The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery(ADV)measurements.The flow characteristics such as the energetics and momentum tr...The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery(ADV)measurements.The flow characteristics such as the energetics and momentum transfer derived from convcntional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch.Consistent with numerous canopy flow experiments,a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin-Helmholtz instabilities after the flow equilibrates with the vegetated layer.These in stabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies.The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed.For a preset water depth,the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities.The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation.The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top.However,the momentum flux carried by sweeps is larger below the top of the canopy.展开更多
随着大数据、云计算等互联网新技术的发展,对传统网络的带宽、速率、安全等方面提出了更高的要求,面对日益复杂的网络环境,传统网络在可扩展性、可编程性、灵活性等方面面临瓶颈。软件定义网络SDN(Software Defined Network)将数据的控...随着大数据、云计算等互联网新技术的发展,对传统网络的带宽、速率、安全等方面提出了更高的要求,面对日益复杂的网络环境,传统网络在可扩展性、可编程性、灵活性等方面面临瓶颈。软件定义网络SDN(Software Defined Network)将数据的控制面和转发面分离,向上为应用层提供可编程接口,向下统一管理网络设备,一经提出便成为下一代互联网研究的热门方向。SDN技术的集中性和开放性也带来新的安全挑战,控制器本身的安全、应用层安全、数据通道安全等目前都缺乏有效的解决方案。本文阐述了SDN的优势与特点,面临的安全风险、一些增强安全的方式;然后提出一种基于SDN技术抗分布式拒绝服务攻击(DDoS,Distributed Denial of Service)的设计,最后对SDN安全问题进行思考和总结。展开更多
The meander channel is one of the most common channel patterns in nature.The characteristics of the flow and sediment in a meander channel which have significant effect on the development of watercourse are important ...The meander channel is one of the most common channel patterns in nature.The characteristics of the flow and sediment in a meander channel which have significant effect on the development of watercourse are important subjects in river dynamics.The transition of the flow patterns in a meander channel concerns with the development mode of the channel pattern and the river regime including the generation conditions of the three-dimensional coherent vortex and secondary flow,the hierarchical scale of coherent vortex in different flow conditions,the large-scale turbulent eddy structure adapted to a meander,etc.In this paper we study the laminar flow instability of the two-dimensional channel in a meander channel.It is essentially different from that in a straight channel:The neutral curve will move forward and the critical Reynolds number will decrease.The flow is unstable in response to a wider range of the disturbance wave number,or the laminar flow instability can happen more easily.The above results could not be obtained in the traditional hydrodynamic stability theory so that our work in this paper would make up for the deficiency and blank in this aspect.展开更多
基金the National Natural Science Foundation of China(Grant Nos.51439007,11672213,11872285 and 51809286).
文摘The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery(ADV)measurements.The flow characteristics such as the energetics and momentum transfer derived from convcntional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch.Consistent with numerous canopy flow experiments,a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin-Helmholtz instabilities after the flow equilibrates with the vegetated layer.These in stabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies.The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed.For a preset water depth,the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities.The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation.The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top.However,the momentum flux carried by sweeps is larger below the top of the canopy.
文摘随着大数据、云计算等互联网新技术的发展,对传统网络的带宽、速率、安全等方面提出了更高的要求,面对日益复杂的网络环境,传统网络在可扩展性、可编程性、灵活性等方面面临瓶颈。软件定义网络SDN(Software Defined Network)将数据的控制面和转发面分离,向上为应用层提供可编程接口,向下统一管理网络设备,一经提出便成为下一代互联网研究的热门方向。SDN技术的集中性和开放性也带来新的安全挑战,控制器本身的安全、应用层安全、数据通道安全等目前都缺乏有效的解决方案。本文阐述了SDN的优势与特点,面临的安全风险、一些增强安全的方式;然后提出一种基于SDN技术抗分布式拒绝服务攻击(DDoS,Distributed Denial of Service)的设计,最后对SDN安全问题进行思考和总结。
基金supported by the National Basic Research Program of China ("973" Program) (Grant No. 2007CB714101)the National Natural Science Foundation of China (Grant Nos. 50979066, 50809045, 51021004)
文摘The meander channel is one of the most common channel patterns in nature.The characteristics of the flow and sediment in a meander channel which have significant effect on the development of watercourse are important subjects in river dynamics.The transition of the flow patterns in a meander channel concerns with the development mode of the channel pattern and the river regime including the generation conditions of the three-dimensional coherent vortex and secondary flow,the hierarchical scale of coherent vortex in different flow conditions,the large-scale turbulent eddy structure adapted to a meander,etc.In this paper we study the laminar flow instability of the two-dimensional channel in a meander channel.It is essentially different from that in a straight channel:The neutral curve will move forward and the critical Reynolds number will decrease.The flow is unstable in response to a wider range of the disturbance wave number,or the laminar flow instability can happen more easily.The above results could not be obtained in the traditional hydrodynamic stability theory so that our work in this paper would make up for the deficiency and blank in this aspect.
