壁面展向匀速运动的槽道湍流的直接数值模拟

为了解展向存在匀速流动时湍流槽道流动的特性,应用直接数值模拟方法研究了壁面的展向运动对湍流槽道流动的影响.采用标准谱方法对固定质量流率条件下,上壁面展向运动速度分别等于零、流向流量平均速度以及两倍于流向流量平均速度等3种情况进行了直接数值模拟.结果表明,壁面的展向运动会导致表面摩擦力的增大,二阶湍流统计量诸如均方根速度脉动、雷诺剪切应力和均方根涡量脉动等都会随着壁面展向运动速度的增大而显著增长,同时,槽道截面上的均方根速度脉动也变得更均匀.      

聚合物槽道湍流的直接数值模拟

应用直接数值模拟方法研究了聚合物对槽道湍流的减阻特性.数值计算方法采用谱方法,时间积分采用二阶精度的时间分裂格式.应用FENE-P模型对聚合物槽道湍流进行定压降的直接数值模拟计算,揭示了聚合物湍流减阻的启动(onset)现象;应用O-B模型进行定流量的直接数值模拟计算,分析了其流动特征.计算结果表明:在低减阻形式中聚合物对湍流的影响主要发生在壁面区,而高减阻形式中聚合物对湍流的影响则遍及全流场.     

TVD格式与高超音速气动加热数值模拟

为了在化学非平衡流动中获得准确的流场解以及表面热流分布,将总变差减小TVD(Total Variation Diminishing)格式中的熵修正函数,由各向同性分布改为各向异性分布,同时让熵修正函数中的参数与流场中的压力梯度分布相关.将改进后的熵修正函数运用到高超音速化学非平衡绕流流场的数值模拟中,获得了较使用原有熵修正函数更为准确的流场参数和表面热流分布.采用改进的熵修正函数,可以提高壁面附近的粘性分辨率,降低热流计算结果对壁面附近法向网格尺度的敏感性.      

涡旋微槽流动与传热特性数值模拟

涡旋微槽散热器具有传输高热流密度的潜力,在解决航空航天高功率密度器件热控制方面具有广泛应用前景.在实验研究的基础上,采用有限体积法对不同体积流量和槽道结构的涡旋微槽中的流动与传热特性进行了数值模拟研究.对涡旋微槽流动的稳定性进行了分析,给出了摩擦因子和Nu数沿流动方向的变化曲线,并采用场协同原理对涡旋微槽强化传热的机理进行了探讨.计算得到的微槽平均传热系数和摩擦阻力系数与实验数据进行了对比.结果表明:涡旋微槽中二次流的出现是涡旋微槽强化传热的机理所在.     

基于唯象模型的等离子激励诱导流场数值模拟

通过在Navier-Stokes方程组中添加体积力源项的方法,模拟了二维不可压流中等离子体激励器对周围流场的诱导作用.计算结果表明等离子体激励器在其下游会产生一股贴近物面的壁面射流,该射流各截面的速度型、最大速度衰减、射流宽度增长等均与典型的层流射流(壁面射流或二维狭缝射流)一致,表明等离子体激励对周围中性流体的诱导是一种势流影响.最后通过积分得到了等离子体诱导壁面射流的吹气系数,通过与传统吹气流动控制技术比较,指出目前等离子体激励产生的壁面射流吹气系数较小,仅适用于低速条件下的流动控制.     

流向磁场作用下二维磁流体槽道湍流直接模拟

对低磁雷诺数近似下流向磁场作用的二维磁流体槽道湍流进行了直接数值模拟(DNS,Direct Numerical Simulation),给出了 Re=10 000时不同磁相互作用数下二维磁流体槽道湍流的近壁速度分布、湍流脉动速度均方根、雷诺应力等统计量的变化,并与中性流体二维槽道湍流进行了比较.结果表明:流向磁场作用会导致对数区上移;雷诺应力最大值随磁相互作用数呈线性变化;随着磁相互作用数的增大,下壁面平均涡量的时间演化由拟周期性向周期性转变,且脉动周期逐渐增大并当流动层流化后下壁面平均涡量成为常值.      

