湍流热对流的高效并行直接求解方法

结合泊松方程直接求解的PDD算法,创建二维湍流热对流DNS模拟的并行直接求解方法（PDM-DNS）.在"天河二号"超级计算机上完成高Ra数湍流热对流大规模DNS计算,计算效果令人惊喜,实现了一年前难以达到的计算规模.高分辨率的湍流热对流计算结果显示,不同高Ra数的湍流热对流流动状态完全不同.为高Ra数湍流热对流大规模并行计算和数值模拟研究提供了有价值的计算方法.     

湍流热对流中的动力学和传热研究

文章介绍了湍流热对流中流动结构和动力学的实验研究．本文作者通过流动示标和粒子成像测速系统（PIV）的测量，清晰地揭示了对流系统的三维流动结构，发现大尺度环流的角向运动在不同的时间尺度上有不同的表现．通过统计分析，本文作者进一步发现大尺度环流方位角的变化具有布朗棘齿的特性，并且从实验上证明在湍流热对流系统中不同的流动模式的确可以得到不同的传热效率．     

湍流大气成像系统分辨率的研究

采用基于Kolmogorov 1941串级湍流模型建立的调制湍流折射率谱模型、简单透镜成像系统传递函数的高斯函数近似和平方近似,从理论上分析了大气成像系统的光学传递函数,研究了有限湍流尺度(湍流内、外尺度)对湍流大气中成像系统光学分辨率的影响。给出了成像系统长曝光光学传递函数和积分分辨率的近似解析关系。结果表明在采用大孔径成像系统成像时,应该考虑湍流尺度对大气系统成像的作用。     

三维湍流Rayleigh-Bénard热对流的高效并行直接求解方法

高Ra数Rayleigh-Bénard热对流的湍流特性研究是当前国际上的一个热门研究课题, DNS模拟计算是研究该课题的重要手段之一. 当计算规模增大而网格数巨大时计算工作难以实现, 高Ra湍流热对流的数值模拟研究面临重大挑战. 本文创建了大规模高效并行计算的三维湍流热对流直接求解方法. 采用FFT变换解耦压力泊松方程, 将其变换成沿z方向上的块三对角方程组, 并利用块三对角方程的MPI与OpenMP联立的大规模高效并行近似解求解方案, 创建了可以高效并行计算的热对流直接求解方法. 通过对该方法并行效率的验证计算, 证明新的直接求解并行计算方法具有很好的并行效率和计算时效. 三维窄方腔热对流的计算结果表明, 本文方法计算的三维热对流特性是合理的. 本文创建的可大规模高效并行计算的三维湍流热对流直接求解方法, 也很可能是关于计算流体力学不可压NS方程大规模高效并行计算在特殊情况中计算技术上的一个突破.     

用于波前补偿实验的对流湍流系统的光学特性

描述一种用于波前补偿实验的对流湍流系统，它用水或乙醇为介质，在采用了自行研制的平板式加热和冷却器后，不仅湍流强度易于控制，温度起伏谱和大气相似，具有较宽的惯性区和较好的重复性，而且比实际大气湍流更平稳。本系统除可用于自适应光学系统的波前补偿试验和湍流与热晕相互作用的研究外，还可用于其它激光通过湍流大气的模拟实验研究。     

湍流热对流DNS多分辨率并行直接求解方法

依据温度标量场与动量计算的空间和时间计算分辨率不同的特点,采用两套网格,建立多分辨率双网格并行直接求解方法,用以解决极高Ra数湍流热对流DNS模拟巨大计算工作量的难题.在两套网格的数据交换上,根据每个细网格都满足连续方程,设计了速度的守恒平移插值方法.二维极高Ra数湍流热对流的计算结果表明,采用多分辨率双网格并行直接求解方法的DNS计算,可以使计算工作量降低近一个量级.瞬时温度场显示,双网格方法的计算结果可以很好地描述极高Ra数下快速运动的小尺寸漩涡团状羽流,得到的结果与原网格一致,不同方法计算得到的传热Nu数误差不超过1%.     

