Calculation of the flow about a sphere and the drag of the sphere under laminar and strongly turbulent conditions

To investigate the influence of a strongly turbulent incoming flow on the hydrodynamic drag of a body and occurrence of the early crisis of drag, a numerical experiment is conducted in which a free gas flow about a sphere is simulated for two cases, namely, for a laminar flow and for a strongly turbulent flow. Turbulence is simulated by assuming a high kinematic coefficient of turbulent viscosity. Calculation data lead us to conclude that the early crisis of drag at Reynolds numbers near 100, which shows up as a considerable (four-to sevenfold) decrease in the hydrodynamic force and the drag coefficient of the body, can be explained by the strong turbulence of the incoming flow.     

Effect of the gas flow geometry and turbulence on the hydrodynamic drag of a body in the flow

The effect of the incoming flow geometry on the hydrodynamic drag of a body is investigated in a numerical experiment simulating a free gas flow past a sphere as well as flows in cylindrical tubes of various radii, in a confuser, and a diffuser. The results of calculations lead to the conclusion that the confinement of the flow by the tube walls, its contraction and expansion may change the hydrodynamic force and the drag acting on the body insignificantly (not more than by 30%). This cannot explain the early drag crisis, in which the values of these quantities decrease by 4–7 times for Reynolds numbers on the order of 100. This phenomenon is explained theoretically by the effect of strong turbulence of the incoming flow to the body.     

折叠翼面展开过程中气动载荷特性数值模拟

针对某飞行器翼面高速展开过程,采用定常和非定常数值模拟两种不同方法,进行折叠翼面展开过程气动载荷分析研究,其中定常方法主要研究翼面从折叠到展开过程中不同展开角下外翼面的气动特性,分析展开角、来流参数对外翼面气动力的影响;非定常方法主要模拟折叠翼面展开角速度变化,从而获得典型工况下翼面展开过程的载荷情况,分析非定常效应对气动载荷的影响.研究发现,当翼面展开速度与来流速度相近时,则非定常效应不可忽略而必须采用非定常模拟方法.      

确定分布的展向Lorentz力调制下的槽道湍流涡结构

采用直接数值模拟方法,对槽道湍流中确定分布的Lorentz力的流动控制与减阻问题进行研究.讨论了Lorentz力作用于槽道湍流后,流场的特性和涡结构的特性,并对此类Lorentz力对槽道湍流的控制与减阻机理进行了讨论.研究发现:1)Lorentz力诱导的层流流场壁面附近存在梯度极大的展向速度剪切层,该剪切层容易形成流向涡结构;2)在给定合适参数的确定分布的Lorentz力作用下,湍流流场仅剩周期分布的准流向涡;3)与未控制流场相比,控制后的流场中,准流向涡的抬升高度大大降低,从而减小猝发强度,使壁面阻力下降.     

Flow characteristics of supersonic gas passing through a circular micro-channel under different inflow conditions

Gas flow in a micro-channel usually has a high Knudsen number. The predominant predictive tool for such a microflow is the direct simulation Monte Carlo(DSMC) method, which is used in this paper to investigate primary flow properties of supersonic gas in a circular micro-channel for different inflow conditions, such as free stream at different altitudes, with different incoming Mach numbers, and with different angles of attack. Simulation results indicate that the altitude and free stream incoming Mach number have a significant effect on the whole micro-channel flow field, whereas the angle of attack mainly affects the entrance part of micro-channel flow field. The fundamental mechanism behind the simulation results is also presented. With the increase of altitude, thr free stream would be partly prevented from entering into micro-channel.Meanwhile, the gas flow in micro-channel is decelerated, and the increase in the angle of attack also decelerates the gas flow. In contrast, gas flow in micro-channel is accelerated as free stream incoming Mach number increases. A noteworthy finding is that the rarefaction effects can become very dominant when the free stream incoming Mach number is low. In other words, a free stream with a larger incoming velocity is able to reduce the influence of the rarefaction effects on gas flow in the micro-channel.     

