一种全堆芯精确到每个通道的子通道并行模拟方法

为实现全堆芯精确到每个通道的并行子通道模拟,本文提出一种基于子通道的并行任务划分和进程映射方法,可对全堆芯或单个组件进行计算任务划分,计算任务和进程的映射可灵活进行.该方法可根据计算机(群)的核数确定恰当的全堆芯子通道的任务划分方式,从而使全堆芯热工水力模拟可在单机、小型集群到超级计算机等不同环境运行.在天河二号超级计算机上进行全堆芯157组件、精确到每个真实流道、轴向划分为125层的稳态模拟,可使用4-6280核实现.使用4核时需约22h,使用6280核时需470s.引入混合编程实现方式后,使用6280核完成模拟需397s.

快堆全堆芯热工流体子通道并行模拟技术研究

采用简化堆芯模型的传统子通道模拟计算结果难以精确反映堆芯的真实运行状况,利用高性能计算技术进行全堆芯精确到每个真实流道的子通道模拟计算成为研究热点。本文抽象描述了快堆堆芯的基础几何结构,在此基础上提出了一种全堆芯子通道建模方法和一种自适应的并行任务划分方法。设计了广度优先划分算法和层次划分算法,实现了全堆芯子通道任意个数求解域的划分,自适应地映射到不同个数的计算核上,从而可利用PC、集群、超算等不同规模的计算资源开展全堆并行模拟。使用针对快堆模拟修改后的子通道模拟软件CTF进行验证,证明了建模方法对于快堆子通道模拟是有效的。基于本文方法在曙光先进计算服务平台上使用两种不同网格规模的算例进行了测试,两组测试最低并行效率在33.02%以上,证明了本文方法的有效性和可用性。

Numerical simulation on magnetohydrodynamic power generation channel of scramjet

The reverse magnetohydrodynamic(MHD)energy bypass technology is a promising energy redis⁃tribution technology in the scramjet system,in augmented with a power generation equipment to supply the neces⁃sary long-distance flight airframe power.In this paper,a computational model of the scramjet magnetohydrody⁃namic channel is developed and verified by using the commercial software Fluent.It is found that when the mag⁃netic induction intensity is 1,2,3,4 T,the power generation efficiency is 22.5%,22.3%,22.0%,21.5%,and decreases with the increase of the magnetic induction intensity,and the enthalpy extraction rate is 0.026%,0.1%,0.21%,0.34%,and increases with the increase of the magnetic induction intensity.The deceleration ef⁃fect of electromagnetic action on the airflow in the power channel increases with the increase of magnetic induc⁃tion intensity.The stronger the magnetic field intensity,the more obvious the decreasing effect of fluid Mach num⁃ber in the channel.The power generation efficiency decreases as the magnetic induction intensity increases and the enthalpy extraction rate is reversed.As the local currents gathering at inlet and outlet of the power generation area,total temperature and enthalpy along the flow direction do not vary linearly,and there are maximum and minimum values at inlet and outlet.Increasing the number of electrodes can effectively regulate the percentage of Joule heat dissipation,which can improve the power generation efficiency.

Molecular dynamics study of viscosity of aqueous NaCl solution confined in nanoscale channels

A physical model of bulk-nanochannel-bulk with buffer baths is built up using nonequilibrium molecular dynamics （MD） simulation to study the effects of vibrating silicon atoms on the viscosity of aqueous NaCl solutions confined in the nanochannel. The simulation is performed under different moving speeds of the upper wall, different heights and different surface charge densities in the nanochannel. The simulation results indicate that with the increase in the surface charge density and the decrease in the nanochannel height and the shear rate, the vibration effect of silicon atoms on the shear viscosity of the confined fluid in the nanochannel cannot be ignored. Compared with still silicon atoms, the vibrating silicon atoms result in the decrease in the viscosity when the height of the nanochannel is less than 0.8 nm and the shear rate is less than 1.0 ×10^11 s^-1, and the effect of the vibrating silicon atoms on the shear viscosity is significant when the shear rate is small. This is due to the fact that the vibrating silicon atoms weaken the interactions between the counter-ions （Na^＋ ） and the charged surface.

Vibrating Carbon Nanotubes as Water Pumps

Nanopumps conducting fluids directionally through nanopores and nanochannels have attracted considerable interest for their potential applications in nanofiltration, water purification, and hydroelectric power generation Here, we demonstrate by molecular dynamics simulations that an excited vibrating carbon nanotube （CNT） cantilever can act as an efficient and simple nanopump. Water molecules inside the vibrating cantilever are driven by centrifugal forces and can undergo a continuous flow from the fixed to free ends of the CNT. Further extensive simulations show that the pumping function holds good not only for a single-file water chain in a narrow （6,6） CNT, but also for bulk-like water columns inside wider CNTs, and that the water flux increases monotonically with increasing diameter of the nanotube.

A DLM/FD/IB Method for Simulating Cell/Cell and Cell/Particle Interaction in Microchannels

A spring model is used to simulate the skeleton structure of the red blood cell (RBC) membrane and to study the red blood cell (RBC) rheology in Poiseuille flow with an immersed boundary method. The lateral migration properties of many cells in Poiseuille flow have been investigated. The authors also combine the above methodology with a distributed Lagrange multiplier/fictitious domain method to simulate the interaction of cells and neutrally buoyant particles in a microchannel for studying the margination of particles.

Assessment of High-Frequency Performance Limits of Graphene Field-Effect Transistors

High frequency performance limits of graphene field-effect transistors （FETs） down to a channel length of 20 nm have been examined by using self-consistent quantum simulations. The results indicate that although Klein band-to-band tunneling is significant for sub-100 nm graphene FETs, it is possible to achieve a good transconductance and ballistic on-off ratio larger than 3 even at a channel length of 20 nm. At a channel length of 20 nm, the intrinsic cut-off frequency remains at a few THz for various gate insulator thickness values, but a thin gate insulator is necessary for a good transconductance and smaller degradation of cut-off frequency in the presence of parasitic capacitance. The intrinsic cut-off frequency is close to the LC characteristic frequency set by graphene kinetic inductance （L） and quantum capacitance （C）, which is about 100 GHz-um divided by the gate length.

Ion specificity in NaCl solution confined in silicon nanochannels

Ion specificity of Na＋ and C1- ions for NaCI solution confined in silicon nanochannels is investigated with molecular dynamics （MD） simulations. The MD simulation results demonstrate that ion specificity for Na＋ and C1- ions exhibits clearly in na- nochannels with high surface charge density. The two types of ions show different density distributions perpendicular to the channel surface due to the ion specificity when they act as countefions near negatively and positively charged surfaces, respec- tively. Both the two counterion distributions cannot be predicted by Poisson-Boltzmann equation within 0.75 nm near the sur- face. In addition, the ion specificity is also demonstrated through affecting the water density distributions. In the nanochannel with negatively charged surfaces, the presence of the Na＋ ions reduces the number of water peaks in water density distribution profile. In comparison, when the C1- ions act as counterions near positively charged surfaces, they do not affect the number of the water peaks. Besides the influence on the water density distribution, ion specificity also exhibits through affecting the wa- ter molecule orientation in the adsorbed layer. It is found that C1- ions make the water molecules in the adsorbed layer align more orderly than Na~ ions do when the two types of ions act as the counterions near the positively and negatively charged surfaces with the same surface charge density.