湍流大气成像系统长曝光传递函数的研究

采用傅里叶变换法对在Kolmogorov谱大气湍流下望远镜的长曝光成像进行了模拟,给出了湍流大气成像系统长曝光传递函数的数值模拟方法,求出了在此情况下的长曝光传递函数。给出了大气湍流在Kolmogorov谱假设下成像系统长曝光传递函数的理论值。基于空间目标为点目标的假设,提出了一种湍流大气成像系统长曝光传递函数的测量方法。对由三种方法获取的大气湍流长曝光传递函数进行了比对分析。结果表明,由三种方法获取的大气湍流长曝光传递函数基本上是一致的,由傅里叶变换法模拟和实际测量得到的传递函数是合理的。     

大气湍流对激光照射精度影响分析和计算

为了评估大气湍流效应对激光照射器照射精度的影响,理论分析和数值计算了由于大气湍流引入的激光照射误差随着照射距离、大气湍流强度、激光光束半径的变化关系。结果表明:随着照射距离和湍流强度的增加,以及激光光束半径的减小,大气湍流引起的激光照射误差增大,大气湍流对现有机载光电稳瞄系统照射精度影响为10 rad~30 rad。     

旋转体系中湍流的LBM模拟

采用基于多松弛时间因子的格子Boltzmann方法对旋转体系中的湍流进行数值研究,考察Rossby数和Ekman数对湍流的影响,包括湍流能量及其耗散率、速度、涡结构及湍流的耗散尺度即Kolmorogov尺度和积分尺度等.研究表明,系统的旋转延缓了湍流能量的衰减速率,逐步破坏初始涡结构的均匀性,与旋转方向相反的涡逐步被抑制,并最终形成若干与旋转同向的涡柱.结果还表明,系统旋转越快,湍流的耗散尺度越小而积分尺度越大.     

激光大气传输湍流扰动仿真技术

为了研究激光大气传输时湍流效应对激光应用技术的影响,对湍流扰动的仿真进行了分析。介绍了在实验室内进行激光大气传输湍流扰动研究的数值仿真技术和仿真系统。阐述了快速傅里叶变换（FFT）和Zernike多项式两种湍流扰动数值仿真方法,并且对比了两种方法的优劣。利用物理相位屏搭建了实物湍流仿真系统,介绍了其理论模型并进行了仿真实验,对激光经湍流系统传输后的光强能量分布进行了研究分析。结果显示,室内湍流仿真系统能够准确地模拟弱起伏条件下湍流对激光传输的影响。     

变物性条件下管内加速流时湍流换热与流动过程的数值模拟

对圆管内变物性流体在加速流时考虑了非定常流的影响,采用新的湍流迁移模型,对换热过程和流动特性进行了数值模拟研究。与实验相比较,得到较一致的结果。模拟计算在很大的参量范围内进行,结果标明:努谢尔特数及阻力系数与其准静态时的量值的比值在所定义的无量纲特征时间上有一最小值;在加速流时变物性的影响是导致不稳定过程加大的因素。     

输油管道油流静电带电数值计算

对输油管道内油品流动带电问题的数值计算进行了研究.紊流条件下的电荷输运方程是一个对流占优的对流扩散反应方程,采用算子分裂法,将该方程分解为纯对流方程、纯扩散方程和纯反应方程,分别采用特征线法和差分法求解.算例证明,该方法能准确描述管道内电荷分布,因而提供了一种获取冲流电流的可靠方法.     

管内对流换热的场协同分析及换热强化

导出管内湍流换热Nu与局域时均参数的关系式,将对流换热的场协同理论扩展至湍流换热。分析了管内对流换热的特点,并根据场协同理论提出强化湍流换热的方法,发展了一种新型强化换热管一交叉椭圆管,既适合于层流换热强化也适合于湍流换热强化,其强化传热效果显著而流阻较小。     

Local Collocation Approach for Solving Turbulent Combined Forced and Natural Convection Problems