Numerical simulation and experimental validation of multiphysics field coupling mechanisms for a high power ICP wind tunnel

We take the established inductively coupled plasma(ICP) wind tunnel as a research object to investigate the thermal protection system of re-entry vehicles. A 1.2-MW high power ICP wind tunnel is studied through numerical simulation and experimental validation. The distribution characteristics and interaction mechanism of the flow field and electromagnetic field of the ICP wind tunnel are investigated using the multi-field coupling method of flow, electromagnetic, chemical, and thermodynamic field. The accuracy of the numerical simulation is validated by comparing the experimental results with the simulation results. Thereafter, the wind tunnel pressure, air velocity, electron density, Joule heating rate, Lorentz force, and electric field intensity obtained using the simulation are analyzed and discussed. The results indicate that for the 1.2-MW ICP wind tunnel, the maximum values of temperature, pressure, electron number density, and other parameters are observed during coil heating. The influence of the radial Lorentz force on the momentum transfer is stronger than that of the axial Lorentz force. The electron number density at the central axis and the amplitude and position of the Joule heating rate are affected by the radial Lorentz force. Moreover, the plasma in the wind tunnel is constantly in the subsonic flow state, and a strong eddy flow is easily generated at the inlet of the wind tunnel.     

Development of the method of an electrohydrodynamic force effect on a boundary layer for active control of aerodynamic flows

The results of investigations on the possibility of an electrohydrodynamic force effect on a gas flow implemented with the help of a barrier discharge are presented. A new method of controlling the laminar flow around a base with suppression of instabilities of the incoming flow due to electrohydrodynamic force action on the boundary layer near the forward edge of a swept wing is proposed. An efficient multidischarge actuator system is developed and created for active control of aerodynamic flows with induced-air-flow characteristics exceeding the world analogues.     

槽道湍流展向振荡电磁力控制的实验研究

对槽道湍流的展向振荡电磁力控制进行了实验研究,讨论了展向振荡电磁力对宏观流场、近壁湍流结构以及壁面阻力的影响.采用谱方法进行了数值模拟的对比.数值模拟和实验结果均表明展向振荡电磁力能够使近壁区域的宏观流场产生周期性振荡,并影响壁湍流的条带结构,使其在展向上发生倾斜,从而使壁面阻力减小.     

平板前缘分离抑制的周期性激励研究

本文采用大涡模拟对平板前缘分离在周期性来流激励作用下的流动结构进行了数值研究。将计算得到的结果与实验数据进行了对比。在此基础上,对涡量等值面和Q准则可视化手段进行了比较。通过研究不同外部周期性激励频率抑制分离的效果,讨论了有效外部扰动频率与分离区域特征频率的关系。结果显示,外部频率为f_(shear)、f_(shed)、1/2f_(shear_时分离抑制明显,其中1/2f_(shear)效果最佳。当分离区域显著减小时,流动具有明显的周期性;当在外部扰动作用下的分离抑制效果很弱时,外部扰动无法直接在分离区域中促成对应尺度的涡生成。     

循环流化床气固曳力模型

气固曳力是稠密气固两相流动,尤其是垂直流动中的主要作用力,相应的模型也是数值模拟中准确描述气固两相运动的关键.为了解决现有经验或半经验模型的普适性问题,合理描述流动中经常发生的颗粒团聚现象及其对气固曳力的影响,从理论分析入手,运用最小能量的概念,将传统的CFD方法与宏观的系统分析方法相结合,建立了一个新的计及颗粒团聚效应的气固曳力理论模型.与现有模型相比,新模型不仅具有相同的函数变化关系,可合理地描述气固两相相互作用的物理过程,而且避免了以往经验系数不准确导致的各种误差,为稠密气固两相流动的数值描述提供了重要依据.     

槽内热磁耦合流动换热数值模拟

数值模拟研究了矩形槽内导电流体由于焦耳热作用和电磁力共同作用引起的流动换热现象.数值结果表明,在给定流体性质情况下,焦耳热作用引起对流为两涡,电磁力作用时获得四涡流动,随Ha数的增加,电磁力驱动对流作用增大,热、磁共同作用时,流场温度场与Ha2Pr/Ra大小有关,从而影响到对流换热的强弱,在临界Ha2Pr/Ra以下,焦耳热引起的对流为主,Ha数增加,减弱换热;在临界日Ha2Pr/Ra以上,电磁力驱动的对流为主,Ha数增加,换热强化.     