非均质壁面对液滴俘获能力的数值模拟研究

为研究不同非均质壁面对液滴的俘获能力,采用界面追踪法（FTM）结合广义滑移边界建立了接触角模型,对液滴在非均匀润湿的非均质壁面上的运动过程进行了数值模拟研究。液滴在倾斜壁面上受到重力作用由均匀润湿部分下滑至非均匀润湿部分,通过改变Bo数、Oh数、非均匀润湿程度研究了液滴在非均匀润湿区域的运动规律。研究表明:Bo数越大,液滴运动受壁面阻力影响越小,液滴下滑的速度越快,液滴越难以被俘获;Oh数越大,液滴运动受壁面阻力影响越小,液滴越难以被俘获;非均匀润湿程度越大,非均质壁面对液滴的阻力越大,液滴越易被俘获。      

三维计算流体力学流场的流线构造

流线的计算与显示是流场可视化中的一项基本技术.通过将CFD(Computational Fluid Dynam ics)的计算网格剖分为四面体单元并设置相邻四面体之间的拓扑关系,运用基于四面体侧面 法矢的"指南针"法进行快速点定位,采用自适应步长的数值积分方法直接在物理空间中进 行流线的追踪,避免了物理空间和计算空间之间的转换以及由此所带来的误差,提高了流线 追踪的精度和效率.      

Shear flow energy redistribution stipulated by the internal-gravity wavy structures in the dissipative ionosphere

The linear mechanism of generation, intensification and further nonlinear dynamics of internal gravity waves (IGW) in stably stratified dissipative ionosphere with non-uniform zonal wind (shear flow) is studied. In case of the shear flows the operators of linear problem are non-selfadjoint, and the corresponding Eigen functions – nonorthogonal. Thus, canonical – modal approach is of less use studying such motions. Non-modal mathematical analysis becomes more adequate for such problems. On the basis of non-modal approach, the equations of dynamics and the energy transfer of IGW disturbances in the ionosphere with a shear flow is obtained. Exact analytical solutions of the linear as well as the nonlinear dynamic equations of the problem are built. The increment of shear instability of IGW is defined. It is revealed that the transient amplification of IGW disturbances due time does not flow exponentially, but in algebraic – power law manner. The effectiveness of the linear amplification mechanism of IGW at interaction with non-uniform zonal wind is analyzed. It is shown that at initial linear stage of evolution IGW effectively temporarily draws energy from the shear flow significantly increasing (by an order of magnitude) own amplitude and energy. With amplitude growth the nonlinear mechanism turns on and the process ends with self-organization of nonlinear solitary, strongly localized IGW vortex structures (the monopole vortex, the transverse vortex chain or the longitudinal vortex street). Accumulation of these vortices in the ionospheric medium can create the strongly turbulent state.     

A retarding potential analyzer for rocket experiments

A Retarding Potential Analyzer (RPA) is designed using a micro-channel plate in front of the collector as a flow amplifier, which makes it possible to detect ion species with very low concentration (<0,5 cm.^?3). A technique is described for using the micro-channel plate at abnormally high pressure (>0.01 N/m²). This allows the precise determination of the mass composition of the low ionosphere. An adaptive retarding potential consisting of 2048 steps is used to increase the sensitivity. This retarding potential is applied in the range where dI/dU ≠ 0. The sensor is hermetically sealed and opens on command.     

北美与东亚中纬地区电离层TEC变化的EOF建模和分析

考察了北美(30°N-50°N, 140°W-50°W)与东亚 (42.5°N-57.5°N, 65°E-140°E) 中纬地区电离层总电子含量 (TEC)的变化. TEC数据来自美国喷气动力实验室(JPL)约15年的全球电离层图 (Global Ionospheric Map, GIM)数据. 利用经验正交函数(Empirical Orthogonal Functions, EOF)得到上述两地区前三阶本征模及相关时间系数, 分别约占TEC 总变化的99.57%和99.79%. 结果表明, 两地区前三阶EOF分量所表现出的TEC变 化基本一致. 第一阶EOF分量表现为受太阳活动调制的半年变化; 第二阶EOF 分量表现为关于零磁偏线的经向电子浓度东西不对称结构, 计算表明该结构 与受地磁偏角控制的热层纬向水平风引起的等离子体向上漂移密切相关; 第三 阶EOF分量表现为磁倾控制的热层子午向风引起的等离子体向上漂移影响.      

基于共振三波的大涡模拟壁面模型

大涡模拟的一个重要发展方向是处理工程流动问题,但高雷诺数下近壁湍流边界层对计算网格的过高要求成为制约其应用的主要瓶颈;要减少计算量,构建合理且能够正确反映近壁湍流动力学特性的壁面模型是一条有效途径.在此背景下,构造了一种计算中便于实现的基于共振三波的壁面模型.通过对充分发展槽道湍流算例的研究,完成了对该模型构造方法、方案的论证和验证计算.结果表明使用壁面模型后,在减少计算量的同时,定性和定量的结果均较为合理,能够得到重要的瞬态特征结构和正确的一阶、二阶统计量,从而部分验证了该方法的有效性.     

