zero and the second potential part of the collision cross-section contributes to the charging.The attenuation characteristics within the modified model are compared with those within the traditional model.The influence of the dusty plasma parameters and the incident angle of EM waves on the attenuation in weakly ionized dusty plasma is further analyzed.Finally,the influence of different reentry heights on the attenuation characteristics of the obliquely incident EM wave is discussed.The results show that the effect of the minimum electron velocity and the second term of the collision cross-section on the attenuation characteristics of EM waves cannot be ignored.When the dust density and dust radius are changed,the trends of the attenuation of obliquely incident EM waves are consistent,but the influence of dust density is weaker than that of dust radius due to the constraint of orbit-limited motion(OLM) theory.The plasma thickness,electron density,and incident angle are proportional to the attenuation amplitude of EM waves.The effect of different reentry heights on the attenuation obliquely incident EM waves is related to the electron density and plasma thickness.
基金the Joint Laboratory of Plasma Application Technology Funding,China(No.JL06120001H)the Key R&D Plan of Anhui Province,China(No.201904a07020013).
文摘Research on terahertz communication under different vehicles has important guiding significance for the design of future hypersonic vehicle and Radio Frequency(RF)blackout.In this paper,a joint simulation model of plasma flow under thermochemical nonequilibrium state and terahertz transmission is developed to investigate the differences in terahertz wave transmission characteristics under different vehicle shapes and the related mechanisms.By comparing the plasma sheath characteristics and terahertz transmission among HIFIRE-5b(Hypersonic International Flight Research Experimentation-5b),RAM C(Radio Attenuation Measurement C)and ARD(Atmospheric Reentry Demonstrator)vehicles,it is found that the sheath thickness and electron density of ARD vehicles is significantly larger than that of HIFIRE-5b and RAM C vehicles,resulting in greater terahertz wave attenuation.Collision absorption plays a major role in the terahertz attenuation of vehicles,and the contribution of reflection effects is only observed in the ARD vehicle due to its larger plasma sheath thickness and spatial structure variation.Based on the above comparison results,a shape design scheme of reducing the vehicle head and tail for mitigating RF blackout is proposed,and the scheme is proved by further analyzing the effects of different vehicle heads and tails on the terahertz communication.With the decrease of the head radius and tail width of hypersonic vehicle,the wave attenuation at the same terahertz frequency decreases,and the contribution of reflection effect to wave attenuation gradually disappears.Therefore,the shape design scheme of reducing the vehicle head and tail can effectively alleviate the RF blackout problem,which provides an important reference value for future hypersonic vehicle design.
文摘高超声速飞行器等离子鞘套由于湍流、姿态调整、非均匀烧蚀等因素的影响,其等离子参数具有一定的随机特性。随机媒质的时域有限差分法(Stochastic Finite Difference Time Domain,SFDTD)可用于快速分析随机媒质中电波传播的统计特性。本文将普通媒质的S-FDTD方法扩展到等离子色散媒质,并采用该方法分析了典型鞘套电子密度分布下,电子密度随机特性对电波传播的影响。结果表明:电子密度的随机变化会引起透射电磁波的幅度与相位的抖动,抖动幅度与电子密度变化幅度具有线性相关性;等离子碰撞频率越高,相同强度的电子密度变化引起的透射系数幅度与相位变化越小;提高入射波频率,有助于有减少透射电磁波幅度与相位抖动。
基金The present work is part of a 3-year research project sponsored by the Engineering and Physical Sciences Research Council(EPSRC)under Grant EP/N021746/1 and the National Natural Science Foundation of China(NSFC)under Grant 51.
文摘Injection-induced rail pressure fluctuations are proven to cause nonuniform spray development.These fluctuations are also responsible for generating lower injection pressures,to the detriment of jet penetration length and break-up timing.Despite the vast literature dealing with such issues,several aspects of rail pressure fluctuations remain unclear.Additionally,the need for compliance with the emission legislation has shed light on the potential of alternative fuels,which represent a pathway for sustainable mobility.This scenario has motivated the present study dealing with the assessment of the time history of rail pressure correlated with fuel properties.Tests have been performed using a last-generation common rail injection equipment under various injection settings,employing diesel and 2-methylfuran-diesel blend.This paper describes the research activity and aims to provide new insights into the correlation of rail pressure fluctuations with fuel properties.