Quasi-periodic relaxation to a steady flow regime after pulsed energy input into the incoming stream

Numerical simulation was used to analyze the quasi-periodic process induced against the steady flow pattern around a blunt forebody by a pulse from an external source located in the incoming stream.     

Hydrodynamic binary coalescence of droplets under air flow in a hydrophobic microchannel

Based on the volume of fluid(VOF) method, we conduct a numerical simulation to study the hydrodynamic binary coalescence of droplets under air flow in a hydrophobic rectangular microchannel. Two distinct regimes, coalescence followed by sliding motion and that followed by detaching motion, are identified and discussed. Additionally, the detailed hydrodynamic information behind the binary coalescence is provided, based on which a dynamic mechanical analysis is conducted to reveal the hydrodynamic mechanisms underlying these two regimes. The simulation results indicate that the sliding motion of droplets is driven by the drag force and restrained by the adhesion force induced by the interfacial tension along the main flow direction. The detachment(i.e., upward motion) of the droplet is driven by the lift force associated with an aerodynamic lifting pressure difference imposed on the coalescent droplet, and also restrained by the adhesion force perpendicular to the main flow direction. Especially, the lift force is mainly induced by an aerodynamic lifting pressure difference imposed on the coalescent droplet. Two typical regimes can be quantitatively recognized by a regime diagram depending on Re and We. The higher Re and We respectively lead to relatively larger lift forces and smaller adhesion forces acting on the droplet, both of which are helpful to detachment of the coalesced droplet.     

Feedback control of instability waves in a transitional flat-plate boundary layer

A semi-automatic and active control of T-S waves and oblique waves in a transitional flat plate boundary layer is carried out in a wind-tunnel experiment and a numerical simulation. An array of piezo-ceramic actuators attached on a surface is used to generate counter waves that cancel the incoming instability waves. The actuator’s operating amplitudes and phases are successively updated using the velocity fluctuations monitored downstream by a rake of hotwires. Experimental results show that the system is effective in weakening these waves when their inclination angles are less than 15 degrees. However, the system encountered difficulty in controlling the waves of large inclination angles. In the numerical simulation, it is shown that the control can be accomplished much easier. The numerical results show that controllability of the large inclination angle waves can be improved by shortening the spanwise length each actuator piece. The danger of pursuing this kind of research solely by a numerical simulation is pointed out.     

A morphological study of vicinal surface growth based on a hybrid algorithm

A hybrid algorithm that combines a phase-field model and a lattice gas model evolving according to a kinetic Monte-Carlo (KMC) simulation scheme is used to investigate the dynamics of vicinal surface growth during vapor phase epitaxy. The algorithm is computationally far more efficient than pure KMC schemes, and this gain in efficiency does not correspond to a loss in information on the kinetics of individual atoms. We present numerical studies on the temperature dependence of macroscopic properties of the growing surface, evaluating the relevant stochastic processes (attachment, detachment, diffusion and island dynamics) as a function of their rates. We show that the temperature at which step flow is replaced by island nucleation depends on incoming flux, diffusion parameters and interstep distance. Moreover, we validate these finding by comparison to experiments and by analytical investigations.     

Bow shock formation in a complex plasma

A bow shock is observed in a two-dimensional supersonic flow of charged microparticles in a complex plasma. A thin conducting needle is used to make a potential barrier as an obstacle for the particle flow in the complex plasma. The flow is generated and the flow velocity is controlled by changing a tilt angle of the device under the gravitational force. A void, microparticle-free region, is formed around the potential barrier surrounding the obstacle. The flow is bent around the leading edge of the void and forms an arcuate structure when the flow is supersonic. The structure is characterized by the bow shock as confirmed by a polytropic hydrodynamic theory as well as numerical simulation.     