二维翼型绕流的CE/SE方法

采用时空守恒元和解元CE/SE(space-time Conservation Element and Solution Element method)法,求解二维Euler方程,开展了翼型绕流的无粘数值模拟研究.用非敏感克朗数计算格式消除克朗数过小引起的数值耗散对解的污染,结合当地时间步长法,解决网格不均匀引起的当地克朗数变化跨度大的问题.对NACA0012翼型的无激波流场进行了二维数值模拟,并与AGARD算例做了对比.结果表明:CE/SE方法的计算结果与AGARD结果吻合得很好,为该数值计算方法对翼型绕流数值模拟的进一步应用奠定了基础.      

Cycle slip detection using multi-frequency GPS carrier phase observations: A simulation study

The detection and repair of the cycle slip or gross error is a key step for high precision global positioning system (GPS) carrier phase navigation and positioning due to interruption or unlocking of GPS signal. A number of methods have been developed to detect and repair cycle slips in the last two decades through cycle slip linear combinations of available GPS observations, but such approaches are subject to the changing GPS sampling and complex algorithms. Furthermore, the small cycle slip and gross error cannot be completely repaired or detected if the sampling is quite longer under some special observation conditions, such as Real Time Kinematic (RTK) positioning. With the development of the GPS modernization or Galileo system with three frequencies signals, it may be able to better detect and repair the cycle slip and gross error in the future. In this paper, the cycle slip and gross error of GPS carrier phase data are detected and repaired by using a new combination of the simulated multi-frequency GPS carrier phase data in different conditions. Results show that various real-time cycle slips are completely repaired with a gross error of up to 0.2 cycles.     

Evaluation of global modeling of M(3000)F2 and hmF2 based on alternative empirical orthogonal function expansions

Global modeling of M(3000)F2 and hmF2 based on three alternative EOF (empirical orthogonal function) expansion methods is described briefly. Data used for the model construction is the monthly median hourly values of M(3000)F2 from the ionosonde/digisonde stations distributed around the world for the period of 1975–1985 and the hmF2 data of the same period converted from the measured M(3000)F2 based on the strong anti-correlation existing between them. Independent data of a low (1965) and a high (1970) solar activity year are used to validate the three alternative models based on different EOF expansion methods. Comparisons between the modeled results and observed data for both the low (1965) and high (1970) solar activity years showed good agreement for both M(3000)F2 and hmF2 parameters. Statistical analysis on the differences between model values and observed data showed that all the three alternative models (Model A, B and C) based on the different EOF expansion methods have better agreement with the observed data than the models currently used in IRI. All three alternative EOF based models have almost the same accuracy. Discussion on the preference of the three alternative EOF based models is given.     

Simulation of the environmental climate conditions on martian surface and its effect on Deinococcus radiodurans

The resistance of terrestrial microorganisms under the thermo-physical conditions of Mars (diurnal temperature variations, UV climate, atmospheric pressure and gas composition) at mid-latitudes was studied for the understanding and assessment of potential life processes on Mars. In order to accomplish a targeted search for life on other planets, e.g. Mars, it is necessary to know the limiting physical and chemical parameters of terrestrial life. Therefore the polyextremophile bacterium Deinococcus radiodurans was chosen as test organism for these investigations. For the simulation studies at the Planetary and Space Simulation Facilities (PSI) at DLR, Cologne, Germany, conditions that are present during the southern summer at latitude of 60° on Mars were applied.We could simulate several environmental parameters of Mars in one single experiment: vacuum/low pressure, anoxic atmosphere and diurnal cycles in temperature and relative humidity, energy-rich ultraviolet (UV) radiation as well as shielding by different martian soil analogue materials. These parameters have been applied both single and in different combinations in laboratory experiments. Astonishingly the diurnal Mars-like cycles in temperature and relative humidity affected the viability of D. radiodurans cells quite severely. But the martian UV climate turned out to be the most deleterious factor, though D. radiodurans is red-pigmented due to carotenoids incorporated in its cell wall, which have been assigned not only a possible role as free radical scavenger but also as a UV-protectant. An additional UV-protection was accomplished by mixing the bacteria with nano-sized hematite.