重大工程引起的珠江河口潮波逆向不稳定传播现象

采用模式ccost-2d对50年前白藤堵海工程及10年之后泥湾门区域的水动力特征变化进行了模拟和分析.结果可见,堵海工程造成泥湾门水道白蕉站潮差变小,高低潮时刻滞后2h以上,低潮水位抬升超过0.5m,引起严重的内涝问题.通过数值解对整个河网区的流动过程分析发现,由于堵海工程堵塞了泥湾门水道直接通向大海的通道,仅留的鸡啼...     

滑移边界对二维Rayleigh-Bénard热对流流态和传热特性的影响

采用并行直接数值模拟（PDM-DNS）计算无滑移和滑移边界二维Rayleigh-Bénard热对流.与无滑移边界形成的随机羽流运动的湍流热对流不同,滑移边界热对流最终形成湍流特征消失,且温度仅分布于四壁的一个大尺度环流的流动形态.平均场近底板的温度分布特性,无滑移边界逐渐变化而滑移边界出现过冲现象.宽高比Γ=1时,Nusselt数（Nu）随Rayleigh数（Ra）的变化具有相同标度指数,Nu~Ra^0.3.滑移边界热对流具有传热增强作用.滑移边界热对流Nu随Γ变化明显,并分为两个阶段,在Γ=0.5时出现Nu_max≈250,是无滑移边界热流Nu的5倍.     

采用交替分层法的河口海岸三维z坐标水流模式

利用河口三维水流数值模式平面变量空间配置的交替网格方法,设计并发展了速度与水位的交替分层法,成功地解决了垂向等平面z坐标模式将实际海底变为台阶状的问题.在ccost-2d水动力模式的基础上,发展了采用垂向z坐标模式而又能保证实际海底底边界模拟的三维水动力数值模式ccost-3d.以珠江西四口门网河和海区的整体计算模拟为算例,求解得到三维流场,水平表层速度分布和口门河道纵向剖面速度分布计算合理.     

二维湍流热对流羽流运动路径对传热特性的影响

本文采用并行直接求解方法计算了0.7和4.3两个Pr数的二维湍流热对流,最高Ra=10~(13).高和极高Ra数的二维湍流热对流传热Nu数与反映羽流运动的大尺度环流路径随Ra数的变化具有很好的相关性,并具有两个Ra数转折点.第一转折点出现在大尺度环流由椭圆变为圆形,其周长C_(LSC)随Ra数变化突然减小,第二转折点出现在大尺度环流周长C_(LSC)最小值处,随后羽流变为旋涡团状而周长C_(LSC)随Ra数增高而增大.Pr数较小对应的转折Ra数较低.传热Nu数的变化规律用Ra~(0.3)补偿后发现, Nu的局部标度律在大尺度环流周长C_(LSC)较小的情况下减小,出现偏离GL理论预测倍数线的现象.当Ra数大于第二转折点后,极高Ra数二维湍流热对流Nu数随Ra数的变化与GL理论预测倍数线符合良好,走向保持一致.     

高雷诺数湍流风场大涡模拟的并行直接求解方法

在环境流体力学中,风场是风沙流、风雪流等自然环境特性问题研究的动力源和基础.通常采用壁湍流模型进行风场大涡模拟(large eddy simulation, LES)计算,但受到计算规模的限制使得高雷诺数风场的模拟计算难以实现.并行计算技术是解决大规模高雷诺数风场大涡模拟的关键技术之一.在不可压湍流风场的LES模拟中,压力泊松方程的并行计算技术是进行规模并行计算的困难点.根据风场流动模拟计算的特点,采用水平网格等距而垂直于地面网格非等距,在解决规模并行计算中求解压力泊松方程的难点问题时,利用FFT解耦三维泊松方程使其变为垂向的一维三对角方程,并利用可并行的三对角方程PDD求解技术,可建立三维泊松方程的直接并行求解技术.结合其它容易并行的动量方程计算,本文建立风场LES模拟的并行直接求解方法 (parallel direct method-LES, PDM-LES).在超级计算机上对新方法进行并行效率测试,并行计算效率达到90%.新的方法可用于进行湍流风场大涡模拟的大规模并行计算.计算结果表明,湍流风场瞬时速度分布近壁面存在条带状的拟序结构,平均场的速度分布符合速度对数律特性,风场湍流特性基本合理.     

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.     