颗粒速度在颗粒流稀疏流-密集流转变中的作用

用实验和计算模拟的方法研究了颗粒流中的颗粒速度与颗粒流特性的关系.实验研究发现当入口流量固定时，在出口上方高速运动的颗粒会使颗粒流由稀疏流向密集流转变的临界出口尺寸变小.当颗粒流转变为密集流后，颗粒速度的作用被出口上方的颗粒堆积区所消耗，最终变得与颗粒速度无关.二维分子动力学模拟计算得到了与实验相同的结论.通过二维分子动力学模拟计算，还给出了不同颗粒速度下体系的密度和速率在空间的分布图.这些分布图显示随着颗粒到达出口上方的瞬间速度的不同，颗粒堆积区的密度和高度均会改变，并最终导致颗粒流流动状态的改变. 关键词： 颗粒流 颗粒气体 分子动力学模拟     

Evolution of melt convection in a liquid metal driven by a pulsed electric current

Gain refinement in metal alloy can be achieved by applying an electric current pulse (ECP) in solidification process. Forced flow inside the melt has been proved to be a key role in grain refinement. In this paper, the fluid flow inside Ga 20 wt%-In 12 wt%-Sn alloy induced by a damping sinusoidal ECP flowing through two parallel electrodes into the cylindrical melt was investigated by both experimental measurements and numerical simulations. Experimental results showed that a strong descending jet was induced beneath the bottom of electrodes under the application of ECP. Besides, it was found that flow intensity increases with the increase of amplitude, frequency, and pulse width, respectively. In order to unlock the formation mechanism of flow pattern and the relevance of flow intensity varied with electrical parameters, a three-dimensional numerical model under the application of ECP was established. Meanwhile, a comparative study was conducted by numerical simulations to reveal the distributions of electromagnetic fields and forced flow. Numerical results showed that the downward Lorentz force induced by ECP was concentrated beneath the bottom of electrodes. This downward Lorentz force induces a descending jet and provokes a global forced flow. According to numerical simulations, the evolution of flow intensity with electrical parameters under the application of ECP can be understood by the time averaged impulse of Lorentz force.     

Calculations of the flow resistance and heat emission of a sphere in the laminar and high-turbulent gas flows

An early drag crisis can occur at high turbulence of incoming gas flow to a sphere. To study the influence of a crisis on heat transfer from a sphere to gas, a numerical experiment was carried out in which the free gas flow around a sphere with a temperature lower than the sphere temperature was simulated for two cases. The flow was laminar in the first case and highly turbulent in the second case. To take into account turbulence, the kinematic coefficient of turbulent viscosity with a value, which is much higher (up to 2000 times) than that for physical viscosity, was introduced. The results of calculations show that the early drag crisis occurs at Reynolds numbers of about 100 and results in considerable (by four to seven times) decrease in the hydrodynamic force and sphere drag coefficient C _d . The early drag crisis is also accompanied by the crisis of heat transfer from a sphere to gas with a decrease in Nusselt numbers Nu by three to six times.     

Passenger management strategy and evacuation in subway station under Covid-19

Under the background of Covid-19 sweeping the world, safe and reasonable passenger flow management strategy in subway stations is an effective means to prevent the spread of virus. Based on the social force model and the minimum cost model, the movement and path selection behavior of passengers in the subway station are modeled, and a strategy for passenger flow management to maintain a safe social distance is put forward. Take Qingdao Jinggangshan Road subway station of China as the simulation scene, the validity of the simulation model is verified by comparing the measured value and simulation value of the time required for passengers from getting off the train to the ticket gate. Simulation results indicate that controlling the time interval between incoming passengers at the entrance can effectively control the social distance between passengers and reduce the risk of epidemic infection. By comparing the evacuation process of passengers under different initial densities, it is found that the greater the initial density of passengers is, the longer the passengers are at risk social distance. In the process of passenger emergency evacuation, the stairs/escalators and ticket gates are bottleneck areas with high concentration of passenger density, which should be strictly disinfected many times on the basis of strictly checking the health code of incoming passengers and controlling the arrival time interval. The simulation results of this paper verify the harmfulness of passenger emergency evacuation without protective measures, and provide theoretical support for the operation and management of subway station under the epidemic situation.