An application of the meshless Local Radial Basis Function Collocation Method (LRBFCM) [22, 30–33] in solution of incompressible turbulent combined forced and natural convection is for the first time explored in the present paper. The turbulent flow equations are described by the low-Re number $k − ε$ model with Launder and Sharma [23] and Abe et al. [1] closure coefficients. The involved temperature, velocity, pressure, turbulent kinetic energy and dissipation fields are represented on overlapping 5-noded sub-domains through the collocation by using multiquadrics Radial Basis Functions (RBF). The involved first and second order partial derivatives of the fields are calculated from the respective derivatives of the RBF's. The involved equations are solved through the explicit time stepping. The pressure-velocity coupling is based on Chorin's fractional step method [11]. The adaptive upwinding technique, proposed by Lin and Atluri [27], is used because of the convection dominated situation. The solution procedure is represented for a 2D upward channel flow with differentially heated walls. The results have been assessed by achieving a reasonable agreement with the direct numerical simulation of Kasagi and Nishimura [20] for Reynolds number 4494, based on the channel width, and Grashof number 9.6 × 10⁵. The advantages of the represented mesh-free approach are its simplicity, accuracy, similar coding in 2D and 3D, and straightforward applicability in non-uniform node arrangements.     

Studying anomalous scaling and heat transport of turbulent thermal convection using a dynamical model

Turbulent thermal convection is a well-studied problem with various issues of interest. In this paper, we review our work which shows the nature and origin of anomalous scaling and heat transport in the limit of very strong thermal forcing, can be gained by studying a dynamical model, known as shell model, of homogeneous turbulent thermal convection in which buoyancy acts directly at most scales. Specifically, we have obtained two results. The first result is that when buoyancy acts directly at most scales such that the dynamics are governed by a cascade of entropy, the scaling behavior is described by Bolgiano and Obukhov scaling plus corrections that are due to the variations of the local entropy transfer rate. This result indicates the validity of the extension of refined similarity hypothesis to turbulent thermal convection. The second result is that when buoyancy is acting directly at most scales, a damping term acting on the largest scale, which has to be added for the system to achieve stationarity, plays a crucial role in heat transport, and that the heat transport depends on the strength of thermal forcing in the same manner as that predicted for the ultimate state of very strong thermal forcing. With our interpretation of the damping term representing the effect of the boundaries, this result indicates that in the ultimate state of turbulent thermal convection, when buoyancy is acting at most scales, boundaries would play a significant role in heat transport.     

Investigation of a turbulent wall jet in forced convection issuing into a directed coflow stream

In the present paper, we have studied numerically the directed coflow stream effects on mean and turbulent flow properties of a turbulent plane wall jet in forced convection emerging into a directed coflow stream. The system of equations governing the studied configuration is solved with a finite difference scheme using a staggered grid for numerical stability, not uniform in the two directions of the flow. The modified version of the first-order low Reynolds number k–? turbulence model is used and compared to existing experimental findings. It is found that predicted results are in satisfactory agreement with the experimental data and that the wall jet fluid decays faster in presence of a directed coflow stream. Results show also that the increase of coflow deviation angles causes an increase of the growth rates of the dynamic and thermal half-width of the jet and enhances the turbulent mixing. It is found that the longitudinal development of normalised forms of the jet characteristics parameters at different directed coflow velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale.     

Rotation curves of galaxies: Missing mass or missing physics

The rotation curves of galaxies are modelled using very special properties of an hydrodynamically turbulent fluid possessing helicity fluctuations. The development of correlations among these fluctuations leads to the formation of organized structures characterized by a new flat branch of the spatial energy spectrum in addition to the well known Kolmogorov spectrum. It is proposed that the flat nature of the rotation curves of galaxies may be a result of the energy cascading processes occuring in turbulent galactic atmospheres. Thus, in this model, there is no need of invoking dark matter to account for the flat rotation curves of galaxies.     

Fractal structures in turbulence

We present a qualitative overview of our work on the issue of fractal structures in turbulence. We explain why fully developed turbulence is not space filling and describe how its fractal dimension can be estimated theoretically. The implications of the fractal nature of turbulence on transport processes like turbulent diffusion and on fluctuations in passive scalars are discussed. The latter affect wave propagation in turbulent media and these effects are examined. In addition we consider clouds in the atmosphere which are claimed to have fractal perimeters (or surfaces) and outline the physical reasons for this phenomenon. The fractal dimension of clouds is tied to the theory of turbulent diffusion and is computed theoretically. Indications of the road ahead are given.