Numerical study of heat-transfer in two-and quasi-two-dimensional Rayleigh–Bénard convection

A detailed comparative numerical study between the two-dimensional(2 D) and quasi-two-dimensional(quasi-2 D)turbulent Rayleigh–B′enard(RB) convection on flow state, heat transfer, and thermal dissipation rate(TDR) is made. The Rayleigh number(Ra) in our simulations ranges up to 5×10~(10) and Prandtl number(Pr) is fixed to be 0.7. Our simulations are conducted on the Tianhe-2 supercomputer. We use an in-house code with high parallelization efficiency, based on the extended PDM–DNS scheme. The comparison shows that after a certain Ra, plumes with round shape, which is called the "temperature islands", develop and gradually dominate the flow field in the 2 D case. On the other hand, in quasi-2 D cases, plumes remain mushroom-like. This difference in morphology becomes more significant as Ra increases, as with the motion of plumes near the top and bottom plates. The exponents of the power-law relation between the Nusselt number(Nu) and Ra are 0.3 for both two cases, and the fitting pre-factors are 0.099 and 0.133 for 2 D and quasi-2 D respectively,indicating a clear difference in magnitude of the heat transfer rate between two cases. To understand this difference in the magnitude of Nu, we compare the vertical profile of the horizontally averaged TDR for both two cases. It is found that the profiles of both cases are nearly the same in the bulk, but they vary near boundaries. Comparing the bifurcation height zb with the thermal boundary layer thickness dq, it shows that zb δ_θ(3 D) δ_θ(2 D) and all three heights obey a universal power-law relation z ~Ra~(-0.30). In order to quantify the difference further, we separate the domain by zb, i.e., define the area between two zb(near top and bottom plates respectively) as the "mid region" and the rest as the "side region", and integrate TDR in corresponding regions. By comparing the integral it is found that most of the difference in TDR between two cases, which is connected to the heat transfer rate, occurs within the thermal boundary layers. We also compare the ratio of contributions to total heat transfer in BL–bulk separation and side–mid separation.     

Characteristics of temperature fluctuation in two-dimensional turbulent Rayleigh-Bénard convection

We study the characteristics of temperature fluctuation in two-dimensional turbulent Rayleigh–Benard convection in′a square cavity by direct numerical simulations.The Rayleigh number range is 1×10⁸≤Ra≤1×10¹³,and the Prandtl number is selected as Pr=0.7 and Pr=4.3.It is found that the temperature fluctuation profiles with respect to Ra exhibit two different distribution patterns.In the thermal boundary layer,the normalized fluctuationθrms/θrms,max is independent of Ra and a power law relation is identified,i.e.,θrms/θrms,max～(z/δ)0.99±0.01,where z/δis a dimensionless distance to the boundary(δis the thickness of thermal boundary layer).Out of the boundary layer,when Ra≤5×10⁹,the profiles ofθrms/θrms,max descend,then ascend,and finally drop dramatically as z/δincreases.While for Ra≥1×10¹⁰,the profiles continuously decrease and finally overlap with each other.The two different characteristics of temperature fluctuations are closely related to the formation of stable large-scale circulations and corner rolls.Besides,there is a critical value of Ra indicating the transition,beyond which the fluctuation hθrmsiV has a power law dependence on Ra,given by hθrmsiV～Ra?0.14±0.01.     

B-C转捩模型的DDES方法模拟转捩大分离流动

根据零压力梯度平板的实验数据对B-C转捩模型中的经验关联式进行标定.标定后的B-C转捩模型在进口湍流度较低的平板算例上可以提供更合理的转捩预测结果.但受限于RANS方法自身特性，B-C转捩模型对转捩后出现的大分离流动计算可靠性较差.结合延迟分离涡模拟SA-DDES方法和B-C转捩模型，发展了转捩-延迟分离涡模拟BC-DDES方法.三维S-K平板的数值模拟表明，BC-DDES方法能得到与B-C转捩模型一致的转捩结果.三维圆柱绕流的数值模拟表明，BC-DDES方法得到与实验符合的平均阻力系数和表面压力分布，并且计算花费比tHRLES方法少.     

高雷诺数湍流风场大涡模拟的并行直接求解方法

在环境流体力学中,风场是风沙流、风雪流等自然环境特性问题研究的动力源和基础. 通常采用壁湍流模型进行风场大涡模拟(large eddy simulation, LES)计算,但受到计算规模的限制使得 高雷诺数风场的模拟计算难以实现. 并行计算技术是解决大规模高雷诺数风场大涡模拟的关键技术之一. 在不可压湍流风场的LES模拟中,压力泊松方程的并行计算技术是进行规模并行计算的困难点. 根据风场流动模拟计算的特点,采用水平网格等距而垂直于地面网格非等距,在解决规模并行计算中求解压力泊松方程的难点问题时,利用FFT解耦三维泊松方程使其变为垂向的一维三对角方程, 并利用可并行的三对角方程PDD求解技术,可建立三维泊松方程的直接并行求解技术. 结合其它容易并行的动量方程计算,本文建立风场LES模拟的并行直接求解方法(parallel direct method-LES, PDM-LES). 在超级计算机上对新方法进行并行效率测试,并行计算效率达到90${\%}$. 新的方法可用于进行湍流风场大涡模拟的大规模并行计算. 计算结果表明,湍流风场瞬时速度分布近壁面存在条带状的拟序结构,平均场的速度分布符合速度对数律特性,风场湍流特性基本合理.